Outline:

1
Heavy quarkonia perspectives with Heavy-Ions in
CMS

• Outline
• The CMS detector and its capabilities for
  heavy quarkonia
• Expected performances for quarkonia studies

CERN / LHCC 2007009 5 March 2007Editor David
dEnterria to be pub. in J. Phys. G
QWG 2007, DESY, Hamburg, October 19, 2007
Pedro Ramalhete, on behalf of CMS
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Phase space coverage of the CMS detector

• CMS (with HF, CASTOR, ZDC) TOTEM almost full ?
  acceptance at the LHC !
• charged tracks and muons ? lt 2.5, full f
• electrons and photons ? lt 3, full f
• jets, energy flow ? lt 6.7 (plus ? gt 8.3 for
  neutrals), full f
• excellent granularity and resolution
• very powerful and flexible High-Level-Trigger

TOTEM
CASTOR
5.2 lt ? lt 6.6
HF
HF
ZDC
h -8 -6 -4 -2 0 2 4 6
8
? gt 8.3 neutrals
3
h, e, g, m measurement in the barrel (? lt
2.5)
Si Tracker ECAL
muon-chambers
CalorimetersECAL PbWO4HCAL Plastic
Sci/Steel sandwich
Si TrackerSilicon micro-stripsand pixels
Muon BarrelDrift Tube Chambers (DT)Resistive
Plate Chambers (RPC)
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Why is quarkonia suppression interesting?
In a deconfined phase the QCD binding potential
is screened and the heavy quarkonia states are
dissolved. Different heavy quarkonium states
have different binding energies and, hence, are
dissolved at successive thresholds in energy
density or temperature of the medium. Their
suppression pattern is a thermometer of the
produced QCD matter.
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A smoking gun signature of QGP formation
The feed-down from higher states leads to
step-wise J/y and ? suppression patterns. It
is very important to measure the heavy quarkonium
yields produced in Pb-Pb collisions at the LHC
energies, as a function of pT and of collision
centrality.
y
cc
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J/y suppression in heavy-ion collisions at the SPS
p-Be p-Pb centralPb-Pb
reference data
The yield of J/y mesons per DY dimuon is
slightly smaller in p-Pb collisions than
inp-Be collisions and is strongly suppressedin
central Pb-Pb collisions
Interpretation strongly bound ccbar pairs (our
probe) are anomalously dissolved by the
deconfined medium created in central Pb-Pb
collisions at SPS energies
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y suppression in heavy-ion collisions at the SPS
The y suppression pattern in S-U and in Pb-Pb
shows a significantly stronger drop than expected
from the normal extrapolation of the p-A data
y
sabs 20 mb
The change of slope at L 4 fm is quite
significant and looks very abrupt...
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Hard Probes at LHC energies

• Experimentally theoretically controlled
  probes of the early phase in the collision
• Very large cross sections at the LHC
• CMS is ideally suited to measure them
• Pb-Pb instant. luminosity 1027 cm-2s-1
• ? Lumi 0.5 nb-1 (1 month, 50 run eff.)
• Hard cross sections Pb-Pb A2 x pp
• ? pp-equivalent ? Lumi 20 pb-1
• ? 1 event limit at 0.05 pb (pp equiv.)

pp ?s 5.5 TeV
1 mb
J/y
1 nb
?
h/h-
jet
Z0jet
gjet
1 pb
gprompt
1 event
? M. Ballitjin, C. Loizides, G. Roland, CMS note
AN-2006/099
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The High Level Trigger

• CMS High Level Trigger 12 000 CPUs of 1.8 GHz
  50 Tflops !
• Executes faster versions of offline
  algorithms (on full events)
• pp design luminosity L1 trigger rate 100 kHz
• Pb-Pb collision rate lt 8 kHz
• ? pp L1 trigger rate gtgt Pb-Pb collision rate
  ? run HLT codes on all Pb-Pb events
• Pb-Pb event size 2.5 MB (up to 9 MB)
• Data storage bandwidth 225 MB/s ? 10100
  Pb-Pb events/s
• HLT reduction factor 3000 Hz ? 100 Hz
• Average HLT time budget per event 4 s
• Using the HLT, the event samples of hard
  processes are statistically enhanced by very
  large factors

