Exploring the QGP withJets at ALICE
Oliver Buschfor the ALICE collaboration

Oliver Busch – TGSW 2015 /09/30

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Outline• introduction• results from pp collisions• jets in heavy-ion collisions• results from Pb-Pb collisions• outlook: LHC run 2

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Introduction

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Jets: seeing quarks and gluons• jet: collimated bunch of hadrons• quasi-free parton scattering at high Q2:
the best available experimental equivalent to quarks and gluons
Oliver Busch – TGSW 2015 /09/30

4Jet fragmentation• initial hard scattering: high-pT partons• cascade of gluons: parton shower• at soft scale (o(ΛQCD)): hadronization

Oliver Busch – TGSW 2015 /09/30

5Jet reconstruction• Establish correspondence between
detector measurements /final state particles / partons• two types of jet finder:- iterative cone
- sequential recombination(e.g. anti-kT)• resolution parameter R

hep-ph/0802.1189

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LHC aerial view

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Jets at ALICE (LHC run 1)• charged particle tracking:- Inner Tracking System (ITS)- Time Projection Chamber- full azimuth, |η |< 0.9
pT > 150 MeV/c• EMCal :- neutral particles- Δφ = 107°, |η|<0.7
cluster ET > 300 MeV• jet trigger with EMCal and TRD• `charged’ (tracking) jets and `full’ jets
• full jets from charged particle tracking and EM energy:

conceptually different and complementary to traditional approach8

Results from pp collisions

Oliver Busch – TGSW 2015 /09/30

9Full jets in pp at= 2.76 TeV• good agreement to NLO calculations for R = 0.2 and R = 0.4• reference for Pb-Pb at same energy
R = 0.4

R = 0.2

Phys. Lett. B 722 (2013) 262
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pp charged jet cross-sections• measured in minimum bias collisions at= 7 TeV• good agreement with ATLAS charged jet measurements(despite slightly different acceptance and track pT range)
R = 0.4

R = 0.6

PRD 91, 112012

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pp jet fragmentation at•

distributionsof charged particles incharged jets

• for z > 0.2 distributions consistent
for all jet pT: ‘scaling’• bulk production at low z:~ 5-10 charged particles per jet

PRD 91, 112012= 7 TeVPID in jets : `TPC coherent fit’
• particle identification via specific ionization in TPC ('dE/dx'):• TPC coherent fit:use energy loss model parameterization as input,
adjust model
parameters and particle
fractions `on the fly’during fit• regularization requiringcontinuity ofparticle fractions• complementary andconsistent:
multi-template fit
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13Particle identified fragmentation• identified charged hadrons in charged jets at= 7 TeV• π, K, p, 5 < pTch jet < 20 GeV/c• scaling for zch > 0.2 for higher jet pT bins

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Particle ratios in jets
• strangeness content strongly enhanced for zch → 1• leading baryons suppressed

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Event generator comparison• comparison to PYTHIA(pT ordered parton shower,Lund string fragmentation)• data reasonably welldescribed• best reproducedby Perugia tune without
color reconnections

Jets and Quark-Gluon Plasma

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QCD phase transition• in heavy-ion collisions at ultra-relativistic energies,

a quasi macroscopic fireball of hot, strongly interacting matterin local thermal equilibrium is created• lattice QCD predicts phase transition to deconfined,chirally symmetric matter• energy densityfrom the lattice:
rapid increase around TC,indicating increase ofdegrees of freedom(pion gas ->quarks and gluons)• TC = 154 +/- 9 MeV

EC = 340 +/- 45 MeV/fm3

HotQCD, PRD 90, 094503

QCD matter at LHC• direct photons:

prompt photons from hard scattering+ thermal radiation from QCD matter• low-pT inverse slope parameter:

