Rare Decays of Heavy Hadrons


Rare decays of hadrons containing heavy quarks are powerful probes of physics beyond the Standard Model. Being induced by flavour-changing neutral currents, their branching ratios are strongly suppressed in the SM and sensitive to new particles in the loop. CP asymmetries can probe non-standard CP violation and angular distributions of the decay products can be used to probe the chirality structure of the fundamental interactions. Beyond the Standard Model, these decays can thus contribute to understanding the flavour structure and eventually the origin of flavour.


In the Standard Model radiative B decays are b → q γ electromagnetic-penguin transitions, with q=s,d, dominated at one-loop level by a virtual intermediate top quark coupling to a W boson. The outcoming photon in this transition is predicted to be left-handed, up to small corrections of order ms/mb. A measurement of an excess of right-handed photons would be a clear sign of new physics, as predicted for left-right symmentric models, SU(2)xU(1) models with exotic fermions or SUSY. The first direct observation of the photon polarization has been perfomed by LHCb studying angular correlations among the decay products in B → Kππγ [1].

The LHCb-IFIC group is involved in the study of the photon polarization information through:

  • Time dependent analyses of the Bs → Φγ decay:
    • The time dependent decay width of Bs → Φγ decays gives information of the fraction of photons with anomalous polarization. A dedicated analysis has been developed at IFIC to measure this quantity using as control channel the Bd → K* γ decay. The first measurement has been performed with Run1 data using untagged events.


  • Radiative decays of b-baryons:
    • The angular distribution of the daughter baryon of these decays is very sensitivy to the photon polarization, due to the spin of the mother particle. Even if these decays are challenging to reconstruct due to the long lifetime of the involved particles, a dedicated radiative trigger has been developed for Run 2. We are focused in the study of the Λb → Λ γ, Ξb → Ξ γ and Ωb → Ω γ decays.