High Q2 probes of the Quark-Gluon Plasma

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Peter JacobsLawrence Berkeley National Laboratory

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QCD: running of aS

Q Q

Confinement

Asymptotic Freedom

coneR

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Low momentum

High momentum

QCD jets in reality…

3

BeamRemnants

BeamRemnants

p=

(uud)

(uud)

p=

{p,K,p,n,…}

Jet

Initial State Radiation(ISR)

Hadronization

Final State Radiation(FSR)

Detector

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4

Modern jet reconstruction algorithms

• Cone algorithms– Mid Point Cone (merging + splitting)– SISCone (seedless, infra-red safe)

• Sequential recombination algorithms

• kT

• anti-kT

• Cambridge/ Aachen

Algorithms differ in recombination metric:different ordering of recombination

different event background sensitivities

Jet

Fragmentation

Hard scatter

Cone jetKT jet

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What everyone uses now: FastJet (M. Cacciari, G. Salam, G. Soyez JHEP 0804:005 (2008))

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Inclusive jet production in proton-proton collisions

Good agreement with pQCD @ (N)NLO over a broad kinematic range

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Phys.Lett. B722 (2013) 262

Phys.Rev. D86 (2012) 014022

ALICE

Now consider “matter”

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Phase structure of water Phase structure of QCD matter

heat

com

pres

s

compress he

at

Explored in detail

Limited exploration to date

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Hot QCD and high energy collisions of heavy nuclei

Model calculation

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Real LHC Pb+Pb collis

ion

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Hot QCD in the laboratory: colliders

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RHIC @ BNL LHC @ CERN

PHENIX STAR

ATLAS

ALICECMS

Au+Au √sNN=200 GeV Pb+Pb √sNN=2.76 TeV

Jets in heavy ion collisions

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Controlled “beams” with well-calibrated intensity

Final-state interactions with colored matter are calculable

“Jet quenching”: quasi-tomographic probe of the Quark-Gluon Plasma

Jet quenching in QCD

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collisional and radiative energy loss

= expectation value of Wilson loop of spatial extent L (incorporates effects of the medium)

L

Second moment:

Total medium-induced jet energy loss (multiple soft scattering):

Probability distribution of

Jet quenching involves processes over wide range of Q2

Complex interplay of perturbative and non-perturbative dynamics

Calculable on the lattice…?

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High Q2 processes in nuclear collisions (A+A):

no nuclear effects Glauber scaling of inclusive cross sections

Quantifying nuclear effects for (inclusive) hard processes:

Jet quenching via high pT hadrons

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Jet fragments (color-charged)

Photons (color-neutral)

Jet quenching

Hadron suppression: RHIC vs LHC

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RHIC p0, h, direct g RHIC/LHC charged hadrons

RHIC/LHC: Qualitatively similar, quantitatively different• interplay between energy loss (~matter density) and spectrum

shape

Measurement of : data + modeling

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JET Collaboration, arXiv:1312.5003

For a 10 GeV light quark at time 0.6 fm/c:

Fit pQCD-based models to single-hadron suppression data at RHIC and LHC

Compared to 5 years ago: significant improvement in precision due to LHC data

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So why bother with fully reconstructed jets?

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High pT hadron suppression is a disappearance measurement: We mainly observe the energetic remnants of those jets that have not interacted

We want a complete dynamical understanding of jet quenching: • Jet energy is not “lost”: where does it go?

Jets and jet quenching are partonic in nature• Hadrons are an annoyance that may screen the essential physics

Jet quenching at the partonic level fully reconstructed jets

STAR

Single hadrons Jets

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Jets in real heavy ion collisionsRHIC/Star

LHC/CMS

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Visual identification of energetic jets above background is fairly easy

Much harder: accurate measurement of jet energy within finite cone• Pb+Pb at LHC: over 100 GeV of uncorrelated background energy in cone R=0.4• Uncorrelated background has complex structure, including multiple overlapping

jets at multiple energy scales• Very challenging…but not intractable

Different experiments take very different approaches to

heavy ion jets

• Experimental sensitivities

• Treatment of backgrounds

• Jet biases

Overall: work in progress

Diversity of approaches is crucial

Dijet Asymmetry AJ

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CMS version:• |hJet|<2

• Lead jet pT,1 > 120 GeV/c

• Subleading jet pT,2 > 50 GeV

• Df1,2 >2p/3

Quantify dijet energy imbalance by asymmetry ratio:

