The leading power asymptotic behaviour of the dimensionally regularized massless on-shell planar triple box diagram in the Regge limit t/s → 0 is analytically evaluated.1 E-mail: smirnov@theory.sinp.msu.ru Systematical analytical evaluation of two-loop Feynman diagrams with four external lines within dimensional regularization [1] began three years ago. In the pure massless case with all end-points on-shell, i.e. p 2 i = 0, i = 1, 2, 3, 4, the problem of analytical evaluation of two-loop four-point diagrams in expansion in ǫ = (4 − d)/2, where d is the space-time dimension, has been completely solved in [2,3,4,5,6,7]. The corresponding analytical algorithms have been successfully applied to the evaluation of two-loop virtual corrections to various scattering processes [8] in the zero-mass approximation.In the case of massless two-loop four-point diagrams with one leg off-shell the problem of the evaluation has been solved in [9,10], with subsequent applications [11] to the process e + e − → 3jets. (See [12] for recent reviews of the present status of NNLO calculations. See [13] for a brief review of results on the analytical evaluation of various double-box Feynman integrals and the corresponding methods of evaluation.) For another three-scale calculational problem, where all four legs are on-shell and there is a non-zero internal mass, a first analytical result was obtained in [14] for the scalar master double box.The purpose of this paper is to turn attention to three-loop on-shell massless four-point diagrams. As a first step, the leading power asymptotic behaviour of the dimensionally regularized massless on-shell planar triple box diagram shown in Fig. 1 in the Regge limit t/s → 0 will be analytically evaluated. This calculation will demonstrate that a three-loop BFKL analysis [15] (at least its virtual part which can be reduced to the evaluation of Regge asymptotics) is possible 2 . The calculation will be based on the technique of alpha parameters and MellinBarnes (MB) representation which was successfully used in [2,4,9,14] and reduces, due to taking residues and shifting contours, to a decomposition of a given MB integral into pieces where a Laurent expansion of the integrand in ǫ becomes possible. At a final stage, summation formulae for series of S 1 (n)S 3 (n)/n 2 , ψ ′′ (n + 1)S 2 (n)/n etc., where S k (n) = n j=1 j −k , are used. A table of such formulae is presented in Appendix.2 In three loops, non-planar diagrams as well as higher terms of expansion of double boxes in ǫ are also needed.
1The general planar triple box Feynman integral without numerator takes the formwhere s = (p 1 + p 2 ) 2 and t = (p 2 + p 3 ) 2 are Mandelstam variables, and k, l and r are loop momenta. Usual prescriptions k 2 = k 2 + i0, s = s + i0, etc. are implied.To evaluate the leading power asymptotic behaviour of the master triple box (1), i.e. for all a i = 1, in the limit t/s → 0 one can use the strategy of expansion by regions [16,17,18]. It shows that in the leading power only (1c-1c-1c) and (2c-2c-2c) region...