# Azimuthal separation in nearly back-to-back jet topologies in inclusive 2- and 3-jet events in pp collisions at $\sqrt{s} =$ 13 TeV

## TL;DR

This paper measures the azimuthal angle between the two most energetic jets in nearly back-to-back configurations in proton-proton collisions at 13 TeV, comparing results with QCD predictions and highlighting discrepancies.

## Contribution

It provides the first detailed measurement of azimuthal correlations in nearly collinear jet events at 13 TeV, testing perturbative QCD predictions against experimental data.

## Key findings

- Predictions generally agree with measurements within 15%
- Discrepancies are largest near 180° azimuthal separation
- No single model simultaneously describes 2- and 3-jet data

## Abstract

A measurement for inclusive 2- and 3-jet events of the azimuthal correlation between the two jets with the largest transverse momenta, $\Delta\phi_{12}$ , is presented. The measurement considers events where the two leading jets are nearly collinear ("back-to-back") in the transverse plane and is performed for several ranges of the leading jet transverse momentum. Proton-proton collision data collected with the CMS experiment at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 35.9 fb$^{-1}$ are used. Predictions based on calculations using matrix elements at leading-order and next-to-leading-order accuracy in perturbative quantum chromodynamics supplemented with leading-log parton showers and hadronization are generally in agreement with the measurements. Discrepancies between the measurement and theoretical predictions are as large as 15%, mainly in the region 177$^\circ$ $<$ $\Delta\phi_{12}$ $<$ 180$^\circ$. The 2- and 3-jet measurements are not simultaneously described by any of models.