5.7. Selectors

Sherpa provides the following selectors that set up cuts at the matrix element level:

Some selectors modify the momenta and flavours of the set of final state particles. These selectors also take a list of subselectors which then act on the modified flavours and momenta. Details are explained in the respective selectors’ description.

5.7.1. Inclusive selectors

The selectors listed here implement cuts on the matrix element level, based on event properties. The corresponding syntax is

SELECTORS:
  - [<keyword>, <parameter 1>, <parameter 2>, ...]
  - # other selectors ...

Parameters that accept numbers can also be given in a form that is understood by the internal algebra interpreter, see Interpreter. The selectors act on all particles in the event. Their respective keywords are

[N, <kf>, <min value>, <max value>]

Minimum and maximum multiplicity of flavour <kf> in the final state.

[PTmis, <min value>, <max value>]

Missing transverse momentum cut (at the moment only neutrinos are considered invisible)

[ETmis, <min value>, <max value>]

Missing transverse energy cut (at the moment only neutrinos are considered invisible)

[IsolationCut, <kf>, <dR>, <exponent>, <epsilon>, <optional: mass_max>]

Smooth cone isolation [Fri98], the parameters given are the flavour <kf> to be isolated against massless partons and the isolation cone parameters.

5.7.2. One particle selectors

The selectors listed here implement cuts on the matrix element level, based on single particle kinematics. The corresponding syntax is

SELECTORS:
  - [<keyword>, <flavour code>, <min value>, <max value>]
  - # other selectors ...

<min value> and <max value> are floating point numbers, which can also be given in a form that is understood by the internal algebra interpreter, see Interpreter. The selectors act on all possible particles with the given flavour. Their respective keywords are

PT

transverse momentum cut

ET

transverse energy cut

Y

rapidity cut

Eta

pseudorapidity cut

PZIN

cut on the z-component of the momentum, acts on initial-state flavours only (commonly used in DIS analyses)

HT

Visible transverse energy cut

E

energy cut

Polar_Angle

Polar Angle cut in radians. An optional boolean can be provided to switch to degrees e.g [<keyword>, <flavour code>, <min value>, <max value>, <radians>]

5.7.3. Two particle selectors

The selectors listed here implement cuts on the matrix element level, based on two particle kinematics. The corresponding is

SELECTORS:
  - [<keyword>, <flavour1 code>, <flavour2 code>, <min value>, <max value>]
  - # other selectors ...

<min value> and <max value> are floating point numbers, which can also be given in a form that is understood by the internal algebra interpreter, see Interpreter. The selectors act on all possible particles with the given flavour. Their respective keywords are

Mass

invariant mass

Q2

DIS-like virtuality

PT2

pair transverse momentum

MT2

pair transverse mass

DY

rapidity separation

DEta

pseudorapidity separation

DPhi

azimuthal separation

DR

angular separation (build from eta and phi)

DR(y)

angular separation (build from y and phi)

INEL

inelasticity, one of the flavours must be in the initial-state (commonly used in DIS analyses)

5.7.4. Decay selectors

The selectors listed here implement cuts on the matrix element level, based on particle decays, see Decay and DecayOS.

DecayMass

Invariant mass of a decaying particle. The syntax is

- [DecayMass, <flavour code>, <min value>, <max value>]

Decay

Any kinematic variable of a decaying particle. The syntax is

- [Decay(<expression>), <flavour code>, <min value>, <max value>]

where <expression> is an expression handled by the internal interpreter, see Interpreter.

Decay2

Any kinematic variable of a pair of decaying particles. The syntax is

- [Decay2(<expression>), <flavour1 code>, <flavour2 code>, <min value>, <max value>]

where <expression> is an expression handled by the internal interpreter, see Interpreter.

Particles are identified by flavour, i.e. the cut is applied on all decaying particles that match <flavour code>. <min value> and <max value> are floating point numbers, which can also be given in a format that is understood by the internal algebra interpreter, see Interpreter.

5.7.5. Particle dressers

5.7.6. Jet selectors

There are two different types of jet finders

NJetFinder

k_T-type algorithm to select on a given number of jets

FastjetFinder

Select on a given number of jets using FastJet algorithms

Their respective syntax and defaults are

SELECTORS:
- NJetFinder:
    N: 0
    PTMin: 0.0
    ETMin: 0.0
    R: 0.4
    Exp: 1
    EtaMax: None
    YMax: None
    MassMax: 0.0
- FastjetFinder:
    Algorithm: kt
    N: 0
    PTMin: 0.0
    ETMin: 0.0
    DR: 0.4
    EtaMax: None
    YMax: None
    Nb: -1
    Nb2: -1

Note that all parameters are optional. If they are not specified, their respective default values as indicated in the above snippet is used. However, at the very least the number of jets, N, should be specified to require a non-zero number of jets.

