Spectroscopy - JLab12

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The study of the hadronic spectrum is one of the key tools to probe the microscopic structure of the matter the surround us and to investigate the underlying dynamics. In the ’60s, it was the study of the hadron spectrum to lead physicists to develop the idea that these could not be elementary particles but had to be complex systems composed by fundamental constituents that were named quarks. Today, we know that hadronic matter is indeed made of baryons and mesons, formed respectively by three quarks and by quark-antiquark pairs interacting via the exchange of gluons.In spite of the significant progresses achieved in the last decades, many open questions remain: what is the origin of the hadronic mass? what is the role of gluons in determining the properties of hadrons? what is the origin of quark confinement? are hadrons with quark-gluon configuration different from ordinary baryons and mesons? Research in this field therefore continues. In the baryon sector most of the information on the excited states of the nucleon has been extracted in the early 70’s from pion-nucleon scattering reactions. The spectrum that emerged is characterized by wide overlapping resonant states in the energy range between 1200 MeV and 2500 MeV.

On the theoretical side constituent quark models, which describe the nucleon in terms of three quarks interacting by means of quantum chromo dynamics inspired potentials, are able to reproduce the baryonic spectrum. They also predict the existence of experimentally unobserved higher mass states in the energy range between 2000 MeV and 3000 MeV, which have been called "missing resonances". The existence of these states has been recently confirmed by Quantum Chromo Dynamics calculations on the Lattice. All models predict that higher energies "missing resonances" have higher probability of decay and/or being formed by the interaction of nucleons with mesons different from the pions: photons, kaons, omega mesons etc. Therefore experiments, which study vector mesons or strange mesons photo-production on the nucleon, have higher chances to confirm the existence of  "missing resonances". Meson spectroscopy is nowadays still a very active field since mesons, being formed by a quark and an antiquark, are the simplest hadronic system and the ideal laboratory to study quark interaction and the role of gluons. In the light quark (u,d,s) sector that is explored at Jefferson Lab, new experiments aim at determining with high accuracy the ordinary meson spectrum and search for evidence of new unconventional states as tetraquarks, formed by two quarks and two antiquarks, glueballs, formed by gluons alone, and hybrids, formed by quark-antiquark-gluon. The discovery of meson state with exotic structure would provide the first evidence for new hadronic matter and would allow physicist to make a big step toward a deep understanding of strong interaction.

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