Exploring Hadron Spectra in Small Collision Systems at PHENIX

Abstract

The paper presents the results on $\pi^{0},\pi^{\pm},K^{\pm},K^{*}$, $\phi$ and $p(\bar{p})$ production in small collision systems at $\sqrt{s_{NN}}=200$ GeV as a function of transverse momentum at midrapidity ($|\eta|<0.35$) measured by the PHENIX experiment.

1 INTRODUCTION

Fig. 1

The $\pi^{0}$, $\pi^{\pm}$, $K^{\pm}$, $K^{*}$, $\phi$ and $p(\bar{p})$ nuclear modification factors in (left) $p+\textrm{Al}$ and (right) ${}^{3}\textrm{He}+\textrm{Au}$ collisions at $\sqrt{s_{NN}}=200$ GeV at midrapidity ($\eta<0.35$).

The quark-gluon plasma QGP [1] formation in the relativistic heavy ion collisions [2] was supported by observation of various QGP effects such as strangeness and baryon enhancement [1]. However, the influence of cold nuclear mater effects (CNM) on particle production in large collision systems is still under consideration [3]. The study of light hadron production in small collision systems might help to interpret the results obtained in large collision systems and additionally provide a study of the minimal collision system size sufficient for observation of the QGP effects. To study effects affecting the particle production in ultrarelativistic collisions, nuclear modification factors $R_{AB}$ are used [2]. The $R_{AB}$ value is defined as a ratio of hadron production in nuclei-nuclei ($A+B$) collisions to its production in $p+p$ collisions, scaled by number of binary collisions. The deviation of $R_{AB}$ value from unity, might indicate the presence of QGP or CNM effects.

2 RESULTS

Figure 1 presents various light hadron ($\pi^{0}$, $\pi^{\pm}$, $K^{\pm}$, $K^{*}$, $\phi$, and $p(\bar{p})$) $R_{AB}$ measured in the most central $p+\textrm{Al}$ and ${}^{3}\textrm{He}+\textrm{Au}$ collisions at $\sqrt{s_{NN}}=200$ GeV. In both collision systems in the whole available $p_{T}$ range the $R_{AB}$ values of $K^{\pm}$, $K^{*}$, and $\phi$ mesons, containing (anti)strange quarks, are consistent with $R_{AB}$ values of $\pi^{0}$, $\pi^{\pm}$ mesons, that contain only first-generation quarks. In central $p+\textrm{Al}$ collisions $R_{AB}$ values of $\bar{p}$ show conformity with light meson $R_{AB}$ values. In ${}^{3}\textrm{He}+\textrm{Au}$ collisions (anti)proton yields are enhanced relatively to the binary scaled yields in $p+p$ collisions.

3 CONCLUSIONS

Values of $R_{AB}$ for all light mesons fall in the same curve in both $p$ +Al and ${}^{3}\textrm{He}+\textrm{Au}$ collisions. This might indicate that CNM effects are not responsible for the differences between light hadron $R_{AB}$ values seen in heavy ion collisions. Nonetheless, the proton $R_{AB}$ values are larger then light meson $R_{AB}$ values in ${}^{3}\textrm{He}+\textrm{Au}$ collisions. This suggests the baryon enhancement might be observed and QGP could be formed in ${}^{3}\textrm{He}+\textrm{Au}$ collisions, while $p+\textrm{Al}$ system size might be insufficient for observation of this effect.