Precision measurement of the ratio of the $\Lambda^0_b$ to $\overline{B}^0$ lifetimes

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The LHCb measurement of the lifetime ratio of the $\Lambda^0_b$ to the $\overline{B}^0$ meson is updated using data corresponding to an integrated luminosity of 3.0 fb$^{-1}$ collected using 7 and 8 TeV centre-of-mass energy $pp$ collisions at the LHC. The decay modes used are $\overline{B}^0\to J/\psi p K^-$ and $\overline{B}^0\to J/\psi \pi^+ K^-$, where the $\pi^+K^-$ mass is consistent with that of the $\overline{K}^{*0}(892)$ meson. The lifetime ratio is determined with unprecedented precision to be $0.974\pm0.006\pm0.004$, where the first uncertainty is statistical and the second systematic. This result is in agreement with original theoretical predictions based on the heavy quark expansion. Using the current world average of the $\overline{B}^0$ lifetime, the $\Lambda^0_b$ lifetime is found to be $1.479 \pm 0.009 \pm 0.010$ ps.

Figures and captions

BDT classifier output for the signal and background. Both training and test samples are shown; their definitions are given in the text.

BDT_re[..].pdf [41 KiB]
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Fits to the invariant mass spectrum of (a) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} pK^-$ and (b) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+K^-$ combinations. The $\Lambda ^0_ b $ and $\overline{ B }{} ^0$ signals are shown by the (magenta) solid curves. The (black) dotted lines are the combinatorial backgrounds, and the (blue) solid curves show the totals. In (a) the $\overline{ B }{} ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K^+K^-$ and $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+K^-$ reflections, caused by particle misidentification, are shown with the (brown) dot-dot-dashed and (red) dot-dashed shapes, respectively, and the (green) dashed shape represents the doubly misidentified $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K^{+}\overline{p}$ final state, where the kaon and proton masses are swapped. In (b) the $ B ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+K^-$ mode is shown by the (red) dashed curve and the (green) dot-dashed shape represents the $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ reflection.

signal[..].pdf [37 KiB]
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Invariant mass distributions of $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} pK^-$ data candidates in the sideband regions $60-200$ MeV on either side of the $\Lambda ^0_ b $ mass peak, reinterpreted as misidentified (a) $\overline{ B }{} ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K^+K^-$ and (b) $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+ K ^- $ combinations through appropriate mass reassignments. The (red) dashed curves show the $\overline{ B }{} ^0 $ contributions and the (green) dot-dashed curves show $\overline{ B }{} ^0_ s $ contributions. The (black) dotted curves represent the polynomial background and the (blue) solid curves the total.

LbRefl.pdf [37 KiB]
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(a) Decay time acceptances (arbitrary scale) from simulation for (green) circles $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} pK^-$, and (red) open-boxes $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \overline{ K }{} ^{*0} (892)$ decays. (b) Ratio of the decay time acceptances between $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} pK^-$ and $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \overline{ K }{} ^{*0} (892)$ decays obtained from simulation. The (blue) line shows the result of the linear fit.

Accept[..].pdf [46 KiB]
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Decay time distributions for $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} pK^-$ shown as (blue) circles, and $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \overline{ K }{} ^{*0} (892)$ shown as (green) squares. For most entries the error bars are smaller than the points.

Yield.pdf [28 KiB]
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Decay time ratio between $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} pK^-$ and $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \overline{ K }{} ^{*0} (892)$ decays, and the fit for $\Delta_{\Lambda B}$ used to measure the $\Lambda ^0_ b $ lifetime.

Ratio_[..].pdf [28 KiB]
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Animated gif made out of all figures.


Tables and captions

Systematic uncertainties on the $\Delta_{\Lambda B}$, the lifetimes ratio $\tau_{\Lambda ^0_ b }/\tau_{\overline{ B }{} ^0 }$ and the $\Lambda ^0_ b $ lifetime. The systematic uncertainty associated with $\Delta_{\Lambda B}$ is independent of the $\overline{ B }{} ^0 $ lifetime.

Table_1.pdf [61 KiB]
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Created on 20 February 2021.