8. Examples¶
Some example set-ups are included in Sherpa, in the
<prefix>/share/SHERPA-MC/Examples/
directory. These may be useful
to new users to practice with, or as templates for creating your own
Sherpa run-cards. In this section, we will look at some of the main
features of these examples.
8.1. Vector boson + jets production¶
To change any of the following LHC examples to production at different collider energies or beam types, e.g. proton anti-proton at the Tevatron, simply change the beam settings accordingly:
BEAMS: [2212, -2212]
BEAM_ENERGIES: 980
8.1.1. W+jets production¶
This is an example setup for inclusive W production at hadron colliders. The inclusive process is calculated at next-to-leading order accuracy matched to the parton shower using the MC@NLO prescription detailed in [HKSS12]. The next few higher jet multiplicities, calculated at next-to-leading order as well, are merged into the inclusive sample using the MEPS@NLO method – an extension of the CKKW method to NLO – as described in [HKSS13] and [GHK+13]. Finally, even higher multiplicities, calculated at leading order, are merged on top of that. A few more things to note are detailed below the example.
# Sherpa configuration for W[lv]+Jets production
# set up beams for LHC run 2
BEAMS: 2212
BEAM_ENERGIES: 6500
# matrix-element calculation
ME_GENERATORS:
- Comix
- Amegic
- OpenLoops
# optional: use a custom jet criterion
#SHERPA_LDADD: MyJetCriterion
#JET_CRITERION: FASTJET[A:antikt,R:0.4,y:5]
# exclude tau (15) from (massless) lepton container (90)
PARTICLE_DATA:
15:
Massive: 1
# pp -> W[lv]+jets
PROCESSES:
- 93 93 -> 90 91 93{4}:
Order: {QCD: 0, EW: 2}
CKKW: 20
# set up NLO+PS final-state multiplicities
2->2-4:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
# make integration of higher final-state multiplicities faster
2->4-6:
Integration_Error: 0.05
SELECTORS:
# Safety cuts to avoid PDF calls with muF < 1 GeV
- [Mass, 11, -12, 1.0, E_CMS]
- [Mass, 13, -14, 1.0, E_CMS]
- [Mass, -11, 12, 1.0, E_CMS]
- [Mass, -13, 14, 1.0, E_CMS]
Things to notice:
The
Order
in the process definition in a multi-jet merged setup defines the order of the core process (here93 93 -> 90 91
with two electroweak couplings). The additional strong couplings for multi-jet production are implicitly understood.The settings necessary for NLO accuracy are restricted to the 2->2,3,4 processes using the
2->2-4
key below the# set up NLO+PS ...
comment. The example can be converted into a simple MENLOPS setup by using2->2
instead, or into an MEPS setup by removing these lines altogether. Thus one can study the effect of incorporating higher-order matrix elements.The number of additional LO jets can be varied through changing the integer within the curly braces in the
Process
definition, which gives the maximum number of additional partons in the matrix elements.OpenLoops
is used here as the provider of the one-loop matrix elements for the respective multiplicities.Tau leptons are set massive in order to exclude them from the massless lepton container (
90
).As both Comix and Amegic are specified as matrix element generators to be used, Amegic has to be specified to be used for all MC@NLO multiplicities using
ME_Generator: Amegic
. Additionally, we specifyRS_ME_Generator: Comix
such that the subtracted real-emission bit of the NLO matrix elements is calculated more efficiently with Comix instead of Amegic. This combination is currently the only one supported for NLO-matched/merged setups.
The jet criterion used to define the matrix element multiplicity in
the context of multijet merging can be supplied by the user. As an
example the source code file
./Examples/V_plus_Jets/LHC_WJets/My_JetCriterion.C
provides such
an alternative jet criterion. It can be compiled via
executing cmake .
in that directory.
The newly created library is linked at run time using
the SHERPA_LDADD
flag. The new jet criterion is then evoked by
JET_CRITERION
.
8.1.2. Z production¶
This is a very basic example at Leading Order for Z production at the LHC. Most of the settings are kept at their default, please refer to the next section for a more sophisticated calculation at higher multiplicity and accuracy.
# Sherpa configuration for Z[ee]+Jets production
# set up beams for LHC run 2
BEAMS: 2212
BEAM_ENERGIES: 6500
# matrix-element calculation
ME_GENERATORS: [Comix]
## 7-point variations
SCALE_VARIATIONS: 4.0*
# pp -> Z[ee]
PROCESSES:
- 93 93 -> 11 -11:
Order: {QCD: 0, EW: 2}
SELECTORS:
- [Mass, 11, -11, 66, E_CMS]
8.1.3. Z+jets production¶
This is an example setup for inclusive Z production at hadron colliders. The inclusive process is calculated at next-to-leading order accuracy matched to the parton shower using the MC@NLO prescription detailed in [HKSS12]. The next few higher jet multiplicities, calculated at next-to-leading order as well, are merged into the inclusive sample using the MEPS@NLO method – an extension of the CKKW method to NLO – as described in [HKSS13] and [GHK+13]. Finally, even higher multiplicities, calculated at leading order, are merged on top of that. A few things to note are detailed below the previous W+jets production example and apply also to this example.
# Sherpa configuration for Z[ee]+Jets production
# set up beams for LHC run 2
BEAMS: 2212
BEAM_ENERGIES: 6500
# matrix-element calculation
ME_GENERATORS:
- Comix
- Amegic
- OpenLoops
## OTF variations
SCALE_VARIATIONS:
- [0.25, 0.25]
- [1.0, 0.25]
- [0.25, 1.0]
- [1.0, 1.0]
- [4.0, 1.0]
- [1.0, 4.0]
- [4.0, 4.0]
PDF_VARIATIONS:
- PDF4LHC21_40_pdfas*
- NNPDF40_nnlo_as_01180
- MSHT20nnlo_as118
- CT18NNLO_as_0118
# EW setup and corrections
EW_SCHEME: alphamZsW
SIN2THETAW: 0.23113
ASSOCIATED_CONTRIBUTIONS_VARIATIONS:
- [EW]
- [EW, LO1]
- [EW, LO1, LO2]
- [EW, LO1, LO2, LO3]
# speed and neg weight fraction improvements
MC@NLO:
PSMODE: 2
RS_SCALE: METS{H_Tp2/4}
# pp -> Z[ee]+jets
PROCESSES:
- 93 93 -> 11 -11 93{5}:
Order: {QCD: 0, EW: 2}
CKKW: 20
# set up NLO+PS final-state multiplicities
2->2-4:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
Associated_Contributions: [EW, LO1, LO2, LO3]
# make integration of higher final-state multiplicities faster
2->4-7:
Integration_Error: 0.05
Max_N_Quarks: 4
Max_Epsilon: 0.01
SELECTORS:
- [Mass, 11, -11, 66, E_CMS]
8.1.4. Zbb with fusing¶
The fusing algorithm as introduced in [HocheKS19] aims for a consistent, combined prediction of a 4FS simulation and a merged 5FS prediction. The new simulation is hereby formed as sum of two components. First, the direct component is given by the 4FS calculation. It is reformulated as if it would be part of a merged simulation and while doing so, the Sudakov rejection by the parton shower will actually remove events. The fragmentation component then is given by the original, merged 5FS simulation. All configurations which are already described by the direct component are removed by an event filter.
Since the event filter operates on combined evolution histories, treating actual emissions from the parton shower and clustered emissions from matrix elements on the same footing, it must be ensured that the same cluster options are used everywhere.
The following parameters summarise all settings which where used for [HocheKS19].
8.1.4.1. The direct component¶
# set up beams for LHC run 2
BEAMS: 2212
BEAM_ENERGIES: 6500
# matrix-element calculation
ME_GENERATORS:
- Comix
- Amegic
- OpenLoops
# Fusing settings direct component
SHERPA_LDADD: SherpaFusing
USERHOOKS: [Fusing_Direct]
FUSING_DIRECT_FACTOR: 1.
PARTICLE_DATA:
5:
Massive: 1
Mass: 4.75 # consistent with PDF set
## TODO fusing: check which of these are still useful:
## for a consistent fusing, the parameters have to be chosen identical between
## direct and fragmentation component
#COMIX_CLUSTER_RS_ORDERED 1;
#COMIX_CLUSTER_ORDERED 1;
#CSS_SCALE_SCHEME 2;
#CSS_EVOLUTION_SCHEME 3;
#PP_RS_SCALE VAR{H_TM2};
#PP_HPSMODE=0;
# OTF variations
SCALE_VARIATIONS:
- [0.25, 0.25]
- [1.0, 0.25]
- [0.25, 1.0]
- [1.0, 1.0]
- [4.0, 1.0]
- [1.0, 4.0]
- [4.0, 4.0]
PDF_VARIATIONS:
- PDF4LHC21_40_pdfas*
- NNPDF40_nnlo_as_01180
- MSHT20nnlo_as118
- CT18NNLO_as_0118
## EW setup and corrections
#EW_SCHEME: alphamZsW
#SIN2THETAW: 0.23113
#ASSOCIATED_CONTRIBUTIONS_VARIATIONS:
# - [EW]
# - [EW, LO1]
# - [EW, LO1, LO2]
# - [EW, LO1, LO2, LO3]
# speed and neg weight fraction improvements
NLO_CSS_PSMODE: 2
MC@NLO:
RS_SCALE: METS{H_Tp2/4}
# pp -> Z[ee]+jets
PROCESSES:
- 93 93 -> 11 -11 5 -5:
Order: {QCD: 2, EW: 2}
CKKW: 100000.0 ## ensures correct Sudakov rejection for Fusing
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
#Associated_Contributions: [EW, LO1, LO2, LO3]
SELECTORS:
- [Mass, 11, -11, 66, E_CMS]
Things to notice:
The PDF and the strong coupling constant are evaluated in the 5FS. This is needed for consistency and to allow cluster configurations with initial-state bottom quarks. The resulting mismatch is corrected by counter-terms, implemented as user hook. If two strong couplings are involved at Born level,
FUSING_DIRECT_FACTOR
must be set to one. If there are four such couplings, as in the case of ttbb, it must be two.From a physical point of view, no merging cut is needed for massive bottom quarks. For technical reasons, it nevertheless must be set to an unreachable value such as
CKKW 100000.0
.
