220 research outputs found
Resolution Effects in the Hybrid Strong/Weak Coupling Model
Within the context of a hybrid strong/weak coupling model of jet quenching,
we study the consequences of the fact that the plasma produced in a heavy ion
collision cannot resolve the substructure of a collimated parton shower
propagating through it with arbitrarily fine spatial resolution. We introduce a
screening length parameter, , proportional to the inverse of the
local temperature in the plasma, estimating a range for the value of the
proportionality constant via comparing weakly coupled QCD calculations and
holographic calculations appropriate in strongly coupled plasma. We then modify
the hybrid model so that when a parton in a jet shower splits, its two
offspring are initially treated as unresolved, and are only treated as two
separate partons losing energy independently after they are separated by a
distance . This modification delays the quenching of partons with
intermediate energy, resulting in the survival of more hadrons in the final
state with in the several GeV range. We analyze the consequences of
different choices for the value of the resolution length, , and
demonstrate that introducing a nonzero results in modifications
to the jet shapes and jet fragmentations functions, as it makes it more
probable for particles carrying a small fraction of the jet energy at larger
angles from the jet axis to survive their passage through the quark-gluon
plasma. These effects are, however, small in magnitude, something that we
confirm via checking for effects on missing- observables.Comment: 32 pages, 7 figure
Jet formation and interference in a thin QCD medium
In heavy-ion collisions, an abundant production of high-energy QCD jets
allows to study how these multiparticle sprays are modified as they pass
through the quark-gluon plasma. In order to shed new light on this process, we
compute the inclusive two-gluon rate off a hard quark propagating through a
color deconfined medium at first order in medium opacity. We explicitly impose
an energy ordering of the two emitted gluons, such that the "hard" gluon can be
thought of as belonging to the jet substructure while the other is a "soft"
emission (which can be collinear or medium-induced). Our analysis focusses on
two specific limits that clarify the modification of the additional angle- and
formation time-ordering of splittings. In one limit, the formation time of the
"hard" gluon is short compared to the "soft" gluon formation time, leading to a
probabilistic formula for production of and subsequent radiation off a
quark-gluon antenna. In the other limit, the ordering of formation is reverted,
which automatically leads to the fact that the jet substructure is resolved by
the medium. We observe in this case a characteristic delay: the jet radiates as
one color current (quark) up to the formation of the "hard" gluon, at which
point we observe the onset of radiation of the new color current (gluon). Our
computation supports a picture in which the in-medium jet dynamics are
described as a collection of subsequent antennas which are resolved by the
medium according to their transverse extent.Comment: 33 page
Angular Structure of Jet Quenching Within a Hybrid Strong/Weak Coupling Model
Within the context of a hybrid strong/weak coupling model of jet quenching,
we study the modification of the angular distribution of the energy within jets
in heavy ion collisions, as partons within jet showers lose energy and get
kicked as they traverse the strongly coupled plasma produced in the collision.
To describe the dynamics transverse to the jet axis, we add the effects of
transverse momentum broadening into our hybrid construction, introducing a
parameter that governs its magnitude. We show that,
because of the quenching of the energy of partons within a jet, even when
the jets that survive with some specified energy in the final state
are narrower than jets with that energy in proton-proton collisions. For this
reason, many standard observables are rather insensitive to . We propose a
new differential jet shape ratio observable in which the effects of transverse
momentum broadening are apparent. We also analyze the response of the medium to
the passage of the jet through it, noting that the momentum lost by the jet
appears as the momentum of a wake in the medium. After freezeout this wake
becomes soft particles with a broad angular distribution but with net momentum
in the jet direction. We show that the particles coming from the response of
the medium to the momentum and energy deposited in it leads to a correlation
between the momentum of soft particles well separated from the jet in angle
with the direction of the jet momentum, and find qualitative but not
quantitative agreement with experimental data on observables designed to
extract such a correlation. By confronting the results that we obtain upon
introducing transverse momentum broadening and the response of the medium to
the jet with available jet data, we highlight the importance of these processes
for understanding the internal, soft, angular structure of high energy jets.Comment: 62 pages, 14 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
Predictions for Boson-Jet Observables and Fragmentation Function Ratios from a Hybrid Strong/Weak Coupling Model for Jet Quenching
We have previously introduced a hybrid strong/weak coupling model for jet
quenching in heavy ion collisions that describes the production and
fragmentation of jets at weak coupling, using PYTHIA, and describes the rate at
which each parton in the jet shower loses energy as it propagates through the
strongly coupled plasma, dE/dx, using an expression computed holographically at
strong coupling. The model has a single free parameter that we fit to a single
experimental measurement. We then confront our model with experimental data on
many other jet observables, focusing here on boson-jet observables, finding
that it provides a good description of present jet data. Next, we provide the
predictions of our hybrid model for many measurements to come, including those
for inclusive jet, dijet, photon-jet and Z-jet observables in heavy ion
collisions with energy ATeV coming soon at the LHC. As the
statistical uncertainties on near-future measurements of photon-jet observables
are expected to be much smaller than those in present data, with about an order
of magnitude more photon-jet events expected, predictions for these observables
are particularly important. We find that most of our pre- and post-dictions do
not depend sensitively on the form we choose for the rate of energy loss dE/dx
of the partons in the shower. This gives our predictions considerable
robustness. To better discriminate between possible forms for the rate of
energy loss, though, we must turn to intrajet observables. Here, we focus on
ratios of fragmentation functions. We close with a suggestion for a particular
ratio, between the fragmentation functions of inclusive and associated jets
with the same kinematics in the same collisions, which is particularly
sensitive to the x- and E-dependence of dE/dx, and hence may be used to learn
which mechanism of parton energy loss best describes the quenching of jets.Comment: 59 pages, 24 figures. v2: minor changes, typos corrected and
references adde
Resolving the spacetime structure of jets with medium
Away from the strictly soft and collinear limit of QCD radiation the choice
of evolution scale in a parton shower algorithm is ambiguous and several
options have been implemented in existing Monte Carlo event generators for
proton-proton collisions. However, the resulting space-time evolution could
result in subtle differences depending on the particular choice. In this work
we quantify measurable consequences of the choice of the evolution variable and
show how the implications of such a choice propagates into jet quenching
observables. We develop a parton shower algorithm for a general evolution
variable, that includes as special cases the virtuality, angle, transverse
momentum and formation time. We study the interplay between the shower history
for different evolution variables and the phase space affected by parton energy
loss. In particular, we implement effects of jet quenching in the dense limit
and highlight the role of color coherence effects. We compare the results of
the different ordering variables to existing Monte Carlo shower implementations
on the parton level by analyzing primary Lund planes. Finally, we study the
sensitivity of quenched jets to the choice of evolution variable by confronting
our results for a certain key observable, such as the jet mass.Comment: 5 pages, 4 figures, 10th International Conference on Hard and
Electromagnetic Probes of High-Energy Nuclear Collisions (Hard Probes 2020
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