Abstract
Events where the two leading jets are separated by a pseudorapidity interval devoid of particle activity, known as jet-gap-jet events, are studied in proton-proton collisions at $\sqrt{s} =$ 13 TeV. The signature is expected from hard color-singlet exchange. Each of the highest transverse momentum ($p_\mathrm{T}$) jets must have $p_\mathrm{T}^\text{jet}$ $\gt$ 40 GeV and pseudorapidity 1.4 $\lt$ $|\eta^\text{jet}|$ $\lt$ 4.7, with $\eta^\text{jet1} \eta^\text{jet2}$ $\lt$ 0, where $\text{jet1}$ and $\text{jet2}$ are the leading and subleading jets in $p_\mathrm{T}$, respectively. The analysis is based on data collected by the CMS and TOTEM experiments during a low luminosity, high-$\beta^*$ run at the CERN LHC in 2015, with an integrated luminosity of 0.66 pb$^{-1}$. Events with a low number of charged particles with $p_\mathrm{T}$ $\gt$ 0.2 GeV in the interval $|\eta|$ $\lt$ 1 between the jets are observed in excess of calculations that assume only color-exchange. The fraction of events produced via color-singlet exchange, $f_\text{CSE}$, is measured as a function of $p_\mathrm{T}^\text{jet2}$, the pseudorapidity difference between the two leading jets, and the azimuthal angular separation between the two leading jets. The fraction $f_\text{CSE}$ has values of 0.4-1.0%. The results are compared with previous measurements and with predictions from perturbative quantum chromodynamics. In addition, the first study of jet-gap-jet events detected in association with an intact proton using a subsample of events with an integrated luminosity of 0.40 pb$^{-1}$ is presented. The intact protons are detected with the Roman pot detectors of the TOTEM experiment. The $f_\text{CSE}$ in this sample is 2.91 $\pm$ 0.70 (stat) $^{+1.08}_{-1.01}$ (syst) times larger than that for inclusive dijet production in dijets with similar kinematics.
Highlights
Quantum chromodynamics (QCD) is the established theory of strong interactions and it is especially successful at very short distances, where physical observables can be computed in a perturbative expansion in powers of the strong coupling, αS
The pp collision data used in this analysis were collected in a combined special rupnffiffi by the CMS and TOTEM experiments in 2015 at s 1⁄4 13 TeV, when the LHC operated in a mode with low probability of overlapping pp interactions in the same bunch crossing
Because of reconstruction inefficiencies and acceptance limitations of the CMS detector, and the use of energy thresholds applied for each PF candidate reconstructed in CMS, these events satisfy instead the inequality ξpðPFÞ − ξpðRPÞ < 0, i.e., the fractional momentum loss is underestimated by the CMS detector
Summary
Quantum chromodynamics (QCD) is the established theory of strong interactions and it is especially successful at very short distances, where physical observables can be computed in a perturbative expansion in powers of the strong coupling, αS. Dynamical effects predicted by the DGLAP evolution equations are largely suppressed in events with pseudorapidity gaps, since the predicted dijet production rate is strongly reduced by way of a Sudakov form factor [48–51] This factor, which accounts for the probability of having no additional parton emissions between the hard partons, is not necessary for BFKL pomeron exchange [38]. The analysis based on CMS and TOTEM data provides a first investigation of dijet events with a central gap and an intact proton This analysis can elucidate the role of soft parton exchanges in the creation and destruction mechanisms of pseudorapidity gaps in strong interactions [63].
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