16599979
Global Properties of Nucleus-Nucleus Collisions
QCD as a theory of extended, strongly interacting matter is familiar from big bang evolution which, within the time interval from electro-weak decoupling (10−12 s) to hadron formation (5 × 10−6 s), is dominated by the expansion of quark–gluon matter, a color conducting plasma that is deconfined. In the 1970s deconfinement was predicted [1–5] to arise from the newly discovered “asymptotic freedom” property of QCD; i.e., the plasma was expected to be a solution of perturbative QCD at asymptotically high square momentum transfer Q 2, or temperature T
120318226
Hadron ratio fluctuations in heavy-ion collisions
18832454
Event-by-Event Fluctuations and the Search for the Critical Point within the NA49 Experiment
In heavy-ion collisions in the energy regime probed at the CERN SPS, experimental hints for the deconfinement phase transition have been seen in numerous inclusive hadronic observables. In order to further characterize this transition, and in the pursuit of indications for the expected critical point of strongly interacting matter, the NA49 collaboration has conducted analyses of the event-by-event fluctuations of various hadronic observables. A selection of these results will be presented and discussed in the light of theoretical predictions. Among these are new results on hadron ratio fluctuations, in particular K/p fluctuations and their potential connection to the correlation between strangeness and baryon number, thus revealing the basic degrees of freedom produced in heavy-ion collisions.
5536378
Hadron formation in relativistic nuclear collisions and the QCD phase diagram.
We study particle production in ultrarelativistic nuclear collisions at CERN SPS and LHC energies and the conditions of chemical freeze-out. We determine the effect of the inelastic reactions between hadrons occurring after hadronization and before chemical freeze-out employing the ultrarelativistic quantum molecular dynamics hybrid model. The differences between the initial and the final hadronic multiplicities after the rescattering stage resemble the pattern of data deviation from the statistical equilibrium calculations. By taking these differences into account in the statistical model analysis of the data, we are able to reconstruct the original hadrochemical equilibrium points in the (T, μ(B)) plane which significantly differ from chemical freeze-out ones and closely follow the parton-hadron phase boundary recently predicted by lattice quantum chromodynamics.
119462879
High pT spectra of identified particles produced in Pb+Pb collisions at 158 A GeV beam energy
Results of the NA49 collaboration on the production of hadrons with large transverse momentum in Pb+Pb collisions at 158 A GeV beam energy are presented. A range of up to pT = 4 GeV/c is covered. The nuclear modification factor RCP is extracted for π±, K± and p, and the baryon to meson ratios and Λ/K0s are studied. All results are compared to other measurements at SPS and RHIC and to theoretical calculations.
120107314
Fluctuation signals and the critical point
We explore the potential of fluctuation measurements to investigate the properties of hot and dense matter created in relativistic heavy-ion collisions to investigate signals expected from the critical point and the first-order phase transition. Contrary to calculations in a grand canonical ensemble we combine the dynamics of the systems with the nonequilibrium propagation of the order parameter of chiral symmetry. We find an enhancement of the fluctuations of the order parameter at the phase transition. Furthermore, we study the effect of exact baryon charge conservation on the net-baryon and the net-proton kurtosis for central (b ? 2.75 fm) Pb+Pb/Au+Au collisions from Elab = 2A GeV to GeV from the UrQMD model. The results are compared to experimentally applied cuts and corrections.
119250225
Comparing efficient computation methods for massless QCD tree amplitudes: Closed analytic formulas versus Berends-Giele recursion
Recent advances in our understanding of tree-level QCD amplitudes in the massless limit exploiting an effective (maximal) supersymmetry have led to the complete analytic construction of tree-amplitudes with up to four external quark-anti-quark pairs. In this work we compare the numerical efficiency of evaluating these closed analytic formulae to a numerically efficient implementation of the Berends-Giele recursion. We compare calculation times for tree-amplitudes with parton numbers ranging from 4 to 25 with no, one, two and three external quark lines. We find that the exact results are generally faster in the case of MHV and NMHV amplitudes. Starting with the NNMHV amplitudes the Berends-Giele recursion becomes more efficient. In addition to the runtime we also compared the numerical accuracy. The analytic formulae are on average more accurate than the off-shell recursion relations though both are well suited for complicated phenomenological applications. In both cases we observe a reduction in the average accuracy when phase space configurations close to singular regions are evaluated. We believe that the above findings provide valuable information to select the right method for phenomenological applications.