Quark mass effects in QCD evolution equations and distribution functions of heavy quarks

QCD evolution equations with accounting of quark masses are considered. Initial conditions of this equations, i.e. quark and gluon distribution functions (DF) are chosen at quite low values of sguared momentum transfer (Q2) that allows one to use only distributions of light (valence) quarks and gluons. On the basis of numerical solution of evolution equations heavy quark DF have been obtained that obey the correct threshold behaviour: the distribution is suppressed at Q2 << m sub(q)sup(2) (m sub(q)-mass of corresponding quark) and effectivey turned on at Q2 approximately 2m sub(q)sup(2), x-dependence of DF where x-is a part of hadron longitudinal momentum is under investigation. It is shown that, for example, for heavy quarks widely used parametrization of type x sup(a)(1-x)sup(β) may lead to the notable difference (approximately 10-15%) in comparing with the exact values