Formal and phenomenological studies in the high energy limit of QCD
PT-CERN Call 2021/2
José Guilherme Milhano
Instituto Superior Técnico (Universidade de Lisboa)
Laboratório de Instrumentação e Física Experimental de Partículas
The subject of this thesis is the theoretical and phenomenological study of the high energy limit of Quantum Chromodynamics (QCD). The understanding of this particular limit, also called Regge limit, in which the colliding energy is much larger than any other hard scale in the process, is one of the most challenging questions in QCD affecting both phenomenology and our formal understanding of QCD as a quantum field theory. The structure of high energy QCD has new effective degrees of freedom, namely, reggeized quarks and gluons (reggeons), which can form bound states, among which the best known are the Pomeron (two reggeons) and the Odderon (three reggeons). The reggeons in the bound states interact with each other via the exchange of gluons. The actual dynamics of these complex interactions is governed by evolution equations which in the case of the Pomeron predict fast increasing cross sections as the energy rises violating unitarity. The best candidate to restore unitarity is parton saturation, where it is assumed that the low-x character of the wave function for an ultrarelativistic hadron or nucleus is governed by an intrinsic hard scale, the saturation scale Qs. Below Qs, the wave functions of the gluons present in the interaction start to overlap which in turn means that gluons begin to recombine leading to a taming of the rise of the cross section. The goals of this research proposal are to study high energy QCD evolution for N-reggeons (N<=4) both with and without saturation, to develop stochastic solutions in momentum space that will be implemented in Monte Carlo codes and to produce theoretical predictions for cross sections which will be contrasted against predictions from multi-purpose Monte Carlos and experimental data. The project will be carried out at LIP within the LIP Phenomenology Group which has members with excellent research record and key contributions to the field of high energy QCD.