5.6. Processes

The process setup takes the following general form

PROCESSES:
# Process 1:
- <process declaration>:
    <parameter>: <value>
    <multiplicities-to-be-applied-to>:
      <parameter>: <value>
      ...
# Process 2:
- <process declaration>
    ...

I.e. PROCESSES followed by a list of process definitions. Each definition consists of a key-value mapping with a single key. The key defines the initial- and final-state particles, and its value gives the optional parameters for this process steering its setup. Most of these parameters can be grouped under a multiplicity key, which can either be a single or a range of multiplicities, e.g. 2->2-4: { <settings-that-affect-only-2->2,2->3 and 2->4 processes> }.

The following parameters are used to steer the process setup:

5.6.1. Process

Each process definition starts with the specification of the (core) process itself. The initial and final state particles are specified by their PDG codes, or by particle containers, see Particle containers. Examples are

- 93 93 -> 11 -11

Sets up a Drell-Yan process group with light quarks in the initial state.

- 11 -11 -> 93 93 93{3}

Sets up jet production in e+e- collisions with up to three additional jets.

The syntax for specifying processes is explained in the following sections:

5.6.1.1. PDG codes

Initial and final state particles are specified using their PDG codes (cf. PDG). A list of particles with their codes, and some of their properties, is printed at the start of each Sherpa run, when the OUTPUT is set at level 2.

5.6.1.2. Particle containers

Sherpa contains a set of containers that collect particles with similar properties, namely

  • lepton (carrying number 90),

  • neutrino (carrying number 91),

  • fermion (carrying number 92),

  • jet (carrying number 93),

  • quark (carrying number 94).

These containers hold all massless particles and anti-particles of the denoted type and allow for a more efficient definition of initial and final states to be considered. The jet container consists of the gluon and all massless quarks, as set by

PARTICLE_DATA:
  <id>:
    Mass: 0
    # ... and/or ...
    Massive: false

A list of particle containers is printed at the start of each Sherpa run, when the OUTPUT is set at level 2.

It is also possible to define a custom particle container using the keyword PARTICLE_CONTAINER. The container must be given an unassigned particle ID (kf-code) and its name (freely chosen by you) and the flavour content must be specified. An example would be the collection of all down-type quarks using the unassigned ID 98, which could be declared as

PARTICLE_CONTAINER:
  98:
    Name: downs
    Flavours: [1, -1, 3, -3, 5, -5]

Note that, if wanted, you have to add both particles and anti-particles.

5.6.1.3. Parentheses

The parenthesis notation allows to group a list of processes with different flavor content but similar structure. This is most useful in the context of simulations containing heavy quarks. In a setup with massive b-quarks, for example, the b-quark will not be part of the jets container. In order to include b-associated processes easily, the following can be used:

PARTICLE_DATA:
  5: {Massive: true}
PARTICLE_CONTAINER:
  98: {Name: B, Flavours: [5, -5]}
PROCESSES:
- 11 -11 -> (93,98) (93,98):
  ...

5.6.1.4. Curly brackets

The curly bracket notation when specifying a process allows up to a certain number of jets to be included in the final state. This is easily seen from an example, 11 -11 -> 93 93 93{3} sets up jet production in e+e- collisions. The matix element final state may be 2, 3, 4 or 5 light partons or gluons.

5.6.2. Decay

Specifies the exclusive decay of a particle produced in the matrix element. The virtuality of the decaying particle is sampled according to a Breit-Wigner distribution. In practice this amouts to selecting only those diagrams containing s-channels of the specified flavour while the phase space is kept general. Consequently, all spin correlations are preserved. An example would be

- 11 -11 -> 6[a] -6[b]:
   Decay:
   - 6[a] -> 5 24[c]
   - -6[b] -> -5 -24[d]
   - 24[c] -> -13 14
   - -24[d] -> 94 94

5.6.3. DecayOS

Specifies the exclusive decay of a particle produced in the matrix element. The decaying particle is on mass-shell, i.e. a strict narrow-width approximation is used. This tag can be specified alternatively as DecayOS. In practice this amouts to selecting only those diagrams containing s-channels of the specified flavour and the phase space is factorised as well. Nonetheless, all spin correlations are preserved. An example would be