The NJetFinder allows to select for kinematic configurations with at least <N> jets that satisfy both, the <PTMin> and the <ETMin> minimum requirements and that are in a pseudo-rapidity region |eta|. The <Exp> (exponent) allows to apply a kt-algorithm (1) or an anti-kt algorithm (-1). As only massless partons are clustered by default, the <MassMax> allows to also include partons with a mass up to the specified values. This is useful e.g. in calculations with massive b-quarks which shall nonetheless satisfy jet criteria.

The second option FastjetFinder allows to use the FastJet plugin, through fjcore. It takes the following arguments: <Algorithm> can take the values kt,antikt,cambridge,eecambridge, <N> is the minimum number of jets to be found, <PTMin> and <ETMin> are the minimum transverse momentum and/or energy, <DR> is the radial parameter. Optional arguments are: <EtaMax> and <YMax> as maximal absolute (pseudo-)rapidity, <Nb> and <Nb2> set the number of required b-jets, where for the former both b and anti-b quarks are counted equally towards b-jets, while for the latter they are added with a relative sign as constituents, i.e. a jet containing b and anti-b is not tagged (default: -1, i.e. no b jets are required). Note that only <Algorithm>, <N> and <PTMin> are relevant for the lepton-lepton collider algorithms.

The selector FastjetVeto allows to use the FastJet plugin to apply jet veto cuts. Its syntax is identical to FastjetFinder.

The momenta and nodal values of the jets found with FastJet can also be used to calculate more elaborate selector criteria. The syntax of this selector is

- FastjetSelector:
    Expression: <expression>
    Algorithm: kt
    N: 0
    PTMin: 0.0
    ETMin: 0.0
    DR: 0.4
    EtaMax: None
    YMax: None
    BMode: 0

wherein Algorithm can take the values kt,antikt,cambridge,eecambridge. In the algebraic <expression>, MU_n2 (n=2..njet+1) signify the nodal values of the jets found and p[i] are their momenta. For details see Scale setters. For example, in lepton pair production in association with jets

- FastjetSelector:
    Expression: Mass(p[4]+p[5])>100
    Algorithm: antikt
    N: 2
    PTMin: 40
    ETMin: 0
    DR: 0.5

selects all phase space points where two anti-kt jets with at least 40 GeV of transverse momentum and an invariant mass of at least 100 GeV are found. The expression must calculate a boolean value. The BMode parameter, if specified different from its default 0, allows to use b-tagged jets only, based on the parton-level constituents of the jets. There are two options: With BMode: 1 both b and anti-b quarks are counted equally towards b-jets, while for BMode: 2 they are added with a relative sign as constituents, i.e. a jet containing b and anti-b is not tagged. Note that only <epression>, <algorithm>, <n> and <ptmin> are relevant when using the lepton-lepton collider algorithms.

NJettiness

NJettiness from [SJW10]. Selects all phase space points that have a value of NJettiness larger than a specified minimum value.

- NJettiness:
    N: <N>
    TauN: <Taucut>
    Frame: lab
    Normalization: jet2
    Algorithm: antikt
    RecombinationScheme: E
    PTMin: 0.0
    ETMin: 0.0
    DR: 1
    EtaMax: None
    YMax: None

where <N> specifies the number of hard directions – jets – that are required, and <Taucut> the minimum value that the observable is allowed to have to be considered a valid phase space point. In the current implementation, the algorithm used to cluster the jets defining the hard directions can be specified using the Algorithm option. The following values are accepted: antikt, kt, genkt, cambridge, and they correspond to those that can be accessed throught the Fastjet implementation. The algorithm is then used to find <N> pseudojet, which are then used to compute the minima that determine the final value of NJetttiness. Note, that in the case of Hadron Colliders, the beam are added internally as additional jets. Further options can be specified to determine the behaviour of this selector. In particular as NJettiness is a frame dependent quantity, the frame in which to compute its value can be specified using the Frame option. Currently only two values are accepted which correspond to the hadronic centre of mass frame (or Laboratory frame), which is the default and referred to as lab, and the partonic centre of mass frame specified as com. Additionally, the normalization of each contribution to NJettiness can be choosen through the Norm parameter between three options: jet, jet2 and com. The first correspond to normalizing the scalar product with the jet energy, the second and default choice is equal to the first but multiplied by a factor two, while the last one corresponds to normalizing each contribution with the centre of mass energy and leads to a Lorentz invariant definition of NJettiness.

Finally a variaty of parameters to control the clustering of the hard directions can be passed, similarly to the FastjetFinder case. In particular the minimum transverse momentum and/or transverse energy (PTMin,ETMin), the radius of the jet (DR), which defaults to 1, and the maximum rapidity and/or pseudorapidity can be specified (YMax,EtaMax).dsd

For example, in the case of vector boson plus jet setup, with 0.1 GeV 0-Jettiness cut, one would use something along the lines of

PROCESSES:
- 93 93 -> 11 -11 93:
  Order: {QCD:1,EW:2}
SELECTORS:
- [Mass, 11, -11, 75.,125.]
- NJettiness:
    N: 0
    TauN: 0.1

