Exotic States

Recently the scientific community has witnessed what is called the golden age for heavy quarkonium physics, dawned a decade ago and initiated by the confluence of exciting advances in Quantum Chromodynamics (QCD) and an explosion of related experimental activity.

On the experimental side, everything started in 2003 when the Belle Collaboration reported the discovery what is now called the X(3872) (with more than 1000 citations, this is the most quoted result of the B-factories). Since then besides BABAR, BES, CLEO, and Fermilab, BESIII and LHC experiments have joined the search and the number of new states has increased dramatically. A wealth of exotic XYZ states, some of which manifestly exotic because charged, have been discovered at or above the strong decay thresholds of charmonium and bottomonium. (see Figure below). New forms of matter such as quark-gluon hybrids, mesonic molecules, and tetraquarks have been proposed. Experiments focused on the abundant production and systematic studies of the XYZ states are needed. Preferably, these should be performed using hadronic probes because the cross sections are expected to be very large. Therefore the LHCb at LHC-CERN and the future PANDA experiment at the Facility for Antiproton and Ion Research (FAIR) are particularly good to study these states.

From a theoretical point of view, heavy quarkonium physics experienced a breakthrough with the development of effective field theories. After the birth of nonrelativistic QCD (NRQCD) in 1986 and, in particular, of potential nonrelativistic QCD (pNRQCD) in 1998, one has been able to give a model-independent, QCD based approach to this field of research, where just the right degrees of freedom appear. These theories have become an important tool to compute physical observables incorporating corrections systematically through a suitable power counting.

Many of the new XYZ states are located close to or above threshold, making this energy region the most interesting one for theoretical and experimental studies. Because situation changes drastically at or above the open-flavor threshold, no EFT description has yet been constructed nor have the appropriate degrees of freedom been clearly identified for most of the new states. Concerning lattice studies, the threshold regions remain troublesome, calculations of excited states have been only recently pioneered and the full treatment of bottomonium on the lattice seems to be tricky.

Our research aims at working out a systematic, model-independent and QCD-based description of the XYZ states. We want to take advantage of the nonrelativistic nature of these states and the appearance of a hierarchy of energy scales to construct adequate Effective Field Theories from the ones that we know at present. For instance, NRQCD is a good EFT for states close to and just above threshold, at least when their binding energies remain much smaller than the heavy-flavor mass.

Written by J. Segovia and J. Tarrus


Nora Brambilla, Antonio Vairo, Jaume Tarrus, Jorge Segovia, Matthias Berwein

Some related publications

N. Brambilla et al..
QCD and strongly coupled gauge theories: challenges and perspectives (Chapter IV)
Eur. Phys. J. C74 (2014) 2981
arXiv:1404.3723 Inspire

N. Brambilla et al.
Heavy quarkonium: progress, puzzles, and opportunities
Eur. Phys. J. C71 (2011) 1534
arXiv:1010.5827 Inspire