INFRARED CONTRIBUTION TO THE DOUBLE-LOGARITHMIC SMALL-X ASYMPTOTICS OF STRUCTURE FUNCTIONS

The role playing by the ''soft'' (k(T) < 1 GeV/c) region in the small- x behaviour of g(1)(x, Q(2)) and the non-singlet structure function f( 1,NS)(x, Q(2)) has been studied with the help of the effective QCD Lag rangian which takes into account the lightest degrees of freedom - the constituent quarks and the pi-mesons (Goldstone bosons). It has been shown that the quark-quark interaction due to the pion exchange has a negative coupling g for the isovector component (I = 1 in the t-channe l) of f(1)(x, Q(2)) and isosinglet component of g(1)(a, Q(2)). Here th e pion induced interaction changes mainly the normalization of the qua rk distribution (it decreases f(1,NS)(I=1)(x(1), Q(2)) two times at x < 3.10(-3)) and changes slightly the effective exponents lambda (f(1,N S), g(1) similar to x(-lambda) at x --> 0). On the other hand due to a positive Value of coupling g the value of lambda increases by 15% for the isovector part of g(1)(x, Q(2)) and up to lambda approximate to 0 .5 (instead of lambda approximate to 0.2 without the pion contribution ) for the isoscalar non-singlet structure function f(1,NS)(I=0)(x,Q(2) ).