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| CMS-FSQ-12-002 ; CERN-PH-EP-2015-309 | ||
| Azimuthal decorrelation of jets widely separated in rapidity in pp collisions at $\sqrt{s} =$ 7 TeV | ||
| CMS Collaboration | ||
| 25 January 2016 | ||
| J. High Energy Phys. 08 (2016) 139 | ||
| Abstract: The decorrelation in the azimuthal angle between the most forward and the most backward jets (Mueller-Navelet jets) is measured in data collected in pp collisions with the CMS detector at the LHC at $\sqrt{s} =$ 7 TeV. The measurement is presented in the form of distributions of azimuthal-angle differences, $ \Delta \phi $, between the Mueller-Navelet jets, the average cosines of $ ( \pi- \Delta \phi) $, $ 2 ( \pi-\Delta\phi) $, and $ 3 ( \pi-\Delta\phi) $, and ratios of these cosines. The jets are required to have transverse momenta, $p_{\mathrm{T}}$, in excess of 35 GeV and rapidities, $| y |$, of less than 4.7. The results are presented as a function of the rapidity separation, $\Delta{y}$, between the Mueller-Navelet jets, reaching $\Delta{y}$ up to 9.4 for the first time. The results are compared to predictions of various Monte Carlo event generators and to analytical predictions based on the DGLAP and BFKL parton evolution schemes. | ||
| Links: e-print arXiv:1601.06713 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Left: Distributions of the azimuthal-angle difference, $\Delta \phi $, between MN jets in the rapidity intervals $ \Delta {y} < $ 3.0 (top row), 3.0 $ < \Delta {y} < $ 6.0 (centre row), and 6.0 $ < \Delta {y} < $ 9.4 (bottom row). Right: Ratios of predictions to the data in the corresponding rapidity intervals. The data (points) are plotted with experimental statistical (systematic) uncertainties indicated by the error bars (the shaded band), and compared to predictions from the LL DGLAP-based MC generators PYTHIA 6, PYTHIA 8, HERWIG++, and SHERPA, and to the LL BFKL-motivated MC generator HEJ with hadronisation performed with ARIADNE (solid line). |
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Figure 1-a:
Distribution of the azimuthal-angle difference, $\Delta \phi $, between MN jets in the rapidity interval $ \Delta {y} < $ 3.0. The data (points) are plotted with experimental statistical (systematic) uncertainties indicated by the error bars (the shaded band), and compared to predictions from the LL DGLAP-based MC generators PYTHIA 6, PYTHIA 8, HERWIG++, and SHERPA, and to the LL BFKL-motivated MC generator HEJ with hadronisation performed with ARIADNE (solid line). |
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Figure 1-b:
Ratio of predictions to the data for the azimuthal-angle difference, $\Delta \phi $, between MN jets in the rapidity interval $ \Delta {y} < $ 3.0. The data (points) are plotted with experimental statistical (systematic) uncertainties indicated by the error bars (the shaded band), and compared to predictions from the LL DGLAP-based MC generators PYTHIA 6, PYTHIA 8, HERWIG++, and SHERPA, and to the LL BFKL-motivated MC generator HEJ with hadronisation performed with ARIADNE (solid line). |
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Figure 1-c:
Distribution of the azimuthal-angle difference, $\Delta \phi $, between MN jets in the rapidity interval 3.0 $ < \Delta {y} < $ 6.0. The data (points) are plotted with experimental statistical (systematic) uncertainties indicated by the error bars (the shaded band), and compared to predictions from the LL DGLAP-based MC generators PYTHIA 6, PYTHIA 8, HERWIG++, and SHERPA, and to the LL BFKL-motivated MC generator HEJ with hadronisation performed with ARIADNE (solid line). |
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Figure 1-d:
Ratio of predictions to the data for the azimuthal-angle difference, $\Delta \phi $, between MN jets in the rapidity interval 3.0 $ < \Delta {y} < $ 6.0. The data (points) are plotted with experimental statistical (systematic) uncertainties indicated by the error bars (the shaded band), and compared to predictions from the LL DGLAP-based MC generators PYTHIA 6, PYTHIA 8, HERWIG++, and SHERPA, and to the LL BFKL-motivated MC generator HEJ with hadronisation performed with ARIADNE (solid line). |
