Charm Jets

Abstract

How did the Universe begin? The Big Bang theory states that the Universe was not the same stars and planets as we see today, but just a very hot liquid called the quark–gluon plasma (QGP) then. Now, what is this liquid and how does it behave? The theory of the strong nuclear force, quantum chromodynamics (QCD), predicts that the quarks and gluons in nuclear matter can deconfine into the QGP when sufficiently high energy densities are reached. The research presented in this thesis is an attempt at furthering our knowledge of strong interactions and the QGP. In ultrarelativistic collisions of protons and heavy ions, quarks and gluons interact with each other and are sometimes scattered to large angles in so-called hard scatterings, and through the process of hadronization, they combine and reorganize themselves to form hadrons, which tend to travel close to each other in narrow tight cones called jets. Jets are a means to understand the original partons. Jets are important to heavy-ion physics because when one of the two back-to-back jets goes through the QGP, it can get quenched and thus lose significant energy. Measuring jets also provides a lens to look into the properties of the astonishingly hot and dense soup of QGP. Unlike light quarks, the heavy quarks (charm and beauty) are mostly produced in high-energy collisions. They either hadronize or decay into lighter quarks which ultimately hadronize. Heavy-flavour jets, which consist of hadrons containing heavy quarks, can provide valuable insights into the behaviour of heavy quarks. Jets can help us better understand perturbative QCD as well as investigate the properties of the QGP by observing energy loss if heavy quarks interact with it. Charm jets are the chosen investigation tool in this thesis. A charm jet was reconstructed by clustering charged particles and a D0 meson (the lightest hadron containing charm quarks) using the anti-kT algorithm. Three analyses were done for charm jets at the centre-of-mass energy per nucleon pair 5.02 TeV. The production cross section of charm jets in pp and p–Pb collisions were measured along with nuclear modification in p–Pb collisions in reference to pp collisions. The fragmentation function of charm jets in pp collisions was also reported by looking into the jet momentum carried by its constituent D0 meson. The LO model with Colour Reconnection and soft processes provides the best description of cross section and fragmentation distribution of charm jets in pp collisions as compared to the LO model with hard processes only. The parton distribution function (PDF) for the proton was obtained from the LHAPDF 6 interpolator with the PDF set CT10nlo, while the nuclear PDF inside a lead ion was taken from EPS09nlo. All measurements in pp and p–Pb collisions agree to NLO theoretical predictions within the uncertainties. Also, the nuclear modification factor RpPb was found to conform to unity indicating an absence of large modifications in the initial parton distributions and an absence of final-state effects on charm-jet production in the measured kinematic region.