Jet Production In Heavy Ion Collisions

Jet Production In Heavy Ion Collisions

Yacine Mehtar-TaniINT, University of Washington

June 10, 2015AGS & RHIC Annual Users’ Meeting, BNL

2

• Jets in vacuum • Jets in medium: general picture • Understanding jet observables in HIC:

1. Nuclear modification factor 2. Missing pT in dijet events 3. Fragmentation Functions

• Summary and outlook

Outline

3

a Jet is an energetic and collimated bunch of particles produced in a high-energy collision

e!

e+

*

q

q_

e!

e+

*

2Q = s

LEP (OPAL)

events

1993

Jets in QCD

4

• Jet ~ parton produced in a hard process at high energy: large separation between short and large distance physics: pT ≫ Λ QCD (QCD-factorization)

• Experimentally jets are reconstructed by clustering particles, whose total energy is pt, within a given cone of size R. An approximate way of reversing the process of fragmentation and compare to theory

R pT

Jets in QCD

5

• Jets originate from energetic partons that successively branch (similarly to an accelerated electron that radiates photons)

• Elementary branching process is enhanced in the collinear region ⟹ Collimated jets

𝜃 ≪ 1 dP ⇠ ↵s CRd✓

✓

d!

!

!

E

branching prob.

Jets substructure in vacuum

hadronspartons

Q0 ⇠ ⇤QCD

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The jet is a coherent object, successive branchings are ordered from larger to smaller angles

Jet transverse mass

Q ⌘ E ✓jet

𝜃1 > 𝜃2 > … > 𝜃n

𝜃1

𝜃2

𝜃3

(coherence leads to destructive interferences between radiations at large angles)

Jets substructure in vacuum

0

1

2

3

4

5

6

7

8

9

0 1 2 3 4 5 6

dN/dl

l

OPAL data, Q = 90 GeVALEPH data, Q = 130 GeV • Perturbative QCD prediction for

the distribution of hadrons in a jet • 2 scales: non-perturbative scale

Q0 = ΛQCD and the jet transverse scale = E 𝜃jet

• Angular Ordering (AO) ⇒ soft gluon emissions (large l ~ small x) are suppressed

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[Dokshitzer, Khoze, Mueller, Troyan, Kuraev, Fong, Webber…80']

Fragmentation Function

l ⌘ ln1

x

x ⌘ Eh/Ejet

D(x) = x

dN

dx

Jets substructure in vacuum

8

What happens when jets are immersed in a very dense QCD medium, the quark-gluon

plasma (QGP)?

An emblematic measurement of Jet quenching

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Nuclear Modification Factor. Suppression of the inclusive hadron and jet yields ~ radiative energy loss

RAA =1

Ncoll

dNAA

dNpp< 1

Jets are excellent probes of the QGP (a lot less sensitive to hadronization physics than inclusive hadron production, quarkonia,etc)…

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• Experimental jet definition depends on experimental procedure: jet reconstruction algorithm, background subtraction, unfolding, etc. But to what extend?

• Theory of jets in the QGP: 2 types of interfering parton showers: vacuum and medium-induced. Multi-scale problem.

• Q: is relevant physics under control? Monte Carlo Event-Generators are available or under construction: MARTINI, JEWEL, Q-PYTHIA, PYQUEN/HYDJET, YAJEM, MATTER

Jets in HIC are more involved than in vacuum

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two main medium effects: • medium-induced radiation off the total jet charge at

large angles: onset of rapid branching & broadening (multiple-scatterings)

• coherent structure (AO) is weakened: decoherent vacuum radiation (quasi-collinear & long form times)

For collimated jets the medium only resolves the total color charge ❬〈 Cjet ❭〉 ≠ 0𝜃jet

General Picture

QGP

Medium-induced QCD cascade and angular broadening

12

𝜔 and 𝛳

E

[Baier, Dokshitzer, Mueller, Peigné, Schiff (1995-2000) Zakharov (1996)]

• Scatterings with the medium can induce gluon radiation

• The radiation mechanism is linked to transverse momentum broadening

� k2? ' q̂ �t

!, k?

E, p?

L

t

13

tbr

Coherent radiation over tbr

Medium-induced radiation

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What is the microscopic mechanism? After a certain number of scatterings coherence between the parent quark and gluon fluctuation is broken and the gluon is formed (decoherence is faster for softer gluons)

tf ⌘ !

hq2?i

' !

q̂ tf ➡

maximum frequency for this mechanism !c =12q̂ L2

tf = tbr ⌘r

!

q̂

corresponding to tbr ⇠ L

[Baier, Dokshitzer, Mueller, Peigné, Schiff (1995-2000) Zakharov (1996)] Guylassy, Levai, Vitev (2001) Arnold, Moore, Yaffe (2001)

Medium-induced radiation

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Spectrum = bremsstrahlung x effective number of scatterers

!dN

d!⇠ L

t⇤(!)

Characteristic time scale: the effective inelastic mean-free path,

t⇤(!) ⇠1

↵stbr(!)

• Multiple branching occur at low momenta and large angles (collinear safety):

✓2⇤(!) ⇠q̂ t⇤(!)

!2> ✓2s ⌘ 1

↵4s

1

q̂L3� ⇥2

jet

! ⌧ !s ⌘ ↵2 q̂ L2

Medium-induced radiation

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In-medium energy distribution obeys a rate equation (inelastic)

𝜔𝜔

𝜔’

@

@tDmed(!) ⌘ Gain + Loss ⇠ Dmed(!)

t⇤(!)

