**
Next: Spin-dependent
parton distributions of Up: Discussion
Previous: Discussion
**

**Contributers: P. Ratcliffe and M. Savage. **

** **

**To determine the strange quark axial matrix element,
,
from deep-inelastic scattering the value of the matrix element of the ``T8''
current,
,
between nucleon states is required. This matrix element cannot be measured
directly but in the limit of exact flavor SU(3) it can be related to axial
matrix elements between baryons in the lowest lying octet of SU(3) (the
N, ,
,
and ).
To make a precise determination of
the
SU(3) breaking in these matrix elements needs to be understood and quantified.
Naive estimates for the size of SU(3) breaking indicate that
is
consistent with zero (for example, [16]).
**

** **

**1.****Constituent quark model approach to SU(3) breaking****2.****The large-Nc expansion****3.****The KTeV measurement of The KTeV experiment is able to cleanly study the decay , and has determined a branching fraction of [20]. The 1999 run will reduce the errors by about a factor two. Angular asymmetries and the muon mode may also become accessible. KTeV will be able to substantially improve the mass measurement, allowing mass relations predicted in the large-Nc expansion to be tested and hadronic models constrained.**

** **

** Since the origin of SU(3) breaking lies in the strange quark mass,
the pattern of SU(3) breaking in hyperon decays might be driven by the
baryon mass hierarchy [17]. The effects
of quark recoil in the decays, accounting for relativistic effects, are
linked to the constituent quark masses and hadronic mass splittings. A
similar approach uses perturbation theory to tie baryon wave-function renormalization
to the mass splittings, parametrizing the effects on the axial form factors.
Such models can successfully describe the decays of the baryon octet and
result in only minor variations in the F and D structure constants. Such
an analysis yields
for .
**

** **

** The large-Nc expansion provides a QCD based framework for a systematic
calculation of corrections to both the hyperon and decuplet decays[18].
One of the significant advantages the large-Nc analysis has over other
methods is that there is a well defined expansion parameter in the theory,
1/Nc. Where the baryon masses are measured with sufficient precision they
obey relations that arise in the large-Nc limit. Precise measurements of
other combinations of baryon masses are needed to provide a significant
test of the large-Nc expansion[19]. **

** The large-Nc limit links SU(3) breaking in the decuplet and octet
baryon decays. Different fits to the measured decay rates yield Vus consistent
with that extracted from Kl3 decays, but show an important variation in
the values of F and D. The results indicate significant corrections to
the vector form factors. This analysis gives
for .
**

** **

** **

Next: Spin-dependent parton distributions of Up: Discussion Previous: Discussion

1999-01-15