1 Heavy Ion Physics at CMS Prashant Shukla Nuclear Physics Division BARC, Mumbai India 28 th February 2011, BARC

Physics motivations of LHC 2 Most publicized motivations of LHC experiments: pp collisions: Searching the particle (Higgs) responsible for masses of the fundamental particles. Plus new physics beyond standard model. PbPb collisions: The matter is made of quarks and gluons always confined inside nucleons. PbPb collisions aim to produce a soup of quarks and gluons. This state is known as Quark Gluon Plasma existing at early universe.

Heavy Ion Collisions at LHC 3 At LHC hottest matter ever created in the laboratory New probes open up High tech detectors for precise measurement Hard probes (approved CMS Results) Dijets and jet quenching (HIN-10-04) Z 0 results (HIN-10-03) Quarkonia (under progress) Soft probes (Under progress) Multplicity Elliptic Flow Charged particle spectra Correlations

4 LHC at CERN Experiments for Heavy Ions CERN CMS ATLAS ALICE Geneva Lake

Heavy Ion plans for the LHC 5 11/23/105 Physics proton-proton run at the LHC Started in November 2009 √s = 0.9, 2.36, 7 TeV. The heavy-ion run started 8 th November 2010 Pb+Pb collisions at √s = 2.76 TeV per nucleon pair CMS LHC run stopped on 5 th December --> Luminosity next slide The second heavy-ion run is expected in the November-December 2011 at the same or little more energy with but increase in luminosity. After the LHC upgrade 2012 we hope to have in 2013 Pb+Pb collisions at √s = 5.5 TeV per nucleon pair

PbPb data taking with CMS 6 11/23/106 PbPb run finished with integrated Luminosity 9.59  b -1 delivered, 8.72  b -1 recorded corresponding pp equivalent ~ 0.3 pb -1 (pp delivered so far 40 pb -1 )

The CMS Experiment Compact Muon Solenoid 7 7 7

The CMS as a Heavy Ion Experiment Silicon Tracker Good efficiency and purity for p T > 1 GeV/c  p/p  1–2 % for p T < 100 GeV/c Calorimeters: high resolution and segmentation Good performance for jet studies Muon Tracking:  from Z0, J/ ,  Wide rapidity coverage: |  | < 2.4 σ m  70 MeV/c 2 at the  mass in |  | < 1 CMS is a superb and versatile detector for heavy ion physics Excellent performance in high p T (E T ) region and for muon pairs Quarkonia, Jet physics, Z0,.... 8

A typical PbPb central event 9 9

A typical PbPb peripheral event 10

The Muon reconstruction 11

Trigger condition 12 Minimum bias trigger: Using information from the two Beam Scintillator Counters (BSC) and Forward Hadronic Calorimeters (HF). Statistics: Number of events = 54,965,553

Centrality Determination 13

Heavy Ion Observables 14

Soft Probes 15

16 Soft probes in progress Multiplicity Elliptic Flow Charged Particle Spectra Two Particle Correlations

The Elliptic Flow v2 17 Anisotropic Flow X y Z pxpx pypy Peripheral Collisions V 2 = phi = atan (py/px)‏  V 2 gives pressure transfer from y to x direction measures collectivity

The Elliptic Flow v2 - v2 measures collectivity - v2 (pT) for |η|<0.8, 0.8 <|η|<1.6, 1.6 <|η|<2, 2<|η|<2.4 – Integrated v2 vs η : for several centralities – v2 (integrated) mid-rapidity scaled by Npart as a function of centrality – v2 results with events selected on the basis of presence of identified jets 18

New observation by CMS in pp collisions 19 “Ridge”-effect in high-multiplicity events at 7 TeV Long-Range Near-Side Angular Correlations in pp Shown alongwith RHIC results

Hard Probes 20

21 Jet Quenching At RHIC Strong quenching effects were observed in single particle spectra and particle correlations. Direct jet reconstruction possible but very difficult with RHIC detectors. Jet suppression is indicated by leading particle.

