The Physics of GlueX


    The observation, nearly four decades ago, that mesons are grouped in nonets, each characterized by unique values of JPC – spin (J), parity (P) and charge conjugation (C) quantum numbers – led to the development of the quark model. Within this picture, mesons are bound states of a quark and an antiquark. The three light quark flavors (up, down and strange) suffice to explain the spectroscopy of most – but not all – of the lighter-mass mesons (below 3 GeV/c2) that do not explicitly carry heavy flavors (charm or beauty). Early observations yielded only those JPC quantum numbers consistent with a fermion-antifermion bound state. The JPC quantum numbers of a meson system with total quark spin, S, and relative angular momentum, L, are determined as follows: J = L + S, P = (-1)L+1 and C = (-1)L+S. Thus JPC quantum numbers such as 0--, 0+-, 1-+ and 2+- are not allowed and are called exotic in this context.


Meson Spectroscopy

Image credit:  D. Leinweber

A long-standing goal of hadron physics has been to understand how the quark and gluonic degrees of freedom that are present in the fundamental QCD Lagrangian manifest themselves in the spectrum of hadrons.  Of particular interest is how the gluon-gluon interactions might give rise to physical states with gluonic excitations.  One class of such states is the hybrid meson, which can be naively thought of as a quark anti-quark pair coupled to a valence gluon.  Recent lattice QCD calculations predict a rich spectrum of hybrid mesons.  A subset of these hybrids has an exotic experimental signature:  angular momentum (J), parity (P), and charge conjugation (C) that cannot be created from just a quark-antiquark pair. 

Our understanding of how gluonic excitations manifest themselves within QCD is maturing thanks to recent results from lattice QCD. This numerical approach to QCD considers the theory on a finite, discrete grid of points in a manner that would become exact if the lattice spacing were taken to zero and the spatial extent of the calculation was made large.  These calculations show a clear and detailed spectrum of exotic JPC mesons, with a lightest 1-+ state lying a few hundred MeV below a 0+- and two 2+- states.

Although the bulk of the non-exotic JPC spectrum in LQCD calculations has the expected systematics of a qq bound state system, some states are only interpolated strongly by operators featuring non-trivial gluonic constructions. One may interpret these states as non-exotic hybrid mesons, and by combining them with the spectrum of exotics, it is possible to isolate the lightest hybrid supermultiplet of (0,1,2)-+ and 1-- states at a mass roughly 1.3 GeV heavier than the ρ meson. The form of the operator that has the strongest overlap onto these states has an S-wave qq pair in a color octet configuration and an exotic gluonic field in a color octet with JgPgCg =1+-, a chromomagnetic configuration. The heavier (0,2)+- states, along with some positive parity non-exotic states, appear to correspond to a P-wave coupling of the qq pair to the same chromomagnetic gluonic excitation.

The primary goal of the GlueX experiment in Hall D is to search for and study these mesons.

Jefferson Lab Program Advisory Committee (PAC) Documents:

  1. “Mapping the spectrum of light quark mesons and gluonic excitations with linearly polarized photons,” A proposal to PAC30, (2006).

  2. “The GlueX Experiment in Hall D,” An update presented to PAC36, (2010).

  3. “A study of meson and baryon decays to strange final states with GlueX in Hall D,” A proposal to PAC39, (2012).

Gluonic Excitations