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Heavy Ion Physics with the CMS
Experiment at the LHC
Gábor Veres
Eötvös Loránd University, Budapest, Hungary
Massachusetts Institute of Technology, Cambridge, USA
for the CMS Collaboration
Strangeness in Quark Matter ’06
UCLA, California, March 26, 2006
CMS HI groups: Adana, Athens, Basel, Budapest, CERN, Demokritos, Dubna, Ioannina, Kiev, Krakow,
Los Alamos, Lyon, MIT, Moscow, Mumbai, New Zealand, Protvino, PSI, Rice, Sofia, Strasbourg, U Kansas, Tbilisi,
UC Davis, UC Riverside, UI Chicago, U Iowa, Yerevan, Warsaw, Zagreb
Gábor Veres
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
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Physics opportunities at the LHC
 LHC: a large increase in collision energy compared to
existing accelerators:
Extended kinematical reach (y, pT) for p+p, p+A, A+A collisions
New properties of initial state, saturation even at mid-rapidity
A hotter and longer lived partonic phase
Increased cross sections of hard probes, heavy quarks
Last but not least: unknown territory/surprises?
 New energy regime will open a new window on hot and
dense matter physics: another large energy jump!
sNN [GeV]
AGS
SPS
RHIC
LHC (Pb+Pb)
5
17
200
5500
3-4
12
28
2.9
5.4
8.7
increase in sNN
y range
Gábor Veres
1.6
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
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Heavy Ion Physics Topics at the LHC
 High pT: modification by the medium
Copious production of high pT
particles
Large jet production cross section
 Different “melting” for the  family
members, depending on binding energy
RHIC
RHIC
Large production cross section for the
J/ψ and  family
 Correlations, scattering in medium
jets clearly identifiable, for the first
time in heavy ion collisions
Gábor Veres
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
LHC
LHC
J/ψ 
CMS, as a heavy ion experiment
Calorimeters: high resolution and
segmentation

Hermetic coverage up to |h|<5
(|h|<7 with the proposed CASTOR)
Zero Degree Calorimeter (approved)

Muon tracking: m from Z0, J/, 
Wide rapidity coverage: |h|<2.4
σm  50 MeV at the  mass in the barrel

Silicon Tracker
Good efficiency and purity for pT>1 GeV
Pixel occupancy: <2% at dNch/dh  3500
Dp/p  2% for pT<70 GeV
CASTOR
(5.32 < η < 6.86)
 DAQ and Trigger
High rate capability for A+A, p+A, p+p
High Level Trigger: real time HI event
reconstruction
ZDC
(z = 140 m)
Functional at the highest expected multiplicities:
studied in detail at dNch/dh 3000-5000 and cross-checked at 7000-8000
Gábor Veres
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
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Data Acquisition and Trigger
 Level 1 hardware trigger
Muon track segments
Calorimetric towers
No tracker data
Output rate (Pb+Pb): 1-2 kHz
comparable to collision rate
Lvl-1
 High level trigger
switch
HLT
Full event information available
Every event accepted by L1 sent to an
online farm of 2000 PCs
Output rate (Pb+Pb):  40 Hz
Trigger algorithm: similar to offline
reconstruction
- Every event must pass the whole chain
- Selectivity depends on available CPU power
Gábor Veres
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
Centrality and forward detectors
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Centrality (impact parameter) determination is needed for physics analysis
Zero Degree Calorimeter
ET [GeV]
Energy in the forward
hadronic calorimeter
Pb+Pb
Tungsten-quartz fibre structure
electromagnetic section: 19X0
hadronic section 5.6λ0
Rad. hard to 20 Grad (AA, pp low lum.)
Energy resolution: 10% at 2.75 TeV
Position resolution: 2 mm (EM sect.)
impact parameter [fm]
Gábor Veres
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
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Charged particle multiplicity
Will be one of the first results,
important for initial energy density,
saturation, detector performance etc.
 high granularity pixel detectors
 pulse height measurement in each pixel
reduces background
 Very low pT reach, pT>26 MeV (counting hits)
Simple extrapolation from RHIC data
W. Busza, CMS Workshop, June 2004
ch
Muon detection, tracking, jet finding performance checked up to dNch/dh5000
Gábor Veres
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
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Track reconstruction
Momentum
Resolution [%]
Efficiency and
fake rate [%]
Transverse Impact
Parameter Resolution
[cm]
%
efficiency
2.0<h<2.5
-0.5<h<0.5
2.0<h<2.5
-0.5<h<0.5
fake rate
-0.5<h<0.5
(Event sample: dn/dy3000 + one 100GeV jet/event)
Excellent performance, even at the highest particle densities
Gábor Veres
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
Quarkonia in CMS Heavy Ions
Mm+m- spectrum,  family
 family
J/
sMY=50 MeV
in the barrel
Expected: 24000 J/ and ~ 18000/5000/3000 /’/’’
After one month of Pb+Pb running at L=1027cm-2s-1
with 50% efficiency
J/ acceptance
Online HLT farm improves acceptance
by 2.5 at high h and low pT
Gábor Veres
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
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Jets – a new observable at LHC
 Hard, perturbative scale: Q>>LQCD. Hard parton production unaffected by medium
 Parton shower development affected by the medium
 At LHC in Pb+Pb collisions:
wider pT range for suppression, quenching studies
jet structure will likely be modified, compared to jets produced in p+p
comparison to p+p and p+A is essential
 Observables:
High pT particles and particle correlations (similar to RHIC analyses)
c
Jet rates: single and multi-jets (quenching studies)
Jet fragmentation and shape:
a
Distance R to leading particle (in h- space)
forward-backward correlation: D(particle, jet axis)
d
Fragmentation function: F(z)=1/NjdNch/dz where z=pt/pjet
correlations with non-hadronic particles: jets+g, jets+Z
Jets originating from heavy quarks (b, c)
Extensive theoretical and experimental preparatory work
presently in progress
Gábor Veres
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
b
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Jet reconstruction in the calorimeters
efficiency
|h|<0.3
1.6<|h|<1.9
|h|<0.3
1.6<|h|<1.9
resolution
Gábor Veres
efficiency
resolution
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
Jet studies using the tracking
Tracking is a very important capability for jet physics
Azimuthal correlations
(as studied at RHIC):
dN/d(D)
dN/dpT
Centrality dependence of pT specra can be studied:
Gábor Veres
pT with respect to jet axis:
1/NjetsdNch/dpTjet
1/NjetsdNch/dz
Fragmentation functions:
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
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Advantages of CMS over other HI experiments
Hermeticity, Resolution, Granularity
Central region: tracker, electromagnetic and
hadronic calorimeters and muon detector
Forward coverage
calorimeters extend to Dh10
Proposed CASTOR calorimeter to Dh14
(5.32 < η < 6.86)
CASTOR
TOTEM
High data taking speed and trigger
versatility
Two-level trigger
Ability to “inspect” every heavy ion event on
the High Level Trigger computer farm
(z = 140 m)
ZDC
Gábor Veres
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
CMS under construction…
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Solenoid
superconducting,
already at 4K
Hadron
Calorimeter
Muon Absorber
Electromagnetic
Calorimeter
DAQ
Gábor Veres
Strangeness in Quark Matter ‘06, UCLA, March 26, 2006
Si tracker &
Pixels