602 research outputs found
On the sensitivity of the dijet asymmetry to the physics of jet quenching
The appearance of monojets is among the most striking signature of jet
quenching in the context of ultrarelativistic heavy-ion collisions.
Experimentally, the disappearance of jets has been quantified by the ATLAS and
CMS collaborations in terms of the dijet asymmetry observable A_J. While the
experimental findings initially gave rise to claims that the measured A_J would
challenge the radiative energy loss paradigm, the results of a systematic
investigation of A_J in different models for the medium evolution and for the
shower-medium interaction presented here suggest that the observed properties
of A_J arise fairly generically and independent of specific model assumptions
for a large class of reasonable models. This would imply that rather than
posing a challenge to any particular model, the observable prompts the question
what model dynamics is not compatible with the data.Comment: 8 pages, 6 figures, added computations of jet R_AA and R=0.2 result
Background subtraction and jet quenching on jet reconstruction
In order to assess the ability of jet observables to constrain the
characteristics of the medium produced in heavy-ion collisions at the LHC, we
investigate the influence of background subtraction and jet quenching on jet
reconstruction, with focus on the dijet asymmetry as currently studied by ATLAS
and CMS. Using a toy model, we examine the influence of different background
subtraction methods on dijet momentum imbalance and azimuthal distributions. We
compare the usual jet-area based background subtraction technique and a variant
of the noise-pedestal subtraction method used by CMS. The purpose of this work
is to understand what are the differences between the two techniques, given the
same event configuration. We analyze the influence of the quenching effect
using the Q-PYTHIA Monte Carlo on the previous observables and to what extent
Q-PYTHIA is able to reproduce the CMS data for the average missing transverse
momentum that seems to indicate the presence of large angle emission of soft
particles.Comment: 4 pages, 3 figures, Proceedings for Hard Probes 201
Cold Nuclear Matter Effects on Dijet Productions in Relativistic Heavy-ion Reactions at LHC
We investigate the cold nuclear matter(CNM) effects on dijet productions in
high-energy nuclear collisions at LHC with the next-to-leading order
perturbative QCD. The nuclear modifications for dijet angular distributions,
dijet invariant mass spectra, dijet transverse momentum spectra and dijet
momentum imbalance due to CNM effects are calculated by incorporating EPS, EKS,
HKN and DS param-etrization sets of parton distributions in nucleus . It is
found that dijet angular distributions and dijet momentum imbalance are
insensitive to the initial-state CNM effects and thus provide optimal tools to
study the final-state hot QGP effects such as jet quenching. On the other hand,
the invariant mass spectra and the transverse momentum spectra of dijet are
generally enhanced in a wide region of the invariant mass or transverse
momentum due to CNM effects with a feature opposite to the expected suppression
because of the final-state parton energy loss effect in the QGP. The difference
of EPS, EKS, HKN and DS parametrization sets of nuclear parton distribution
functions is appreciable for dijet invariant mass spectra and transverse
momentum spectra at p+Pb collisions, and becomes more pronounced for those at
Pb+Pb reactions.Comment: 10 pages, 11 figure
Light and heavy flavor dijet production and dijet mass modification in heavy ion collisions
Back-to-back light and heavy flavor dijet measurements are promising
experimental channels to accurately study the physics of jet production and
propagation in a dense QCD medium. They can provide new insights into the path
length, color charge, and mass dependence of quark and gluon energy loss in the
quark-gluon plasma produced in reactions of ultra-relativistic nuclei. To this
end, we perform a comprehensive study of both light and heavy flavor dijet
production in heavy ion collisions. We propose the modification of dijet
invariant mass distributions in such reactions as a novel observable that shows
enhanced sensitivity to the QGP transport properties and heavy quark mass
effects on in-medium parton showers. This is achieved through the addition of
the jet quenching effects on the individual jets as opposed to their
subtraction. The latter drives the subtle effects on more conventional
observables, such as the dijet momentum imbalance shifts, which we also
calculate here. Results are presented in Pb+Pb collisions at =
5.02 TeV for comparison to data at the Large Hadron Collider and in Au+Au
collisions at = 200 GeV to guide the future sPHENIX program at
the Relativistic Heavy Ion Collider.Comment: 17 pages, 16 figures, 1 table. v2: subfigures and references added,
version published by PR
An analysis of the influence of background subtraction and quenching on jet observables in heavy-ion collisions
Subtraction of the large background in reconstruction is a key ingredient in
jet studies in high-energy heavy-ion collisions at RHIC and the LHC. Here we
address the question to which extent the most commonly used subtraction
techniques are able to eliminate the effects of the background on the most
commonly discussed observables at present: single inclusive jet distributions,
dijet asymmetry and azimuthal distributions. We consider two different
background subtraction methods, an area-based one implemented through the
FastJet pack- age and a pedestal subtraction method, that resemble the ones
used by the experimental collaborations at the LHC. We also analyze different
ways of defining the optimal parame- ters in the second method. We use a toy
model that easily allows variations of the background characteristics: average
background level and fluctuations and azimuthal structure, but cross- checks
are also done with a Monte Carlo simulator. Furthermore, we consider the
influence of quenching using Q-PYTHIA on the dijet observables with the
different background subtrac- tion methods and, additionally, we examine the
missing momentum of particles. The average background level and fluctuations
affect both single inclusive spectra and dijet asymmetries, although
differently for different subtraction setups. A large azimuthal modulation of
the background has a visible effect on the azimuthal dijet distributions.
