CMS-SMP-17-009

CMS-SMP-17-009 ; CERN-EP-2018-344
Azimuthal separation in nearly back-to-back jet topologies in inclusive 2- and 3-jet events in pp collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
12 February 2019
Eur. Phys. J. C 79 (2019) 773
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.
Links: e-print arXiv:1902.04374 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Normalized inclusive 2-jet distributions as a function of the azimuthal separation of the two leading jets ${\Delta \phi _{12}}$ for different ${{p_{\mathrm {T}}} ^{\text {max}}}$ regions (left and right). The data are represented by the markers and the theory by histograms. Overlaid with the data are predictions from the HERWIG++ event generator (solid lines) and PH-2J + PYTHIA 8 (dotted lines). The total experimental uncertainty is depicted as error bars on the predictions.
png pdf	Figure 1-a: Normalized inclusive 2-jet distributions as a function of the azimuthal separation of the two leading jets ${\Delta \phi _{12}}$ for different ${{p_{\mathrm {T}}} ^{\text {max}}}$ regions. The data are represented by the markers and the theory by histograms. Overlaid with the data are predictions from the HERWIG++ event generator (solid lines) and PH-2J + PYTHIA 8 (dotted lines). The total experimental uncertainty is depicted as error bars on the predictions.
png pdf	Figure 1-b: Normalized inclusive 2-jet distributions as a function of the azimuthal separation of the two leading jets ${\Delta \phi _{12}}$ for different ${{p_{\mathrm {T}}} ^{\text {max}}}$ regions. The data are represented by the markers and the theory by histograms. Overlaid with the data are predictions from the HERWIG++ event generator (solid lines) and PH-2J + PYTHIA 8 (dotted lines). The total experimental uncertainty is depicted as error bars on the predictions.
png pdf	Figure 2: Ratios of the normalized inclusive 2-jet distributions for the PYTHIA 8, HERWIG++, and MadGraph + PYTHIA 8 predictions to data as a function of the azimuthal separation of the two leading jets ${\Delta \phi _{12}}$, for all the ${{p_{\mathrm {T}}} ^{\text {max}}}$ regions. The solid band indicates the total experimental uncertainty and the error bars on the MC points represent the statistical uncertainty of the simulated data.
png pdf	Figure 3: Ratios of the normalized inclusive 2-jet distributions for the PH-2J + PYTHIA 8, PH-3J + PYTHIA 8, and PH-2J + HERWIG++ predictions to data as a function of the azimuthal separation of the two leading jets ${\Delta \phi _{12}}$, for all the ${{p_{\mathrm {T}}} ^{\text {max}}}$ regions. The solid band indicates the total experimental uncertainty and the error bars on the MC points represent the statistical uncertainty of the simulated data. The PH-3J prediction is not shown for the highest bin in ${{p_{\mathrm {T}}} ^{\text {max}}}$ because of the large statistical fluctuations.
png pdf	Figure 4: Normalized inclusive 3-jet distributions as a function of the azimuthal separation of the two leading jets ${\Delta \phi _{12}}$ for different ${{p_{\mathrm {T}}} ^{\text {max}}}$ regions (left and right). The data are represented by the markers and the theory by histograms. Overlaid with the data are predictions from the HERWIG++ event generator (solid lines) and PH-2J + PYTHIA 8 (dotted lines). The total experimental uncertainty is depicted as error bars on the predictions.
png pdf	Figure 4-a: Normalized inclusive 3-jet distributions as a function of the azimuthal separation of the two leading jets ${\Delta \phi _{12}}$ for different ${{p_{\mathrm {T}}} ^{\text {max}}}$ regions. The data are represented by the markers and the theory by histograms. Overlaid with the data are predictions from the HERWIG++ event generator (solid lines) and PH-2J + PYTHIA 8 (dotted lines). The total experimental uncertainty is depicted as error bars on the predictions.
png pdf	Figure 4-b: Normalized inclusive 3-jet distributions as a function of the azimuthal separation of the two leading jets ${\Delta \phi _{12}}$ for different ${{p_{\mathrm {T}}} ^{\text {max}}}$ regions. The data are represented by the markers and the theory by histograms. Overlaid with the data are predictions from the HERWIG++ event generator (solid lines) and PH-2J + PYTHIA 8 (dotted lines). The total experimental uncertainty is depicted as error bars on the predictions.
png pdf	Figure 5: Ratios of the normalized inclusive 3-jet distributions for the PYTHIA 8, HERWIG++, and MadGraph + PYTHIA 8 predictions to data as a function of the azimuthal separation of the two leading jets ${\Delta \phi _{12}}$, for all the ${{p_{\mathrm {T}}} ^{\text {max}}}$ regions. The solid band indicates the total experimental uncertainty and the error bars on the MC points represent the statistical uncertainty of the simulated data.
png pdf	Figure 6: Ratios of the normalized inclusive 3-jet distributions for the PH-2J + PYTHIA 8, PH-3J + PYTHIA 8, and PH-2J + HERWIG++ predictions to data as a function of the azimuthal separation of the two leading jets ${\Delta \phi _{12}}$, for all ${{p_{\mathrm {T}}} ^{\text {max}}}$ regions. The solid band indicates the total experimental uncertainty and the error bars on the MC points represent the statistical uncertainty of the simulated data. The PH-3J prediction is not shown for the highest bin in ${{p_{\mathrm {T}}} ^{\text {max}}}$ because of the large statistical fluctuations.
png pdf	Figure 7: Ratios of the normalized inclusive 2-jet distributions for the PH-2J + PYTHIA 8 predictions to data as a function of the azimuthal separation of the two leading jets ${\Delta \phi _{12}}$, for all ${{p_{\mathrm {T}}} ^{\text {max}}}$ regions. The solid beige band indicates the total experimental uncertainty and the hatched band represents the total theoretical uncertainty.
png pdf	Figure 8: Ratios of the normalized inclusive 3-jet distributions for the PH-2J + PYTHIA 8 predictions to data as a function of the azimuthal separation of the two leading jets ${\Delta \phi _{12}}$, for all ${{p_{\mathrm {T}}} ^{\text {max}}}$ regions. The solid beige band indicates the total experimental uncertainty, the hatched band represents the total theoretical uncertainty.

