# 2019 Paul Dirac Medal and Prize

Professor R Keith Ellis for his seminal work in quantum chromodynamics (QCD) where he performed many of the key calculations that led to the acceptance of QCD as the correct theory of the strong interaction.

Keith Ellis has made numerous seminal contributions to the theory of the strong interaction, QCD. He performed many of the key calculations that led to the acceptance of QCD as the correct theory.

A key ingredient was the development of theoretical techniques that led directly to the first quantitatively reliable predictions of fundamental strong interaction processes observed in proton- (anti-)proton colliders, namely the Drell-Yan production of lepton pairs. To perform this type of calculation a regulator is required to control the divergences associated with soft and collinear parton emission. Ellis and collaborators showed how to regulate the singularities using dimensional regularization. This is now a textbook calculation, and established the method used to calculate all higher radiative corrections to hard processes in QCD.

The results were themselves striking. In the late 1970s, the data showed an excess over the tree level prediction of the Drell-Yan model by a factor of about two at the energies which were probed at that time. Ellis and collaborators showed that the radiative corrections to the Drell-Yan process were large, and led to a good description of the data. Soon afterwards, the W and Z bosons were discovered at CERN and Ellis's calculations were pivotal in successfully describing the total cross sections for W and Z production, as well as the transverse momentum and rapidity distributions for the produced bosons within the QCD improved parton model.

Other key results in establishing QCD as the correct theory were the radiative corrections to event shapes in electron-positron annihilation, and to the production of heavy quarks in proton-(anti)proton collisions. These results were essential for the analysis of the properties of gluons and the discovery of the top quark.

Ellis continues to play a leading role in the development of new techniques for precision calculations in quantum field theory, through the automation of one-loop corrections and through even more precise second order corrections that are so important for interpreting the highly precise data from the Large Hadron Collider.