Observation of b → dγ decays and determination of |V_td/V_ts|

Abstract

The flavor changing neutral current process b → dγ is a sensitive probe to the Standard Model of elementary particle physics. Using a sample of 386 × 10⁶ B meson pairs accumulated by the Belle detector at the KEKB e⁺e⁻ collider, we measure the branching fractions for the exclusive modes B⁻ → ρ⁻γ, B̅⁰ → ρ⁰γ and B̅⁰ → ωγ as follows: B(B⁻ → ρ⁻γ) = 0.55 −0.36+0.42−0.08+0.09 B(B̅⁰ → ρ⁰γ) = 1.25 −0.33+0.37−0.06+0.07 B(B̅⁰ → ωγ) = 0.56 −0.27+0.34−0.10+0.05 where the first error on each value is statistical and the second is systematic. Assuming that these three modes are related by isospin conservation rules, we find the combined branching fraction B(B̅ → (ρ,ω)γ) = 1.32 −0.31+0.34−0.090.10. This result is used to determine the ratio of CKM matrix elements, |Vtd/Vts| = 0.199 −0.025+0.026−0.0150.018.