G. Chachamis

Instituto de Física Corpuscular, Paterna, Valencia, Spain

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Publications (37)53.94 Total impact

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    ABSTRACT: We review the recent developments of the loop-tree duality method, focussing our discussion on analysing the singular behaviour of the loop integrand of the dual representation of one-loop integrals and scattering amplitudes. We show that within the loop-tree duality method there is a partial cancellation of singularities at the integrand level among the different components of the corresponding dual representation. The remaining threshold and infrared singularities are restricted to a finite region of the loop momentum space, which is of the size of the external momenta and can be mapped to the phase-space of real corrections to cancel the soft and collinear divergences.
    07/2014;
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    ABSTRACT: We analyze the singular behaviour of one-loop integrals and scattering amplitudes in the loop momentum space and show that within the loop--tree duality method there is a partial cancellation of singularities at the integrand level among the different components of the corresponding dual representation. The remaining threshold and infrared singularities are restricted to a finite region of the loop momentum space, which is of the size of the external momenta and can be mapped to the phase-space of real corrections to cancel the soft and collinear divergences.
    05/2014;
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    Grigorios Chachamis
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    ABSTRACT: We discuss the result with full mass dependence for the virtual NNLO QCD corrections to the W boson pair production in the quark-anti-quark annihilation channel. We also report on our progress regarding the treatment of the double-real radiative corrections which, along with the virtual-real corrections, are the other two necessary ingredients for a theoretical prediction of the total cross section for W+ W- production to NNLO accuracy.
    12/2013;
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    Grigorios Chachamis, Michal Deak, German Rodrigo
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    ABSTRACT: We comment on the calculation of the finite part of the heavy quark impact factor at next-to-leading logarithmic (NLx) accuracy. The result is presented in a form suitable for phenomenological studies such as the calculation of the cross-section for single heavy quark production at the LHC within the kT-factorization scheme.
    10/2013;
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    Grigorios Chachamis, Michal Deak, German Rodrigo
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    ABSTRACT: We present the calculation of the finite part of the heavy quark impact factor at next-to-leading logarithmic accuracy in a form suitable for phenomenological studies such as the calculation of the cross-section for single bottom quark production at the LHC within the kT-factorization scheme.
    10/2013;
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    ABSTRACT: We discuss aspects of our recent derivation of the gluon Regge trajectory at two loop from Lipatov's high energy effective action. We show how the gluon Regge trajectory can be rigorously defined through renormalization of the high energy divergence of the reggeized gluon propagator. We furthermore provide some details on the determination of the two-loop reggeized gluon self-energy.
    08/2013;
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    G. Chachamis, A. Sabio Vera
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    ABSTRACT: We discuss the solution to the BFKL equation in the adjoint representation at LO and NLO accuracy for the N = 4 SUSY theory. We use Monte Carlo techniques to study numerically the Gluon Green's function at LO and NLO directly written in the transverse momentum space which allows for the factorization of its infrared divergencies. Finally, we discuss the applicability of our approach to phenomenological searches for the BKP Odderon at the LHC.
    07/2013;
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    ABSTRACT: We discuss computational details of our recent result, namely, the first derivation of the two-loop gluon Regge trajectory within the framework of Lipatov's high energy effective action. In particular, we elaborate on the direct evaluation of Feynman two-loop diagrams by using the Mellin-Barnes representations technique. Our result is in precise agreement with previous computations in the literature, providing this way a highly non-trivial test of the effective action and the proposed subtraction and renormalization scheme combined with our approach for the treatment of the loop diagrams.
    07/2013;
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    Grigorios Chachamis, Michal Deak, German Rodrigo
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    ABSTRACT: We calculate the finite part of the heavy quark impact factor at next-to-leading logarithmic accuracy in a form suitable for phenomenological studies such as the calculation of the cross-section for single bottom quark production at the LHC.
    07/2013;
