L. Cunqueiro

An analysis of the influence of background subtraction and quenching on jet observables in heavy-ion collisions

A bstractSubtraction 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 package 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 parameters 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 subtraction 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 implemented in Q-PYTHIA, substantially affects the di-jet asymmetry but little the azimuthal dijet distributions. Besides, the missing momentum characteristics observed in the experiment are qualitatively reproduced by Q-PYTHIA.

Background subtraction and jet quenching on jet reconstruction

Abstract 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.