Probing the high-energy dynamics of QCD: selected theoretical and phenomenological studies

Abstract

The center-of-mass energies available at modern accelerators, such as the Large Hadron Collider (LHC), and at future generation accelerators, such as the Electron-Ion Collider (EIC) and Future Circular Collider (FCC), offer us a unique opportunity to investigate hadronic matter under the most extreme conditions ever reached. In particular, we can access the Regge-Gribov (or semi-hard) limit of QCD, characterized by the scale hierarchy s ≫ {Q2} ≫ Λ2 QCD, where √s is the center-of-mass energy, {Q} a set of hard scales characterizing the process and ΛQCD is the QCD mass scale. In this limit, large logarithmic corrections can affect both parton densities and hard scattering cross sections. The Balitsky-Fadin-Kuraev-Lipatov (BFKL) approach represents the established tool to resum to all orders, both in the leading (LLA) and the next-to-leading (NLA) approximation, these large-energy logarithmic contributions. However, it is well known that at very low values of the Bjorken-x, the density of partons, per unit transverse area, in hadronic wavefunctions becomes very large leading to the so-called saturation effects. The evolution of densities is then described by non-linear generalizations of the BFKL equation. Among these, the most general is represented by the Balitsky-JIMWLK hierarchy of equations, which is needed to describe the scattering of a dilute projectile on a dense target, or also the scattering of two dense systems. The dense system condition can be achieved by a very small-x proton, but is more easily achieved for large nuclei. It is clear that a detailed comparison with experimental data requires precision predictions that can only be achieved in the next-to-leading logarithmic approximation or beyond. We face this task from two different perspectives. On the one hand developing analytical calculations that allow to increase the theoretical accuracy that can be reached in predictions, and on the other, by proposing phenomenological analyzes that can be directly tested experimentally. In particular, within the BFKL approach we calculate the full NLO impact factor for the Higgs production. This is the necessary ingredient to study the inclusive forward emissions of a Higgs boson in association with a backward identified jet. We claim that this result should necessarily supplement pure fixed-order calculations entering in the collinear factorization framework, which cannot be able to describe the entire kinematic spectrum in the Higgs-plus-jet channel. The result can be as well used to describe the inclusive hadroproduction of a forward Higgs in the limit of small Bjorken x. Moreover, using the knowledge of already known impact factors we propose a series of new semi-hard reactions that can be used to investigate BFKL dynamics at the LHC. We investigate all observables used so far to study BFKL, including: total cross sections, azimuthal coefficients, azimuthal distributions and pT -differential distributions. In the context of linear evolution, we consider also the problem of extending BFKL beyond the NLLA. To this aim, we compute the Lipatov vertex in QCD with higher ϵ- accuracy, where ϵ = (D − 4)/2. This ingredient enters the BFKL kernel at next-to-NLA (NNLLA) accuracy. In fact, the NNLLA formulation of BFKL requires not only two and three-loop calculations, but also higher ϵ-accuracy of the one-loop results, for instance, in the part of the kernel containing the product of two one-loop Lipatov vertices. Finally, in the saturation framework, and more specifically in the Shockwave approach, we calculate the diffractive double hadron photo- or electroproduction cross sections with full NLL accuracy. These results are usable to detect saturation effects, at both the future EIC or already at LHC, using Ultra Peripheral Collisions.