The Gluex Experiment

Project Outline

A long-standing goal of hadron physics has been to understand how the quark and gluonic degrees of freedom that are present in the fundamental QCD Lagrangian manifest themselves in the spectrum of hadrons. Conventional mesons are made from ud, and quarks and are grouped in multiplets, each characterized by a given JPC combination (of angular momentum (J), parity (P), and charge conjugation (C)), as determined by the relative orientation of the spins of the quarks and their relative orbital angular momentum. Hybrid mesons result from the addition of angular momentum quantum numbers from a gluonic component to those of the two quarks and are pictured as a q-qbar-state. Among the hybrids, a subset is predicted to have exotic JPC combinations that are not allowed in the simple quark model: JPC = 0−−0+1+2+, etc.; these do not mix with any other states and thus can be unambiguously identified based on their JPC quantum numbers. Our understanding is maturing thanks to results from Lattice QCD (LQCD) calculations that predict a clear and detailed spectrum of hybrid mesons. On the experimental side, the contemporary landscape of hadron spectroscopy that has emerged is exciting, in that it includes solid evidence for new types of mesons in heavy-quark systems: tetraquark and pentaquark candidates in the charmonium sector) and in the light-quark sector (potential hybrids, probed in pion production, which possess exotic quantum numbers .

GlueX’s place in the global spectroscopy program is pivotal. The GlueX program — using 8- 9 GeV linearly polarized photons at JLab — is complementary to efforts at other facilities; its focus is on the exploration of the light-quark domain, potentially accessing hybrid mesons with exotic JPC quantum numbers produced in photon-proton collisions. The photon is expected to be a particularly effective probe towards the uncontroversial identification of exotic hybrid mesons in hybrid multiplets. The reason is that it has the same quantum numbers as a vector meson (qq̄ with aligned spins, for a total spin S⃗ = 1) and, when coupled with a gluonic component ((JPC)= 1+, mass ≈ 1.− 1.5 GeV) both ordinary and exotic JPC combinations are possible, in contrast toS⃗ = 0 probes (π±, K±). Our plan is to map out hadron spectrum patterns and test the features of theoretical models. To achieve this goal, we must systematically study all possible decay modes of conventional and hybrid mesons by conducting an amplitude analysis of many different hadronic final states.  A vigorous collaboration with theorists is leading to the necessary analysis and theoretical tools that will be necessary to extract timely physics results from the GlueX data.

The experiment is in full data-taking mode, which commenced in the spring of 2017 and will continue for several years. Our first publication is: 

Our group at Regina designed and constructed one of the largest sub-systems (the electromagnetic barrel calorimeter - BCAL) and plays an active role in the collaboration.

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