First measurement of near-threshold J/ψ exclusive photoproduction off the proton
We report on the measurement of the γp → J/ψp cross section from Eγ=11.8 GeV down to the threshold at 8.2 GeV using a tagged photon beam with the GlueX experiment. We find the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section dσ/dt has an exponential slope of 1.67 ± 0.39 GeV−2 at 10.7 GeV average energy. The LHCb pentaquark candidates P+c can be produced in the s-channel of this reaction. We see no evidence for them and set model-dependent upper limits on their branching fractions B(P+c → J/ψp).
- Accepted by Physical Review Letters
- arXiv:1905.10811 [nucl-ex] (2017).
In summary, we have made the first measurement of the J/ψ exclusive photoproduction cross section from Eγ = 11.8 GeV down to the threshold, which provides important inputs to models of the gluonic structure of the proton at high x. The data were collected by the GlueX experiment located in Hall D at Jefferson Lab during 2016 and 2017, representing about 25% of the total data accumulated by the experiment to date. We measure the exclusive reaction γp → J/ψp, and identify the J/ψ by its decay into an electron-positron pair. We normalize the J/ψ total cross section to that of the non-resonant Bethe-Heitler process in the invariant mass range 1.20−2.50 GeV, thus canceling uncertainties from factors like luminosity and common detector efficiencies. The measured cross section is also used to set model-dependent upper limits on the branching fraction of the LHCb P+c states 90% confidence level of <4.6%, <2.3%, and <3.8% for P+c (4312), P+c(4440), and P+c(4457), respectively, which allow to discriminate between different pentaquark models.
|Electron-positron invariant mass spectrum from the data. The insert shows the J/ψ region fitted with a linear polynomial plus a Gaussian (fit parameters shown).|
|GlueX results for the J/ψ total cross section vs beam energy, compared to previous data [15,16], theoretical predictions [11, 13], and the JPAC model  for B(P+c(4440) → J/ψp) = 1.6% and JP = 3/2−. All curves are fitted/scaled to the GlueX data only. For our data the quadratic sums of statistical and systematic errors are shown; the overall normalization uncertainty is 27%.|
|Differential cross section for J/ψ photoproduction as a function of −(t − tmin) for 10.00 < Eγ < 11.80 GeV.|
|GlueX results for the J/ψ total cross section vs beam energy, Cornell , and SLAC , data compared to the JPAC model  corresponding to B(P+c(4312) → J/ψp) = 2.9%, B(P+c(4440) → J/ψp) = 1.6%, and B(P+c(4457) → J/ψp) = 2.7% for the JP = 3/2− case discussed in the paper.|
|The tagged photon luminosity as a function of beam energy.|
|p/E distribution of the two leptons. The background slice (2σ < p/E-1 < 4σ cut on the y-axis), and the slice containing the signal (- 3σ < p/E-1 < 2σ cut on the y-axis) are indicated with horizontal lines.|
| Left plot: the signal slice from Fig. 4 projected on the x-axis(black points) fitted with a background shape times a normalization parameter pnorm (blue line) plus a Gaussian (red line); the background shape is a polynomial fit of the projection of the background slice from Fig. 4 (blue points normalized by pnorm).
Right plot: the difference of the black and blue points from the left plot representing the electron/positron signal fitted with a Gaussian. The BH yield is assigned to the number of events within (-3σ,+2σ) of the peak (shaded histogram).