Jet studies for PbPb and pPb collisions with CMS
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Jet studies for PbPb and pPb collisions with CMS. Dipanwita Dutta Nuclear Physics Division, BARC, Mumbai India-CMS Meet, SINP, Kolkata 20 th Dec, 2012. 1. Jets : Probe of QGP in HI. Indirect jet quenching @ RHIC. Jet Quenching. QGP.

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Jet studies for PbPb and pPb collisions with CMS

Dipanwita Dutta

Nuclear Physics Division, BARC, Mumbai

India-CMS Meet, SINP, Kolkata

20th Dec, 2012

1


Jets : Probe of QGP in HI

Indirect jet quenching @ RHIC

Jet Quenching

QGP

Jets suppressed at Au+Au but not in pp or d+Au

Direct jet quenching @ CMS, LHC

  • Confirm production of QGP @ LHC

  • Important probe to study QGP properties


What do we expect for the jet shapes?

r

Low- particles tend to be closerto the jet axis; high- particles extend to large distances (radii).

low 

=ln(pjet/p||track)

high 

Jet shape:

pT-flow vs. - distancefrom the jet axis (r)

Excess pT-flow is expected at largeradii, and no change at r0(compared to pp collisions).

3


Background estimation

Eta reflection

Event mixing

Dijet events are matched to randomly selected

minimum bias events with consistent centrality,

event-plane and position of the primary vertex.

Then, the particles from minbias events falling into the jet axis are counted to estimate the average contribution from underlying event.

pseudo-rapidity(η) of the background jet is

determined by reflecting η of the original jet

cone around η =0. phi (ϕ) of the background

jet is the same as of the two leading jets. Δη<0.3

region is not used in order to avoid overlay

between the leading jets and background jet.

4


Changing jet shapes vs. centrality

Pb

Pb

Pb

Pb

Compared to pp: same pT-flow close to the jet axis;

more pT-flow at large radii; and a bit less in between.

CMS PAS HIN-12-013

5


Jet studies in pPb collisions

Pb

Pb

p Pb

p

p

pPb collisions run at s = 5.02 TeV in Jan-Feb 2013

  • Have we created a medium in pPb collisions?

?

6


Ridge observed in pPb

pPb and pp ridge compared paper for “apples-apples” comparison

pPb ridge much larger than pp

Average over

ridge region

(2<|Δη|<4)

7


Several unanswered question for pPb

  • What is the physics origin of ridge:

    • Collective effect?

    • Modification of jet structure?

  • Is there any medium produced in pPb

    • Jet quenching?

ALICE result on ridge conclude:

  • Near side and away side same ridge

  • No pT dependence ( not from jet)

  • The ridge subtracted yield comparable for all multiplicities in pPb (5.02 TeV) and pp at 2.76 and 7 TeV, no change in jet structure observed - in contrast to RHIC result for dAu at 0.2 TeV

8


Jet energy imbalance in PbPb

Pb

Pb

pp

p

p

Pb

Pb

  • Parton energy loss is observed as a pronounced dijet energy imbalance in central PbPb collisions

PRC 84 (2011) 024906

9


Dijet asymmetry in pPb (pilot run)

  • Fake jets contribution with centrality

  • Centrality is determined by NTrack

Lead Jet pT>40 GeV/c,

SubLead pT>20 GeV/c

Lead Jet pT > 60 GeV/c,

SubLead pT >30 GeV/c

Ruchi Chudasama

10


Dijet asymmetry(contbn from fake jets)

Fake jets contribution to Aj from side bands <1/3

Ruchi Chudasama

11



Dijet asymmetry in pPb

Lead Jet pT>40 GeV/c,

SubLead pT>20 GeV/c

Lead Jet pT > 60 GeV/c,

SubLead pT >30 GeV/c

Hijing

Pythia

AMPT

Hydjet

Ruchi Chudasama

13


Future Plan

  • Study of fake jet contribution (fake pair from underlying events):

    • From side band in jet event (<1/3)

    • From mix event (jet from jet triggered event and paired with fake jets from min bias event)

    • Any other method

  • Efficiency correction and unfolding of jet distribution

Acknowledgement:

  • Heavy ion High pT group

  • A. K. Mohanty

14


Jets shape theoretical study for lhc

Jet broadening can be studied in the differential shape through tot(r/R)/vac(r/R)

Jets Shape : Theoretical study for LHC

QGP effects are manifested as a suppression near the core and enhancement near the periphery for central collisions


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