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Precise measurement of the $Λ$-binding energy difference between $^3_Λ$H and $^4_Λ$H via decay-pion spectroscopy at MAMI

Ryoko Kino, Sho Nagao, Patrick Achenbach, Satoshi N. Nakamura, Josef Pochodzalla, Takeru Akiyama, Ralph Böhm, Mirco Christmann, Michael O. Distler, Luca Doria, Anselm Esser, Julian Geratz, Christian Helmel, Matthias Hoek, Tatsuhiro Ishige, Masashi Kaneta, Pascal Klag, David Markus, Harald Merkel, Masaya Mizuno, Ulrich Müller, Kotaro Nishi, Ken Nishida, Kazuki Okuyama, Jonas Pätschke, Björn Sören Schlimme, Concettina Sfienti, Tianhao Shao, Daniel Steger, Marcell Steinen, Liguang Tang, Michaela Thiel, Philipp Vonwirth, Luca Wilhelm

TL;DR

This study addresses the hypertriton puzzle by directly measuring the $\Lambda$ binding energy of $^3_\Lambda H$ via decay-pion spectroscopy at MAMI. By comparing the monochromatic $\pi^-$ momenta from the two-body decays of $^3_\Lambda H$ and $^4_\Lambda H$, and calibrating the momentum scale with $^4_\Lambda H$ data, the authors extract $B_\Lambda(^3_\Lambda H)=0.523\pm0.013\;\mathrm{stat.}\pm0.075\;\mathrm{(syst.)}$ MeV, indicating a deeper binding than earlier measurements and aligning with the STAR result. The analysis employs high-precision tracking in the A1 spectrometer, meticulous energy-loss corrections, and a relative momentum approach that cancels many common systematics. The result provides stringent constraints on hyperon–nucleon interactions ($YN$) and three-body forces, with implications for the possible existence of exotic hypernuclei and for tuning theoretical models in chiral EFT.

Abstract

We performed high-precision decay-pion spectroscopy of light $Λ$ hypernuclei at the Mainz Microtron (MAMI) using the A1 spectrometer facility. By measuring the monochromatic $π^-$ momentum from the two-body weak decay $^3_Λ\mathrm{H} \to {}^3\mathrm{He} + π^-$ and referencing it to the $^4_Λ\mathrm{H} \to {}^4\mathrm{He} + π^-$ decay, we determined the $Λ$ binding energy of $^3_Λ\mathrm{H}$ with unprecedented accuracy. The obtained value, $B_Λ(^3_Λ\mathrm{H}) = 0.523 \pm 0.013~(\mathrm{stat.}) \pm 0.075~(\mathrm{syst.})$~MeV, is consistent with the STAR result, but indicates a significantly deeper binding than inferred from earlier measurements. This result implies a stronger $Λ$-deuteron interaction and provides stringent constraints on hyperon-nucleon interactions.

Precise measurement of the $Λ$-binding energy difference between $^3_Λ$H and $^4_Λ$H via decay-pion spectroscopy at MAMI

TL;DR

This study addresses the hypertriton puzzle by directly measuring the binding energy of via decay-pion spectroscopy at MAMI. By comparing the monochromatic momenta from the two-body decays of and , and calibrating the momentum scale with data, the authors extract MeV, indicating a deeper binding than earlier measurements and aligning with the STAR result. The analysis employs high-precision tracking in the A1 spectrometer, meticulous energy-loss corrections, and a relative momentum approach that cancels many common systematics. The result provides stringent constraints on hyperon–nucleon interactions () and three-body forces, with implications for the possible existence of exotic hypernuclei and for tuning theoretical models in chiral EFT.

Abstract

We performed high-precision decay-pion spectroscopy of light hypernuclei at the Mainz Microtron (MAMI) using the A1 spectrometer facility. By measuring the monochromatic momentum from the two-body weak decay and referencing it to the decay, we determined the binding energy of with unprecedented accuracy. The obtained value, ~MeV, is consistent with the STAR result, but indicates a significantly deeper binding than inferred from earlier measurements. This result implies a stronger -deuteron interaction and provides stringent constraints on hyperon-nucleon interactions.
Paper Structure (4 sections, 5 equations, 4 figures, 1 table)

This paper contains 4 sections, 5 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Experiment
  3. Data analysis
  4. Result and Discussion

Figures (4)

  • Figure 1: Schematic top view of the experimental setup in the A1 spectrometer facility.
  • Figure 2: Deviation $\Delta E_x$ from reference excitation energies versus relative momentum $\delta p= (p_{\mathrm{meas.}}/p_0 - 1)\times100$ for the calibration measurements with the $^{12}$C target.
  • Figure 3: Decay $\pi^-$ momentum distribution measured by SpecA. The lower panel shows the spectra for true (red) and accidental (blue) coincidences. The upper panels display magnified views of the regions 113.2-114.4 MeV/$c$ and 132.4-133.2 MeV/$c$, together with the results of unbinned fits using a Landau-Gaussian convolution function.
  • Figure 4: Measured difference between $B_\Lambda(^4_\Lambda\mathrm{H})$ and $B_\Lambda(^3_\Lambda\mathrm{H})$ in comparison with independent measurements by J-PARC E07 Kasagi2025, ALICE Acharya2023ALICE:2024djx, STAR Shao2022STAR2020, and emulsion experiments Juric1973. For the data points, the inner (colored) error bars represent statistical uncertainties only, while the outer error bars show the quadratic sum of statistical and systematic uncertainties. For the present work, the black line denotes the central value of the momentum difference, and the red band indicates the total uncertainty, calculated as the quadratic sum of the statistical and systematic errors.