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

Antchev, G. and Aspell, P. and Atanassov, I and Avati, V and Baechler, J. and Barrera, C. Baldenegro and Berardi, V and Berretti, M. and Bossini, E. and Bottigli, U. and Bozzo, M. and Bruce, R. and Burkhardt, H. and Cafagna, F. S. and Catanesi, M. G. and Csanád, Máté and Csörgő, Tamás and Deile, M. and De Leonardis, F. and D'Orazio, A. and Doubek, M. and Druzhkin, D. and Eggert, K. and Eremin, V and Ferro, F. and Fiergolski, A. and Garcia, F. and Morales, H. Garcia and Georgiev, V and Giani, S. and Nemes, Frigyes and Novák, Tamás and Sziklai, János István (2019) First determination of the ρ parameter at √s=13 TeV -- probing the existence of a colourless three-gluon bound state. EUROPEAN PHYSICAL JOURNAL C, 79 (9). ISSN 1434-6044

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Abstract

The TOTEM experiment at the LHC has performed the first measurement at √s = 13 TeV of the ρ parameter, the real to imaginary ratio of the nuclear elastic scattering amplitude at t = 0, obtaining the following results: ρ = 0.09 ± 0.01 and ρ = 0.10 ± 0.01, depending on different physics assumptions and mathematical modelling. The unprecedented precision of the ρ 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 ρ results obtained by TOTEM are compatible with the predictions, from alternative theoretical models both in the Regge-like framework and in the QCD framework, of a colourless 3-gluon bound state exchange in the t-channel of the proton-proton elastic scattering. On the contrary, if shown that the 3- gluon bound state t-channel exchange is not of importance for the description of elastic scattering, the ρ 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-|t| 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 σtot = (110.3 ± 3.5) mb, completely independent from the previous TOTEM determination. Combining the two TOTEM results yields σtot = (110.5 ± 2.4) mb.

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