Measurement of neutral mesons and direct photons in pp and Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

The new state of matter, called quark-gluon plasma (QGP), created by the high-energy heavy-ion collision has been studied for more than 40 years. Partons originating from initial hard scatterings lose their energy in the hot and dense QCD medium, which results in suppression of hadron production at high transverse momentum ($p_{\rm T}$), compared to pp collisions at the same center-of-mass energy $\sqrt{s_{\rm NN}}$. Light flavor particles are excellent probes to study the suppression in a wide $p_{\rm T}$ range with high precision. Especially, neutral mesons such as $\pi^{0}$ and $\eta$ mesons that decay into two photons can be reconstructed and identified by a fine-segmented electro-magnetic calorimeter in a wide $p_{\rm T}$ range. In this thesis, the suppression of $\pi^{0}$ and $\eta$ mesons in Pb--Pb collisions at the highest energy $\sqrt{s_{\rm NN}} = 5.02$ TeV is reported. By increasing the collision energy, $p_{\rm T}$ spectra of $\pi^{0}$ meson become harder than that at $\sqrt{s_{\rm NN}} = 2.76$ TeV in both pp and Pb-Pb collisions. Nevertheless, the suppression of $\pi^{0}$ meson in Pb-Pb collisions compared to pp collisions is the same level, which is by a factor of up to 8. This indicates the larger energy-loss at the higher collision energy. Comparing light and heavy flavor hadrons, namely $\pi^{0}$ and D mesons, the suppression of D mesons at low $p_{\rm T}$ is weaker than that of $\pi^{0}$ meson. This is interpreted as the smaller energy-loss for charm quarks than for up, down quarks. The suppression pattern of $\eta$ meson seems to be similar to $K^{\pm}$ meson consisting of a strange quark, though uncertainties for the $\eta$ meson measurement is large. Direct photons that are defined as photons not originating from hadron decays are also discussed in this thesis. Direct photons are unique probes to study the space-time evolution of the QGP, since they are not involved in strong interaction and can carry information when they are produced. When focusing on direct photons, $\pi^{0}$ and $\eta$ mesons contribute as huge backgrounds. To subtract decay photon yields, the cocktail simulation where $p_{\rm T}$ spectra of neutral mesons are inputs has been performed. Direct photon spectra or upper limits at the 90% of confidence level have been extracted. Finally, $R_{\rm AA}$ of direct photons has been determined and is consistent with unity at high $p_{\rm T}$ which justifies the measurement. On the other hand, the excess beyond the pQCD calculation is observed at low $p_{\rm T}$ by a factor of up to 4 in central Pb--Pb collisions. This indicates thermal photon emissions from the hot and dense QCD medium. The obtained effective temperature $T_{\rm eff}$ is $345 \pm 222 {\rm (total \ unc.)}$ MeV in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV for centrality 0-10%. This is the first measurement and setting upper limits on direct photons in pp and Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV.