探秘正宇称粲介子的内部结构

Resolving the mysteries of the positive-parity charm mesons

2003年,标量粲-奇异介子D_(s0)^(*)(2317)和轴矢粲-奇异介子Ds1(2460)的发现拉开了含重夸克奇特强子态研究热潮的序幕.随后,又有实验报道了不含奇异夸克的标量和轴矢粲介子D0*(2300)和D1(2430).它们的发现带来了数个谜题,引发了大量理论和实验研究.近年来,随着有效场论和格点量子色动力学对相关系统的研究,以及LHCb实验对数个B介子三体衰变的精确测量,这些谜题已经在很大程度上得到了自然的解答;D_(s0)^(*)(2317)和Ds1(2460)的衰变宽度是确定这些粲介子内部结构的重要物理量,理论的定量预言正等待实验的验证.本文将概述对这些正宇称粲介子内部结构研究的进展.

In 2003,two positive-parity charm-strange mesons,the scalar meson D_(s0)^(*)(2317)and the axial-vector meson Ds1(2460),were discovered,followed by the report of two charm-nonstrange meson resonances,the D0*(2300)and D1(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 Ds1(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 Ds1(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 Ds1(2460),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 Ds1(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 Ds1(2460)in different models,and these widths are expected to be at the order of 100 keVin 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 Ds1(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 D0*(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 D0*(2300)and D1(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.