BESIII Collaboration has performed the first measurement of the lineshape of X(3872) via a coupled-channel analysis. This research has been published in Physics Review Letters on 12, April 2024 [Phys. Rev. Lett. 132, 151903].
The challenge on understanding the mechanism of quark confinement has aroused great interest in the study of hadronic states. Among all the hadrons, the X(3872), aka the χ
c1(3872), has played a pivot role over the last two decades. Its decay into π
+π
-J/ψ indicates a ccbar constituent, however, the narrow width and the mass consistent with the D
*0 Dbar
0 threshold suggest an unexpected structure inside. Therefore, X(3872) is regarded as the first exotic candidate beyond the traditional quarkonium model. Its observation has prompted extensive theoretical investigation and refined experimental measurements, yet the precise values of its fundamental parameters remain a subject of ongoing debate.
The invariant mass spectrum of its decay products encodes information on the intrinsic properties of X(3872). Extracting its parameters requires an accurate description of the spectrum's lineshape. This study obtains the intact lineshape of the X(3872) by employing a coupled-channel amplitude and conducting a simultaneous fit to previously established datasets collected at BESIII in the π+π-J/ψ and D*0 Dbar0π0 final states. Over two decades after its discovery, the particle finally reveals its shape for the first time (see the figures below). From the fit, the coupling between X(3872) and D*0 Dbar0, locations of poles, compositeness, scattering length and effective range parameters are extracted, which suggest a predominance of the molecular state, as well as a compact short-range core within its wave function.
Fit results for (a) D*0 Dbar0π0 and (b) π+π-J/ψ channels (c) X(3872) lineshape
URL:
https://link.aps.org/doi/10.1103/PhysRevLett.132.151903DOI: 10.1103/PhysRevLett.132.151903
