Simulating travelling rhadrons

- 2 grids in the Whitepaper where the LLP carries SM (except weak) charge. We need to know how to simulate these.
- Important to let G4 interact the rhadron because it can hadronize as it goes through all the material (travels through more than a bhadron because of lifetime) so cannot decide how to decay it before g4 does its thing (unlike bhadrons, as is done in evtgen, where assumption that bhadron wont change state is reasonable given lifetime/material).
- G4 can decay slepton, but not things like rhadrons ...  need to use pythia.
- Have been technical issues in being able to reinitialize pythia after G4 runs and make it decay an rhadron that’s given to it (Reported by Jennifer, Somewhat resolved by Steve?).
- How do we pass configuration information from pythia->G4->pythia in this chain ... ATLAS moving away from HepMC, making it harder to do this in a unified way, each collaboration just deals with it themself?
- The current mass spectrum ("generic" or "intermediate") used by ATLAS for rhadrons is a hacked-together incoherent(?) mess. Its a choice. Last update was 5 years ago, before that was 15 years ago. Store deltas (are masses relative to the input gluino state). Rather than adding up the constituent quark masses to get Rhadron mass in pythia, we should just specify the masses for each Rhadron. 
- Regge model determines hadronic interactions - is what is in use in ATLAS atm. Not the same as “regge model” mass spectrum that is used in pythia6. CMS uses the "cloud model" for the hadronic interactions, which we hope is the same thing.
- At the moment spin correlations are completely neglected - nobody can think of an important case where it matters.

Simulating stopping rhadrons

- At the moment ATLAS uses a simple stopping threshold criteria (Beta < A^(-2/3)). This doesnt simulate the deceleration of stopping rhadrons, which would occur as they drag ions along. 

- Concern that efficiencies measured in stopping particle searches are overestimated (by factors of maybe 2, not 10-100). Would like to be able to simulate these correctly.

Simulating quirks

- Chris Hill made his argument for why quirks are basically non-existent (all but shortest string lengths were killed by fractional charge search).
- Could reinterpret Heavy Stable Charged Particle searches in terms of (short length) quirks if we had a reliable simulation. But it's going to be driven by best effort, and effort seems lacking.

Fast simulation

- There was reasonable interest in understanding how far validity of fast simulation can go when it comes to LLPs (non-pointing tracks/photons etc).

Action items

- It is very important that each simplified model in the white paper that features long-lived interacting (SM-charged) particles should come with a corresponding description of the interaction model. Experts should be assembled to define/document for each case (rhadrons, HIP, monopole, ..) where relevent:

   - Particle mass spectrum
   - Particle fragmentation function
   - G4 interaction model
   - Other relevant parameters of the model (e.g. initial qluino/gluon fraction)

- At very least there needs to be effort to compare/contrast what is currently done by ATLAS and CMS and LHCb to check for differences

- Would like to call for studies on both sides that compare fast and full sim in LLP scenarios to try to quantify geometric/phase space where fast sim is reasonable

Participants in discussion

Zach Marshall (ATLAS)
Larry Lee (ATLAS)
Chris Hill (CMS)
Steve Mrenna (CMS/Pythia)
Jennifer Roloff (ATLAS)
...