Formation and lifetime measurements of light hypernuclei in Ag+Ag collisions at $\mathrm{\sqrt{s_{NN}}}$ = 2.55 GeV

TL;DR

This study reports the first observation of light hypernuclei in Ag+Ag collisions at 2.55 GeV, measuring their yields and lifetimes, revealing differences from free lambda decay and providing high-precision data to inform hypernuclear physics.

Contribution

It introduces the first detection of $ ext{^{3}_ extLambda H}$ and $ ext{^{4}_ extLambda H}$ in Ag+Ag collisions at this energy, with novel lifetime measurements and yield comparisons.

Findings

Hypernuclei are observed around mid-rapidity.

The yield of $ ext{^{4}_ extLambda H}$ exceeds that of $ ext{^{3}_ extLambda H}$.

Lifetimes are measured with high precision, showing deviations from free lambda decay.

Abstract

We present the first observation of $\mathrm{^{3}_\Lambda H}$ and $\mathrm{^{4}_\Lambda H}$ in Ag+Ag collisions at $\mathrm{\sqrt{s_{NN}}}$ = 2.55 GeV, emitted around mid-rapidity. The hypernuclei are reconstructed via their two-body decay channels and identified through their weak-decay topology, employing an artificial neural network for enhanced discrimination. The analysis methodology is validated using $\Lambda$ hyperons. The resulting rapidity distributions, dN/dy, exhibit a bell shape centered at mid-rapidity. The yield of $\mathrm{^{4}_\Lambda H}$ is equal to or exceeds that of $\mathrm{^{3}_\Lambda H}$, which contrasts the measurement from the STAR collaboration at $\mathrm{\sqrt{s_{NN}}}$ = 3 GeV and is consistent with a scenario in which hypernuclei receive feed-down from excited states. The data enable a high-precision measurement of the hypernuclei lifetimes. For the $\mathrm{^{3}_\Lambda H}$, a lifetime of $\mathrm{\tau_{^{3}_\Lambda H}}$ = $(239 \pm 23{\mathrm{(stat)}} \pm 18{\mathrm{(sys)}})\,\mathrm{ps}$, is extracted, consistent on the 1$\sigma$ level with that of the free $\Lambda$. In contrast, the $\mathrm{^{4}_\Lambda H}$ lifetime of $\mathrm{\tau_{^{4}_\Lambda H}}$ = $(209 \pm 7{\mathrm{(stat)}} \pm 10{\mathrm{(sys)}})\,\mathrm{ps}$, shows a 4.5 $\sigma$ deviation from the free $\Lambda$ lifetime. The results consolidate the available world data.