Hard probes (mainly jets) at LHC
Yaxian MAO
Central China Normal University

Tsukuba Global Science Week
Tsukuba, 2015
Yaxian MAO
Central China Normal University

TGSW2015, September 30th, 2015 Tsukuba

The QCD phase transition and HI collision• QCD calculations (on the lattice) indicate that the phasetransition occurs at a critical energy density
We can thus create a system of deconfined quarks and gluons➡ by heating (T)➡ by compression (matter density)

2The QCD phase transition and HI collision• QCD calculations (on the lattice) indicate that the phasetransition occurs at a critical energy density
We can thus create a system of deconfined quarks and gluons➡ by heating (T)➡ by compression (matter density)

so#	  physics	  regimehard	  (high-­‐pT)	  probes2Two labs to create QGP
AGS : 1986 – 2000
• Si and Au beams ; √s ~ 5 GeV• only hadronic variables
RHIC : 2000 –• Au beams ; up to √s = 200 GeV• 4 experiments (only two remain)

SPS : 1986 – 2003
• O, S, In, Pb beams ; √s ~ 20 GeV• hadrons, photons and dileptons
LHC : 2010 –• Pb beams ; up to √s = 5500 GeV• ALICE, CMS and ATLAS

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Probing QGP
We study the QCD matter produced in HI collisions by looking how thewell understood probes are modified, as a function of temperature(centrality of the collisions)

Matter under study

Probe

QGP$?$
Calibrated“probe meter”

Calibrated“probe source”
Calibrated
heat source

4Centrality of the collisions
Controls the volume, shape and energy density of the system➡ Multiplicity and energy of produced particles are correlated withgeometry of collisions

nAbnB

Npart = nA + nB
Ncoll = nA ⊗ nB

Npart/2 ~ ASoft processes: long timescale, large σ, σtot ∝Npart
Hard processes: short timescale, large σ, σtot ∝Ncoll
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Hard scattering and jet production:QCDStudy the strong force using jet production
Final%State%Radia.on%%(FSR)%{π,K,p,nDetector%,…
}"HadIni.al%State%Radia.on%%(ISR)%

Jet%za3on"

Beam%
Remnants%p%="(uud)"Beam%
Remnants%

ronip%="(uud)"Jetz1 z2phadronz=p jet• Jets of particles produced in high energy parton-parton scattering• Jet fragmentation function (FF): hadron distribution as a function of z,(momentum fraction taken by hadron from the jet)

6Jet-quenching in QGP
Study the transport properties of the QGP
a color charge (g, q, Q) in a colored medium
Final%State%Radia.on%%(FSR)%{π,K,p,nDetector%,…
}

q"

Had

Ini.al%State%Radia.on%%(ISR)%

Jet%

roniza3on"="Pb(uud)"

Hadrons

Beam%
Remnants%

p%

Beam%
Remnants%qHigh-energy
parton(from hard scattering)p%="Pb(uud)"• Partons loose energy ΔE when traversing the medium• Jet(E) → Jet (E’ = E-ΔE) + soft particles(ΔE)• Jet quenching measures ‘stopping power' of QGP

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Jet quenching evidences at LHC• R : Nuclear modification factor
AA2σ ineldN AA /dp T dηpp
R AA =
 N coll  d 2 σ pp /dp T dη

1.8
PbPb

1.6

∫

1.4

Jet RAA

1.2

sNN = 2.76 TeV

L dt = 129 µ b
100 < p100 < pjet
Tjet
T-1

|η|<2

Bayesian
Uncorr statistical(GeV/c) < 110(GeV/c) < 300

Total systematics

10.8
0.6
0.4

CMS Preliminary

0.2

Anti-kT Particle Flow Jets

00

100

CMS PAS HIN-12-004

200

R = 0.3300

PLB 712 (2012) 176

400Npart• About 50% of jets (R ~ 0.5) are lost at a given p in most central PbPb
• Dijet p ratio is imbalanced in most central PbPb collisions
AATT➡ Jets (light flavour) are quenched in central PbPb collisions, how aboutb-jets?

8b-jet suppression

PRL 113 (2014) 132301• Evidence of b-jet suppression in central PbPb collisions• b-jet RAA favours strong jet-medium coupling from pQCD model9Anatomy of jets• Understand how jets interact with the QGP medium

Vacuum(pp reference)

Jets in Medium(jet broadening)by studying the energy flow inside the jet➡ question to address: is the jet energy in PbPb
redistributed radially?

r = √(ηjet-ηch)2+(φjet-φch)2

ρ (r) =1 1 1
fch δr N jetpT (r − δ r / 2, r + δ r / 2)∑,jetjets
pT10

Anatomy of jets

• Understand how jets interact with the QGP medium

Vacuum(pp reference)

Jets in Medium(jet broadening)by studying the energy flow inside the jet➡ question to address: is the jet energy in PbPbredistributed radially?

