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

MC and quarkonium physics
studies for LHCb
Marianne Bargiotti
CERN
Marianne Bargiotti
International Workshop on Heavy Quarkonium 2007
17-20 October 2007, DESY Hamburg
Status of PYTHIA simulation: what
has been done so far
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Brand new NRQCD processes introduced and tested in PYTHIA
6.324 onward:
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color octet mechanism introduced for:
 S wave for cc and bb;
color singlet new process:
 implementation of g-g, q-g, q-q channels for cc 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:
T. Sjostrand et al.
pT4
Phys. Rev. D36:2019, 1987
W ( pT , pT 0 )  2
JHEP 0403:053, 2004
( p  p 2 )2
T0
T
together with a dampening of the strong coupling
Marianne Bargiotti
 s ( pT2 )   s ( pT2 0  pT2 )
International Workshop on Heavy Quarkonium 2007
17-20 October 2007, DESY Hamburg
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Most effective approach:
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Issues :
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Cross section smoothed reweighting 
reweight event cross section by process
type and kinematics of the hard scattering
best pTMIN = 2.85 GeV
Br ( J /   ) d / dpT (nb / GeV )
Status of PYTHIA simulation:
what has been done so far
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-2007-042,3
Jun 2007
Quarkonium physics studies at LHCb
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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) :
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production cross sections for J/ 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/ vertices.
Spectroscopy with cc states
Possible investigations on reconstruction of (3770) --> DD.
Inital studies of quarkonium states will certainly focus on two
muon final states. However, we should not forget the electron
channels
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PYTHIA foreseen studies
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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.
Marianne Bargiotti
International Workshop on Heavy Quarkonium 2007
17-20 October 2007, DESY Hamburg
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