Journeys Through the Precision Frontier: Amplitudes for by Lance Dixon, Frank Petriello

By Lance Dixon, Frank Petriello

This quantity is a compilation of the lectures at TASI 2014. The insurance specializes in glossy calculational innovations for scattering amplitudes, and at the phenomenology of QCD in hadronic collisions. Introductions to taste physics, darkish topic, and physics past the traditional version also are supplied. The lectures are available to graduate scholars on the preliminary phases in their study careers.

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Dφn+1 = . . d 3 pa , 2Ea (2π)3 d 3 pc d 3 pb . 2Eb (2π)3 2Ec (2π)3 Since pc = pa − pb , we have d3 pc = d3 pa for fixed pb . For small θ this yieldsc dφn+1 = dφn = dφn 1 2(2π)3 Eb dEb θb dθb dφ dz δ(z − Eb /Ea )dtδ(t − Ea Eb θ2 ) 1−z 1 dtdzdφ. 4(2π)3 Since the matrix element and the phase space factorize, so does the cross section: dt dφ αs CF F (z). dσn+1 = dσn dz t 2π 2π Therefore, multiple emission processes like c One observes that dφn+1 = . . d3 pb d3 p c Ea d3 p b = dφn 3 3 2Eb (2π) 2Ec (2π) Ec (2π)3 2Eb dφn Ea Eb dEb 1 Eb dEb θb dθb dφ = dφn θb dθb dφ.

DGLAP equations The (unphysical) bare quark distribution q(x) describes the probability of finding a quark without any accompanying gluon radiation. It is therefore independent of the mass factorization scale μ2F introduced to discriminate between resolved and unresolved radiation: d μ2F 2 q(x) = 0. dμF Differentiating Eq. (38) with respect to log μ2F we thus obtain an evolution equation for the quark distribution: ∂q(x, μ2F ) αs = ∂ log μ2F 2π 1 x x dξ q(ξ, μ2F )Pqq . ξ ξ (39) This implies that naive scaling invariance is violated logarithmically.

15) and (19) to see that the spinor products are complex square roots of the momentum-invariants, √ √ i j = sij eiφij , [i j] = sij e−iφij , (20) where φij is some phase. We will see later that this complex square-root property allows the spinor products to capture perfectly the singularities of amplitudes as two massless momenta become parallel (collinear). This fact is one way of understanding why helicity amplitudes can be so compact when written in terms of spinor products. We collect here some useful spinor product identities: anti-symmetry : squaring : i j = − j i , [i j] = − [j i] , i j [j i] = sij , i i = [i i] = 0, (21) (22) n i j [j k] = 0, (23) ij kl − ik j l = il kj .

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