University of Durham
Measurements at the Large Hadron Collider (LHC) are achieving small measurement errors, in part
because of the large numbers of events observed for standard model processes. It is therefore important
for theory to follow suit to match the precision of the experimental measurements.
Standard model processes at high energy are calculated as an expansion in the strong coupling constant,
the equivalent of the fine structure constant of electrodynamics (alpha_QED). Famously the value of
alpha_QED is 1/137. However the value of the equivalent quantity (alpha_S) in the theory of quarks and gluons (called QCD)
is about 15 times larger at high energy. QCD is the theory which governs the interactions of the quarks and gluons,
collectively called partons, and at high energy the proton can be viewed as a dilute agglomeration of quarks and gluons.
Because of the larger value of alpha_S the effects of gluon and quark radiation are much more important than the radiation of photons.
With each additional power of alpha_S, one more parton is exchanged and the calculations become more challenging.
On the other hand, the theoretical precision of the calculation increases as the neglected terms involve higher and higher
powers of the strong coupling.
In order for the calculations to be useful for the experimenters they must be delivered in the form of a monte carlo
program, so that the fact that the experiment has limitations in its ability to cover all regions can be taken into account.
I am one of the principal authors of the program MCFM.
Interests and expertise (Subject groups)