Jet Production In Heavy Ion Collisions
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Jet Production In Heavy Ion Collisions Yacine Mehtar-Tani INT, University of Washington June 10, 2015 AGS & RHIC Annual Users’ Meeting, BNL
Transcript of 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
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• 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
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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
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• 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
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• 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
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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
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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
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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
