NA2-Small-x: Physics at the LHC and future DIS experiments

We aim at strengthening the communication and collaboration between the European groups involved in theoretical and phenomenological studies in small-x physics. Understanding the dynamics of the strong interaction at high energies is one of the central topics in nuclear and particle physics. From a fundamental point of view, Quantum Chromodynamics (QCD) is today the only quantum field theory that may be studied experimentally in all regimes from strong to weak coupling and from confined to deconfined matter. From an applied point of view, QCD processes constitute the main background and source of uncertainties for the precise determination of parameters in the Standard Model and for searches of new physics. Increasing our understanding of the dynamics of QCD at high energies is of extreme importance for several reasons:

• A genuinely new, non-linear regime of QCD is expected to occur at high energies. Here the Color Glass Condensate (CGC) effective theory provides a controlled weak coupling description applicable in spite of the nonperturbatively large gluonic field strengths. Hints of the breaking of fixed-order perturbation theory have been found at HERA and RHIC, but only data from the LHC and future DIS experiments can unambiguously establish the existence of the nonlinear regime.
• The behavior of gluons carrying only a small fraction x of the proton and nuclear momentum determines the bulk of particle production in relativistic heavy ion collisions. These gluons provide the initial conditions for the subsequent evolution towards a quark-gluon plasma in nuclear collisions at the LHC. A good control of these initial conditions is paramount for understanding the emergence of collectivity at the macroscopic level from the microscopic QCD dynamics.
• New developments in high energy QCD also have implications for rarer observables in the hard domain, where parton densities are not large but linear small-x physics, beyond fixed-order perturbation theory, may nevertheless play a crucial role.
• The nuclear wave function at small x determines the cross sections for interactions of high-energy cosmic rays in the atmosphere, thus the determination of their energy and composition.

In recent years there has been a large outburst of precision calculations in the CGC for evolution equations, particle production and correlations, and of improvements in the determination of nuclear parton densities (nPDFs) using all available sets of data including those from the Large Hadron Collider LHC at CERN. Many of these advances have been pioneered by participants in this proposal. Besides, experiments at the LHC and the Relativistic Heavy-Ion Collider RHIC at BNL have shown that small collision systems, pp and pA, show hints of a collective behavior that must be understood in order to characterise the partonic matter created in heavy-ion collisions and to model hadronic and nuclear collisions for both theoretical and experimental studies. The experimental program will be further pursued at RHIC and, above all, at the LHC in the next decade with pp, pPb and PbPb runs. New electron- proton/nucleus deep-inelastic scattering (DIS) experiments are under consideration in the US (the Electron Ion Collider EIC identified as the highest priority for new facility construction after FRIB in the 2015 Nuclear Physics LRP of the DOE) and at CERN (the Large Hadron-electron Collider LHeC and the Future Circular Collider FCC).