MadGraph5_aMC@NLO

Pythia 8 not recognizing anti-particle code

Asked by Kayla McLean

I am trying to simulate ATLAS dark matter simplified models with a vector mediator. I am generating using MadGraph 5 and then simulating the underlying event and hadronization with Pythia 8. The model I'm using consists of a dark matter particle (chi) and a mediator (xi). The MadGraph param_card.dat contains the following particle info:

Block QNUMBERS 1000022 # chi
        1 0 # 3 times electric charge
        2 2 # number of spin states (2S+1)
        3 1 # colour rep (1: singlet, 3: triplet, 8: octet)
        4 1 # Particle/Antiparticle distinction (0=own anti)

Block QNUMBERS 101 # xi
        1 0 # 3 times electric charge
        2 3 # number of spin states (2S+1)
        3 1 # colour rep (1: singlet, 3: triplet, 8: octet)
        4 0 # Particle/Antiparticle distinction (0=own anti)

I successfully generate events with MadGraph. Then I use one of the Pythia 8 examples (main42) to read in that MadGraph .lhe file and subsequently output a .hep file. However when I run main42 it gives an error in the initialization:

-------- End LHA initialization information --------
PYTHIA Error in Pythia::check: unknown particle code , i = 312, id = -1000022
PYTHIA Error in Pythia::next: check of event revealed problems
PYTHIA Abort from Pythia::next: parton+hadronLevel failed; giving up
Event generation aborted prematurely, owing to error!

I tried adding in QNUMBERS for the chi anti-particle explicitly, but this did not help. As far as I know, Pythia recognizes the chi and xi particles. I thought for sure that if Pythia knows about the chi particle, then it would know about the chi~ particle. I'm not sure how to proceed.

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Kayla McLean
Solved:
Last query:
Last reply:
Revision history for this message
Kayla McLean (kaylamc) said : 		#1

I've stumbled across something else... When I set

SLHA:readFrom = 0

in main42, Pythia then complains that it does not recognize the particle with id = 101 (the xi mediator). Is Pythia confusing my particles with SUSY particles?

Revision history for this message
Kayla McLean (kaylamc) said : 		#2

I think I'm understanding the SLHA input more. I definitely want Pythia to read the SLHA section of param_card.dat because this contains my custom particles.

When I remove the SLHA:readFrom = 0 flag, I see that Pythia shows the following warning:

PYTHIA Warning in SLHAinterface::initSLHA: ignoring QNUMBERS for id = 1000022 (already exists)

And it still does not recognize the particle id = -1000022. Is there a way to overwrite this particle so that it has the properties I want?

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#3

Dear Kayla,

Please contact the pythia8 author directly on that point.

Cheers,

Olivier

Revision history for this message
Stephen Mrenna (stephen-mrenna) said : 		#4

The neutralino (1000022) is defined to be a Majorana fermion,
whereas it is treated as a Dirac fermion in the LHE file.

In that case, there would be 3 solutions:
1. use sed on your file to change -1000022 to 1000022 (messy)

2. use sed to change 1000022 to, say, 18 (a 4th gen neutrino).
(also messy)

3. redefine the neutralino in your Pythia8 config to be a fermion, i.e.
add the following line to your config file:
'1000022:all = chidm chidm_bar 2 0 0 10.0 0.0 0.0 10000.0 0.00',

(the actual mass should not matter, because it is updated in the SLHA part of
the LHE file)

--Steve

Revision history for this message
Kayla McLean (kaylamc) said : 		#5

Hi Steve,

Thank you very much for your answer. Implementing step #3 worked.

Cheers,
Kayla