Jet Experiments at LHC

Gunther Roland

JET Collaboration Symposium Montreal June 2015

(as opposed to “Jet Physics at RHIC”)

Gunther Roland JET SymposiumJet Experiments at LHC 2

Heavy-ion jet results at LHCDijet asymmetries Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at sqrt(s) = 2.76 TeV with the ATLAS Detector at the LHC (ATLAS; 1011.6182) Observation and studies of jet quenching in PbPb collisions at sqrt(s) =2.76TeV (CMS; 1102.1957) Jet momentum dependence of jet quenching in PbPb collisions at sNN =2.76TeV (CMS; 1202.5022v2) Photon-jet asymmetries Studies of jet quenching using isolated- photon + jet correlations in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV (CMS; 1205.0206) Single-jet suppression Measurement of the jet radius and transverse momentum dependence of inclusive jet suppression in lead-lead collisions at sNN = 2.76 TeV with the ATLAS detector (ATLAS; 1208.1967v2) Measurement of charged jet suppression in Pb-Pb collisions at sNN = 2.76 TeV (ALICE; 1311.0633) Measurement of jet suppression in central Pb–Pb collisions at √sNN= 2.76 TeV (ALICE; 1502.01689v1.pdf) Jet structure Measurement of jet fragmentation into charged particles in pp and PbPb collisions at sqrt(s[NN]) = 2.76 TeV (CMS; 1205.5872) Modification of jet shapes in PbPb collisions at sqrt(s) = 2.76TeV (CMS; 1310.0878) Measurement of inclusive jet charged-particle fragmentation functions in Pb+Pb collisions at√sNN =2.76TeV with the ATLAS detector (ATLAS; 1406.2979v2) Measurement of jet fragmentation in PbPb and pp collisions at√sNN =2.76TeV (CMS;1406.0932v2) B-tagged jets Evidence of b-jet quenching in PbPb collisions at √sNN =2.76TeV (CMS;1312.4918) Jet-hadron correlations Measurement of jet quenching with semi-inclusive hadron-jet distributions in central Pb–Pb collisions at √sNN = 2.76 TeV (ALICE; 1506.03984v1) Jets in pPb Studies of dijet transverse momentum balance and pseudorapidity distributions in pPb collisions at sNN =5.02TeV (CMS; 1401.4433v2.pdf) Measurements of the Nuclear Modification Factor for Jets in Pb+Pb Collisions at √sNN = 2.76 TeV with the ATLAS Detector (ATLAS; 1411.2357v2) Centrality and rapidity dependence of inclusive jet production in √sNN = 5.02 TeV proton–lead collisions with the ATLAS detector (ATLAS; 1412.4092v1) Measurement of charged jet production cross sections and nuclear modification in p–Pb collisions at sNN = 5.02 TeV (ALICE; 1503.00681v1) Measurement of dijet kT in p–Pb collisions at √sNN = 5.02 TeV (ALICE; 1503.03050v1)

Some key results on jets in LHC HI collisions

Some comments on what to expect from Run II

Some comments on experiment/theory interactions

Jet Experiments at LHC

Gunther Roland JET SymposiumJet Experiments at LHC 4

Jets as tools to characterize QGP

Medium effects on jets allow extraction of QGP transport coefficients:

• q: transverse momentum diffusion (radiative energy loss)

• e: longitudinal drag (collisional energy loss)^

^

Jets as tools to understand QGP

How does the strongly coupled liquid emerge from QCD? • Jets probe QGP at different (controllable) length scales • Scattering sensitive to quasi-particle nature of the medium

“pQCD plasma”

“AdS/CFT goo”

Jets as tools to manipulate QGP

How does QGP respond to local energy deposition by jets?

