Radiative B-decay as a test of CKM unitarity

Abstract

We point out that $R\equiv Br(b\to d\gamma )/Br(b\to s\gamma )$ is a sensitive probe of possible violation of CKM unitarity. We compute $R$ in a minimal extension of the Standard Model containing an additional isosinglet charge ($-1/3$) quark, which leads to a deviation from CKM unitarity.

Comments: 11 pages (Latex) + 4 postscript figures (compressed, tared, uuencoded) included at the end of the paper - [please check before printing that the uncompressed fig2.ps and fig3.ps files do not contain blank lines in the beginning], CERN-TH.7294/94, MPI-PhT/94-32