Amplitude analysis of the radiative decay B_s⁰→ K ⁺K ⁻γ

A search for radiative decay of B⁰_s   mesons to orbitally excited K⁺K⁻ states is performed using proton proton collisions recorded by the LHCb experiment, corresponding to an integrated luminosity of 9 fb⁻1. The dikaon spectrum in the mass range m_KK < 2400 MeV/c² is dominated by the φ(1020) resonance that accounts for almost 70% of the decay rate. Considering the possible contributions of f₂(1270), f′₂(1525) and f₂(2010) meson states, the overall tensor contribution to the amplitude is measured to be F_{f2}=16.8 ± 0.5 (stat.) ± 0.7(syst.) %, mostly dominated by the f′₂(1525) state. Several statistically equivalent solutions are obtained for the detailed resonant structure depending on whether the smaller amplitudes interfere destructively or constructively with the dominant amplitude. The preferred solution that corresponds to the lowest values of the fit fractions along with constructive interference leads to the relative branching ratio measurement B(B⁰_s→f′₂γ)/B(B⁰_s→φγ)=19.4^+0.9_−0.8(stat.) ^+1.4_−0.5 (syst.) ± 0.5(B)%, where the last uncertainty is due to the ratio of measured branching fractions to the K⁺K⁻ final state. This result represents the first observation of the radiative B⁰_s → f′₂(1525)γ decay, which is the second radiative transition observed in the B⁰_s sector.