5.7.7. Isolation selector

Instead of the simple IsolationCut (Inclusive selectors), you may also use the more flexible Isolation_Selector to require photons (or other particles) with a smooth cone isolation and additionally apply further criteria to them. Example:

SELECTORS:
- Isolation_Selector:
    Isolation_Particle: 22
    Rejection_Particles: [93]
    Isolation_Parameters:
      R: 0.1
      EMAX: 0.1
      EXP: 2
      PT: 0.
      Y: 2.7
    NMin: 2
    Remove_Nonisolated: true
    Subselectors:
    - VariableSelector:
        Variable: PT
        Flavs: [22]
        Ranges:
        - [20, E_CMS]
        - [18, E_CMS]
        Ordering: [PT_UP]
    - [DR, 22, 22, 0.2, 10000.0 ]
    #for integration efficiency: m_yy >= sqrt(2 pTmin1 pTmin2 (1-cos dR))
    - [Mass, 22, 22, 3.7, E_CMS]

5.7.8. Universal selector

The universal selector is intended to implement non-standard cuts on the matrix element level. Its syntax is

SELECTORS:
- VariableSelector:
    Variable: <variable>
    Flavs: [<kf1>, ..., <kfn>]
    Ranges:
    - [<min1>, <max1>]
    - ...
    - [<minn>, <maxn>]
    Ordering: [<order1>, ..., <orderm>]

The Variable parameter defines the name of the variable to cut on. The keywords for available predefined can be figured by running Sherpa SHOW_VARIABLE_SYNTAX: true. Or alternatively, an arbitrary cut variable can be constructed using the internal interpreter, see Interpreter. This is invoked with the command Calc(...). In the formula specified there you have to use place holders for the momenta of the particles: p[0] … p[n] hold the momenta of the respective particles kf1 … kfn. A list of available vector functions and operators can be found here Interpreter.

<kf1>,.., specify the PDG codes of the particles the variable has to be calculated from. The ranges [<min>, <max>] then define the cut regions.

If the Ordering parameter is not given, the order of cuts is determined internally, according to Sherpa’s process classification scheme. This then has to be matched if you want to have different cuts on certain different particles in the matrix element. To do this, you should put enough (for the possible number of combinations of your particles) arbitrary ranges at first and run Sherpa with debugging output for the universal selector: Sherpa 'FUNCTION_OUTPUT: {"Variable_Selector::Trigger": 15}'. This will start to produce lots of output during integration, at which point you can interrupt the run (Ctrl-c). In the Variable_Selector::Trigger(): {...} output you can see, which particle combinations have been found and which cut range your selector has held for them (vs. the arbitrary range you specified). From that you should get an idea, in which order the cuts have to be specified.

If the fourth argument is given, particles are ordered before the cut is applied. Possible orderings are PT_UP, ET_UP, E_UP, ETA_UP and ETA_DOWN, (increasing p_T, E_T, E, eta, and decreasing eta). They have to be specified for each of the particles, separated by commas.

Examples

SELECTORS:
# two-body transverse mass
- VariableSelector:
    Variable: mT
    Flavs: [11, -12]
    Ranges:
    - [50, E_CMS]

# cut on the pT of only the hardest lepton in the event
- VariableSelector:
    Variable: PT
    Flavs: 90
    Ranges:
    - [50, E_CMS]
    Ordering: [PT_UP]

# using bool operations to restrict eta of the electron to |eta| < 1.1 or
# 1.5 < |eta| < 2.5
- VariableSelector:
    Variable: Calc(abs(Eta(p[0]))<1.1||(abs(Eta(p[0]))>1.5&&abs(Eta(p[0]))<2.5))
    Flavs: 11
    Ranges:
    - [1, 1]  # NOTE: this means true for bool operations

# requesting opposite side tag jets in VBF
- VariableSelector:
    Variable: Calc(Eta(p[0])*Eta(p[1]))
    Flavs: [93, 93]
    Ranges:
    - [-100, 0]
    Ordering: [PT_UP, PT_UP]

# restricting electron+photon mass to be outside of [87.0,97.0]
- VariableSelector:
    Variable: Calc(Mass(p[0]+p[1])<87.0||Mass(p[0]+p[1])>97.0)
    Flavs: [11, 22]
    Ranges:
    - [1, 1]

# in ``Z[lepton lepton] Z[lepton lepton]``, cut on mass of lepton-pairs
# produced from Z's
- VariableSelector:
    Variable: m
    Flavs: [90, 90]
    # here we use knowledge about the internal ordering to cut only on the
    # correct lepton pairs
    Ranges:
    - [80, 100]
    - [0, E_CMS]
    - [0, E_CMS]
    - [0, E_CMS]
    - [0, E_CMS]
    - [80, 100]

5.7.9. Minimum selector

This selector can combine several selectors to pass an event if either those passes the event. It is mainly designed to generate more inclusive samples that, for instance, include several jet finders and that allows a specification later. The syntax is

SELECTORS:
- MinSelector:
    Subselectors:
    - <selector 1>
    - <selector 2>
    ...

The Minimum selector can be used if constructed with other selectors mentioned in this section