Decay pion spectroscopy of electro-produced light Lambda hypernuclei at MAMI

The hypertriton ³_ΛH, the lightest hypernucleus, consists of a proton, a neutron, and a Lambda. This simplest Lambda hypernucleus has been used as an essential benchmark of hypernuclear physics, including understanding Λ-N interaction. Its Lambda-binding energy (B_Λ) is approximately 130 ± 50 keV based on emulsion data summarized by Jurič et al. The shallow binding energy suggests a ³_ΛH lifetime similar to that of a Lambda particle in free space(τ_Λ = 263 ps). However, recent heavy ion collision experiments have observed lifetimes and binding energies of hypernuclei, sparking significant interest.

After 2010, the STAR and ALICE collaborations reported a 20-30% shorter lifetime for ³_ΛH. These groups also reported binding energies of B_Λ = 406 ± 120(stat.) ± 110(syst.) keV and preliminarily B_Λ = 102 ± 63(stat.) ± 67(syst.) keV, respectively. For understanding the correlation between hypertriton's lifetime and binding energy, accurate measurement of these values is crucial.

In October 2022, we conducted decay-pion spectroscopy experiments on s-shell hypernuclei (³_ΛH and ⁴_ΛH) at Mainz Microtron (MAMI). Some electro-produced hypernuclei form bound states and decay via two-body weak decay, emitting pions with monochromatic momentum. By measuring this momentum with high-resolution magnetic spectrometers (Δp/p ~ 10^-4) and combining it with a Kaon spectrometer to tag strangeness production and suppress background events, we successfully measured the ⁴_ΛH mass with an accuracy of 80 keV in 2014.

In 2022, to enhance the yield of ³_ΛH and reduce background events, we changed the target from ⁹Be to ⁷Li in this experiment. The ⁷Li target was positioned at a 90-degree angle to increase luminosity and mitigate the effects of the pion energy struggle. Additionally, we introduced a new technology for beam energy measurement to achieve hypernuclear mass determination with 10-20 keV accuracy.

In this presentation, I will discuss this experimental method and provide an update on the latest analysis.