Monte Carlo onia simulations at ATLAS

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					        Monte Carlo quarkonium simulations at ATLAS

        Darren Price,

        International Workshop on Heavy Quarkonium 2007

Darren Price – Monte Carlo onia simulations at ATLAS   QWG Workshop – 20/10/2007   Page 1
      Monte Carlo onia samples at ATLAS

 Official high statistics production samples at ATLAS currently use NRQCD octet
  processes only. Originally written by S. Wolf, now incorporated into Pythia.

 Colour octet NRQCD matrix elements describe non-perturbative onia evolution
      Matrix elements set to values derived from Tevatron data

   PYTHIA     NRQCD matrix                      Value
 parameter      element                                   Much interest at ATLAS in producing
                O ( J  )[ 3 S1 (1)]
  PARP(141)                                      1.16
                                                           samples of 2S and 3S states.
  PARP(142)     O ( J  )[ 3 S1 (8)]            0.0119
  PARP(143)     O ( J  )[ 1 S 0 (8)]            0.01
                                                              In Pythia, requires separate production runs
              O ( J  )[ P0 (8)] m
                                                               so this has not been done yet
                           3                2
  PARP(144)                                 c    0.01

  PARP(145)   O (  c 0 )[ 3 P0 (1)] m c2        0.05
  PARP(146)       O (  )[ S1 (1)]
                                                 9.28         Currently ME’s for these higher states have
  PARP(147)      O (  )[ S1 (8)]
                                                 0.15          not been added into ATLAS Pythia tuning but
  PARP(148)      O (  )[ S 0 (8)]
                                                               should be possible based on hep-ph/0003142
  PARP(149)    O (  )[ P0 (8)] m
                       3                2
                                        b        0.02
  PARP(150)   O (  b 0 )[ P0 (1)] m
                           3            2
                                        b       0.085     Are updated ME values available?
         Based on hep-ph/0003142

Darren Price – Monte Carlo onia simulations at ATLAS             QWG Workshop – 20/10/2007            Page 2
      Extending processes available in Pythia

  Only LO order NRQCD processes are implemented in Pythia (as of 6.413)

  Desirable to extend range of processes we are able to study at ATLAS

        Pythia can handle showering/hadronisation if provided with short distance cross-

        NNLO calculations should be added, MadOnia interfaced, kt factorisation approach?

  Theoretical cross-sections and distributions for various processes need to be
   run through detector simulation and reconstruction software to see if we can
   really see what we hope to see!

  Can investigate feasibility of excluding or proving various production models
   at the LHC if we have these processes simulated sooner rather than later

        After discussions with Lansberg, considering associated charm/beauty production
        Will be able to look for J/ produced with , but need processes in Pythia!

  Would like to be able to compare different MC generators, such as Herwig++

Darren Price – Monte Carlo onia simulations at ATLAS   QWG Workshop – 20/10/2007        Page 3
      Polarisation and octet evolution in Pythia

 As standard Pythia does not contain information about octet state spin-
  alignment so this cannot be determined in the final analysis
       Would be useful to be able to include and keep track of this polarisation information
        throughout the evolution
       Would allow us to run normal simulations through the detector and see effect of
        detector acceptance with pT, h etc.
       Reducing dependence on MC templates important, can reduce any unknown
        systematics and correlations that may be otherwise present

 Octet states in Pythia currently very simplistic -- states have correct quantum
  numbers but are assumed to all evolve as either:
                 J/(8)->g+J/(1) or (8)->g+(1)
  with the emitted gluon taking away colour having 4 MeV phase space!

 By changing the mass of the octet quarkonia, can give this gluon a better
  chance of doing something (anything!) Would like to know if there are any
  predictions for what values would be sensible, or upper limits?

 This effects how likely we are to see associated hadronic activity
       If gluon has ~O(MeV) phase space, have no chance of differentiating octet/singlet
        evolution based on associated hadronic activity near onia direction after reconstruction

Darren Price – Monte Carlo onia simulations at ATLAS   QWG Workshop – 20/10/2007          Page 4

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