Monte Carlo quarkonium simulations at ATLAS
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)]
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
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
PARP(150) O ( b 0 )[ P0 (1)] m
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