Measurement of the Lifetime and $\mathrm{\Lambda }$ Separation Energy of ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$

The most precise measurements to date of the ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ lifetime $\tau$ and $\mathrm{\Lambda }$ separation energy ${\mathrm{B}}_{\mathrm{\Lambda }}$ are obtained using the data sample of Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ collected by ALICE at the LHC. The ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ is reconstructed via its charged two-body mesonic decay channel (${}_{\mathrm{\Lambda }}{}^3\mathrm{H}\to {}^3\mathrm{He}+{\pi }^{-}$ and the charge-conjugate process). The measured values $\tau =[253\pm 11(\mathrm{stat})\pm 6(\mathrm{syst})]\mathrm{ps}$ and ${\mathrm{B}}_{\mathrm{\Lambda }}=[102\pm 63(\mathrm{stat})\pm 67(\mathrm{syst})]\mathrm{keV}$ are compatible with predictions from effective field theories and confirm that the ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ structure is consistent with a weakly bound system.

Figure 1

Left: exponential decay spectrum as a function of the proper decay length for ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$, the blue points represent the measured yield, while the orange line represents the best fit to the measurement. Right: ${\mathrm{B}}_{\mathrm{\Lambda }}$ measurement as a function of the proper decay length. Only statistical uncertainties are shown; see the text for a description of the determination of the systematic uncertainties. The fit probability computed with a Pearson test is reported.

Figure 2

The ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ lifetime relative to the free $\mathrm{\Lambda }$ lifetime as a function of the ${\mathrm{B}}_{\mathrm{\Lambda }}$ for pionless EFT [19] (green), $\chi \mathrm{EFT}$ [20] (light blue), and the original $\pi$ exchange calculations [50] (blue). The red point represents the measurement presented in this Letter with the statistical and total uncertainties depicted with lines and an ellipse, respectively.

Figure 3

Collection of the ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ lifetime (left) [21, 22, 23, 24, 25, 26, 51, 52, 53, 54, 55, 56] and ${\mathrm{B}}_{\mathrm{\Lambda }}$ (right) [27, 55, 57, 58, 59, 60, 61, 62] measurements obtained with different experimental techniques. The horizontal lines and boxes are the statistical and systematic uncertainties, respectively. The dashed and dash-dotted lines are the corresponding theoretical predictions [10, 17, 19, 20, 50, 63, 64, 65]. Two predictions are reported in [20]: prediction A assumes ${\mathrm{B}}_{\mathrm{\Lambda }}=130\mathrm{keV}$, while prediction B assumes ${\mathrm{B}}_{\mathrm{\Lambda }}=69\mathrm{keV}$.