Charged Kaon Production in p+p and d+Au Collisions, the Baseline Comparison Systems for Understanding Au+Au Collisions at RHIC

One of the primary challenges in modern nuclear physics is to understand the properties of hot nuclear matter. The expectation is that at sufficiently high energy densities, nuclear matter undergoes a phase transition where individual nucleons ‘dissolve’ and a plasma of freely moving quarks and gluons is formed. To accomplish this in the laboratory, normal nuclear matter is heated and compressed through collisions of heavy nuclei at relativistic energies.

The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory is a dedicated particle accelerator, capable of colliding nuclear beams to energies up to 100 GeV per nucleon per beam. Particle species ranging from protons (A=1) to gold (A=197) are accelerated in this state-of-the-art facility and collide at selected intersection points.

In this dissertation, a detailed transverse momentum (p_T) analysis is made at central rapidities, using the STAR Time Projection Chamber (TPC). The data set is comprised of about 10 million d+Au and about 6 million p+p events at 200 GeV. Previously analyzed data from a 2002 Au+Au run are also used. This work concentrates on the study of identified charged kaons (K⁺, K^-), which are the lightest strange mesons and hence the particles that dominate strangeness production. Charged kaons are identified using a topological reconstruction method which has relatively large p_T coverage.

In this dissertation, we present p_T and yield systematics. We find that the particle to anti-particle ratio is p_T independent in all colliding systems studied, an indication that in the p_T range studied, the pQCD regime is not reached yet. The ratios, close to unity, signal a rather net-baryon-free mid-rapidity region. The <p_T> in central d+Au collisions is larger than in peripheral Au+Au collisions, which might hint at the presence of ‘Cronin effect’ in the dAu system as explained.

We also obtain results on nuclear modification factors (R^dA_CP - central to peripheral ratio, R_dA, R_AA - geometrically scaled Au+Au(d+Au) to p+p ratios) which are presented for various mesons and baryons. In d+Au collisions, an enhancement compared to binary scaling of both R^dA_CP and R_dA is observed, an experimental observation called ‘Cronin effect’. This result is thought to be an initial-state effect. In contrast, the same ratio in central Au+Au collisions exhibits a suppression instead of an enhancement. This was understood in terms of a dense partonic medium which induces energy loss via gluon radiation by a high-energy parton traversing the medium, and leads, after fragmentation, to hadrons with lower <p _T >. The meson-baryon differences, first observed in Au+Au R^AA_CP, also exist in d+Au collisions.