On the critical point of the Random Walk Pinning Model in dimension d=3

We consider the Random Walk Pinning Model studied in [3,2]: this is a random walk X on Z^d, whose law is modified by the exponential of \beta times L_N(X,Y), the collision local time up to time N with the (quenched) trajectory Y of another d-dimensional random walk. If \beta exceeds a certain critical value \beta_c, the two walks stick together for typical Y realizations (localized phase). A natural question is whether the disorder is relevant or not, that is whether the quenched and annealed systems have the same critical behavior. Birkner and Sun proved that \beta_c coincides with the critical point of the annealed Random Walk Pinning Model if the space dimension is d=1 or d=2, and that it differs from it in dimension d\ge4 (for d\ge 5, the result was proven also in [2]). Here, we consider the open case of the marginal dimension d=3, and we prove non-coincidence of the critical points.Comment: 23 pages; v2: added reference [4], where a result similar to Th. 2.8 is proven independently for the continuous-time mode

Bootstrapping One-Loop QCD Amplitudes

We review the recently developed bootstrap method for the computation of high-multiplicity QCD amplitudes at one loop. We illustrate the general algorithm step by step with a six-point example. The method combines (generalized) unitarity with on-shell recursion relations to determine the not cut-constructible, rational terms of these amplitudes. Our bootstrap approach works for arbitrary configurations of gluon helicities and arbitrary numbers of external legs.Comment: 18 pages, 9 figures; extended version of talks given at the 7th Workshop On Continuous Advances In QCD, 11-14 May 2006, Minneapolis, Minnesota; at SUSY06: 14th International Conference On Supersymmetry And The Unification Of Fundamental Interactions, 12-17 Jun 2006, Irvine, California; at the LoopFest V: Radiative Corrections For The International Linear Collider: Multi-Loops And Multi-Legs, 19-21 Jun 2006, SLAC, Menlo Park, California; and at the Vancouver Linear Colliders Workshop (ALCPG 2006), 19-22 Jul 2006, Vancouver, British Columbi

Power Corrections to e+e- Dijet Event Shapes

We discuss a class of event shapes for e+e- dijet events that include the thrust as a special case. Large logarithmic corrections to the corresponding cross sections can be resummed to all logarithmic orders at leading power. However, irrespective of the order up to which the perturbative expansion is calculated, it has to be supplemented by nonperturbative corrections due to its at best asymptotic nature. We find that the leading power corrections are universal for the class of event shapes discussed here. Based on these findings, we provide sample numerical predictions for the distributions of the new event shapes.Comment: 3 pages, 1 figure; presented by C.F. Berger at the International Europhysics Conference on High-Energy Physics (HEP 2003), Aachen, Germany, 17-23 July 200

Scaling of Power Corrections for Angularities from Dressed Gluon Exponentiation

We study power corrections to a recently introduced family of event shapes, the class of angularities, within the formalism of dressed gluon exponentiation (DGE). We find that the universal scaling rule for the leading power corrections deduced from resummation also holds when taking renormalon enhancements into account. The scaling is due to boost invariance of eikonal dynamics in the two-jet limit, which we recover in the context of DGE. Furthermore, dressed gluon exponentiation provides an ansatz for non-leading power corrections that violate the scaling. These non-leading corrections are further suppressed by non-integer powers of the hard scale.Comment: 24 page

Multi-Parton Scattering Amplitudes via On-Shell Methods

We present an overview of recent developments, based on on-shell techniques, in the calculation of multi-parton scattering amplitudes at one loop that are relevant for phenomenological studies at hadron colliders. These new on-shell methods make efficient use of the physical properties of the hard scattering, such as unitarity and factorization.Comment: Invited review, Annual Review of Nuclear and Particle Science; v2: references adde

Hard exclusive photoproduction of Φ\Phi and J/ΨJ/\Psi mesons

We present predictions for differential cross sections for the reaction $\gamma p \to \Phi p$ and give an outlook to which extent our calculations may be generalized to the photoproduction of $J/\Psi$ mesons. Our results are obtained within perturbative QCD treating the proton as a quark-diquark system.Comment: 4 pages, 1 figure, uses Elsevier style espcrc1.st