Exotic Tetraquark states with two $\bar{b}$-quarks and $J^P=0^{+}$ and $1^{+}$ $B_s$ states in a nonperturbatively-tuned Lattice NRQCD setup

Abstract

We use $n_f=2+1$ Wilson-clover gauge-field ensembles from the CLS consortium in a Lattice NRQCD setup to predict the binding energy of a $I(J^P)=0(1^{+})$ $ud\bar{b}\bar{b}$ tetraquark and a $\frac{1}{2}(1^{+})$ $\ell s\bar{b}\bar{b}$ tetraquark. We determine the binding energies with respect to the relevant $B{B}^{\ast }$ and $B_s{B}^{\ast }$ thresholds respectively to be $112.0(13.2)$ MeV for the $ud\bar{b}\bar{b}$, and $46.4(12.3)$ MeV for the $\ell s\bar{b}\bar{b}$. We also determine the ground-state $J^P=0^{+}$ $B_{s0}^{\ast }$ and $1^{+}$ $B_{s1}$ mesons to lie $75.4(14.0)$ and $78.7(13.9)$ MeV below the $BK$ and $B^{\ast }K$ thresholds respectively. Our errors are entirely dominated by systematics due to discretisation effects. To achieve these measurements, we performed a neural network based nonperturbative tuning of the Lattice NRQCD Hamiltonian's parameters against the basic bottomonium spectrum. For all lattice spacings considered we can reproduce the continuum splittings of low-lying bottomonia. It is worth remarking that our nonperturbative tuning parameters deviate from 1 by significant amounts, particularly the term $c_2$.

Comments: 48 pages, 16 figures