Coulomb gauge QCD as a tool for the excited spectrum

TL;DR

This paper discusses how Coulomb gauge QCD, formulated with physical gluons and quarks, can be used to study the excited spectrum of hadrons, revealing insights into quark mass behavior and decay dynamics.

Contribution

It highlights two applications: probing the running quark mass via parity doubling in excited baryons and mapping heavy quark velocities through decay product distributions, inspired by molecular physics principles.

Findings

Parity doubling in excited baryons reflects the mid-infrared quark mass.

Decoupling rate of fast quarks indicates asymptotic freedom behavior.

Velocity distribution of decay products can reveal heavy quark dynamics.

Abstract

A distinct feature of Coulomb gauge QCD is that it can be formulated in terms of physical, transverse gluons and quarks alone. The state-counting is then transparent, and the gauge is suited for studies of the excited spectrum. Leaving aside exotic spectroscopy, which has been the subject of other publications, in this note I call attention on two recent applications. One is that the running quark mass in the mid-infrared can be probed from excited baryons thanks to parity doubling, a consequence of insensitivity to chiral symmetry breaking. Fast quarks are asymptotically free and behave as massless, so hadrons containing fast quarks decouple from the condensate. Their (power-law) rate of decoupling reflects on the rate of decreasing parity splittings, which can be measured. The second is that, in analogy with the Franck-Condon principle of molecular physics, the velocity distribution of the heavy quarks inside a heavy hadron can be mapped out by the velocity distribution of its open-flavor decay products. This is exemplified by recent data from the Belle collaboration taken at the Upsilon(10860).