Multi-fragment decays of 129 Xe, 197 Au, and 238 U projectiles in collisions with Be, C, Al, Cu, In, Au, and U targets at energies between E/A = 400 MeV and 1000 MeV have been studied with the ALADIN forward-spectrometer at SIS. By adding an array of 84 SiCsI(Tl) telescopes the solid-angle coverage of the setup was extended to θ lab = 16 • . This permitted the complete detection of fragments from the projectile-spectator source.The dominant feature of the systematic set of data is the Z bound universality that is obeyed by the fragment multiplicities and correlations. These observables are invariant with respect to the entrance channel if plotted as a function of Z bound , where Z bound is the sum of the atomic numbers Z i of all projectile fragments with Z i ≥ 2. No significant dependence on the bombarding energy nor on the target mass is observed. The dependence of the fragment multiplicity on the projectile mass follows a linear scaling law.The reasons for and the limits of the observed universality of spectator fragmentation are explored within the realm of the available data and with model studies. It is found that the universal properties should persist up to much higher bombarding energies than explored in this work and that they are consistent with universal features exhibited by the intranuclear cascade and statistical multifragmentation models.

The centrality dependence of transverse momentum distributions and yields for ± , K ± , p, and p in Au + Au collisions at ͱ s NN = 200 GeV at midrapidity are measured by the PHENIX experiment at the Relativistic Heavy Ion Collider. We observe a clear particle mass dependence of the shapes of transverse momentum spectra in central collisions below ϳ2 GeV/ c in p T. Both mean transverse momenta and particle yields per participant pair increase from peripheral to midcentral and saturate at the most central collisions for all particle species. We also measure particle ratios of − / + , K − / K + , p / p, K / , p / , and p / as a function of p T and collision centrality. The ratios of equal mass particle yields are independent of p T and centrality within the experimental uncertainties. In central collisions at intermediate transverse momenta ϳ1.5-4.5 GeV/ c, proton and antiproton yields constitute a significant fraction of the charged hadron production and show a scaling behavior different from that of pions.

Measurements of the midrapidity transverse energy distribution, dET /dη, are presented for p+p, d+Au, and Au+Au collisions at √ s N N = 200 GeV and additionally for Au+Au collisions at √ s N N = 62.4 and 130 GeV. The dET /dη distributions are first compared with the number of nucleon participants Npart, number of binary collisions N coll , and number of constituent-quark participants Nqp calculated from a Glauber model based on the nuclear geometry. For Au+Au, dET /dη /Npart increases with Npart, while dET /dη /Nqp is approximately constant for all three energies. This indicates that the two component ansatz, dET /dη ∝ (1 − x)Npart/2 + xN coll , which has been used to represent ET distributions, is simply a proxy for Nqp, and that the N coll term does not represent a hard-scattering component in ET distributions. The dET /dη distributions of Au+Au and d+Au are then calculated from the measured p+p ET distribution using two models that both reproduce the Au+Au data. However, while the number-of-constituent-quark-participant model agrees well with the d+Au data, the additive-quark model does not.

No abstract

High energy proton-and deuteron-nucleus collisions provide an excellent tool for studying a wide array of physics effects, including modifications of parton distribution functions in nuclei, gluon saturation, and color neutralization and hadronization in a nuclear environment, among others. All of these effects are expected to have a significant dependence on the size of the nuclear target and the impact parameter of the collision, also known as the collision centrality. In this article, we detail a method for determining centrality classes in p(d) + A collisions via cuts on the multiplicity at backward rapidity (i.e., the nucleus-going direction) and for determining systematic uncertainties in this procedure. For d+Au collisions at √ s N N = 200 GeV we find that the connection to geometry is confirmed by measuring the fraction of events in which a neutron from the deuteron does not interact with the nucleus. As an application, we consider the nuclear modification factors R p(d)+A , for which there is a bias in the measured centrality-dependent yields due to auto correlations between the process of interest and the backward-rapidity multiplicity. We determine the bias-correction factors within this framework. This method is further tested using the hijing MC generator. We find that for d+Au collisions at √ s N N = 200 GeV, these bias corrections are small and vary by less than 5% (10%) up to pT =10 (20) GeV/c. In contrast, for p+Pb collisions at √ s N N = 5.02 TeV we find these bias factors are an order of magnitude larger and strongly pT dependent, likely due to the larger effect of multiparton interactions.