neutrino induced: T2K 0pi on water

Here we give a short guide on how to obtain the results discussed in the paper:

21. Mosel, K.Gallmeister, Muon-neutrino-induced charged current scross section without pions on water: theoretical analysis, ...

A first step consists in downloading both the source-code and some input files by following the detailed instructions and compiling it as described here.

All input into GiBUU happens via so-called job-cards that contain Fortran namelists for input. These namelists control a large number of switches. Among the obvious for specifying target and energy (or flux) of the incoming neutrino there are many more switches. For some of the more relevant cases sample jobcards are part of the download.

The job cards also allow to specify the reaction type.

Electron results

First, the electron results shown in this paper were all obtained by choosing the mode

# namelist: neutrino_induced
   nuXsectionMode  =  1 !  1: dSigmadCosThetadElepton

('neutrino' here stands generically for lepton). Energy and angle are then specified in the namelist

# namelist: nl_dSigmadCosThetadElepton
   ...

In this namelist the incoming electron energy, the cosine of the scattering angle and a startenergy for the outgoing electron together with an energy-stepwidth are specified. How many energy steps are calculated is controlled by the parameter

# namelist: input
   num_Energies=30

Since we are dealing here with inclusive cross sections, the number of timesteps, controlled by

# namelist: input
   numTimeSteps=0

This be set to 0 since one does not have to follow the reaction through its dynamical development. The statistics, i.e. the number of events, is controlled by

# namelist: input
   numEnsembles = ...

For C12 or O16 a typical number is 4000. Obviously, inclusive cross sections need much smaller numbers of events than those for complicated final states.

GiBUU produces on one hand large files with many events (which are unnecessary for inclusive processes) which could be analyzed further in any other self-written analysis routine. In addition, GiBUU also produces many cross section files in ASCII format that can directly be 'gnuplotted'. To get these files for the inclusive processes it is essential to set the switch

# namelist: neutrino_induced
   printAbsorptionXS = T

The output file neutrino_absorption_cross_section_ALL.dat then contains all the numbers necessary to produce the plots of Fig. 1 of the given paper. (For information on the various output files consult e.g. the online doku.) Running times are typically of the order of 1 hour on a PC.

Neutrino results, 0-pion

Switching from electron to neutrino is not the main issue here; whether one has inclusive events or a specific final state decision (as e.g.: "0 pions") makes the difference.

The neutrino results for 0-pion cross sections on water were obtained by choosing first the mode and the T2K flux

# namelist: neutrino_induced
   nuXsectionMode  =  6
   nuExp           =  9 !  9: T2K-OA2.5-ND280

The various elementary reaction types, such as QE, 2p2h, DIS, etc can be turned on by setting.e.g., includeQE = T in that same namelist. Since we do now need information on the final state interactions and the time-development of the reaction the number of time steps now has to be set to some non-zero number. For \( ^{16}{\rm O}\) we have worked with numTimeSteps=130. By setting in addition num_runs_SameEnergy = 100 will increase statistics for semi-inclusive events, such as 0-pion events. Typical running times with these parameters are about 12 hours on a PC, if DIS is turned off. The latter is quite time-consuming at the lower energies treated here.

For the results in this paper we have prepared two runs:

• one with all reaction types turned on and
• one in which only QE and 2p2h were turned on.

Both runs produce among many others also an output file diff_000_d2Sigma_dEkindcost_lepton_no_pi.dat, which contains the double-differential cross section for 0-ion events as a function of cos(theta) and of the kinetic energy of the outgoing lepton. This file could be directly used for producing contour plots with gnuplot, but could also be manipulated to produce the binned cross sections for this paper. This output file from the run with only QE and 2p2h turned on during event generation then gives just that: the cross section for QE and 2p2h only, without any stuck-pion events. This output file from the more complete run has in addition also the contributions from stuck-pion events.