HEP Weekly Seminar: The Discovery of the Odderon: from Theory to the Experimental Results by TOTEM/D0 (Yuri Kovchegov, The Ohio State University & Christophe Royon, The University of Kansas)
Abstract:We will give an overview of our theoretical understanding of the QCD odderon, a C-odd t-channel exchange in high-energy scattering. We will argue that at the lowest non-trivial order in perturbation theory the odderon is simply a 3-gluon exchange. We will then describe the decades-long quest to understand the high-energy asymptotics of the odderon by studying the small-x evolution (quantum) corrections to the 3-gluon exchange. The quest culminated in the early 2000s: surprisingly, the high-energy behavior of the odderon with all the quantum corrections turned out to be independent of energy, very similar to the lowest-order 3-gluon exchange amplitude. While this result was obtained at the leading order in the evolution kernel, since then several arguments have been constructed indicating that this constant in energy behavior persists at higher orders in the strong coupling and even in the non-perturbative strong-coupling regime. If time allows, we will argue that at very high energies the high gluon densities in the colliding hadrons should lead to the odderon amplitude becoming a decreasing function of energy.
In the second part of the talk, we will describe the odderon discovery by the TOTEM and D0 experiments. The analysis compares the p pbar elastic cross section as measured by the D0 Collaboration at a center-of-mass energy of 1.96 TeV to that in pp collisions as measured by the TOTEM Collaboration at 2.76, 7, 8, and 13 TeV using a model-independent approach. The TOTEM cross sections, extrapolated to a center-of-mass energy of 1.96 TeV, are compared with the D0 measurement in the region of the diffractive minimum and the second maximum of the pp cross section. The two data sets disagree at the 3.4 sigma level and thus provide evidence for the t-channel exchange of a colorless, C-odd gluonic compound, also known as the odderon. We combine these results with a TOTEM analysis of the same C-odd exchange based on the total cross section and the ratio of the real to imaginary parts of the forward elastic strong interaction scattering amplitude in pp scattering. The combined significance is larger than 5 sigma and is interpreted as the first observation of the exchange of a colorless, C-odd gluonic compound.