Gunther Roland JET SymposiumJet Experiments at LHC 5

LHC jet experiments (in heavy ion collisions)

Achieved good experimental control • Reconstruction efficiency (close to 100%@50+ GeV) • Jet energy scale (2-4% above 30 GeV) • Resolution and UE fluctuations (~15%@100GeV)

Jet measurement approach: • Clustering with anti-kT algorithm • Subtraction of event-by-event background energy

estimate • Jet energy corrections using pp calibration and MC

correction factors

This was not obvious a-priori; success enabled by nature of observed jet modifications}

Complementary approaches to reconstruction biases • At low pT: efficiency vs fake jets • Restrict measurement to high pT (CMS/ATLAS) • Ensemble-based measurement with combinatorial

correction (ALICE/STAR)

For sufficiently high trigger pT, all away-side jets are found

Gunther Roland JET SymposiumJet Experiments at LHC 6

+ RHIC

Jets in LHC Run-2 (and beyond)

PbPb √s = 5TeV

Collision rate ≈ 20kHz Lint ≈ 3/nb

√s = 5.5TeV Collision rate ≈ 50kHz Lint ≈ 10/nb (run 2+3)

LHC schedule

Compared to LHC Run1: x60 due to higher luminosity; x3 due to higher √s

HI

HI

HI

HI HI HI

Gunther Roland JET SymposiumJet Experiments at LHC 7

“RA

A”

“x+J

et”

JetsHadrons

b Jets

RHIC today LHC today RHIC tomorrow LHC tomorrow

Dijets (pT,1)

γ+jets (pTγ)

Z0+jets (pTZ)

Double b-tag (pT,1)

Kinematic reach: Now and tomorrow(*)(*)Artist’s impression

B mesonsD mesons

Gunther Roland JET SymposiumJet Experiments at LHC 8

Kinematic reach: Now and tomorrow(*)“R

AA”

“x+J

et”

D mesons

Hadrons

B mesonsb Jets

RHIC today LHC today RHIC tomorrow LHC tomorrow

Dijets (pT,1)

γ+jets (pTγ)

Z0+jets (pTZ)

Double b-tag (pT,1)

New observables; smaller uncertainties; broader pT reach RHIC+LHC overlap

Jets

(*)Artist’s impression

Ensemble-based measurements and x+hadron correlations

add low pT reach

Gunther Roland JET SymposiumJet Experiments at LHC 9

The importance of photon-jet correlations

40 < pTγ < 50GeV 50 < pTγ < 60GeV 60 < pTγ < 80GeV pTγ > 80GeV

Using isolated photons to tag away-side jets • determines initial parton energy to ≈ 15% • determines initial direction of the parton • tags parton to be a light quark

photon pT <50GeV: Spectrum of PbPb jets is

suppressed vs pp Biased jet selection

photon pT >80GeV: Spectrum of PbPb jets is

shifted vs pp Unbiased jet selection

jet pT > 30 GeV

Sufficiently high γ pT or sufficiently low jet pT yield unbiased selection of jets

Gunther Roland JET SymposiumJet Experiments at LHC 10

q determined with about 35% uncertainty Combined RHIC and LHC data: • Test model consistency • First hint of temperature dependence Quantitative extraction of e awaits more precise heavy flavor data

^

^

Similar information from jet RAA, dijet asymmetries, γ-jet

momentum shift

Extracting QGP transport coefficients

Gunther Roland JET SymposiumJet Experiments at LHC 11

RAA now and in Run-2

Essential to not let systematic, quantitative exp/theo comparison go stale • Add new Run-1 data • Predict/prepare for Run-2 data

(GeV/c)T

Charged Particle p20 40 60 80 100 120 140 160

AA R

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1| < 0.8ηALICE, || < 1.0ηATLAS, |

| < 1.0ηCMS, |

| < 0.8ηALICE, || < 1.0ηATLAS, |

| < 1.0ηCMS, |PbPb 0 - 5%, 2.76 TeVpp, 2.76 TeV

300

LHC Run-2

200 250

?

?ATLAS; 1504.04337

Gunther Roland JET SymposiumJet Experiments at LHC 12

~ Agreement betweenCMS and ATLASat high jet pT

Agree with charged particle RAA at high pT

It would be nice to have low pT CMS data / ATLAS RAA with R=0.2 / ALICE high pT data

RAA for charged hadrons and jets

ATLAS; 1504.04337

Gunther Roland JET SymposiumJet Experiments at LHC 13

Collisional energy loss

Radiative energy loss

The B’s are coming…

This is what google thinks of dead cones…

Gunther Roland JET SymposiumJet Experiments at LHC 14

Flavor Dependence of Jet Quenching

π

D

b-jet

bàJ/ψ

Pb+Pb

Flavor/mass dependence: Yes, no, maybe?

