meson production in Al, Au, Au, and HeAu collisions at GeV

Acharya, U. A.*; Hasegawa, Shoichi   ; Imai, Kenichi*; Sako, Hiroyuki   ; Sato, Susumu  ; Tanida, Kiyoshi   ; PHENIX Collaboration*; 365 of others*

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 Al, Au, and HeAu, as well as Au, at 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 GeV. However, the volume and the lifetime of the produced medium may be insufficient for observing strangeness-enhancement and jet-quenching effects. Comparison with calculations suggests that the main production mechanisms of mesons at midrapidity may be different in Al versus HeAu collisions at GeV. While thermal quark recombination seems to dominate in HeAu collisions, fragmentation seems to be the main production mechanism in Al collisions.