Model-independent extraction of the pole and Breit-Wigner resonance parameters

Strong Polarization of a J=1/2 to 1/2 Transition Arising from Unexpectedly Large Quantum Interference

We experimentally show that the 1s^{2}2s^{2}2p_{1/2}-1s2s^{2}2p_{1/2}^{2} transition in Pb^{77+} emitted in dielectronic recombination of Pb^{78+} is strongly polarized, although it is an intrinsically unpolarized J=1/2 to 1/2 transition. This unanticipated polarization is shown to be due to quantum interference with radiative recombination. The interference effect has been studied on an asymmetric resonance profile but has never been studied on polarization. In this Letter, we show that the effect on polarization can arise from a different cross term than that responsible for asymmetry, resulting in unexpectedly large polarization even for a nearly symmetric resonance suggesting a small interference.

Impact of the decay width in Breit-Wigner formula on Maxwellian-averaged cross section for neutron capture on 16O

Neutron capture on 16O may serve as a crucial neutron poison reaction in weak s-process occurred in massive stars. In our previous study [Zhang, S. S., Xu, S. Z., He, M. et al. Eur. Phys. J. A 57, 114 (2021)], we found that the contribution from low-lying resonances to Maxwellian-averaged cross section (MACS) progressively increases as the energy goes beyond 70 keV. In Breit-Wigner formula for resonant cross section, the decay width is a decisive quantity. In this paper, we discuss the impact of three kinds of decay widths, i.e. a constant width and two energy-dependent widths, on reaction cross sections and MACSs. The penetration factor adopts semi-classical WKB approximation and the asymptotic solution, respectively. We clarify that energy-dependent width are necessary for a reasonable behavior of resonance cross section around resonance peak and low-energy region far from the peak. The difference of two energy-dependent widths decreases from 3.7 to 1.5 with the energy increasing from 0.01 keV to 1000 keV. It results in similar behavior of resonance cross sections, but a slight difference by 1% for MACSs when E > 50 keV.

Clues about the Nature of Multiquark States

We present a comprehensive study of the decay width of multiquark states containing different color singlet components in a coupled-channel approach. We show how the decay width can provide in-depth information about the nature of a coupled-channel resonance. An unexpected behavior of the decay width of a multiquark state could be pointing to a relevant role of the coupled-channel dynamics, aiming at the channel responsible for the formation of the resonance. The symmetrical situation between meson- and baryon-like multiquarks is highlighted. Our study accounts for the existence of narrow resonances with large phase spaces. In the case of resonances far from their detection channel, it is the mass difference with the formation channel that determines their decay width. The larger the binding, the larger the decay width, even though the phase space to the detection channel gets reduced. The trends noticed cast doubts on the molecular assignment of some multiquark candidates. Finally, we wonder about the existence and properties of multiquark partners in other flavor sectors.

Fundamental properties of resonances

All resonances, from hydrogen nuclei excited by the high-energy gamma rays in deep space to newly discovered particles produced in Large Hadron Collider, should be described by the same fundamental physical quantities. However, two distinct sets of properties are used to describe resonances: the pole parameters (complex pole position and residue) and the Breit-Wigner parameters (mass, width, and branching fractions). There is an ongoing decades-old debate on which one of them should be abandoned. In this study of nucleon resonances appearing in the elastic pion-nucleon scattering we discover an intricate interplay of the parameters from both sets, and realize that neither set is completely independent or fundamental on its own.