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Observation of a new $\Xi_b^-$ resonance

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Abstract

From samples of $pp$ collision data collected by the LHCb experiment at $\sqrt{s}=7$, $8$ and $13$ TeV corresponding to integrated luminosities of 1.0, 2.0 and 1.5 fb$^{-1}$, respectively, a peak in both the $\Lambda_b^0K^-$ and $\Xi_b^0\pi^-$ invariant mass spectra is observed. In the quark model, radially and orbitally excited $\Xi_b^-$ resonances with quark content $bds$ are expected. Referring to this peak as $\Xi_b(6227)^-$, the mass and natural width are measured to be $m_{\Xi_{b}(6227)^-}=6226.9\pm2.0\pm0.3\pm0.2$ MeV/$c^2$ and $\Gamma_{\Xi_b(6227)^-}=18.1\pm5.4\pm1.8$ MeV/$c^2$, where the first uncertainty is statistical, the second is systematic, and the third, on $m_{\Xi_b(6227)^-}$, is due to the knowledge of the $\Lambda_b^0$ baryon mass. Relative production rates of the ${\Xi_b(6227)^-\to\Lambda_b^0K^-}$ and ${\Xi_b(6227)^-\to\Xi_b^0\pi^-}$ decays are also reported.

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Invariant mass spectra for (top) $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $, (middle) $\Lambda ^+_ c $ from $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \mu ^- X$, and (bottom) $\Xi ^+_ c $ from $\Xi ^0_ b \rightarrow \Xi ^+_ c \mu ^- X$ candidate decays. The left column is for $7,\,8$ TeV and the right is for 13 TeV data. Fits are overlaid, as described in the text. Here, the $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \mu ^- X$ mode has been prescaled by a factor of ten.

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Spectra of mass differences for $\Xi _ b (6227)^{-} $ candidates, reconstructed in the final states (top) $\Lambda ^0_ b K ^- $, with $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $, (middle) $\Lambda ^0_ b K ^- $, with $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \mu ^- X$, and (bottom) $\Xi ^0_ b \pi ^- $, with $\Xi ^0_ b \rightarrow \Xi ^+_ c \mu ^- X$, along with the results of the fits. The left column is for 7, 8 TeV and the right is for 13 TeV data. The symbol $M^*$ represents the mass after the constraint $(p_{H_c^+}+p_{\mu ^- }+p_{\rm miss})^2=m_{H_b^0}^2$ is applied, as described in the text.

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Distribution of (left) $M^*(\Lambda ^0_ b K ^- )-M^*(\Lambda ^0_ b )$ for simulated $\Xi _ b (6227)^{-} \rightarrow \Lambda ^0_ b K ^- $ decays, where $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \mu ^- X$, and (right) $M^*(\Xi ^0_ b \pi ^- )-M^*(\Xi ^0_ b )$ for simulated $\Xi _ b (6227)^{-} \rightarrow \Xi ^0_ b \pi ^- $ decays, where $\Xi ^0_ b \rightarrow \Xi ^+_ c \mu ^- X$. The symbol $M^*$ represents the mass after the constraint $(p_{H_c^+}+p_{\mu ^- }+p_{\rm miss})^2=m_{H_b^0}^2$ is applied, as described in the text. The natural width used in the simulation is set to a negligible value, so that these spectra are due entirely to the mass resolution. Fits to the sum of a nonrelativistic Breit-Wigner function and a Crystal Ball function \cite{Skwarnicki:1986xj} with a common mean value are overlaid.

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Uncorrected $\Xi _ b (6227)^{-} $ and $H_b^0$ signal yields for $7,\,8$ and 13 TeV data. The $H_b^0$ yields are limited to the signal regions used to form $\Xi _ b (6227)^{-} $ candidates (see text).

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Relative efficiencies ($\epsilon_{\rm rel}^{(\prime)}$) for the SL modes. Uncertainties are due only to the finite size of the simulated samples.

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Summary of systematic uncertainties on $R(\Lambda ^0_ b K ^- )$ and $R(\Xi ^0_ b \pi ^- )$, in units of $10^{-3}$.

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Measured ratios $R(\Lambda ^0_ b K ^- )$ and $R(\Xi ^0_ b \pi ^- )$ for $7\,,8$ and 13 TeV data, in units of $10^{-3}$. The uncertainties are statistical (first) and systematic (second).

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Supplementary Material [file]

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