We present an improved measurement of the $\eta'$ meson energy spectrum in $\Upsilon(1S)$ decays, using 1.2 fb^{ -1} of data taken at the $\Upsilon(1S)$ center-of-mass energy with the CLEO III detector. We compare our results with models of the $\eta'$ gluonic form factor that have been suggested to explain the unexpectedly large $B\to \eta 'X_s$ rate. Models based on perturbative QCD fail to fit the data for large $\eta '$ energies, and thus an explanation outside the realm of the Standard Model or an improved understanding of non-perturbative QCD effects may be needed to account for this large rate.
Categories: hep-ex