Mixing effects on 1S and 2S state heavy mesons in the light-front quark model

TL;DR

This paper analyzes the mass spectra and wave functions of 1S and 2S heavy mesons using the light-front quark model, emphasizing mixing effects and comparing results with experimental and lattice data.

Contribution

It introduces a detailed analysis of 1S and 2S heavy meson states including mixing effects, and provides predictions consistent with experimental observations.

Findings

Mixing angle between 1S and 2S states is around 12 degrees.

Mass of the 2S pseudoscalar D_s meson predicted to be 2600 MeV.

Results agree well with experimental data and lattice simulations.

Abstract

The mass spectra and wave functions of both $1S$ and $2S$ state heavy pseudoscalar ($P$) and vector ($V$) mesons are analyzed within the light-front quark model. Important empirical constraints employed in our analysis of the mass spectra and wave functions are the experimental mass-gap relation, $\Delta M_P > \Delta M_V$, where $\Delta M_{P(V)}=M^{2S}_{P(V)}-M^{1S}_{P(V)}$ and the hierarchy of the decay constants, $f_{1S}>f_{2S}$, between $1S$ and $2S$ meson states. We maintain the orthogonality of the trial wave functions of the $1S$ and $2S$ states in our variational calculation of the Hamiltonian with the Coulomb plus confining potentials and treat the hyperfine interaction perturbatively for the heavy-heavy and heavy-light $P$ and $V$ mesons due to the nature of the heavy quark symmetry. Realizing that the empirical constraints cannot be satisfied without mixing of the $1S$ and $2S$ states, we find the lower bound of the mixing angle $\theta$ between $1S$ and $2S$ states as $\theta_c = {\rm cot}^{-1}(2\sqrt{6})/2\simeq 6^{\circ}$ and obtain the optimum value of the mixing angle around $12^\circ$ to cover both the charm and bottom flavors of the heavy quark. The mixing effects are found to be more significant to the $2S$ state mesons than to the $1S$ state mesons. The properties of $1S$ and $2S$ state mesons including the mass spectra, decay constants, twist-2 distribution amplitudes, and electromagnetic form factors are computed. Our results are found to be in a good agreement with the available data and lattice simulations. In particular, the $2S$ state pseudoscalar $D_s$ meson is predicted to have a mass of $2600$ MeV, which is very close to the mass of the newly discovered $D_{s0}(2590)^+$ meson by the LHCb Collaboration. This supports the interpretation of the observed state as a radial excitation of the $D^+_s$ meson.