[Paper Review] Heavy Quarkonium Production at Threshold: from JLab to EIC

S. Joosten, Z.-E. Meziani|arXiv (Cornell University)|Feb 7, 2018

Particle physics theoretical and experimental studies24 references3 citations

TL;DR

This paper proposes using elastic electro- and photo-production of $J/\psi$ and $\Upsilon$ quarkonia at Jefferson Lab 12 GeV and a future Electron-Ion Collider (EIC) to probe the origin of proton mass and spin, measure color Van der Waals forces between hadrons, and investigate the nature of the LHCb pentaquark. By combining threshold and high-mass kinematics, the study enables experimental access to quark and gluon contributions in nucleon mass decomposition via generalized parton distributions (GPDs), with predictions validated using a new Monte Carlo generator, LIGEN.

ABSTRACT

In this contribution we present opportunities to address questions about the origin of mass and spin, probe the existence and nature of the LHCb pentaquark state, and probe the color Van der Waal forces among two color neutral hadrons. The key reaction is elastic production of heavy quarkonia (J/psi and Upsilon) on the nucleon from threshold to large nucleon-quarkonium invariant masses. This is possible when combining the energy range of two high luminosity facilities, Jefferson Lab 12 GeV and an electron ion collider (EIC).

Motivation & Objective

• To experimentally probe the origin of nucleon mass and spin by measuring elastic $J/\psi$ and $\Upsilon$ production at threshold and high invariant mass.
• To access the gluonic contribution to nucleon spin and mass via generalized parton distributions (GPDs) at EIC.
• To determine the transverse spatial profile of gluon densities and the gluonic radius of the nucleon.
• To test the existence and properties of the LHCb pentaquark state through exclusive $J/\psi$ production in $\gamma p \to P_c \to J/\psi p$ processes.
• To measure color Van der Waals forces between two color-singlet hadrons using $t$-channel $J/\psi$ and $\Upsilon$ production.

Proposed method

• Utilizes a new Monte Carlo event generator, LIGEN, to simulate $l$-A reactions for both fixed-target and collider kinematics.
• Models the $t$-channel cross section for $J/\psi$ and $\Upsilon$ production via two-gluon exchange, fitted to world data on photo-production.
• Applies the s-channel helicity conservation (SCHC) framework to describe angular distributions of decay leptons in $J/\psi \to e^+e^-$ and $\Upsilon \to e^+e^-$, $\mu^+\mu^-$ decays.
• Uses a dipole-like form factor $D = \left(\frac{M_V^2}{M_V^2 + Q^2}\right)^{n_2}$ to parameterize $Q^2$-dependence of the electro-production cross section.
• Implements exact virtual photon flux and polarization using the formalism from Ref. [44], with $R = \sigma_L / \sigma_T$ modeled via a power-law form.
• Normalizes GPDs to the CT14 gluon PDF using LHAPDF to enable quantitative comparison with lattice QCD and experimental data.

Experimental results

Research questions

• RQ1 What fraction of the proton’s mass arises from gluonic energy and the trace anomaly, as opposed to quark masses?
• RQ2 How do the orbital angular momentum of quarks and the total angular momentum of gluons contribute to the proton’s spin?
• RQ3 What is the spatial extent of the gluon distribution in the nucleon, and how does it relate to the gluonic radius?
• RQ4 Does the LHCb pentaquark state manifest in exclusive $\gamma p \to P_c \to J/\psi p$ production at threshold?
• RQ5 What is the strength and range of color Van der Waals forces between two color-singlet hadrons, as probed by $t$-channel $J/\psi$ and $\Upsilon$ exchange?

Key findings

• The $t$-channel cross section for $J/\psi$ and $\Upsilon$ production is well described by a two-gluon exchange model fitted to existing photo-production data.
• The LIGEN generator successfully simulates realistic $J/\psi$ and $\Upsilon$ electro- and photo-production rates across a wide kinematic range.
• The angular distribution of $J/\psi \to e^+e^-$ decay products follows the s-channel helicity conservation prediction, with $\mathcal{W}(\cos\theta_{\text{CM}}) = \frac{3}{8}(1 + \cos^2\theta_{\text{CM}})$ in real photo-production.
• The $Q^2$-dependence of the electro-production cross section is modeled using a tuned dipole-like form factor $D = \left(\frac{M_V^2}{M_V^2 + Q^2}\right)^{n_2}$, optimized for exclusive $\rho$ production and adapted to $J/\psi$ and $\Upsilon$.
• GPDs are normalized to the CT14 gluon PDF, enabling direct comparison with lattice QCD calculations and extraction of gluonic radius and spin contributions.
• The study provides a pathway to measure the gluonic radius and total angular momentum of gluons via $\Upsilon$ production at EIC, with sensitivity to $x$-dependent transverse profiles.