8.1.4.2. The fragmentation component¶
# set up beams for LHC run 2
BEAMS: 2212
BEAM_ENERGIES: 6500
# matrix-element calculation
ME_GENERATORS:
- Comix
- Amegic
- OpenLoops
# Fusing settings direct component
SHERPA_LDADD: SherpaFusing
USERHOOKS: [Fusing_Fragmentation]
PARTICLE_DATA:
5:
Mass: 4.75 # consistent with PDF set
# to set event weights instead of rejecting events use:
FUSING_FRAGMENTATION_STORE_AS_WEIGHT: 1
## TODO fusing: check which of these are still useful:
## for a consistent fusing, the parameters have to be chosen identical between
## direct and fragmentation component
#COMIX_CLUSTER_RS_ORDERED 1;
#COMIX_CLUSTER_ORDERED 1;
#CSS_SCALE_SCHEME 2;
#CSS_EVOLUTION_SCHEME 3;
#PP_RS_SCALE VAR{H_TM2};
#PP_HPSMODE=0;
# OTF variations
SCALE_VARIATIONS:
- [0.25, 0.25]
- [1.0, 0.25]
- [0.25, 1.0]
- [1.0, 1.0]
- [4.0, 1.0]
- [1.0, 4.0]
- [4.0, 4.0]
PDF_VARIATIONS:
- PDF4LHC21_40_pdfas*
- NNPDF40_nnlo_as_01180
- MSHT20nnlo_as118
- CT18NNLO_as_0118
## EW setup and corrections
#EW_SCHEME: alphamZsW
#SIN2THETAW: 0.23113
#ASSOCIATED_CONTRIBUTIONS_VARIATIONS:
# - [EW]
# - [EW, LO1]
# - [EW, LO1, LO2]
# - [EW, LO1, LO2, LO3]
# speed and neg weight fraction improvements
NLO_CSS_PSMODE: 2
MC@NLO:
RS_SCALE: METS{H_Tp2/4}
# pp -> Z[ee]+jets
PROCESSES:
- 93 93 -> 11 -11 93{4}:
Order: {QCD: 0, EW: 2}
CKKW: 20
# set up NLO+PS final-state multiplicities
2->2-4:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
#Associated_Contributions: [EW, LO1, LO2, LO3]
# make integration of higher final-state multiplicities faster
2->4-7:
Integration_Error: 0.05
Max_N_Quarks: 4
Max_Epsilon: 0.01
SELECTORS:
- [Mass, 11, -11, 66, E_CMS]
Things to notice:
The event filter can either be performed directly (default), or the veto information can be stored in HepMC. The latter requires
HEPMC_USE_NAMED_WEIGHTS=1
andFUSING_FRAGMENTATION_STORE_AS_WEIGHT=1
. The correspending weight is set to zero in case there is a fusing-induced event veto but remains unchanged if not.
8.1.5. W+bb production¶
This example is currently broken. Please contact the Authors for more information.
8.1.6. Zbb production¶
BEAMS: 2212
BEAM_ENERGIES: 6500
# general settings
EVENTS: 1M
# me generator settings
ME_GENERATORS: [Comix, Amegic, LHOLE]
HARD_DECAYS:
Enabled: true
Mass_Smearing: 0
Channels:
23,11,-11: {Status: 2}
23,13,-13: {Status: 2}
PARTICLE_DATA:
5:
Massive: true
Mass: 4.75 # consistent with MSTW 2008 nf 4 set
23:
Width: 0
Stable: 0
MI_HANDLER: None
FRAGMENTATION: None
MEPS:
CORE_SCALE: VAR{H_T2+sqr(91.188)}
PDF_LIBRARY: MSTW08Sherpa
PDF_SET: mstw2008nlo_nf4
PROCESSES:
- 93 93 -> 23 5 -5:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: LHOLE
Order: {QCD: 2, EW: 1}
SELECTORS:
- FastjetFinder:
Algorithm: antikt
N: 2
PTMin: 5.0
DR: 0.5
EtaMax: 5
Nb: 2
Things to notice:
The matrix elements are interfaced via the Binoth Les Houches interface proposal [B+10], [A+b], External one-loop ME.
The Z-boson is stable in the hard matrix elements. It is decayed using the internal decay module, indicated by the settings
HARD_DECAYS:Enabled: true
andPARTICLE_DATA:23:Stable: 0
.fjcore from FastJet is used to regularize the hard cross section. We require two b-jets, indicated by
Nb: 2
at the end of theFastjetFinder
options.Four-flavour PDF are used to comply with the calculational setup.
8.2. Jet production¶
8.2.1. Jet production¶
To change any of the following LHC examples to production at the Tevatron simply change the beam settings to
BEAMS: [2212, -2212]
BEAM_ENERGIES: 980
8.2.1.1. MC@NLO setup for dijet and inclusive jet production¶
This is an example setup for dijet and inclusive jet production at hadron colliders at next-to-leading order precision matched to the parton shower using the MC@NLO prescription detailed in [HKSS12] and [HS12]. A few things to note are detailed below the example.
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
TAGS:
LOOPGEN: <my-loop-gen>
# settings fot ME generators
ME_GENERATORS: [Amegic, Comix, $(LOOPGEN)]
# scale definitions
MEPS:
CORE_SCALE: VAR{0.25*H_T2}
SCALE_VARIATIONS:
- [0.25, 0.25]
- [1.0, 0.25]
- [0.25, 1.0]
- [1.0, 1.0]
- [4.0, 1.0]
- [1.0, 4.0]
- [4.0, 4.0]
PROCESSES:
- 93 93 -> 93 93:
Order: {QCD: 2, EW: 0}
NLO_Order: {QCD: 1, EW: 0}
NLO_Mode: MC@NLO
ME_Generator: Amegic
Loop_Generator: $(LOOPGEN)
RS_ME_Generator: Comix
SELECTORS:
- FastjetFinder:
Algorithm: antikt
N: 1
PTMin: 20
DR: 0.4
- FastjetFinder:
Algorithm: antikt
N: 2
PTMin: 10
DR: 0.4
Things to notice:
Asymmetric cuts are implemented (relevant to the RS-piece of an MC@NLO calculation) by requiring at least two jets with pT > 10 GeV, one of which has to have pT > 20 GeV.
Both the factorisation and renormalisation scales are set to the above defined scale factors times a quarter of the scalar sum of the transverse momenta of all anti-kt jets (R = 0.4, pT > 20 GeV) found on the ME-level before any parton shower emission. See SCALES for details on scale setters.
The resummation scale, which sets the maximum scale of the additional emission to be resummed by the parton shower, is set to the above defined resummation scale factor times half of the transverse momentum of the softer of the two jets present at Born level.
The external generator OpenLoops provides the one-loop matrix elements.
The
NLO_Mode
is set toMC@NLO
.
8.2.1.2. MEPS setup for jet production¶
BEAMS: 2212
BEAM_ENERGIES: 6500
PROCESSES:
- 93 93 -> 93 93 93{0}:
Order: {QCD: 2, EW: 0}
CKKW: 20
Integration_Error: 0.02
SELECTORS:
- NJetFinder:
N: 2
PTMin: 20.0
ETMin: 0.0
R: 0.4
Exp: -1
Things to notice:
Order
is set to{QCD: 2, EW: 0
}. This ensures that all final state jets are produced via the strong interaction.An
NJetFinder
selector is used to set a resolution criterion for the two jets of the core process. This is necessary because the “CKKW” tag does not apply any cuts to the core process, but only to the extra-jet matrix elements, see Multijet merged event generation with Sherpa.
8.2.2. Jets at lepton colliders¶
This section contains two setups to describe jet production at LEP I, either through multijet merging at leading order accuracy or at next-to-leading order accuracy.
8.2.2.1. MEPS setup for ee->jets¶
This example shows a LEP set-up, with electrons and positrons colliding at a centre of mass energy of 91.2 GeV.
BEAMS: [11, -11]
BEAM_ENERGIES: 45.6
ALPHAS(MZ): 0.1188
ORDER_ALPHAS: 1
PROCESSES:
- 11 -11 -> 93 93 93{3}:
CKKW: pow(10,-2.25/2.00)*E_CMS
Order: {QCD: 0, EW: 2}
Things to notice:
The running of alpha_s is set to leading order and the value of alpha_s at the Z-mass is set.
Note that initial-state radiation is enabled by default. See ISR parameters on how to disable it if you want to evaluate the (unphysical) case where the energy for the incoming leptons is fixed.
8.2.2.2. MEPS@NLO setup for ee->jets¶
This example expands upon the above setup, elevating its description of hard jet production to next-to-leading order.
# collider setup
BEAMS: [11, -11]
BEAM_ENERGIES: 45.6
TAGS:
# tags for process setup
YCUT: 2.0
# tags for ME generators
LOOPGEN0: Internal
LOOPGEN1: <my-loop-gen-for-3j>
LOOPGEN2: <my-loop-gen-for-4j>
LOOPMGEN: <my=loop-gen-for-massive-2j>
# settings for ME generators
ME_GENERATORS:
- Comix
- Amegic
- $(LOOPGEN0)
- $(LOOPGEN1)
- $(LOOPGEN2)
- $(LOOPMGEN)
AMEGIC: {INTEGRATOR: 4}
# model parameters
MODEL: SM
ALPHAS(MZ): 0.118
PARTICLE_DATA: {5: {Massive: true}}
HADRON_DECAYS:
Max_Proper_Lifetime: 100
PROCESSES:
- 11 -11 -> 93 93 93{3}:
CKKW: pow(10,-$(YCUT)/2.00)*E_CMS
Order: {QCD: 0, EW: 2}
RS_Enhance_Factor: 10
2->2: { Loop_Generator: $(LOOPGEN0) }
2->3: { Loop_Generator: $(LOOPGEN1) }
2->4: { Loop_Generator: $(LOOPGEN2) }
2->2-4:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
- 11 -11 -> 5 -5 93{3}:
CKKW: pow(10,-$(YCUT)/2.00)*E_CMS
Order: {QCD: 0, EW: 2}
Loop_Generator: $(LOOPMGEN)
RS_Enhance_Factor: 10
2->2:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
- 11 -11 -> 5 5 -5 -5 93{1}:
CKKW: pow(10,-$(YCUT)/2.00)*E_CMS
Order: {QCD: 2, EW: 2}
Cut_Core: 1
Things to notice:
the b-quark mass has been enabled for the matrix element calculation (the default is massless) because it is not negligible for LEP energies
the b b-bar and b b b-bar b-bar processes are specified separately because the
93
particle container contains only partons set massless in the matrix element calculation, see Particle containers.model parameters can be modified in the config file; in this example, the value of alpha_s at the Z mass is set.
