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Observation of a new $\Xi_b^0$ state

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Abstract

Using a proton-proton collision data sample collected by the LHCb experiment, corresponding to an integrated luminosity of 8.5 fb$^{-1}$, the observation of a new excited $\Xi_b^0$ resonance decaying to the $\Xi_b^-\pi^+$ final state is presented. The state, referred to as $\Xi_b(6227)^0$, has a measured mass and natural width of $m(\Xi_b(6227)^0) = 6227.1^{ +1.4}_{ -1.5}\pm0.5$ MeV, $\Gamma(\Xi_b(6227)^0) = 18.6^{ +5.0}_{ -4.1}\pm1.4$ MeV, where the uncertainties are statistical and systematic. The production rate of the $\Xi_b(6227)^0$ state relative to that of the $\Xi_b^-$ baryon in the kinematic region $2<\eta<5$ and $p_{\rm T}<30$ GeV is measured to be $\frac{f_{\Xi_b(6227)^0}}{f_{\Xi_b^-}}{\mathcal{B}}(\Xi_b(6227)^0\to\Xi_b^-\pi^+) = 0.045\pm0.008\pm0.004$, where ${\mathcal{B}}(\Xi_b(6227)^0\to\Xi_b^-\pi^+)$ is the branching fraction of the decay, and $f_{\Xi_b(6227)^0}$ and $f_{\Xi_b^-}$ represent fragmentation fractions. Improved measurements of the mass and natural width of the previously observed $\Xi_b(6227)^-$ state, along with the mass of the $\Xi_b^-$ baryon, are also reported. Both measurements are significantly more precise than, and consistent with, previously reported values.

Figures and captions

Invariant-mass spectra for (left) $\Xi ^-_ b \rightarrow \Xi ^0_ c \pi ^- $ and (right) $\Xi ^-_ b \rightarrow \Xi ^0_ c \pi ^- \pi ^+ \pi ^- $ candidates after all selection requirements. Projections of the fits to the data are overlaid.

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Invariant-mass spectra for (left) $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ and (right) $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- \pi ^+ \pi ^- $ candidates after all selection requirements. Projections of the fits to the data are overlaid.

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Distribution of reconstructed $\delta M_{\pi}=M(\Xi ^-_ b \pi ^+ )-M(\Xi ^-_ b )$ in $\Xi _ b (6227)^{0} \rightarrow \Xi ^-_ b \pi ^+ $ candidate decays, with (top) $\Xi ^-_ b \rightarrow \Xi ^0_ c \pi ^- $ decays, and (bottom) $\Xi ^-_ b \rightarrow \Xi ^0_ c \pi ^- \pi ^+ \pi ^- $ decays. The left column shows the right-sign candidates and the right column shows the wrong-sign candidates. The fit projections are overlaid.

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Distribution of reconstructed $\delta M_K = M(\Lambda ^0_ b K ^- )-M(\Lambda ^0_ b )$ in $\Xi _ b (6227)^{-} \rightarrow \Lambda ^0_ b K ^- $ candidate decays, with (left) $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ and (right) $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- \pi ^+ \pi ^- $ candidates. The fit projections are overlaid.

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Distribution of (left) $\Xi ^0_ c \pi ^- $ and (right) $\Lambda ^+_ c \pi ^- $ invariant mass for the combined Run 1 and Run 2 data sets, with extra PID selection requirements with respect to the samples shown in Figs. 1 and 2. The fit projections are overlaid.

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Animated gif made out of all figures.

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Tables and captions

Signal yields of $\Xi ^-_ b $ and $\Xi _ b (6227)^{0} $ decays for the full data set after all selection requirements, and the corresponding Run 2 signal yields used for the measurement of $R(\Xi ^-_ b \pi ^+ )$ at 13 $\text{ Te V}$ .

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Signal yields of $\Lambda ^0_ b $ and $\Xi _ b (6227)^{-} $ decays for the full data set after all selection requirements.

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The fitted signal yields and masses of the $\Xi ^-_ b $ and $\Lambda ^0_ b $ peaks and the mass differences, $m_{\rm diff}\equiv m_{\rm fit}(\Xi ^-_ b )-m_{\rm fit}(\Lambda ^0_ b )$, for each center-of-mass energy and for the full data sample. For the last row, the known $\Lambda ^0_ b $ mass [62] is used. Uncertainties are statistical only.

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Summary of systematic uncertainties on quantities related to the $\Xi _ b (6227)^{0} $ ($\delta m^{\rm peak}_{\pi}$, $\Gamma(\Xi _ b (6227)^{0} )$, $R(\Xi ^-_ b \pi ^+ )$), the $\Xi _ b (6227)^{-} $ ($\delta m^{\rm peak}_{K}$, $\Gamma(\Xi _ b (6227)^{-} $), and the $\Xi ^-_ b $ mass ($m_{\rm diff}$) measurements. The statistical uncertainties are also reported for comparison.

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2020-032. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps : The figures in various formats

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Created on 26 February 2021.