Production of light neutral mesons in the ALICE experiment at the LHC

Abstract

Light pseudo-scalar neutral mesons have been studied in e+eˉ, hadron-hadron, hadron-nucleus and nuclear collisions for decades. The results associated with neutral mesons provide information that is as good as that which comes from charged pions or hadrons. However, they have the advantage that they are not affected by Coulomb forces as charged pions. Due to the fact that the differential cross-section can be factorized into short distance (perturbative) and long distance (non-perturbative) terms when sufficiently large momentum transfer occurs, the measurement of neutral meson spectra provides a chance to test NLO or NNLO pQCD calculations or to constrain parton distribution and fragmentation functions. The existence of the Quark-Gluon Plasma (QGP) state was postulated in seventies of the last century. The QGP is a highly dense and hot QCD medium in thermal equilibrium where quarks and gluons are almost free. The strongly interacting QGP state has been discovered at RHIC experiments in heavy-ion collisions and is also formed in nuclear collisions at the LHC. The QGP properties can be probed with neutral mesons. Hard (high pT) partons are produced in hard scatterings at the early stage of a collision. Hard partons interact with the dense QGP medium, lose energy, and emerge as quenched jets in the final state after hadronization. Modification of a jet energy results also in reduction of the energy of leading hadrons and in particular neutral mesons. This phenomenon, known as jet quenching, can be quantified via the nuclear modification factor which shows a large suppression of hadron yields at high pT in the most central AA collisions compared to pp collisions scaled by the number of nucleon-nucleon binary collisions. Thus, the measurement of neutral meson spectra constrains mechanisms of parton energy loss in dense systems via studies of the nuclear modification factor. The ALICE experiment has been designed to study the Quark-Gluon Plasma. However, not only heavy-ion collisions but also pp and pA collisions can be studied there. The π° and η mesons are reconstructed in ALICE mainly via their photonic decays. Photons can be directly measured in ALICE electromagnetic alorimeters, EMCal and PHOS or reconstructed from e+eˉ pairs which come from photon conversion in the central barrel detectors’ material. Calibration of detectors in which photons and electrons are identified plays a very important role. In this context, alibration of two detectors, the Time Projection Chamber (TPC) and the Electromagnetic Calorimeter (EMCal), of the ALICE detector is explained. The overview of the world results for many hadron-hadron, nucleus-nucleus and hadron-nucleus collisions collected in a very wide range of center-of-mass collision energy, from few GeV to few TeV, is given in the paper. Thanks to the LHC accelerator which provided pp collisions in the energy range √s = 0.9 to 13 TeV, p-Pb collisions at √sNN = 5.02 and 8.16 TeV, as well as Pb-Pb collisions at √sNN = 2.76 and 5.02 TeV, the ALICE experiment could extend neutral meson measurements to new energies unavailable before the LHC era. Inclusive π° and η meson production spectra have been measured in a wide pT range at mid-rapidity. The nuclear modification factor shows the large suppression in the central Pb-Pb collisions. The existence of strongly interacting QGP state of matter is confirmed. No suppression is observed in p-Pb collisions, which supports the thesis that suppression is a final state effect. The measured η/ π° ratio shows universal behaviour among systems and energies. However, deviation from mT scaling in very low pT range is indicated. The results from the ALICE experiment give new constraints on various models in both perturbative and non-perturbative regimes. Data collected by ALICE in pp collisions at √s = 13 TeV can provide very precise measurement of neutral meson spectra and information about jet fragmentation. Future measurements of neutral meson production spectra and isolated photon - jet correlations in pp collisions at √s = 13 TeV are discussed.