# Evidence for the strangeness-changing weak decay $\Xi_b^-\to\Lambda_b^0\pi^-$

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## Abstract

Using a $pp$ collision data sample corresponding to an integrated luminosity of 3.0 fb$^{-1}$, collected by the LHCb detector, we present the first search for the strangeness-changing weak decay $\Xi_b^-\to\Lambda_b^0\pi^-$. No $b$ hadron decay of this type has been seen before. A signal for this decay, corresponding to a significance of 3.2 standard deviations, is reported. The relative rate is measured to be ${{f_{\Xi_b^-}}\over{f_{\Lambda_b^0}}}{\cal{B}}(\Xi_b^-\to\Lambda_b^0\pi^-) = (5.7\pm1.8^{+0.8}_{-0.9})\times10^{-4}$, where $f_{\Xi_b^-}$ and $f_{\Lambda_b^0}$ are the $b\to\Xi_b^-$ and $b\to\Lambda_b^0$ fragmentation fractions, and ${\cal{B}}(\Xi_b^-\to\Lambda_b^0\pi^-)$ is the branching fraction. Assuming $f_{\Xi_b^-}/f_{\Lambda_b^0}$ is bounded between 0.1 and 0.3, the branching fraction ${\cal{B}}(\Xi_b^-\to\Lambda_b^0\pi^-)$ would lie in the range from $(0.57\pm0.21)\%$ to $(0.19\pm0.07)\%$.

## Figures and captions

 Invariant mass spectrum for selected $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^-$ candidates in data. Lb2LcP[..].pdf [27 KiB] HiDef png [242 KiB] Thumbnail [174 KiB] *.C file Fit to the $\delta m$ spectra in data: (top left) RS low S/B, (top right) WS low S/B, (bottom left) RS high S/B and (bottom right) WS high S/B. Xib2Lb[..].pdf [22 KiB] HiDef png [270 KiB] Thumbnail [199 KiB] *.C file Xib2Lb[..].pdf [21 KiB] HiDef png [252 KiB] Thumbnail [181 KiB] *.C file Xib2Lb[..].pdf [19 KiB] HiDef png [293 KiB] Thumbnail [219 KiB] *.C file Xib2Lb[..].pdf [18 KiB] HiDef png [256 KiB] Thumbnail [197 KiB] *.C file Animated gif made out of all figures. PAPER-2015-047.gif Thumbnail

## Tables and captions

 Relative systematic uncertainties (in percent) on the signal yield determination and on the quantity $\left(f_{\Xi ^-_ b }/f_{\Lambda ^0_ b }\right)\cal{B}(\Xi ^-_ b \rightarrow \Lambda ^0_ b \pi ^- )$, as described in the text. Table_1.pdf [50 KiB] HiDef png [116 KiB] Thumbnail [52 KiB] tex code

Created on 10 July 2020.