Heavy Ion Physics with
CMS
Bolek Wyslouch
MIT
Holy Grail of High Energy Nuclear
Physics: Quark Gluon Plasma
• quarks and gluons, the fundamental constituents of
matter, are no longer confined within the dimensions of
the nucleon, but free to move around over a volume in
which a high enough temperature and/or density
prevails
• plasma also exhibits the so-called "chiral symmetry"
which in normal nuclear matter is spontaneously
broken, resulting in effective quark masses which are
much larger than the actual masses.
• Basically: physics of condensed nuclear matter: phase
changes due to multiple interactions of energetic quarks
and gluons
May 19 01
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Particle production in Heavy Ion collision
The “probes”
m
t
freeze-out
hadronization
chiral symmetry
thermal equilibrium
chemical equilibrium
deconfinement
m
p
n
k p
hadrons
mixed
plasma
partons
thermalisation
z
•
•
•
•
•
mass and width of resonances (r, w)
thermal photons or dileptons (e+e- , m+m-)
strangeness enhancement (K, f, L, X, W)
energy loss of initial partons (jet quenching)
suppression of heavy-quark bound states (J/y , ’s  m+m-)
May 19 01
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Hard probes: cc, bb, jets
• High mass or high momentum objects created
during formation phase via high Q2 parton
scattering
• Penetrate hot and dense matter
• Sensitive to state of hot and dense matter
– Color screening
– dE/dx by strong interactions
Quarkonia,
Jets
High pt particles
Vacuum
QGP
?????
J/Y suppressed ?
’/ ratio ?
May 19 01
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Today’s Experiments
• Relativistic Heavy Ion Collider on Long Island, a
dedicated facility:
– Collisions of Au-Au with up to 100 AGeV in each beam
– Four Experiments:
• Brahms, Phenix, Phobos, Star
•
•
•
•
So far about 1-2 weeks of data
This year ~9 months
Running 6-9 months per year
Building on CERN/SPS results
May 19 01
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Recent results from RHIC
• Global features: charged multiplicity, particle
production asymmetry (flow) indicate
significant increase of energy density
compared to pp
• Indication of importance of “hard probes”
– Jet production may be suppressed (quenched),
production of high pt particles is smaller than
expected from pp
• Looking forward to see J/psi production at
RHIC, suppression observed at SPS…
May 19 01
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Charged multiplicity at mid-rapidity

May 19 01
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dN dy mT
 o  pR 2
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Missing high pt particles at RHIC ?
Effect seems to be stronger than
“Cronin effect” observed in pA
May 19 01
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What to expect from RHIC within the next
few years
• A very detailed study of most if not all
“QGP signatures” at RHIC
• Many beam energies, ion species, very
high luminosity
• But: maximum energy at RHIC is
limited by ring radius and magnets
• Some measurements, especially high pt
probes may require higher energies:
LHC
May 19 01
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What To Expect from LHC
Ion
• LHC (Large Heavy ion
Collider) is expected to
provide pA and AA
collisions.
• Energy: 5.5 GeV per
nucleon pair in PbPb
• 6 weeks/year
• First heavy ion run in
March 2007
May 19 01
208
Lmax
Pb82 1.0 1027 4.2 1026
120
50
Sn
84
36
Kr
40
<L>
18
Ar
16
Bolek Wyslouch
O8
28
1.7 10
28
6.6 10
30
1.0 10
27
7.6 10
28
3.2 10
29
5.2 10
3.1 1031 1.4 1031
10
Main emphasis:
Identified particles at low pt
Add-ons for high pt physics
May 19 01
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CMS: High pt edge of HI physics
May 19 01
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Some CMS Assets of interest to HI
• CMS has excellent muon detection
capabilities:
– |h|<1.3 for barrel and |h|<2.4 with endcaps.
– Good mass resolution: 46 MeV for the Upsilon.
– Efficient suppression of background from p/K
decays:
• Electromagnetic calorimeter at 1.3 m from beam
axis.
• PT threshold at 3.5 GeV/c for a single muon to reach
the m-chambers.
• Large calorimeter coverage with good jet
reconstruction capabilities.
