The hypertriton (3ΛH), the lightest bound hypernucleus, has long served
as a benchmark in hypernuclear physics. However, its fundamental
properties, such as the Λ binding energy and lifetime, have remained
uncertain. In particular, since the 2010s, new measurements of both
quantities have revealed inconsistencies, drawing renewed attention to
the so-called “hypertriton puzzle.” This situation underscores the need
for direct and precise binding-energy measurements.

To address this issue, we carried out a high-precision measurement of
the Λ binding energy of 3ΛH using decay pion spectroscopy at the Mainz
Microtron (MAMI). Building on the method successfully applied in the 4ΛH
study at MAMI, the experiment introduced a newly designed lithium target
with low atomic number and optimized geometry. The target was elongated
to ensure high luminosity while keeping its transverse thickness small
to minimize pion energy loss, thereby reducing both electromagnetic and
hyperfragment-induced backgrounds. This configuration enabled the first
statistically significant observation of a distinct decay-pion momentum
peak from 3ΛH.

The seminar will focus on the experimental methodology and analysis:
target development, spectrometer setup, calibration strategies using
both elastic electron scattering and a novel undulator-based beam energy
measurement, and the procedures employed to achieve high statistical
precision. The results provide crucial input toward resolving the
apparent inconsistencies between lifetime and binding-energy
measurements.
The possible implications of the observed decay pion momentum difference
between 3ΛH and 4ΛH for Λ–N interactions will also be briefly discussed.