8.3. Higgs boson + jets production¶
8.3.1. H production in gluon fusion with interference effects¶
This is a setup for inclusive Higgs production through gluon fusion at hadron colliders. The inclusive process is calculated at next-to-leading order accuracy, including all interference effects between Higgs-boson production and the SM gg->yy background. The corresponding matrix elements are taken from [BDS02] and [DL].
# collider parameters
BEAMS: 2212
BEAM_ENERGIES: 6500
# generator parameters
EVENTS: 1M
EVENT_GENERATION_MODE: Weighted
AMEGIC: {ALLOW_MAPPING: 0}
ME_GENERATORS: [Amegic, Higgs]
SCALES: VAR{Abs2(p[2]+p[3])}
# physics parameters
PARTICLE_DATA:
4: {Yukawa: 1.42}
5: {Yukawa: 4.92}
15: {Yukawa: 1.777}
EW_SCHEME: 3
RUN_MASS_BELOW_POLE: 1
PROCESSES:
- 93 93 -> 22 22:
NLO_Mode: Fixed_Order
Order: {QCD: 2, EW: 2}
NLO_Order: {QCD: 1, EW: 0}
Enable_MHV: 12
Loop_Generator: Higgs
Integrator: PS2
RS_Integrator: PS3
SELECTORS:
- HiggsFinder:
PT1: 40
PT2: 30
Eta: 2.5
MassRange: [100, 150]
- [IsolationCut, 22, 0.4, 2, 0.025]
Things to notice:
This calculation is at fixed-order NLO.
All scales, i.e. the factorisation, renormalisation and resummation scales are set to the invariant mass of the di-photon pair.
Dedicated phase space generators are used by setting
Integrator: PS2
andRS_Integrator: PS3
, cf. Integrator.
To compute the interference contribution only, as was done in
[DL], one can set HIGGS_INTERFERENCE_ONLY:
1
. By default, all partonic processes are included in this
simulation, however, it is sensible to disable quark initial states at
the leading order. This is achieved by setting
HIGGS_INTERFERENCE_MODE: 3
.
One can also simulate the production of a spin-2 massive graviton in
Sherpa using the same input card by setting
HIGGS_INTERFERENCE_SPIN: 2
. Only the massive graviton case
is implemented, specifically the scenario where k_q=k_g. NLO
corrections are approximated, as the gg->X->yy and qq->X->yy loop
amplitudes have not been computed so far.
8.3.2. H+jets production in gluon fusion¶
This is an example setup for inclusive Higgs production through gluon fusion at hadron colliders used in [HKS]. The inclusive process is calculated at next-to-leading order accuracy matched to the parton shower using the MC@NLO prescription detailed in [HKSS12]. The next few higher jet multiplicities, calculated at next-to-leading order as well, are merged into the inclusive sample using the MEPS@NLO method – an extension of the CKKW method to NLO – as described in [HKSS13] and [GHK+13]. Finally, even higher multiplicities, calculated at leading order, are merged on top of that. A few things to note are detailed below the example.
# collider parameters
BEAMS: 2212
BEAM_ENERGIES: 6500
# settings for ME generators
ME_GENERATORS: [Comix, Amegic, Internal, OpenLoops]
# settings for hard decays
HARD_DECAYS:
Enabled: true
Channels:
25,22,22: {Status: 2}
Apply_Branching_Ratios: false
Use_HO_SM_Widths: false
# model parameters
MODEL: HEFT
PARTICLE_DATA:
25: {Mass: 125, Width: 0}
PROCESSES:
- 93 93 -> 25 93{2}:
Order: {QCD: 2, EW: 0, HEFT: 1}
CKKW: 30
2->1-2: { Loop_Generator: Internal }
2->3: { Loop_Generator: OpenLoops }
2->1-3:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Things to notice:
The example can be converted into a simple MENLOPS setup by replacing
2->1-3
with2->1
, or into an MEPS setup with2->0
, to study the effect of incorporating higher-order matrix elements.Providers of the one-loop matrix elements for the respective multiplicities are set using
Loop_Generator
. For the two simplest cases Sherpa can provide it internally. Additionally, MCFM is interfaced for the H+2jet process, cf. MCFM interface.To enable the Higgs to decay to a pair of photons, for example, the hard decays are invoked. For details on the hard decay handling and how to enable specific decay modes see Hard decays.
8.3.3. H+jets production in gluon fusion with finite top mass effects¶
This is example is similar to H+jets production in gluon fusion but with finite top quark mass taken into account as described in [B+15] for all merged jet multiplicities. Mass effects in the virtual corrections are treated in an approximate way. In case of the tree-level contributions, including real emission corrections, no approximations are made concerning the mass effects.
# collider parameters
BEAMS: 2212
BEAM_ENERGIES: 6500
# settings for ME generators
ME_GENERATORS: [Amegic, Internal, OpenLoops]
# settings for hard decays
HARD_DECAYS:
Enabled: true
Channels:
25,22,22: {Status: 2}
Apply_Branching_Ratios: false
Use_HO_SM_Widths: false
# model parameters
MODEL: HEFT
PARTICLE_DATA:
25: {Mass: 125, Width: 0}
# finite top mass effects
KFACTOR: GGH
OL_IGNORE_MODEL: true
OL_PARAMETERS:
preset: 2
allowed_libs: pph2,pphj2,pphjj2
psp_tolerance: 1.0e-7
PROCESSES:
- 93 93 -> 25 93{1}:
Order: {QCD: 2, EW: 0, HEFT: 1}
CKKW: 30
Loop_Generator: Internal
2->1-2:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
Things to notice:
One-loop matrix elements from OpenLoops [CMP12] are used in order to correct for top mass effects. Sherpa must therefore be compiled with OpenLoops support to run this example. Also, the OpenLoops process libraries listed in the run card must be installed.
The maximum jet multiplicities that can be merged in this setup are limited by the availability of loop matrix elements used to correct for finite top mass effects.
The comments in H+jets production in gluon fusion apply here as well.
8.3.4. H+jets production in associated production¶
This section collects example setups for Higgs boson production in association with vector bosons
8.3.4.1. Higgs production in association with W bosons and jets¶
This is an example setup for Higgs boson production in association with a W boson and jets, as used in [HKP+]. It uses the MEPS@NLO method to merge pp->WH and pp->WHj at next-to-leading order accuracy and adds pp->WHjj at leading order. The Higgs boson is decayed to W-pairs and all W decay channels resulting in electrons or muons are accounted for, including those with intermediate taus.
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
ME_GENERATORS: [Comix, Amegic, OpenLoops]
# define custom particle container for easy process declaration
PARTICLE_CONTAINERS:
900: {Name: W, Flavs: [24, -24]}
901: {Name: lightflavs, Flavs: [1, -1, 2, -2, 3, -3, 4, -4, 21]}
MC@NLO:
DISALLOW_FLAVOUR: 5
# particle properties (ME widths need to be zero if external)
PARTICLE_DATA:
24: {Width: 0}
25: {Mass: 125.5, Width: 0}
15: {Stable: 0, Massive: true}
# hard decays setup, specify allowed decay channels, ie.:
# h->Wenu, h->Wmunu, h->Wtaunu, W->enu, W->munu, W->taunu, tau->enunu, tau->mununu + cc
HARD_DECAYS:
Enabled: true
Channels:
25,24,-12,11: {Status: 2}
25,24,-14,13: {Status: 2}
25,24,-16,15: {Status: 2}
25,-24,12,-11: {Status: 2}
25,-24,14,-13: {Status: 2}
25,-24,16,-15: {Status: 2}
24,12,-11: {Status: 2}
24,14,-13: {Status: 2}
24,16,-15: {Status: 2}
-24,-12,11: {Status: 2}
-24,-14,13: {Status: 2}
-24,-16,15: {Status: 2}
15,16,-12,11: {Status: 2}
15,16,-14,13: {Status: 2}
-15,-16,12,-11: {Status: 2}
-15,-16,14,-13: {Status: 2}
Decay_Tau: 1
Apply_Branching_Ratios: 0
PROCESSES:
- 901 901 -> 900 25 901{2}:
Order: {QCD: 0, EW: 2}
CKKW: 30
2->2-3:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
Things to notice:
Two custom particle container, cf. Particle containers, have been declared, facilitating the process declaration.
As the bottom quark is treated as being massless by default, a five flavour calculation is performed. The particle container ensures that no external bottom quarks, however, are considered to resolve the overlap with single top and top pair processes.
OpenLoops [CMP12] is used as the provider of the one-loop matrix elements.
To enable the decays of the Higgs, W boson and tau lepton the hard decay handler is invoked. For details on the hard decay handling and how to enable specific decay modes see Hard decays.
8.3.4.2. Higgs production in association with Z bosons and jets¶
This is an example setup for Higgs boson production in association with a Z boson and jets, as used in [HKP+]. It uses the MEPS@NLO method to merge pp->ZH and pp->ZHj at next-to-leading order accuracy and adds pp->ZHjj at leading order. The Higgs boson is decayed to W-pairs. All W and Z bosons are allowed to decay into electrons, muons or tau leptons. The tau leptons are then allowed to decay into all possible partonic channels, leptonic and hadronic, to allow for all possible trilepton signatures, unavoidably producing two and four lepton events as well.
