Hadron Spectroscopy in the Light Quark Sector at GlueX

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Subatomic physics seeks to understand nature - and all its laws - at the most fundamental level, down to its elementary constituents and their interactions. Tremendous strides have been made in the last several decades, using accelerators and astrophysical observations, which have strengthened our conviction that the basic model, known as the Standard Model, is essentially a valid description of the subatomic world but not necessarily a complete one. Among the four forces of the Standard Model, the strong force inside an atomic nucleus is understood by a theory called Quantum Chromodynamics (QCD). QCD describes the physics underlying the interaction of quarks and gluons and it seems to permit a rich spectrum of allowed constituents, whereas nature prefers to reveal a narrower set of quark and gluonic composite states. Features of QCD include six flavors of quarks with various masses, strongly interacting quarks and gluons, and the phenomena of asymptotic freedom and confinement. To date only colourless "quark model" states have been observed - such as quark-antiquark pairs (mesons) and quark triplets (baryons) - while gluonic degrees of freedom are difficult to observe or suppressed. But QCD predicts more types of states than just mesons and baryons. The goal of the GlueX Experiment is to produce and identify unseen exotic forms of matter that exhibit unique signatures among all other particles, which will allow QCD to be tested in a constraining manner. Specifically, hybrid mesons result from the addition of gluon quantum numbers to those of the quarks and a subset of hybrids is predicted to have exotic quantum number combinations that are not allowed in the simple quark model. This is a difficult task requiring experimental conditions that have recently become possible in accelerator and detector technology. GlueX is addressing the former by employing a unique, high-intensity, polarized photon beam and a hermetic detector. The latter has benefited by the recent development of large silicon-based photo sensors that are completely immune to magnetic fields. Our group played a defining role in guiding industry towards their development and these devices are now available commercially to the subatomic, medical and nuclear safety fields. The GlueX plan is to map out the QCD spectrum by systematically studying all decay modes of conventional and hybrid mesons. This effort needs to reconcile state-of-the-art experimental results with state-of-the-art experimental interpretation. The latter is limited by uncertainties in obtaining a robust theoretical description rather than our understanding of the detector or the statistical precision of the data. Our group will pursue the analysis of low-mass mesons, in regions where there is sparse or no real-world data. This research will provide data in controversial (past experiments with low statistical power) as well as unexplored domains of meson spectroscopy.