Measurement of event background fluctuations for charged particle jet reconstruction in Pb-Pb collisions at $ \sqrt {{{s_{\text{NN}}}}} = {2}{.76}\,{\text{TeV}} $

Abstract

The effect of event background fluctuations on charged particle jet reconstruction in Pb-Pb collisions at √ s NN = 2.76 TeV has been measured with the ALICE experiment. The main sources of non-statistical fluctuations are characterized based purely on experimental data with an unbiased method, as well as by using single high p t particles and simulated jets embedded into real Pb-Pb events and reconstructed with the anti-k t jet finder. The influence of a low transverse momentum cut-off on particles used in the jet reconstruction is quantified by varying the minimum track p t between 0.15 GeV/c and 2 GeV/c. For embedded jets reconstructed from charged particles with p t > 0.15 GeV/c, the uncertainty in the reconstructed jet transverse momentum due to the heavy-ion background is measured to be 11.3 GeV/c (standard deviation) for the 10% most central PbPb collisions, slightly larger than the value of 11.0 GeV/c measured using the unbiased method. For a higher particle transverse momentum threshold of 2 GeV/c, which will generate a stronger bias towards hard fragmentation in the jet finding process, the standard deviation of the fluctuations in the reconstructed jet transverse momentum is reduced to 4.8-5.0 GeV/c for the 10% most central events. A non-Gaussian tail of the momentum uncertainty is observed and its impact on the reconstructed jet spectrum is evaluated for varying particle momentum thresholds, by folding the measured fluctuations with steeply falling spectra.

Multiparticle azimuthal correlations inp-Pb and Pb-Pb collisions at the CERN Large Hadron Collider

“…Track quality is ensured by requiring tracks to have at least 70 TPC clusters out of a maximum of 159, and a χ 2 per TPC cluster less than 4 for the track fit. In addition, the distances of closest approach to the primary vertex in the xy plane and z direction are required to be less than 2.4 and 3.2 cm respectively [28].…”

Background estimator for jet studies in p+p and A+A collisions

“…Although highly successful at estimating p jet T,bkg on average, this method lacks control over background fluctuations. This translates into a (p jet T,reco − p jet T,truth )distribution centered at zero but, as shown in Section III, with a sizable standard deviation [10].…”

“…In high-energy hadronic collisions two competing mechanisms are relevant: pileup interactions, ubiquitous in high-luminosity facilities and the underlying event whose importance grows with the collision system size, i.e. from p+p to A+A [10]. Both of them populate the low-p T sector of the event and are commonly referred to as background.…”

Jet measurements in proton-proton and PbPb collisions with the ALICE experiment at LHC

“…To reduce the contribution from the background in heavy-ion collisions, a commonly used value of the radius parameter is 0.4. This value is also used in pp collisions for comparison with heavy-ion collisions [4]. All jets taken for analysis have their jet axis within |η| < 0.5.…”