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
ME_GENERATORS: [Comix, Amegic, OpenLoops]
# define custom particle container for easy process declaration
PARTICLE_CONTAINERS:
901: {Name: lightflavs, Flavs: [1, -1, 2, -2, 3, -3, 4, -4, 21]}
MC@NLO:
DISALLOW_FLAVOUR: 5
# particle properties (ME widths need to be zero if external)
PARTICLE_DATA:
23: {Width: 0}
25: {Mass: 125.5, Width: 0}
15: {Stable: 0, Massive: true}
# hard decays setup, specify allowed decay channels
# h->Wenu, h->Wmunu, h->Wtaunu, W->enu, W->munu, W->taunu,
# Z->ee, Z->mumu, Z->tautau, tau->any + cc
HARD_DECAYS:
Enabled: true
Channels:
25,24,-12,11: {Status: 2}
25,24,-14,13: {Status: 2}
25,24,-16,15: {Status: 2}
25,-24,12,-11: {Status: 2}
25,-24,14,-13: {Status: 2}
25,-24,16,-15: {Status: 2}
24,12,-11: {Status: 2}
24,14,-13: {Status: 2}
24,16,-15: {Status: 2}
23,15,-15: {Status: 2}
-24,-12,11: {Status: 2}
-24,-14,13: {Status: 2}
-24,-16,15: {Status: 2}
15,16,-12,11: {Status: 2}
15,16,-14,13: {Status: 2}
-15,-16,12,-11: {Status: 2}
-15,-16,14,-13: {Status: 2}
15,16,-2,1: {Status: 2}
15,16,-4,3: {Status: 2}
-15,-16,2,-1: {Status: 2}
-15,-16,4,-3: {Status: 2}
Decay_Tau: 1
Apply_Branching_Ratios: 0
PROCESSES:
- 901 901 -> 23 25 901{2}:
Order: {QCD: 0, EW: 2}
CKKW: 30
2->2-3:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
Things to notice:
A custom particle container, cf. Particle containers, has been declared, facilitating the process declaration.
As the bottom quark is treated as being massless by default, a five flavour calculation is performed. The particle container ensures that no external bottom quarks, however, are considered to resolve the overlap with single top and top pair processes.
OpenLoops [CMP12] is used as the provider of the one-loop matrix elements.
To enable the decays of the Higgs, W and Z bosons and tau lepton the hard decay handler is invoked. For details on the hard decay handling and how to enable specific decay modes see Hard decays.
8.3.4.3. Higgs production in association with lepton pairs¶
This is an example setup for Higgs boson production in association with an electron-positron pair using the MC@NLO technique. The Higgs boson is decayed to b-quark pairs. Contrary to the previous examples this setup does not use on-shell intermediate vector bosons in its matrix element calculation.
BEAMS: 2212
BEAM_ENERGIES: 6500
ME_GENERATORS: [Comix, Amegic, OpenLoops]
MEPS:
CORE_SCALE: VAR{Abs2(p[2]+p[3]+p[4])}
PARTICLE_DATA:
5: {Massive: true}
15: {Massive: true}
25: {Stable: 0, Width: 0.0}
# hard decays setup, specify allowed decay channels h->bb
HARD_DECAYS:
Enabled: true
Channels:
25 -> 5 -5: {Status: 2}
Apply_Branching_Ratios: false
PROCESSES:
- 93 93 -> 11 -11 25:
Order: {QCD: 0, EW: 3}
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
Loop_Generator: OpenLoops
ME_Generator: Amegic
RS_ME_Generator: Comix
Integration_Error: 0.1
Things to notice:
The central scale is set to the invariant mass of the Higgs boson and the lepton pair.
As the bottom quark is set to be treated massively, a four flavour calculation is performed.
OpenLoops [CMP12] is used as the provider of the one-loop matrix elements.
To enable the decays of the Higgs the hard decay handler is invoked. For details on the hard decay handling and how to enable specific decay modes see Hard decays.
8.3.5. Associated t anti-t H production at the LHC¶
This set-up illustrates the interface to an external loop matrix element generator as well as the possibility of specifying hard decays for particles emerging from the hard interaction. The process generated is the production of a Higgs boson in association with a top quark pair from two light partons in the initial state. Each top quark decays into an (anti-)bottom quark and a W boson. The W bosons in turn decay to either quarks or leptons.
BEAMS: 2212
BEAM_ENERGIES: 6500
ME_GENERATORS: [Comix, Amegic, OpenLoops]
MEPS:
CORE_SCALE: VAR{sqr(175+125/2)}
PARTICLE_DATA:
5: {Yukawa: 4.92}
6: {Stable: 0, Width: 0.0}
24: {Stable: 0}
25: {Stable: 0, Width: 0.0}
# hard decays setup, specify allowed decay channels h->bb
HARD_DECAYS:
Enabled: true
Channels:
25,5,-5: {Status: 2}
Apply_Branching_Ratios: false
PROCESSES:
- 93 93 -> 25 6 -6:
Order: {QCD: 2, EW: 1}
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
Loop_Generator: OpenLoops
ME_Generator: Amegic
RS_ME_Generator: Comix
Integration_Error: 0.1
Things to notice:
The virtual matrix elements is interfaced from OpenLoops.
The top quarks are stable in the hard matrix elements. They are decayed using the internal decay module, indicated by the settings in the
HARD_DECAYS
andPARTICLE_DATA
blocks.Widths of top and Higgs are set to 0 for the matrix element calculation. A kinematical Breit-Wigner distribution will be imposed a-posteriori in the decay module.
The Yukawa coupling of the b-quark has been set to a non-zero value to allow the H->bb decay channel even despite keeping the b-quark massless for a five-flavour-scheme calculation.
Higgs BRs are not included in the cross section (
Apply_Branching_Ratios: false
) as they will be LO only and not include loop-induced decays
8.4. Top quark (pair) + jets production¶
8.4.1. Top quark pair production¶
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# scales
EXCLUSIVE_CLUSTER_MODE: 1
MEPS:
CORE_SCALE: TTBar
# me generator settings
ME_GENERATORS: [Comix, Amegic, OpenLoops]
# decays
HARD_DECAYS:
Enabled: true
Channels:
24,2,-1: {Status: 0}
24,4,-3: {Status: 0}
-24,-2,1: {Status: 0}
-24,-4,3: {Status: 0}
# particle properties (width of external particles of the MEs must be zero)
PARTICLE_DATA:
6: {Width: 0}
# on-the-fly variations
SCALE_VARIATIONS: 4.0* # 7-point scale variations
PROCESSES:
- 93 93 -> 6 -6 93{3}:
Order: {QCD: 2, EW: 0}
CKKW: 20
2->2-3:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
2->5-8:
Max_N_Quarks: 6
Integration_Error: 0.05
Things to notice:
We use OpenLoops to compute the virtual corrections [CMP12].
We match matrix elements and parton showers using the MC@NLO technique for massive particles, as described in [HHL+13].
A non-default METS core scale setter is used, cf. Scale setting in multi-parton processes (METS)
We enable top decays through the internal decay module using
HARD_DECAYS:Enabled: true
We calculate on-the-fly a 7-point scale variation, cf. On-the-fly event weight variations.
8.4.2. Top quark pair production incuding approximate EW corrections¶
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# scales
EXCLUSIVE_CLUSTER_MODE: 1
MEPS:
CORE_SCALE: TTBar
# me generator settings
ME_GENERATORS: [Comix, Amegic, OpenLoops]
# decays
HARD_DECAYS:
Enabled: true
Channels:
24,2,-1: {Status: 0}
24,4,-3: {Status: 0}
-24,-2,1: {Status: 0}
-24,-4,3: {Status: 0}
# particle properties (width of external particles of the MEs must be zero)
PARTICLE_DATA:
6: {Width: 0}
# on-the-fly variations (QCD)
SCALE_VARIATIONS: 4.0* # 7-point scale variations
# on-the-fly variations (EWapprox)
ASSOCIATED_CONTRIBUTIONS_VARIATIONS:
- [EW]
- [EW, LO1]
- [EW, LO1, LO2]
- [EW, LO1, LO2, LO3]
OL_PARAMETERS:
ew_renorm_scheme: 1
PROCESSES:
- 93 93 -> 6 -6 93{3}:
Order: {QCD: 2, EW: 0}
CKKW: 20
2->2-3:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
Associated_Contributions: [EW, LO1, LO2, LO3]
2->5-8:
Max_N_Quarks: 6
Integration_Error: 0.05
Things to notice:
In addition to the setup in Top quark pair production we add approximate EW corrections, cf. [GLS18].
Please note: this setup only works with OpenLoops v.2 or later.
The approximate EW corrections are added as additional variations on the event weight.
8.4.3. Production of a top quark pair in association with a W-boson¶
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# me generator settings
ME_GENERATORS: [Comix, Amegic, OpenLoops]
# settings for hard decays
HARD_DECAYS:
Enabled: true
Channels:
24,2,-1: {Status: 0}
24,4,-3: {Status: 0}
24,16,-15: {Status: 0}
# model parameters
PARTICLE_DATA:
6: {Width: 0}
24: {Width: 0}
# technical parameters
EXCLUSIVE_CLUSTER_MODE: 1
PROCESSES:
- 93 93 -> 6 -6 24:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
Order: {QCD: 2, EW: 1}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
Things to notice:
Hard decays are enabled through
HARD_DECAYS:Enabled: true
.Top quarks and W bosons are final states in the hard matrix elements, so their widths are set to zero using
Width: 0
in theirPARTICLE_DATA
settings.Certain decay channels are disabled using
Status: 0
in theChannels
sub-settings of theHARD_DECAYS
setting.
8.5. Single-top production in the s, t and tW channel¶
In this section, examples for single-top production in three different channels are described. For the channel definitions and a validation of these setups, see [BSK].
8.5.1. t-channel single-top production¶
# SHERPA run card for t-channel single top-quark production at MC@NLO
# and N_f = 5
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# me generator settings
ME_GENERATORS: [Comix, Amegic, OpenLoops]
# scales
# CORESCALE SingleTop:
# use Mandelstam \hat{t} for s-channel 2->2 core process
MEPS:
CORE_SCALE: SingleTop
# disable hadronic W decays
HARD_DECAYS:
Enabled: true
Channels:
24,2,-1: {Status: 0}
24,4,-3: {Status: 0}
-24,-2,1: {Status: 0}
-24,-4,3: {Status: 0}
# choose EW Gmu input scheme
EW_SCHEME: 3
# required for using top-quark in ME
PARTICLE_DATA: { 6: {Width: 0} }
PROCESSES:
- 93 93 -> 6 93:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
Order: {QCD: 0, EW: 2}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
Min_N_TChannels: 1 # require t-channel W
Things to notice:
We use OpenLoops to compute the virtual corrections [CMP12].
