Exotic hadrons: multiquarks, glueballs and hybrids

7 problems

• Are near-threshold exotic hadrons compact multiquarks or hadronic molecules?

  Contested

  Many exotic hadrons have masses very close to the combined mass of two ordinary hadrons, which suggests a loosely bound molecule of two mesons or of a meson and a baryon. Others argue they are compact objects in which four or five quarks are tightly bound together.

• Spectrum of light hybrid mesons with exotic quantum numbers

  Open

  Hybrid mesons contain a quark, an antiquark and an excited gluon field, and some have spin-parity combinations impossible for a plain quark-antiquark pair. A few candidates exist, and whether they form the complete family predicted by lattice QCD is open.

• Spin and parity of the Pc(4440) and Pc(4457) pentaquarks

  Open

  LHCb found in 2019 that one pentaquark signal is two close peaks, at 4440 and 4457 MeV, both just below the energy of a charmed baryon plus a charmed meson. Molecular models predict spins $1/2$ and $3/2$ for this pair while compact models give other patterns, so measuring the spins tests the pictures.

• Is X(2370) the lightest pseudoscalar glueball?

  Open

  In 2024 the BESIII experiment in Beijing found that the X(2370) particle has the spin and parity expected of the lightest pseudoscalar glueball and roughly the predicted mass. Whether it is made mainly of gluons is not established.

• Which observed scalar mesons contain the lightest glueball?

  Contested

  QCD predicts particles made only of gluons, called glueballs, with the lightest near 1.7 GeV. It mixes with ordinary quark-antiquark states, and which observed particles contain it is disputed.

• Pole of the doubly charmed $T_{cc}+$ from lattice QCD

  Open

  The $T_{cc}+$, found in 2021, contains two charm quarks and two light antiquarks and lies just below the energy of a $D*+$ and D0 meson pair. Lattice QCD (QCD computed numerically on a space-time grid) calculations near the real quark masses disagree on whether it is a true bound state or a virtual state (a near-bound state that does not quite bind).

• Compositeness of chi_c1(3872) from its line shape

  Open

  The chi_c1(3872), also called $X(3872)$, lies within about 0.2 MeV of the combined mass of two charmed mesons, $D0$ and anti-D*0. Measuring its exact position and line shape decides how much of it is a loose two-meson molecule.