The PHENIX experiment has performed a systematic study of identified charged-hadron ($π^±$, $K^±$, $p$, $\overline{p}$) production at midrapidity in $p$ + Al, 3He +Au, and Cu + Au collisions at ...$\sqrt{s_{NN}}$ = 200 GeV and U + U collisions at $\sqrt{s_{NN}}$ = 193 GeV. Identified charged-hadron invariant transverse-momentum ($p_T$) and transverse-mass ($m_T$) spectra are presented and interpreted in terms of radially expanding thermalized systems. The particle ratios of $K/π$ and $p/π$ have been measured in different centrality ranges of large (Cu + Au and U + U) and small ($p$ + Al and 3He +Au) collision systems. The values of $K/π$ ratios measured in all considered collision systems were found to be consistent with those measured in $p + p$ collisions. However, the values of $p/π$ ratios measured in large collision systems reach the values of ≈0.6, which is a factor of ≈2 larger than in $p + p$ collisions. These results can be qualitatively understood in terms of the baryon enhancement expected from hadronization by recombination. Identified charged-hadron nuclear-modification factors ($R_{AB}$) are also presented. Enhancement of proton $R_{AB}$ values over meson $R_{AB}$ values was observed in central 3He +Au, Cu + Au, and U + U collisions. Here, the proton $R_{AB}$ values measured in the $p$ + Al collision system were found to be consistent with $R_{AB}$ values of $Φ$, $π^±$, $K^±$, and $π^0$ mesons, which may indicate that the size of the system produced in $p$ + Al collisions is too small for recombination to cause a noticeable increase in proton production.
We present a measurement of the transverse single-spin asymmetry for π0 and η mesons in p↑+ p collisions in the pseudorapidity range |η| < 0.35 and at a center-of-mass energy of 200 GeV with the ...PHENIX detector at the Relativistic Heavy Ion Collider. In comparison with previous measurements in this kinematic region, these results have factor-of-3-smaller uncertainties. As hadrons, π0 and η mesons are sensitive to both initial- and final-state nonperturbative effects for a mix of parton flavors. Comparisons of the differences in their transverse single-spin asymmetries have the potential to disentangle the possible effects of strangeness, isospin, or mass. These results can constrain the twist-3 trigluon collinear correlation function as well as the gluon Sivers function.
The PHENIX Collaboration has measured the ratio of the yields of ψ(2S) to ψ(1S) mesons produced in p+p, p+Al, p+Au, and He3+Au collisions at sNN=200 GeV over the forward and backward rapidity ...intervals 1.2<|y|<2.2. We find that the ratio in p+p collisions is consistent with measurements at other collision energies. In collisions with nuclei, we find that in the forward (p-going or He3-going) direction, the relative yield of ψ(2S) mesons to ψ(1S) mesons is consistent with the value measured in p+p collisions. However, in the backward (nucleus-going) direction, the ψ(2S) meson is preferentially suppressed by a factor of ∼2. This suppression is attributed in some models to the breakup of the weakly bound ψ(2S) meson through final-state interactions with comoving particles, which have a higher density in the nucleus-going direction. These breakup effects may compete with color screening in a deconfined quark-gluon plasma to produce sequential suppression of excited quarkonia states.
Small nuclear collisions are mainly sensitive to cold-nuclear-matter effects; however, the collective behavior observed in these collisions shows a hint of hot-nuclear-matter effects. The ...identified-particle spectra, especially the ϕ mesons which contain strange and antistrange quarks and have a relatively small hadronic-interaction cross section, are a good tool to study these effects. The PHENIX experiment has measured ϕ mesons in a specific set of small collision systems p+Al, p+Au, and 3He+Au, as well as d+Au Adare et al., Phys. Rev. C 83, 024909 (2011), at √sNN=200 GeV. The transverse-momentum spectra and nuclear-modification factors are presented and compared to theoretical-model predictions. The comparisons with different calculations suggest that quark-gluon plasma may be formed in these small collision systems at √sNN=200 GeV. However, the volume and the lifetime of the produced medium may be insufficient for observing strangeness-enhancement and jet-quenching effects. The comparison with calculations suggests that the main production mechanisms of ϕ mesons at midrapidity may be different in p+Al versus p/d/3He+Au collisions at √sNN=200 GeV. While thermal quark recombination seems to dominate in p/d/3He+Au collisions, fragmentation seems to be the main production mechanism in p+Al collisions.
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