Charged Antiparticle to Particle Ratios Near Midrapidity in d+Au and p+p Collisions at sqrt(s_NN) = 200 GeV

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Experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory are designed to investigate the behavior of strongly interacting matter at high temperatures and densities. The conditions created during a heavy ion collision at RHIC energies are predicted to be sufficient to form a quark-gluon plasma. As part of this investigation, smaller collision systems need to be studied to aid in the understanding and interpretation of results from the more complicated heavy ion collisions.

This thesis reports the ratios of the yields of antiparticles to particles for primary charged pions, kaons, and protons emitted in p+p and d+Au collisions at sqrt{s_NN}=200GeV. In the d+Au collision system the results are measured as a function of collision centrality. The data analyzed were collected by the PHOBOS detector during the 2003 run of the Relativistic Heavy Ion Collider. Comparison of the results obtained in this thesis with the antiparticle to particle ratios measured in 200GeV Au+Au collisions allows the effects of final state interactions on the produced particle yields to be inferred. Furthermore, measurement of the antiproton to proton ratio allows the relative influence of the baryon number transport and the antibaryon-baryon pair production mechanisms on the collision process to be investigated.

The measured antiparticle to particle ratios represent the ratio of the yields averaged over the rapidity range of 0.1<y_pi<1.3 and 0<y_{K,p}<0.8, and for transverse momenta of 0.1<p_{T}^{pi,K}<1.0GeV/c and 0.3<p_{T}^p<1.0GeV/c. In the d+Au collision system it is found that the relative yields of antiparticles to particles are independent of centrality for all three particle species, pions, kaons and protons. The <pi-/pi+> ratio at all centralities is consistent with one. In the top 10% most central events <pi-/pi+>=1.016+/-0.007(stat.)+/-0.019(syst.) and in the 60-100% most peripheral events <pi-/pi+>=0.996+/-0.008(stat.)+/-0.013(syst.). The <K-/K+> ratio ranges from 0.97+/-0.03(stat.)+/-0.04(syst.) in the top 10% most central events to 1.00+/-0.04(stat.)+/-0.03(syst.) in the 60-100% most peripheral events. The <pbar/p> ratio ranges from 0.86+/-0.02(stat.)+/-0.04(syst.) in the top 10% most central events to 0.85+/-0.02(stat.)+/-0.03(syst.) in the 60-100% most peripheral events. The results obtained for the p+p collision system are consistent with the values measured in d+Au collisions. Ratios of <pi-/pi+>=1.000+/-0.012(stat.)+/-0.019(syst.), <K-/K+>=0.93+/-0.05(stat.)+/-0.03(syst.), and <pbar/p>=0.85+/-0.04(stat.)+/-0.03(syst.) have been measured. The data are compared to results from model calculations, other collision systems and other collision energies.