Di Jet events First hours of LHC running We have seen di-jet events We have seen di-jets with unbalanced energy 22

Study of Di Jet events Leading jet is required to have at least 120 GeV Above trigger threshold Sub-leading jet is required to have at least 50 GeV Above background fluctuations Select back-to-back jets  phi > 2.5 To study jet quenching effects use jet energy asymmetry A J = (P T,1 – P T,2 ) / (P T,1 + P T,2 ) arXiv: [nucl-ex] 23

Di Jet energy imbalance A significant dijet imbalance, well beyond that expected from unquenched MC embedded in real data, appears with increasing collision centrality 24 arXiv: [nucl-ex]

Quantifying the imbalance Fraction of unbalanced dijets Fraction of jets with imbalance smaller than 0.15 Plot as a function of number of participating nucleons (volume) averaged over centrality bin 25 arXiv: [nucl-ex]

Jet quenching 26 What we conclude: A significant dijet energy imbalance. The imbalance is well beyond that expected from unquenched MC embedded in real data. The imbalance increases with collision centrality The robustness checks: Imbalance MC with and without embedding in data. By smearing the jet resolution by 10 to 50 % in simulation. Leading jet cut off ( 120, 130, 140). Sub leading jet cut off ( 35, 50, 55)

QGP probes are modified in the medium: a baseline needed. Z0 does not interact with medium (like photons) - Probe of initial state effects: shadowing (10-20 %), multi-parton scattering (2 %), Isospin (3 %) Z0 bosons 27 First Z0 Candidate in HI collisions

Z0 analysis framework 28 MuSkim: Event passed DiMuon Trigger HLT_HIL2DoubleMu3_Core DiMuonSkim: Three exclusive dimuon categories. i) DiMuonsGlobal ii) DiMuonsGlobalSTA iii) DiMuonsSTA DiMuon2DPlots 2D histograms (M vs pT), (M vs Y), (M vs Cent) for each dimuon category. Z0MassFit V. Kumar + P Shukla

Z 0   +  - signal in PbPb 29 All heavy ion statistics between [30,120] GeV/c 2, with some loose quality criteria Resolution comparable to p+p 2.9 pb -1 [60,120] GeV/c 2

RAA for Z 0   +  - in PbPb 30

RAA for Z 0   +  - in PbPb 31

Z-> ee Candidate 32

Future Study : gamma+jet 33

Future Study : Z0+jet 34

Quarkonia as probes of QGP 35 Large Masses produced early in the collisions via gluon fusion Strongly bound and weakly coupled to light mesons Quarkonia should melt in QGP: SPS: J/ψ suppression seen. But there are alternative explainations. RHIC: Suppresion vs rapidity not completely understood. LHC: Regeneration of J/ψ from the (large) number of uncorrelated ccbar pairs. Upsilons open up.

High p T J/    +  - 36 Subset of the statistics in HI, dimuon pT in [6.5, 30] GeV/c2 Very good resolution also in HI collisions ! Background in HI already low with basic quality criteria in this pT window

High p T Y   +  - 37 Subset of the statistics in HI, single muon pT in [4, 30] GeV/c2 Good resolution also in HI collisions Background is more than J/psi

Quarkonia 38 The goal of the first analysis: absolute cross sections/ RCP All Physics Data Set Crucial to separate prompt and non-prompt J/ψ Need to tune muon identification cuts  Abdulla Abdulsalam

Summary and Outlook 39 CMS has collected a good quality data with heavy ion collisions. The detector has shown excellent performance in all major sectors. Observation of new phenomena in heavy ion collisions Large number of dijets with unbalanced energies indicative of jet quenching Z0 measurement: Within uncertainty no modification was observed. At higher luminosity it can be used to study the modifications in PDFs in the initial state. Reconstruction of J/psi and Upsilon with similar mass resolutions as in pp. Rigorously pursued now. Soft Probes: Interesting results are expected soon for: Multplicity Elliptic Flow Charged particle spectra Correlations

CMS Heavy Ion Crew 40

Back Up 41

Reconstruction of Jets 42