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Measurement of the mass and production rate of $\Xi_b^-$ baryons

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Abstract

The first measurement of the production rate of $\Xi_b^-$ baryons in $pp$ collisions relative to that of $\Lambda_b^0$ baryons is reported, using data samples collected by the LHCb experiment, and corresponding to integrated luminosities of $1$, $2$ and $1.6$ fb$^{-1}$ at $\sqrt{s}=7, 8$ and $13$ TeV, respectively. In the kinematic region $2 < \eta < 6$ and $p_{\rm T}<20$ GeV/$c$, we measure \begin{align*} \frac{f_{\Xi_b^-}}{f_{\Lambda_b^0}}\frac{{\mathcal{B}}(\Xi_b^-\to J/\psi\Xi^-)}{{\mathcal{B}}(\Lambda_b^0\to J/\psi\Lambda)} &= (10.8\pm0.9\pm0.8)\times10^{-2} [\sqrt{s}=7,8 {\rm TeV}],\\ \frac{f_{\Xi_b^-}}{f_{\Lambda_b^0}}\frac{{\mathcal{B}}(\Xi_b^-\to J/\psi\Xi^-)}{{\mathcal{B}}(\Lambda_b^0\to J/\psi\Lambda)} &= (13.1\pm1.1\pm1.0)\times10^{-2} [\sqrt{s}=13 {\rm TeV}], \end{align*} where $f_{\Xi_b^-}$ and $f_{\Lambda_b^0}$ are the fragmentation fractions of $b$ quarks into $\Xi_b^-$ and $\Lambda_b^0$ baryons, respectively, ${\mathcal{B}}$ represents branching fractions, and the uncertainties are due to statistical and experimental systematic sources. The values of $f_{\Xi_b^-}/f_{\Lambda_b^0}$ are obtained by invoking SU(3) symmetry in the $\Xi_b^-\to J\psi\Xi^-$ and $\Lambda_b^0\to J/\psi\Lambda$ decays. Production asymmetries between $\Xi_b^-$ and $\bar{\Xi}_b^+$ baryons are also reported. The mass of the $\Xi_b^-$ baryon is also measured relative to that of the $\Lambda_b^0$ baryon, from which it is found that \begin{align*} m(\Xi_b^-) &= 5796.70\pm0.39\pm0.15\pm0.17 {\rm MeV}/c^2, \end{align*} where the last uncertainty is due to the precision on the known $\Lambda_b^0$ mass. This result represents the most precise determination of the $\Xi_b^-$ mass.

Figures and captions

Invariant-mass distributions for (top) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ candidates, (middle) $\Xi ^-_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Xi ^- $, $\Xi ^- \rightarrow \Lambda \pi^-_{\rm L}$, and (bottom) $\Xi ^-_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Xi ^- $, $\Xi ^- \rightarrow \Lambda \pi^-_{\rm D}$. The subscript on the $\pi ^- $ refers to whether the corresponding track is long or downstream. The left column shows the combined 7 and 8 $\text{ Te V}$ data and the right one shows the 13 $\text{ Te V}$ data. The fitted probability distributions functions (PDF) are overlaid.

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MassFit_LbDD_Run1_constr.pdf
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MassFit_XibDDL_Run1_constr.pdf
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MassFit_XibDDL_Run2_constr.pdf
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MassFit_XibDDD_Run1_constr.pdf
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MassFit_XibDDD_Run2_constr.pdf

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Tables and captions

Fitted signal yields and peak position of the $\Lambda ^0_ b $ signal peak, as obtained from the fit described in the text. The subscript on the $\pi ^- $ refers to whether the corresponding track is long or downstream. The uncertainties shown are statistical only.

Table_1.pdf [71 KiB]
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Selection efficiencies as obtained from the simulation of $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ and $\Xi ^-_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Xi ^- $, $\Xi ^- \rightarrow \Lambda \pi^-_{\rm L}$ decays at $\sqrt{s}=7, 8$ $\text{ Te V}$ and 13 $\text{ Te V}$ . The efficiencies ($\epsilon$) listed are those associated with the detector acceptance (acc), the reconstruction and selection (sel), the trigger (trig), their product, and the relative efficiency.

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Yields of $\Lambda ^0_ b $ and $\Xi ^-_ b $ decays, split by the charge of the final state, and their asymmetries, for the combined 7, 8 $\text{ Te V}$ data samples and the 13 $\text{ Te V}$ data sample. Uncertainties are statistical only.

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Summary of relative systematic uncertainties on the production ratio $R$.

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Created on 14 June 2019.Citation count from INSPIRE on 14 June 2019.