May 19 01
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Selected Physics Topics
first physics studies by CMS
Studies conducted by existing HI group in CMS
Pablo Yepes will present the details
• Event Characterization (centrality)
Muon
detector
• Quarkonium Production: Upsilon and J/Y in
the barrel
• Z detection
• Jet Production:
Calorimetry
May 19 01
– Single/Double jet ratios, jet quenching
– Z and g tagged jets
• Ultra-Peripheral Collisions: gg and gPomeron
Bolek Wyslouch
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Stretching CMS
Pb-Pb
mm
Detector designed for pp. However due to
flexible design offers unique capabilities for AA
May 19 01
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Centrality:
Participants vs. Spectators
The collision geometry (i.e. the impact parameter) determines
the number of nucleons that participate in the collision
“Spectators”
Only ZDCs measure Npart
Zero-degree
Calorimeter
“Spectators”
Many things scale with Npart:
• Transverse Energy
• Particle Multiplicity
• Particle Spectra
May 19 01
“Participants”
Bolek Wyslouch
Detectors
at 90o
N part  A - N spec
16
Event Characterization
May 19 01
Bolek Wyslouch
In spite of very strong
magnetic field
(4 Tesla) there is a
good correlation
between centrality
and transverse
energy.
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Alternative “centrality trigger”
• “Zero degree calorimeter”
– Small fast calorimeters sensitive to forward
neutrons
– Very forward region, between two vacuum
beam lines
– Working at RHIC
– Same design can be adopted for LHC
• Could be essential for some
measurements
May 19 01
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Will the DAQ cope with the HI ?
• Pb-Pb at L=1027cm-2s-1 (2 experiments
running at the time)
• LHC HI is relatively low luminosity but
the occupancy can be huge, O(100,000)
particles
• Expected event size ~1.5 M Bytes (or
larger, needs study)
• With mass storage of 100 MBytes/s one
can write ~70 events/s
May 19 01
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Needs dedicated, continuing studies
• DAQ and trigger: configuration, length
of DAQ buffers, trigger thresholds
– Most likely need to be reconfigured for the
HI run
• Lower magnetic field ?
– Better jet acceptance
– Lower pt threshold for J/psi (?)
– functioning of trigger etc
May 19 01
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Software for Heavy Ion Collisions
A real challenge !
Example: Small MC PRODUCTION in 2001
PbPb min bias
Multiplicity: dNch/dh=1500
time: 6 hours/ev
space: 500Mb/ev
For 3000 events (0.5 sec LHC)
~3 years for PII 400 MHz
~2Tb disk space.
-> minimum 36 computers (for 1 month run).
The most interesting events are central
dNch/dy=8000 (dNch/dh=6000).
The time and the space will be
~24h/ev and ~2Gb/ev.
May 19 01
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US Politics
• CMS has a great potential to do good physics
during Heavy Ion runs at LHC
• US Nuclear physics community endorsed US
participation in LHC heavy ion program (NSAC
Long Range Plan)
• Positive signs from DoE/Nuclear
• But no miracles: RHIC is big and very hungry
• Some of our colleagues prefer Alice
CMS Heavy ion group: Lyon, Moscow, St. Petersburg, Dubna
From the US: Rice, UC Davis
Several US groups are considering participation (MIT, UIC, your colleagues)
Schedule and level of participation are driven by RHIC commitments
May 19 01
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Schedule and Resources
• First HI run will be in 2007
• Needed to get good HI physics in 2007:
– Physics studies
– DAQ, Trigger and software preparation
– Zero-degree calorimeter construction
• There will be solid physics base learned
at RHIC
May 19 01
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Possible plan
• Involvement in direct simulation and
physics preparation work, ramping up in
2003-2004
• Involvement in US CMS computing
together with HEP groups
• DoE Nuclear funding for Tier-2(+) size
computing resources
May 19 01
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Summary
• LHC will be a natural continuation of the
series of Heavy Ion accelerators
• CMS will have unique capabilities at the
high transverse momentum frontier
– , Z0, g, high pt jets
• CMS can provide a natural place to do
these measurements in the late-RHIC
and post-RHIC era
• Complementary to RHIC
May 19 01
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