We match matrix elements and parton showers using the MC@NLO technique for massive particles, as described in [HHL+13].
A non-default METS core scale setter is used, cf. Scale setting in multi-parton processes (METS)
We enable top and W decays through the internal decay module using
HARD_DECAYS:Enabled: true
. The W is restricted to its leptonic decay channels.By setting
Min_N_TChannels: 1
, only t-channel diagrams are used for the calculation
8.5.2. t-channel single-top production with N_f=4¶
# SHERPA run card for t-channel single top-quark production at MC@NLO
# and N_f = 4
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# me generator settings
ME_GENERATORS: [Comix, Amegic, OpenLoops]
# scales
# muR = transverse momentum of the bottom
# muF = muQ = transverse momentum of the top
MEPS:
CORE_SCALE: VAR{MPerp2(p[2])}{MPerp2(p[3])}{MPerp2(p[2])}
# disable hadronic W decays
HARD_DECAYS:
Enabled: true
Channels:
24,2,-1: {Status: 0}
24,4,-3: {Status: 0}
-24,-2,1: {Status: 0}
-24,-4,3: {Status: 0}
# choose EW Gmu input scheme
EW_SCHEME: 3
PARTICLE_DATA:
6: {Width: 0} # required for using top-quark in ME
5: {Massive: true, Mass: 4.18} # mass as in NNPDF30_nlo_as_0118_nf_4
# configure for N_f = 4
PDF_LIBRARY: LHAPDFSherpa
PDF_SET: NNPDF30_nlo_as_0118_nf_4
ALPHAS: {USE_PDF: 1}
PROCESSES:
- 93 93 -> 6 -5 93:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
Order: {QCD: 1, EW: 2}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
Min_N_TChannels: 1 # require t-channel W
Things to notice:
We use an Nf4 PDF and use its definition of the bottom mass
See t-channel single-top production for more comments
8.5.3. s-channel single-top production¶
# SHERPA run card for s-channel single top-quark production at MC@NLO
# and N_f = 5
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# me generator settings
ME_GENERATORS: [Comix, Amegic, OpenLoops]
# scales
# CORESCALE SingleTop:
# use Mandelstam \hat{s} for s-channel 2->2 core process
MEPS:
CORE_SCALE: SingleTop
# disable hadronic W decays
HARD_DECAYS:
Enabled: true
Channels:
24,2,-1: {Status: 0}
24,4,-3: {Status: 0}
-24,-2,1: {Status: 0}
-24,-4,3: {Status: 0}
# choose EW Gmu input scheme
EW_SCHEME: 3
# required for using top-quark in ME
PARTICLE_DATA: { 6: {Width: 0} }
# there is no bottom in the initial-state in s-channel production
PARTICLE_CONTAINERS:
900: {Name: lj, Flavs: [1, -1, 2, -2, 3, -3, 4, -4, 21]}
PROCESSES:
- 900 900 -> 6 93:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
Order: {QCD: 0, EW: 2}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
Max_N_TChannels: 0 # require s-channel W
Things to notice:
By excluding the bottom quark from the initial-state at Born level using
PARTICLE_CONTAINERS
, and by settingMax_N_TChannels: 0
, only s-channel diagrams are used for the calculationSee t-channel single-top production for more comments
8.5.4. tW-channel single-top production¶
# SHERPA run card for tW-channel single top-quark production at MC@NLO
# and N_f = 5
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# me generator settings
ME_GENERATORS: [Comix, Amegic, OpenLoops]
# scales
# mu = transverse momentum of the top
MEPS:
CORE_SCALE: VAR{MPerp2(p[3])}{MPerp2(p[3])}{MPerp2(p[3])}
# disable hadronic W decays
HARD_DECAYS:
Enabled: true
Channels:
24,2,-1: {Status: 0}
24,4,-3: {Status: 0}
-24,-2,1: {Status: 0}
-24,-4,3: {Status: 0}
# choose EW Gmu input scheme
EW_SCHEME: 3
# required for using top-quark/W-boson in ME
PARTICLE_DATA:
6: {Width: 0}
24: {Width: 0}
PROCESSES:
- 93 93 -> 6 -24:
No_Decay: -6 # remove ttbar diagrams
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
Order: {QCD: 1, EW: 1}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
Things to notice:
By setting
No_Decay: -6
, the doubly-resonant TTbar diagrams are removed. Only the singly-resonant diagrams remain as required by the definition of the channel.See t-channel single-top production for more comments
8.6. Vector boson pairs + jets production¶
8.6.1. Dilepton, missing energy and jets production¶
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# me generator settings
ME_GENERATORS: [Comix, Amegic, OpenLoops]
METS: { CLUSTER_MODE: 16 }
# define parton container without b-quarks to
# remove 0 processes with top contributions
PARTICLE_CONTAINERS:
901: {Name: lightflavs, Flavs: [1, -1, 2, -2, 3, -3, 4, -4, 21]}
MC@NLO:
DISALLOW_FLAVOUR: 5
PROCESSES:
- 901 901 -> 90 91 90 91 901{3}:
Order: {QCD: 0, EW: 4}
CKKW: 30
2->4-5:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
2->5-7:
Integration_Error: 0.05
SELECTORS:
- VariableSelector:
Variable: PT
Flavs: 90
Ranges: [[5.0, E_CMS], [5.0, E_CMS]]
Ordering: [PT_UP]
- [Mass, 11, -11, 10.0, E_CMS]
- [Mass, 13, -13, 10.0, E_CMS]
- [Mass, 15, -15, 10.0, E_CMS]
8.6.2. Dilepton, missing energy and jets production (gluon initiated)¶
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# scales
MEPS:
CORE_SCALE: VAR{Abs2(p[2]+p[3]+p[4]+p[5])/4.0}
# me generator settings
ME_GENERATORS: [Amegic, OpenLoops]
AMEGIC: { ALLOW_MAPPING: 0 }
# the following phase space libraries have to be generated with the
# corresponding qq->llvv setup (Sherpa.tree.yaml) first;
# they will appear in Process/Amegic/lib/libProc_fsrchannels*.so
SHERPA_LDADD: [Proc_fsrchannels4, Proc_fsrchannels5]
PROCESSES:
- 93 93 -> 90 90 91 91 93{1}:
CKKW: $(QCUT)
Enable_MHV: 10
Loop_Generator: OpenLoops
2->4:
Order: {QCD: 2, EW: 4}
Integrator: fsrchannels4
2->5:
Order: {QCD: 3, EW: 4}
Integrator: fsrchannels5
Integration_Error: 0.02
SELECTORS:
- [Mass, 11, -11, 10.0, E_CMS]
- [Mass, 13, -13, 10.0, E_CMS]
- [Mass, 15, -15, 10.0, E_CMS]
8.6.3. Four lepton and jets production¶
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# me generator settings
ME_GENERATORS: [Comix, Amegic, OpenLoops]
METS: { CLUSTER_MODE: 16 }
PROCESSES:
- 93 93 -> 90 90 90 90 93{3}:
Order: {QCD: 0, EW: 4}
CKKW: 30
2->4-5:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
2->5-7:
Integration_Error: 0.05
SELECTORS:
- VariableSelector:
Variable: PT
Flavs: 90
Ranges: [[5.0, E_CMS], [5.0, E_CMS]]
Ordering: [PT_UP]
- [Mass, 11, -11, 10.0, E_CMS]
- [Mass, 13, -13, 10.0, E_CMS]
- [Mass, 15, -15, 10.0, E_CMS]
8.6.4. Four lepton and jets production (gluon initiated)¶
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# scales
MEPS:
CORE_SCALE: VAR{Abs2(p[2]+p[3]+p[4]+p[5])/4.0}
# me generator settings
ME_GENERATORS: [Amegic, OpenLoops]
AMEGIC: { ALLOW_MAPPING: 0 }
# the following phase space libraries have to be generated with the
# corresponding qq->llll setup (Sherpa.tree.yaml) first;
# they will appear in Process/Amegic/lib/libProc_fsrchannels*.so
SHERPA_LDADD: [Proc_fsrchannels4, Proc_fsrchannels5]
PROCESSES:
- 93 93 -> 90 90 90 90 93{1}:
CKKW: $(QCUT)
Enable_MHV: 10
Loop_Generator: OpenLoops
2->4:
Order: {QCD: 2, EW: 4}
Integrator: fsrchannels4
2->5:
Order: {QCD: 3, EW: 4}
Integrator: fsrchannels5
Integration_Error: 0.02
SELECTORS:
- [Mass, 11, -11, 10.0, E_CMS]
- [Mass, 13, -13, 10.0, E_CMS]
- [Mass, 15, -15, 10.0, E_CMS]
8.6.5. WZ production with jets production¶
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# scales
MEPS:
CORE_SCALE: VAR{Abs2(p[2]+p[3])/4.0}
# me generator settings
ME_GENERATORS: [Comix, Amegic, OpenLoops]
HARD_DECAYS:
Enabled: true
Channels:
24,2,-1: {Status: 2}
24,4,-3: {Status: 2}
-24,-2,1: {Status: 2}
-24,-4,3: {Status: 2}
23,12,-12: {Status: 2}
23,14,-14: {Status: 2}
23,16,-16: {Status: 2}
PARTICLE_DATA:
23: {Width: 0}
24: {Width: 0}
PROCESSES:
- 93 93 -> 24 23 93{3}:
Order: {QCD: 0, EW: 2}
CKKW: 30
2->2-3:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
2->3-7:
Integration_Error: 0.05
- 93 93 -> -24 23 93{3}:
Order: {QCD: 0, EW: 2}
CKKW: 30
2->2-3:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
2->3-7:
Integration_Error: 0.05
8.6.6. Same sign dilepton, missing energy and jets production¶
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# choose EW Gmu input scheme
EW_SCHEME: 3
# tags for process setup
TAGS:
NJET: 1
QCUT: 30
# scales
MEPS:
CORE_SCALE: VAR{Abs2(p[2]+p[3]+p[4]+p[5])}
EXCLUSIVE_CLUSTER_MODE: 1
# solves problem with dipole QED modeling
ME_QED: { CLUSTERING_THRESHOLD: 10 }
# improve integration performance
PSI: { ITMIN: 25000 }
INTEGRATION_ERROR: 0.05
PROCESSES:
- 93 93 -> 11 11 -12 -12 93 93 93{$(NJET)}:
Order: {QCD: 0, EW: 6}
CKKW: $(QCUT)
- 93 93 -> 13 13 -14 -14 93 93 93{$(NJET)}:
Order: {QCD: 0, EW: 6}
CKKW: $(QCUT)
- 93 93 -> 15 15 -16 -16 93 93 93{$(NJET)}:
Order: {QCD: 0, EW: 6}
CKKW: $(QCUT)
- 93 93 -> 11 13 -12 -14 93 93 93{$(NJET)}:
Order: {QCD: 0, EW: 6}
CKKW: $(QCUT)
- 93 93 -> 11 15 -12 -16 93 93 93{$(NJET)}:
Order: {QCD: 0, EW: 6}
CKKW: $(QCUT)
- 93 93 -> 13 15 -14 -16 93 93 93{$(NJET)}:
Order: {QCD: 0, EW: 6}
CKKW: $(QCUT)
- 93 93 -> -11 -11 12 12 93 93 93{$(NJET)}:
Order: {QCD: 0, EW: 6}
CKKW: $(QCUT)
- 93 93 -> -13 -13 14 14 93 93 93{$(NJET)}:
Order: {QCD: 0, EW: 6}
CKKW: $(QCUT)
- 93 93 -> -15 -15 16 16 93 93 93{$(NJET)}:
Order: {QCD: 0, EW: 6}
CKKW: $(QCUT)
- 93 93 -> -11 -13 12 14 93 93 93{$(NJET)}:
Order: {QCD: 0, EW: 6}
CKKW: $(QCUT)
- 93 93 -> -11 -15 12 16 93 93 93{$(NJET)}:
Order: {QCD: 0, EW: 6}
CKKW: $(QCUT)
- 93 93 -> -13 -15 14 16 93 93 93{$(NJET)}:
Order: {QCD: 0, EW: 6}
CKKW: $(QCUT)
SELECTORS:
- [PT, 90, 5.0, E_CMS]
- NJetFinder:
N: 2
PTMin: 15.0
ETMin: 0.0
R: 0.4
Exp: -1
8.6.7. Polarized same-sign \(\mathrm{W}^+\) boson pair production in association with two jets at LO+PS¶
This is an example for the simulation of polarized cross sections for pure electroweak same-sign \(\mathrm{W}^+\) boson pair production in association with two jets at LO+PS.
