Jets in QCD matter: Monte Carlo approaches

Monte Carlo approaches are a powerful tool in collider physics as they allow to make theory-data comparison on complex multi-particle observables, otherwise difficult for perturbative calculations. In heavy-ion collisions, there is a multitude of Monte Carlo approaches that try to address jet quenching phenomena, name given to the collection of medium-induced modifications that high momentum particles and jets undergo when traversing the hot and dense medium that is produced in such collisions. These models are being continuous developed alongside the theoretical efforts to understand and accurately describe experimental results provided by both RHIC and the LHC. In this manuscript, it is given a general overview about the fundamental building blocks that these tools have to address to describe jets in heavy-ion collisions. It follows a comparison on the latest results provided by some of the jet quenching Monte Carlo models to jet and intra-jet observables. A final personal outlook is presented at the end of the manuscript.Comment: Hard Probes Proceeding

Recent progress on the understanding of the medium-induced jet evolution and energy loss in pQCD

Motivated by the striking modifications of jets observed both at RHIC and the LHC, significant progress towards the understanding of jet dynamics within QGP has occurred over the last few years. In this talk, I review the recent theoretical developments in the study of medium-induced jet evolution and energy loss within a perturbative framework. The main mechanisms of energy loss and broadening will be firstly addressed with focus on leading particle calculations beyond the eikonal approximation. Then, I will provide an overview of the modifications of the interference pattern between the different parton emitters that build up the parton shower when propagating through an extended coloured medium. I will show that the interplay between color coherence/decoherence that arises from such effects is the main mechanism for the modification of the jet core angular structure. Finally, I discuss the possibility of a probabilistic picture of the parton shower evolution in the limit of a very dense or infinite medium.Comment: XIIth Quark Confinement and the Hadron Spectrum Proceeding

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

Sub-jet structure as a discriminating quenching probe

In this work, we propose a new class of jet substructure observables which, unlike fragmentation functions, are largely insensitive to the poorly known physics of hadronization. We show that sub-jet structures provide us with a large discriminating power between different jet quenching Monte Carlo implementations.Comment: 4 pages, 3 figures, Quarks Matter conference 201

Energy loss and (de)coherence effects beyond eikonal approximation

The parton branching process is known to be modified in the presence of a medium. Colour decoherence processes are known to determine the process of energy loss when the density of the medium is large enough to break the correlations between partons emitted from the same parent. In order to improve existing calculations that consider eikonal trajectories for both the emitter and the hardest emitted parton, we provide in this work, the calculation of all finite energy corrections for the gluon radiation off a quark in a QCD medium that exist in the small angle approximation and for static scattering centres. Using the path integral formalism, all particles are allowed to undergo Brownian motion in the transverse plane and the offspring allowed to carry an arbitrary fraction of the initial energy. The result is a general expression that contains both coherence and decoherence regimes that are controlled by the density of the medium and by the amount of broadening that each parton acquires independently.Comment: 4 pages, to appear in the proceedings of the Quark Matter 2014 conferenc

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 $\Delta S_{12}$ 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 $\Delta S_{12}$ 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