Search for the Familon in Two-Body B Meson Decays

Abstract

A tremendous effort has been devoted to finding the deep physical reason for the existence of the mass hierarchy and weak mixing among the quarks and leptons. Despite the enormous amount of data accumulated during the past three decades we still appear to be far from the true understanding of the family structure of fermions and the origin of particle generations (families).

A possible explanation based on a spontaneously broken continuous global family symmetry was suggested some time ago. One of the most important consequence of the spontaneous breakdown of this family symmetry is the existence of neutral massless Nambu-Goldstone bosons, called familons. Familons can have flavor-conserving as well as flavor-changing couplings with the fermions and the coupling strength is suppressed by the energy scale at which the flavor symmetry is broken. In contrast to the first two generations, experimental constraints on familons coupled to the third generation of fermions are quite modest, the only weak constraint is in the leptonic sector from tau->e/mu X0 decay.

Flavor-changing couplings between the b quark and the familon (f) would lead to the decay B->hf (h=pi,K) through vector coupling and B->Vf (V=rho,K*) through axial coupling. We have searched for the two-body decay of the B meson to a light pseudoscalar meson h=pi,K,K0-short and a massless neutral weakly-interacting particle X0 such as the familon. We find no significant signal by analyzing a data sample containing 9.7 million BB mesons collected with the CLEO detector at the Cornell Electron Storage Ring, and set a 90% C.L. upper limit of 4.9x10^{-5} and 5.3x10^{-5} on the branching fraction for the decays B+->h+X^0$ and B0->K0S X0, respectively. These upper limits correspond to a lower bound of approximately 10^8 GeV on the family symmetry breaking scale (with vector coupling) for the third generation of quarks.