Associate 2007-08

Aida X El-Khadra



The standard model of particle physics describes the strong (QCD) and electroweak interactions of elementary particles (i.e., the six flavors ofquarks, three charged leptons, and three neutrinos, as well as the gauge bosons that mediate the interactions). Currently in physics there is a worldwide experimental effort to measure precisely the couplings between heavy quarks and weak bosons in the standard model. It is hoped that these experiments at the precision frontier, combined with future discoveries at the energy frontier, will ultimately yield a deeper understanding of the nature of elementary particles and their interactions.

Because quarks are confined into hadrons (bound states of quarks and bosons), experiments don’t measure quark couplings directly. Instead they measure weak decays of hadrons. Theoretical input is needed in order to determine the quark-level couplings from hadron-level measurements. This requires a quantitative understanding of the nonperturbative QCD effects that confine the quarks inside the hadrons. Such calculations are notoriously difficult and currently limit the accuracy with which the standard model parameters can be determined from experiment.

Lattice field theory offers a systematic approach to solving QCD nonperturbatively, and it is in this area that Professor El-Khadra has focused her attention. About five years ago the main obstacle to obtaining quantitative results from lattice QCD simulations (i.e., the computational effort associated with the proper inclusion of sea quark effects) was removed, through the introduction of a highly improved light quark action that can be simulated efficiently. Professor El-Khadra’s current research project builds on this breakthrough in latticeQCD.

In the last few years her group has started a program for calculating the most important hadronic parameters relevant to the experimental program. Their initial calculations yielded promising results, and a few predictions at the 10 percent level that were subsequently verified experimentally. They plan to reduce the overall uncertainty of their calculations to the few-percent level in the coming years. This new theoretical effort, with new numerical simulations and additional perturbative calculations to improve the heavy quark action beyond its current level of accuracy, will be Professor El-Khadra’s focus during her Center appointment.