Scientists observed the mixing of two light scalar mesons, a00(980) and f0(980), using data samples accumulated with the Beijing Spectrometer (BESIII). This observation should help clear up what the 980 MeV scalar mesons are made of (e.g., a tetraquark, meson molecule, quark-gluon hybrid). The results were published recently in Physical Review Letters as a letter of “Editors’ Suggestion”.
Although light scalar mesons a00(980) and f0(980) are well-established in experiment, explanations about the nature of them have been controversial for several decades. These two states, with similar masses but different decay modes and isospins, are difficult to accommodate in the traditional quark-antiquark model and many alternative formulations have been proposed to explain their internal structure, including tetraquark state, KK molecule, or quark-antiquark gluon hybrid.
The mixing mechanism in the system of a00(980) -f0(980), which was first proposed in the late 1970s, is considered to be an essential approach to clarify the nature of these two states. However, the direct measurement of the mixing signals is only made possible with the world largest data samples of 1.31×109J/ψ and 4.48×108ψ(3686) events accumulated with the BESIII detector. The signals of f0(980)→a00(980) and a00(980)→f0(980) are first observed with statistical significances of 7.4σ and 5.5σ, respectively. The statistical significance of the mixing signal versus the values of the coupling constants, ga0K+K- and gf0K+K- is shown in the following figure.
The statistical significance of the signal scanned in the two-dimensional space of ga0K+K- and gf0K+K-. (Image by BESIII Collaboration)
This direct measurement of a00(980) -f0(980) mixing is a sensitive probe to the internal structure of those ground state scalars and sheds important light on their nature. The new results from BESIII provide critical constraints to the development of theoretical models for a00(980) and f0(980) . It is theorists' turn to refine the calculations to understand the inner structure of the a00(980) and f0(980) mesons.
The BESIII experiment at the Beijing Electron Positron Collider is composed of about 450 physicists from 66 institutions from 14 countries. The BESIII experiment contributes to China’s world-leading role in τ-charm physics. To date, the collaboration has published more than 200 papers.