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# ME-Generator settings
ME_GENERATORS:
- Comix
COMIX_DEFAULT_GAUGE: 0
# scales
MEPS:
CORE_SCALE: VAR{0.5*Abs2(p[2]+p[3])}
# width 0 for the stable W bosons in the hard matrix element
# width 0 for Z boson to preserve SU(2) Ward Identities
PARTICLE_DATA:
24: {Width: 0}
23: {Width: 0}
WIDTH_SCHEME: Fixed
# decay channels & polarization settings
HARD_DECAYS:
Enabled: true
Channels:
24,12,-11: {Status: 2}
24,14,-13: {Status: 2}
Pol_Cross_Section:
Enabled: true
Reference_System: [Lab, COM]
# vector boson production process
PROCESSES:
- 93 93 -> 24 24 93 93:
Order: {QCD: 0, EW: 4}
# cuts on PROCESSES final state particles
SELECTORS:
- FastjetSelector:
Expression: Mass(p[4]+p[5])>500
Algorithm: antikt
N: 2
PTMin: 20.0
EtaMax: 5.0
- FastjetSelector:
Expression: abs(Eta(p[4])-Eta(p[5]))>2.5
Algorithm: antikt
N: 2
PTMin: 20.0
EtaMax: 5.0
Things to notice:
The
COMIX_DEFAULT_GAUGE
is set to a special value to get the polarization vectors mentioned in section Simulation of polarized cross sections for intermediate particles .The width of the \(\mathrm{W}^\pm\) boson is set to zero to retain gauge invariance since it is considered as stable during the hard scattering process (
PROCESSES
). To preserve SU(2) Ward Identities also the width of the Z boson need to be set to zero then.The process is divided into the production of the vector bosons (
PROCESSES
) and their decays (HARD_DECAYS
), all matrix elements are calculated with on-shell vector bosons (narrow-width approximation).Mass_Smearing
andSPIN_CORRELATIONS
are enabled per default.
8.6.8. Polarized \(\mathrm{W}^+\) Z boson pair production -pair production at nLO+PS¶
This is an example for the simulation of polarized cross sections for pure electroweak \(\mathrm{W}^+\) Z boson pair production at nLO+PS. The resulting unpolarized cross section contains all NLO QCD corrections while for polarized cross sections the effect of virtual corrections on the polarization fractions is neglected.
# collider setup
BEAMS: 2212
BEAM_ENERGIES: 6500
# settings matrix-element generation
ME_GENERATORS:
- Comix
- Amegic
- OpenLoops
COMIX_DEFAULT_GAUGE: 0
# scale setting
SCALES: METS{0.25*sqr(80.352+91.153)}
# width 0 for the stable vector bosons in the hard matrix element
PARTICLE_DATA:
24:
Width: 0
23:
Width: 0
WIDTH_SCHEME: Fixed
# speed and neg weight fraction improvements
MC@NLO:
PSMODE: 2
# vector boson production part pp -> WZ
PROCESSES:
# leading order
- 93 93 -> 24 23:
Order: {QCD: 0, EW: 2}
# NLO QCD corrections
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
Loop_Generator: OpenLoops
# vector boson decays
HARD_DECAYS:
Enabled: true
Channels:
24,12,-11: {Status: 2}
23,13,-13: {Status: 2}
# settings for polarized cross sections
Pol_Cross_Section:
Enabled: true
Reference_System: [Lab, COM]
8.7. Event generation in the MSSM using UFO¶
This is an example for event generation in the MSSM using Sherpa’s UFO
support. In the corresponding Example directory
<prefix>/share/SHERPA-MC/Examples/BSM/UFO_MSSM/
, you will find a
directory MSSM
that contains the UFO output for the MSSM
(https://feynrules.irmp.ucl.ac.be/wiki/MSSM). To run the example,
generate the model as described in UFO Model Interface by
executing
$ cd <prefix>/share/SHERPA-MC/Examples/BSM/UFO_MSSM/
$ <prefix>/bin/Sherpa-generate-model MSSM
An example run card will be written to the working directory. Use this run card as a template to generate events.
8.8. Deep-inelastic scattering¶
8.8.1. DIS at HERA¶
This is an example of a setup for hadronic final states in deep-inelastic lepton-nucleon scattering at a centre-of-mass energy of 300 GeV. Corresponding measurements were carried out by the H1 and ZEUS collaborations at the HERA collider at DESY Hamburg.
# collider setup
BEAMS: [-11, 2212]
BEAM_ENERGIES: [27.5, 820]
PDF_SET: [None, Default]
# technical parameters
TAGS:
QCUT: 5
SDIS: 1.0
LGEN: BlackHat
ME_GENERATORS:
- Comix
- Amegic
- $(LGEN)
RESPECT_MASSIVE_FLAG: true
SHOWER:
KIN_SCHEME: 1
PROCESSES:
- -11 93 -> -11 93 93{4}:
CKKW: $(QCUT)/sqrt(1.0+sqr($(QCUT)/$(SDIS))/Abs2(p[2]-p[0]))
Order: {QCD: 0, EW: 2}
Max_N_Quarks: 6
Loop_Generator: $(LGEN)
2->2-3:
NLO_Mode: MC@NLO
NLO_Order: {QCD: 1, EW: 0}
ME_Generator: Amegic
RS_ME_Generator: Comix
2->3:
PSI_ItMin: 25000
Integration_Error: 0.03
SELECTORS:
- [Q2, -11, -11, 4, 1e12]
Things to notice:
the beams are asymmetric with the positrons at an energy of 27.5 GeV, while the protons carry 820 GeV of energy.
the multi-jet merging cut is set dynamically for each event, depending on the photon virtuality, see [CGH10].
there is a selector cut on the photon virtuality. This cut implements the experimental requirements for identifying the deep-inelastic scattering process.
8.9. Fixed-order next-to-leading order calculations¶
8.9.1. Production of NTuples¶
Root NTuples are a convenient way to store the result of cumbersome
fixed-order calculations in order to perform multiple analyses. This
example shows how to generate such NTuples and reweighted them in
order to change factorisation and renormalisation scales. Note that
in order to use this setup, Sherpa must be configured with option
-DSHERPA_ENABLE_ROOT=ON
, see Event output formats. If Sherpa has not been configured with Rivet analysis
support, please disable the analysis using -a0
on the
command line, see Command Line Options.
When using NTuples, one needs to bear in mind that every calculation involving jets in the final state is exclusive in the sense that a lower cut-off on the jet transverse momenta must be imposed. It is therefore necessary to check whether the event sample stored in the NTuple is sufficiently inclusive before using it. Similar remarks apply when photons are present in the NLO calculation or when cuts on leptons have been applied at generation level to increase efficiency. Every NTuple should therefore be accompanied by an appropriate documentation.
NTuple compression can be customized using the parameter
ROOTNTUPLE_COMPRESSION
, which is used to call
TFile::SetCompressionSettings
. For a detailed documentation of
available options, see http://root.cern.ch
This example will generate NTuples for the process pp->lvj, where l is an electron or positron, and v is an electron (anti-)neutrino. We identify parton-level jets using the anti-k_T algorithm with R=0.4 [CSS08]. We require the transverse momentum of these jets to be larger than 20 GeV. No other cuts are applied at generation level.