Ratio reduces uncertainty in Jet Energy Scale

LHC Pb+Pb: Dijet energy imbalance

March 3,2014 QCD jets in QCD matter 18

Enhanced energy asymmetry in central Pb+Pb collisions

Qualitative indication of jet quenching

Hot QCD Matter 19

Compare PbPb to pp data

Anti-kT jets with R = 0.2, 0.3, 0.4

If PbPb = superposition of pp

CMS PAS HIN-12-004

University of Tennessee April 8, 2013

Inclusive Jet RAA: Pb+Pb @ LHC

Central (“head-on”) collisions: factor ~2 yield suppressionFull jet energy not recovered: still a disappearance measurement !?

Radiation scattered to large angle? softened below experimental sensitivity?

x2

Inclusive jets: Au+Au at RHIC

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pTthresh=5 GeV pT

thresh=7 GeV

Lower jet energies, larger S/B than LHC: more difficult measurementBackground suppression: require hard leading hadron in jet

J. Rusnak, Hard Probes 2013

Trigger bias

First systematically well-controlled heavy ion jet spectrum at RHICData on tape: pT

jet > 50 GeV/c

Jet suppression: RHIC vs LHC

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Markedly larger jet suppression observed at LHC than at RHICDifferent jet quenching dynamics? Larger “out of cone” radiation at LHC?

RHIC: central Au+Au LHC: central Pb+Pb

Hadron vs jet suppression at RHIC

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Jets are markedly less suppressed than hadrons at RHIC• Contrast LHC, where jet and hadron suppression are

similarLess out-of-cone radiation at RHIC?

Instructive to compare and contrast similar jet measurements at RHIC and LHC• Data-driven guidance on the nature of jet quenching• Strong constraints on theory/modelling

Hadrons Jets

Infrared+collinear safe jets in heavy ion collisions: hadron+jet correlations

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Semi-inclusive yield of jets recoiling from a high pT hadron trigger

Measured Calculable in fixed-order pQCD

Recoil jet spectrum in p+p collisions

Consider two different trigger pT intervals

Higher pTtrigger

biases towards higher Q2 processesharder recoil jet spectrum

Recoil jet

Recoil jets

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IRC-safe jet quenching: h+jet

Bkgd-corrected jet pT:r: event-wise bkgd density estimateAi = jet area (Fastjet)

Central Pb+Pb (ALICE data)

8<pTtrig<9 GeV/c

20<pTtrig<50 GeV/c

nois

e je

ts

Trigger-correlated jets

But: no jet-by-jet rejection; retain complete jet population (signal +bkgd) S/B discrimination done entirely at the event-ensemble level

p+p

IRC-safe jets: intra-jet broadening due to quenching?

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Ratio of differential recoil yields R=0.2/R=0.5

Compare ratios for central Pb+Pb and p+p

No evidence of intra-jet broadening due to quenching within jet radius R~0.5

Large-angle scattering off the QGP

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d’Eramo et al, arXiv:1211.1922

Look at the rate of large-angle deflections(DIS-like scattering off the QGP)

• what are the quasi-particles?Weak coupling: pQCD calculationStrong coupling: AdS/CFT calculation

Strong coupling: Gaussian distribution

Weak coupling: hard tail

Discrete scattering centers or effectively continuous medium?

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h+jet @ LHC: medium-induced acoplanarity?

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Df

Compare to p+p

No evidence of medium-induced acoplanarity

d’Eramo et al, arXiv:1211.1922

“DIS off the QGP”: look at rate of large-angle scattering…?

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CMS : photon-jet angular correlation

pp

Azimuthal angle difference between photon and jet

PbPb

pppp

“QGP Rutherfold experiment”

Jet

Jet

Photon

Photon“Backscattering?”

