Measurment of neutral pion and eta mesons in proton+proton collisions with alice at the large hadron collider

Abstract

The quantum eld theory for strong interaction is called Quantum Chromodynamics (QCD). QCD has two peculiar properties called con nement and asymptotic freedom. Con nement 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 con ned inside hadrons can be de-con ned. This de-con ned state of quarks and gluons is termed as Quark-Gluon Plasma (QGP).QGP can be experimentally created in the laboratory by ultrarelativistic heavy-ion collisions. The experimental search for de-con ned state of quarks and gluons started with the rst 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 xed target experiment at the SPS, proton and lead ions were accelerated. The SPS reached an energy of p s = 30 GeV for proton+proton (pp) collisions and for Pb+Pb collisions upto p sNN = 17:3 GeV. The observed suppression of J= and enhancement in strangeness in Pb+Pb collisions indicated the formation of de-con ned state of quarks and gluons. Relativistic Heavy Ion Collider (RHIC) at BNL is the rst collider which accelerates protons, deuterons, copper ions and gold ions. The energy of the collision is upto p s = 510 GeV for pp collisions and p sNN = 200 GeV for Au+Au collisions. At RHIC, various signatures like jet quenching, azimuthal anisotropy, J= suppression, strangeness enhancement indicated the creationof QGP. The created matter in heavy-ion collisions at RHIC was found to behave like a strongly interacting liquid with =s ratio close to the AdS/CFT bound of 1=4 . At the Large Hadron Collider (LHC), the colliding particles energy is in TeV scale, which is 10 times higher than RHIC. A Large Ion Collider Experiment (ALICE) at LHC is speci cally 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 (pT) 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-pT neutral pion and eta mesons at LHC energies allows validating the QCD predictions. The particles with low-pT 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-pT 0 and can be used to tune various phenomenological models. The 0 and are the main sources of decay photons, it is very important to have precise estimation of 0 and 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 p s = 7 TeV is done. The 0 and invariant yield and cross-section measurements are presented using PHOS detector in ALICE, with a phase space coverage of j j < 0:13 and 260 < < 320 . The results are combined with the measurements using Electromagnetic Calorimeter (EMCal) and Photon Conversion Method (PCM). The xT scaling is studied for neutral pion and eta mesons production at LHC energies. Both neutral pions and eta mesons follows xT scaling at high-xT where the cross-section has the contribution from hard processes. We nd the value of exponent n for neutral pions at LHC is lower than RHIC. The mT scaling is also studied for neutral pion and eta mesons production at LHC energies. We observe the mT scaling violation for pT < 2 GeV=c. The study of energy dependence of neutral meson production is performed. We have also made prediction for 0 production cross-section in pp collisions at p s = 5:02 and 13 TeV.In brief, the thesis is organized as below: In Chapter 1, an introduction to Standard Model, QCD, QGP and its various signatures are given. The relativistic heavy-ion and pp collisions are also discussed brie y. The motivation for pp collisions and in particular 0 and measurements are discussed.In Chapter 2, the experimental facilities at LHC are explained. We discuss ALICE experiment and its di erent detectors brie y. The PHOS detector used for this analysis is described in details. In Chapter 3, the details of data analysis methodology are discussed. The good run selection, clusters selection, bad map construction and 0 and yield extraction methods are explained. We describe di erent corrections from non-linearity, pileup, feed-down and reconstruction e ciencies. The invariant spectra are obtained after applying these corrections. Lastly, we discuss sources of various systematic errors in the pion and eta mesons analysis.In Chapter 4, we present the nal results of the analysis. We show the 0 and invariant yield and cross-sections with systematic errors. We present results from the xT and mT scaling and show the energy evolution of 0 and mesons production at LHC energies. In Chapter 5, we summarise the results with important ndings.