Measurements of identified charged hadrons in Au+Au and d+Au collisions at 200 GeV

Abstract

In this thesis, the transverse momentum (pT) spectra and ratios of identified charged hadrons π±, K±, p and ṗ produced in √sNN = 200 GeV Au+Au and d+Au collisions are reported as a function of centrality, which characterizes the number of nucleons that participate in the collisions, Npart. The measurements of pions and protons are reported up to pT = 6 GeV/c (5 GeV/c), and the measurements of kaons are reported up to pT = 4 GeV/c (3.5 GeV/c) in Au+Au (d+Au) collisions. In the intermediate pT region, between 2-4 GeV/c, a significant enhancement of baryon to meson ratios compared to those measured in p+p collisions is observed. This enhancement is present in both Au+Au and d+Au collisions, and increases as Npart increases.

In Au+Au collisions, meson production is strongly suppressed at intermediate and high pT, relative to the production in p+p collisions. The suppression is understood in terms of the partons losing energy as they traverse the hot and dense nuclear medium created in the collisions. Baryons, on the other hand, do not exhibit any suppression in the intermediate pT region, and particle-species differences are observed up to the highest pT measured.

In d+Au collisions, there is an enhancement of particle production relative to the production in p+p collisions in the intermediate pT region. The enhancement of mesons is quite small and may be systematically insignificant; conversely, the enhancement of the baryons is found to be quite large.

These results are discussed in the context of the recombination model of hadronization, in which the baryon vs meson dynamics emerge in a natural way as a consequence of the different number of valence quarks in the hadrons.