QCD corrections to J/psi polarisation in pp collisions at RHIC

TL;DR

Problem: predicting the J/psi cross section and polarization in pp collisions at RHIC using perturbative QCD. Approach: CS Model calculations including NLO gg/gq, LO cg, and NNLO* leading-P_T contributions, with a data-driven treatment of chi_c feed-down and ISR effects discussed. Findings: NLO corrections drive the J/psi polarization toward longitudinal in the direct yield; cg fusion contributes notably to yield with distinct polarization; prompt J/psi polarization is in good agreement with PHENIX data within uncertainties. Significance: supports the viability of the CS framework at RHIC, highlights the role of feed-down and high-order corrections, and motivates CNM studies and more complete resummation.

Abstract

We update the study of the polarisation of J/psi produced in proton-proton collisions at RHIC at sqrt(s)=200 GeV using the QCD-based Colour-Singlet Model (CSM), including next-to-leading order partonic matrix elements from gluon and light quark fusion and leading-order contributions from charm-quark initiated processes. To do so, we also evaluate the corresponding cross section differential in P_T which agrees qualitatively with the measurements of PHENIX in the central and forward regions at low P_T -- for instance below 2 GeV --, while emphasising the need for Initial State Radiation (ISR) resummation. At mid P_T, we also compare the measurements from PHENIX and STAR with the same evaluation complemented with the dominant alphaS^5 contributions (NNLO*). We find a reasonable agreement with the data. Regarding the polarisation, as shown for previous studies at larger sqrt(s) and P_T, the polarisation pattern from gluon and light quark fusion in the helicity frame is drastically modified at NLO and is shown to be increasingly longitudinal. The yield from charm-gluon fusion is found to be slightly transversally polarised. Combining both these contributions with a data-driven range for the polarisation of J/psi from chi_c, we eventually provide an evaluation of the polarisation of the prompt J/psi yield which is in a good agreement with the experimental data from PHENIX both in the central and forward regions.

Figures (10)

• Figure 1: Representative diagrams contributing to $^3S_1$ quarkonium (denoted ${\cal Q}$) hadroproduction in the CSM by gluon fusion at orders $\alpha_S^3$ (a), $\alpha_S^4$ (b,c,d) and initiated by a charm quark at orders $\alpha_S^3$ (e), $\alpha_S^4$ (f). The quark and antiquark attached to the ellipsis are taken as on-shell and their relative velocity $v$ is set to zero.
• Figure 2: $d\sigma/dy \times {\rm Br}$ in $pp$ collisions at $\hbox{$\sqrt{s_{NN}}$}=200\mathrm{~GeV}$ at LO and NLO accuracy compared to the PHENIX data Adare:2006kfRHIC2009. The theoretical-error bands for LO and NLO come from combining the uncertainties resulting from the choice of $\mu_f$, $\mu_r$, $m_q$. We have used the LO set CTEQ6_LPumplin:2002vw for the yield at LO, the NLO set CTEQ6_MPumplin:2002vw for the yield at NLO.
• Figure 3: $d\sigma/dP_T/dy \times {\rm Br}$ from the $gg$ and $gq$ fusion channels in $pp$ collisions at various order in $\alpha_S$ at $\hbox{$\sqrt{s_{NN}}$}=200\mathrm{~GeV}$: a) in the central ($|y|<0.35$), b) in the forward ($1.2 < |y|< 2.2$) regions compared to the PHENIX Adare:2006kf and STAR Abelev:2009qa data. The theoretical-error bands come from combining the uncertainties resulting from the choice of $\mu_f$, $\mu_r$, $m_q$, see text.
• Figure 4: $\alpha(P_T)$ at LO and NLO from $gg$ and $gc$ fusion in $pp$ at $\hbox{$\sqrt{s_{NN}}$}=200\mathrm{~GeV}$: a) in the central ($|y|<0.35$), b) in the forward ($1.2 < |y|< 2.2$) regions.
• Figure 5: $d\sigma/dy \times {\rm Br}$ from $cg$ fusion in $pp$ at $\hbox{$\sqrt{s_{NN}}$}=200\mathrm{~GeV}$ using a sealike charm distribution compared to the PHENIX data Adare:2006kf. The theoretical-error bands for LO and NLO come from combining the uncertainties resulting from the choice of $\mu_f$, $\mu_r$, $m_q$.