• Understand how jets interact with theQGP medium by studying the particle
longitudinal momenta➡ question to address: is the jet energyin PbPb redistributed in momentum
space?

r = √(ηjet-ηchρ (r) =

1 1 1fch δr N jet)2+(φjet-φch)2ξ = ln(1/z) = ln(pjet/p||)low ξ

pT (r − δ r / 2, r + δ r / 2)∑,jetjets
pThigh ξ10Jet shape analysis
PYTHIA 100 GeV inclusive jet
Anti-kT R=0.3 jet
Charged particle energy fraction~95% of energy
in r < 0.2

PLB 730 (2014) 243~95% of energy
with pT > 4GeVparticles• Core of the jet (dominated by high pT particles) → no changes• Intermediate r (intermediate pT particles) → depletion/narrower• Large radii (low pT particles) →excess/broadening➡Jet energy is redistributed inside jet cone11

Inclusive jet FF vs. pT and ξ

PRC 90 (2014) 024908

Ejet = 100 GeV
10050

10432

1pT (GeV/c)

ξ = ln(1/z) = ln(pjet/p||track)• high pT particles → no changes• intermediate pT particles → depletion• low pT particles →excess➡Jet fragmentation is modified

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Controlled Experiment: p + Pb ?
pPb collisions

PbPb collisions

Jet

Jet

QGPJet• Clear signs of Quark-Gluon Plasma(QGP)• Strongly interacting particles
affected by the presence of QGP• quenched jets and high pT• modified jet structure

Jet

QGP? CGC?

• Can we understand the baseline forPbPb?
• How do strongly interacting
particles behave in cold nuclear
matter? quenching?

• Can we see nuclear structure?
13PbNN

1.4

1.4

1.4

1.2

1.2

1.2-2.0 < η

CM

0.4

1.8

10.8

0.80.6RpPb

1.61100300

p [GeV/c]

400

T

1.6-1.5 < η

CM

0.4

2005001.8

1001200300

p [GeV/c]

400

500
1.8

1.2

1.2

0.4

RpPb

1.2

RpPb

1.4

1CM

1000.6< 0.50.5 < ηCM0.4200300

p [GeV/c]
T

400

500

100200< -0.5300

p [GeV/c]

400

500

T

10.8

0.8

-0.5 < ηCM

1.6

1.40.6-1.0 < η

0.4

1.40.8Luminosity uncertainties

0.6< -1.0T

1.6

1CMS-HIN-14-001

0.80.6< -1.5-1

1.8

1.8
Anti-kT Particle Flow Jets: R = 0.3
1.6
1.6

total uncertainties

RpPb

RpPb

1.8RpPb

pJet quenching in p + Pb?
CMS Preliminary
pPb s = 5.02 TeV∫L dt = 35 nb

100200< 1.0300

p [GeV/c]T• No strong jet p dependence observed• Consistent with EPS09 description

0.6

1.0 < ηCM

0.4
400

500

100200< 1.5300

p [GeV/c]400500TT14Charged particles RpApPb

EPJC 75 (2015) 237• No suppression observed for charged hadron production in pPb collisions• High pT charged particles (50 < pT < 100) RpPb >1 using interpolated pp
reference• EPS09 calculation is under predicted the data → possible baryon/meson
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difference?

Dijet η asymmetry
EPJC 74 (2014) 2951

pPb

ηdijet =η1+η22• Agreement between data and EPS09
calculation with systematics

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Dijet η asymmetry
François Arleo and Jean-Philippe Guillet http://lapth.cnrs.fr/npdfgenerator/

EPJC 74 (2014) 2951

pPb

ηdijet =η1+η22x = pparton/pbeam• Agreement between data and EPS09
calculation with systematics

16

Dijet η asymmetry
François Arleo and Jean-Philippe Guillet http://lapth.cnrs.fr/npdfgenerator/

EPJC 74 (2014) 2951

pPb

ηdijet =η1+η2
2

x = pparton/pbeam

EMC

anti-shadowing• Agreement between data and EPS09
calculation with systematics

shadowing16

Probing nPDF with jets and hadrons
x - fractional momentum from a colliding nucleon carried by the parton“backward”

pPb“forward”

JHEP 0904 (2009) 065

large x from PbnPDF
R=PDFsmall x from Pbx• Different p and η region can probe different x-range
T17

Probing nPDF with jets and hadrons
x - fractional momentum from a colliding nucleon carried by the parton“backward”

pPb“forward”

