Study of hard color singlet exchange in dijet events with proton-proton collisions at $\sqrt{s}= 13$ TeV

A study of pp collision events where the two leading jets are separated by a pseudorapidity gap void of radiation, is presented. Both jets have $p_\text{T, jet} > 40$ GeV and $1.4 < |\eta_\text{jet}| < 4.7$, with $\eta_\text{jet-1}\times \eta_\text{jet-2} < 0$. The analysis is based on data collected by the CMS experiment during a low luminosity high-$\beta^*$ run in 2015 at $\sqrt{s} = 13$ TeV, with an integrated luminosity of $0.66~\mathrm{pb}^{-1}$. The number of charged particles with transverse momentum $p_\text{T} > 200$ MeV in $-1 < \eta < 1$ is used to discriminate color-singlet exchange (CSE) dijet events from color-exchange dijet events. The fraction of CSE dijet events to all dijet events with similar kinematics, $f_\text{CSE}$, is presented as a function of various kinematic variables of interest. The results are compared to previous measurements and to perturbative quantum chromodynamics predictions. A first study of jet-gap-jet events with a leading proton using data collected jointly by the CMS and TOTEM experiments is presented. The protons are detected with the Roman pot detectors of TOTEM. The ratio $f_\text{CSE}$ in this sample is found to be $2.91 \pm 0.70~\mathrm{(stat)} ^{+ 1.02}_{- 0.94}~\mathrm{(syst)}$ times larger than in inclusive dijet production.