Resolving the mysteries of the positive-parity charm mesons

In 2003, two positive-parity charm-strange mesons, the scalar meson D-s0(*)(2317) and the axial-vector meson D-s1(2460), were discovered, followed by the report of two charm-nonstrange meson resonances, the D-0(*)(2300) and D-s1(2430). These discoveries pioneered the observations of tens of new hadron resonant structures in the last two decades by various experimental collaborations and opened a new chapter in the study of exotic hadrons. Their properties are at odds with expectations from the quark model, and several mysteries were heavily discussed over the years. The mysteries include: (1) Why the D-s0(*)(2317) and the D-s1(2460) mesons are significantly lighter than the quark model predictions of the lightest charm-strange mesons with the same quantum numbers? (2) Why is the mass difference between the D-s1(2460) and the D-s0(*)(2317) almost the same as that between the ground state vector meson D* and pseudoscalar meson D? (3) Why are the masses of the charm-strange mesons almost the same as those of the charm-nonstrange mesons with the same spin and parity, or even lighter depending on the experiments? For the charm-strange mesons D-s0(*)(2317) and D (2460) D-s1, different models were proposed, including modifying the quark model of cs mesons, the chiral doublet model where they are proposed to be chiral partners of the ground state pseudoscalar and vector charm-strange mesons, interpreting them as tetraquark states or hadronic molecules. In the hadronic molecular model, the D-s0(*)(2317) and the D-s1( 2460) are explained as composite hadron systems with the main component being DK and D K* S-wave bound states, respectively. Vastly different predictions were made on the hadronic decay widths of the D-s0(*)(2317) and the D-s1(2460) in different models, and these widths are expected to be at the order of 100 keV in the hadronic molecular model, much larger than the predictions based on other models. However, so far only upper limits have been reported, and the absolute values may be measured at the PANDA experiment that is still under construction. Despite of that, these mesons were investigated by studying the interaction between a ground state charm meson and a light pseudoscalar meson in an S-wave in unitarized chiral perturbation theory which is based on chiral symmetry and unitarity. With parameters determined from calculations of lattice quantum chromodynamics (QCD), various nontrivial predictions were made, such as the existence of a virtual state in the isoscalar pair of the D meson and anti-kaon. These predictions are consistent with very recent lattice QCD results, and the hadronic molecular picture of the D-s0(*)(2317) and the D-s1 (2460) receives strong support. In addition, the precise data of the angular moments for several three-body decays of the B mesons measured by the LHCb Collaboration can be well described in the same picture. In particular, the results from unitarized chiral perturbation theory suggest that there are two scalar charm-nonstrange mesons with masses below 2. 5 GeV, instead of only one D-0*( 2300), and the situation for the axialvector charm-nonstrange mesons is similar; the lightest charm-nonstrange scalar and axial-vector mesons should have masses significantly smaller than those of the D-0*( 2300) and D-1( 2430) reported by experiments, which is also supported by recent lattice QCD calculations. With these developments, all the three mentioned mysteries can be naturally explained. This paper is devoted to reviewing the progress in understanding these positive-parity charm mesons.

Automated summary

This paper reviews the progress in understanding positive-parity charm mesons, which have properties that contradict the expectations from the quark model and have puzzled researchers with several mysteries. Different models and experimental measurements have been proposed to explain these mysteries.