1 First results from PbPb collisions at CMS Prashant Shukla Nuclear Physics Division BARC, Mumbai India CMS collaboration India-CMS Meeting, 17 January, TIFR

Heavy Ion Collisions at LHC 2 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

Heavy Ion plans for the LHC 3 11/23/103 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 4 11/23/104 Run finished with integrated 8.58  b -1 delivered, 7.84  b -1 recorded, certified 6.88  b -1

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 Good low p T reach using pixels Calorimeters: high resolution and segmentation Hermetic coverage up to |  | < 5.2 –5.2  < –6.6 with CASTOR Zero Degree Calorimeter Muon Tracking:  from Z0, J/ ,  Wide rapidity coverage: |  | < 2.4 σ m  70 MeV/c 2 at the  mass in |  | < 1 DAQ and Trigger High rate capability for A+A, p+A, p+p High Level Trigger: real time HI event reco 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,.... 5

Centrality Determination 6 6

Heavy Ion Observables 7

Soft Probes 8

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

The Multiplicity Two analyses: – Clusters – Tracklets ● Deviations between the methods: – <1% for peripheral – ~6% for central collisions Consistency between B=0 and 4 T pixel counting results: ~3% ● Significant differences between CMS and ALICE results 10

The Elliptic Flow v2 11 Anisotropic Flow X y Z pxpx pypy Peripheral Collisions V 2 =  = 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 12

The elliptic Flow (v2): Eta, Centrality 13 Small change in v2 as rapidity increases

The elliptic Flow (v2): comparison with PHENIX 14 Closed symbols: CMS |η| < 0.8 Open symbols: PHENIX |η| < 0.35

The charged particle spectra 15 From JET50U TRIGGERED sample Spectra in different centralities RAA (or RCP) up to ~ 80 GeV/c

New observation by CMS in pp collisions 16 “Ridge”-effect in high-multiplicity events at 7 TeV Long-Range Near-Side Angular Correlations in pp Shown alongwith RHIC results

Correlations in PbPb collisions 17

Hard Probes 18

19 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 20

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 21

Di Jet energy imbalance A significant dijet imbalance, well beyond that expected from unquenched MC embedded in real data, appears with increasing collision centrality 22

Quantifying the imbalance Fraction of unbalanced dijets Fraction of jets with imbalance larger than 0.24 Plot as a function of number of participating nucleons (volume) averaged over centrality bin 23

Jet quenching 24 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)

Z0 does not interact with medium (like photons) - Probe of initial state effects: shadowing (10-20 %), multi-parton scattering (2 %), Isospin (3 %) Z0 bosons 25 First Z0 Candidate in HI collisions

Measurement of the Z in PbPb collisions at 2 : 76 TeV The inclusive cross section pT, y and centrality dependance Confirmation that Z are not suppressed in the QGP: RAA Event Selection: Eq. numb. of minbias : 51 ; 016 ; 587 Luminosity L = micbarn(-1) (Glauber) Z0 paper 26

Z 0   +  - signal in PbPb 27

RAA for Z 0   +  - in PbPb 28 Vineet Kumar for detailed analysis

Z-> ee Candidate 29

Future Study : gamma+jet 30

Future Study : Z0+jet 31

Quarkonia 32 Quarkonia should melt in the Quark Gluon Plasma... J/ψ suppression has been seen at SPS and RHIC. Alternative explainations at SPS. Effect of increasing collision energy. Regeneration of J/ψ from the (large) number of uncorrelated ccbar pairs would be a golden probe of reconfinement (thus deconfinement) at LHC. Very few Upsilons at RHIC CMS will see many Successive melting of the three bound states could act as a thermometer

J/    +  - 33 The goal of the first paper: absolute cross sections/ RCP AllPhysics Data Set Crucial to separate prompt and non-prompt J/ψ Need to tune muon identification cuts In addition to cuts over Z0 Dimuon quality cut (e.g. vtxProb > 0.001)

High p T J/    +  - 34 Abdulla for details

High p T Y   +  - 35

Summary and Outlook 36 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 Z 0 production Large number of dijets with unbalanced energies indicative of jet quenching Reconstruction of J/psi and Upsilon with similar mass resolutions as in pp. Regourously pursued now. Soft Probes: Interesting results are expected soon for: Multplicity Elliptic Flow Charged particle spectra Correlations

CMS Heavy Ion Crew 37

Back Up 38

The charged Particle Multiplicity Predictions for PbPb N.Armesto: PP at 7 TeV 39

Reconstruction of Jets 40

Jet trigger efficiency 41 Corrected jet energy Raw jet energy

Aj in pp collisions at 7 TeV 42