Hyperfine splitting of B mesons and B_s production at the Υ(5S)

Using the Columbia University–Stony Brook (CUSB-II) detector we have studied the inclusive photon spectrum from 2.9×10⁴ Υ(5S) decays. We observe a strong signal due to B^*→Bγ decays. From this we obtain (i) the average B^*-B mass difference, 46.7±0.4 MeV, (ii) the photon yield per Υ(5S) decay, 〈γ/Υ(5S)〉=1.09±0.06, and (iii) the average velocity of the B^*’s, 〈β〉=0.156±0.010, for a mix of nonstrange (B) and strange (B_s) B^* mesons from Υ(5S) decays. From the shape of the photon line, we find that both B and B_s mesons are produced with nearly equal values for the hyperfine splitting of the B and B_s meson systems.