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Register a new userNuclear-modification factor of charged hadrons at forward and backward rapidity in $p$$+$Al and $p$$+$Au collisions at $sqrt{s_{_{NN}}}=200$ GeV
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The PHENIX experiment has studied nuclear effects in $p$$+$Al and $p$$+$Au collisions at $sqrt{s_{_{NN}}}=200$ GeV on charged hadron production at forward rapidity ($1.4<eta<2.4$, $p$-going direction) and backward rapidity ($-2.2<eta<-1.2$, $A$-going direction). Such effects are quantified by measuring nuclear modification factors as a function of transverse momentum and pseudorapidity in various collision multiplicity selections. In central $p$$+$Al and $p$$+$Au collisions, a suppression (enhancement) is observed at forward (backward) rapidity compared to the binary scaled yields in $p$+$p$ collisions. The magnitude of enhancement at backward rapidity is larger in $p$$+$Au collisions than in $p$$+$Al collisions, which have a smaller number of participating nucleons. However, the results at forward rapidity show a similar suppression within uncertainties. The results in the integrated centrality are compared with calculations using nuclear parton distribution functions, which show a reasonable agreement at the forward rapidity but fail to describe the backward rapidity enhancement.
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We report on the nuclear dependence of transverse single-spin asymmetries (TSSAs) in the production of positively-charged hadrons in polarized $p^{uparrow}+p$, $p^{uparrow}+$Al and $p^{uparrow}+$Au collisions at $sqrt{s_{_{NN}}}=200$ GeV. The measurements have been performed at forward rapidity ($1.4<eta<2.4$) over the range of $1.8<p_{T}<7.0$ GeV$/c$ and $0.1<x_{F}<0.2$. We observed a positive asymmetry $A_{N}$ for positively-charged hadrons in polpp collisions, and a significantly reduced asymmetry in $p^{uparrow}$+$A$ collisions. These results reveal a nuclear dependence of charged hadron $A_N$ in a regime where perturbative techniques are relevant. These results provide new opportunities to use polpA collisions as a tool to investigate the rich phenomena behind TSSAs in hadronic collisions and to use TSSA as a new handle in studying small-system collisions.
Charmonium is a valuable probe in heavy-ion collisions to study the properties of the quark gluon plasma, and is also an interesting probe in small collision systems to study cold nuclear matter effects, which are also present in large collision systems. With the recent observations of collective behavior of produced particles in small system collisions, measurements of the modification of charmonium in small systems have become increasingly relevant. We present the results of $J/psi$ measurements at forward and backward rapidity in various small collision systems, $p$$+$$p$, $p$$+$Al, $p$$+$Au and $^3$He$+$Au, at $sqrt{s_{_{NN}}}$=200 GeV. The results are presented in the form of the observable $R_{AB}$, the nuclear modification factor, a measure of the ratio of the $J/psi$ invariant yield compared to the scaled yield in $p$$+$$p$ collisions. We examine the rapidity, transverse momentum, and collision centrality dependence of nuclear effects on $J/psi$ production with different projectile sizes $p$ and $^3$He, and different target sizes Al and Au. The modification is found to be strongly dependent on the target size, but to be very similar for $p$$+$Au and $^{3}$He$+$Au. However, for 0%--20% central collisions at backward rapidity, the modification for $^{3}$He$+$Au is found to be smaller than that for $p$$+$Au, with a mean fit to the ratio of $0.89pm0.03$(stat)${pm}0.08$(syst), possibly indicating final state effects due to the larger projectile size.
The fraction of $J/psi$ mesons which come from B-meson decay, $textrm{F}_{B{rightarrow}J/psi}$, is measured for J/$psi$ rapidity mbox{$1.2<|y|<2.2$} and $p_T>0$ in $p$+$p$ and Cu+Au collisions at $sqrt{s_{_{NN}}}$=200 GeV with the PHENIX detector. The extracted fraction is $textrm{F}_{B{rightarrow}J/psi}$ = 0.025 $pm$ 0.006(stat) $pm$ 0.010(syst) for $p$+$p$ collisions. For Cu+Au collisions, $textrm{F}_{B{rightarrow}J/psi}$ is 0.094 $pm$ 0.028(stat) $pm$ 0.037(syst) in the Au-going direction ($-2.2<y<-1.2$) and 0.089 $pm$ 0.026(stat) $pm$ 0.040(syst) in the Cu-going direction ($1.2<y<2.2$). The nuclear modification factor, $R_{rm CuAu}$, of B mesons in Cu+Au collisions is consistent with binary scaling of measured yields in $p$+$p$ at both forward and backward rapidity.
Event-by-event fluctuations of the ratio of inclusive charged to photon multiplicities at forward rapidity in Au+Au collision at $sqrt{s_{NN}}$=200 GeV have been studied. Dominant contribution to such fluctuations is expected to come from correlated production of charged and neutral pions. We search for evidences of dynamical fluctuations of different physical origins. Observables constructed out of moments of multiplicities are used as measures of fluctuations. Mixed events and model calculations are used as baselines. Results are compared to the dynamical net-charge fluctuations measured in the same acceptance. A non-zero statistically significant signal of dynamical fluctuations is observed in excess to the model prediction when charged particles and photons are measured in the same acceptance. We find that, unlike dynamical net-charge fluctuation, charge-neutral fluctuation is not dominated by correlation due to particle decay. Results are compared to the expectations based on the generic production mechanism of pions due to isospin symmetry, for which no significant (<1%) deviation is observed.
Semi-inclusive charged jet spectra per trigger at STAR are presented binned by event activity (EA) as determined by the Beam Beam Counter (BBC) signal in the Au-going direction. The selected EA determination is motivated by correlations between the number of charged tracks in the Time Projection Chamber (TPC) ($|eta|<1$) and EA ($eta_mathrm{EA}in[-5,-2]$) which are also presented. The jet spectra per trigger at high EA are suppressed relative to the spectra at low EA. A PYTHIA investigation refutes that the suppression results from a trivial autocorrelation between jet kinematics and the acceptance of the EA and the TPC.
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