PLB 718 (2013) 773

Anti-kT jet R = 0.3

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Compare CMS g+jet/ALICE h+jet

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Many differences: trigger, jet kinematics, jet selection bias, parton flavor bias,…

But still: distributions are similar• Difference in tails….?

h+jet @ RHIC (STAR)

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Au+Au peripheral Au+Au central

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Similar analysis as ALICE@LHC: • background correction done entirely at event-ensemble level

Systematically well-controlled measurement of small jet signal in large combinatorial background

h+jet recoil yield suppression: RHIC vs LHC

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R=0.5

STAR central Au+Au ALICE central Pb+Pb

Are these consistent?

Convert vertical suppression into horizontal shift: energy loss out of jet cone

RHIC: DE ~ 5 GeV

LHC: DE ~ 7 GeVDirect, IRC-safe measurement of partonic energy loss

40<pTcorr<60 GeV

h+jet azimuthal distributions: RHIC vs LHC

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Df Au+Au 0-10%

Au+Au 0-10%Au+Au 60-80% Pb+Pb 2.76 TeV 0-10%

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RHIC/LHC: No evidence yet of large-angle scattering • (N)NLO pQCD calculations underway for baseline expectation

LHC Run 2: significant improvement in statistical precision

Outlook

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Jet quenching is a deep problem in QCD• Field-theoretical basis, several theory/modeling approaches• Experiment: challenging measurements, promising new

ideas

Techniques in place for precise measurements of IRC-safe jets (not an approximation) in nuclear collisions at both RHIC and LHC, over the full kinematic range available

Needed for statistical precision: • LHC Run 2 • future RHIC running + upgrades

We are approaching the long-standing goal of applying jets as precisely controlled tomographic probes of hot QCD matter

Extra slides

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Finite Temperature QCD on the lattice (mB=0)

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42

30TgDOF

4T

Cross-over, not sharp phase transition (like ionization of atomic plasma)

Slow convergence to non-interacting Steffan-Boltzmann limitWhat carries energy - complex bound states of q+g? “strongly-coupled” plasma?

Energy density

Temperature [MeV]

S. B

orsa

nyi e

t al.,

JH

EP

1011

, 077

(20

10)

Inclusive jet spectrum: isolation of hard jet component

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G. De Barros et al., arXiv:1208.1518

unbiased

biased

Require leading hadron of each jet candidate to be above pT threshold• Imposition of momentum scale discriminating hard from non-hard jets• Infrared-safe: large fraction of jet energy can still be carried by very soft

radiation (down to ~200 MeV)• Collinear-unsafe: minimize pT cut and vary it to assess its effect

Inclusive jets at RHIC: compare Au+Au and p+p

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p+p spectrum with leading hadron bias biased Au+Au/unbiased p+p

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

Ratio of heavy ion jet yield to p+p jet cross section

Bias persists to ~few times hadron pT threshold

Bias in Au+Au not markedly different than in p+p

Vacuum-like jets?

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Background density estimate

Jet candidate pT corrected event-wise for median background density:

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~half the jet population has pT

<corr> < 0

• Not interpretable as physical jets• But we do not reject this component explicitly by

a cut in pT<corr>:

• Contains crucial information about background or “combinatorial” jets

• Rejected at later step by imposition of a specific (transparent) bias on candidates

For each event:• Run jet finder, collect all jet candidates• Tabulate jet energy pT,i

jet and area Ai jet

• Event-wide median energy density:

STAR Preliminary

STAR Preliminary

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True and measured jet spectra

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STAR Preliminary

ATLAS/CMS algorithm:• reject jet candidates based on

pT<corr>

• Correct for missing yield by simulation

STAR/ALICE: • keep entire pT

<corr> distribution• Reject background based on other

observables

simulation

Analysis steps:1. Isolate the real hard jet component and suppress combinatorial component2. “Unfold” the effects of energy smearing on the hard jet component

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Vacuum reference: NLO vs MC shower

MC shower and NLO differ

Compare ALICE p+p@7 TeV (not shown): MC shower strongly favored

Important lesson for jet quenching via pQCD@NLO

NLO: D. de Florian arXiv:0904.4402

p+p √s=2.76 TeV