A. Sabio Vera

Scale choice and collinear contributions to Mueller-Navelet jets at LHC energies

Abstract We investigate the stability under variation of the renormalization, factorization and energy scales entering the calculation of the cross section, at next-to-leading order in the BFKL formalism, for the production of Mueller–Navelet jets at the Large Hadron Collider, following the experimental cuts on the tagged jets. To find optimal values for the scales involved in this observable it is possible to look for regions of minimal sensitivity to their variation. We show that the scales found with this logic are more natural, in the sense of being more similar to the squared transverse momenta of the tagged jets, when the BFKL kernel is improved with a resummation of collinear contributions than when the treatment is at a purely next-to-leading order. We also discuss the good perturbative convergence of the ratios of azimuthal angle correlations, which are quite insensitive to collinear resummations and well described by the original BFKL framework.

Quark contribution to the gluon Regge trajectory at NLO from the high energy effective action

The two loop (NLO) diagrams with quark content contributing to the gluon Regge trajectory are computed within the framework of Lipatov’s eective action for QCD, using the regularization procedure for longitudinal divergencies recently proposed by two of us in [1]. Perfect agreement with previous results in the literature is found, providing a robust check of the regularization prescription and showing that the high energy eective action is a very useful computational tool in the quasi-multi-Regge limit.

A comparative study of small x Monte Carlos with and without QCD coherence effects

We compare two Monte Carlo implementations of resummation schemes for the description of parton evolution at small values of Bjorken x. One of them is based on the Balitsky-FadinKuraev-Lipatov (BFKL) evolution equation and generates fully differential parton distributions in momentum space making use of reggeized gluons. The other one is based on the Catani-CiafaloniFiorani-Marchesini (CCFM) partonic kernel where QCD coherence effects are introduced. It has been argued that both approaches agree with each other in the x → 0 limit. We show that this is not the case for azimuthal angle dependent quantities since at high energies the BFKL approach is dominated by its zero conformal spin component while the CCFM gluon Green function receives contributions from all conformal spins even at very small x.

The Colour Octet Representation of the Non-Forward BFKL Green Function

Abstract We factorize the infrared divergences of the non-forward BFKL Green function for a general t-channel projection of the color quantum numbers and study the properties of the infrared finite remainder in the case of color octet exchange. The adjoint Green function is compared with the singlet case for different values of the momentum transfer. The adjoint Green function plays an important role in the calculation of the finite remainder of the two-loop six-point MHV planar amplitude in N = 4 SYM as it was demonstrated by Bartels, Lipatov and Sabio Vera (2009, 2010) [1] , [2] . A comparison with regularizations preserving conformal invariance at large momentum transfer is shown.

The NLO N=4 SUSY BFKL Green function in the adjoint representation

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 (2012) in [1] 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.

Multi-Regge kinematics and azimuthal angle observables for inclusive four-jet production

We evaluate differential cross sections for production of four jets in multi-Regge kinematics at a hadron collider. The main focus lies on the azimuthal angle dependences. As in previous studies, the ratios of correlation functions of products of cosines of azimuthal angle differences among the tagged jets offer us the cleanest quantities to compare with the experimental data. The calculations are based on the jet production from a single BFKL ladder with a convolution of three BFKL Green functions where we always have two forward/backward jets tagged in the final state. We also demand the tagging of two further jets in more central regions of the detectors with a relative separation in rapidity from each other, plus the inclusive production of an arbitrary number of mini-jets. We show that dependences on the transverse momenta and rapidity of the two central jets can be a distinct signal of the onset of BFKL dynamics.

Forward jet production & quantum corrections to the gluon Regge trajectory from Lipatov's high energy effective action

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.

Next-to-leading order corrections to the gluon-induced forward jet vertex from the high energy effective action

We acknowledge partial support from the European Commission under Contract No. LHCPhenoNet (PITNGA- 2010-264564), the Comunidad de Madrid through Proyecto HEPHACOS ESP-1473, MICINN (FPA2010- 17747), and Spanish MINECO’s ‘‘Centro de Excelencia Severo Ochoa’’ Programme under Grant No. SEV-2012- 0249. M.H. acknowledges support from the German Academic Exchange Service (DAAD), the U.S. Department of Energy under Contract No. DE-AC02- 98CH10886 and a BNL ‘‘Laboratory Directed Research and Development’’ Grant (No. LDRD 12-034). G. C. thanks the partial support of the Research Executive Agency (REA) of the European Union under the Grant Agreement No. PIEF-GA-2011-298582 and MICINN (No. FPA2011-23778, No. FPA2007-60323, and No. CSD2007-00042 CPAN)

Inclusive four-jet production at 7 and 13 TeV: Azimuthal profile in multi-Regge kinematics

Recently, new observables in LHC inclusive events with three tagged jets were proposed. Here, we extend that proposal to events with four tagged jets. The events are characterized by one jet in the forward direction, one in the backward direction with a large rapidity distance Y from the first one and two more jets tagged in more central regions of the detector. In our setup, non-tagged associated mini-jet multiplicity is present and needs to be accounted for by the inclusion of BFKL gluon Green functions. The projection of the cross section on azimuthal-angle components opens up the opportunity for defining new ratios of correlation functions of the azimuthal-angle differences among the tagged jets that can be used as probes of the BFKL dynamics.

BFKL Azimuthal Imprints in Inclusive Three-jet Production at 7 and 13 TeV

Abstract We propose the study of new observables in LHC inclusive events with three tagged jets, one in the forward direction, one in the backward direction and both well-separated in rapidity from the each other (Mueller–Navelet jets), together with a third jet tagged in central regions of rapidity. Since non-tagged associated mini-jet multiplicity is allowed, we argue that projecting the cross sections on azimuthal-angle components can provide several distinct tests of the BFKL dynamics. Realistic LHC kinematical cuts are introduced.