S = {minus}1 and S = {minus}2 few-body hypernuclei Page: 5 of 7
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ducing quark degrees of freedom, may only be observable in the S=0 sector.
This was addressed in the 1993 US/Japan seminar on the hyperon-nucleon
interaction. [3]
Let me briefly reprise the triton results, which suggest an approach to A
hypernuclei with a somewhat surprising outcome. The Argonne V14 potential
[4] has been thoroughly studied and yields a triton binding energy of ~ 7.7
MeV. The V14 model is particularly interesting because there also exists a V28
model, one which includes NN - NA coupling and is fitted to the same NN
data set. Surprisingly, the triton binding energy is essentially unchanged.
Why is the octect-decuplet (NN - NA) coupling so well modeled implicitly
by V14? Can one extend this approach to the S=-1 octet-octet coupling? That
is, can one represent the hyperon-nucleon coupled-channel NA potential
VNA VNX
VNX VNE]
by an effective single-channel potential VNA?
In the non strange sector we observe that the ratio of neutron separation
energies for neighboring s-shell nuclei is approximatly 3: Bn(3H)/Bn(2H) ~
3/2 = 3, and B"(4He)/Bn(3H) ~ 20/6 ~ 3. If the physics of few-body systems
is similar, then we might anticipate a factor of 3 in the ratio of A separa-
tion energies for neighboring A hypernuclei. Using BA(4H) ~ 2 MeV as our
basis, we would then predict BA('He) ~ 3xBA('H) ~ 6 MeV and BA(3H)
~ }x BA(4H) ~ 3 MeV. Simple, central force calculations using VNA fitted
to BA(AH) plus low-energy scattering data confirm this simple analysis. [5]
However, the real world is more complex. Experimentally [6] we know that
BA('He) ~ 3.1 MeV and BA(SH) ~ 0.13 MeV. Our S=0 model experience does
not extrapolate to S=-1. Explicit NA - NE coupling is required. Moreover,
there exist ir+ decay data that suggest the importance of explicit NA - NE
coupling in A hypernuclei. The open channels for A mesonic decay are A -}
ir-p and A -4 7rOn. However, experimentaly [7] there is observed a 5% braching
ratio for 4He - 7r++X. Second order pion processes such as charge exchange
(7r-pp -+ 7r+nn) are too small to explain more than 1%. [8] The virtual pA -+
nE+ transition followed by E+N -+ r+nN is the key.
Furthermore, explicit NA - NE coupling was demonstrated in Ref. [9] to
play a crucial role in driving the hypertriton A separation energy from 2/3
MeV toward 0.1 MeV. Gloeckle and coworkers [10] have since shown that the
S=-1 Nijmegen soft-core potential yields a value for BA ('H) which agrees with
experiment. Additionally, a correct ordering of the A=4 isodoublet 0+ and
1+ states appears to require explicit NA - NE coupling. [11] Finally, Monte
Carlo calculations [12] have indicated that suppression of A4He++ E4He*
coupling, because of the large excitation energy of the T=1 even parity 4He*
states that result when the T=0 A converts to a T=1 E, can account for the
anomalously low value of BA(IHe) = 3.1 MeV. In the S=-1 sector we now
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Gibson, B. F. S = {minus}1 and S = {minus}2 few-body hypernuclei, article, November 1997; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc689542/m1/5/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.