Abstract
The isovector axial-vector coupling constant {ital g}{sub {ital A}} is determined by using the method of QCD sum rules. A sum rule for ({ital g}{sub {ital A}}{minus}1) is obtained, and it is shown that, with standard values of the quark condensates, {ital g}{sub {ital A}}=1.26{plus minus}0.08. It is also shown that the isovector axial-vector coupling ({ital g}{sub {ital A}}{minus}1)=0 in the limit in which chiral symmetry is restored, and the quark condensate vanishes. A sum rule is also obtained for the isoscalar'' axial-vector coupling constant {ital g}{sub {ital A}}{sup {ital S}}, which is found to be 0.13 if the isovector values of susceptibilities are used. On the other hand, {ital g}{sub {ital A}}{sup {ital S}}={minus}0.68 if the quark condensate is set to zero while {ital g}{sub {ital A}}{sup {ital S}}={minus}1.00 if both the quark and gluon condensates vanish in the event of chiral-symmetry restoration. The values of {ital g}{sub {ital A}} and {ital g}{sub {ital A}}{sup {ital S}} allow us to deduce {Delta}{ital u} and {Delta}{ital d} in the proton.
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