Separation of soft and collinear infrared limits of QCD squared matrix elements

We present a simple way of separating the overlap between the soft and collinear factorization formulae of QCD squared matrix elements. We check its validity explicitly for single and double unresolved emissions of tree-level processes. The new method makes possible the definition of helicity-dependent subtraction terms for regularizing the real contributions in computing radiative corrections to QCD jet cross sections. This implies application of Monte Carlo helicity summation in computing higher order corrections

One-loop Singular Behaviour of QCD and SUSY QCD Amplitudes with Massive Partons

We discuss the structure of infrared and ultraviolet singularities in on-shell QCD and supersymmetric QCD amplitudes at one-loop order. Previous results, valid for massless partons, are extended to the case of massive partons. Using dimensional regularization, we present a general factorization formula that controls both the singular epsilon-poles and the logarithmic contributions that become singular for vanishing masses. We introduce generalized Altarelli--Parisi splitting functions and discuss their relations with the singular terms in the amplitudes. The dependence on the regularization scheme is also considered

Three-jet cross sections in hadron-hadron collisions at next-to-leading order

We present a new QCD event generator for hadron collider which can calculate one-, two- and three-jet cross sections at next-to-leading order accuracy. In this letter we study the transverse energy spectrum of three-jet hadronic events using the kT algorithm. We show that the next-to-leading order correction significantly reduces the renormalization and factorization scale dependence of the three-jet cross section.Comment: 4 pages, 4 figures, REVTEX

Multi-jet cross sections in deep inelastic scattering at next-to-leading order

We present the perturbative prediction for three-jet production cross section in DIS at the NLO accuracy. We study the dependence on the renormalization and factorization scales of exclusive three-jet cross section. The perturbative prediction for the three-jet differential distribution as a function of the momentum transfer is compared to the corresponding data obtained by the H1 collaboration at HERA.Comment: 5 pages, 3 figure

Four-jet angular distributions and color charge measurements: leading order versus next-to-leading order

We present the next-to-leading order perturbative QCD prediction to the four-jet angular distributions used by experimental collaborations at LEP for measuring the QCD color charge factors. We compare our results to ALEPH data corrected to parton level. We perform a leading order ``measurement'' of the QCD color factor ratios by fitting the leading order perturbative predictions to the next-to-leading order result. Our result shows that in an experimental analysis for measuring the color charge factors the use of the O($\alpha_s^3$) QCD predictions instead of the O($\alpha_s^2$) results may shift the center of the fit by a relative factor of 1+2\as in the $T_R/C_F$ direction.Comment: 14 pages, 10 tables, 5 figures, revtex, eps style