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    ABSTRACT: We present the derivation of the two-loop gluon Regge trajectory using Lipatov's high energy effective action and a direct evaluation of Feynman diagrams. Using a gauge invariant regularization of high energy divergences by deforming the light-cone vectors of the effective action, we determine the two-loop self-energy of the reggeized gluon, after computing the master integrals involved using the Mellin-Barnes representations technique. The self-energy is further matched to QCD through a recently proposed subtraction prescription. The Regge trajectory of the gluon is then defined through renormalization of the reggeized gluon propagator with respect to high energy divergences. Our result is in agreement with previous computations in the literature, providing a non-trivial test of the effective action and the proposed subtraction and renormalization framework.
    Nuclear Physics B 07/2013; 876(2). · 4.33 Impact Factor
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    ABSTRACT: In the context of evolution equations and scattering amplitudes in the high energy limit of the N=4 super Yang-Mills theory we investigate in some detail the BFKL gluon Green function at next-to-leading order. In particular, we study its collinear behaviour in terms of an expansion in different angular components. We also perform a Monte Carlo simulation of the different final states contributing to such a Green function and construct the diffusion pattern into infrared and ultraviolet modes and multiplicity distributions, making emphasis in separating the gluon contributions from those of scalars and gluinos. We find that the combined role of the non-gluonic degrees of freedom is to improve the collinear behavior and reduce the diffusion into ultraviolet regions while not having any effect on the average multiplicities or diffusion into the infrared. In terms of growth with energy, the non-zero conformal spin components are mainly driven by the gluon terms in the BFKL kernel. For zero conformal spin (Pomeron) the effect of the scalar and gluino sectors is to dramatically push the Green function towards higher values.
    Physics Letters B 05/2013; 724(s 1–3). · 4.57 Impact Factor
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    ABSTRACT: We determine both real and virtual next-to-leading order corrections to the gluon induced forward jet vertex, from the high energy effective action proposed by Lipatov. For these calculations we employ the same regularization and subtraction formalism developed in our previous work on the quark-initiated vertex. We find agreement with previous results in the literature.
    Physical review D: Particles and fields 12/2012; 87(7).
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    G. Chachamis, A. Sabio Vera, C. Salas
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    ABSTRACT: Using Monte Carlo integration techniques, we investigate running coupling effects compatible with the high energy bootstrap condition to all orders in the strong coupling in evolution equations valid at small values of Bjorken x in deep inelastic scattering. A model for the running of the coupling with analytic behavior in the infrared region and compatible with power corrections to jet observables is used. As a difference to the fixed coupling case, where the momentum transfer acts as an effective strong cut-off of the diffusion to infrared scales, in our running coupling study the dependence on the momentum transfer is much milder.
    Physical review D: Particles and fields 11/2012; 87(1).
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    ABSTRACT: We review Lipatov's high energy effective action and show that it is a useful computational tool to calculate scattering amplitudes in (quasi)-multi-Regge kinematics. We explain in some detail our recent work where a novel regularization and subtraction procedure has been proposed that allows to extend the use of this effective action beyond tree level. Two examples are calculated at next-to-leading order: forward jet vertices and the gluon Regge trajectory.
    11/2012;
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    ABSTRACT: Lipatov's high-energy effective action is a useful tool for computations in the Regge limit beyond leading order. Recently, a regularisation/subtraction prescription has been proposed that allows to apply this formalism to calculate next-to-leading order corrections in a consistent way. We illustrate this procedure with the computation of the gluon Regge trajectory at two loops.
    06/2012;
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    ABSTRACT: We investigate dijet production at large rapidity separation in QCD and N = 4 SYM, showing that both theories give similar predictions for observables only sensitive to conformal properties of the scattering such as ratios of azimuthal angle correlations. Renormalization prescriptions are important in this comparison.
    06/2012;
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    G. Chachamis, A. Sabio Vera
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    ABSTRACT: We study the solution of the BFKL equation in the adjoint representation for the N=4 SUSY theory at NLO accuracy. Consistency with the large momentum transfer solution obtained by Fadin and Lipatov is found. We investigate, for large and small values of the momentum transfer, the growth with energy of the Green function, its collinear behaviour and the expansion in azimuthal angle Fourier components.
    Physics Letters B 06/2012; 717(s 4–5). · 4.57 Impact Factor
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    G. Chachamis, A. Sabio Vera, C. Salas
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    ABSTRACT: An iterative solution best suited for a Monte Carlo implementation is presented for the non-forward BFKL equation in a generic color representation. We introduce running coupling effects compatible with bootstrap to all orders in perturbation theory. A numerical analysis is given showing a smooth transition from a hard to a soft pomeron when accounting for running effects.
    06/2012;