Pb-Pb at 5.5 TeV design luminosity
ET reach x2
jets
x35
x35
? M. Ballitjin, C. Loizides, G. Roland, CMS note
AN-2006/099
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Impact of the HLT on the pT reach of the RAA
Nuclear modification factor AA-yield / pp-yield
QCD medium / QCD vacuum
Pb-Pb (PYQUEN) 0.5 nb-1
HLT
Important measurement to compare with parton
energy loss models and derive the initial parton
density, dNg/dy, and the medium transport
coefficient, ltqgt

? C. Roland et al., CMS note AN-2006/109
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Quarkonia studies in CMS
So far, only the dimuon decay channel has been
considered. The physics performance has been
evaluated with the 4 T field (2 T in return yoke)
and requiring a good track in the muon chambers.
The good momentum resolution results from the
matching of the muon tracks to the tracks in the
silicon tracker.
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Pb-Pb ? ? X event in CMS
dNch/dh 3500
? ? ???-
Pb-Pb event simulated using the official CMS
software framework (developed for pp)
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? ? mm- acceptances and mass resolutions
CMS has a very good acceptance for dimuons in the
Upsilon mass region The dimuon mass resolution
allows usto separate the three Upsilon states
54 MeV within the barrel and 86 MeV when
including the endcaps
? O. Kodolova, M. Bedjidian, CMS note 2006/089
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J/y ? mm- acceptances and mass resolutions

• The material between the silicon tracker and the
  muon chambers (ECAL, HCAL, magnets iron)
  prevents hadrons from giving a muon tag but
  impose a minimum muon momentum of 3.54.0 GeV/c.
  This is no problem for the Upsilons, given their
  high mass, but sets a relatively high threshold
  on the pT of the detected J/ys.
• The low pT J/y acceptance is better at forward
  rapidities.
• The dimuon mass resolution is 35 MeV, in the
  full h region.

barrel endcaps
J/y
Acceptance
barrel endcaps
barrel
pT (GeV/c)
? O. Kodolova, M. Bedjidian, CMS note 2006/089
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pT reach of quarkonia measurements (for 0.5 nb-1)
? produced in 0.5 nb-1 rec. if dN/dh 2500 ?
rec. if dN/dh 5000
Expected rec. quarkonia yields J/y 180 000
? 26 000
J/y
Statistical accuracy (with HLT) of expected? /
? ratio versus pT ? model killer...
?
curves from
Nucl. Phys. B492 (1997) 301337
? O. Kodolova, M. Bedjidian, CMS note 2006/089
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? production in Ultra-Peripheral Pb-Pb Collisions

• CMS will also study Upsilon photo-production,
  which occurs when the electromagnetic fields of
  the 82 protons of each nuclei interact with
  each other
• This measurement (based on neutron tagging in
  the ZDCs) allows us, in particular, to study
  the gluon distribution function in the Pb
  nucleus
• Around 500 events are expected after 0.5 nb-1,
  adding the ee- and mm- decay channels

?
?
? D. dEnterria, A. Hees, CMS note AN-2006/107
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Summary

• The CMS detector has excellent capabilities to
  study the dense QCD matter produced in
  very-high-energy heavy-ion collisions, through
  the use of hard probes such as high-ET (fully
  reconstructed) jets and heavy quarkonia
• With a high granularity inner tracker (full
  silicon, analog readout), a state-of-the-art
  crystal ECAL, large acceptance muon stations, and
  a powerful DAQ HLT system, CMS has the means to
  measure charged hadrons, jets, photons, electron
  pairs, dimuons, quarkonia, Z0, etc!
• This opens the door to high-quality measurements
  that so far lived only in the realm of dreams
  (maybe even including the study of cc ? J/y g
  using the ECAL)