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C. Setyadi

PhD student
C. Setyadi

In high-energy scattering processes in which subatomic particles, such as protons, collide, the interaction is usually through the exchange of many particles, in particular gluons, which are the force carriers of the strong nuclear interaction. As the collision energy increases the number of gluons increases and the scattering depends on the combined effect of many gluons, encoded by the gluon field integrated along a trajectory, a line or sometimes even a loop. The scattering is then described in terms of so-called Wilson lines or Wilson loops.

 Wilson lines/loops are exponentials of integrals over the elementary gluonic degrees of freedom. They are the QCD analogues of the potential scattering phases that arise in scattering off an electromagnetic potential in QED. These extended (nonlocal) quantities are the relevant degrees of freedom in certain kinematic regimes, in particular, in the regime where the gluons carry very little of the initial proton’s momentum (referred to as the small- x region) and the process is sensitive to the transverse momentum of the gluons (where the gluon distribution is a transverse momentum dependent distribution, a so-called TMD). In the small-x region an effective field theory in terms of Wilson lines has been employed, describing the physics of a new phenomenon: gluon saturation, where the density of gluons becomes maximal and glass-like properties result. The gluons form what is called the Color Glass Condensate. Another kinematic region where Wilson lines/loops arise is in diffractive scattering, where the process exhibits a rapidity gap, a region where no particles are produced. This is also described in terms of the combined effect of multiple gluons.

The aim of this project is to investigate the properties of Wilson line correlators and to find new ways to learn about these fundamental quantities, for example by defining well-defined quantities that could be calculated by others using lattice gauge theory methods. Another aim of the project is to arrive at a unified description and understanding of the role of these non-local degrees of freedom in high-energy scattering processes, specifically by relating the small-x region description to the one of diffractive scattering. Further extensions can be made to double parton scattering and to in-medium scattering, which is relevant for the scattering with heavy ions, such as (also) performed at the LHC.

Last modified:14 June 2018 09.45 a.m.

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