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Precision measurement of the mass and lifetime of the $\Xi_b^-$ baryon

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Abstract

We report on measurements of the mass and lifetime of the Xi_b^- baryon using about 1800 Xi_b^- decays reconstructed in a proton-proton collision data set corresponding to an integrated luminosity of 3.0 fb^{-1} collected by the LHCb experiment. The decays are reconstructed in the Xi_b^- --> Xi_c^0 pi^-, Xi_c^0 --> pK-K-pi+ channel and the mass and lifetime are measured using the Lambda_b^0 --> Lambda_c^+ pi- mode as a reference. We measure M(Xi_b^-) - M(Lambda_b^0) = 178.36 +- 0.46 +- 0.16 MeV/c^2 and tau(Xi_b^-)/tau(Lambda_b^0) = 1.089 +- 0.026 +- 0.011, where the uncertainties are statistical and systematic, respectively. These results lead to a factor of two better precision on the Xi_b^- mass and lifetime compared to previous best measurements, and are consistent with theoretical expectations.

Figures and captions

Invariant mass spectrum, along with the fit projections, for (left) $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ and (right) $\Xi ^-_ b \rightarrow \Xi ^0_ c \pi ^- $ candidates.

Lb2LcP[..].pdf [22 KiB]
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Lb2LcPi_LbFit_def_Y0_07302014.pdf
Xib2Xi[..].pdf [30 KiB]
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Xib2XicPi_LbFit_def_Y0_07302014.pdf

Ratio of the $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ to the $\Xi ^-_ b \rightarrow \Xi ^0_ c \pi ^- $ selection efficiencies as a function of decay time. The uncertainties are due to the finite size of the simulated samples.

TauEff[..].pdf [13 KiB]
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TauEffRatioPlots_Xib2Lb_def.pdf

Corrected yield ratio, $N_{\rm cor}(\Xi ^-_ b )/N_{\rm cor}(\Lambda ^0_ b )$ in bins of decay time, along with the exponential fit. The uncertainties are statistical only.

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CorrYieldRatio_XibOverLb_def.pdf

Animated gif made out of all figures.

PAPER-2014-048.gif
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Tables and captions

Fitted yields of $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^- $ and $\Xi ^-_ b \rightarrow \Xi ^0_ c \pi ^- $ in each time bin. Uncertainties are statistical only.

Table_1.pdf [50 KiB]
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Table_1.pdf

Supplementary Material [file]

Supplementary material full pdf

supple[..].pdf [129 KiB]
supplementary.pdf

Contained are supplementary figures for LHCb-PAPER-2014-048 Fig 4: Invariant mass spectrum for $\Xicz\to p\Km\Km\pip$ candidates that form a $\Xibm$ candidate within 80\mevcc of the $\Xibm$ fitted mass. Fig 5: Invariant mass spectrum for $\Bm\to\Dz\pim$ candidates, with $\Dz\to\Kp\Km\pip\pim$. The analysis is redone using this mode as a normalization mode to provide a cross-check of our nominal results using the $\Lb\to\Lc\pim$ decay mode as a reference. The fitted mass difference is $M(\Xibm)-M(\Bm)=518.72\pm0.51$\,(stat)\mevcc, resulting in a $\Xibm$ mass of $M(\Xibm)=5797.98\pm0.51$\,(stat) \mevcc. Fig 6: Corrected yield ratio, $N_{\rm cor}(\Xibm)/N_{\rm cor}(\Bm)$ in bins of decay time, along with the exponential fit. The uncertainties are statistical only. The fitted slope $\beta=-0.0123\pm0.0172$\,(stat) ps$^{-1}$. The resulting lifetime using the $\Bm$ mode for normalization is $\tau(\Xibm)=1.609\pm0.045$\,(stat). Fig. 7: Comparison of measurements of the $\Xibm$ and $\Xibz$ masses, along with this measurement. Fig 8: Comparison of measurements of the $\Xibm$ and $\Xibz$ lifetimes, along with this measurement.

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