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Observation of two new $\Xi_b^-$ baryon resonances

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Abstract

Two structures are observed close to the kinematic threshold in the $\Xi_b^0 \pi^-$ mass spectrum in a sample of proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb$^{-1}$ recorded by the LHCb experiment. In the quark model, two baryonic resonances with quark content $bds$ are expected in this mass region: the spin-parity $J^P = \frac{1}{2}^+$ and $J^P=\frac{3}{2}^+$ states, denoted $\Xi_b^{\prime -}$ and $\Xi_b^{*-}$. Interpreting the structures as these resonances, we measure the mass differences and the width of the heavier state to be $m(\Xi_b^{\prime -}) - m(\Xi_b^0) - m(\pi^{-}) = 3.653 \pm 0.018 \pm 0.006$ MeV$/c^2$, $m(\Xi_b^{*-}) - m(\Xi_b^0) - m(\pi^{-}) = 23.96 \pm 0.12 \pm 0.06$ MeV$/c^2$, $\Gamma(\Xi_b^{*-}) = 1.65 \pm 0.31 \pm 0.10$ MeV, where the first and second uncertainties are statistical and systematic, respectively. The width of the lighter state is consistent with zero, and we place an upper limit of $\Gamma(\Xi_b^{\prime -}) < 0.08$ MeV at 95% confidence level. Relative production rates of these states are also reported.

Figures and captions

Distribution of $m_{\mathrm{cand}}(\Xi ^0_ b )$ for $\Xi ^0_ b \pi_{\mathrm{s}}^- $ candidates passing the full selection apart from the $m_{\mathrm{cand}}(\Xi ^0_ b )$ requirement. Inset: The subset of candidates that lie in the $\delta m$ signal regions of $3.0 < \delta m < 4.2$ $ {\mathrm{\,MeV\!/}c^2}$ and $21 < \delta m < 27$ $ {\mathrm{\,MeV\!/}c^2}$ .

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Distribution of the mass difference, $\delta m$, for $\Xi ^0_ b \pi_{\mathrm{s}}^- $ candidates in data. The points with error bars show right-sign candidates in the $\Xi ^0_ b $ mass signal region, and the hatched histogram shows wrong-sign candidates with the same selection. The curve shows the nominal fit to the right-sign candidates. Inset: detail of the region $2.0$--$5.5$ $ {\mathrm{\,MeV\!/}c^2}$ .

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Tables and captions

Systematic uncertainties, in units of $ {\mathrm{\,MeV\!/}c^2}$ (masses) and $\mathrm{\,MeV}$ (width). The statistical uncertainties are also shown for comparison.

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Supplementary Material [file]

Supplementary material full pdf

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supplementary.pdf

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2014-061. The files are: README This file Supplementary.pdf An overview of the extra content Fig* The figures in various formats helicity-table.tex The table showing results of the fits to the angular distribution under various hypotheses

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Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.