Overall momentum balance and redistribution of
lost energy in asymmetric dijet events in Pb-Pb
collisions with AMPT model
Hanzhong Zhang
Institute of Particle Physics, Central China Normal University,
Wuhan, China
Collaborated with Zhan Gao, Guoliang Ma and Guang-You Qin
arXiv: 1612.02548
Huzhou Workshop, Dec. 17, 2016
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Outline
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Introduction – Jet quenching and full jets
Four stages in AMPT model
Jet quenching in overall momentum balance
Redistribution of lost energy in dijet events
Summary
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Evidence for the formation of Quark Matter
Jet quenching:
X.-N. Wang and M. Gyulassy, Phys.Rev.Lett.68,1480(1992)
Originates from the energy loss experienced by the hard
partonic jets initially produced from early scatterings
as they traverse and interact with the hot medium.
leading
particle
hadrons
Leading
particle
suppressed
hadrons
q
q
q
hadrons
q
hadrons
leading particle
N-N collision
leading
particle
suppressed
A-A collision
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Confirmed by single, di-/gamma-hadron at RHIC/LHC
single hadron @RHIC
H.Z. Zhang et al, PRL 98(2007)212301
H.Z. Zhang et al, PRL 103(2009)032302
X.F. Chen, H.Z. Zhang et al, PRC 84(2011)034902
H.Z. Zhang et al, JPG38(2011)124115
Within HT energy loss formalism,
simultaneously fit to data.
1) JQ is consistent for single di-h. …
2) An evidence for JQ description
Di-/gamma-hadron @RHIC
single hadron @LHC
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Extracting Jet quenching parameter
 Five different approaches
 Study of data on R_AA
 Extracting q̂
Jet Collaboration, PRC 90,014909(2014)
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Simultaneously extract q̂0 and 0
Z. Liu，H. Z. Zhang, B. W. Zhang
and E. Wang, Eur. Phys. J. C76 (2016) 20
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Fully reconstructed jets modified by Jet quenching
 Include both leading and subleading fragments of the parton showers
 Expect to provide more detailed information than hadrons on interactions between jets and the medium.
Internal structures of the full jets
N.-B. Chang, G.-Y. Qin, PRC94 (2016) , 024902
CMS, Phys. Lett. B730 (2014) 243-263
Single inclusive full jet suppression
X.-N. Wang, S.-Y Wei, H. Zhang,
arXiv: 1611.07211
ATLAS, PRL114 (2015) 072302
Dijet, hadron-jet correlations
p_T asymmetry distributions
See S.-Y. Wei’s talk
See Lin Chen’s talk
G.-L. Ma, PRC87 (2013) , 064901
CMS, PRC84 (2011) , 024906
L. Chen et al, arXiv: 1607.01932
1612.04202
STAR, NPA956(2016)641-644
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Question: how lost energy evolves with medium
and where it goes
 Jets not only lose energy due to jet-medium interaction,
but also induce medium excitations
 Deposit energy loss into the medium
 Modify the subsequent medium evolution
 Manifest in the final-state hadron distributions and correlations
 Analyze projected p_T by all hadrons in asymmetric dijet events
 Check JQ in the overall p_T balance
 Study the redistribution of the lost energy
 A recent study on the balance Y. Tachibana and T. Hirano, PRC90 (2014) , 021902
 3+1d hydrodynamic model
 Simplified energy deposition profile
 Main difference : we simultaneously simulate jet propagation and
medium evolution
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Four stages in a multi-phase transport (AMPT) model
Z.-W. Lin, C. M. Ko, B.-A. Li, B. Zhang, and S. Pal, PRC72 (2005) , 064901
(1) Initial condition
HIJING model: minijet partons and soft string excitations.
Jet triggering technique for dijets, qq-qq, qg-qg, gg-gg, etc.
(2) Zhang parton cascade (ZPC)
Describes elastic partonic collisions among medium partons
and jet partons. Strength is controlled by partonic cross section
(3) Hadronization
Coalescence : two nearest quarks into a meson and three
nearest quarks into a baryon
(4) Hadronic rescatterings
Simulated via a relativistic transport (ART) model
Fastjet package is used to reconstruct the full jets with the anti-kt
algorithm from the AMPT output for hadrons
M. Cacciari, G. P. Salam, and G. Soyez, Eur. Phys. J. C72, 1896 (2012)
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Transverse momentum asymmetry in dijet events
The asymmetry variable
AJ 
pT ,1  pT , 2
pT ,1  pT , 2
Numerical simulations fit data
well, with   1.5mb in ZPC
In 50-100% PbPb,
 No JQ !
