First Measurement of Near-Threshold J/ψ Exclusive Photoproduction off the Proton

Area of Science:

• High-energy physics
• Quantum chromodynamics
• Particle physics

Background:

• The J/ψ meson is a probe of the strong force.
• Understanding J/ψ meson production in photon-proton interactions is crucial for probing the internal structure of protons and the nature of the strong interaction.
• Previous models, such as two-gluon exchange, predicted a steeper fall in the cross section near the production threshold.

Purpose of the Study:

• To measure the γp→J/ψp cross section from 11.8 GeV down to the 8.2 GeV threshold.
• To compare the measured cross section with predictions from theoretical models, particularly two-gluon exchange.
• To search for evidence of LHCb pentaquark candidates (Pc+) produced via s-channel J/ψp production and set limits on their properties.

Main Methods:

• Utilized a tagged photon beam at the GlueX experiment.
• Measured the J/ψp cross section across a range of photon energies (8.2–11.8 GeV).
• Analyzed differential cross section data (dσ/dt) to determine its energy dependence.

Main Results:

• The total cross section falls less steeply toward the threshold than predicted by two-gluon exchange models.
• The differential cross section dσ/dt exhibits an exponential slope of 1.67±0.39 GeV⁻² at an average energy of 10.7 GeV.
• No evidence for the production of LHCb pentaquark candidates (Pc+) was observed.

Conclusions:

• The observed cross section behavior challenges current theoretical descriptions of J/ψ meson production near threshold.
• Model-dependent upper limits were established for the branching fractions and production cross sections of Pc+ pentaquarks.
• Further theoretical and experimental work is needed to fully understand J/ψ production mechanisms and the existence of exotic hadrons.