Mid-rapidity Charged Multiplicity Research Articles

Monte Carlo Glauber model with meson cloud: predictions for 5.44 TeV Xe + Xe collisions

We study, within the Monte-Carlo Glauber model, centrality dependence of the midrapidity charged multiplicity density dN_{ch}/deta and of the anisotropy coefficients varepsilon _{2,3} in Pb + Pb collisions at sqrt{s}=5.02 TeV and in Xe + Xe collisions at sqrt{s}=5.44 TeV. Calculations are performed for versions with and without nucleon meson cloud. The fraction of the binary collisions, alpha , has been fitted to the data on dN_{ch}/deta in Pb + Pb collisions. We obtain alpha approx 0.09(0.13) with (without) meson cloud. The effect of meson cloud on the dN_{ch}/deta is relatively small. For Xe + Xe collisions for 0–5% centrality bin we obtain dN_{ch}/deta approx 1149 and 1134 with and without meson cloud, respectively. We obtain varepsilon _2(mathrm {Xe})/varepsilon _2(mathrm {Pb})sim 1.45 for most central collisions, and varepsilon _2(mathrm {Xe})/varepsilon _2(mathrm {Pb}) close to unity at . We find a noticeable increase of the eccentricity in Xe + Xe collisions at small centralities due to the prolate shape of the Xe nucleus. The triangularity in Xe + Xe collisions is bigger than in Pb + Pb collisions at . We obtain varepsilon _3(text{ Xe })/varepsilon _3(text{ Pb })sim 1.3 at .

Constituent quarks and systematic errors in mid-rapidity charged multiplicity dNch/dη distributions

Centrality definition in A + A collisions at colliders such as RHIC and LHC suffers from a correlated systematic uncertainty caused by the efficiency of detecting a p + p collision ([Formula: see text] for PHENIX at RHIC). In A + A collisions where centrality is measured by the number of nucleon collisions, [Formula: see text], or the number of nucleon participants, [Formula: see text], or the number of constituent quark participants, [Formula: see text], the error in the efficiency of the primary interaction trigger (Beam–Beam Counters) for a p + p collision leads to a correlated systematic uncertainty in [Formula: see text], [Formula: see text] or [Formula: see text] which reduces binomially as the A + A collisions become more central. If this is not correctly accounted for in projections of A + A to p + p collisions, then mistaken conclusions can result. A recent example is presented in whether the mid-rapidity charged multiplicity per constituent quark participant [Formula: see text] in Au + Au at RHIC was the same as the value in p + p collisions.

Monte Carlo Glauber wounded nucleon model with meson cloud

We study the effect of the nucleon meson cloud on predictions of the Monte Carlo Glauber wounded nucleon model for $AA$, $pA$, and $pp$ collisions. From the analysis of the data on the charged multiplicity density in $AA$ collisions we find that the meson-baryon Fock component reduces the required fraction of binary collisions by a factor of $\sim 2$ for Au+Au collisions at $\sqrt{s}=0.2$ TeV and $\sim 1.5$ for Pb+Pb collisions at $\sqrt{s}=2.76$ TeV. For central $AA$ collisions the meson cloud can increase the multiplicity density by $\sim 16-18$\%. We give predictions for the midrapidity charged multiplicity density in Pb+Pb collisions at $\sqrt{s}=5.02$ TeV for the future LHC run 2. We find that the meson cloud has a weak effect on the centrality dependence of the ellipticity $\epsilon_2$ in $AA$ collisions. For collisions of the deformed uranium nuclei at $\sqrt{s}=0.2$ TeV we find that the meson cloud may improve somewhat agreement with the data on the dependence of the elliptic flow on the charged multiplicity for very small centralities defined via the ZDCs signals. We find that the meson cloud may lead to a noticeable reduction of $\epsilon_2$ and the size of the fireball in $pA$ and $pp$ collisions.