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Register a new userTransverse Velocity Scaling in Au+Au Fragmentation
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Invariant transverse-velocity spectra of intermediate-mass fragments were measured with the 4-pi multi-detector system INDRA for collisions of Au on Au at incident energies between 40 and 150 MeV per nucleon. Their scaling properties as a function of incident energy and atomic number Z are used to distinguish and characterize the emissions in (i) peripheral collisions at the projectile and target rapidities, and in (ii) central and (iii) peripheral collisions near mid-rapidity. The importance of dynamical effects is evident in all three cases and their origin is discussed.
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We report measurements of charmed hadron production from hadronic ($D^{0}rightarrow Kpi$) and semileptonic ($mu$ and $e$) decays in 200 GeV Au+Au collisions at RHIC. Analysis of the spectra indicates that charmed hadrons have a different radial flow pattern from light or multi-strange hadrons. Charm cross sections at mid-rapidity are extracted by combining the three independent measurements, covering the transverse momentum range that contributes to $sim$90% of the integrated cross section. The cross sections scale with number of binary collisions of the initial nucleons, a signature of charm production exclusively at the initial impact of colliding heavy ions. The implications for charm quark interaction and thermalization in the strongly interacting matter are discussed.
Transverse momentum spectra of pions, kaons, protons and antiprotons from Au+Au collisions at sqrt(s_(NN)) = 62.4 GeV have been measured by the PHOBOS experiment at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The identification of particles relies on three different methods: low momentum particles stopping in the first detector layers; the specific energy loss (dE/dx) in the silicon Spectrometer, and Time-of-Flight measurement. These methods cover the transverse momentum ranges 0.03-0.2, 0.2-1.0 and 0.5-3.0 GeV/c, respectively. Baryons are found to have substantially harder transverse momentum spectra than mesons. The pT region in which the proton to pion ratio reaches unity in central Au+Au collisions at sqrt(s_(NN)) = 62.4 GeV fits into a smooth trend as a function of collision energy. At low transverse momenta, the spectra exhibit a significant deviation from transverse mass scaling, and when the observed particle yields at very low pT are compared to extrapolations from higher pT, no significant excess is found. By comparing our results to Au+Au collisions at sqrt(s_(NN)) = 200 GeV, we conclude that the net proton yield at midrapidity is proportional to the number of participant nucleons in the collision.
Multiparticle production at high energies is described in terms of color strings stretched between the projectile and target. As string density increases, overlap among the strings leads to cluster formation. At some critical density a macroscopic cluster appears, spanning the entire system. This marks the percolation phase transition. Data from p+p, d+Au and Au+Au collisions at 200 GeV has been analyzed using the STAR detector to obtain the percolation density parameter, $eta$. For 200 GeV Au+Au collisions, the value of $eta$ is found to lie above the critical percolation threshold, while for 200 GeV d+Au collisions it is below the critical value. This supports the idea of string percolation, which at high enough string density is a possible mechanism to explore the hadronic phase transition to a quark-gluon plasma.
New measurements by the PHENIX experiment at RHIC for eta production at midrapidity as a function of transverse momentum (p_T) and collision centrality in sqrt(s_NN) = 200 GeV Au+Au and p+p collisions are presented. They indicate nuclear modification factors (R_AA) that are similar both in magnitude and trend to those found in earlier pi^0 measurements. Linear fits to R_AA in the 5--20 GeV/c p_T region show that the slope is consistent with zero within two standard deviations at all centralities although a slow rise cannot be excluded. Having different statistical and systematic uncertainties the pi^0 and eta measurements are complementary at high p_T; thus, along with the extended p_T range of these data they can provide additional constraints for theoretical modeling and the extraction of transport properties.
The jet fragmentation function is measured with direct photon-hadron correlations in p+p and Au+Au collisions at sqrt(s_NN)=200 GeV. The p_T of the photon is an excellent approximation to the initial p_T of the jet and the ratio z_T=p_T^h/p_T^gamma is used as a proxy for the jet fragmentation function. A statistical subtraction is used to extract the direct photon-hadron yields in Au+Au collisions while a photon isolation cut is applied in p+p. I_ AA, the ratio of jet fragment yield in Au+Au to that in p+p, indicates modification of the jet fragmentation function. Suppression, most likely due to energy loss in the medium, is seen at high z_T. The fragment yield at low z_T is enhanced at large angles. Such a trend is expected from redistribution of the lost energy into increased production of low-momentum particles.
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