Measurement of the $Λ$ hyperon lifetime

A new, more precise measurement of the $\Lambda$ hyperon lifetime is performed using a large data sample of Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV with ALICE. The $\Lambda$ and $\overline{\Lambda}$ hyperons are reconstructed at midrapidity using their two-body weak decay channel $\Lambda \rightarrow \mathrm{p} + \pi^{-}$ and $\overline{\Lambda} \rightarrow \overline{\mathrm{p}} + \pi^{+}$. The measured value of the $\Lambda$ lifetime is $\tau_{\Lambda} = [261.07 \pm 0.37 \ ( \rm stat.) \pm 0.72 \ (\rm syst.) ]\ \rm ps$. The relative difference between the lifetime of $\Lambda$ and $\overline{\Lambda}$, which represents an important test of CPT invariance in the strangeness sector, is also measured. The obtained value $(\tau_{\Lambda}-\tau_{\overline{\Lambda}})/\tau_{\Lambda} = 0.0013 \pm 0.0028 \ (\mathrm{stat.}) \pm 0.0021 \ (\mathrm{syst.})$ is consistent with zero within the uncertainties. Both measurements of the $\Lambda$ hyperon lifetime and of the relative difference between $\tau_{\Lambda}$ and $\tau_{\overline{\Lambda}}$ are in agreement with the corresponding world averages of the Particle Data Group and about a factor of three more precise.

 

Submitted to: PRD
e-Print: arXiv:2303.00606 | PDF | inSPIRE
CERN-EP-2023-020
Figure group

Figure 1

Invariant mass spectra of p$\pi$ pairs measured in central collisions (0--10$\%$) at low (left) and large (right) $L_{\rm proper}$. The green area indicates the peak region.

Figure 2

Reconstruction efficiency of primary $\Lambda$ ($\overline{\Lambda}$) (left) and fraction of secondary $\Lambda$ ($\overline{\Lambda}$) (right) in central Pb--Pb collisions (0--10$\%$).

Figure 3

$L_{\rm proper}$ spectrum of $\Lambda$ and $\overline{\Lambda}$ and exponential fit for the lifetime extraction. Only statistical uncertainties are shown for each data point and for the mean lifetime extracted from the exponential fit.

Figure 4

$\Lambda$ lifetime measured in different centrality intervals. Only statistical uncertainties are shown.

Figure 5

ALICE measurement of the $\Lambda$ lifetime in comparison with previous measurements  and the current world average taken from the PDG . Statistical and systematic uncertainties are added in quadrature.

Figure 6

Asymmetry parameter $( \tau_{\Lambda} - \tau_{\overline{\Lambda}}) / \tau_{\Lambda}$ in comparison with the PDG value . Statistical and systematic uncertainties are added in quadrature.