? meson production in p plus Al, p plus Au, d + Au, and He-3 +Au collisions at ?sNN=200 GeV

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 ?? 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 ?? 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 ?? 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.