# Measurement of the ratio of branching fractions $B(B^0 \to K^{\ast 0} \gamma) / B(B^0_s \to \phi \gamma)$ and the direct $C P$ asymmetry in $B^0 \to K^{\ast 0} \gamma$

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## Abstract

The ratio of branching fractions of the radiative B decays B0 -> K*0 gamma and Bs0 phi gamma has been measured using an integrated luminosity of 1.0 fb-1 of pp collision data collected by the LHCb experiment at a centre-of-mass energy of sqrt(s)=7 TeV. The value obtained is BR(B0 -> K*0 gamma)/BR(Bs0 -> phi gamma) = 1.23 +/- 0.06(stat.) +/- 0.04(syst.) +/- 0.10(fs/fd), where the first uncertainty is statistical, the second is the experimental systematic uncertainty and the third is associated with the ratio of fragmentation fractions fs/fd. Using the world average value for BR(B0 -> K*0 gamma), the branching fraction BR(Bs0 -> phi gamma) is measured to be (3.5 +/- 0.4) x 10^{-5}. The direct CP asymmetry in B0 -> K*0 gamma decays has also been measured with the same data and found to be A(CP)(B0 -> K*0 gamma) = (0.8 +/- 1.7(stat.) +/- 0.9(syst.)). Both measurements are the most precise to date and are in agreement with the previous experimental results and theoretical expectations.

## Figures and captions

 Invariant-mass distributions of the (a) $K ^{*0}$ and (b) $\phi$ resonance candidates. The black points represent the data and the fit result is represented as a solid blue line. The fit is described in the text. The regions outside the vector meson invariant-mass window are shaded. The Poisson $\chi^2$ residuals [28] are shown below the fits with the $\pm2 \sigma$ confidence-level interval delimited by solid red lines. fig1a.eps [220 KiB] HiDef png [319 KiB] Thumbnail [239 KiB] *.C file fig1b.eps [198 KiB] HiDef png [317 KiB] Thumbnail [247 KiB] *.C file Invariant-mass distributions of the (a) $B ^0 \rightarrow K ^{*0} \gamma$ and (b) $B ^0_ s \rightarrow \phi \gamma$ candidates. The black points represent the data and the fit result is represented as a solid blue line. The signal is fitted with a double-sided Crystal Ball function (short-dashed green line). The combinatorial background is modelled with an exponential function (long-dashed red line). In decreasing amplitude order, the exclusive background contributions to $B ^0 \rightarrow K ^{*0} \gamma$ are $B^{+(0)} \rightarrow K^{*0}\pi^{+(0)}\gamma$ (short-dotted black), $B\rightarrow K^{*0}(\phi)\pi^0X$ (long-dashed blue), $B ^0_ s \rightarrow K^{*0}\gamma$ (dotted short-dashed green), $\Lambda ^0_ b \rightarrow \Lambda ^*\gamma$ (double-dotted dashed pink), $B ^0 \rightarrow K^+\pi^-\pi^0$ (dotted long-dashed black) and $B ^0_ s \rightarrow K^+\pi^-\pi^0$ (long-dotted blue). The background contributions to $B ^0_ s \rightarrow \phi \gamma$ are $B^{+(0)} \rightarrow \phi K^{+(0)}\gamma$ (dotted black), $\Lambda ^0_ b \rightarrow \Lambda ^*\gamma$ (double-dotted dashed pink) and $B ^0_ s \rightarrow K^+K^-\pi^0$ (dotted-dashed black). No significant contribution to $B ^0_ s \rightarrow \phi \gamma$ is found from partially reconstructed $B\rightarrow K^{*0}(\phi)\pi^0X$ decays. The Poisson $\chi^2$ residuals [28] are shown below the fit with the $\pm 2 \sigma$ confidence-level interval delimited by solid red lines. fig2a.eps [202 KiB] HiDef png [335 KiB] Thumbnail [286 KiB] *.C file fig2b.eps [166 KiB] HiDef png [292 KiB] Thumbnail [260 KiB] *.C file Invariant-mass distributions of the (a) $\overline{ B }{} ^0$ $\rightarrow$ $\overline{ K }{} ^{*0}$ $\gamma$ and (b) $B ^0 \rightarrow K ^{*0} \gamma$ decay candidates. The black points represent the data and the fit result is represented as a solid blue line. The different background components are also shown. The Poisson $\chi^2$ residuals [28] are shown below the fits with the $\pm2 \sigma$ confidence-level interval delimited by solid red lines. fig3a.eps [27 KiB] HiDef png [321 KiB] Thumbnail [263 KiB] *.C file fig3b.eps [33 KiB] HiDef png [352 KiB] Thumbnail [294 KiB] *.C file Animated gif made out of all figures. PAPER-2012-019.gif Thumbnail

## Tables and captions

 Expected contributions to the $B ^0 \rightarrow K ^{*0} \gamma$ and $B ^0_ s \rightarrow \phi \gamma$ yields in the $\pm$1 ${\mathrm{ Ge V /}c^2}$ mass window from the exclusive background channels: radiative decays, $h^+h'^-\gamma$ (top), charmless b decays involving energetic $\pi^0$, $h^+h'^-\pi^0$ (middle) and partially reconstructed decays (bottom). The average measurement (exp.) or theoretical (theo.) branching fraction is given where available. Each exclusive contribution above 0.1% is included in the fit model, with a fixed shape determined from simulation. The amplitude of the partially reconstructed backgrounds is left to vary in the fit while the $h^+h'^-\gamma$ and $h^+h'^-\pi^0$ contributions are fixed to their expected level. Table_1.pdf [59 KiB] HiDef png [104 KiB] Thumbnail [49 KiB] tex code Summary of the individual contributions to the relative systematic uncertainty on the ratio of branching fractions as defined in Eq. ???. Table_2.pdf [55 KiB] HiDef png [51 KiB] Thumbnail [23 KiB] tex code $C P$ asymmetry and total number of signal candidates measured for each magnet polarity. Table_3.pdf [53 KiB] HiDef png [50 KiB] Thumbnail [23 KiB] tex code Corrections to the raw asymmetry and corresponding systematic uncertainties. Table_4.pdf [56 KiB] HiDef png [71 KiB] Thumbnail [34 KiB] tex code

## Supplementary Material [file]

 Supplementary material full pdf supple[..].pdf [67 KiB] This ZIP file contains supplemetary material for the publication LHCb-PAPER-2012-019. The files are: supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps : The figures in variuous formats fig4a.pdf [30 KiB] HiDef png [400 KiB] Thumbnail [333 KiB] *C file fig4b.pdf [21 KiB] HiDef png [360 KiB] Thumbnail [311 KiB] *C file

Created on 11 July 2020.