Extracting the longitudinal structure function FL(x, Q2) at small x from a Froissart-bounded parametrization of F2(x, Q2)

We present a method to extract, in the leading- and next-to-leading-order approximations, the longitudinal deep inelastic scattering structure function FL(x, Q2) from the experimental data by relying on a Froissart-bounded parametrization of the transversal structure function F2(x, Q2) and, partially, on the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi equations. Particular attention is paid to the kinematics of low and ultralow values of the Bjorken variable x, x ~ 10−5–10−2. Analytical expressions for FL(x, Q2) in terms of the effective parameters of the parametrization of F2(x, Q2) are presented explicitly. We argue that the obtained structure functions FL(x, Q2) within both, the leading- and next-to-leading-order approximations, manifestly obey the Froissart boundary conditions. Numerical calculations and comparison with available data from the ZEUS and H1 collaborations at HERA demonstrate that the suggested method provides reliable structure functions FL(x, Q2) at low x in a wide range of the momentum transfer (1 GeV2< Q2 < 3000 GeV2) and can be applied as well in analyses of ultrahigh-energy processes with cosmic neutrinos.