Quarkonium at the Frontiers of High Energy Physics: A Snowmass White Paper

TL;DR

Heavy quarkonium provides a precise testing ground for QCD, highlighting two forward-looking frontiers: spectroscopy of states above open-flavor thresholds and production at large $p_T$. The paper surveys experimental discoveries of XYZ and charged tetraquark states, and maps them to phenomenological models (conventional quarkonia, molecules, hybrids, tetraquarks) and first-principles QCD approaches (lattice QCD, lattice NRQCD, Born-Oppenheimer). It emphasizes NRQCD factorization and NLP fragmentation as central frameworks for quarkonium production, while acknowledging outstanding issues with polarization, feeddown, and large higher-order corrections. The authors outline a coordinated program across BESIII, Belle II, LHC upgrades, and PANDA to test factorization, refine matrix elements, and explore potential new-physics sensitivities through quarkonium processes and Higgs couplings, ultimately advancing our understanding of the QCD spectrum and bound-state formation.

Abstract

In this Snowmass White Paper, we discuss physics opportunities involving heavy quarkonia at the intensity and energy frontiers of high energy physics. We focus primarily on two specific aspects of quarkonium physics for which significant advances can be expected from experiments at both frontiers. The first aspect is the spectroscopy of charmonium and bottomonium states above the open-heavy-flavor thresholds. Experiments at e^+ e^- colliders and at hadron colliders have discovered many new, unexpected quarkonium states in the last 10 years. Many of these states are surprisingly narrow, and some have electric charge. The observations of these charged quarkonium states are the first definitive discoveries of manifestly exotic hadrons. These results challenge our understanding of the QCD spectrum. The second aspect is the production of heavy quarkonium states with large transverse momentum. Experiments at the LHC are measuring quarkonium production with high statistics at unprecedented values of p_T. Recent theoretical developments may provide a rigorous theoretical framework for inclusive production of quarkonia at large p_T. Experiments at the energy frontier will provide definitive tests of this framework. Experiments at the intensity frontier also provide an opportunity to understand the exclusive production of quarkonium states.