Using b → sγ to Probe Top Quark Couplings ∗

Possible anomalous couplings of the top-quark to on-shell photons and gluons are constrained by the recent results of the CLEO Collaboration on both inclusive and exclusive radiative B decays. We find that the process b → sγ can lead to reasonable bounds on both the anomalous electric and magnetic dipole moments of the top-quark, while essentially no limits are obtained on the corresponding chromoelectric and chromomagnetic moments, which enter the expression for the decay rate only through operator mixing. ∗Research supported by the U.S. Department of Energy, Division of High Energy Physics, Contract W-31-109-ENG-38. The Standard Model (SM) of electroweak interactions is in very good agreement with present experimental data[1]. Nonetheless, it is believed to leave many questions unanswered, and this belief has resulted in numerous theoretical and experimental attempts to discover a more fundamental underlying theory. Various types of experiments may expose the existence of physics beyond the SM, including the search for direct production of exotic particles at high-energy colliders. A complementary approach in hunting for new physics is to examine its indirect effects in higher order processes. For example, even though the top-quark has yet to be discovered, it has already made its presence known through loop order processes, such as rare decays of the b-quark. Since the top-quark is far more massive than the other SM fermions, its interactions may be quite sensitive to new physics originating at a higher scale. If there are any deviations from SM expectations in the properties of the top-quark, they may indirectly lead to modifications in the anticipated branching fractions for these one-loop induced b-quark decays. The possibility of having top-quarks with anomalous couplings to various gauge bosons has been discussed in the literature[2, 3]. Strong bounds can be placed on these anomalous couplings at future colliders, such as the SSC/LHC[2] and the next linear ee linear collider (NLC)[3], which rely on direct production of top-quark pairs. Since the tquark has yet to appear at the Tevatron, it is clear that any restrictions on these couplings at present can only be obtained indirectly. In this paper we examine the effects of anomalous couplings of the top-quark to on-shell photons and gluons on the process b → sγ. If the t-quark has large anomalous couplings, then the resulting prediction of the rate for b → sγ would conflict with experiment. The CLEO collaboration has recently[4] observed the exclusive decay B → K∗γ with a branching fraction of (4.5± 1.5± 0.9)× 10 and has placed an upper limit on the inclusive quark-level process b → sγ of B(b → sγ) < 5.4 × 10 at 95% CL. Using a conservative estimate of the ratio of exclusive to inclusive decay rates[5],