EVENTS: 100k
EVENT_GENERATION_MODE: Weighted
TAGS:
LGEN: BlackHat
ME_GENERATORS: [Amegic, $(LGEN)]
# Analysis (please configure with -DSHERPA_ENABLE_RIVET=ON & -DSHERPA_ENABLE_HEPMC3=ON)
ANALYSIS: Rivet
ANALYSIS_OUTPUT: Analysis/HTp/BVI/
# NTuple output (please configure with '-DSHERPA_ENABLE_ROOT=ON')
EVENT_OUTPUT: EDRoot[NTuple_B-like]
BEAMS: 2212
BEAM_ENERGIES: 3500
SCALES: VAR{sqr(sqrt(H_T2)-PPerp(p[2])-PPerp(p[3])+MPerp(p[2]+p[3]))/4}
EW_SCHEME: 0
WIDTH_SCHEME: Fixed # sin\theta_w -> 0.23
DIPOLES: {ALPHA: 0.03}
PARTICLE_DATA:
13: {Massive: true}
15: {Massive: true}
PROCESSES:
# The Born piece
- 93 93 -> 90 91 93:
Order: {QCD: 1, EW: 2}
NLO_Order: {QCD: 1, EW: 0}
NLO_Mode: Fixed_Order
NLO_Part: B
# The virtual piece
- 93 93 -> 90 91 93:
Order: {QCD: 1, EW: 2}
NLO_Order: {QCD: 1, EW: 0}
NLO_Mode: Fixed_Order
NLO_Part: V
Loop_Generator: $(LGEN)
# The integrated subtraction piece
- 93 93 -> 90 91 93:
Order: {QCD: 1, EW: 2}
NLO_Order: {QCD: 1, EW: 0}
NLO_Mode: Fixed_Order
NLO_Part: I
SELECTORS:
- FastjetFinder:
Algorithm: antikt
N: 1
PTMin: 20
ETMin: 0
DR: 0.4
RIVET:
--analyses: ATLAS_2012_I1083318
USE_HEPMC_SHORT: 1
--ignore-beams: 1
Things to notice:
NTuple production is enabled by
EVENT_OUTPUT: Root[NTuple_B-like]
, see Event output formats.The scale used is defined as in [BBD+09].
EW_SCHEME: 0
andWIDTH_SCHEME: Fixed
are used to set the value of the weak mixing angle to 0.23, consistent with EW precision measurements.DIPOLES:ALPHA: 0.03
is used to limit the active phase space of dipole subtractions.13:Massive: true
and15:Massive: 1
are used to limit the number of active lepton flavours to electron and positron.The option
USE_HEPMC_SHORT: 1
is used in the Rivet analysis section as the events produced by Sherpa are not at particle level.
8.9.1.1. NTuple production¶
Start Sherpa using the command line
$ Sherpa Sherpa.B-like.yaml
Sherpa will first create source code for its matrix-element calculations. This process will stop with a message instructing you to compile. Do so by running
$ ./makelibs -j4
Launch Sherpa again, using
$ Sherpa Sherpa.B-like.yaml
Sherpa will then compute the Born, virtual and integrated subtraction contribution to the NLO cross section and generate events. These events are analysed using the Rivet library and stored in a Root NTuple file called NTuple_B-like.root. We will use this NTuple later to compute an NLO uncertainty band.
The real-emission contribution, including subtraction terms, to the NLO cross section is computed using
$ Sherpa Sherpa.R-like.yaml
Events are generated, analysed by Rivet and stored in the Root NTuple file NTuple_R-like.root.
The two analyses of events with Born-like and real-emission-like kinematics need to be merged, which can be achieved using scripts like yodamerge. The result can then be plotted and displayed.
8.9.1.2. Usage of NTuples in Sherpa¶
Next we will compute the NLO uncertainty band using Sherpa. To this
end, we make use of the Root NTuples generated in the previous steps.
Note that the setup files for reweighting are almost identical to
those for generating the NTuples. We have simply replaced
EVENT_OUTPUT
by EVENT_INPUT
.
We re-evaluate the events with the scale variation as defined in the Reweight configuration files:
$ Sherpa Sherpa.Reweight.B-like.yaml
$ Sherpa Sherpa.Reweight.R-like.yaml
The contributions can again be combined using yodamerge.
8.9.2. MINLO¶
The following configuration file shows how to implement the MINLO procedure from [HNZ]. A few things to note are detailed below. MINLO can also be applied when reading NTuples, see Production of NTuples. In this case, the scale and K factor must be defined, see SCALES and KFACTOR.
BEAMS: 2212
BEAM_ENERGIES: 6500
EVENT_GENERATION_MODE: W
MEPS:
CORE_SCALE: VAR{Abs2(p[2]+p[3])+0.25*sqr(sqrt(H_T2)-PPerp(p[2])-PPerp(p[3])+PPerp(p[2]+p[3]))}
PROCESSES:
- 93 93 -> 11 -12 93:
Scales: MINLO
KFactor: MINLO
ME_Generator: Amegic
Loop_Generator: BlackHat
Order: {QCD: 1, EW: 2}
SELECTORS:
- [Mass, 11, -12, 2, E_CMS]
- FastjetFinder:
Algorithm: antikt
N: 1
PTMin: 1.0
ETMin: 1.0
DR: 0.4
Things to notice:
The R parameter of the flavour-based kT clustering algorithm can be changed using
MINLO:DELTA_R
.Setting
MINLO: {SUDAKOV_MODE: 0
} defines whether to include power corrections stemming from the finite parts in the integral over branching probabilities. It defaults to 1.The parameter
MINLO:SUDAKOV_PRECISION
defines the precision target for integration of the Sudakov exponent. It defaults to1e-4
.
8.10. Soft QCD: Minimum Bias and Cross Sections¶
8.10.1. Calculation of inclusive cross sections¶
Note that this example is not yet updated to the new YAML input format. Contact the Authors for more information.
(run){
OUTPUT = 2
EVENT_TYPE = MinimumBias
SOFT_COLLISIONS = Shrimps
Shrimps_Mode = Xsecs
deltaY = 1.5;
Lambda2 = 1.7;
beta_0^2 = 20.0;
kappa = 0.6;
xi = 0.2;
lambda = 0.3;
Delta = 0.4;
}(run)
(beam){
BEAM_1 = 2212; BEAM_ENERGY_1 = 450.;
BEAM_2 = 2212; BEAM_ENERGY_2 = 450.;
}(beam)
(me){
ME_SIGNAL_GENERATOR = None
}(me)
Things to notice:
Inclusive cross sections (total, inelastic, low-mass single-diffractive, low-mass double-diffractive, elastic) and the elastic slope are calculated for varying centre-of-mass energies in pp collisions
The results are written to the file InclusiveQuantities/xsecs_total.dat and to the screen. The directory will automatically be created in the path from where Sherpa is run.
The parameters of the model are not very well tuned.
8.10.2. Simulation of Minimum Bias events¶
Contact the Authors for more information.
BEAMS: 2212
BEAM_ENERGIES: 3500
EVENT_TYPE: MinimumBias
ME_GENERATORS: None
SOFT_COLLISIONS: Shrimps
Shrimps_Mode: Inelastic
FRAGMENTATION: Ahadic
YFS:
MODE: None
ANALYSIS: Rivet
ANALYSIS_OUTPUT: Shrimps
RIVET:
--analyses: [ATLAS_2010_S8918562, ATLAS_2010_S8894728, ATLAS_2011_S8994773, ATLAS_2012_I1084540, TOTEM_2012_I1115294, CMS_2011_S8978280, CMS_2011_S9120041, CMS_2011_S9215166]
Things to notice:
The SHRiMPS model is not properly tuned yet – all parameters are set to very natural values, such as for example 1.0 GeV for infrared parameters.
Elastic scattering and low-mass diffraction are not included.
A large number of Minimum Bias-type analyses is enabled.
8.11. Setups for event production at B-factories¶
8.11.1. QCD continuum¶
Example setup for QCD continuum production at the Belle/KEK collider. Please note, it does not include any hadronic resonance.
# collider setup
BEAMS: [11, -11]
BEAM_ENERGIES: [7., 4.]
# model parameters
ALPHAS(MZ): 0.1188
ORDER_ALPHAS: 1
PARTICLE_DATA:
4: {Massive: 1}
5: {Massive: 1}
MASSIVE_PS: 3
PROCESSES:
- 11 -11 -> 93 93:
Order: {QCD: 0, EW: 2}
- 11 -11 -> 4 -4:
Order: {QCD: 0, EW: 2}
- 11 -11 -> 5 -5:
Order: {QCD: 0, EW: 2}
Things to notice:
Asymmetric beam energies, photon ISR is switched on per default.
Full mass effects of c and b quarks computed.
Strong coupling constant value set to 0.1188 and two loop (NLO) running.
8.11.2. Signal process¶
Example setup for B-hadron pair production on the Y(4S) pole.
# collider setup
BEAMS: [11, -11]
BEAM_ENERGIES: [7., 4.]
# model parameters
ALPHAS(MZ): 0.1188
ORDER_ALPHAS: 1
PARTICLE_DATA:
4: {Massive: 1}
5: {Massive: 1}
MASSIVE_PS: 3
ME_GENERATORS: Internal
MEPS:
CORE_SCALE: VAR{sqr(91.2)}
PROCESSES:
#
# electron positron -> Y(4S) -> B+ B-
#
- 11 -11 -> 300553[a]:
Decay: "300553[a] -> 521 -521"
Order: {QCD: 0, EW: 0}
#
# electron positron -> Y(4S) -> B0 B0bar
#
- 11 -11 -> 300553[a]:
Decay: "300553[a] -> 511 -511"
Order: {QCD: 0, EW: 0}
Things to notice:
Same setup as QCD continuum, except for process specification.
Production of both B0 and B+ pairs, in due proportion.
8.11.3. Single hadron decay chains¶
This setup is not a collider setup, but a simulation of a hadronic decay chain.
# collider setup
BEAMS: [11, -11]
BEAM_ENERGIES: [7., 4.]
# general settings
EVENT_TYPE: HadronDecay
# specify hadron to be decayed
DECAYER: 511
# initialise rest for Sherpa not to complain
# model parameters
SCALES: VAR{sqr(91.2)}
PROCESSES:
- 11 -11 -> 13 -13:
Order: {QCD: 0, EW: 2}
Things to notice:
EVENT_TYPE
is set toHadronDecay
.DECAYER
specifies the hadron flavour initialising the decay chain.A place holder process is declared such that the Sherpa framework can be initialised. That process will not be used.
8.12. Calculating matrix element values for externally given configurations¶
8.12.1. Computing matrix elements for individual phase space points using the Python Interface¶
Sherpa’s Python interface (see Python Interface) can be used to compute matrix elements for individual phase space points. Access to a designated class “MEProcess” is provided by interface to compute matrix elements as illustrated in the example script.
