Measurements of $\pi^{0}$-jet correlations in √s = 7 TeV pp collisions and in √sNN = 2.76 TeV central Pb-Pb collisions at ALICE experiment

Quarks and gluons are confined in a hadron by the strong interaction described by Quantum Chromodynamics (QCD). However, at high temperature, the quarks and gluons move freely beyond the boundary of hadrons. Such state is called Quark-Gluon Plasma (QGP) and believed to exist at a few micro second after the Big Bang of the universe. The ultra relativistic heavy ion collision is unique tool to create the QGP state on the earth. Several results indicates a creation of QGP state by the ultra relativistic heavy ion collisions. One of them is the suppression of high momentum particles (jet quenching) which is observed by the two particle correlation measurements at the Relativistic Heavy Ion Collider (RHIC) and di-jet measurements at the Large Hadron Collider (LHC). This modification is caused by the energy loss of the partons in the QGP. Jets are produced from the two body scattering of partons with large momentum transfer. In theoretical approach, it is indicated that the energy loss of jets in QGP is related to the properties of QGP particularly gluon density and initial temperature of the QGP. Therefore, jet measurements play a critical role in probing the QGP matter created in heavy ion collisions through parton energy loss via the observation of the jet structure modification or jet suppression. The energy loss of energetic partons also depends on the path length in the QGP matter. For example, jet pairs with a large energy asymmetry in the final states can be produced due to the difference of path-length between leading jet and sub-leading jet. While leading jets escape mostly from the surface in the medium, recoil jets traverse in the matter with losing its energy in a hot and dense matter. We use this surface bias to acquire deeper insights into the matter properties longer than that for leading jets. The stronger the surface bias, the larger the path length in the QGP is for the recoiling jet at the opposite azimuth. By measuring jets directly rather than measuring inclusive high transverse momentum (pT) hadrons as is done in the previous study, we can perform a more comprehensive and direct study of jet interactions in the matter. In this thesis, we report measurements of neutral pion and charged jet correlations in pp collisions at √s = 7 TeV and in central Pb-Pb collisions at √sNN = 2.76 TeV from the LHC-ALICE experiment. The LHC at CERN is the largest energy accelerator in the world. The ALICE detector was built to exploit the unique physics in nucleus-nucleus interactions at the LHC and are capable of studying jet quenching effects via jet reconstruction and particle identification. For the neutral pion identification, we use an electro-magnetic calorimeter (EMCal) in ALICE. In addition, EMCal is used as a trigger detector to enrich the high pT neutral pion sample. The opening angle of two photons decayed from the neutral pion become smaller, when the neutral meson energy is high due to the Lorentz boost. In order to identify π0 at higher pT range, we use the cluster splitting method which identifies a single cluster as a neutral meson via the parameter λ0. This parameter is the length of long axis of the ellipse with two overlapping showers on the surface of calorimeter. We can identify π0 from 8 up to 40 GeV/c by using the cluster splitting method. Jets are reconstructed by using charged particles which are measured by the Inner Tracking System (ITS) and Time-Projection Chamber (TPC) with jet constituents pT,ch > 0.15 GeV/c, with jet resolution parameter R = 0.4. Underlying event backgrounds are subtracted by the event-by-event background subtraction method which takes into account the event plane dependence of the background coming from the event anisotropy. The measured pT range of the jets is from 10 to 80 GeV/c. Azimuthal correlations between trigger π0’s and reconstructed jets have been measured at √s = 7 TeV pp collisions as a baseline, and at √sNN = 2.76 TeV Pb-Pb central 0−10 % collisions to see a modification of jet in heavy ion collisions. We observe jet-like peaks in near and away side in pp and Pb-Pb collisions, and the away side peak becomes sharp with increasing pT of the associated jets in pp collisions. This result indicates that the high pT π0 production is strongly associated with a jet production. Next we take a ratio of per trigger yields IAA which is ratio between per trigger yield in Pb-Pb and that in pp. We measure an enhancement for near side jet for jet pT,ch.jet > 40 GeV/c, and a suppression for all measured jet pT,ch.jet range for away side. Triggering higher pT π0 would select near side jets produced close to the surface in a medium, while the path-length of its away side jets become longer than the near side jets. In the near and away side widths measurement, we observe an indication of a jet broadening effect in the near-side with requiring the lower pT of trigger π0 and/or pT leading particle in a jet in Pb-Pb collisions. This effect decreases with increasing pT of the leading particle in a jet. These results may suggest a possible jet broadening effect in central Pb-Pb collision at the LHC energy depending on the path-length of a initial parton in a medium.