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High Q 2 probes of the Quark-Gluon Plasma. Peter Jacobs Lawrence Berkeley National Laboratory. QCD: running of a S. Asymptotic Freedom. Confinement. High momentum. Low momentum. QCD jets in reality…. Final State Radiation (FSR). Detector. Initial State Radiation (ISR).

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high q 2 probes of the quark gluon plasma
High Q2 probes of the Quark-Gluon Plasma

Peter Jacobs

Lawrence Berkeley National Laboratory

ICNFP 2014

qcd running of a s
QCD: running of aS

Asymptotic Freedom


High momentum

Low momentum

ICNFP 2014

qcd jets in reality
QCD jets in reality…

Final State Radiation



Initial State Radiation















ICNFP 2014

modern jet reconstruction algorithms
Modern jet reconstruction algorithms

KT jet

Cone jet

  • 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



Hard scatter

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

ICNFP 2014

inclusive jet production in proton proton collisions
Inclusive jet production in proton-proton collisions

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

Phys.Rev. D86 (2012) 014022

Phys.Lett. B722 (2013) 262


ICNFP 2014

now consider matter
Now consider “matter”

Phase structure of water

Phase structure of QCD matter


Limited exploration to date

Explored in detail




ICNFP 2014

h ot qcd and high energy collisions of heavy nuclei
Hot QCD and high energy collisions of heavy nuclei

Model calculation

Real LHC Pb+Pb collision

ICNFP 2014

hot qcd in the laboratory colliders
Hot QCD in the laboratory: colliders

Au+Au √sNN=200 GeV

Pb+Pb √sNN=2.76 TeV








ICNFP 2014

jets in heavy ion collisions
Jets in heavy ion collisions
  • Controlled “beams” with well-calibrated intensity
  • Final-state interactions with colored matter are calculable
  • “Jet quenching”: quasi-tomographic probe of the Quark-Gluon Plasma

ICNFP 2014

jet quenching in qcd
Jet quenching in QCD

collisional and radiative energy loss

Jet quenching involves processes over wide range of Q2

Complex interplay of perturbative and non-perturbative dynamics

Calculable on the lattice…?


Probability distribution of

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

Second moment:

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

ICNFP 2014

High Q2processes in nuclear collisions (A+A):
    • no nuclear effects Glauber scaling of inclusive cross sections

Quantifying nuclear effects for (inclusive) hard processes:

ICNFP 2014

jet quenching via high p t hadrons
Jet quenching via high pT hadrons

Photons (color-neutral)

Jet quenching

Jet fragments (color-charged)

ICNFP 2014

hadron suppression rhic vs lhc
Hadron suppression: RHIC vs LHC

RHIC/LHC charged hadrons

RHIC p0, h, direct g

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

ICNFP 2014

measurement of data modeling
Measurement of : data + modeling

JET Collaboration, arXiv:1312.5003

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

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

ICNFP 2014

so why bother with fully reconstructed jets
So why bother with fully reconstructed jets?

Single hadrons



  • High pThadron 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

ICNFP 2014

jets in real heavy ion collisions
Jets in real heavy ion collisions



  • 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
  • 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

ICNFP 2014

dijet asymmetry a j
Dijet Asymmetry AJ
  • 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

ICNFP 2014

lhc pb pb dijet energy imbalance
LHC Pb+Pb: Dijet energy imbalance

Enhanced energy asymmetry in central Pb+Pb collisions

Qualitative indication of jet quenching

QCD jets in QCD matter

inclusive jet r aa pb pb @ lhc
Inclusive Jet RAA: Pb+Pb @ LHC

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

  • Central (“head-on”) collisions: factor ~2 yield suppression
  • Full jet energy not recovered: still a disappearance measurement !?
    • Radiation scattered to large angle? softened below experimental sensitivity?

Hot QCD Matter

i nclusive jets au au at rhic
Inclusive jets: Au+Au at RHIC

J. Rusnak, Hard Probes 2013

Lower jet energies, larger S/B than LHC: more difficult measurement

Background suppression: require hard leading hadron in jet

pTthresh=7 GeV

pTthresh=5 GeV

Trigger bias

First systematically well-controlled heavy ion jet spectrum at RHIC

Data on tape: pTjet > 50 GeV/c

ICNFP 2014

j et suppression rhic vs lhc
Jet suppression: RHIC vs LHC

RHIC: central Au+Au

LHC: central Pb+Pb

Markedly larger jet suppression observed at LHC than at RHIC

Different jet quenching dynamics? Larger “out of cone” radiation at LHC?

