Christian W. Bauer

Christian W. Bauer is a theoretical particle physicist associated with Lawrence Berkeley National Laboratory. His laboratory theory-group entry and personal research page identify his work in high-energy physics.^[ref]

With Sean Fleming, Dan Pirjol and Iain W. Stewart, Bauer coauthored a2001 paper constructing an effective field theory for energetic quarks interacting with soft and collinear gluons. The paper matches currents between theories and evolves their Wilson coefficients using the renormalization group.^[ref] This is a foundational contribution to soft-collinear effective theory, which organizes strongly interacting processes involving separated momentum scales.

An original toy expansion illustrates the general logic of retaining controlled low-energy information. Consider an exchange contribution proportional toThe geometric series separates a leading coefficient from corrections with additional powers of momentum. If $q^2/M^2=0.01$, replacing the exact expression by $1/M^2$ gives a relative error of1percent; retaining the next term reduces that relative error to0.01percent. This simple exchange model is not the Lagrangian or a numerical result of the2001 paper. It shows why specifying an expansion parameter and truncation order gives an approximation more content than merely calling it a low-energy model.

In the actual collinear problem, an energetic particle can still have a small invariant mass. The relevant low-energy description therefore retains selected energetic modes instead of removing every field carrying a large energy. The explicit hierarchy and degrees of freedom in soft-collinear effective theory encode this distinction.