CERN Accelerating science

The quantum field theory for strong interaction is called Quantum Chromodynamics (QCD). QCD has two peculiar properties called confinement and asymptotic freedom. Confinement is the reason for the bound state of quarks which are called as hadrons, for example - protons and neutrons which together with electrons make up the visible matter of the Universe. In contrary, asymptotic freedom suggests, at high temperatures or high baryon densities the quarks and gluons confined inside hadrons can be de-confined. This de-confined state of quarks and gluons is termed as Quark-Gluon Plasma (QGP). QGP can be experimentally created in the laboratory by ultra-relativistic heavy-ion collisions. The experimental search for de-confined state of quarks and gluons started with the first heavy-ion collisions at Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory (BNL) in 1986 and at Super Proton Synchrotron (SPS) at CERN. In the fixed target experiment at the SPS, proton and lead ions were accelerated. The SPS reached an energy of $\sqrt{s} = 30$ GeV for proton+proton ($pp$) collisions and for Pb+Pb collisions upto $\rm{\it}\sqrt{s_{NN}} = 17.3$ GeV. The observed suppression of $\rm{\it}J/\psi$ and enhancement in strangeness in Pb+Pb collisions indicated the formation of de-confined state of quarks and gluons. Relativistic Heavy Ion Collider (RHIC) at BNL is the first collider which accelerates protons, deuterons, copper ions and gold ions. The energy of the collision is upto $\sqrt{s} = 510$ GeV for $pp$ collisions and $\sqrt{s_{\rm{\it}NN}} = 200$ GeV for Au+Au collisions. At RHIC, various signatures like jet quenching, azimuthal anisotropy, $\rm{\it}J/\psi$ suppression, strangeness enhancement indicated the creation of QGP. The created matter in heavy-ion collisions at RHIC was found to behave like a strongly interacting liquid with $\eta/s$ ratio close to the AdS/CFT bound of $1/4\pi$. At the Large Hadron Collider (LHC), the colliding particles energy is in TeV scale, which is $\sim~10$ times higher than RHIC. A Large Ion Collider Experiment (ALICE) at LHC is specifically designed for precise measurement of QGP and study its properties. One of the features of heavy-ion collisions is suppression of high energy particles. High transverse momentum ($p_{\rm{\it} T}$) partons are produced in hard scattering processes in the early stages of the collisions. The produced partons undergo interaction with the created hot and dense medium and lose energy. This appears as a suppression for neutral pion and eta mesons in heavy-ion collisions. To study this suppression, measurement of neutral pion and eta mesons in $pp$ collisions are required. Similarly, other observables in $pp$ collisions act as a baseline for the study of nuclear collisions. Measurement of high-$p_{\rm{\it} T}$ neutral pion and eta mesons at LHC energies allows validating the QCD predictions. The particles with low-$p_{\rm{\it} T}$ are produced by ``soft'' processes, and cannot be described by using perturbative QCD (pQCD) calculations. It requires various phenomenological models to describe the production mechanism of these particles. So, the low-$p_{\rm{\it} T}$ $\pi^{0}$ and $\eta$ can be used to tune various phenomenological models. The $\pi^{0}$ and $\eta$ are the main sources of decay photons, it is very important to have a precise estimation of $\pi^{0}$ and $\eta$ yields, before any measurement of direct photons, is done. In the present thesis, analysis of neutral pion and eta meson production in $pp$ collisions at $\sqrt{s} = 7$ TeV is done. The $\pi^{0}$ and $\eta$ invariant yield and cross-section measurements are presented using PHOS detector in ALICE, with a phase space coverage of $|\eta|<0.13$ and $260^{\circ}<\phi<320^{\circ}$. The results are combined with the measurements using Electromagnetic Calorimeter (EMCal) and Photon Conversion Method (PCM). The $x_{\rm{\it} T}$ scaling is studied for neutral pion and eta mesons production at LHC energies. Both neutral pions and eta mesons follows $x_{\rm{\it} T}$ scaling at high-$x_{\rm{\it} T}$ where the cross-section has the contribution from hard processes. We find the value of exponent $n$ for neutral pions at LHC is lower than RHIC. The $m_{\rm{\it} T}$ scaling is also studied for neutral pion and eta mesons production at LHC energies. We observe the $m_{\rm{\it} T}$ scaling violation for $p_{\rm{\it} T}<2$ GeV$/c$. The study of energy dependence of neutral meson production is performed. We have also made prediction for $\pi^{0}$ production cross-section in $pp$ collisions at $\sqrt{s} = 5.02$ and 13 TeV.