Study of strange mesons in p+p, d+Au and Cu+Cu collisions at = 200 GeV

The PHENIX experiment at RHIC has measured invariant transverse momentum (pT) spectra and nuclear modification factor (RAB) of strange mesons K*0 and K0S, in p+p, d+Au and Cu+Cu collisions at center of mass energy 200 GeV. The pT range of these measurements spans from 1.1 to 8.5 GeV/c for K*0 and from 2 to 13 GeV/c for K0S. Similar to the other light-quark mesons measurements, both these strange mesons show no cold-nuclear- matter effects in the measured pT range in d+Au collisions. The nuclear modification factor for d+Au (RdAu) ≈ 1 and is almost constant as a function of pT. In case of Cu+Cu peripheral collisions, no suppression is registered with respect to the p+p yields scaled with number of binary collisions. In central Cu+Cu collisions, both mesons suffer substantial amount of suppression at high pT (> 5 GeV/c), which is similar to the suppression suffered by light-quark mesons. In the intermediate pT range (2 < pT < 5 (GeV/c)), the strange mesons are less suppressed than light-quark mesons (π0) and more suppressed than baryons (p, p).


Introduction
Lattice QCD calculations predict phase transition from hadronic phase to quark gluon plasma (QGP) above an energy density of 1 GeV/fm 3 [1]. The high energy heavy ion colliders, RHIC and LHC, provide the means to create a quark gluon plasma in the laboratory and study its properties. Particles traversing this hot/dense medium suffer significant energy loss which results in the modification of fragmentation functions and softening of the particle spectra. The modification of the spectra due to the medium can be quantified by the "nuclear modification factor", defined as the ratio of the yield in heavy ion collisions (A+B) to the yield in p+p collisions scaled by the number of binary collisions (N coll ). Analytically, it is expressed as, R AB = d 2 N AB /dy dp T N coll . d 2 Npp/dy dp T . The deviation of the value of R AB from 1, is a manifestation of medium effects. In heavy ion collisions, the energy loss by high p T partons is reflected by the suppression of the yield (i.e. R AB < 1). Figure 1 shows the R AA for different particles for the most central Au+Au collisions at √ s N N = 200 GeV. It is seen that for p T > 5 GeV/c, the R AA value is around 0.2 which is similar for all particles irrespective of quark content. At intermediate p T (2 < p T GeV/c < 5), the medium-effects for particles with different quark content are different. Different bulk phenomenons (e.g. flow, recombination, Cronin effect) may play important roles towards hadron production in this p T range. The measurement of strange hadrons is important to study collective effects in the intermediate p T and the parton energy-loss effects in the high p T range. These proceedings present the overview of K 0 S and K * 0 measurements at midrapidity by PHENIX detector in p+p, d+Au and Cu+Cu collisions at √ s N N = 200 GeV. The measurements Figure 1. (color online) Nuclear modification factor as a function of p T for π 0 , p, γ, η, φ, J/ψ, ω, e ± , K ± for the most central Au+Au collisions at √ s N N = 200 GeV. Plot taken from Ref. [3].
include the invariant yield transverse momentum spectra and nuclear modification factor for different centrality bins [2]. This article mainly focuses on the results related to the nuclear modification factor obtained in d+Au and Cu+Cu collisions.

PHENIX detector
The results presented here are obtained with the help of global, tracking and PID detectors of PHENIX [4].

Analysis
The K 0 S and K * 0 mesons are reconstructed from K 0 S → π 0 (→ γγ)π 0 (→ γγ) and K * 0 → K ± π ∓ channels respectively. K 0 S reconstruction is done in two steps. Initially, π 0 s are reconstructed from photon pairs after applying kinematic and analysis cuts. Then K 0 S meson is reconstructed from π 0 pairs within the same event. For K * 0 reconstruction, a pair of oppositely charged tracks with p T > 0.3 GeV/c is required. To have measurement in a wider p T range, three different exclusive and statistically independent techniques are used for K * 0 analysis. In one of the techniques both tracks are identified in TOF as pions and kaons; it is used to measure K * 0 production at low p T . In other approach, tracks are not identified but matched in PC3 and given the masses of kaons and pions. This technique extends the measurement to highest possible p T . The intermediate p T range is obtained by identifying kaon in TOF and matching the other track in PC3 before assigning the mass of pion. For analysis details one may refer [2].  [7] are also shown.

Results
The nuclear modification factor for K 0 S and K * 0 in d+Au collisions at √ s N N = 200 GeV for the most central and the most peripheral bins are shown in Fig. 2. A comparison with π 0 [5], φ [6] and protons [7] are also shown. The statistical and systematic errors are shown with error bars and boxes respectively. It is observed that R dAu is consistent with unity within uncertainties for all measured centrality bins for p T > 1 GeV/c. In the most central bin, there is a slight hint of Cronin enhancement in the intermediate p T (2-5 GeV/c) range and suppression at p T > 6-8 GeV/c. The comparisons with π 0 and φ mesons show that cold nuclear matter effects do not play a significant role in these mesons productions. However, the proton shows an enhancement in the central collisions at intermediate p T , which can be explained by recombination models [8].
The nuclear modification factor for K 0 S and K * 0 along with π 0 [9] and φ [6] in are also shown. Right panel : Comparison of the nuclear modification factor of π 0 [9], φ [6], and K * 0 in Cu+Cu collisions and proton [7] and kaon [7] in Au+Au collisions at √ s N N = 200 GeV.
For details see text. GeV/c.

Conclusions
Measurements of K 0 S and K * 0 mesons via π 0 π 0 and K ± π ∓ decays respectively, in p+p, d+Au and Cu+Cu collisions at √ s N N = 200 GeV, with PHENIX are presented. In d+Au collisions, in the measured p T range, no significant cold nuclear matter effect is seen for both mesons. In Cu+Cu collisions, no suppression is observed in peripheral collisions. In the most central collisions, for p T > 5 GeV/c, K 0 S and K * 0 show a suppression similar to π 0 and φ within uncertainties. However, for 2 < p T (GeV/c) < 5, baryons show no suppression, π 0 s are the most suppressed species and particles with strange quark content show an intermediate behavior. These results provide additional constraints to the modelisation of hadron energy loss in medium.