Semi-leptonic flavour-changing neutral-current decays of the fourth generation b′-quark

Abstract

We study the decay modes b′ → bℓ +ℓ −, b′ → bν ν for min b′ ⪅M Z including virtual γ, Z 0, and box diagram contributions, the latter complicating the computation considerably. Due to the effect of the Z 0 propagator and the finite Z 0 width, these decay rates are substantial. The box diagram is subleading but also not negligible. As a consequence, the lepton pair mass distribution is rather flat and high mass ℓ +ℓ − or ν ν pairs are much more abundant than suggested from phase space. It is found that b′ → bZ ∗ (loosely denoting the sum of if b′ → bℓ +ℓ −, b′ → ν ν and b′ → bq q ) is of similar order as the b′ → bg decay mode, and is likely to dominate in case m t′ is very heavy. With three light neutrinos, the decay b′ → bν ν is similar order as b′ → b γ, however, the b′ → bℓ +ℓ − mode are relatively suppressed. The flavour-changing neutral-current b′ decay scenario is thus shown to be quite rich. A comprehensive, but theoretical, discussion is given regarding b′ discovery potentials at present and future colliders.