Low-x evolution of parton densities

It is shown that a Bessel-like behaviour of the structure function F2 at small x, obtained for a flat initial condition in the DGLAP evolution equations, leads to good agreement with the deep inelastic scattering experimental data from HERA.Comment: 8 pages, 3 figures, in Proc. of the third International Workshop on Multiple Partonic Interactions at the LHC (21-25 November 2011, DESY, Hamburg

Calculating four-loop tadpoles with one non-zero mass

An efficient method to calculate tadpole diagrams is proposed. Its capability is demonstrated by analytically evaluating two four-loop tadpole diagrams of current interest in the literature, including their $O(\epsilon)$ terms in $D=4-2\epsilon$ space-time dimensions.Comment: 12 pages, 3 figure

Ultrahigh-energy neutrino-nucleon deep-inelastic scattering and the Froissart bound

We present a simple formula for the total cross section sigma^{nu N} of neutral- and charged-current deep-inelastic scattering of ultrahigh-energy neutrinos on isoscalar nuclear targets, which is proportional to the structure function F_2^{nu N}(M_V^2/s, M_V^2), where M_V is the intermediate-boson mass and s is the square of the center-of-mass energy. The coefficient in the front of F_2^{nu N}(x, Q^2) depends on the asymptotic low-x behavior of F_2^{nu N}. It contains an additional ln(s) term if F_2^{nu N} scales with a power of ln(1/x). Hence, an asymptotic low-x behavior F_2^{nu N} propto ln^2(1/x), which is frequently assumed in the literature, already leads to a violation of the Froissart bound on sigma^{nu N}.Comment: 5 pages, 2 figures, to appear in Physical Review Letter

Small-x behavior of the structure function F_2 and its slope partial ln(F_2)/partial ln(1/x) for "frozen" and analytic strong-coupling constants

Using the leading-twist approximation of the Wilson operator product expansion with "frozen" and analytic versions of the strong-coupling constant, we show that the Bessel-inspired behavior of the structure function F_2 and its slope\break partial ln(F_2)/partial ln(1/x) at small values of x, obtained for a flat initial condition in the DGLAP evolution equations, leads to good agreement with experimental data of deep-inelastic scattering at DESY HERA.Comment: new curves added to Figs. 1 and 2, minor changes to the text, accepted for publication in Phys. Lett.

Heavy-quark contributions to the ratio F_L/F_2 at low x

We study the heavy-quark contribution to the proton structure functions F_2^i(x,Q^2) and F_L^i(x,Q^2), with i=c,b, for small values of Bjorken's x variable at next-to-lading order and provide compact formulas for their ratios R_i=F_L^i/F_2^i that are useful to extract F_2^i(x,Q^2) from measurements of the doubly differential cross section of inclusive deep-inelastic scattering at DESY HERA. Our approach naturally explains why R_i is approximately independent of x and the details of the parton distributions in the small-x regime.Comment: 11 pages, 1 figur

On DIS Wilson coefficients in N=4 super Yang-Mills theory

In this Letter we evaluate Wilson coefficients for “deep inelastic scattering” (DIS) in N = 4SYMtheory at NLO in perturbation theory, using as a probe an R -symmetry conserved current. They exhibit uniform transcendentality and coincide with the piece of highest transcendentality in the corresponding QCD Wilson coefficients. We extract from the QCD result a NNLO prediction for the N = 4SYMWilson coefficient, and comment on the features of its Regge limit asymptotics

Analogs of noninteger powers in general analytic QCD

In contrast to the coupling parameter in the usual perturbative QCD (pQCD), the coupling parameter in the analytic QCD models has cuts only on the negative semiaxis of the Q^2-plane (where q^2 = -Q^2 is the momentum squared), thus reflecting correctly the analytic structure of the spacelike observables. The Minimal Analytic model (MA, named also APT) of Shirkov and Solovtsov removes the nonphysical cut (at positive Q^2) of the usual pQCD coupling and keeps the pQCD cut discontinuity of the coupling at negative Q^2 unchanged. In order to evaluate in MA the physical QCD quantities whose perturbation expansion involves noninteger powers of the pQCD coupling, a specific method of construction of MA analogs of noninteger pQCD powers was developed by Bakulev, Mikhailov and Stefanis (BMS). We present a construction, applicable now in any analytic QCD model, of analytic analogs of noninteger pQCD powers; this method generalizes the BMS approach obtained in the framework of MA. We need to know only the discontinuity function of the analytic coupling (the analog of the pQCD coupling) along its cut in order to obtain the analytic analogs of the noninteger powers of the pQCD coupling, as well as their timelike (Minkowskian) counterparts. As an illustration, we apply the method to the evaluation of the width for the Higgs decay into b+(bar b) pair.Comment: 29 pages, 5 figures; sections II and III extended, appendix B is ne