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Figure 1-e:
Distribution of the azimuthal-angle difference, $\Delta \phi $, between MN jets in the rapidity interval 6.0 $ < \Delta {y} < $ 9.4. The data (points) are plotted with experimental statistical (systematic) uncertainties indicated by the error bars (the shaded band), and compared to predictions from the LL DGLAP-based MC generators PYTHIA 6, PYTHIA 8, HERWIG++, and SHERPA, and to the LL BFKL-motivated MC generator HEJ with hadronisation performed with ARIADNE (solid line). |
png pdf |
Figure 1-f:
Ratio of predictions to the data for the azimuthal-angle difference, $\Delta \phi $, between MN jets in the rapidity interval 6.0 $ < \Delta {y} < $ 9.4. The data (points) are plotted with experimental statistical (systematic) uncertainties indicated by the error bars (the shaded band), and compared to predictions from the LL DGLAP-based MC generators PYTHIA 6, PYTHIA 8, HERWIG++, and SHERPA, and to the LL BFKL-motivated MC generator HEJ with hadronisation performed with ARIADNE (solid line). |
png pdf |
Figure 2:
Left: Average $< \cos(n(\pi -\Delta \phi )) >$ ($n =$ 1, 2, 3) as a function of $ \Delta {y} $ compared to LL DGLAP MC generators. In addition, the predictions of the NLO generator POWHEG interfaced with the LL DGLAP generators PYTHIA 6 and PYTHIA 8 are shown. Right: Comparison of the data to the MC generator SHERPA with parton matrix elements matched to a LL DGLAP parton shower, to the LL BFKL inspired generator HEJ with hadronisation by ARIADNE, and to analytical NLL BFKL calculations at the parton level (4.0 $< \Delta {y} <$ 9.4). |
png pdf |
Figure 2-a:
Average $< \cos(\pi -\Delta \phi ) >$ as a function of $ \Delta {y} $ compared to LL DGLAP MC generators. In addition, the predictions of the NLO generator POWHEG interfaced with the LL DGLAP generators PYTHIA 6 and PYTHIA 8 are shown. |
png pdf |
Figure 2-b:
Average $< \cos(\pi -\Delta \phi ) >$ as a function of $ \Delta {y} $ compared to the MC generator SHERPA with parton matrix elements matched to a LL DGLAP parton shower, to the LL BFKL inspired generator HEJ with hadronisation by ARIADNE, and to analytical NLL BFKL calculations at the parton level (4.0 $< \Delta {y} <$ 9.4). |
png pdf |
Figure 2-c:
Average $< \cos(2(\pi -\Delta \phi )) >$ as a function of $ \Delta {y} $ compared to LL DGLAP MC generators. In addition, the predictions of the NLO generator POWHEG interfaced with the LL DGLAP generators PYTHIA 6 and PYTHIA 8 are shown. |
png pdf |
Figure 2-d:
Average $< \cos(2(\pi -\Delta \phi )) >$ as a function of $ \Delta {y} $ compared to the MC generator SHERPA with parton matrix elements matched to a LL DGLAP parton shower, to the LL BFKL inspired generator HEJ with hadronisation by ARIADNE, and to analytical NLL BFKL calculations at the parton level (4.0 $< \Delta {y} <$ 9.4). |
png pdf |
Figure 2-e:
Average $< \cos(3(\pi -\Delta \phi )) >$ as a function of $ \Delta {y} $ compared to LL DGLAP MC generators. In addition, the predictions of the NLO generator POWHEG interfaced with the LL DGLAP generators PYTHIA 6 and PYTHIA 8 are shown. |
png pdf |
Figure 2-f:
Average $< \cos(3(\pi -\Delta \phi )) >$ as a function of $ \Delta {y} $ compared to the MC generator SHERPA with parton matrix elements matched to a LL DGLAP parton shower, to the LL BFKL inspired generator HEJ with hadronisation by ARIADNE, and to analytical NLL BFKL calculations at the parton level (4.0 $< \Delta {y} <$ 9.4). |
png pdf |
Figure 3:
Left: The measured ratios $C_2/C_1$ (top row) and $C_3/C_2$ (bottom row) as a function of rapidity difference $ \Delta {y} $ are compared to LL DGLAP parton shower generators and to the NLO generator POWHEG interfaced with PYTHIA 6 and PYTHIA 8. Right: Comparison of the ratios to the MC generator SHERPA with parton matrix element matched to a LL DGLAP parton shower, to the LL BFKL-inspired generator HEJ with hadronisation by ARIADNE, and to analytical NLL BFKL calculations at the parton level. |
png pdf |
Figure 3-a:
