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Register a new userHadron yields and fluctuations at the CERN Super Proton Synchrotron: system size dependence from Pb+Pb to p+p collisions
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The kaon to pion ratio $K^+/pi^+$ and the scaled variance $omega^-$ for fluctuations of negatively charged particles are studied within the statistical hadron resonance gas (HRG) model and the Ultra relativistic Quantum Molecular Dynamics (UrQMD) transport model. The calculations are done for p+p, Be+Be, Ar+Sc, and Pb+Pb collisions at the CERN Super Proton Synchrotron energy range to reveal the system size dependence of hadron production. For the HRG calculations the canonical ensemble is imposed for all conserved charges. In the UrQMD simulations the centrality selection in nucleus-nucleus collisions is done by calculating the forward energy $E_{rm F}$ deposited in the Projectile Spectator Detector, and the acceptance maps of the NA61/SHINE detectors are used. A comparison of the HRG and UrQMD results with the data of the NA61/SHINE Collaboration is done. To understand a difference of the event-by-event fluctuations in p+p and heavy ion collisions the centrality selection procedure in the sample of all inelastic p+p events is proposed and analyzed within the UrQMD simulations.
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Effects of strong longitudinal colour electric fields (SCF), shadowing, and quenching on the open prompt charm mesons (D$^0$, D$^+$, D$^{*+}$, D${_s}{^+}$) production in central Pb + Pb collisions at $sqrt{s_{rm NN}}$ = 2.76 TeV are investigated within the framework of the {small HIJING/B=B v2.0} model. We compute the nuclear modification factor $R_{rm PbPb}^{rm D}$, and show that the above nuclear effects constitute important dynamical mechanisms in the description of experimental data. The strength of colour fields (as characterized by the string tension $kappa$), partonic energy loss and jet quenching process lead to a suppression factor consistent with recent published data. Predictions for future beauty mesons measurements have been included. Ratios of strange to non-strange prompt charm mesons in central Pb + Pb and minimum bias (MB) $ p + p$ collisions at 2.76 TeV are also discussed. Minimum bias $p + p$ collisions which constitute theoretical baseline in our calculations are studied at the centre of mass energies $sqrt{s}$ = 2.76 TeV and 7 TeV.
In a framework of a semi-analytic model with longitudinally extended strings of fluctuating end-points, we demonstrate that the rapidity spectra and two-particle correlations in collisions of Pb-Pb, p-Pb, and p-p at the energies of the Large Hadron Collider can be universally reproduced. In our approach, the strings are pulled by wounded constituents appearing in the Glauber modeling at the partonic level. The obtained rapidity profile for the emission of hadrons from a string yields bounds for the distributions of the end-point fluctuations. Then, limits for the two-particle-correlations in pseudorapidity can be obtained. Our results are favorably compared to recent experimental data from the ATLAS Collaboration.
Collinear factorized perturbative QCD model predictions are compared for p+Pb at 4.4A TeV to test nuclear shadowing of parton distribution at the Large Hadron Collider (LHC). The nuclear modification factor (NMF), R_{pPb}(y=0,p_T<20 GeV/c) = dn_{p Pb} /(N_{coll}(b)dn_{pp}), is computed with electron-nucleus (e+A) global fit with different nuclear shadow distributions and compared to fixed Q^2 shadow ansatz used in Monte Carlo Heavy Ion Jet Interacting Generator (HIJING) type models. Due to rapid DGLAP reduction of shadowing with increasing Q^2 used in e+A global fit, our results confirm that no significant initial state suppression is expected (R_{pPb} (p_T) = 1 pm 0.1) in the p_T range 5 to 20 GeV/ c. In contrast, the fixed Q^2 shadowing models assumed in HIJING type models predict in the above p_T range a sizable suppression, R_{pPb} (p_T) = 0.6-0.7 at mid-pseudorapidity that is similar to the color glass condensate (CGC) model predictions. For central (N_{coll} = 12) p+ Pb collisions and at forward pseudorapidity (eta = 6) the HIJING type models predict smaller values of nuclear modification factors (R_{pPb}(p_T)) than in minimum bias events at mid-pseudorapidity (eta = 0). Observation of R_{pPb}(p_T= 5-20 GeV/c) less than 0.6 for minimum bias p+A collisions would pose a serious difficulty for separating initial from final state interactions in Pb+Pb collisions at LHC energies.
The elliptic and triangular flow coefficients ($mathrm{v_n, , n=2,3}$) measured in Pb+Pb ($sqrt{s_{_{rm NN}}} = 2.76$ TeV) and p+Pb ($sqrt{s_{_{rm NN}}} = 5.02$ TeV) collisions, are studied as a function of initial-state eccentricity ($varepsilon_n$), and dimensionless size characterized by the cube root of the mid-rapidity charged hadron multiplicity density $mathrm{left< N_{ch} right>^{1/3}}$. The results indicate that the influence of eccentricity ($mathrm{v_n} propto varepsilon_n$) observed for large $mathrm{left< N_{ch} right>}$, is superseded by the effects of viscous attenuation for small $mathrm{left< N_{ch} right>}$, irrespective of the colliding species. Strikingly similar acoustic scaling patterns of exponential viscous modulation, with a damping rate proportional to $mathrm{n^2}$ and inversely proportional to the dimensionless size, are observed for the eccentricity-scaled coefficients for the two sets of colliding species. The resulting scaling parameters suggest that, contrary to current predilections, the patterns of viscous attenuation, as well as the specific shear viscosity $left<frac{eta}{s}(text{T})right>$ for the matter created in p+Pb and Pb+Pb collisions, are comparable.
New measurements by the NA49 experiment of the centrality dependence of event-by-event fluctuations of the particle yield ratios (K$^{+}$+K$^{-}$)/($pi^{+}+pi^{-}$), (p+$bar{text p}$)/($pi^{+}+pi^{-}$), and (K$^{+}$+K$^{-}$)/(p+$bar{text p}$) are presented for Pb+Pb collisions at 158$A$ GeV. The absolute values of the dynamical fluctuations of these ratios, quantified by the measure $sigma_{text{dyn}}$, increase by about a factor of two from central to semi-peripheral collisions. Multiplicity scaling scenarios are tested and found to apply for both the centrality and the previously published energy dependence of the (K$^{+}$+K$^{-}$)/($pi^{+}+pi^{-}$) and (p+$bar{text p}$)/($pi^{+}+pi^{-}$) ratio fluctuations. A description of the centrality and energy dependence of (K$^{+}$+K$^{-}$)/(p+$bar{text p}$) ratio fluctuations by a common scaling prescription is not possible since there is a sign change in the energy dependence.
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