  The process pp$\to\ell^+\ell^-$p$^{(*)}$, with $\ell^+\ell^-$ a muon or an
electron pair produced at midrapidity with mass larger than 110 GeV, has been
observed for the first time at the LHC in pp collisions at $\sqrt{s} =$ 13 TeV.
One of the two scattered protons is measured in the CMS-TOTEM precision proton
spectrometer (CT-PPS), which operated for the first time in 2016. The second
proton either remains intact or is excited and then dissociates into a low-mass
state p$^{*}$, which is undetected. The measurement is based on an integrated
luminosity of 9.4 fb$^{-1}$ collected during standard, high-luminosity LHC
operation. A total of 12 $\mu^+\mu^-$ and 8 e$^+$e$^-$ pairs with
$m(\ell^{+}\ell^{-}) >$ 110 GeV, and matching forward proton kinematics, are
observed, with expected backgrounds of 1.49 $\pm$ 0.07 (stat) $\pm$ 0.53 (syst)
and 2.36 $\pm$ 0.09 (stat) $\pm$ 0.47 (syst), respectively. This corresponds to
an excess of more than five standard deviations over the expected background.
The present result constitutes the first observation of proton-tagged
$\gamma\gamma$ collisions at the electroweak scale. This measurement also
demonstrates that CT-PPS performs according to the design specifications.
