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

R. Abou Yassine, J. Adamczewski-Musch, C. Asal, M. Becker, A. Belounnas, A. Blanco, C. Blume, L. Chlad, P. Chudoba, I. Ciepał, J. Dreyer, W. A. Esmail, L. Fabbietti, H. Floersheimer, J. Förtsch, P. Fonte, J. Friese, I. Fröhlich, T. Galatyuk, R. Greifenhagen, M. Grunwald, M. Gumberidze, S. Harabasz, T. Heinz, C. Höhne, F. Hojeij, R. Holzmann, H. Huck, M. Idzik, B. Kämpfer, K-H. Kampert, B. Kardan, V. Kedych, S. Kim, I. Koenig, W. Koenig, M. Kohls, J. Kolas, G. Kornakov, R. Kotte, I. Kres, W. Krueger, A. Kugler, R. Lalik, S. Lebedev, S. Linev, F. Linz, L. Lopes, M. Lorenz, A. Malige, J. Markert, T. Matulewicz, S. Maurus, V. Metag, J. Michel, A. Molenda, C. Müntz, M. Nabroth, L. Naumann, K. Nowakowski, A. Opíchal, J. Orliński, J. -H. Otto, M. Parschau, C. Pauly, D. Pawlowska-Szymanska, V. Pechenov, O. Pechenova, D. Pfeifer, K. Piasecki, J. Pietraszko, T. Povar, K. Prościński, A. Prozorov, W. Przygoda, K. Pysz, B. Ramstein, N. Rathod, J. Ritman, A. Rost, A. Rustamov, P. Salabura, J. Saraiva, K. Scharmann, N. Schild, E. Schwab, F. Scozzi, F. Seck, I. Selyuzhenkov, U. Singh, L. Skorpil, J. Smyrski, S. Spies, A. Sreejith, H. Ströbele, J. Stroth, K. Sumara, O. Svoboda, M. Szala, P. Tlusty, M. Traxler, S. Treliński, I. C. Udrea, F. Ulrich-Pur, C. Ungethum, V. Wagner, A. A. Weber, C. Wendisch, J. Wirth, A. Władyszewska, H. P. Zbroszczyk, E. Zherebtsova, M. Zieliński, P. Zumbruch

TL;DR

This work reports the first observation of $^{3}_{\Lambda}$H and $^{4}_{\Lambda}$H produced in central $\mathrm{Ag+Ag}$ collisions at $\sqrt{s_{NN}} = 2.55$ GeV, reconstructed via two-body weak decays with an artificial neural network to suppress background. The study provides multi-differential yields and bell-shaped rapidity distributions centered at mid-rapidity, and extracts lifetimes by analyzing decay curves, finding $\tau(^{3}_{\Lambda}$H) = $(239 \pm 23_{stat} \pm 18_{sys})$ ps and $\tau(^{4}_{\Lambda}$H) = $(209 \pm 7_{stat} \pm 10_{sys})$ ps; the former agrees with the free $\Lambda$ within $1\sigma$, while the latter deviates by $4.5\sigma$. The integrated yields show $^{4}_{\Lambda}$H being equal to or more abundant than $^{3}_{\Lambda}$H, contrasting some higher-energy results and hinting at feed-down from excited $A=4$ states. These results extend the available world data on light hypernuclei and provide new constraints on hyperon–nucleon interactions at high baryon density.

Abstract

We present the first observation of $\mathrm{^{3}_ΛH}$ and $\mathrm{^{4}_ΛH}$ 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 $Λ$ hyperons. The resulting rapidity distributions, dN/dy, exhibit a bell shape centered at mid-rapidity. The yield of $\mathrm{^{4}_ΛH}$ is equal to or exceeds that of $\mathrm{^{3}_ΛH}$, 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 $\mathrm{^{3}_ΛH}$, a lifetime of $\mathrm{τ_{^{3}_ΛH}}$ = $(239 \pm 23{\mathrm{(stat)}} \pm 18{\mathrm{(sys)}})\,\mathrm{ps}$, is extracted, consistent on the 1$σ$ level with that of the free $Λ$. In contrast, the $\mathrm{^{4}_ΛH}$ lifetime of $\mathrm{τ_{^{4}_ΛH}}$ = $(209 \pm 7{\mathrm{(stat)}} \pm 10{\mathrm{(sys)}})\,\mathrm{ps}$, shows a 4.5 $σ$ deviation from the free $Λ$ lifetime. The results consolidate the available world data.