Qualitatively not too dissimilar wrt. expectations Quantitatively there are many subtleties

Gunther Roland JET SymposiumJet Experiments at LHC 15

b jets D mesons

▪ Non-negligible for both jets and hadrons! ▪ Even more important for charm than for bottom at LHC energy!

Gluon Splitting Contribution

~40-50% ~20%

▪ HF studies: matched partons are not necessary heavy quarks!Non-Prompt J/ψ

~30-40%

Estimated from PYTHIA 6

Not all heavy quarks are created equalfrom Yen-Jie Lee

RHIC users meeting

Gunther Roland JET SymposiumJet Experiments at LHC 16 25

2.0<|η|<2.5

0<|η|<0.5

Flavor/mass dependence: Yes, no, maybe?

Forward spectra are steeper (RAA smaller) But it’s 85% quark jets!

It might be good to look at a global analysis of all data relevant to q/g flavor dependence and mass dependence to verify some of

the basic beliefs

from Yen-Jie Lee RHIC users

meeting

Gunther Roland JET SymposiumJet Experiments at LHC 17

Energy flow and medium response

Do we have a medium, if there’s no medium response?

Experimental and theoretical challenges: • Strength and angular structure of medium response

unknown • Jets are correlated with the complex e-by-e flow fields

through quenching • How to distinguish medium-response from modified jet

branching?

Neufeld, Vitev PRC 2012Ma, Wang PRL 2011

Gunther Roland JET SymposiumJet Experiments at LHC 18

Jet-track correlations in CMSEnhancement of low-pT correlated yields at large angles to jet axis

Is this “trivial” jet-medium coupling or signature of modified QCD branching (“turbulent flow”)?AJ > 0.22

PbPb –pp

PbPb 0-30%

CMS dijet “missing pT”

Jet-track correlations for leading/subleading jet Common interpretation with MPT analysis is highly non-trivial Need jet MC to translate one to the other

Gunther Roland JET SymposiumJet Experiments at LHC 19

Jet measurements as QGP microscope

Gunther Roland JET SymposiumJet Experiments at LHC 20

Jet measurements as QGP microscope

“pQCD plasma”

“AdS/CFT goo”

Kurkela, Wiedemann, arXiv:1407.0293

Angular and momentum structure of intra-jet parton cascadeLarge angle inter-jet scattering

D’Eramo, Rajagopal, ‘12 updated calculation underway

Differential (event-by-event) measurements may allow glimpse at “QCD Rutherford scattering” off QGP constituents

Need ≈ x10 better precision

CMS γ+jet data

STAR jet-hadron correlations (1506.03984v1)

Gunther Roland JET SymposiumJet Experiments at LHC 21

We have observed medium modifications of jet momentum and angular structure

Broadening/excess at large r, low pT

(~2% of jet energy)

Narrowing/depletion at intermediate r, pT

Little change at small r, high pT

Radius r

Jet shapes Fragmentation functions RAA radius dependence

ΔRecoil radius dependence

Gunther Roland JET SymposiumJet Experiments at LHC 22

Qualitative consistent results between CMS and ATLAS

But: Experiment-comparison suggests that we do NOT YET have sufficient control over systematics necessary for our decadal programHigh pT particles Low pT particles

CMS: PRC 90 (2014) 024908ATLAS: PLB 739 (2014) 320-342

Z = p||Trk / pJet

But there is a long road ahead…

Gunther Roland JET SymposiumJet Experiments at LHC 23

Jet structure in pp and PbPbLiliana Apolinario

Lisbon HI jet workshop

Q-Pythia

Jewel

Jet structure variables sensitive to (in-medium) shower evolution

Critical effort in pp highest sensitivity searches (q/g and boosted object discrimination)

Stability vs pp pileup is major issue

Convergence of AA and pp needs

Jesse Thaler, Boost 2014

Dani Pablos Lisbon HI jet workshop

Gunther Roland JET SymposiumJet Experiments at LHC 24

• Rich (overwhelming?) harvest of jet results from LHC Run-1• Theory/experiment dialogue is essential

• JET collaboration qhat extraction from RAA is paradigmatic• Needs to be continuous process, not one and done (e.g. new RAA

measurements)• Complex questions will need global analyses (e.g. flavor dependence)• Full MC essential for interpretation of complex measurements (e.g. jet-medium

correlations) • Step forward with LHC Run-2 may prove as big as LHC Run-1• Will experiment or theory lead?

Summary