Abstract
We report on a polarization measurement of inclusive J/ψ mesons in the di-electron decay channel at mid-rapidity at 2<pT<6 GeV/c in p+p collisions at s=200 GeV. Data were taken with the STAR detector at RHIC. The J/ψ polarization measurement should help to distinguish between different models of the J/ψ production mechanism since they predict different pT dependences of the J/ψ polarization. In this analysis, J/ψ polarization is studied in the helicity frame. The polarization parameter λθ measured at RHIC becomes smaller towards high pT, indicating more longitudinal J/ψ polarization as pT increases. The result is compared with predictions of presently available models.
Highlights
The J /ψ is a bound state of charm (c) and anti-charm (c) quarks
A high-predict different transverse momentum (pT) J /ψ polarization measurement is of particular interest since it can help to discriminate between the models
The trend seen in the STAR and PHENIX results is towards longitudinal J /ψ polarization with increasing pT, and a linear fit to the Relativistic Heavy Ion Collider (RHIC) data has a negative slope parameter
Summary
The J /ψ is a bound state of charm (c) and anti-charm (c) quarks. Charmonia physical states have to be colorless, they can be formed via a color-singlet or a color-octet intermediate cc state. For the lower pT range at RHIC energies, the LO NRQCD calculations [26] and NLO+ CSM [28] have similar predictions regarding the J /ψ polarization, which is longitudinal, and describe the experimental results [27] well. These models predict different pT dependence: in the case of the NRQCD prediction, the trend is towards the transverse polarization with increasing pT , while the NLO+ CSM shows almost no pT dependence. The obtained result is compared with predictions of NLO+ CSM [28] and LO NRQCD calculations (COM) [26]
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