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This page aims to outline how to use NEUT to create a sample of interactions on your favourite target. We'll also outline how to feed this generated sample through the NUISANCE framework and get good results.

Compiling NEUT and running neutroot2

The main neutrino-nucleus interaction generator lives in $NEUT_ROOT/src/neutsmpl/neutroot. NEUT has two dependencies: ROOT and the lesser known CERNLIB. You need to have $ROOTSYS, $CERN and $CERN_LEVEL environment variables set-up to compile NEUT. These are all specified in $NEUT_ROOT/src/neutsmpl/Envmakeneutsmpl.csh.

Compilation is done (somewhat inconveniently) by running ./Makeneutsmpl in src/neutsmpl/: this calls a host of makefiles, sets up symlinks and so on. If you have some time on your hands and want to fix this, please contact us or Hayato-san.

Running the main executable is done by ./neutroot2 NEUT_CARD_FILE.card OUTPUT_ROOT_FILE.root, where CARD_FILE.card is a card file compliant with your chosen NEUT version, and OUTPUT_ROOT_FILE.root is the output file where the events are to be saved.

Examples of NEUT card files can be found in the $NEUT_ROOT/src/neutsmpl/Cards directory, or have a look at what we used for 2016 at https://nuisance.hepforge.org/trac/wiki/Winter2016 and ​https://imperialcollegelondon.app.box.com/files/0/f/13226969736/neut_cards

Hot tip: NEUT can be quite verbose and you might want to pipe the output to file (or /dev/null...) rather than dump it to screen. This can actually speed up NEUT considerably.

Structure of card files:

NEUT has a load of defaults set on run-time if the user does not supply them, which means you're running with "reference" NEUT.

Some things are good to specify though, and here's a list for generating all modes on T2K H2O. We can specify how many events we want, the incoming neutrino type, the flux file, number of bound protons and neutrons and free protons, what interaction modes, and so on.

The "C" at the start of each line is just a comment and will not be parsed by NEUT.

C Number of events ; EVCT-NEVT
EVCT-NEVT  100000
C
C Particle Code ; EVCT-IDPT
EVCT-IDPT 14
EVCT-MPOS 2
EVCT-RAD  100.
EVCT-MDIR 1
EVCT-DIR 0. 0. 1.
C
EVCT-MPV 3
EVCT-FILENM  'nuisance_fluxes/t2kflux_2016_plus250kA.root'
EVCT-HISTNM  'enu_nd280_numu'
EVCT-INMEV 0
C
C **** TARGET INFORMATION ****
NEUT-NUMBNDN 8
NEUT-NUMBNDP 8
NEUT-NUMFREP 2
NEUT-NUMATOM 16
C
C **** WHAT INTERACTION MODES DO WE WANT? (0 = ALL) ****
NEUT-MODE   0 
C **** WHAT CCQE MODEL ARE WE USING (2 = RFG WITH BBBA05) ***
NEUT-MDLQE 2
C
C **** RANDOM NUMBER AND IF WE WANT NEUT TO BE QUIET ****
NEUT-RAND 1
NEUT-QUIET 2

NEUT inputs for NUISANCE

The output of neutroot, a neutvect, can be read in by NUISANCE to generate predictions. neutroot also saves the predicted flux and event rate predictions automatically into the neutvect files so NUISANCE already has everything it needs.

To read NEUT inputs the NEUT type just needs to be specified.

sample MiniBooNE_CCQE_XSec_1DQ2_nu  NEUT:/path/to/neut/input/file.root

Using NEUT \ReWeight?

Joint Flux Inputs

neutroot does not yet natively support combined beams or targets so if you want to use the measurement classes that require this (e.g. MINERvA CC0pi nue+nuebar) you will have to setup a JOINT input. See HowToJointInput? for more information on how to achieve this.