Observation of Photon Polarization in the $b\to s\gamma$ Transition

This Letter presents a study of the flavor-changing neutral current radiative $B^{\pm }\to K^{\pm }{\pi }^{\mp }{\pi }^{\pm }\gamma$ decays performed using data collected in proton-proton collisions with the LHCb detector at 7 and 8 TeV center-of-mass energies. In this sample, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$, nearly 14 000 signal events are reconstructed and selected, containing all possible intermediate resonances with a $K^{\pm }{\pi }^{\mp }{\pi }^{\pm }$ final state in the $[1.1,1.9]\mathrm{GeV}/c^2$ mass range. The distribution of the angle of the photon direction with respect to the plane defined by the final-state hadrons in their rest frame is studied in intervals of $K^{\pm }{\pi }^{\mp }{\pi }^{\pm }$ mass and the asymmetry between the number of signal events found on each side of the plane is obtained. The first direct observation of the photon polarization in the $b\to s\gamma$ transition is reported with a significance of $5.2\sigma$.

Figure 1

Mass distribution of the selected $B^{+}\to K^{+}{\pi }^{-}{\pi }^{+}\gamma$ candidates. The blue solid curve shows the fit results as the sum of the following components: signal (red solid), combinatorial background (green dotted), missing pion background (black dashed), and other partially reconstructed backgrounds (purple dash-dotted).

Figure 2

Background-subtracted $K^{+}{\pi }^{-}{\pi }^{+}$ mass distribution of the $B^{+}\to K^{+}{\pi }^{-}{\pi }^{+}\gamma$ signal. The four intervals of interest, separated by dashed lines, are shown.

Figure 3

Distributions of $\cos \widehat{\theta }$ for $B^{+}\to K^{+}{\pi }^{-}{\pi }^{+}\gamma$ signal in four intervals of $K^{+}{\pi }^{-}{\pi }^{+}$ mass. The solid blue (dashed red) curves are the result of fits allowing all (only even) Legendre components up to the fourth power.