ICNFP 2014

hadron vs jet suppression at rhic
Hadron vs jet suppression at RHIC



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

ICNFP 2014

infrared collinear safe jets in heavy i on c ollisions hadron jet correlations
Infrared+collinear safe jets in heavy ion collisions: hadron+jet correlations

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


Calculable in fixed-order pQCD

Recoil jet

Recoil jet spectrum in p+pcollisions

  • Consider two different trigger pT intervals
  • Higher pTtrigger
    • biases towards higher Q2 processes
    • harder recoil jet spectrum

ICNFP 2014

irc safe jet quenching h jet
IRC-safe jet quenching: h+jet

Central Pb+Pb (ALICE data)

noise jets




Recoil jets

Trigger-correlated jets

Bkgd-corrected jet pT:

r: event-wise bkgd density estimate

Ai = jet area (Fastjet)

But: no jet-by-jet rejection; retain complete jet population (signal +bkgd)

S/B discrimination done entirely at the event-ensemble level

ICNFP 2014

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

ICNFP 2014

large angle scattering off the qgp
Large-angle scattering off the QGP

d’Eramo et al, arXiv:1211.1922

Discrete scattering centers or effectively continuous medium?

  • Look at the rate of large-angle deflections
  • (DIS-like scattering off the QGP)
    • what are the quasi-particles?
  • Weak coupling: pQCD calculation
  • Strong coupling: AdS/CFT calculation

Strong coupling:

Gaussian distribution

  • Weak coupling:
    • hard tail

ICNFP 2014

h jet @ lhc medium induced acoplanarity
h+jet @ LHC: medium-induced acoplanarity?


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

Compare to p+p

No evidence of medium-induced acoplanarity

ICNFP 2014

d’Eramo et al, arXiv:1211.1922

cms photon jet angular correlation
CMS : photon-jet angular correlation

“QGP Rutherfold experiment”

Anti-kT jet R = 0.3










Azimuthal angle difference between photon and jet

PLB 718 (2013) 773

ICNFP 2014

compare cms g jet alice h jet
Compare CMS g+jet/ALICE h+jet
  • Many differences: trigger, jet kinematics, jet selection bias, parton flavor bias,…
  • But still: distributions are similar
  • Difference in tails….?

ICNFP 2014

h jet @ rhic star
[email protected] RHIC (STAR)

Au+Au peripheral

Au+Au central

  • Similar analysis as [email protected]:
    • background correction done entirely at event-ensemble level
  • Systematically well-controlled measurement of small jet signal in large combinatorial background

ICNFP 2014

h jet recoil yield suppression rhic vs lhc
h+jet recoil yield suppression: RHIC vs LHC

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 GeV


Direct, IRC-safe measurement of partonic energy loss

ICNFP 2014

h jet azimuthal distributions rhic vs lhc
h+jet azimuthal distributions: RHIC vs LHC


Au+Au 0-10%

Au+Au 60-80%

Au+Au 0-10%

Pb+Pb 2.76 TeV 0-10%


  • 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

ICNFP 2014

  • 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

ICNFP 2014

extra slides
Extra slides

ICNFP 2014

finite temperature qcd on the lattice m b 0
Finite Temperature QCD on the lattice (mB=0)

Slow convergence to non-interacting Steffan-Boltzmann limit

What carries energy - complex bound states of q+g? “strongly-coupled” plasma?

Energy density

S. Borsanyi et al., JHEP 1011, 077 (2010)

Temperature [MeV]

Cross-over, not sharp phase transition

(like ionization of atomic plasma)

ICNFP 2014

inclusive jet spectrum isolation of hard jet component
Inclusive jet spectrum: isolation of hard jet component

G. De Barros et al., arXiv:1208.1518

  • 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



ICNFP 2014

inclusive jets at rhic compare au au and p p
Inclusive jets at RHIC: compare Au+Au and p+p

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

p+p spectrum with leading hadron bias

biased Au+Au/unbiased p+p

Bias persists to ~few times hadron pT threshold

  • Bias in Au+Au not markedly different than in p+p
    • Vacuum-like jets?

ICNFP 2014

b ackground density estimate
Background density estimate
  • For each event:
  • Run jet finder, collect all jet candidates
  • Tabulate jet energy pT,ijetand area Ai jet
  • Event-wide median energy density:

STAR Preliminary

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

  • ~half the jet population has pT < 0
  • Not interpretable as physical jets
  • But we do not reject this component explicitly by a cut in pT:
    • Contains crucial information about background or “combinatorial” jets
    • Rejected at later step by imposition of a specific (transparent) bias on candidates

STAR Preliminary

ICNFP 2014

true and measured jet spectra
True and measured jet spectra
  • ATLAS/CMS algorithm:
  • reject jet candidates based on pT
  • Correct for missing yield by simulation


STAR Preliminary

  • keep entire pT distribution
  • Reject background based on other observables

Analysis steps:

Isolate the real hard jet component and suppress combinatorial component

“Unfold” the effects of energy smearing on the hard jet component

ICNFP 2014

vacuum reference nlo vs mc shower
Vacuum reference: NLO vs MC shower

NLO: D. de Florian arXiv:0904.4402

MC shower and NLO differ

Compare ALICE [email protected] TeV (not shown): MC shower strongly favored

Important lesson for jet quenching via [email protected]

p+p √s=2.76 TeV

ICNFP 2014