Heavy quark spin in an S-wave meson-antimeson state

The newly observed exotic mesons above [Formula: see text] threshold were widely discussed in the molecular picture. To understand deeper their structures, we here discuss the spin ([Formula: see text]) of a heavy quark-antiquark pair in an S-wave meson-antimeson system by constructing explicitly the spin wave function. One finds two selection rules for [Formula: see text] (a) the total angular momentum J is larger than the maximum angular momentum of the light degree of freedom or smaller than the minimum one; (b) JC = 1+, 2-, 3+, ⋯ if the two mesons are different but belong to the same doublet. This feature may be used to constrain possible strong decay channels.

HYPERFINE MIXING AND THE SEMILEPTONIC DECAYS OF DOUBLE-HEAVY BARYONS IN A QUARK MODEL

The semileptonic decays of the lowest-lying double-heavy baryons are treated in a quark model. For the Ξbb, hyperfine mixing in the spin wave function leaves the total rate for decay into the lowest lying daughter baryons essentially unchanged, but changes the relative rates into the Ξbc and [Formula: see text]. The same pattern is obtained in the decays of the Ωbb. For the Ξbc, this mixing leads to factor of about 17 suppression in the decay rate to the [Formula: see text] when wave functions truncated to the largest components are used, but the total semileptonic decay rate of the parent baryon remains essentially unchanged. For the Ωbc, the decay to the [Formula: see text] is suppressed by a factor of more than 25 from the unmixed case. When the full wave functions are used, the large suppression of the decays to the [Formula: see text] and [Formula: see text] persists.