617 research outputs found
Jet Production at RHIC and LHC
Recent results on jet production in heavy ion collisions at RHIC and the LHC
are discussed, with emphasis on inclusive jet yields and semi-inclusive
hadron-triggered and vector boson-triggered recoil jet yields as well as their
azimuthal angular correlations. I will also discuss the constraints that these
observables impose on the opacity of the medium, the flavour dependence of
energy loss, the interplay of perturbative and non perturbative effects and the
change of the degrees of freedom of the medium with the resolution of the
probe.Comment: 8 pages, 7 figures, proceedings of Quark Matter 2017 conferenc
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
Dead-cone searches in heavy-ion collisions using the jet tree
We explore the possibility of using the dead cone of heavy quarks as a region
of the Lund plane where medium-induced gluon radiation can be isolated and
characterised. The filling of the dead cone by medium-induced gluons is
expected to be the result of the interplay between the minimum angle of such
radiation due to transverse momentum broadening and the dead-cone angle. Since
the measurement of a fully corrected Lund plane in heavy-ion collisions is
currently challenging, we propose to use jet grooming techniques to identify a
particular splitting in the jet tree that is both perturbative and sensitive to
the dead-cone effect. To that end, we propose a new jet substructure groomer,
dubbed Late-, that selects the most collinear splitting in a QCD jet above
a certain transverse momentum cutoff . The role of
is to guarantee perturbative splittings, while selecting the
most collinear splitting enhances the sensitivity to mass effects. As a proof
of concept, we study the angular distribution of the splitting tagged by
Late- both analytically and with Monte Carlo simulations. First, we derive
the logarithmic resummation structure in vacuum and demonstrate its capability
to distinguish between inclusive and heavy-flavoured jets. Next, we extend the
calculation for in-medium jets and show that medium-induced emissions lead to
an enhancement of collinear emissions below the dead cone angle. Numerically,
we demonstrate an excellent resilience of Late- against uncorrelated
thermal background, thus confirming this observable as a potential candidate to
unveil medium dynamics around the dead cone regime
Isolating perturbative QCD splittings in heavy-ion collisions
We define a new strategy to scan jet substructure in heavy-ion collisions.
The scope is multifold: (i) test the dominance of vacuum jet dynamics at early
times, (ii) capture the transition from coherent to incoherent jet energy loss,
and (iii) study elastic scatterings in the medium, which are either hard and
perturbative or soft and responsible for jet thermalisation. To achieve that,
we analyse the angular distribution of the hardest splitting, , above a transverse momentum scale, , in high- jets.
Sufficiently high values of target the regime in which the
observable is uniquely determined by vacuum-like splittings and energy loss,
leaving the jet substructure unmodified compared to proton-proton collisions.
Decreasing enhances the sensitivity to the relation between
energy loss and the intra-jet structure and, in particular, to observe
signatures of colour decoherence at small angles. At wider angles it also
becomes sensitive to hard elastic scatterings with the medium and, therefore,
the perturbative regime of medium response. Choosing
leads to order one effects of non-perturbative origin such as hadronisation
and, potentially, soft scatterings responsible for jet thermalisation. We
perform a comprehensive analysis of this observable with three state-of-the-art
jet-quenching Monte Carlo event generators. Our study paves the way for
defining jet observables in heavy-ion collisions dominated by perturbative QCD
and thus calculable from first principles.Comment: 18 pages, 11 figure
Monte Carlo for Jet Showers in the Medium
The most commonly employed formalisms of radiative energy loss have been
derived in the high- energy approximation. In its present form, it is reliable
only for the medium modifications of inclusive particle spectra. Modifications
to this formalism are expected to be important for less inclusive measurements.
This is especially relevant for reconstructed jets in heavy-ion collisions,
which are becoming available only recently. We present some ideas to overcome
this limitation. Specifically, we show an implementation of radiative energy
loss within a jet parton shower. This implementation has been done within the
PYTHIA Monte Carlo event generator. We present the publicly available routine
Q-PYTHIA and discuss some of the obtained physics results.Comment: 4 pages, 1 figures - To appear in the conference proceedings for
Quark Matter 2009, March 30 - April 4, Knoxville, Tennessee. Final version
with minor typos correcte
Implementation of a medium-modified parton shower algorithm
We present a Monte Carlo implementation of medium-induced gluon radiation in
the final-state branching process. Medium effects are introduced through an
additive term in the splitting functions. We have implemented such modification
within PYTHIA. We show the medium effects on the hump-backed plateau, and the
transverse momentum and angular distributions with respect to the parent
parton. As expected, with increasing medium densities there is an increase
(decrease) of partons with small (large) momentum fraction, and angular
broadening is observed. The effects on the transverse-momentum distributions
are more involved, with an enhancement of low- and intermediate- partons
and a decrease at large , which is related to energy conservation, and to
the lack of momentum exchange with the medium in our approach.Comment: LaTeX, 6 pages, 2 eps figures; proceedings of the 3rd International
Conference on Hard and Electromagnetic Probes in High-Energy Nuclear
Collisions - Hard Probes 2008 (Illa de A Toxa, Spain, June 8th-14th 2008
Angular-ordered parton showers with medium-modified splitting functions
Modified Altarelli-Parisi splitting functions were recenty proposed to model
multi-parton radiation in a dense medium and describe jet quenching, one of
most striking features of heavy-ion collisions. We implement medium-modified
splitting functions in the HERWIG parton shower algorithm, which satisfies the
angular ordering prescription, and present a few parton-level results, such as
transverse momentum, angle and energy-fraction distributions, which exhibit
remarkable medium-induced effects. We also comment on the comparison with
respect to the results yielded by other implementations of medium-modified
splitting functions in the framework of virtuality-ordered parton cascades.Comment: 19 pages, 8 figures, 1 table. Minor changes after referee repor
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