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Search for the radiative $\Xi_b^-\to\Xi^-\gamma$ decay

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Abstract

The first search for the rare radiative decay $\Xi_b^- \to \Xi^- \gamma$ is performed using data collected by the LHCb experiment in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb$^{-1}$. The $\Xi_b- \to \Xi^- J/\psi$ channel is used as normalization. No $\Xi_b^- \to \Xi^- \gamma$ signal is found and an upper limit of $\mathcal{B}(\Xi_b^- \to \Xi^- \gamma) < 1.3 \times 10^{-4}$ at 95 confidence level is obtained.

Figures and captions

The $b \rightarrow s\gamma$ penguin diagram, mediated by SM particles (left) and BSM particles (right).

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Topology of the $\Xi ^-_ b \rightarrow \Xi ^- \gamma $ decay, including three displaced vertices, with $c\tau$ of each particle given.

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Mass distribution $m(\pi ^- \pi ^- p )$ showing the $\Xi ^- \rightarrow \Lambda \pi ^- $ signal for events satisfying the trigger and the offline requirements described in the text for $\Xi ^-_ b \rightarrow \Xi ^- \gamma $ decays.

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Distribution of (top) mass $m(\pi ^- \pi ^- p \gamma)$ for selected $\Xi ^-$ $\gamma $ ( $\Xi ^- \rightarrow \Lambda \pi ^- $ ) and (bottom) $m(\pi ^- \pi ^- p \mu ^+ \mu ^- )$ for selected $\Xi ^-$ $ { J \mskip -3mu/\mskip -2mu\psi }$ ( $ { J \mskip -3mu/\mskip -2mu\psi } \rightarrow \mu ^+ \mu ^- $ ) candidates. The projections of the simultaneous fit are overlaid.

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Confidence interval at 95% CL showing the upper limit for $\mathcal{B} (\Xi ^-_ b \rightarrow \Xi ^- \gamma )$ as a function of the $\frac{N(\Xi ^-_ b \rightarrow \Xi ^- \gamma )}{N(\Xi ^-_ b \rightarrow \Xi ^- { J \mskip -3mu/\mskip -2mu\psi } )}$ ratio. The green line represents the relation between the yield and the branching fractions. The interval considering only the statistical uncertainty is shown by the dashed blue lines, while the full blue lines also includes the systematic uncertainties. The measured ratio of the yields and the upper limit on the branching fraction are represented by the red line.

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Animated gif made out of all figures.

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Tables and captions

Input parameters used to compute the branching fraction $\mathcal{B} (\Xi ^-_ b \rightarrow \Xi ^- { J \mskip -3mu/\mskip -2mu\psi } )$.

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Dominant systematic uncertainties on the measurement of the branching fraction $\mathcal{B}$ ( $\Xi ^-_ b \rightarrow \Xi ^- \gamma $ ).

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Created on 27 April 2024.