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

TL;DR

This work reports the first observation of H and H produced in central collisions at GeV, reconstructed via two-body weak decays with an artificial neural network to suppress background. The study provides multi-differential yields and bell-shaped rapidity distributions centered at mid-rapidity, and extracts lifetimes by analyzing decay curves, finding H) = ps and H) = ps; the former agrees with the free within , while the latter deviates by . The integrated yields show H being equal to or more abundant than H, contrasting some higher-energy results and hinting at feed-down from excited states. These results extend the available world data on light hypernuclei and provide new constraints on hyperon–nucleon interactions at high baryon density.

Abstract

We present the first observation of and in Ag+Ag collisions at = 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 hyperons. The resulting rapidity distributions, dN/dy, exhibit a bell shape centered at mid-rapidity. The yield of is equal to or exceeds that of , which contrasts the measurement from the STAR collaboration at = 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 , a lifetime of = , is extracted, consistent on the 1 level with that of the free . In contrast, the lifetime of = , shows a 4.5 deviation from the free lifetime. The results consolidate the available world data.

Paper Structure

This paper contains 5 sections, 5 equations, 6 figures.

Table of Contents

  1. Introduction
  2. Experimental Setup and Reconstruction
  3. Transverse Momentum Spectra, Rapidity Distributions and Yields
  4. Decay Curves
  5. Summary

Figures (6)

  • Figure 1: Upper panel: Invariant mass spectra for the same-event (black) and mixed-event (blue) combinations of 3He- and $\pi$---tracks. The mixed-event is normalized to the same-event outside of a ± 5$\sigma$ region around the nominal 3$\Lambda$H mass. Lower panel: Same, but for combinations of 4He- and $\pi$---tracks.
  • Figure 2: Acceptance- and efficiency-corrected transverse momentum $p_{\mathrm{T}}$ spectra for subsequent slices of rapidity $y_{\mathrm{cm}}$ for 3$\Lambda$H (upper panel) and 4$\Lambda$H (lower panel) nuclei. Displayed is the yield per event, per transverse momentum interval, and per unit of rapidity for the 0--25% most central Ag(1.58A GeV)+Ag collisions. For a better representation, the spectra are scaled by consecutive factors of 10 for each rapidity slice. The boxes indicate the systematic uncertainties, while the curves correspond to fits using Eq. (1), see text for details.
  • Figure 3: Rapidity distributions $dN/dy$ for the 3$\Lambda$H (upper panel) and the 4$\Lambda$H (lower panel), obtained by integrating the data as a function of transverse momentum $p_{\mathrm{T}}$ and using fit functions to extrapolate into $p_{\mathrm{T}}$ regions not covered by our detector. The boxes indicate the systematic uncertainties, while the curves correspond to distributions motivated by the thermal ansatz fitted to the data; see text for details.
  • Figure 4: Compilation of energy excitation function of 3$\Lambda$H (empty symbols) and 4$\Lambda$H (fat symbols) yields emitted around mid-rapidity in central heavy-ion collisions from bib:STAR_Hypertriton2022bib:STAR_Hypertriton2025bib:ALICE_Hypertriton2019, the preliminary STAR data bib:STAR_Hypertriton2025, and the present work.
  • Figure 5: Decay curves of 3$\Lambda$H (upper plot, black dots) and 4$\Lambda$He (lower plot, black dots). For comparison also the decay curve for $\Lambda$ hyperons (blue dots) is plotted. The decay curves are fitted with an exponential (solid curve) to extract values for the mean lifetime $\tau$, see text for details.
  • ...and 1 more figures