- 11 -11 -> 6[a] -6[b]:
    DecayOS:
    - 6[a] -> 5 24[c]
    - -6[b] -> -5 -24[d]
    - 24[c] -> -13 14
    - -24[d] -> 94 94

5.6.4. No_Decay

Remove all diagrams associated with the decay/s-channel of the given flavours. Serves to avoid resonant contributions in processes like W-associated single-top production. Note that this method breaks gauge invariance! At the moment this flag can only be set for Comix. An example would be

- 93 93 -> 6[a] -24[b] 93{1}:
    Decay: 6[a] -> 5 24[c]
    DecayOS:
    - 24[c] -> -13 14
    - -24[b] -> 11 -12
    No_Decay: -6

5.6.5. Scales

Sets a process-specific scale. For the corresponding syntax see SCALES.

5.6.6. Couplings

Sets process-specific couplings. For the corresponding syntax see COUPLINGS.

5.6.7. CKKW

Sets up multijet merging according to [HKSS09]. The additional argument specifies the parton separation criterion (“merging cut”) Q_{cut} in GeV. It can be given in any form which is understood by the internal interpreter, see Interpreter. Examples are

  • Hadronic collider: CKKW: 20

  • Leptonic collider: CKKW: pow(10,-2.5/2.0)*E_CMS

  • DIS: CKKW: $(QCUT)/sqrt(1.0+sqr($(QCUT)/$(SDIS))/Abs2(p[2]-p[0]))

5.6.8. Process_Selectors

Using Selectors: [<selector 1>, <selector 2>] in a process definition sets up process-specific selectors. They use the same syntax as describes in Selectors.

5.6.9. Order

Sets a process-specific coupling order. Orders are counted at the amplitude level. For example, the process 1 -1 -> 2 -2 would have orders {QCD: 2, EW: 0}, {QCD: 1, EW: 1} and {QCD: 0, EW: 2}. There can also be a third entry that is model specific (e.g. for HEFT couplings). Half-integer orders are so far supported only by Comix. The word “Any” can be used as a wildcard.

Note that for decay chains this setting applies to the full process, see Decay and DecayOS.

5.6.10. Max_Order

Sets a process-specific maximum coupling order. See Order for the syntax and additional information.

5.6.11. Min_Order

Sets a process-specific minimum coupling order. See Order for the syntax and additional information.

5.6.12. Min_N_Quarks

Limits the minimum number of quarks in the process to the given value.

5.6.13. Max_N_Quarks

Limits the maximum number of quarks in the process to the given value.

5.6.14. Min_N_TChannels

Limits the minimum number of t-channel propagators in the process to the given value.

5.6.15. Max_N_TChannels

Limits the maximum number of t-channel propagators in the process to the given value.

5.6.17. Name_Suffix

Defines a unique name suffix for the process.

5.6.18. Integration_Error

Sets a process-specific relative integration error target. An example to specify an error target of 2% for 2->3 and 2->4 processes would be:

- 93 93 -> 93 93 93{2}:
    2->3-4:
      Integration_Error: 0.02

5.6.19. Max_Epsilon

Sets epsilon for maximum weight reduction. The key idea is to allow weights larger than the maximum during event generation, as long as the fraction of the cross section represented by corresponding events is at most the epsilon factor times the total cross section. In other words, the relative contribution of overweighted events to the inclusive cross section is at most epsilon.

5.6.20. Enhance_Factor

Sets a process specific enhance factor.

5.6.21. RS_Enhance_Factor

Sets an enhance factor for the RS-piece of an MC@NLO process.

5.6.22. Enhance_Function

Sets a process specific enhance function.

Note

This feature can only be used when generating weighted events.

Note that the convergence of the Monte Carlo integration can be worse if enhance functions are employed and therefore the integration can take significantly longer. The reason is that the default phase space mapping, which is constructed according to diagrammatic information from hard matrix elements, is not suited for event generation including enhancement. It must first be adapted, which, depending on the enhance function and the final state multiplicity, can be an intricate task.

If Sherpa cannot achieve an integration error target due to the use of enhance functions, it might be appropriate to locally redefine this error target, see Integration_Error.

