Lepton universality violation from neutral pion decays in $R_{K^{(\ast )}}$ measurements

Abstract

I show that the neutral pion decay in $B\to K^{(\ast )}{\pi }^0\gamma$, with ${\pi }^0\to ee\gamma$, might generate large sources of lepton flavor universality violation (LFUV) in measurements of the ratios, $R_{K^{(\ast )}}$: If the photons in the $K^{(\ast )}ee\gamma \gamma$ final state are reconstructed as Bremsstrahlung, the recovered electron-positron invariant mass can be pushed into the $1$-$6$ GeV${}^2$ signal region, artificially enhancing the measured $B\to K^{(\ast )}ee$ branching ratio compared to $B\to K^{(\ast )}\mu \mu$. I present a conservative estimate and simulation of the $B\to K{\pi }^0\gamma$ LFUV background at LHCb, that together suggest this effect could reduce the recovered $R_K$ up to several percent. A reliable assessment of the size of this effect will require dedicated simulations within experimental frameworks themselves.

Comments: 6 pages, 6 figures, 1 table; published version; includes estimates for effects of lepton $p_T$ thresholds and magnetic bending power enhancements