Information geometry of DGLAP evolution

Abstract

Quark and gluon splittings shift proton momentum toward smaller fractions with increasing resolution. Momentum conservation makes the summed momentum-weighted quark and gluon densities, when nonnegative, a probability distribution. We apply information geometry, the geometry of probability distributions, to this probabilistic form of Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution. We evaluate, to our knowledge for the first time, the speed at which the shape of the proton's momentum distribution changes, measured by how well distributions at neighboring resolutions can be told apart statistically. At leading order, with one-loop splitting functions and running coupling, five fixed quark flavors, nonnegative quark and gluon densities and sufficiently regular inputs, we obtain a closed-form lower bound on this speed. Once the coupling is specified, three numbers of the input (the distribution at the starting resolution) fix the bound at every scale without computing the evolved distribution. For three of the five model inputs at four scales from 2 GeV to 10 TeV, the squared bound is at least 94.8 percent of the squared speed. The bound is reached when the distribution's relative rate of change is linear in the logarithm of the inverse momentum fraction. For these three model inputs at the four scales, the relative rate of change is close to this linear form in mean square weighted by momentum.