Negative-pion trapping by a metastable state in liquid helium

Abstract

We have found a long-lived metastable state of stopped m in liquid helium by measuring the time spectra of two different delayed products: (i) protons emitted after m absorption by He nuclei and (ii) 70-MeV electrons originating from free m~e v, decay. The lifetime and fraction of delayed~absorption obtained by using the emitted protons are 7.26+0. 12 nsec and 1.66+0.05%, respectively. The free-decay fraction of a pion in liquid helium was obtained to be 0.64+0.03% from this result, which is consistent with the observed free-m decay fraction. These results imply that 2. 30+0.07%%uo of stopped are trapped in the metastable state which have an overall lifetime of 10. 1+0.2 nsec. The same experimental and analysis were performed for stopped vr in liquid neon, where no evidence for trapping was found.

Spectroscopy of antiprotonic helium atoms and its contribution to the fundamental physical constants

“…We did find a similar anomaly in the case of π − -helium, 9) but the most spectacular was the discovery of p̄ longevity in helium. 10) Shown in Fig.…”

Search for free decay of negative pions in water and light materials

“…The free decay fraction of π − 's was measured to be 8.7 × 10 −5 [4] in Hydrogen and is expected to decrease with the atomic number. However, there is an exception in the π − He atom system [5,6], where the π − is trapped in a meta-stable state that has little wave-function overlap with the nucleus and the Auger processes are suppressed [7]. It is therefore important to confirm the absence of other mechanisms that may cause free decays of π − 's.…”

“…Limits for the fraction of the free-decay component exist for H 2 and He [4,5,6] (data in Ne is only applicable to a meta-stable state with a lifetime longer than ∼ 1 ns). This paper reports the results of a search for a free decay component of π − in water and light materials from analyses of electron yields and proton spectra.…”

Reply to ‘‘Analysis of the lifetimes and fractions of antiprotons trapped in metastable states of antiprotonic helium’’

“…. , n -1) for each n is degen-V(R)=U(R)+e (R), (3) where er(R ) arises from the electron involved. Shimamura [8] and (previously) Ahlrichs et al [9] as well as (more recently) Greenland and Thiirwachter [10] solved this equation.…”