First determination of the ρ parameter at √s=13 TeV: probing the existence of a colourless C-odd three-gluon compound state

The TOTEM experiment at the LHC has performed the first measurement at root s = 13 TeV of the rho parameter, the real to imaginary ratio of the nuclear elastic scattering amplitude at t = 0, obtaining the following results: rho = 0.09 +/- 0.01 and rho = 0.10 +/- 0.01, depending on different physics assumptions and mathematical modelling. The unprecedented precision of the rho measurement, combined with the TOTEM total cross-section measurements in an energy range larger than 10 TeV (from 2.76 to 13 TeV), has implied the exclusion of all the models classified and published by COMPETE. The rho results obtained by TOTEM are compatible with the predictions, from other theoretical models both in the Regge-like framework and in the QCD framework, of a crossing-odd colourless 3-gluon compound state exchange in the t-channel of the proton-proton elastic scattering. On the contrary, if shown that the crossing-odd 3-gluon compound state t-channel exchange is not of importance for the description of elastic scattering, the rho value determined by TOTEM would represent a first evidence of a slowing down of the total cross-section growth at higher energies. The very low-vertical bar t vertical bar reach allowed also to determine the absolute normalisation using the Coulomb amplitude for the first time at the LHC and obtain a new total proton-proton cross-section measurement sigma(tot) = (110.3 +/- 3.5) mb, completely independent from the previous TOTEM determination. Combining the two TOTEM results yields sigma(tot) = (110.5 +/- 2.4) mb.