The measured ratio $C_2/C_1$ as a function of rapidity difference $ \Delta {y} $ are compared to LL DGLAP parton shower generators and to the NLO generator POWHEG interfaced with PYTHIA 6 and PYTHIA 8. |
png pdf |
Figure 3-b:
The measured ratio $C_2/C_1$ as a function of rapidity difference $ \Delta {y} $ are compared to the MC generator SHERPA with parton matrix element matched to a LL DGLAP parton shower, to the LL BFKL-inspired generator HEJ with hadronisation by ARIADNE, and to analytical NLL BFKL calculations at the parton level. |
png pdf |
Figure 3-c:
The measured ratio $C_3/C_2$ as a function of rapidity difference $ \Delta {y} $ are compared to LL DGLAP parton shower generators and to the NLO generator POWHEG interfaced with PYTHIA 6 and PYTHIA 8. |
png pdf |
Figure 3-d:
The measured ratio $C_3/C_2$ as a function of rapidity difference $ \Delta {y} $ are compared to the MC generator SHERPA with parton matrix element matched to a LL DGLAP parton shower, to the LL BFKL-inspired generator HEJ with hadronisation by ARIADNE, and to analytical NLL BFKL calculations at the parton level. |
png pdf |
Figure 4:
Average $< \cos{(\pi - \Delta \phi )}> $, $< \cos{2(\pi - \Delta \phi )}> $ and $< \cos{3(\pi - \Delta \phi )} > $ compared to PYTHIA 6 with and without MPI. |
png pdf |
Figure 4-a:
Average $< \cos{(\pi - \Delta \phi )}> $ compared to PYTHIA 6 with and without MPI. |
png pdf |
Figure 4-b:
Average $< \cos{2(\pi - \Delta \phi )}> $ compared to PYTHIA 6 with and without MPI. |
png pdf |
Figure 4-c:
Average $< \cos{3(\pi - \Delta \phi )} > $ compared to PYTHIA 6 with and without MPI. |
png pdf |
Figure 5:
Measured ratios $C_2/C_1$ (left) and $C_3/C_2$ (right) compared to PYTHIA 8 with and without MPI. |
png pdf |
Figure 5-a:
Measured ratios $C_2/C_1$ compared to PYTHIA 8 with and without MPI. |
png pdf |
Figure 5-b:
Measured ratios $C_3/C_2$ compared to PYTHIA 8 with and without MPI. |
| Tables | |
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Table 1:
Systematic and statistical uncertainties (%) of the observables measured in this work. |
| Summary |
|
The first measurement of the azimuthal decorrelation of the most-forward and backward jets in the event (called Mueller-Navelet dijets), with rapidity separations up to $\Delta{y}=$ 9.4, is presented for proton-proton collisions at $\sqrt{s}=$ 7 TeV. The measured observables include azimuthal-angle distributions, moments of the average cosines of the decorrelation angle, $< \cos(n(\pi - \Delta \phi)) >$ for $n=$ 1, 2, 3, as well as ratios of the average cosines, as a function of the rapidity separation $\Delta{y}$ between the MN jets. The predictions of the DGLAP-based MC generator HERWIG++ 2.5, improved with leading-log (LL) parton showers and colour-coherence effects, exhibit satisfactory agreement with the data for all measured observables. Other MC generators of this type, such as PYTHIA 6 Z2, PYTHIA 8 4C, and SHERPA 1.4, provide a less accurate description of all measurements. The MC generator POWHEG, with NLO matrix elements interfaced with the LL parton shower of PYTHIA 6 and PYTHIA 8, does not improve the overall agreement with the data compared to the description provided by PYTHIA 6 and 8 alone. The MC generator HEJ, based on LL BFKL matrix elements combined with ARIANDE for parton shower and hadronisation, predicts a stronger decorrelation than observed in the data. An analytical BFKL calculation at next-to-leading logarithmic (NLL) accuracy with an optimised renormalisation scheme and scale, provides a satisfactory description of the data for the measured jet observables at $\Delta{y}>$ 4. The observed sensitivity to the implementation of the colour-coherence effects in the DGLAP MC generators and the reasonable data-theory agreement shown by the NLL BFKL analytical calculations at large $\Delta{y}$, may be considered as indications that the kinematical domain of the present study lies in between the regions described by the DGLAP and BFKL approaches. Possible manifestations of BFKL signatures are expected to be more pronounced at increasing collision energies. |
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