Quenching, as imple- mented in Q-PYTHIA, substantially affects the dijet
asymmetry but little the azimuthal dijet distributions. Besides, the missing
momentum characteristics observed in the experiment are qualitatively
reproduced by Q-PYTHIA.Comment: 29 pages, 43 figures Accepted by JHE
A Hybrid Strong/Weak Coupling Approach to Jet Quenching
We propose and explore a new hybrid approach to jet quenching in a strongly
coupled medium. The basis of this phenomenological approach is to treat physics
processes at different energy scales differently. The high- processes
associated with the QCD evolution of the jet from production as a single hard
parton through its fragmentation, up to but not including hadronization, are
treated perturbatively. The interactions between the partons in the shower and
the deconfined matter within which they find themselves lead to energy loss.
The momentum scales associated with the medium (of the order of the
temperature) and with typical interactions between partons in the shower and
the medium are sufficiently soft that strongly coupled physics plays an
important role in energy loss. We model these interactions using qualitative
insights from holographic calculations of the energy loss of energetic light
quarks and gluons in a strongly coupled plasma, obtained via gauge/gravity
duality. We embed this hybrid model into a hydrodynamic description of the
spacetime evolution of the hot QCD matter produced in heavy ion collisions and
confront its predictions with jet data from the LHC. The holographic expression
for the energy loss of a light quark or gluon that we incorporate in our hybrid
model is parametrized by a stopping distance. We find very good agreement with
all the data as long as we choose a stopping distance that is comparable to but
somewhat longer than that in supersymmetric Yang-Mills theory. For
comparison, we also construct alternative models in which energy loss occurs as
it would if the plasma were weakly coupled. We close with suggestions of
observables that could provide more incisive evidence for, or against, the
importance of strongly coupled physics in jet quenching.Comment: 47 pages, 10 figures, typos corrected. Small mistake in the
implementation of the Gluaber Monte-Carlo for the collision geometry
corrected. Correction results in small changes to values of fitted parameters
and to plots. No changes to any discussion or conclusion
Evolution of the Mean Jet Shape and Dijet Asymmetry Distribution of an Ensemble of Holographic Jets in Strongly Coupled Plasma
Some of the most important probes of the quark-gluon plasma (QGP) produced in
heavy ion collisions come from the analysis of how the shape and energy of jets
are modified by passage through QGP. We model an ensemble of back-to-back
dijets to gain a qualitative understanding of how the shapes of the individual
jets and the asymmetry in the energy of the pairs of jets are modified by
passage through an expanding droplet of strongly coupled plasma, as modeled in
a holographic gauge theory. We do so by constructing an ensemble of strings in
the gravitational description of the gauge theory. We model QCD jets in vacuum
using strings whose endpoints move "downward" into the gravitational bulk
spacetime with some fixed small angle that represents the opening angle (ratio
of jet mass to jet energy) that the QCD jet would have in vacuum. Such strings
must be moving through the gravitational bulk at (close to) the speed of light;
they must be (close to) null. This condition does not specify the energy
distribution along the string, meaning that it does not specify the shape of
the jet being modeled. We study the dynamics of strings that are initially not
null and show that strings with a wide range of initial conditions rapidly
accelerate and become null and, as they do, develop a similar distribution of
their energy density. We use this distribution of the energy density along the
string, choose an ensemble of strings whose opening angles and energies are
distributed as in perturbative QCD, and show that we can then fix one model
parameter such that the mean jet shape in our ensemble matches that measured in
p-p collisions reasonably well. We send our strings through the plasma,
choosing the second model parameter to get a reasonable suppression in the
number of jets, and study how the mean jet shape and the dijet asymmetry are
modified, comparing both to data from LHC heavy ion collisions.Comment: References added; 34 pages, 11 figure
The angular structure of jet quenching within a hybrid strong/weak coupling model
Building upon the hybrid strong/weak coupling model for jet quenching, we
incorporate and study the effects of transverse momentum broadening and medium
response of the plasma to jets on a variety of observables. For inclusive jet
observables, we find little sensitivity to the strength of broadening. To
constrain those dynamics, we propose new observables constructed from ratios of
differential jet shapes, in which particles are binned in momentum, which are
sensitive to the in-medium broadening parameter. We also investigate the effect
of the back-reaction of the medium on the angular structure of jets as
reconstructed with different cone radii R. Finally we provide results for the
so called "missing-pt", finding a qualitative agreement between our model
calculations and data in many respects, although a quantitative agreement is
beyond our simplified treatment of the hadrons originating from the
hydrodynamic wake.Comment: 4 pages, 4 figures, proceedings for conference Hard Probes 201
Novel subjet observables for jet quenching in heavy-ion collisions
Using a novel observable that relies on the momentum difference of the two
most energetic subjets within a jet we study the internal
structure of high-energy jets simulated by several Monte Carlo event generators
that implement the partonic energy-loss in a dense partonic medium. Based on
inclusive jet and di-jet production we demonstrate that is an
effective tool to discriminate between different models of jet modifications
over a broad kinematic range. The new quantity, while preserving the colinear
and infrared safety of modern jet algorithms, it is experimentally attractive
because of its inherent resiliance against backgrounds of heavy-ion collisions.Comment: v1: 10 pages. v2: Includes (i) additional discussion about best
discriminant by calculating the RSD (ii) new section about hadronization
effects on the reconstructed subjets; version to be published in European
Physical Journal
Jet quenching pattern at LHC in PYQUEN model
The first LHC data on high transverse momentum hadron and dijet spectra in
PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed
in the frameworks of PYQUEN jet quenching model. The presented studies for the
nuclear modification factor of high-pT hadrons and the imbalance in dijet
transverse energy support the supposition that the intensive wide-angular
("out-of-cone") medium-induced partonic energy loss is seen in central PbPb
collisions at the LHC.Comment: 5 pages including 4 figures as EPS-files; prepared using LaTeX
package for publication in the European Physical Journal
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