Please note that the process in the script must be compatible with the one specified in the Sherpa configuration file in the working directory. A random phase space point for the process of interest can be generated as shown in the example.
If AMEGIC++ is used as the matrix element generator, executing the
script will result in AMEGIC++ writing out libraries and exiting.
After compiling the libraries using ./makelibs
, the script must be
executed again in order to obtain the matrix element.
#!/usr/bin/env python
@LOADMPIFORPY@
import sys
sys.path.append('@PYLIBDIR@')
import Sherpa
# Add this to the execution arguments to prevent Sherpa from starting the cross section integration
sys.argv.append('INIT_ONLY: 2')
Generator=Sherpa.Sherpa(len(sys.argv),sys.argv)
try:
Generator.InitializeTheRun()
Process=Sherpa.MEProcess(Generator)
# Incoming flavors must be added first!
Process.AddInFlav(11);
Process.AddInFlav(-11);
Process.AddOutFlav(1);
Process.AddOutFlav(-1);
Process.Initialize();
# First argument corresponds to particle index:
# index 0 correspons to particle added first, index 1 is the particle added second, and so on...
Process.SetMomentum(0, 45.6,0.,0.,45.6)
Process.SetMomentum(1, 45.6,0.,0.,-45.6)
Process.SetMomentum(2, 45.6,0.,45.6,0.)
Process.SetMomentum(3, 45.6,0.,-45.6,0.)
print('\nSquared ME: ', Process.CSMatrixElement())
# Momentum setting via list of floats
Process.SetMomenta([[45.6,0.,0.,45.6],
[45.6,0.,0.,-45.6],
[45.6,0.,45.6,0.],
[45.6,0.,-45.6,0.]])
print('\nSquared ME: ', Process.CSMatrixElement())
# Random momenta
E_cms = 500.0
wgt = Process.TestPoint(E_cms)
print('\nRandom test point ')
print('Squared ME: ', Process.CSMatrixElement(), '\n')
except Sherpa.SherpaException as exc:
print(exc)
exit(1)
8.12.2. Computing matrix elements for individual phase space points using the C++ Interface¶
Matrix elements values for user defined phase space points can also be
quarried using a small C++ executable provided in
Examples/API/ME2
. It can be compiled using the provided
Makefile
. The test program is then run typing (note: the
LD_LIBRARY_PATH
must be set to include
<Sherpa-installation>/lib/SHERPA-MC
)
$ ./test <options>
where the usual options for Sherpa are passed. An example configuration file, giving both the process and the requested phase space points looks like
BEAMS: [11, -11]
BEAM_ENERGIES: 45.6
EVENTS: 0
INIT_ONLY: 2
PDF_LIBRARY: None
PROCESSES:
- 11 -11 -> 2 -2 21 21 21 21: {}
MOMENTA:
- [
[ 11, 45.6, 0.0, 0.0, 45.6 ],
[ -11, 45.6, 0.0, 0.0, -45.6 ],
[ 21, 10.0, 0.0, 0.0, -10.0, 1, 2 ],
[ 21, 10.0, 0.0, 0.0, 10.0, 2, 3 ],
[ 21, 10.0, 10.0, 0.0, 0.0, 3, 1 ],
[ 21, 10.0, -10.0, 0.0, 0.0, 1, 3 ],
[ 2, 25.6, 0.0, 25.6, 0.0, 3, 0 ],
[ -2, 25.6, 0.0, -25.6, 0.0, 0, 1 ]
]
- [
[ 11, 45.6, 0.0, 0.0, 45.6 ],
[ -11, 45.6, 0.0, 0.0, -45.6 ],
[ 21, 12.0, 0.0, 0.0, -12.0, 1, 2 ],
[ 21, 12.0, 0.0, 0.0, 12.0, 2, 3 ],
[ 21, 12.0, 12.0, 0.0, 0.0, 3, 1 ],
[ 21, 12.0, -12.0, 0.0, 0.0, 1, 3 ],
[ 2, 21.6, 0.0, 21.6, 0.0, 3, 0 ],
[ -2, 21.6, 0.0, -21.6, 0.0, 0, 1 ]
]
- [
[ 11, 45.6, 0.0, 0.0, 45.6 ],
[ -11, 45.6, 0.0, 0.0, -45.6 ],
[ 21, 14.0, 0.0, 0.0, -14.0, 1, 2 ],
[ 21, 14.0, 0.0, 0.0, 14.0, 2, 3 ],
[ 21, 14.0, 14.0, 0.0, 0.0, 3, 1 ],
[ 21, 14.0, -14.0, 0.0, 0.0, 1, 3 ],
[ 2, 17.6, 0.0, 17.6, 0.0, 3, 0 ],
[ -2, 17.6, 0.0, -17.6, 0.0, 0, 1 ]
]
- [
[ 11, 45.6, 0.0, 0.0, 45.6 ],
[ -11, 45.6, 0.0, 0.0, -45.6 ],
[ 21, 16.0, 0.0, 0.0, -16.0, 1, 2 ],
[ 21, 16.0, 0.0, 0.0, 16.0, 2, 3 ],
[ 21, 16.0, 16.0, 0.0, 0.0, 3, 1 ],
[ 21, 16.0, -16.0, 0.0, 0.0, 1, 3 ],
[ 2, 13.6, 0.0, 13.6, 0.0, 3, 0 ],
[ -2, 13.6, 0.0, -13.6, 0.0, 0, 1 ]
]
Please note that both the process and the beam specifications need to be present in order for Sherpa to initialise properly. The momenta need to be given in the proper ordering employed in Sherpa, which can be read from the process name printed on screen. For each entry the sequence is the following
[<pdg-id>, <E>, <px>, <py>, <pz>, triplet-index, antitriplet-index]
with the colour indices ranging from 1..3 for both the triplet and the antitriplet index in the colour-flow basis. The colour information is only needed if Comix is used for the calculation as Comix then also gives the squared matrix element value for this colour configuration. Otherwise, the last two arguments can be omitted. In any case, the colour-summed value is printed to screen.
8.13. Using the Python interface¶
8.13.1. Generate events using scripts¶
This example shows how to generate events with Sherpa using a Python wrapper script. For each event the weight, the number of trials and the particle information is printed to stdout. This script can be used as a basis for constructing interfaces to own analysis routines.
#!/usr/bin/env python
@LOADMPIFORPY@
import sys
sys.path.append('@PYLIBDIR@')
import Sherpa
Generator=Sherpa.Sherpa(len(sys.argv),sys.argv)
try:
Generator.InitializeTheRun()
Generator.InitializeTheEventHandler()
for n in range(1,1+Generator.NumberOfEvents()):
Generator.GenerateOneEvent()
blobs=Generator.GetBlobList();
print("Event",n,"{")
## print blobs
print(" Weight ",blobs.GetFirst(1)["Weight"]);
print(" Trials ",blobs.GetFirst(1)["Trials"]);
for i in range(0,blobs.size()):
print(" Blob",i,"{")
## print blobs[i];
print(" Incoming particles")
for j in range(0,blobs[i].NInP()):
part=blobs[i].InPart(j)
## print part
s=part.Stat()
f=part.Flav()
p=part.Momentum()
print(" ",j,": ",s,f,p)
print(" Outgoing particles")
for j in range(0,blobs[i].NOutP()):
part=blobs[i].OutPart(j)
## print part
s=part.Stat()
f=part.Flav()
p=part.Momentum()
print(" ",j,": ",s,f,p)
print(" } Blob",i)
print("} Event",n)
if ((n%100)==0): print(" Event ",n)
Generator.SummarizeRun()
except Sherpa.SherpaException as exc:
exit(1)
8.13.2. Generate events with MPI using scripts¶
This example shows how to generate events with Sherpa using a Python
wrapper script and MPI. For each event the weight, the number of
trials and the particle information is send to the MPI master node and
written into a single gzip’ed output file. Note that you need the
mpi4py module to run this Example. Sherpa
must be configured and installed using -DSHERPA_ENABLE_MPI
, see
MPI parallelization.
#!/usr/bin/env python
@LOADMPIFORPY@
import sys
sys.path.append('@PYLIBDIR@')
import Sherpa
import gzip
class MyParticle:
def __init__(self,p):
self.kfc=p.Flav().Kfcode()
if p.Flav().IsAnti(): self.kfc=-self.kfc
self.E=p.Momentum()[0]
self.px=p.Momentum()[1]
self.py=p.Momentum()[2]
self.pz=p.Momentum()[3]
def __str__(self):
return (str(self.kfc)+" "+str(self.E)+" "
+str(self.px)+" "+str(self.py)+" "+str(self.pz))
Generator=Sherpa.Sherpa(len(sys.argv),sys.argv)
try:
Generator.InitializeTheRun()
Generator.InitializeTheEventHandler()
comm=MPI.COMM_WORLD
rank=comm.Get_rank()
size=comm.Get_size()
if rank==0:
outfile=gzip.GzipFile("events.gz",'w')
for n in range(1,1+Generator.NumberOfEvents()):
for t in range(1,size):
weight=comm.recv(source=t,tag=t)
trials=comm.recv(source=t,tag=2*t)
parts=comm.recv(source=t,tag=3*t)
outfile.write("E "+str(weight)+" "+str(trials)+"\n")
for p in parts:
outfile.write(str(p)+"\n")
if (n%100)==0: print(" Event",n)
outfile.close()
else:
for n in range(1,1+Generator.NumberOfEvents()):
Generator.GenerateOneEvent()
blobs=Generator.GetBlobList();
weight=blobs.GetFirst(1)["Weight"]
trials=blobs.GetFirst(1)["Trials"]
parts=[]
for i in range(0,blobs.size()):
for j in range(0,blobs[i].NOutP()):
part=blobs[i].OutPart(j)
if part.Stat()==1 and part.HasDecBlob()==0:
parts.append(MyParticle(part))
comm.send(weight,dest=0,tag=rank)
comm.send(trials,dest=0,tag=2*rank)
comm.send(parts,dest=0,tag=3*rank)
Generator.SummarizeRun()
except Sherpa.SherpaException as exc:
exit(1)