Bootstrapping multiparton loop amplitudes in QCD
ABSTRACT We present a new method for computing complete one-loop amplitudes, including their rational parts, in nonsupersymmetric gauge theory. This method merges the unitarity method with on-shell recursion relations. It systematizes a unitarity-factorization bootstrap approach previously applied by the authors to the one-loop amplitudes required for next-to-leading order QCD corrections to the processes e+e-→Z,γ*→4 jets and pp→W+2 jets. We illustrate the method by reproducing the one-loop color-ordered five-gluon helicity amplitudes in QCD that interfere with the tree amplitude, namely A5;1(1-,2-,3+,4+,5+) and A5;1(1-,2+,3-,4+,5+). Then we describe the construction of the six- and seven-gluon amplitudes with two adjacent negative-helicity gluons, A6;1(1-,2-,3+,4+,5+,6+) and A7;1(1-,2-,3+,4+,5+,6+,7+), which uses the previously-computed logarithmic parts of the amplitudes as input. We present a compact expression for the six-gluon amplitude. No loop integrals are required to obtain the rational parts.
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ABSTRACT: Generalized-unitarity calculations of two-loop amplitudes are performed by expanding the amplitude in a basis of master integrals and then determining the coefficients by taking a number of generalized cuts. In this paper, we present a complete classification of the solutions to the maximal cut of integrals with the double-box topology. The ideas presented here are expected to be relevant for all two-loop topologies as well. We find that these maximal-cut solutions are naturally associated with Riemann surfaces whose topology is determined by the number of states at the vertices of the double-box graph. In the case of four massless external momenta we find that, once the geometry of these Riemann surfaces is properly understood, there are uniquely defined master contours producing the coefficients of the double-box integrals in the basis decomposition of the two-loop amplitude. This is in perfect analogy with the situation in one-loop generalized unitarity. In addition, we point out that the chiral integrals recently introduced by Arkani-Hamed et al. can be used as master integrals for the double-box contributions to the two-loop amplitudes in any gauge theory. The infrared finiteness of these integrals allow for their coefficients as well as their integrated expressions to be evaluated in strictly four dimensions, providing significant technical simplification. We evaluate these integrals at four points and obtain remarkably compact results.Journal of High Energy Physics 05/2012; 2012(10). · 5.62 Impact Factor
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ABSTRACT: Calculation of amplitudes in perturbative quantum field theory involve large loop integrals. The complexity of those integrals, in combination with the large number of Feynman diagrams, make the calculations very difficult. Reduction methods proved to be very helpful, lowering the number of integrals that need to be actually calculated. Especially, the reduction at the integrand level technique, improves the speed and set-up of these calculations. In this article we demonstrate, by counting the numbers of tensor structures and independent coefficients, how to write such relations at the integrand level for one- and two-loop amplitudes. We clarify their connection to the so-called spurious terms at one loop and discuss their structure in the two-loop case. This method is also applicable to higher loops, and the results obtained apply to both planar and non-planar diagrams.Journal of High Energy Physics 06/2012; 2012(12). · 5.62 Impact Factor
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ABSTRACT: By the unitarity cut method, analytic expressions of one-loop coefficients have been given in spinor forms. In this paper, we present one-loop coefficients of various bases in Lorentz-invariant contraction forms of external momenta. Using these forms, the analytic structure of these coefficients becomes manifest. Firstly, coefficients of bases contain only second-type singularities while the first-type singularities are included inside scalar bases. Secondly, the highest degree of each singularity is correlated with the degree of the inner momentum in the numerator. Thirdly, the same singularities will appear in different coefficients, thus our explicit results could be used to provide a clear physical picture under various limits (such as soft or collinear limits) when combining contributions from all bases.Journal of High Energy Physics 01/2013; 2013(5). · 5.62 Impact Factor