# Observation of photon polarization in the $b \to s\gamma$ transition

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## Abstract

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 $3\,\text{fb}^{-1}$, 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 $[1.1, 1.9]\,$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 $5.2\,\sigma$.

## Figures and captions

 Mass distribution of the selected $B ^+ \!\rightarrow K ^+ \pi ^- \pi ^+ \gamma$ 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). Fig1.pdf [15 KiB] HiDef png [267 KiB] Thumbnail [76 KiB] *.C file Background-subtracted $K ^+\pi ^-\pi ^+$ mass distribution of the $B ^+ \!\rightarrow K ^+ \pi ^- \pi ^+ \gamma$ signal. The four intervals of interest, separated by dashed lines, are shown. Fig2.pdf [19 KiB] HiDef png [142 KiB] Thumbnail [33 KiB] *.C file Distributions of $\cos\hat{{\theta}}$ for $B ^+ \!\rightarrow K ^+ \pi ^- \pi ^+ \gamma$ 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. Fig3_a.pdf [10 KiB] HiDef png [152 KiB] Thumbnail [53 KiB] *.C file Fig3_b.pdf [10 KiB] HiDef png [150 KiB] Thumbnail [53 KiB] *.C file Fig3_c.pdf [10 KiB] HiDef png [160 KiB] Thumbnail [55 KiB] *.C file Fig3_d.pdf [10 KiB] HiDef png [149 KiB] Thumbnail [53 KiB] *.C file Animated gif made out of all figures. PAPER-2014-001.gif Thumbnail

## Tables and captions

 Legendre coefficients obtained from fits to the normalized background-subtracted $\cos\hat{{\theta}}$ distribution in the four $K ^+\pi ^-\pi ^+$ mass intervals of interest. The up-down asymmetries are obtained from Eq. 4. The quoted uncertainties contain statistical and systematic contributions. The $K ^+\pi ^-\pi ^+$ mass ranges are indicated in ${\mathrm{\,GeV\!/}c^2}$ and all the parameters are expressed in units of $10^{-2}$. The covariance matrices are given in the supplementary material. Table_1.pdf [33 KiB] HiDef png [78 KiB] Thumbnail [13 KiB] tex code Legendre coefficients obtained from fits to the normalized background-subtracted $\cos\hat{{\theta}}$ distribution, using the alternative normal $\vec{p}_{\pi ^- }\times\vec{p}_{\pi ^+ }$ for defining the direction of the photon, in the four $K ^+\pi ^-\pi ^+$ mass intervals of interest. The up-down asymmetries are obtained from Eq. 4. The quoted uncertainties contain statistical and systematic contributions. The $K ^+\pi ^-\pi ^+$ mass ranges are indicated in ${\mathrm{\,GeV\!/}c^2}$ and all the parameters are expressed in units of $10^{-2}$. The covariance matrices are given in the supplementary material. Table_2.pdf [40 KiB] HiDef png [81 KiB] Thumbnail [14 KiB] tex code Covariance matrices (in units of $10^{-3}$) for the fitted values of $c_1$, $c_2$, $c_3$ and $c_4$ of Table 1, for the four $K ^+\pi ^-\pi ^+$ mass intervals. Table_3.pdf [41 KiB] HiDef png [15 KiB] Thumbnail [2 KiB] tex code Covariance matrices (in units of $10^{-3}$) for the fitted values of $c^{\prime}_1$, $c^{\prime}_2$, $c^{\prime}_3$ and $c^{\prime}_4$ of Table 2, for the four $K ^+\pi ^-\pi ^+$ mass intervals. Table_4.pdf [41 KiB] HiDef png [16 KiB] Thumbnail [2 KiB] tex code

## Supplementary Material [file]

 This ZIP file contains supplementary material for the publication LHCb-PAPER-2014-001. The files are: LHCb-PAPER-2014-001-supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps *.C : The figures in various formats Fig4.pdf [53 KiB] HiDef png [119 KiB] Thumbnail [29 KiB] *C file Fig5.pdf [13 KiB] HiDef png [80 KiB] Thumbnail [21 KiB] *C file Fig6.pdf [13 KiB] HiDef png [80 KiB] Thumbnail [21 KiB] *C file LHCb-P[..].pdf [379 KiB] HiDef png [274 KiB] Thumbnail [91 KiB] *C file

Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.