Beyond the Conventional Quark Model: Theoretical Studies of QCD Exotics

点击翻译按钮获取中文摘要

The conventional quark model classifies mesons as quark-antiquark ($q\bar q$) states and baryons as $qqq$ three-quark states. Although the conventional quark model represents the simplest combinations of quarks that can form colour singlets, there is a rich range of other colour-singlet possibilities such as gluonium (consisting entirely of gluons), hybrids (a conventional hadron combined with gluonic content), and multiquark states such as four-quark $q\bar q q\bar q$ mesons and pentaquark $qqq\bar q q$ baryons. These configurations that lie beyond the conventional quark model are collectively known as exotic hadrons. Until 2003, evidence for exotic hadrons was limited to hints of gluonium and a hybrid within the light mesons. However, the 2003 discovery of the X(3872) marked the beginning of a new era in hadron spectroscopy with the discovery of a multitude of XYZ mesons that cannot be described within the conventional quark model.  The long-term passion of my research program is to advance our understanding of exotic hadrons from the theoretical perspective of quantum chromodynamics (QCD): the fundamental theory of the strong interactions. Despite intense theoretical and experimental activity, the internal structure of the XYZ mesons remains an open question. Scenarios for four-quark states include meson-meson molecules, diquark-antidiquark clusters binding into tetraquarks, and hadro-quarkonium with a heavy quark-antiquark core and a cloud of light quarks. These distinct forms of quark internal clustering are expected to manifest themselves in hadronic properties. One of the key themes in the proposed research is to develop and use new QCD sum-rule operators consisting of meson or diquark operators at separate spacetime points to model the internal molecular or tetraquark structure. The goal is to determine the effect of the internal structure on hadronic properties to gain new insights on possible internal configurations. Another main theme of the proposed research is to advance the precision of our theoretical calculations for exotic hadrons to ensure that important physics insights are not obscured by theoretical uncertainties. The vast majority of QCD sum-rule studies of exotics have been performed at leading order, so the proposed research will examine higher-loop effects on the mass spectrum and decays in a variety of exotic systems including hybrid mesons, multiquark states, and hybrid baryons. Mixed interpretations involving hybrid, quark-antiquark, and multi-quark components will be an area of investigation. My research program has pushed the boundaries of QCD sum-rule analytic calculations for exotic hadrons, and the next frontier is the use of sector decomposition methods for numerical loop-integration techniques implemented in pySecDec. Combined with novel theoretical approaches, the proposed research will pioneer the use of these numerical methods in QCD sum-rule studies of exotic hadrons.