5.6.23. Enhance_Observable

Allows for the specification of a ME-level observable in which the event generation should be flattened. Of course, this induces an appropriate weight for each event. This option is available for both weighted and unweighted event generation, but for the latter as mentioned above the weight stemming from the enhancement is introduced.

An example would be:

- 93 93 -> 11 -11 93{1}:
    2->3:
      Enhance_Observable: VAR{log10(PPerp(p[2]+p[3]))}|1|3

Here, the 1-jet process is flattened with respect to the logarithmic transverse momentum of the lepton pair in the limits 1.0 (10 GeV) to 3.0 (1 TeV). For the calculation of the observable one can use any function available in the algebra interpreter (see Interpreter).

Note that the convergence of the Monte Carlo integration can be worse if enhance observables are employed and therefore the integration can take significantly longer. The reason is that the default phase space mapping, which is constructed according to diagrammatic information from hard matrix elements, is not suited for event generation including enhancement. It must first be adapted, which, depending on the enhance function and the final state multiplicity, can be an intricate task.

If Sherpa cannot achieve an integration error target due to the use of enhance functions, it might be appropriate to locally redefine this error target, see Integration_Error.

5.6.24. NLO_Mode

This setting specifies whether and in which mode an NLO calculation should be performed. Possible values are:

Fixed_Order

perform a fixed-order next-to-leading order calculation

MC@NLO

perform an MC@NLO-type matching of a fixed-order next-to-leading order calculation to the resummation of the parton shower

The usual multiplicity identifier apply to this switch as well. Note that this setting implies NLO_Part: BVIRS for the relevant multiplicities. This can be overridden by setting NLO_Part explicitly in case of fixed-order calculations.

Note that Sherpa includes only a very limited selection of one-loop corrections. For processes not included external codes can be interfaced, see External one-loop ME

5.6.25. NLO_Part

In case of fixed-order NLO calculations this switch specifies which pieces of a NLO calculation are computed. Possible choices are

B

born term

V

virtual (one-loop) correction

I

integrated subtraction terms

RS

real correction, regularized using Catani-Seymour subtraction terms

Different pieces can be combined in one processes setup. Only pieces with the same number of final state particles and the same order in alpha_S and alpha can be treated as one process, otherwise they will be automatically split up.

5.6.26. NLO_Order

Specifies the relative order of the NLO correction wrt. the considered Born process. For example, NLO_Order: @{QCD: 1, EW: 0@} specifies a QCD correction while NLO_Order: @{QCD: 0, EW: 1@} specifies an EW correction.

5.6.27. Subdivide_Virtual

Allows to split the virtual contribution to the total cross section into pieces. Currently supported options when run with @url{http://projects.hepforge.org/blackhat,BlackHat} are LeadingColor and FullMinusLeadingColor. For high-multiplicity calculations these settings allow to adjust the relative number of points in the sampling to reduce the overall computation time.

5.6.28. ME_Generator

Set a process specific nametag for the desired tree-ME generator, see ME_GENERATORS.

5.6.29. RS_ME_Generator

Set a process specific nametag for the desired ME generator used for the real minus subtraction part of NLO calculations. See also ME_GENERATORS.

5.6.30. Loop_Generator

Set a process specific nametag for the desired loop-ME generator. The only Sherpa-native option is Internal with a few hard coded loop matrix elements. Other loop matrix elements are provided by external libraries.

5.6.31. Integrator

Sets a process-specific integrator, see INTEGRATOR.

5.6.32. PSI_ItMin

Sets the number of points per optimization step, see PSI.

5.6.33. RS_PSI_ItMin

Sets the number of points per optimization step in real-minus-subtraction parts of fixed-order and MC@NLO calculations, see PSI.

5.6.34. Special Group

Allows to split up individual flavour processes within a process group for integrating them separately. This can help improve the integration/unweighting efficiency. Note: Only works with Comix so far. Example for usage:

Process 93 93 -> 11 -11 93
Special Group(0-1,4)
[...]
End process
Process 93 93 -> 11 -11 93
Special Group(2-3,5-7)
[...]
End process

The numbers for each individual process can be found using a script in the AddOns directory: AddOns/ShowProcessIds.sh Process/Comix.zip

5.6.35. End process

Completes the setup of a process or a list of processes with common properties.