Raghav K E Rutgers University, NJ 1

Recent Jet Results from the CMS experiment

Raghav Kunnawalkam Elayavalli (Rutgers University, NJ USA)

Nuclear Seminar

Monday, February 2nd, 2015

Raghav K E Rutgers University, NJ

Overview •  https://twiki.cern.ch/twiki/bin/view/CMSPublic/

PhysicsResultsHIN

•  Motivations and background –  QCD phase diagram

•  Experimental overview –  LHC , CMS HI beams –  Technicalities

•  Results –  Nuclear modification factors

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HI collisions

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Motivations

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•  Production and effects of the QGP (quark gluon plasma) in high temperature and low baryon density environment.

•  Initial (Cold) vs Final (Hot) nuclear matter effects.

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Phase Diagram

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•  µb = 0: Lattice QCD predicts analytic crossover

•  Large µb The transition between QGP and hadron gas is expected to be a first order phase transition

•  Critical point expected at the end of the first order line.

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What we measure

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Lead Ion beams at the LHC

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CERN Yellow Report CERN-2010-001, pp. 393-416

Detlef Kuchler, a physicist in CERN's Beams department, with

the container holding the purified sample of lead used to create heavy ions for the LHC http://www.symmetrymagazine.org/breaking/2010/11/05/the-skinny-on-the-lhcs-heavy-ions

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CMS detector

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η = 0

η = 2.4

Beam

η= -ln (tan θ/2)

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What are jets

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CMS is fine-tuned to look at jets using the calorimeters and trackers combined.

Collimated stream of particles produced by hadronization of quark or gluon from high impact collisions (evidence for quarks!) Example: quark/lepton/gluon jets

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Jets in CMS

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Calorimeter (CALO) Jets: Using Calorimeter energy deposits. Particle Flow (PF) Jets: Combines information from all sub detectors to make PF candidates, which are then clustered.

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Data Sets

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PRC 84 (2011) 024906

PbPb – 2.76 TeV pPb – 5.02 TeV

pp – 2.76 TeV

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Technicalities in Heavy Ions •  Event classifications

–  Centrality classes & glauber model •  Background subtraction

–  Iterative pile up –  Flow modulated Voronoi subtraction

•  PP reference “data” at 5.02 TeV •  Identifying B jets

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Event Classification

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Ann.Rev.Nucl.Part.Sci.57:205-243,2007

Heavy Ion collisions are split up in terms of the number of colliding nuclei.

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Centrality in Data

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JHEP 1108 (2011) 141 Shengquan Tuo – pA centrality workshop: Feb 14 2014

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Background subtraction

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1.  Background (bkg) energy per

tower calculated in strips of η 2. Jet finder run on subtracted (sub) towers 3. Background energy recalculated excluding jets 4. Rerun Jet algo on bkg-sub towers without jets -> get the final jets.

EPJC (2007) 117.

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pp Reference spectra @ 5.02 TeV

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Extrapolated from pp data at 7 TeV. 1.  Dependence of the

jet radius and the √s on the cross section

2.  The above effect was extracted from Pythia and compared with NLO (next to leading order) calculations

3.  Applied to generated spectra to derive the reference at 5.02 TeV

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Identifying b-jets

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b-jet tagging

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The Simple Secondary Vertex (SSV) tagger is more robust against a combinatorial background due to the secondary vertex requirement

template fit to the SV invariant mass distribution in pPb collisions for jets of 90 < pT < 110 GeV/c, where b-jets dominate after 2 GeV/c2

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Results - Nuclear modification factor

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<Ncoll> - No of participating nuclei per event σ - cross section <TAA> - Average value of the nuclear ‘thickness’ function RAA > 1 : Enhancement RAA = 1 : no medium effect RAA < 1 : Suppression/quenching

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RpA & RAA – tracks vs Jets

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RpA & RAA: inclusive vs b-jets

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[GeV/c]T

b-jet p0 100 200 300 400

Nucle

ar M

odific

ation

Fac

tor0

0.5

1

1.5

2

2.5

|<2η, (0-10%), |AA

b-jet R

<1.6CMη, -2.4<PYTHIA

pAb-jet R

pPb Luminosity Unc.

pPb Reference Unc.

CMS Preliminary -1bµ; PbPb L = 150 -1pPb L = 35 nb

[GeV/c]T

p0 20 40 60 80 100

Nucle

ar M

odific

ation

Fac

tor

0

0.5

1

1.5

2

2.5| < 1

CMη |

pACharged particles R

| < 1η (0-5%) |AA

Charged particles R

-1bµ; PbPb L = 150 -1 CMS Preliminary pPb L = 35 nb

[GeV/c]T

p0 50 100 150 200 250 300 350 400

Nucle

ar M

odific

ation

Fac

tor

0

0.5

1

1.5

2

2.5| < 0.5

CMη (0-100%) |

pAInclusive jet R

| < 2η (0-5%) |AA

Inclusive jet R

= 2.76 TeVNNs = 5.02 TeV PbPb NNs pPb

No observable difference between inclusive and b-jets in the explored pT range

CMS-HIN-14-001

CMS-HIN-12-004

CMS-HIN-14-007

arXiv:1312.4198

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Missing pT

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Missing pT- II

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Sum charged particles for unbalanced AJ>0.22 dijets in central (0-30%) PbPb •  35 GeV/c of high pT tracks

missing from away side jet at ΔR=0.2

•  Balanced by low pT particles up to very large ΔR=2.0

•  PbPb-pp : result shows a different pT distribution

•  Take the pT cumulative of all tracks – total angular pattern is similar in PbPb and pp

CMS-HIN-14-010

Able to recover the lost energy by going to Large ΔR in the away side jet

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Where does the energy go?

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Jet shape

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PLB 730 (2014) 243

Broader jet shapes in PbPb in most central collisions

First experimental Jet Shapes in HI

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CMS fragmentation function

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z = ptrack|| / pjet High pT – no modification Suppression of

intermediate pT in the cone

Enhancement at low pT

Tracks inside a jet cone of R=0.3

Phys.Rev. C90 (2014) 024908

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Summary

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•  Many observables showing independent confirmation of modification of jets in the medium (final state interactions)

•  Jets are heavily quenched in most central PbPb collisions

•  Initial state in pPb collisions can be described by nPDF •  Inclusive jets are not quenched

•  Flavor dependence: So far no glaring differences between tagged and inclusive jets (in the explored pT range). Need results from fully reconstructed D, B mesons (in both PbPb and pPb)

•  Quenched energy recovered by going to higher radii.

•  Lost energy carried away by low pT particles away from the jet cone (Jet+Track measurements)

•  Jet Structure modification: •  Excess of low pT particles inside the jet cone (AJ measurements) •  Observe quenching of intermediate range pT particles (Jet

Fragmentation & jet Shape)

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Backup slides

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Evolution of HI collisions

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https://ww

w.bnl.gov/rhic/new

s/082807/story3.asp

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New HF/Voronoi algorithm

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A Voronoi diagram in the (eta, phi)- plane is used to associate an unique area to each particle such that the UE density can be removed particle-by-particle

Voronoi tessellated HYDJET/GEANT particle-flow event (combined tracks and calorimeter towers) before (left) and after (right) subtraction. Non physical negative particle/areas are “equalized” to maximally approximate to the original (real) jet distribution of radius R. (backup slides) Flow (v2,..,v5) accounted for by projecting the expectation from the HF

CMS-DP-2013-018