 Peak at AJ  0 symmetry!
 Related with dominative LO
In 0-10% PbPb,
 With JQ
 Peak at AJ  0.3 asymmetry!
 Leading jets: surface emission bias
 Away-side sub-leading jets:
longer passage, more energy loss
Asymmetry is generated by JQ !
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Overall transverse momentum balance in dijets events
p_T by all hadrons projected in a special
direction: leading jet.
Event average
+ - value: sub- and leading direction
Qualitatively describe data for
 different pT bin,
 different AJ bin,
 different centrality bin.
In pp and PbPb,
 “-”
by pT  8.0GeV imbalance !
 “+”
by pT  8.0GeV imbalance !
 total “-” is balanced by total “+”
due to momentum conservation
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Imbalance is enlarged by Jet quenching
 For example, “-” imbalance
for pT  8.0GeV
 The “-” contribution increases
from peripheral to central PbPb
 “-” imbalance = NET contribution
by hadrons with pT  8.0GeV in both
leading and subleading jet directions
 Relatively, the subleading jets
tend to hadronize into less hard
fragments due to stronger jetmedium interaction and energy loss
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Lost energy is carried by soft hadrons
 In contrast, “+” contribution
for pT  0.5 - 2.0GeV
 “+” contribution by soft hadrons
gradually increases from peripheral
to central PbPb
 Soft “+” domination in the most
central PbPb collisions
 A large portion of the lost energy
from the jets is carried by the final
state soft hadrons
 Only elastic processes in AMPT,
the transportation of the lost energy
from hard jets into the soft hadrons
are mainly caused by elastic
scatterings
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How and when energy loss happens ?
Check “+” contributions by pT  8.0GeV
for 4 different evolution stages in AMPT
(i) initial state by HIJING
(ii) parton cascade
(iii) hadronization
(iv) hadron rescattering
Panel (a) : in pp, no change, with parton
interaction   1.5mb
Panel (b): contrast to pp, in AA
contributions
to
after ZPC
dramatically change due to jet quenching.
Panel (b): in AA, after parton evolution,
almost no change in following 2 stages.
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No parton interaction, no energy loss
 Once turn off parton interaction:   0
the result from central PbPb collisions is
very similar to pp collisions
 The dijet asymmetry is mainly due to jetmedium interactions and jet energy loss in
the partonic phase
 The lost energy from the jets is carried
by the partons which are fragmented into
final state soft hadrons
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To check where lost energy goes
In the  - space, define almost back-to-back di-cone with jet axis
out-of-cone
Subleading
in-cone
The cone size:
in-cone
Leading jet axis
R  ( -  J ) 2  ( - J ) 2
The  - space is divided into two regions: in-cone, out-of-cone
The di-jet is inside the di-cone with the same axis if R is big enough
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Large angle scattering for Jet quenching
R  0.5
in-cone
out-of-cone
R  1.2
in-cone
out-of-cone
 In-cone by large p_T hadrons from hard fragments of leading and subleading jets
 Out-of-cone by soft hadrons in central PbPb even if large cone size is chosen
 Lost energy carried by soft hadrons is redistributed at large angles away
from dijet axis by elastic collisions.
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Overestimation asks for radiative energy loss ?
 For the case R  0.8 ,
compared to CMS data
in-cone
out-of-cone
AMPT simulation somehow
overestimates individual positive
and negative contributions
 Radiative energy loss is
dominated by collinear radiation
while elastic energy loss is with
large scattering angles
For similar amount of jet energy loss and dijet asymmetry,
we have included more contributions from elastic collisions in the simulation
than the case when radiative processes are included
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Summary
 A detailed analysis is performed for
within AMPT model, to study the
overall pT balance and the redistribution of the lost energy from hard jets for
asymmetric dijet events.
 We find that
in the leading jet direction is mainly contributed by hard
hadrons, while the opposite direction in central collisions is dominated by soft
hadrons, which obeys momentum conservation.
 A large amount of the lost energy from hard jets occurred in the partonic
stage is transported to soft partons (hadrons) at large angles by elastic
collisions in AMPT model.
 Future studies including also inelastic processes should be helpful in
understanding the overestimation of in-cone and out-of-cone imbalances, and
shed light on different roles played by radiative and collisional processes in the
redistribution of the lost energy from hard jets.
Thank for your attention!
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