112 research outputs found
Parton energy loss in QCD matter
QCD jets, produced copiously in heavy-ion collisions at LHC and also at RHIC,
serve as probes of the dynamics of the quark-gluon plasma (QGP). Jet
fragmentation in the medium is interesting in its own right and, in order to
extract pertinent information about the QGP, it has to be well understood. We
present a brief overview of the physics involved and argue that jet
substructure observables provide new opportunities for understanding the nature
of the modifications.Comment: 6 pages, 1 figure; plenary talk at the 8th International Conference
on Hard and Electromagnetic Probes of High-energy Nuclear Collisions (Hard
Probes 2016), Wuhan, China, September 23-27, 201
Dissipative Axial Inflation
We analyze in detail the background cosmological evolution of a scalar field
coupled to a massless abelian gauge field through an axial term
, such as in the case of an axion. Gauge
fields in this case are known to experience tachyonic growth and therefore can
backreact on the background as an effective dissipation into radiation energy
density , which which can lead to inflation without the need of a flat
potential. We analyze the system, for momenta smaller than the cutoff
, including numerically the backreaction. We consider the evolution
from a given static initial condition and explicitly show that, if
is smaller than the field excursion by about a factor of at least
, there is a friction effect which turns on before that the
field can fall down and which can then lead to a very long stage of inflation
with a generic potential. In addition we find superimposed oscillations, which
would get imprinted on any kind of perturbations, scalars and tensors. Such
oscillations have a period of 4-5 efolds and an amplitude which is typically
less than a few percent and decreases linearly with . We also stress
that the comoving curvature perturbation on uniform density should be sensitive
to slow-roll parameters related to rather than ,
although we postpone a calculation of the power spectrum and of non-gaussianity
to future work and we simply define and compute suitable slow roll parameters.
Finally we stress that this scenario may be realized in the axion case, if the
coupling to U(1) (photons) is much larger than the coupling
to non-abelian gauge fields (gluons), since the latter sets the range
of the potential and therefore the maximal allowed .Comment: 22 pages, 27 figure
Massless Mode and Positivity Violation in Hot QCD
We calculate the quark self-energy at one-loop level at high temperature,
taking into account contributions from both the (chromo)electric scale and
the (chromo)magnetic scale . While reproducing standard massive
excitations due to the electric scale, we uncover a novel massless excitation
ascribable to the magnetic scale. The residue of this massless excitation is
nonpositive at all temperatures, which consequently gives rise to positivity
violation in the quark spectral functions. This demonstrates the profound
impact of confinement effects on thermal quark collective excitations, which
manifest genuine long-range correlations in the system.Comment: 5 pages, 2 figures; v3 - published versio
Sudakov suppression of jets in QCD media
We compute modifications to the jet spectrum in the presence of a dense
medium. We show that in the large- approximation and at leading
logarithmic accuracy the jet nuclear modification factor factorizes into a
quenching factor associated to the total jet color charge and a Sudakov
suppression factor which accounts for the energy loss of jet substructure
fluctuations. This factor, called the jet collimator, implements the fact that
subjets, that are not resolved by the medium, lose energy coherently as a
single color charge, whereas resolved large angle fluctuations suffer more
quenching. For comparison, we show that neglecting color coherence results in a
stronger suppression of the jet nuclear modification factor.Comment: 5 pages, 1 figur
Radiative energy loss of neighboring subjets
We compute the in-medium energy loss probability distribution of two
neighboring subjets at leading order, in the large- approximation. Our
result exhibits a gradual onset of color decoherence of the system and accounts
for two expected limiting cases. When the angular separation is smaller than
the characteristic angle for medium-induced radiation, the two-pronged
substructure lose energy coherently as a single color charge, namely that of
the parent parton. At large angular separation the two subjets lose energy
independently. Our result is a first step towards quantifying effects of energy
loss as a result of the fluctuation of the multi-parton jet substructure and
therefore goes beyond the standard approach to jet quenching based on single
parton energy loss. We briefly discuss applications to jet observables in
heavy-ion collisions.Comment: 34 pages, 15 figure
Groomed jets in heavy-ion collisions: sensitivity to medium-induced bremsstrahlung
We argue that contemporary jet substructure techniques might facilitate a
more direct measurement of hard medium-induced gluon bremsstrahlung in
heavy-ion collisions, and focus specifically on the "soft drop declustering"
procedure that singles out the two leading jet substructures. Assuming coherent
jet energy loss, we find an enhancement of the distribution of the energy
fractions shared by the two substructures at small subjet energy caused by hard
medium-induced gluon radiation. Departures from this approximation are
discussed, in particular, the effects of colour decoherence and the
contamination of the grooming procedure by soft background. Finally, we propose
a complementary observable, that is the ratio of the two-pronged probability in
Pb-Pb to proton-proton collisions and discuss its sensitivity to various energy
loss mechanisms.Comment: 20 pages, 5 figure
Parton energy loss: new theoretical progress
The physics of jet quenching combines the dynamics of the QCD parton shower with bremsstrahlung radiation and decoherence processes induced by interactions with an underlying medium. Here we present a brief overview of the established features of medium-induced bremsstrahlung spectrum in a deconfined QCD plasma, highlight the aspect of rapid jet showering inside the medium and compute the resulting energy lost out of the jet cone in heavy-ion collisions.publishedVersio
Jet (de)coherence in Pb-Pb collisions at the LHC
We study the modifications of jets created in heavy-ion collisions at LHC
energies. The inherent hierarchy of scales governing the jet evolution allows
to distinguish a leading jet structure, which interacts coherently with the
medium as a single color charge, from softer sub-structures that will be
sensitive to effects of color decoherence. We argue how this separation comes
about and show that this picture is consistent with experimental data on
reconstructed jets at the LHC, providing a quantitative description
simultaneously of the jet nuclear modification factor, the missing energy in
di-jet events and the modification of the fragmentation functions. In
particular, we demonstrate that effects due to color decoherence are manifest
in the excess of soft particles measured in fragmentation functions in Pb-Pb
compared to proton-proton collisions.Comment: 5 pages, 2 figure
Towards tomography of quark-gluon plasma using double inclusive forward-central jets in Pb-Pb collision
We propose a new framework, merging High Energy Factorization with
final-state jet quenching effects due to interactions in a quark-gluon plasma,
to compute di-jet rates at mid- and forward rapidity. It allows to consistently
study the interplay of initial-state effects with medium interactions, opening
the possibility for understanding the dynamics of hard probes in heavy-ion
collisions and the QGP evolution in rapidity.Comment: 10 pages, 5 figure
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