Tables
png pdf	Table 1: Monte Carlo event generators, parton densities, and underlying event tunes used for comparison with measurements.
png pdf	Table 2: The integrated luminosity for each trigger sample in the analysis, and trigger used for each ${{p_{\mathrm {T}}} ^{\text {max}}}$ range.

Summary

Measurements of the normalized inclusive 2- and 3-jet distributions as a function of the azimuthal separation ${\Delta\phi_{12}}$ between the two jets with the highest transverse momentum ${p_{\mathrm{T}}}$, in the collinear back-to-back region, are presented for several ${p_{\mathrm{T}}^{\mathrm{max}}}$ ranges of the leading jet. The measurements are performed using data collected with the CMS experiment at the LHC, corresponding to an integrated luminosity of 35.9 fb$^{-1}$ of pp collisions at a center-of-mass energy of $13 TeV$.

The measured ${\Delta\phi_{12}}$ distributions generally agree with predictions from PYTHIA 8, HERWIG++, MadGraph + PYTHIA 8, PH-2J + HERWIG++, and POWHEG (PH-2J and PH-3J) matched to PYTHIA 8. Discrepancies between the measurement and theoretical predictions are as large as 15%, mainly in the region 177$^\circ$ $< {\Delta\phi_{12}} <$ 180$^\circ$. The predictions agree better with the measurements for larger ${p_{\mathrm{T}}^{\mathrm{max}}}$ and smaller ${\Delta\phi_{12}}$, where the contribution of resummation effects becomes smaller. The 2- and 3-jet measurements are not simultaneously described by any of models.

The tree-level multijet event generator MadGraph in combination with PYTHIA 8 for showering, hadronization, and multiparton interactions, shows deviations from the measured ${\Delta\phi_{12}}$ for the inclusive 2-jet case, and even larger deviations for the 3-jet case. The PYTHIA 8 and HERWIG++ predictions show deviations (up to 10%) for the 2-jet inclusive distributions, whereas their predictions are in reasonable agreement with the inclusive 3-jet distributions.

The next-to-leading-order PH-2J + PYTHIA 8 prediction does not describe the data and a different trend compared to PYTHIA 8 and HERWIG++ towards ${\Delta\phi_{12}} =$ 180$^{\circ}$ is observed. The PH-3J + PYTHIA 8 predictions agree with the measurements except for the last bin in the low ${p_{\mathrm{T}}^{\mathrm{max}}}$ intervals. The PH-2J + HERWIG++ prediction agrees well with the measurement in the highest ${p_{\mathrm{T}}^{\mathrm{max}}}$ ranges. For the inclusive 3-jet case, PH-2J + PYTHIA 8 performs similarly to PYTHIA 8 and HERWIG++ in the whole ${\Delta\phi_{12}}$ range for high ${p_{\mathrm{T}}^{\mathrm{max}}}$ intervals. MadGraph + PYTHIA 8, PH-3J + PYTHIA 8, and PH-2J + HERWIG++ show deviations from the measurements of up to 15%.

The measurement of correlations for collinear back-to-back dijet configurations probes the multiple scales involved in the event and, therefore, the differences observed between predictions and the measurements illustrate the importance of improving the models of soft parton radiation accompanying the hard process.

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