Very precise data on elastic proton-proton scattering at $\sqrt{ s}=7$, $8$ and $13$ TeV have been obtained by the TOTEM group at LHC in the near-forward region (momentum transfers down to $|t| = 6 \times 10^{ -4}\ { \rm GeV}^2$ at $\sqrt{ s}=8$ TeV and to $|t| = 8 \times 10^{ -4}\ { \rm GeV}^2$ at $\sqrt{ s}=13$ TeV). The Coulomb-nuclear interference has been measured with sufficient accuracy for TOTEM to establish the falloff of the $\rho$ parameter with increasing energy. The predictions from a previously studied model are shown to be in good agreement with the data and thus allow us to draw rather firm conclusions about the structure of the near-forward nuclear amplitude. We point out that due to a zero in the real part of the nuclear amplitude occurring at a very small momentum transfer--that can migrate into the Coulomb-nuclear interference (CNI) region at higher energies--much care is needed in extracting the numerical value of $\rho$ for such energies. Thus, the true value of $\rho$ would be higher than the TOTEM value for $\rho$ found under the hypothesis that the real part of the elastic nuclear amplitude is devoid of such a zero in the CNI region.
Categories: hep-ph hep-ex