Thesis

Holography, QCD and Regge Theory

• Call:

  IDPASC Portugal - PHD Programme 2015

• Academic Year:

  2015 / 2016

• Domains:

  Theoretical Particle Physics | Experimental Particle Physics

• Supervisor:

  Miguel Costa

• Co-Supervisor:

  Nuno Castro

• Institution:

  Universidade do Porto

• Host Institution:

  Universidade do Porto & Universidade do Minho

• Abstract:

  The gauge/gravity duality has been used in recent years to develop a new, intrinsically non-perturbative, QCD phenomenology. The best known example is that of the quark-gluon plasma (QGP) produced in heavy ion physics collisions. Holographic methods proved very helpful because the hydrodynamical description of the QGP arises from a strongly coupled thermalized plasma. The interest in understanding the regime where hadrons or nucleus are dominated by a large gluonic component is actually much wider. It is important for the LHC forward physics programme, which refers to the Regge kinematic limit of large Mandelstam variable s and fixed t. In this limit the gluon partonic distribution function grows with s. This growth can be understood using BFKL pomeron techniques, that describe the exchange of a ladder of Reggeized gluons. However, the BFKL evolution has some severe shortcomings like violation of unitarity at high energy and insensitivity to the non-perturbative infrared physics and to the running of the coupling. The best established phenomenological method to include non-linear effects due to the dense gluon medium is through the BK equation, which allows for gluons recombination, therefore taming the growth of the gluon density. The gauge/gravity duality also serves as a new tool in the above exploration, since the pomeron Regge trajectory is dual to the graviton Regge trajectory in AdS space. This gives a new phenomenology to model the growth of cross sections and the onset of saturation in processes involving pomeron exchange. We pioneered this approach, consistently reproducing available low-x data from deep inelastic scattering, deeply virtual Compton scattering and vector meson production, producing better fits that those that use the above conventional QCD techniques. Furthermore, recent developments suggest that AdS/QCD can resolve a long standing problem in pomeron physics, the unification of the soft and the hard pomeron. The above successes call for a new testing ground within the LHC forward physics programme, which extends HERA data to even smaller value of x (by a factor of ten), becoming a predecessor of a future e + A collider. For instance, the question of whether gluon saturation has already been observed in DIS data, at ALICE recent measurements and in cosmic ray data is a subject of current debate. In this thesis we propose to build a phenomenological model that will reach the community of QCD practitioners, in close connection with the Portuguese participation at the ATLAS experiment, focusing on new observables relevant for the forward physics program. This thesis is a joint proposal between the theory group at Porto University, with strong expertise in the topic of the gauge/gravity duality, and the particle physics group at Minho University, which integrates the ATLAS collaboration and is therefore in a privileged position to explore new physics results that will be obtained at LHC second operation phase and in the forthcoming upgrade.