Measurement of neutral mesons in pp and Pb–Pb collisions at mid-rapidity with ALICE

Abstract

One of the key signatures of the Quark-Gluon Plasma (QGP), is the modification of hadron transverse momentum differential cross-sections in heavy-ion collisions (HIC) as compared to proton-proton (pp) collisions. Suppression of hadron production at high transverse momenta (p T ) in HIC has been explained by the energy loss of the partons produced in the hard scattering processes which traverse the deconfined quantum chromodynamic (QCD) matter. The dependence of the observed suppression on the p T of the measured hadron towards higher p T is an important input for the theoretical understanding of jet quenching effects in the QGP and the nature of the energy loss. The ALICE experiment at the Large Hadron Collider (LHC) performs measurements of neutral meson inclusive spectra at mid-rapidity in a wide p T range in pp, p-Pb and Pb-Pb collisions. Neutral mesons (π 0 , η, ω) are reconstructed via complementary methods, using the ALICE electromagnetic calorimeters, PHOS and EMCal, and by the central tracking system, identifying photons converted into e + e − pairs in the material of the inner barrel detectors (TPC and ITS). In this presentation, an overview of current π 0 and η measurements by the ALICE experiment in HIC and pp collisions will be given.

Neutral meson production and correlation with charged hadrons in pp and Pb-Pb collisions with the ALICE experiment at the LHC

“…Comparisons with NLO pQCD calculations lead to the same disagreement for the η than with the π 0 [5] when CTEQ PDFs and AES [4] for the fragmentation function are used, but the discrepancy is much reduced for π 0 when MSTW PDFs with DSS14 [3] for the fragmentation function are used. Combined with the Pb-Pb measurement at √ s NN = 2.76 TeV [2], these results will allow us to obtain the nuclear modification factor, R AA , for the η meson as well as to extend the π 0 R AA measurement [6] up to 20 GeV/c. Yet differential measurements such as π 0 -hadron correlations are also needed to have a better understanding of the in-medium energy loss.…”

“…2 for pp collisions at √ s = 2.76 TeV [1]. The cross section for π 0 results from a combination of PCM, PHOS and EMCal measurements from 2011 [2] and 2013 data while the η cross section results from a combination of PCM and EMCal measurements. The new EMCal measurement using the single cluster analysis allows us to extend the p T reach from 20 GeV/c up to 40 GeV/c for π 0 , and the new measurement allows us to reach a p T up to 20 GeV/c for the η.…”