A report from the ALICE experiment
Hypernuclei are bound states of nucleons and hyperons. Studying their properties is one of the best ways to investigate hyperon–nucleon interactions and offers insights into the high-density inner cores of neutron stars, which favour the creation of the exotic nuclear states. Constraining such astrophysical models requires detailed knowledge of hyperon–nucleon and three-body hyperon–nucleon–nucleon interactions. The strengths of these interactions can be determined in collider experiments by precisely measuring the lifetimes of hypernuclei.
Hypernuclei are produced in significant quantities in heavy-ion collisions at LHC energies. The lightest, the hypertriton, is a bound state of a proton, a neutron and a Λ. With a Λ-separation energy of only ~130 keV, the average distance between the Λ and the deuteron core is 10.6 fm. This relatively large separation implies only a small perturbation to the Λ wavefunction inside the hypernucleus, and therefore a hypertriton lifetime close to that of a free Λ, 263.2 ± 2.0 ps. Most calculations predict the hypertriton lifetime to be in the range 213 to 256 ps.
The measured lifetimes were systematically below theoretical predictions
The first measurements of the hypertriton lifetime were performed in the 1960s and 1970s with imaging techniques such as photographic emulsions and bubble chambers, and were based on very small event samples, leading to large statistical uncertainties. In the last decade, however, measurements have been performed using the larger data samples of heavy-ion collisions. Though compatible with theory, the measured lifetimes were systematically below theoretical predictions: thus the so-called “lifetime puzzle”.
The ALICE collaboration has recently reported a new measurement of the hypertriton lifetime using Pb–Pb collisions at √sNN = 5.02 TeV, which were collected in 2015. The lifetime of the (anti-)hypertriton is determined by reconstructing the two-body decay channel with a charged pion, namely 3ΛH →3 He + π– (3Λ̅ H →3 He + π+). The branching ratio of this decay channel, taken from the theoretical calculations, is 25%. The measured lifetime is 242+34–38(stat) ± 17 (syst) ps. This result shows an improved statistical resolution and reduced systematic uncertainty compared to previous measurements and is currently the most precise measurement. It is also in agreement with both theoretical predictions and the free-Λ lifetime, even within the statistical uncertainty. Combining this ALICE result with previous measurements gives a weighted average of 206+15–13ps (figure 1).
This result represents an important step forward in solving the longstanding hypertriton lifetime puzzle, since it is the first measurement with a large data sample that is close to theoretical expectations. Larger and more precise data sets are expected to be collected during LHC Runs 3 and 4, following the ongoing major upgrade of ALICE. This will allow a significant improvement in the quality of the present lifetime measurement, and the determination of the Λ binding energy with high precision. The combination of these two measurements has the potential to constrain the branching ratio for this decay, which cannot be determined directly without access to the neutral and non-mesonic decay channels. This will be a crucial step towards understanding if the now partially confirmed theoretical description of the hypertriton is finally resolved.
ALICE Collab. 2019 arXiv:1907.06906 (Phys. Lett. B 797 134905).