Heavy Ion Physics with CMS

Bolek WyslouchMIT

May 19 01 Bolek Wyslouch 2

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 Bolek Wyslouch 3

p n

z

partonspartons

plasmaplasmamixedmixed

thermalisation

hadronizationfreeze-out

chiral symmetrythermal equilibriumchemical equilibriumdeconfinement

k

t

hadronshadrons

• mass and width of resonances ()• thermal photons or dileptons (e+e +)• strangeness enhancement (K • energy loss of initial partons (jet quenching)• suppression of heavy-quark bound states (J/ , ’s +)

Particle production in Heavy Ion collisionThe “probes”

May 19 01 Bolek Wyslouch 4

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

Vacuum

QGP ?????

Quarkonia,JetsHigh pt particles

J/ suppressed ?’/ ratio ?

May 19 01 Bolek Wyslouch 5

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 Bolek Wyslouch 6

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 Bolek Wyslouch 7

Charged multiplicity at mid-rapidity

2R

mdydN

o

T

May 19 01 Bolek Wyslouch 8

Missing high pt particles at RHIC ?

Effect seems to be stronger than“Cronin effect” observed in pA

May 19 01 Bolek Wyslouch 9

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 Bolek Wyslouch 10

What To Expect from LHC

• 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

Ion Lmax <L>208Pb82 1.0 1027 4.2 1026

120Sn50 1.7 1028 7.6 1027

84Kr36 6.6 1028 3.2 1028

40Ar18 1.0 1030 5.2 1029

16O8 3.1 1031 1.4 1031

May 19 01 Bolek Wyslouch 11

Main emphasis: Identified particles at low ptAdd-ons for high pt physics

May 19 01 Bolek Wyslouch 12

CMS: High pt edge of HI physics

May 19 01 Bolek Wyslouch 13

Some CMS Assets of interest to HI

• CMS has excellent muon detection capabilities:– ||<1.3 for barrel and ||<2.4 with endcaps.– Good mass resolution: 46 MeV for the Upsilon.– Efficient suppression of background from /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 -chambers.

• Large calorimeter coverage with good jet reconstruction capabilities.

May 19 01 Bolek Wyslouch 14

Selected Physics Topicsfirst physics studies by CMS

• Event Characterization (centrality)

• Quarkonium Production: Upsilon and J/in the barrel

• Z detection

• Jet Production:– Single/Double jet ratios, jet quenching

– Z and tagged jets

• Ultra-Peripheral Collisions: and -Pomeron

Muon detector

Calorimetry

Studies conducted by existing HI group in CMSPablo Yepes will present the details

May 19 01 Bolek Wyslouch 15

Stretching CMS

Pb-Pb

Detector designed for pp. However due to flexible design offers unique capabilities for AA

May 19 01 Bolek Wyslouch 16

Centrality: Participants vs. Spectators

“Spectators”

Zero-degreeCalorimeter

“Spectators”

Many things scale with Npart:• Transverse Energy• Particle Multiplicity• Particle Spectra

“Participants”

Only ZDCs measure Npart

specpart NAN

Detectors at 90o

The collision geometry (i.e. the impact parameter) determines the number of nucleons that participate in the collision

May 19 01 Bolek Wyslouch 17

Event Characterization

In spite of very strong magnetic field(4 Tesla) there is a good correlation between centrality and transverse energy.

May 19 01 Bolek Wyslouch 18

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 Bolek Wyslouch 19

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 Bolek Wyslouch 20

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 Bolek Wyslouch 21

Software for Heavy Ion CollisionsA real challenge !

PbPb min bias PbPb min bias Multiplicity: dNch/dh=1500 time: 6 hours/ev space: 500Mb/evFor 3000 events (0.5 sec LHC) ~3 years for PII 400 MHz ~2Tb disk space.-> minimum 36 computers (for 1 month run).

Example: Small MC PRODUCTION in 2001

The most interesting events are centraldNch/dy=8000 (dNch/dh=6000). The time and the space will be ~24h/ev and ~2Gb/ev.

May 19 01 Bolek Wyslouch 22

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, DubnaFrom 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 Bolek Wyslouch 23

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 Bolek Wyslouch 24

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 Bolek Wyslouch 25

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, , high pt jets

• CMS can provide a natural place to do these measurements in the late-RHIC and post-RHIC era

• Complementary to RHIC