Measurement of the near-threshold J$/ψ$ photoproduction cross section with the CLAS12 experiment

Abstract

We present measurements of the total and differential cross sections for near-threshold J/$ψ$ photoproduction obtained with the CLAS12 detector at the Thomas Jefferson National Accelerator Facility. The results are based on data collected during the Fall 2018 and Spring 2019 running periods, using electron beams with energies of 10.6 and 10.2 GeV, respectively, scattered off a liquid-hydrogen target. Near-threshold J$/ψ$ photoproduction offers a unique sensitivity to the strong interaction in the non-perturbative regime of Quantum Chromodynamics (QCD). The energy dependence of the cross section constrains the underlying J$/ψ$ production mechanisms, including multi-gluon exchange and potential baryonic excitations. Additionally, the $t$-dependence of the differential cross section can be related to the transverse spatial distribution of gluons in the proton, providing critical input for theoretical descriptions of the gluonic structure of the proton. An interpretation of the results in terms of the gluon content of the proton is presented, providing new experimental constraints on QCD-inspired models of the proton structure and the role of gluonic degrees of freedom in hadronic mass generation.

Figures (11)

• Figure 1: Electron-photon angular correlation for radiative events (top) and missing mass spectrum of misidentified positron events (bottom). The green spectra were obtained without any additional BDT-based lepton identification. The orange spectra were obtained after the selection cut was applied on the output of the BDT-based lepton identification algorithm.
• Figure 2: Reconstructed incoming photon virtuality ${Q}^2$ as a function of the squared missing mass $M_X^2$ for selected photoproduction events in the Fall 2018 inbending dataset.
• Figure 3: Invariant mass of the electron-positron pair (top) and photon energy (bottom) for the selected photoproduction events in the Fall 2018 inbending dataset. Data points are the black circles. The MC contributions to the J$/\psi$ signal, the BH continuum, and the mixed-event background are represented using stacked histograms. The bottom panel of each plot shows the ratio of the data to the summed MC contributions.
• Figure 4: Invariant mass spectra of the electron-positron pair for selected photoproduction events in the J$/\psi$ mass region, combining all three datasets, and for three photon-energy bins. The J$/\psi$ yields were extracted by fitting the J$/\psi$ peak with a Gaussian signal and an exponentially decaying background, providing reasonable $\chi^2/\rm{ndf}$ (where $\rm ndf$ denotes the number of degrees of freedom of the fit). The bottom panel of each plot shows the pulls, defined as the difference between the measured and fitted yield, normalized by the statistical uncertainty of each data point.
• Figure 5: Total cross sections for J$/\psi$ photoproduction measured with the CLAS12 experiment (orange squares). The results from the GlueX experiment PhysRevC.108.025201 are shown as blue diamonds, and the single data point from Cornell PhysRevLett.35.1616 is shown as a red triangle. Vertical error bars represent statistical uncertainties, while the boxes indicate systematic uncertainties. For the GlueX measurement, the 19.5% normalization uncertainty reported in Ref.PhysRevC.108.025201 has been added in quadrature to the quoted systematic uncertainties.