Teff = 297 +/- 12stat. +/- 42syst. MeV/c• indicates initial temperature way
above TC

arXiv 1509.07324 [nucl-ex]
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Partons in heavy-ion collisions• hard partons are produced early and traverse the hot and dense QGP• expect enhanced parton energy loss : `jet quenching’ (mostly) due tomedium-induced gluon radiation• ‘vacuum’ expectation calculable by pQCD : ‘calibrated probe of QGP’
• jets sensitive to properties ofthe medium (energydensity, , mean free path,coupling ... )• ... but also jet-medium

interaction not trivial(strong / weak coupling,parton mass / type,fireball dynamics ...)
JET collaboration, arXiv: 1312.5003

Hadrons in heavy-ion collisions
PLB 720 (2013) 250• high- pT hadrons `proxy’ for jet• jet quenching for charged hadrons,

Pb-Pb collisions at √sNN = 2.76 TeV• hadron observables biased towards
leading fragment→ study the effect for fully reconstructed jets

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Underlying event in heavy-ion collisions• jet reconstruction in heavy-ion collisions :difficult due to the high underlying event backgroundnot related to hard scattering• correct spectra for background fluctuations and detector effects
via unfolding• not possible down to lowest jet pT

centraljet area ~ 0.5 (R = 0.4)
peripheral

Jet nuclear modification factor• strong suppression observed, similar to hadron RAA-> parton energy not recovered inside jet cone
• increase of suppression
with centrality• weak pT
dependence• JEWEL andYaJEM
jet quenchingmodels reproduce
suppression

nucl-ex/1502.01689
JEWEL: PLB 735 (2014)
YaJEM:PRC 88 (2013) 014905

Reaction plane dependence• different medium thickness in- and out-of plane• sensitive to path length dependence of jet
quenching:pQCD radiative E-loss : ~L2collisional E-loss : ~Lstrong coupling (ADS/CFT) : ~L3

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Jet v2 : results• charged jets, R = 0.2• quantify azimuthal asymmetry via 2nd Fourier harmonic v2• non-zero v2 jet in semi-central collisionsarXiv:1509.07334 [nucl-ex]
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Jet Structure• different observables, e.g. radial moment g, pTD• comparison to PYTHIA pp reference shows collimationof jet core (R=0.2)

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Jet Structure : Model Comparison• trends reproduced by JEWEL jet quenching model

JEWEL: K.C. Zapp, F. Kraus, U.A. Wiedemann, JHEP 1303 (2013) 080
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Perspectives for LHC run 2

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LHC run 2• LHC run 2: 2015 - 2017, heavy-ion run November 2015

• increased CMS energy for Pb-Pb collisions from 2.76 → 5.1 TeV• quenching strength ~ s ~ ε3/4
• expect (modest) increase in ε, T→ measure energy density
dependence of jet quenching• note: also a dependence onparton ‘input spectrum’(increased RAA ??? )Oliver Busch – TGSW 2015 /09/30

ALICE, PRL 105, 252301

ALICE in run 2: DCal

• run 2: DCal upgrade- significantly extended jet acceptance- back-to-back in azimuth (di-jet topology)
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Summary• jet cross sections and properties in pp• identified jet fragmentation in pp• strong jet suppression observed in Pb-Pb collisions• non-zero jet v2 indicates path-length dependence of jet quenching• first look at jet shapes• looking forward to LHC run 2 !

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- Backup -Oliver Busch – Tsukuba 2014 /03/13

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Jet finder comparison""kT: sequential
recombination
SISCone: cone algorithm

nucl-ex/1411.4969
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Multi Template Fit• TPC multi-template fit- best possible description of dE/dx from external reference- parametrize dependences on η, TPC nClusters- templates in transverse momentum (z, xi) slices• dE/dx in one z slice (0.6 < z < 0.65), 10-15 GeV/c fitted with
4 templates

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Method comparison• uncorrected hadron fractions from Multi-Template Fit and
TPC Coherent Fit• 2 complementary methods
obtain consistent results

Jet structure• ‘jet structure ratio’ R=0.2 / R=0.3 for charged jets

• sensitive to potential broadening of jet shape• consistent with PYTHIA pp:
no modification observed
within small radii(jet core)

JHEP 03 (2014) 013