Gluon cascade and energy flow

0.001 0.01 0.1 1

x0.0001

0.001

0.01

0.1

1

10

100

1000

D(x

,t)

t=0.01 t=0.5 t=1 t=1 1.5

Energy FlowConstant flow of energy

x ⌘ !/p?

1/px

Energy loss: Baier, Mueller, Schiff, Son (2001), Jeon, Moore (2005) Energy recovery: Blaizot, Iancu, YMT, PRL 111 (2013)

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• Due to color transparency the in-cone jet structure is expected to be vacuum-like

• Possible alteration of the distribution in the soft sector due to decoherence of vacuum radiation

𝜃jet

QGP

Decoherence of vacuum radiation

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• Consider two subsequent splittings

dP2 ⇠ ↵sd✓

✓

d!

!⇥(✓1 � ✓)

• Color coherence yields the angular ordering constraint on the second branching

✓1

✓

Decoherence of vacuum radiation

Decoherence of vacuum radiation

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✓1

✓

dP2 ⇠ ↵sd✓

✓

d!

![⇥(✓1 � ✓) +�med ⇥(✓ � ✓1)]

• The interaction with the medium alters angular ordering

�med ! 1�med ! 0coherence: decoherence:

decoherence parameter:

�med ⌘ 1� e�q̂L r2?/12

• Consider two subsequent splittings

r? ⇠ ✓1L

[YMT, Salgado, Tywoniuk, PRL106 (2011) PLB707 (2012) ]

Color transparency: Well defined limit in pQCD 20

How to treat Interferences between Vacuum and in-medium shower?

A Multi-Scale Problem

Q ⌘ ✓jet E

Qmed ⌘ (q̂L)1/2

r�1? ⌘ (✓jetL)

�1

Q0 ⇠ ⇤QCD

Jet mass

pt-broadeding

Jet size

𝜃jet

QGP

𝜃jet

Transparency limit: Jets as coherent objets Q � r�1

? � Qmed � Q0

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The medium interacts mostly with the total charge (original parton)

x

dN

dx⌘ D

coh(x) ⌘Z

1

x

dz

z

D

vac

(x/z,Q)Dmed

(z, p?)

In-cone parton dist: Convolution of in-medium and vacuum evolution

x =E

parton

E

Q ⌘ p? ⇥jet

Sub-leading corrections due to partial decoherence of vacuum radiation ~ r2

Dtot ⌘ Dcoh +�Ddecoh

Understanding Jet Observables

(I) Nuclear modification factor (II) Missing pT in asymmetric dijet events (III) Medium-modified Fragmentation Function

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[YMT, Tywoniuk, PLB744 (2015) , YMT, Blaizot, Torres, PRL114(2015)]

( I ) Nuclear Modification Factor

L = 2 - 3 fm q̂ = 2.5 - 6 GeV2/fm

RAA = dNAA / ( dNpp x Ncoll )

Solving the evolution equation for D convolved with an initial power spectrum p�n

?

0

0.2

0.4

0.6

0.8

1

1.2

50 100 150 200 250 300 350 400

Rjet

AA

p⊥ [GeV]

ωc = 60-100 GeV, ωBH = 1.5 GeVωc = 80 GeV, ωBH = 0.5-2.5 GeV

CMS Prelim.

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dNAAjet

d2p?⌘ N

coll

Z1

0

dx

x

D

med

(x, p?/x)dNpp

jet

d2p?(p?/x)

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• Selection of dijet events with large energy Imbalance pT1>120 GeV and pT2>50 GeV

E flow

CMS: energy is lost in soft particles at large angles

pT1pT2

pT1

pT2

( II ) Missing pT in dijet events

0.2 0.4 0.6 0.8 1Θ

-40

-30

-20

-10

0

Dije

t ene

rgy

imba

lanc

e (G

eV)

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CMS DATAMomentum imbalance

Leading jet: L1= 1 fm, Subleading jet: L2= 5 fm,

IR-cutoff (200 - 800 MeV)

no-IR-cutoff

Cumulative Energy

with E = 120 GeV , q̂= 2 GeV2/fm, 𝛼s=0.3

( II ) Missing pT in dijet events

l = � lnx

• vacuum baseline (blue) • medium-induced energy loss

at large angles depletes energy inside the cone. responsible for dip in the ratio

• small angle soft radiation due to decoherence of vacuum radiation: possibly responsible for enhancement at large l = shift of humpbacked plateau!

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0

1

2

3

4

5

dN

/

dℓ

0.5

1

1.5

2

0 1 2 3 4 5

Ratio

ℓ

CohCoh + Decoh

VacuumCMS Prelim.: medium, 0-10%

CMS Prelim.: vacuum

CohCoh + Decoh

CMS Preliminary, 0-10%

( III ) Fragmentation Functions

NB: the different suppression of quark and gluon jets does not explain the soft enhancement [M. Spousta, B. Cole (2015)]

Dtot ⌘ Dcoh +�Ddecoh

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• Jets in HIC are composed of a coherent inner core and large angle decoherent gluon cascades that are characterized by a constant energy flow from large to low momenta down to the QCD scale where energy is dissipated. Geometrical separation: The cascade develop at parametrically large angles away from the jet axis.

• Excess of soft particles within the jet cone might be a signature of color decoherence.

• Comparison with data: consistent picture, but need a Complete Monte Carlo Event Generator (to deal with experimental biases) and a realistic treatment of the geometry of the collision, particle content, hadronization, etc, for quantitative studies.

Summary and outlook