Uncovering QCD at high parton densities with hadron-hadron collisions at the LHC – DenseQCDatLHC

One of the main goals of the experiments carried out at the CERN Large Hadron Collider is the study of new forms of hadronic matter, the quark-gluon plasma (QGP) and the color glass condensate (CGC), characterized by high parton density and strong non-linear phenomena. These forms of matter are also important for other high-energy experiments, like the electron-ion collider which is currently under project in both USA and at CERN, and also for understanding the evolution of the Early Universe.

A good understanding of the fundamental theory of strong interactions, Quantum Chromodynamics (QCD), in the high-energy regime is mandatory in order to control the production and the properties of these new forms of matter. At high density, the QCD coupling is moderately weak, but naive perturbation theory nevertheless breaks down due to strong non-linearities: the relevant expansion parameter is the product of the coupling by the gluon occupation number, which can reach values of order one. A powerful strategy to circumvent this problem is to reorganize the perturbative expansion, by identifying and resumming to all orders the dominant effects of the interactions — i.e. those that are enhanced by the high-density effects. Within perturbative QCD at high energy, the most elaborate framework for performing these resummations is the CGC effective theory, that has been in particular introduced by members of the team and which is continuously evolving.

Despite a lot of conceptual progress, the current status of the CGC effective theory is still unsatisfactory from the viewpoint of phenomenology. Its perturbative accuracy is still too crude to allow for realistic comparisons with data and the factorization schemes are not sufficiently developed, nor accurate enough, to deal with more exclusive final states, that are nevertheless measured in experiments. So far, all phenomenological applications of the CGC have departed from a rigorous first-principle approach by incorporating elements of modeling and ad-hoc free parameters in areas that should in principle be under perturbative control in QCD.

In this project, our main goal is to develop the CGC effective theory to a level that allows reliable predictions and explicit comparisons with the phenomenology, without unnecessary modeling. This includes higher-order perturbative calculations, more sophisticated factorization schemes, and innovative numerical and analytical techniques. In particular, we plan to improve the accuracy of the CGC effective theory not only by including next-to-leading order perturbative corrections, but also by performing all-order resummations of special classes of radiative corrections, that are enhanced by 'collinear' logarithms. Albeit formally of higher order, such corrections are known to be numerically large and to lead to pathologies (like negative cross-sections) in fixed-order calculations. Members of our team have obtained important preliminary results on these resummations over the last months.

Some of the cutting-edge problems that we intend to address include the calculation of single- and multiple-particle production in hadron-hadron collisions beyond leading order, the formation and thermalization of the quark-gluon plasma in ultrarelativistic nucleus-nucleus collisions, and the role of parton number fluctuations in hadron-hadron collisions. The members of the team have pioneering contributions on several among these topics. We also plan to compute less inclusive observables such as transverse spin asymmetries in high-energy collisions with polarized beams, and to elucidate some striking correlations, like the `ridge', that have been observed in multi-particle production in proton-proton and proton-nucleus collisions at the LHC.