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MCEventWeight

This wiki page describes how to generate event weights in LArSoft for the study of beam, interaction model or other systematics.

GENIE re-weighting

For the parameters that is possible to vary, check the GENIE User Manual: https://arxiv.org/abs/1510.05494 (Chapter 9).
For information on how to save or access these data, see the AnalysisTree page.

You can decide how many GENIE parameters you want to vary. This can be done defining a function in a fcl file.
For example, if we want to vary the Axial Mass for CC quasi elastic interactions we'll define something like this:

 genie_qema: {
   type: Genie
   random_seed: 1
   parameter_list:        ["QEMA"]
   parameter_sigma:       [1     ]
   mode: pm1sigma
   number_of_multisims: 0
 }

where parameter_list contains the string "QEMA". For a list of the possible strings that can be used see the tape below. In parameter_sigma you need to specify the number of sigmas you want to change that parameter. There are currently three possibilities for mode:

mode Description
pm1sigma This will ignore the parameter_sigma and number_of_multisims inputs and change the specified parameters by + and - 1 sigma. This allows to understand the impact one parameter's uncertainty on the final measurement.
multisim If multisim is specified the sigma used to vary the parameter is generated according to a gaussian distribution with mean zero and standard deviation equal to one. Many weights are generated based on the number specified in number_of_multisims. Note that the generated gaussian number is multiplied by the values in parameter_sigma. So the sigma used to vary the parameter is parameter_sigma x Gauss(0,1).
Everything else you may write In this case one weight for each parameter in parameter_list will be generated. The sigma used to vary the parameter is the one that you specify in parameter_sigma. Important is to write the right number_of_multisims.

Many functions are already defined in
uboone/EventWeight/jobs/jobs_geniereweight/genie_eventweight_microboone.fcl
which also defines a job that can be run in LArSoft.

If you want to evaluate the neutrino interaction systematics for your measurement, you will most likely use:

genie_all: {
          type:                  Genie
          random_seed:           1
          parameter_list:        ["QEMA", "NCELaxial", "NCELeta", "CCResAxial", 
             "CCResVector", "NCResAxial", "NCResVector", "CohMA", "CohR0", 
             "NonResRvp1pi", "NonResRvbarp1pi", "NonResRvp2pi", "NonResRvbarp2pi", "ResDecayGamma", 
             "ResDecayEta", "NC", "DISAth", "DISBth", 
             "DISCv1u", "DISCv2u", "AGKYxF", "AGKYpT", "FormZone", 
             "FermiGasModelKf", "IntraNukeNmfp", "IntraNukeNcex", "IntraNukeNel", 
             "IntraNukeNinel", "IntraNukeNabs", "IntraNukeNpi", "IntraNukePImfp", "IntraNukePIcex", 
             "IntraNukePIel", "IntraNukePIinel", "IntraNukePIabs", "IntraNukePIpi"]
          parameter_sigma:       [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 
              1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 ]
          mode: multisim
          number_of_multisims:   1000
 }

which will vary all the parameters at the same time, generating a gaussian distributed sigma, 1000 times. 1000 weights will be stored in the ART event.

What follows is the list of reweighting parameters present in LArSoft.
Parameters with a (*) contain more that one reweighing parameter at the same time.

Parameter name Description
NCELaxial Axial mass for NC elastic
NCELeta Strange axial form factor for NC elastic
QEMA Axial mass for CC quasi-elastic
QEVec Choice of CCQE vector form factor (sigma = 0 => BBA05; sigma = 1 => Dipole)
CCResAxial Axial mass for CC resonance neutrino production
CCResVector Vector mass for CC resonance neutrino production
ResGanged CC Res && NC Res (NOT ACTIVE)
NCResAxial Axial mass for NC resonance neutrino production
NCResVector Vector mass for NC resonance neutrino production
CohMA Axial mass for CC and NC coherent pion production
CohR0 Nuclear size param. controlling pi absorption in Rein-Sehgal model
NonResRvp1pi v+p and vbar + n (1 pi) type interactions (*)
NonResRvbarp1pi v+n and vbar + p (1 pi) type interactions (*)
NonResRvp2pi v+p and vbar + n (2 pi) type interactions (*)
NonResRvbarp2pi v+n and vbar + p (2 pi) type interactions (*)
ResDecayGamma BR for radiative resonance decay
ResDecayEta BR for single-eta resonance decay
ResDecayTheta Pion angular distibution in Delta -> pi N (sigma = 0 => isotropic; sigma = 1 => RS)
DISAth Ath higher twist param in BY model scaling variable xi_w
DISBth Bth higher twist param in BY model scaling variable xi_w
DISCv1u Cv1u u valence GRV98 PDF correction param in BY model
DISCv2u Cv2u u valence GRV98 PDF correction param in BY model
DISnucl NOT IMPLEMENTED IN GENIE
AGKYxF Pion Feynman x for Npi states in AGKY
AGKYpT Pion transverse momentum for Npi states in AGKY
FormZone Hadron Formation Zone
FermiGasModelKf CCQE Pauli Suppression via changes in Fermi level kF
FermiGasModelSf Choice of model (sigma = 0 => FermiGas; sigma = 1 => SF (spectral function))
IntraNukeNmfp Nucleon mean free path (total rescattering probability)
IntraNukeNcex Nucleon charge exchange probability
IntraNukeNel Nucleon elastic reaction probability
IntraNukeNinel Nucleon inelastic reaction probability
IntraNukeNabs Nucleon absorption probability
IntraNukeNpi Nucleon pi-production probability
IntraNukePImfp Pi mean free path (total rescattering probability)
IntraNukePIcex Pi charge exchange probability
IntraNukePIel Pi elastic reaction probability
IntraNukePIinel Pi inelastic reaction probability
IntraNukePIabs Pi absorption probability
IntraNukePIpi Pi pi-production probability

Example: Reweight RES parameters by +-3 sigma each

The GENIE function to write is this case is:

genie_all: { 

          type:                  Genie 
          random_seed:           1 
          parameter_list:        ["CCResAxial", “CCResVector", “NCResAxial”] 
          parameter_sigma:       [-3, 3,-3, 3,-3, 3] 
          mode: whatever 
          number_of_multisims:   2 
}

Beam re-weighting