First observation of the inclusive decay B → Xη′ with the Belle detector

Abstract

The decay of a B meson into an η′ meson and an inclusive charmless state with unit strangeness, B → Xsη′, has stimulated significant theoretical interest since it was first observed in 1998. A number of models were proposed to explain the unexpectedly large branching fraction and the spectrum of Xs mass, which peaks above 2.0 GeV/c2. One of the most initially appealing explanations was the QCD anomaly model, in which the η′ couples strongly to two gluons. However, despite a number of theoretical calculations and improved measurements of the decay, no explanation has been universally accepted as accounting for available experimental data. The unique relationship between the η′ and the η suggest that the complimentary decay, B → Xsη, could elucidate the nature of the Xsη′ process. We report the first measurement of inclusive B → Xsη decays, based on a pseudoinclusive reconstruction technique using a sample of 657×106 B ¯B pairs accumulated with the Belle detector at the KEKB e+e− collider. ForMXs < 2.6 GeV/c2, we measure a branching fraction of (26.1±3.0(stat)+1.9−2.1(syst)+4.0−7.1(model))×10−5 and a direct CP asymmetry of ACP = −0.13 ± 0.04+0.02−0.03. Over half of the signal occurs in the range MXs > 1.8 GeV/c2, above all currently known exclusive contributions to B → Xsη. The lack of significant suppression of this decay relative to its η′ counterpart indicates that explanations for the B → Xsη′ signal that invoke mechanisms specific to the η′ are disfavored.