R-PARITY VIOLATING SUPERSYMMETRY SUBGROUP

• Goals

       We do not have any event generator to deal with cascade
       decays of squarks (e.g. decays into charginos are missing
       and can be of high importance).
  
       => Project = write a (user friendly) tool for Rp Susy
          generation in ep collisions (something like SUSYGEN
          which exists for e+e-, or ISASUSY for pp/ppbar)
  
          We should be able not to start from scratch, but use
          stuff from available packages (SUSYGEN,SPYTHIA,GRACE..)
  
          We hope to agree on a common strategy by next (June) meeting.
          It would be fine to have a first version of this complete tool
          by the end of the year.
  
  

• Starting Point

  At the startup meeting of 29.04.98, we agreed on a specific process :

     e+ + d    ->   \tilde{u}_L  -> d + \chi^+_1
  
     Followed by :
  
          \chi^+_1   ->   W^+  + \chi^0_1
  
     and :
  
           W^+       -> \mu^+ + nu
  
           \chi^0_1  -> e^+  + d + \bar{u}  through a virtual \tilde{u} 
                                            decaying via \lambda'_{111}
   

  We do not have for the while any complete generator which allows to generate this process. We decided, for the next June meeting, to simulate this using the following tools/frames :

  • in the frame of SPYTHIA : leptoquark production (which is in PYTHIA) can be used to code the squark production. Decays of chargino and neutralino are then performed using only phase space.

  • in the frame of SUSYGEN : a squark is produced and decays to quark + chargino. The decay chain of the chargino is then performed using routines from SUSYGEN. Matrix elements are used for the 2-body decay of the chargino and 3-body decay of the neutralino. Spin correlation are not taken into account.

  • using complete matrix element provided by GRACE system. Here all decays are performed properly and spin correlations are taken into account.

  We agreed on the following masses : M_squark = 240 GeV, M_chargino = 160 GeV, M_neutralino = 70 GeV

  We should compare at our next meeting what we get from different methods and the difficulties we had to face, since :

  • many approximations with SPYTHIA but should be quite simple.
  • less approximation with SUSYGEN, but no interface for ep physics
  • no approximation with the GRACE matrix element, but performing the hadronization should require quite some work.
  These comparisons should allow us to define a common strategy to write a general tool for Rp-susy processes in ep collisions.

• SPYTHIA

  Supersymmetry has been incorporated in PYTHIA by S. Mrenna. Unfortunately, it does not yet allow to produce susy events in ep collisions. However, squark production via R-parity violation can be coded from leptoquark production. In SPYTHIA, all decays of sparticles are performed using phase space element. Matrix elements are used only for the calculation of cross-sections and branching ratios. R-parity violation is not included in SPYTHIA.

  Link to SPYTHIA Home Page.

• SUSYGEN

  SUSYGEN is a general tool developped by S. Kastanevas and P. Moravitz to generate SUSY events in e+e- collisions. Decays of sparticles are performed using 2-body or 3-body matrix elements. R-parity violation through one coupling lambda, lambda' or lambda'' is possible. SUSYGEN is interfaced with JETSET. Many routines can from SUSYGEN can be used to write a tool for SUSY processes in ep collisions.

  Link to SUSYGEN Home Page.

• GRACE

  This short description has been written by F. Boudjema.

  GRACE is a package for the automatic computation and event generation of any elementary process once model files containing the various particles, vertices and propagators have been given. The package provides automatic Feynman graph generation as well as codes in symbolic languages. GRACE with the addition of a counter-term library is able to compute complete one-loop corrections even for reactions such as 2-->3-body electroweak processes. Helicity amplitudes are provided throughout. Model files containing R-parity breaking terms of type (LQD) have been added. A complete version of GRACE-SUSY is being developed, so that any process in SUSY (not just e+e- annihilation) can be computed. Among other things this will allow the automatic implementation of the correct spin-correlation in the decay of a supersymmetric particle with spin.

  Link to GRACE Home Page.

• People

  Ascii file of onvolved people with email adresses can be found here. Send us a mail if you wish to contribute to this subgroup.