New Particles in MadGraph/Pythia

Asked by Matthew Low on 2012-03-14

Hi,

I'm trying to simulate a model where I have added two new particles via the model_v4 method (i.e. modified particles.dat, interactions.dat, ran the Conversion script, etc.). Param_card.dat correctly shows the following information about the new particles:

BLOCK QNUMBERS 77 # xv
         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)

BLOCK QNUMBERS 79 # xf
         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)

The process I generate has one jet and two xf particles in the final state and MadGraph does this fine. When I generate events what I would expect to happen is for Pythia to ignore the xf particles and for them to appear as missing energy after PGS has run. My problem is that rather than this happening it seems that they are recognized as jets after PGS has run.

Looking at the unweighted_events.root file the xf particles appear correctly (i.e. there are particle with PID 79 in the file). Looking at pythia_events.root (from the STDHEP file) there are no particles with PID=79 in the events, however, there are several (6) entries with PID=0 about where I would have thought to see the PID=79 entries. Then PGS seems to identify these as jets.

I'm wondering if you have any insight as to what I need to do to have the PID=79 particles be ignored by Pythia and appear as missing energy in PGS.

Thanks in advance for your help,
- Matthew

Question information

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MadGraph5_aMC@NLO Edit question
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Last query:
2012-03-14
Last reply:
2012-03-16

Hi Matthew,

I don't know the answer to this question (I hope that Johan does, otherwise we will contact Pythia author)

But just want to check one point: Did you put the width of your new particles to zero?

Cheers,

Olivier

Matthew Low (mattlow) said : #2

Hi Olivier,

Thanks for the response. I kept the widths of the new particles at 1 GeV.

- Matthew

Hi Matthew,

Please put it to zero and retry.

A non zero value requires for pythia that you provide the branching ratio of the decay.
That you probably didn't provide. So in short put a width is not coherent for pythia and might results to weird output.

Cheers,

Olivier

Matthew Low (mattlow) said : #4

Hi Olivier,

I tried putting the width to zero for the new stable particle xf (and keeping the width at 1 GeV for the new unstable particle xv). The results are still the same. Even though there are no xv particles in the final state I have not been asked to specify branching ratios for the xv particle anywhere. Perhaps do I need to specify that somewhere to make sure Pythia understands the new particles?

Thanks,
- Matthew

Hi Matthew,

Few days ago, I've created a tutorial (for an upcoming conferences) which is in fact quite close to your situation:
https://server06.fynu.ucl.ac.be/projects/madgraph/wiki/MC4BSM

The only point is that I stop before running pythia, since they will use either pythia8 or herwig for the shower.
But one of the part of the tutorial is to generate a valid param_card both for MG and for Pythia/Herwig.
Please tell me if any point is unclear such that I can improve it before the conferences.

But since your process doesn't have any xv in the final states, this shouldn't be the problem. I propose to wait for Johan, and if he doesn't know, we will point this topic to Pythia author.

Cheers,

Olivier

Johan Alwall (johan-alwall) said : #6

Hello Matthew,

The only way for PGS to know that a new particle should be considered as missing energy is to set it's PDG code to that of an MSSM neutralino1 (1000022).

All the best,
Johan

Matthew Low (mattlow) said : #7

Hi Johan,

Thanks for the help. In fact I tried the model setting the new stable particle xf to have PID=1000022 and this PID made it through Pythia unharmed and PGS correctly identified it as missing energy. The problem I have here is that I'm forced to make xf have the same properties as a neutralino when I would look to have the freedom to make it a Dirac fermion or even a scalar.

It seems that if any new PID codes are passed to Pythia in the STDHEP output then they are all given PID=0. I realize that for my instance I could simply manually replace all final state PID=0 cases with PID=1000022 and then manually run PGS so that PGS could correctly identify missing energy. However, I think it would be preferable (and more flexible) if the new PID codes simply went through Pythia "as is". I wonder if you have any recommendations on how to do this?

Thanks for your help,
- Matthew

P.S. Thanks Olivier for pointing me to your tutorial, I intend to read it/try it soon!

Johan Alwall (johan-alwall) said : #8

Hello Matthew,

The only properties of 1000022 that need to be the same as MSSM are color and electric charge. There are no cross-checks in Pythia or PGS for spin, Majorana/Dirac, etc. So as long as it's neutral, you can give it any properties you want.

All the best,
Johan

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