Neutral pion measurement in p-Pb collisions at ${\sqrt{s_{\rm{NN}}}}$ = 5.02 TeV

Quarks and gluons which are thought to be a elementary particle are confined in the nucleon, and it is impossible to extract only quark from the nucleon. However they are freed from confined state with a condition of extreme high temperature and high density. There was the high temperature and dense condition immediately after the big bang of the birth of the universe, and quarks and gluons could move around freely. This state is called Quark-Gluon Plasma (QGP). A high-energy heavy-ion collisions experiment can create the QGP experimentally. Particles passing through the QGP lose energy by the strong force. In the results, produced particles are modified their own yields. Strong suppression of high-${p_{\rm T}}$ particles has been observed in heavy-ion collisions at LHC energies, which can be interpreted by invoking various processes involving transport properties of the QCD medium and initial-state effects. Proton-nucleus (p-A) collisions are intermediate between proton-proton (pp) and nucleus-nucleus (A-A) collisions in terms of system size and number of produced particles. Comparing particle production in pp, p-A, A-A reactions has frequently been used to separate initial-state effects of colliding nuclei from final-state effects in quark matter created by the collisions. The study of neutral meson production in proton-lead (p-Pb) collisions at ${\sqrt{s_{\rm{NN}}}}$ = 5.02 TeV is of importance to confirm that the strong suppression observed in central lead-lead (Pb-Pb) collisions is a final-state effect of the produced hot and dense medium. This paper will presents ${\pi^{0}}$ and ${\eta}$ meson production in p-Pb collisions at ${\sqrt{s_{\rm{NN}}}}$ = 5.02 TeV and nuclear modification factor ($R_{\rm pPb}$) for ${\pi^{0}}$ from the LHC-ALICE experiment for the first time. The $\pi^{0}$ meson is measured in $p_{\rm T}$ range of $0.3 - 20$ GeV/$c$ via completely 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, called photon conversion method (PCM) In addition, PCM via $\gamma$-Dalitz decay channel is denoted as PCM-Dalitz. The $\eta$ meson is measured in $p_{\rm T}$ range of $0.7 - 20$ GeV/$c$ via EMCal and PCM. The $\pi^{0}$ and $\eta$ meson final spectra are achieved via combination of individual analyses with weight according to their uncertainties. Both $\pi^{0}$ and $\eta$ meson invariant yields are a nice agreement by the Tsallis fit and all measurements are consistent with each other within the uncertainties. EPOS3 event generator based on hydrodynamical calculation reproduces well in the almost entire $p_{\rm T}$ region for $\pi^{0}$ and intermediate-$p_{\rm T}$ region for $\eta$ meson. The $\eta$/$\pi^{0}$ ratio increases at $p_{\rm T}$ $<$ 4 GeV/$c$ and arrives a plateau of 0.47 $\pm$ 0.02 at $p_{\rm T}$ $>$ 4 GeV/$c$. It is consistent with the ALICE pp and Pb-Pb measurements and the world results. The $m_{\rm T}$ scaling for the $\eta$/$\pi^{0}$ ratio is good description at $p_{\rm T}$ $>$ 4 GeV/$c$, but discrepancy is observed in ow-$p_{\rm T}$ region. The EPOS3 generator is good reproduction for data in low-$p_{\rm T}$ region and is closer than the $m_{\rm T}$ scaling prediction. But it fails to reproduce data in high-$p_{\rm T}$ region. The $\pi^{0}$ nuclear modification factor in p-Pb collisions ($R_{\rm pPb}$) increases with $p_{\rm T}$ in low-$p_{\rm T}$ region and consists with unity at $p_{\rm T}$ $>$ 2 GeV/$c$. It is not observed particle yield suppression as observed in Pb-Pb collisions. In addition, the $\pi^{0}$ nuclear modification factor in p-Pb collisions at LHC energy and in d-Au collisions at RHIC energy have no obvious difference. Theoretical model via using EPS09s NLO calculations and CGC model calculation are able to describe $R_{\rm pPb}$. These results provide direction that strong suppression of high-$p_{\rm T}$ $\pi^{0}$ observed in Pb-Pb collisions comes from final-state effects due to parton energy loss in the hot QCD medium rather than initial-state effects.