Heavy Ion Physics with CMS

Russell Betts - UIC

Studying QCD with Heavy Ions

• Quark Gluon Plasma:– QCD at High T, High Density– Phase Diagram of QCD

• Strongly-Interacting Systems– Evolution of Colliding Nuclei– Study Early-Stage Dynamics

hadrons quark/gluon

(F. Karsch, hep-lat/0106019)

3/26.0~

200150~

fmGeV

MeVT

c

c

The Physics Landscape: Pb+Pb Collisions SPS->RHIC->LHC

d

Extrapolation of RHIC results favors low values

Production of High pT Particles: (e.g 0)

Enormous Increase at High pT over RHIC and SPS

Hot Nuclear Matter Diagnostics

Leading Particle

Hadrons

q

q

Hadrons

Leading Particle

Jet cross-section calculable in QCD

Study fate of jets in hot, dense matter in Au+Au

Hadrons

q

q

Hadrons

Leading Particle

Leading Particle

Correlations as Function of Collision Geometry

(Thickness Traversed by Particles)

Suppression Patterns of Quarkonia

J/

family

A+A/p+p

Distinguish Between Scenarios of Suppression

CMS as a Detector for Heavy Ion Physics

Si Trackerincluding Pixels

ECALHCAL

chambers

•Fine Grained High Resolution CalorimeterHermetic coverage up to ||<5 (||<7 proposed using CASTOR)Zero Degree Calorimeter (proposed)

•Tracking from Z0, J/, Wide rapidity range ||<2.4

σm ~50 MeV at

•Silicon Tracker Preliminary Results Very Promising

•DAQ and Trigger

High rate capability for AA, pA, pp

High level trigger can reconstruct most AA events in real time

• Excellent Detector for High pT Probes:– Quarkonia (J/ ,) and Heavy Quarks (bb)

– High pT Jets

– High Energy Photons

– Z0

• Hermetic Calorimetry - Correlation of Jets with Jets, , Z0

• Global Event Characterization– Energy Flow in Wide Rapidity Range

– Charged Particle Multiplicity – dN/d– and Flow

– Centrality (Collision Geometry)

• CMS can use Full Luminosities for both AA & dA

Physics Measurements in CMS

-

CMS Detector in the Heavy Ion EnvironmentHigh Multiplicity of Low pT Hadrons

Occupancies still Reasonable

Large Event Size but Lower Event Rate

Jets are easily seen in pp

… and also in PbPb

1. Subtract average pileup2. Find jets with sliding window3. Build a cone around Etmax 4. Recalculate pileup outside the cone5. Recalculate jet energy

Spatial resolution: σφ = 0.032 ση = 0.028

Efficiency, purity

Jet energy resolution

Full Jet Reconstruction in Central Pb-Pb Collision

HIJING, dNch/d = 5000

High Mass Dimuon, Z0 Production

• Z0 can be reconstructed with high efficiency.

• Dimuon continuum dominated by b decays– Heavy quark energy loss

• High statistics (1 month):

Channel (M10GeV) Barrel+Endcap

Z 1.1104

BB Pt()5 GeV 1.2105

B J/ Pt()5 GeV 1.3105

Channel Barrel+Endcap

Jet+Jet, ET(Jet) >100 GeV 8.7106

+Jet, ET(Jet) >100 GeV 6103

Z(Jet, ET(Jet),PT(Z) >100 GeV 90

Z(Jet, ET(Jet),PT(Z) >50 GeV 600

Balancing or Z0 vs Jets: Quark Energy Loss

# E

ven

ts/4

GeV

ET//0-ET

Jet (GeV)

<E>=8 GeV<E>=4 GeV<E>=0 GeV

Background

Isol. 0+jet

, Z0

1 month at1027 cm-2s-

1

Pb+Pb

Jet+Z0

Quarkonia from Different Ion Species

J/ family

Quarkonia in CMS

J/ family Yield/month (k events, 50% eff)Nominal luminosity for each ion

species

Pb+Pb, 1 month at L=1027

  

Pb+Pb Kr+Kr Ar+Ar

L 1027 7×1028 1030

J/ 29 470 2200

´ 0.8 12 57

23 320 1400

´ 12 180 770

´´ 7 100 440

Si Tracker Performance with Heavy Ions

6 layersOuterBarrel

4 layersInnerBarrel

3 disks 9 disks in the End Cap

1 Single Detector

2 Detectors Back to Back

Pixel Layers Crucial for Heavy Ions

Construct “Tracklets” using two outer pixel layers - straight lines in R-z - and calculate zBEAM for each.

Iterative histogramming draws out peak

Fit with Gaussian + constant to find z vertex

Optimize pT (min) and range to

balance statistics vs. combinatorics

Tracklets constructed from triplets reduce combinatorial background

Primary Vertex Reconstruction

Multiplicity (dN/d) 2000 3000 5000 7000

RMS (m) 16.3 14.1 13.0 12.6

Tracking with CMS pp Track Finder

Uses “Triplet” Track Seeds

“Reconstructable” Tracks have 8

Layers Hit Including 3 Pixel

Layers

pT Inside a Jet

100 GeV

Heavy Ion Specific Additions

• Zero Degree Calorimeters

• CASTOR

• High Level Trigger Software

Beam pipe splits 140m from IR

ZDC LOCATION

BEAMS

b2R ~ 15fm

Spectators

Spectators

Participant Region

Measure Spectator

Neutrons at 0°

Correlate EZDC

with Impact Parameter

CASTOR and T2

CASTOR

5.32 < η < 6.86

T2 Tracker

5.32 < η < 6.71

New Opportunity: Forward Silicon Counters and CalorimetryFull Multiplicity Coverage up to of 6.7

Heavy Ion Trigger

• Main types of trigger as required by physics:

– multiplicity/centrality:”min-bias”, “central-only”

– high pT probes: muons, jets, photons, quarkonia etc.

• High occupancy but low luminosity !

– many low level trigger objects may be present, but less isolated than in p+p, Level 1 might be difficult for high pT particles

– but we can read most of the events up to High Level Trigger and do partial reconstruction

• HLT for HI needs significant software/simulation effort.

L1

HLT

People and Institutions

• Russia: Moscow State University, Dubna

• France: Lyon

• Georgia: Tbilisi

• New Zealand: Auckland

• Greece: Athens, Demokritos, Ioannina

• USA NP: Rice, UC Davis, MIT, UI Chicago, U Iowa, UC Riverside, U Kansas

• Presently ~30 people involved directly in studies/discussions etc.

• Expect to grow to ~100 by the time LHC starts

SUMMARY

• CMS will Function very well as a Detector for HI Physics.

• Present RHIC data indicates that high pT HI physics is likely to be a rich field at LHC, which plays to strength of CMS.

• Number of Heavy Ion Physicists in CMS is growing. Group will be very active.

• Additional forward detectors for HI will also be interesting and useful for p+p.