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    ABSTRACT: After years of waiting, and after six Les Houches workshops, the era of LHC running is finally upon us, albeit at a lower initial center-of-mass energy than originally planned. Thus, there has been a great sense of anticipation from both the experimental and theoretical communities. The last two years, in particular, have seen great productivity in the area of multi-parton calculations at leading order (LO), next-to-leading order (NLO) and next-to-next-to-leading order (NNLO), and this productivity is reflected in the proceedings of the NLM group. Both religions, Feynmanians and Unitarians, as well as agnostic experimenters, were well-represented in both the discussions at Les Houches, and in the contributions to the write-up. Next-to-leading order (NLO) is the first order at which the normalization, and in some cases the shape, of perturbative cross sections can be considered reliable. This can be especially true when probing extreme kinematic regions, as for example with boosted Higgs searches considered in several of the contributions to this writeup. A full understanding for both standard model and beyond the standard model physics at the LHC requires the development of fast, reliable programs for the calculation of multi-parton final states at NLO. There have been many advances in the development of NLO techniques, standardization and automation for such processes and this is reflected in the contributions to the first section of this writeup. Many calculations have previously been performed with the aid of semi-numerical techniques. Such techniques, although retaining the desired accuracy, lead to codes which are slow to run. Advances in the calculation of compact analytic expressions for Higgs + 2 jets have resulted in the development of much faster codes, which extend the phenomenology that can be conducted, as well as making the code available to the public for the first time. A prioritized list of NLO cross sections was assembled at Les Houches in 2005 and added to in 2007. This list includes cross sections which are experimentally important, and which are theoretically feasible (if difficult) to calculate. Basically all 2-3 cross sections of interest have been calculated, with the frontier now extending to 2 {yields} 4 calculations. Often these calculations exist only as private codes. Since 2007, two additional calculations have been completed: t{bar t}b{bar b} and W+3 jets, reflecting the advance of the NLO technology to 2 {yields} 4 processes. In addition, the cross section for b{bar b}b{bar b} has been calculated for the q{bar q} initial state with the gg initial state calculation in progress. Final states of such complexity usually lead to multi-scale problems, and the correct choice of scales to use can be problematic not only at LO, but also at NLO. The size of the higher order corrections and of the residual scale dependence at NLOcan depend strongly on whether the considered cross section is inclusive, or whether a jet veto cut has been applied. Depending on the process, dramatically different behavior can be observed upon the application of a jet veto. There is a trade-off between suppressing the NLO cross section and increasing the perturbative uncertainty, with application of a jet veto sometimes destroying the cancellation between infra-red logs of real and virtual origin, and sometimes just suppressing large (and very scale-sensitive) tree-level contributions. So far, there is no general rule predicting the type of behavior to be expected, but this is an important matter for further investigation. From the experimental side, an addition to the above wish-list that will be crucial is the determination of the accuracy to which each of the calculations needs to be known. This is clearly related to the experimental accuracy at which the cross sections can be measured at the LHC, and can determine, for example, for what processes it may be necessary to calculate electo-weak corrections, in addition to the higher order QCD corrections. On the theoretical side, it would also be interesting to categorize the impact of a jet veto on the size and stability of each of the NLO cross sections. The technology does exist to carry out a calculation for W/Z production at NNLO (QCD) and at NLO (EW). This process was placed on the wish-list in 2007 and it is unfortunate that the combined calculation has not yet been carried out, as this precision benchmark will be very useful and important at the LHC.
    04/2012
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    ABSTRACT: The 2011 Les Houches workshop was the first to confront LHC data. In the two years since the previous workshop there have been significant advances in both soft and hard QCD, particularly in the areas of multi-leg NLO calculations, the inclusion of those NLO calculations into parton shower Monte Carlos, and the tuning of the non-perturbative parameters of those Monte Carlos. These proceedings describe the theoretical advances that have taken place, the impact of the early LHC data, and the areas for future development.
    03/2012;

Publication Stats

171 Citations
53.94 Total Impact Points

Institutions

  • 2012–2013
    • Instituto de Física Corpuscular
      Paterna, Valencia, Spain
    • University of Valencia
      Valenza, Valencia, Spain
  • 2009–2012
    • Paul Scherrer Institut
      Aargau, Switzerland
  • 2011
    • University of Florence
      Florens, Tuscany, Italy
  • 2008
    • University of Wuerzburg
      • Department of Theoretical and Astrophysics
      Würzburg, Bavaria, Germany
  • 2005–2006
    • University of Hamburg
      • II. Institut für Theoretische Physik
      Hamburg, Hamburg, Germany