Nuclear Physics Research and Education with GlueX

University Of Connecticut, Storrs CT

Investigators

Abstract

In the standard model of nuclear physics, individual protons and neutrons are described as bound assemblies of much smaller particles called quarks and gluons. While the types and numbers of quarks that belong to one proton or neutron are tightly constrained within the model, no restrictions are placed on the configuration and number of gluons that make up the bound state. One way to study gluon structure within the nucleon is through the process of deep inelastic scattering (DIS), where individual gluons are knocked out in a hard collision with a point-like probe. Experiments of this type at the CERN Large Hadron Collider (LHC) and at the future Electron Ion Collider (EIC) yield new information about the gluon structure of the nucleon. Another way to study the way gluons are arranged in stable nuclear matter is by passing high energy photons through stable nuclear matter and looking for the remnants that are produced when the photon is absorbed. While such interactions do disturb the quiescent state of the gluons inside the target, these excitations are much softer those explored in DIS experiments, and can provide complementary information about the way gluons are arranged in stable nuclear matter. One particularly interesting outcome from such spectroscopic studies would be the discovery of so-called exotic mesons in the photon remnants after the collision. The PI and students at the University of Connecticut are participating in the Gluonic Excitations Experiment (GlueX) at Jefferson Lab in Newport News, Virginia to search for a specific class of exotic mesons called hybrids that carry a unique experimental signature that allows them to be distinguished from other less exotic forms of nuclear matter. The UConn group is responsible for the source of polarized photons (gamma rays) which are produced by passing the 12 GeV electron beam from the Jefferson Lab accelerator through a carefully crafted and oriented diamond crystal. Polarized gamma rays from the source are directed onto a liquid hydrogen target where resonances are produced, whose subsequent decays are detected and identified in the GlueX spectrometer surrounding the target. The PI and students work with leaders in the diamond industry to improve the quality of the very thin sections of single-crystal diamond needed to meet the strict demands of this application. In parallel with this effort, the UConn group is also responsible for key aspects of the GlueX detector instrumentation, and for the Monte Carlo simulation that is essential to the interpretation of the experimental results. This includes improving the particle ID capability with machine learning algorithms The GlueX experiment aims at clarifying the role played by gluonic degrees of freedom in the excitation spectrum of light-quark hadrons. Data collected in pursuit of this goal also shed light on a number of additional topics in nuclear physics, including near-threshold J/ѱ photoproduction, rare decays of the (550) meson, the 2 decay width of the and ’ mesons, and the polarizability of the charged and neutral pions (CPP, NPP). GlueX has now completed 70% of its approved data collection for GlueX Phase 2, and has received approval for a third phase of running at a factor 2-3 higher intensity. The UConn group plays a critical role in quantifying and reducing systematics related to the properties of the photon beam, in addition to supporting the ongoing operation of the photon beamline and tagger. Support under this grant enables the PI and one PhD student to provide and enhance the quality of diamond radiators for Hall D experiments, maintain and operate the tagger microscope, and lead the ongoing development of the physics simulation for the GlueX experiment. In parallel with these efforts, UConn students will also carry out physics analysis in line with the primary GlueX physics program in hybrid spectroscopy and related topics. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.