The polarization of photons produced in radiative $B^{0}_{s}$ decays is studied for the first time. The data are recorded by the LHCb experiment in $pp$ collisions corresponding to an integrated luminosity of 3fb$^{1}$ at centerofmass energies of $7$ and $8$TeV. A timedependent analysis of the $B^{0}_{s} \to \phi \gamma$ decay rate is conducted to determine the parameter ${\mathcal{A}}^\Delta$, which is related to the ratio of right over lefthanded photon polarization amplitudes in $b \to s \gamma$ transitions. A value of ${\mathcal{A}}^\Delta=0.98^{ +0.46 +0.23}_{ 0.52 0.20}$ is measured. This result is consistent with the Standard Model prediction within two standard deviations.
Fits to the invariant mass distributions of the $ B ^0$ (top) and $ B ^0_ s $ (bottom) candidates. 
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Backgroundsubtracted decaytime distributions for $ B ^0 \rightarrow K ^{*0} \gamma $ (top) and $ B ^0_ s \rightarrow \phi \gamma $ (bottom) decays with the fit projections overlaid and normalized residuals shown below. The projections of a fit with $ {\mathcal{A}}^\Delta$ fixed to the central value of the SM prediction [2] are also shown. For display purposes, the PDF is shown as a histogram, integrated across each decaytime interval. 
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Decaytime dependence of the ratio of the yields of $ B ^0_ s \rightarrow \phi \gamma $ and $ B ^0 \rightarrow K ^{*0} \gamma $ , with the fit overlaid. The expected distribution for the central value of the SM prediction [2] is also shown. 
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Animated gif made out of all figures. 
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Supplementary material full pdf 
supple[..].pdf [141 KiB] 

This ZIP file contains supplemetary material for the publication LHCbPAPER2016034. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps, *.C : The figures in variuous formats 
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Created on 20 February 2021.