AN HISTORICAL OVERVIEW OF THE IMPORTANCE OF THE WEAK DECAY OF HYPERNUCLEI Page: 3 of 8
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enhanced by an order of magnitude over the DI = 3/2 amplitude. The practical result is
F(A -+ p7r-)/F(A -+ nir ) ~ 1.9 , (2)
or approximately 2, which is the square of the ratio of C-G coefficients in the I = a
11/2, -1/2 > = 2/3p7r- > - 1/3Inr0 > . (3)
There is no universal explanation for this DI = 1/2 rule that governs free hyperon decay
as well as CP violation in kaon decay.
Hypernuclei along with their weak decay were discovered in 1952 . Key questions
regarding hypernuclear weak decay include: (1) How should the weak interactions of
baryons be described when embedded in a strongly interacting many-baryon system? (2)
Does the empirical DI = 1/2 rule, which governs the Y -+ N7r mesonic decay, characterize
the NA -+ NN transition of nonmesonic decay? (3) Does the internal structure of baryons
play a significant role in the decay of hypernuclei? Quarks and gluons are assumed to
be the fundamental degrees of freedom in describing hadrons, but are they the relevant
degrees of freedom in the realm of nonperturbative QCD? Electrons and photons are
fundamental degrees of freedom in condensed matter, but BCS pairs are the relevant
degrees of freedom in a Type I superconductor.
2. Mesonic Weak Decay of Hypernuclei
Hypernuclei provide a laboratory for studying the medium modifications of hyperon
weak decay. The free A decays into a nucleon and a pion, the nucleon having a momen-
tum of approximately 100 MeV/c or about 5 MeV of kinetic energy . Even neglecting
the binding, a A at rest in nuclear matter cannot decay into a nucleon of this momentum,
because the nucleon Fermi momentum is kF ~ 270 MeV/c. That is, the process is Pauli
blocked. (The effect of Pauli blocking in medium and heavy hypernuclei has received sig-
nificant attention - see the discussion by Oset.) Mesonic weak decay should be significant
only in the light hypernuclei :
r,-/r~ ~ 1/2 for 5He,
F,-/FA ~ 1/10 for A= 11, 12.
Therefore, let us look more closely at the lightest hypernucleus, the hypertriton.
From 3H investigations  it is well known that obtaining the correct binding energy
[B(3H) = 8.48 MeV] in a model calculation leads to a correct description of the low energy
physical observables such as the r.m.s. radius and the Coulomb energy in 3He. Because 3H
is weakly bound [BA(3H) = 130 keV], one has a A-deuteron molecular-like halo structure
in which the distance of the A from the c.m. of the deuteron core is 7x that of the NN
separation in the deuteron. The large size is not typical of hypernuclei, but j3H is one
hypernucleus whose lifetime we can calculate. One expects r(3H) ~ TA. Experimentally
one observes -(H) = 2.2o3 x 10"0s and T(IH) = 2.640.2 x 10-0s . Because the
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GIBSON, B. F. AN HISTORICAL OVERVIEW OF THE IMPORTANCE OF THE WEAK DECAY OF HYPERNUCLEI, article, January 9, 2001; Los Alamos, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc722718/m1/3/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.