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Study of the kinematic dependences of $\Lambda_b^0$ production in $pp$ collisions and a measurement of the $\Lambda_b^0 \rightarrow \Lambda_c^+ \pi^-$ branching fraction

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Abstract

The kinematic dependences of the relative production rates, $f_{\Lambda_b^0}/f_d$, of $\Lambda_b^0$ baryons and $\bar{B}^0$ mesons are measured using $\Lambda_b^0 \rightarrow \Lambda_c^+ \pi^-$ and $\bar{B}^0 \rightarrow D^+ \pi^-$ decays. The measurements use proton-proton collision data, corresponding to an integrated luminosity of 1 fb$^{-1}$ at a centre-of-mass energy of 7 TeV, recorded in the forward region with the LHCb experiment. The relative production rates are observed to depend on the transverse momentum, $p_T$, and pseudorapidity, $\eta$, of the beauty hadron, in the studied kinematic region $1.5 < p_T < 40$ GeV/$c$ and $2 < \eta < 5$. Using a previous LHCb measurement of $f_{\Lambda_b^0}/f_d$ in semileptonic decays, the branching fraction $\mathcal{B}(\Lambda_b^0 \rightarrow \Lambda_c^+ \pi^-) = \Big( 4.30 \pm 0.03 \,\, ^{+0.12}_{-0.11} \pm 0.26 \pm 0.21 \Big) \times 10^{-3}$ is obtained, where the first uncertainty is statistical, the second is systematic, the third is from the previous LHCb measurement of $f_{\Lambda_b^0}/f_d$ and the fourth is due to the $\bar{B}^0 \rightarrow D^+ \pi^-$ branching fraction. This is the most precise measurement of a $\Lambda_b^0$ branching fraction to date.

Figures and captions

Ratio of total selection efficiencies in bins of the (a) $p_{\rm T}$ and (b) $\eta$ of the $b$ hadron. The horizontal error bars indicate the range of each bin in $p_{\rm T}$ or $\eta$ respectively.

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Invariant mass distributions of (a,c) $\Lambda ^+_ c \pi ^- $ candidates and (b,d) $ D ^+ \pi ^- $ candidates for specific ranges in $p_{\rm T}$ and $\eta$ of the $b$ hadron, with fit projections overlaid. The different components are defined in the legend, where "part reco" refers to the sum of partially reconstructed decays.

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(a) Dependence of the efficiency-corrected ratio of yields, $\mathcal{R}$, between \LbLcPi and $\overline{ B }{} ^0 \rightarrow D ^+ \pi ^- $ decays on the $p_{\rm T}$ of the beauty hadron, fitted with an exponential function. The error bars on the data show the statistical and systematic uncertainties added in quadrature. (b) The resulting parametrisation is then fitted to the rescaled $ f_{\Lambda ^0_ b }/f_ d $ measurements from the semileptonic analysis [7], to obtain the scale factor $\mathcal{S}$. The error bars include only the statistical uncertainty.

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Dependence of $ f_{\Lambda ^0_ b }/f_ d $ on the (a) $p_{\rm T}$ and (b) $\eta$ of the beauty hadron. To obtain this figure, the ratio of efficiency-corrected event yields is scaled to the absolute value of $ f_{\Lambda ^0_ b }/f_ d $ from the semileptonic analysis [7]. The error bars include the statistical and systematic uncertainties associated with the hadronic measurement. The dashed red lines indicate the uncertainty on the scale of $ f_{\Lambda ^0_ b }/f_ d $ from the semileptonic analysis.

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Animated gif made out of all figures.

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Tables and captions

Relative systematic uncertainties for the measurements of $\mathcal{R}(x)$ (first five columns) and $\cal B$ (\LbLcPi) (last column). The uncertainties from the various sources are uncorrelated and added in quadrature to obtain the total uncertainty. Sample size refers to the size of the simulated events sample.

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Supplementary Material [file]

Supplementary material full pdf

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This zip files contains supplementary material for the publication LHCb-PAPER-2014-004. The files are: supplementary.pdf: An overview of the extra material *pdf,*png,*eps,*C: Fig1 in supplementary.pdf in various formats *txt: Tables 1 and 2 in supplementary.pdf

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.