Angular analysis of the $B^0 \rightarrow K^{*0} e^+ e^-$ decay in the low-$q^2$ region

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Abstract

An angular analysis of the $B^0 \rightarrow K^{*0} e^+ e^-$ decay is performed using a data sample, corresponding to an integrated luminosity of 3.0 ${ fb^{-1}}$, collected by the LHCb experiment in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV during 2011 and 2012. For the first time several observables are measured in the dielectron mass squared ($q^2$) interval between 0.002 and 1.120${\mathrm{\,GeV^2\!/}c^4}$. The angular observables $F_{\mathrm{L}}$ and $A_{\mathrm{T}}^{\mathrm{Re}}$ which are related to the $K^{*0}$ polarisation and to the lepton forward-backward asymmetry, are measured to be $F_{\mathrm{L}}= 0.16 \pm 0.06 \pm0.03$ and $A_{\mathrm{T}}^{\mathrm{Re}} = 0.10 \pm 0.18 \pm 0.05$, where the first uncertainty is statistical and the second systematic. The angular observables $A_{\mathrm{T}}^{(2)}$ and $A_{\mathrm{T}}^{\mathrm{Im}}$ which are sensitive to the photon polarisation in this $q^2$ range, are found to be $A_{\mathrm{T}}^{(2)} = -0.23 \pm 0.23 \pm 0.05$ and $A_{\mathrm{T}}^{\mathrm{Im}} =0.14 \pm 0.22 \pm 0.05$. The results are consistent with Standard Model predictions.

Figures and captions

 Dominant Standard Model Feynman graphs for the electroweak loop and box diagrams involved in the $B ^0 \!\rightarrow K ^{*0} e ^+ e ^-$ decay. Fig1a.pdf [15 KiB] HiDef png [139 KiB] Thumbnail [23 KiB] *.C file Fig1b.pdf [15 KiB] HiDef png [144 KiB] Thumbnail [24 KiB] *.C file Fig1c.pdf [14 KiB] HiDef png [138 KiB] Thumbnail [23 KiB] *.C file Resolution on the $\tilde\phi$ angle as a function of the $e ^+ e ^-$ invariant mass as obtained from LHCb simulated events. Fig2.pdf [14 KiB] HiDef png [71 KiB] Thumbnail [33 KiB] *.C file Invariant mass distribution for (a) the $B ^0 \!\rightarrow K ^{*0} \gamma _{ e ^+ e ^- }$ and (b) the $B ^0 \!\rightarrow K ^{*0} e ^+ e ^-$ decay modes and the three trigger categories grouped together. The dashed line is the signal PDF, the light grey area corresponds to the combinatorial background and the dark grey area is the PR background. The solid line is the total PDF. The two vertical dotted lines on the $B ^0 \!\rightarrow K ^{*0} e ^+ e ^-$ plot indicate the signal window that is used in the angular fit. Fig3a.pdf [22 KiB] HiDef png [287 KiB] Thumbnail [88 KiB] *.C file Fig3b.pdf [21 KiB] HiDef png [255 KiB] Thumbnail [79 KiB] *.C file Distributions of the $K^+\pi^- e ^+ e ^-$ invariant mass, $\cos{\theta_\ell}$ , $\cos{\theta_K}$ and $\tilde\phi$ variables for the $B ^0 \!\rightarrow K ^{*0} e ^+ e ^-$ decay mode and the three trigger categories grouped together. The dashed line is the signal PDF, the light grey area corresponds to the combinatorial background, the dark grey area is the PR background. The solid line is the total PDF. Fig4a.pdf [17 KiB] HiDef png [248 KiB] Thumbnail [81 KiB] *.C file Fig4b.pdf [15 KiB] HiDef png [150 KiB] Thumbnail [50 KiB] *.C file Fig4c.pdf [15 KiB] HiDef png [166 KiB] Thumbnail [56 KiB] *.C file Fig4d.pdf [15 KiB] HiDef png [136 KiB] Thumbnail [48 KiB] *.C file Distribution of the reconstructed $q^2$ from an sPlot of data (black points). The dashed line represents the $B ^0 \!\rightarrow K ^{*0} e ^+ e ^-$ contribution and the grey area corresponds to the $3.8\%$ $B ^0 \!\rightarrow K ^{*0} \gamma _{ e ^+ e ^- }$ contamination. The solid line is the sum of the two. Fig5.pdf [14 KiB] HiDef png [138 KiB] Thumbnail [51 KiB] *.C file Animated gif made out of all figures. PAPER-2014-066.gif Thumbnail

Tables and captions

 Fit results for the angular observables $F_{\mathrm{L}}$ , $A_{\mathrm{T}}^{(2)}$ , $A_{\mathrm{T}}^{\mathrm{Im}}$ and $A_{\mathrm{T}}^{\mathrm{Re}}$ . The second column corresponds to the uncorrected values directly obtained from the fit while the third column gives the final results after the correction for the $(3.8 \pm 1.9)$% of $B ^0 \!\rightarrow K ^{*0} \gamma _{ e ^+ e ^- }$ contamination and for the small fit biases due to the limited size of the data sample. The first uncertainty is statistical and the second systematic. Table_1.pdf [38 KiB] HiDef png [77 KiB] Thumbnail [13 KiB] tex code Summary of the systematic uncertainties. Table_2.pdf [50 KiB] HiDef png [102 KiB] Thumbnail [17 KiB] tex code

Created on 22 February 2019.Citation count from INSPIRE on 22 February 2019.