Measurement of $J/ψ$ helicity distributions in inelastic photoproduction at HERA

TL;DR

This study measures the J/ψ decay helicity distributions in inelastic photoproduction at HERA using the ZEUS detector, extracting the polar and azimuthal parameters λ and ν in the target frame as functions of p_T and z. By leveraging a large data set and a detailed MC background treatment, the analysis compares the results to LO CS, LO CS+CO, k_T-factorisation, and an NLO CS calculation. The findings show that no single theoretical framework simultaneously describes the observed λ and ν across the explored kinematic range, challenging current understandings of J/ψ production mechanisms and constraining NRQCD and related approaches. The work provides precise helicity measurements that sharpen tests of perturbative QCD in photoproduction and guide future refinements of quarkonium production models.

Abstract

The $J/ψ$ decay angular distributions have been measured in inelastic photoproduction in $e p$ collisions with the ZEUS detector at HERA, using an integrated luminosity of 468 $pb^{-1}$. The range in photon-proton centre-of-mass energy, $W$, was 50 $< W <$ 180 $GeV$. The $J/ψ$ mesons were identified through their decay into muon pairs. The polar and azimuthal angles of the $μ^+$ were measured in the $J/ψ$ rest frame and compared to theoretical predictions at leading and next-to-leading order in QCD.

Figures (4)

• Figure 1: Dimuon invariant mass, m$_{\mu \mu}$, spectrum in the phase-space region $50 < W < 180$ GeV, $z > 0.1$ and $p_T >$ 1 GeV. The continuous line represents the fitted background.
• Figure 2: The helicity parameter $\lambda$, measured in the target frame, as a function of (a) $p_T$, and (b) $z$. The measurement is performed in the kinematic range $50 < W < 180$ GeV, $0.1 < z < 1$ and $p_T >$ 1 GeV. The measurement as a function of $p_T$ is restricted to the kinematic range $0.4 < z < 1$. The inner (outer) error bars correspond to the statistical (total) uncertainty. The theoretical curves are described in the text.
• Figure 3: The helicity parameter $\nu$, measured in the target frame, as a function of (a) $p_T$, and (b) $z$. The measurement is performed in the kinematic range $50 < W < 180$ GeV, $0.1 < z < 1$ and $p_T >$ 1 GeV. The measurement as a function of $p_T$ is restricted to the kinematic range $0.4 < z < 1$. The inner (outer) error bars correspond to the statistical (total) uncertainty. The theoretical curves are described in the text.
• Figure 4: Distributions of the helicity parameters (a), (c) $\lambda$ and (b), (d) $\nu$ as a function of $z$, measured in the target frame, for $50 < W < 180$ GeV, $0.1 < z < 0.9$ and (a), (b) $p_T >$ 2 GeV and (c), (d) $p_T >$ 3 GeV. The inner (outer) error bars correspond to the statistical (total) uncertainty. The theoretical bands are described in the text.