MC and quarkonium physics studies for LHCb

1. MC and quarkonium physics studies for LHCb Marianne Bargiotti CERN International Workshop on Heavy Quarkonium 2007 17-20 October 2007, DESY Hamburg

2. Status of PYTHIA simulation: what has been done so far • Brand new NRQCD processes introduced and tested in PYTHIA 6.324 onward: • color octet mechanism introduced for: • S wave for cc and bb; • color singlet new process: • implementation of g-g, q-g, q-q channels forcc and cb • QCD perturbation theory breaks down at low pT values, confinement is not taken into account: • This is implemented in Pythia by the introduction of a pT min. cut-off, that can be an abrupt one, or an appropriately smoothed cut-off defined by a reweight of the cross section: together with a dampening of the strong coupling • T. Sjostrand et al. • Phys. Rev. D36:2019, 1987 • JHEP 0403:053, 2004 International Workshop on Heavy Quarkonium 2007 17-20 October 2007, DESY Hamburg

3. Status of PYTHIA simulation: what has been done so far • Most effective approach: • Cross section smoothed reweighting  reweight event cross section by process type and kinematics of the hard scattering • best pTMIN = 2.85 GeV • Issues : • Colour Singlet cross section at high pT exceeds what one would expect, and this leads to a slight excess of the sum of the two contributions CSM+COM • pT0 value slightly on the large side, one would hope e.g. for 2-2.5 GeV similarly to the value used in the Multiple Parton Interaction scheme in Pythia, which adopts the same concept and mechanism for dampening the divergent cross sections at low pT • No recent result on LHCb side: total lack of manpower on this side • Recent studies carried on by ATLAS/CMS (comparison among diff. generators: PYTHIA, COMPHEP and Alpgen) Total Color octet Color-singlet CTEQ6L|y|<0.6 • Full description of the approach used in: • Heavy quarkonia sector in PYTHIA 6.324: Tuning, validation and perspectives at LHC(b).M. Bargiotti (CERN) , V. Vagnoni (INFN, Bologna) . CERN-LHCB-2007-042, Jun 2007

4. Quarkonium physics studies at LHCb • All decays are handled by EvtGen that handles spin correlations in the same way • No production polarization is taken into account at the moment. • Foreseen productions and investigations (depending on manpower availability) : • production cross sections for J/y and Upsilon, as a function (or in a certain range) pT, y or xF. • study on the polarization (spin alignment) as a function of the kinematics, and possibility to extract also a measurement for the b-cross section by looking for detached J/y vertices. • Spectroscopy with cc states • Possible investigations on reconstruction of y(3770) --> DD. • Inital studies of quarkonium states will certainly focus on two muon final states. However, we should not forget the electron channels

5. PYTHIA foreseen studies • Polarization production: test of polarization contribution from PYTHIA 6.4 onward • Y production testing • cc,b contribution testing (explicitly useful for cc studies foreseen in near future) • Comparison of various MC tuning on cross section reweighting in order to extract a coherent value of pTMIN. International Workshop on Heavy Quarkonium 2007 17-20 October 2007, DESY Hamburg