Jeppe R. Andersen

Inclusive spectra in charmless semileptonic B decays by dressed gluon exponentiation

The triple differential spectrum in Xul is computed by Dressed Gluon Exponentiation (DGE). In this framework the on-shell calculation, converted into hadronic variables, can be directly used as an approximation to the meson decay spectrum, without involving a leading–power non-perturbative function. Sudakov resummation for the fully differential Xul width is formulated in moment space, where moments are defined using the ratio between the lightcone momentum components of the partonic jet p+/p− and the hard scale is p−. In these variables the correspondence with the Xsγ case is transparent. The Sudakov exponent is known to next–to–next–to–leading logarithmic accuracy. Further constraints are put on its Borel sum using the cancellation of the leading renormalon ambiguity and the absence of the next–to–leading one, which was proven in the large–β0 limit and assumed here to be general. Based on the resummed spectrum, matched to the fully differential NLO result, we calculate the event fraction associated with experimental cuts on the hadronic mass (or the small lightcone component) as well as on the lepton energy. Finally, we extract |Vub| from recent measurements by Belle and analyze the theoretical uncertainty.

Multiple Jets at the LHC with High Energy Jets

We present a flexible Monte Carlo implementation of the perturbative framework of High Energy Jets, describing multi-jet events at hadron colliders. The description includes a resummation which ensures leading logarithmic accuracy for large invariant mass between jets, and is matched to tree-level accuracy for multiplicities up to 4 jets. The resummation includes all-order hard corrections, which become important for increasing centre-of-mass energy of the hadronic collision.We discuss observables relevant for confronting the perturbative framework with 7 TeV data from the LHC, and the impact of the perturbative corrections on several dijet and trijet observables which are relevant in the search for new physics.

Small x Phenomenology - summary of the 3rd Lund Small x Workshop in 2004

A third workshop on small-x physics, within the Small-x Collaboration, was held in Hamburg in May 2004 with the aim of overviewing recent theoretical progress in this area and summarizing the experimental status.

Constructing all-order corrections to multi-jet rates

We discuss the universal behaviour of scattering cross sections in the limit of infinite rapidity separation between all produced particles, and illustrate the behaviour explicitly for the production of n jets, W + n jets, Z + n jets for n = 2, 3, 4, and for H + 2, 3 jets. We give a set of rules for constructing scattering cross sections, which are exact in the given limit, and order-by-order reproduce well the full fixed order results when applied to LHC phenomenology. The approximation includes both real and virtual corrections, and is sufficiently simple to allow for the regulated all-order perturbative sum to be explicitly constructed. We test the expected accuracy by comparing results orderby-order with full fixed-order perturbation theory for the processes mentioned above.

Loop induced interference effects in Higgs Boson plus two jet production at the LHC.

We calculate the order (α2αs3) interference effect between the gluon fusion and weak boson fusion processes allowed at the one-loop level in Higgs boson plus 2 jet production at the LHC. The corresponding one-loop amplitudes, which have not been considered in the literature so far, are evaluated analytically using dimensional regularisation and the necessary master integrals with massive propagators are reported. It is discussed in detail how various mechanisms conspire to make this contribution numerically negligible for experimental studies at the LHC.

W plus multiple jets at the LHC with high energy jets

A bstractWe study the production of a W boson in association with n hard QCD jets (for n ≥ 2), with a particular emphasis on results relevant for the Large Hadron Collider (7 TeV and 8 TeV). We present predictions for this process from High Energy Jets, a framework for all-order resummation of the dominant contributions from wide-angle QCD emissions. We first compare predictions against recent ATLAS data and then shift focus to observables and regions of phase space where effects beyond NLO are expected to be large.

High energy scattering in gravity and supergravity

We investigate features of perturbative gravity and supergravity by studying scattering in the ultra-Planckian limit, and sharpen arguments that the dynamics is governed by long-distance physics. A simple example capturing aspects of the eikonal resummation suggests why short-distance phenomena and, in particular, divergences or nonrenormalizability do not necessarily play a central role in this regime. A more profound problem is apparently unitarity. These considerations can be illustrated by showing that known gravity and supergravity amplitudes have the same long-distance behavior, despite the extra light states of supergravity, and this serves as an important check on long-range dynamics in a context where perturbative amplitudes are finite. We also argue that these considerations have other important implications: they obstruct both probing the conjectured phenomenon of asymptotic safety through a physical scattering process, and ultra-Planckian scattering exhibiting Regge behavior. These arguments sharpen the need to find a nonperturbative completion of gravity with mechanisms which restore unitarity in the strong gravity regime.

Small-x phenomenology - summary and status 2002

Abstract.A second workshop on small-x physics, within the Small-x Collaboration, was held in Lund in June 2002 with the aim of over-viewing recent theoretical progress in this area and summarizing the experimental status.

Solving the BFKL equation in the next-to-leading approximation

The Balitsky–Fadin–Kuraev–Lipatov equation in the next-to-leading logarithmic approximation is solved using an iterative method. We derive the solution for forward scattering with all conformal spins. A discussion of the infrared finiteness of the results is included.

Higgs Boson Production in Association with Multiple Hard Jets

We elucidate a new technique for estimating the production of multiple (at least two) hard jets in Higgs production via gluon-gluon fusion. The approach is based upon high energy factorisation, with the region of applicability extended by constraints on the analytic behaviour of the scattering amplitudes stemming from known all-order results. The method approximates both real and virtual corrections, and allows for the resummation in an n-parton inclusive event sample of the terms dominant in the high energy limit. The resulting approximation is matched to the known tree level matrix elements for the production of a Higgs boson in association with 2 and 3 jets, and implemented in a Monte Carlo generator. Example results are presented and characteristic radiation patterns discussed.