Pentaquark Spectroscopy in a Bethe Salpeter Approach

Abstract:One of the spectacular successes of this millennium's hadron spectroscopy experiments is the discovery of many exotic meson states in the heavy-quark energy region, which do not seem to fit into the conventional quark model picture. Our traditional understanding of hadrons is that they can be grouped into mesons (quark-antiquark states) and baryons (which are made of three valence quarks). Moreover, over the last years the LHCb collaboration has found prominent peaks in the J/ψ-proton invariant mass spectrum. This implies a minimal quark content uudc-cbar and thus constitutes the first clear experimental evidence for pentaquarks. The proximity of those peaks to meson-baryon thresholds suggests a molecular explanation in terms of meson-baryon molecules, in analogy to several exotic meson candidates in the charmonium sector.

The goal of this PhD project is to establish a theoretical description of pentaquarks from a covariant five-body Bethe-Salpeter equation in QCD, namely as five-body systems made of four quarks and one antiquark where the (anti-)quarks interact through gluons. This is based on a body of preceding work on mesons, baryons and tetraquarks from 2-, 3- and 4-body equations, which have been successful in describing the spectra and internal dynamics of these states. Therefore, the generalization of the approach to the five-body problem will substantially advance the status quo and deliver predictions on the spectrum and structure of pentaquarks directly from QCD.

In this seminar, I will mainly focus on three works: I will present the technical basis for treating the five-body equation for a scalar system, which provides the starting point for detailed studies in view of the resonance behavior of pentaquarks, their internal structure, and also light-quark spectroscopy to investigate meson-cloud effects on baryons. I will also present the ongoing work on the five-body equation using the leading tensor components. Secondly, I will present the two-body equation for a pentaquark as a meson-baryon system and describe how the Dyson-Schwinger and Bethe-Salpeter equations work. Finally, I will briefly discuss another project I have been working on, namely the QCD contributions to flavor changing neutral currents which may pose constraints on flavor anomalies.