CERN Accelerating science

Quantum Chromodynamics (QCD) describes quarks and gluons interactions. Due to asymptotic freedom, a sufficiently high energy density can lead matter to transition into the Quark-Gluon Plasma (QGP), a deconfined form of matter. In heavy-ion collisions, a medium governed by strong interactions is formed, and its evolution can be studied by the produced particles. The primary objective of the heavy-ion programme at the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland, is to search for the possible formation of the QGP. The ALICE experiment at the LHC is designed to study the hot and dense medium produced in ultrarelativistic heavy-ion collisions. Hadronic resonances have shorter lifetimes relative to other stable hadrons, which is analogous to the time taken by the dense nuclear matter to reach its final state. This can be used to explore the hadronic phase's characteristics that results from heavy-ion collisions. The yield of resonances might be modified with respect to the expectations due to in-medium effects such as re-scattering and regeneration. The study of resonance production in p-Pb collisions fills the gap between pp and heavy-ion (Pb-Pb, Xe-Xe) collisions and helps us understand initial state effects due to cold nuclear matter. In this thesis, we study the $\phi$ (1020) mesons production using invariant mass reconstruction from their decay daughters (kaons) in p-Pb collisions. The kaon particles are identified by the Time Projection Chamber (TPC) and Time of Flight (TOF) in the ALICE experiment. The details of ALICE detector and its subsystems are described in this thesis. We present the measurement of $\phi$ mesons production in p-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 8.16 TeV using the ALICE detector. Resonances are reconstructed via their hadronic decay channels in the rapidity interval -0.5 $< y <$ 0 and the transverse momentum spectra are measured up to $p_{\rm{T}}$ = 16 GeV/$c$ for various multiplicity classes (0-5$\%$, 5-10$\%$, 10-20$\%$, 20-40$\%$, 40-60$\%$, 60-80$\%$, 80-100$\%$ and 0-100$\%$). The $x_{\rm{T}}$ scaling for the $\phi$ mesons production is newly tested in p-Pb collisions and found to hold in the high $p_{\rm{T}}$ region at LHC energies. The nuclear modification factors $(R_{\rm{pPb}})$ as a function of $p_{\rm{T}}$ for $\phi$ mesons at $\sqrt{s_{\rm{NN}}}$ = 8.16 TeV are presented and compared with $R_{\rm{pPb}}$ measurements of K$^{*0}$, $\phi$, $\Xi$, and $\Omega$ at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV. At intermediate $p_{\rm{T}}$ (2-8 GeV/$c$), $R_{\rm{pPb}}$ of $\Xi$, $\Omega$ show a Cronin-like enhancement, while K$^{*0}$ and $\phi$ show no or little nuclear modification. At high $p_{\rm{T}}$ ($>$ 8 GeV/$c$), the $R_{\rm{pPb}}$ values of all hadrons are consistent with unity within uncertainties. The $R_{\rm{pPb}}$ of $\phi$ mesons at $\sqrt{s_{\rm{NN}}}$ = 8.16 and 5.02 TeV show no significant energy dependence. The results are compared with EPOS-LHC, DPMJET and HIJING models predictions. We also present the first measurement of multiplicity and rapidity dependence of $\phi$ mesons production in p-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV. The transverse-momentum ($p_{\rm{T}}$) spectra of $\phi$ mesons measured with the ALICE detector up to $p_{\rm{T}}$ = 16 GeV/$c$ for various multiplicity classes in five rapidity intervals (-1.2 $< y <$ -0.9, -0.9 $< y <$ -0.6, -0.6 $< y <$ -0.3, -0.3 $< y <$ 0 and 0 $< y <$ 0.3) in the rapidity range −1.2 $< y <$ 0.3. The measured $p_{\rm{T}}$ distributions show a dependence on both multiplicity and rapidity at low $p_{\rm{T}}$, whereas no significant dependence is observed at high $p_{\rm{T}}$. A rapidity dependence is observed in the $p_{\rm{T}}$-integrated yield (d$N$/d$y$), whereas the mean transverse momentum ($\langle p_{\rm{T}}\rangle$) shows a flat behavior as a function of rapidity. The rapidity asymmetry $(Y_{\rm{asym}})$ at low $p_{\rm{T}}$ ( $<$ 5 GeV/$c$) is more significant for higher multiplicity classes. At high $p_{\rm{T}}$, no significant rapidity asymmetry is observed in any of the multiplicity classes. The nuclear modification factor $(Q_{\rm{CP}})$ as a function of $p_{\rm{T}}$ shows a Cronin-like enhancement at intermediate $p_{\rm{T}}$, which is more prominent at higher rapidities (Pb-going direction) and in higher multiplicity classes. At high $p_{\rm{T}}$ ($>$ 5 GeV/$c$), the $(Q_{\rm{CP}})$ values are greater than unity and no significant rapidity dependence is observed. The results are compared with EPOS-LHC, DPMJET, Pythia8, EPOS3 and HIJING models predictions.