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Jet studies in PbPb collisionsPawan Kumar Netrakanti

BARC, MumbaiINDIA-CMS week 3rd-4th Aug 2012

PbPbpp

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Outline:● Motivation● Jet selection and dataset● Analysis and results● Summary● Analysis related with this talk:

● Di-jet momentum imbalance (HIN-11-013)

PLB 710 (2012) 256● Gamma+jet (HIN-11-010) (submitted to PLB)● Jet RAA : HIN-12-004 (Approved)

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Motivation : Heavy-ion collision

Jets :➔Good probes to understand the medium effects.

➔High pT particle suppression observed at RHIC (BNL).

➔Direct measure of the lost energy: γ+jet Z+jet

➔Constrains lot of theoretical modelsfor the energy loss mechanism in dense medium.

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CMS Jets: PbPb and pp collisionspp 2.76 TeV data 2011 Luminosity 231/nb (HLT_Jet60_v1)PbPb 2.76 TeV data 2011 Luminosity 150/µb (HLT_HIJet80_v1)

pp MC : PYTHIA QCD dijet pT-hat 30+, 50+, 80+, 120+, 170+, 200+, 250+, 300+

PbPb MC : Above PYTHIA embedded into HYDJET for background.

Jet Selection: Jet Algorithm : Anti-k

T, Particle Flow jets, R = 0.3

(for systematics also used R=0.2 and 0.4)

Vertex : |vz|<15 cm

Jet p

T : > 100 GeV/c (sub-leading > 30 GeV/c)

Jet eta : |η|<2

pp jets are reconstructed with the PbPb tunned jet algorithm.

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Centrality

Most peripheral 70-100%, 50-70%, 30-50%, 20-30%, 10-20%, 0-10% Most central

Npart : Number of participating nucleons in eventNcol l : Number of binary interactions in eventTransverse energy in the forward calorimeter is correlated to Npart

Rare probes occur more frequently in central events (Ncoll scaling)

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Jet energy resolution and scale

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Jet Background from PbPb

<Background pT> = (photonSum + neutralSum + chargedSum) – raw pT

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Di-jet imbalance : centrality dependence

150 μb-1 ~ 20 times more data than in 2010!!! Able to perform same measurements differentially in pT

pp data at the same √s available, more statistics welcome!

https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIN11013

PLB 712 (2011) 176

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Energy loss apparent at all jet pT !

Reference itself has an increasing trend

No significant dependence on jet pT

pT-dependence of the dijet imbalance

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Photon+Jet in PbPb

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Isolated photon+jet momentum imbalance

Momentum ratio shifts/decreases with centralityRed/Shaded boxes try to indicate possible, anti-correlated systematic variations

https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIN11010

Distribution consistent between pp and PYTHIA (Z2) + HYDJET

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Width, mean and fraction

● Angular width σ (∆φJγ) is consistent, both PbPb to pp and PbPb

to PYTHIA tune Z2 + HYDJET

● Significant deviation of <XJγ> PbPb compared to PYTHIA

tune Z2 + HYDJET, significance of PbPb vs. pp is weaker

● The centrality dependence is mostly visible in RJγ (jet p

T shifting

below the 30 GeV/c threshold)

https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIN11010

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Inclusive jet spectra : 2.76 TeV

|η|<2

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Nuclear Modification factor for inclusive jets (RAA

)

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Comparison of inclusive jets (RAA

)

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RAA

Vs. No. of participating nucleons

More suppression of jets in central collisions

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RAA

: cross-checks

Smearing method

Systematics for Bayesian only. Different jet pT ranges averagedspectra and then take R

AA

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pp PbPb

ConclusionsDi-jet:●Enhanced imbalance exists at all p

T.

●The fraction of the energy that a jet loses does not dramaticallychange with jet pT.

Isolated-photon + jet●First measurement of isolated prompt photon-jet in heavy-ion collisions

●Direct observation of jet energy loss vs. initial parton energy●Shift of associated jet towards lower jet p

T with centrality

Inclusive jets:●Observation of suppression of high p

T jets in central PbPb collisions

in comparison to peripheral collisions.●Consistent observation with jet quenching and high p

T charged

particle RAA

More details of the analysis can be found at:

Di-jet : AN-11-478 & HIN-11-013Isolated photon+jet : AN-11-435 & HIN-11-010Jet Raa : AN-11-393 & HIN-12-004

https://twiki.cern.ch/twiki/bin/view/CMS/HighPtFirstYear2011

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High pT working group

BARC : P. K. Netrakanti, D. Dutta, D. K. Mishra and A.K. Mohanty

CERN : Y. J. Lee

LLR : M. Nguyen

MIT : Y. S. Lai, Y. Kim, F. Ma, Y. Yilmaz, D. Valicanu, I. A. Cali C. Roland, G. Roland, G. S. F. Stephans

UM : Y. Lu and M. Tjones

RU : J. Robels and S. Salur

VU : P. Kurt, Y. Mao and J. Velkovska

Current topics pursued in the group are: 1. Jet shape analysis 2. Jet track correlations 3. Photon-jet track correlations 4. b+jets 5. Fragmentation function analysis

One plenary and 6 parallel talks in Quark Matter – 2012 (Aug).

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Back up

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Isolated photon selection

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Photon isolation in data