JHEP 0904 (2009) 065

large x from Pb

nPDF
R=PDF

small x from Pb

xPYTHIA Z2

s= 5.02 TeV: Generator Level

<xPb>

110−1

-2.0 < η < -1.5
-1.5 < ηCM < -1.0
-1.0 < ηCM < -0.5-0.5 < ηCM < 0.50.5 < η CM< 1.01.0 < ηCM < 1.5CM10−250100200300 400

jetp [GeV/c]T• Different p and η region can probe different x-range
T17

Probing nPDF with jets and hadrons
x - fractional momentum from a colliding nucleon carried by the parton“backward”

pPb“forward”

JHEP 0904 (2009) 065

large x from Pb

nPDF
R=PDF

small x from Pb

xPYTHIA Z2

s= 5.02 TeV: Generator Level

<xPb>

1

10−1-2.0 < η < -1.5
-1.5 < ηCM < -1.0-1.0 < ηCM < -0.5-0.5 < ηCM < 0.50.5 < η CM< 1.01.0 < ηCM < 1.5CM

10−250100200300 400

jetp [GeV/c]
T• Different p and η region can probe different x-range
T17

Charged particle and jet production asymmetrypPb

PARTICLE YIELDLEAD GOING SIDECMS Preliminary, pPb sNN = 5.02 TeVYieldPb going/Yieldp going

1.5∫L dt = 35 nb

PARTICLE YIELDPROTON GOING SIDE

jets

-1

charged hadrons

Anti-kT Particle Flow Jets: R = 0.3

10.5-1.5<η <-1.0/1.0< η <1.5CMCM-1.0<η <-0.5/0.5< η <1.0050100CMCM200300 400

p [GeV/c]T• Charged hadrons: Y > 1 (p < 10 GeV/c); Y ~ 1 (p > 10 GeV/c)• Decreasing trend of Y for both charged hadron and jets at very high pasym

TasymTasymT18

dijet η and charged hadron asymmetryηdijet =η1+η2
2

EPJC 74 (2014) 2951

pPb

• “Peripheral” (low HF activity): dijets shifted top-going side, expect Yasym < 1• “Central” (high HF activity): dijets shifted to
Pb-going side, expect Yasym > 119

dijet η and charged hadron asymmetryηdijet =η1+η2
2

EPJC 74 (2014) 2951

pPb

• “Peripheral” (low HF activity): dijets shifted top-going side, expect Yasym < 1• “Central” (high HF activity): dijets shifted toPb-going side, expect Yasym > 119

Summary and outlook• AA: consistent picture about jet quenching in PbPb collisions from different
experiments• high p jets are strongly suppressed• heavy quark jets behave similarly as light quark jets
• Jet fragmentation patterns are modifiedT• pA: too complex to serve as reference but interesting to explore• no jet quenching observed• pQCD calculation including nPDF effects can described data in general
but not hadrons• But…still left with questions…
• can be addressed and checked by higher statistics LHC RunII and RunIII
data with more differential measurements➡ flash in the next few slides with the questions in my mind…
Yaxian MAO
Central China Normal University

TGSW2015, September 30th, 2015 Tsukuba

20How low pT Jet quenched?

Jet RAA

How are the low pT jets suppressed?

• High p R is in good agreement, however low p behavior isTAATdifferent➡ different jet cone size, can be revisited with RunII data under samecondition
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Flavour dependent energy loss?
Is the energy loss depending on the quark flavour as predicted?
partonhot and dense medium

ENERGY LOSSlightM.Djordjevic PRL 94 (2004)• R (b-jet) ≃ R (inclusive-jet) at high p , no strong flavour dependence• R (J/ψ←B)>R (D)≃R (π)
AAAAAAAATAA➡ More measurements down to low pT needed to conclude
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Longitudinal or transverse broadening?
Which direction of the jet broadening occurred?

r = √(ηjet-ηch)2+(φjet-φch)2• Di-hadron correlation study observed:• Broadening mainly in transverse direction and no increase inlongitudinal direction➡Need precise study with jets for Δη and Δφ broadening
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Color charge dependent jet FF and modifications?

PbPb/pp

FFHigh z fragments modified? Different partons have different modification?

ATLAS: PLB 739 (2014) 320
CMS: PRC 90 (2014) 024908
PRD 84 (2011) 014034• FF Rcp shows difference hints at high z between experiments➡ need precise measurements with coming LHC data

• Theory predicted jet fragmentation pattern modified differently fordifferent parton mass➡ can be checked at LHC with coming data24

Thank you for your attention!

Of course there
are much more…

Please stay tuned...
Yaxian MAO
Central China Normal University

TGSW2015, September 30th, 2015 Tsukuba

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backup

Yaxian MAO
Central China Normal University

Mini-workshop on jet physics, Wuhan April 14-15

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