First determination of the rho parameter at sqrt(s) = 13 TeV: probing the existence of a colourless C-odd three-gluon compound state


Experiment TOTEM at the LHC at CERN has performed the first measurement of the rho parameter, the real to imaginary ratio of the nuclear elastic scattering amplitude, at center-of-mass collision energy 13 TeV. The unprecedented precision, combined with other TOTEM proton-proton elastic scattering data, favors alternative elastic scattering models that require, in addition to the standard C-even colourless two-gluon compound state (Pomeron) an existence of additional colourless C-odd three-gluon compound state.

Total elastic cross section and rho parameter
Comparison of TOTEM measurements (red solid markers) with predictions of COMPETE models for several values of center-of-mass energies of proton collisions. None of the models, based only on existence of two-gluon colourless Pomeron state is able to describe simultaneously the energy dependence of total elastic cross section (left) and rho parameter (right).