ar X iv : h ep - p h / 00 11 31 2 v 1 2 5 N ov 2 00 0 1 Hyperon semileptonic decay constants and quark - spin content of the octet baryons

We present the recent investigation on the effect of the SU(3) symmetry breaking in the spin structure of the octet baryons. The spin structure of the nucleon has been a hot issue since the measurement of the first moment of the proton spin structure function g p 1 [1] by the European Muon Collaboration (EMC). The EMC result indicates that the strange quark is polarized in the opposite direction to other light quarks. It implies that the spin of the nucleon is not carried by quarks only. While it is now known that a large fraction of the nucleon spin is provided by gluons and their orbital angular momenta, it is still very important to understand the mechanism of how the quarks carry the nucleon spin. The information on the nucleon spin carried by quarks comes from two different sources: the first moment of the spin structure function g p 1 (x) of the proton and the experimental data of hyperon semileptonic decay (HSD). With SU(3) symmetry assumed, one obtains two reduced matrix elements F and D. Taking for F = 0.46 and for D = 0.80, one gets for the proton: ∆u p = 0.79, ∆d p = −0.47 and ∆s p = −0.13, which implies ∆Σ p = 0.19, quite a small number as compared with the naive expectation from the quark model: ∆Σ p = 1. Recently, the spin content of the Λ hyperon has drawn attention to us. In contrast to the nucleon, the nonrelativistic quark model predicts that the spin of the Λ comes solely from the strange quark, since the up and down quarks constitute the spin singlet. Furthermore , the ∆q Λ is deeply related to the fragmentation funtion which can be measured experimentally [2]. However, note that ∆Σ p and ∆Σ Λ are not directly measured. The standard way to extract ∆Σ B is to assume the SU(3) symmetry for HSD. In this case it is enough to take any two decays and data of deep inelastic scattering as input. With SU(3) symmetry assumed, one can pick out two different values of the constants of HSD among the existing data. One normally uses neutron and Σ − β decays as an input. However, if the SU(3)