Singlet structure function F₁ in double-logarithmic approximation

The conventional ways to calculate the perturbative component of the DIS singlet structure function F₁ involve approaches based on BFKL which account for the single-logarithmic contributions accompanying the Born factor 1/x. In contrast, we account for the double-logarithmic (DL) contributions unrelated to 1/x and because of that they were disregarded as negligibly small. We calculate the singlet F₁ in the double-logarithmic approximation (DLA) and account at the same time for the running α_s effects. We start with a total resummation of both quark and gluon DL contributions and obtain the explicit expression for F₁ in DLA. Then, applying the saddle-point method, we calculate the small-x asymptotics of F₁, which proves to be of the Regge form with the leading singularity ω₀ = 1.066. Its large value compensates for the lack of the factor 1/x in the DLA contributions. Therefore, this Reggeon can be identified as a new Pomeron, which can be quite important for the description of all QCD processes involving the vacuum (Pomeron) exchanges at very high energies. We prove that the expression for the small-x asymptotics of F₁ scales: it depends on a single variable Q²/x² only instead of x and Q² separately. Finally, we show that the small-x asymptotics reliably represent F₁ at x ≤ 10^-6. (orig.)