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CMS-EXO-16-046 ; CERN-EP-2018-036
Search for new physics in dijet angular distributions using proton-proton collisions at $\sqrt{s} = $ 13 TeV and constraints on dark matter and other models
Eur. Phys. J. C 78 (2018) 789
Abstract: A search is presented for physics beyond the standard model, based on measurements of dijet angular distributions in proton-proton collisions at $\sqrt{s} = $ 13 TeV. The data collected with the CMS detector at the LHC correspond to an integrated luminosity of 35.9 fb$^{-1}$. The observed distributions, corrected to particle level, are found to be in agreement with predictions from perturbative quantum chromodynamics that include electroweak corrections. Constraints are placed on models containing quark contact interactions, extra spatial dimensions, quantum black holes, or dark matter, using the detector-level distributions. In a benchmark model where only left-handed quarks participate, contact interactions are excluded at the 95% confidence level up to a scale of 12.8 or 17.5 TeV, for destructive or constructive interference, respectively. The most stringent lower limits to date are set on the ultraviolet cutoff in the Arkani-Hamed-Dimopoulos-Dvali model of extra dimensions. In the Giudice-Rattazzi-Wells convention, the cutoff scale is excluded up to 10.1 TeV. The production of quantum black holes is excluded for masses below 5.9 and 8.2 TeV, depending on the model. For the first time, lower limits between 2.0 and 4.6 TeV are set on the mass of a dark matter mediator for (axial-)vector mediators, for the universal quark coupling ${g_{\mathrm{q}}} =$ 1.0.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Normalized $ {\chi _{\text {dijet}}} $ distributions in the three highest mass bins. Unfolded data are compared to NLO predictions (black dotted line). The error bars represent statistical and experimental systematic uncertainties combined in quadrature. The ticks on the error bars correspond to the experimental systematic uncertainties only. Theoretical uncertainties are indicated as a gray band. Also shown are the predictions for various CI, ADD, QBH, and DM scenarios. The lower panels show the ratio of the unfolded data distributions and NLO predictions.

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Figure 1-a:
Normalized $ {\chi _{\text {dijet}}} $ distribution in mass bin $ M_{jj} > $ 6.6 TeV. Unfolded data are compared to NLO predictions (black dotted line). The error bars represent statistical and experimental systematic uncertainties combined in quadrature. The ticks on the error bars correspond to the experimental systematic uncertainties only. Theoretical uncertainties are indicated as a gray band. Also shown are the predictions for various CI, ADD, QBH, and DM scenarios. The lower panel shows the ratio of the unfolded data distributions and NLO predictions.

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Figure 1-b:
Normalized $ {\chi _{\text {dijet}}} $ distribution in mass bin 5.4 $ < M_{jj} < $ 6.6 TeV. Unfolded data are compared to NLO predictions (black dotted line). The error bars represent statistical and experimental systematic uncertainties combined in quadrature. The ticks on the error bars correspond to the experimental systematic uncertainties only. Theoretical uncertainties are indicated as a gray band. Also shown are the predictions for various CI, ADD, QBH, and DM scenarios. The lower panel shows the ratio of the unfolded data distributions and NLO predictions.

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Figure 1-c:
Normalized $ {\chi _{\text {dijet}}} $ distribution in mass bin 4.8 $ < M_{jj} < $ 5.4 TeV. Unfolded data are compared to NLO predictions (black dotted line). The error bars represent statistical and experimental systematic uncertainties combined in quadrature. The ticks on the error bars correspond to the experimental systematic uncertainties only. Theoretical uncertainties are indicated as a gray band. Also shown are the predictions for various CI, ADD, QBH, and DM scenarios. The lower panel shows the ratio of the unfolded data distributions and NLO predictions.

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Figure 2:
Normalized $ {\chi _{\text {dijet}}} $ distributions in the four lower mass bins. Unfolded data are compared to NLO predictions (black dotted line). The error bars represent statistical and experimental systematic uncertainties combined in quadrature. The ticks on the error bars correspond to the experimental systematic uncertainties only. Theoretical uncertainties are indicated as a gray band. Also shown are the predictions for various CI, ADD, and DM scenarios. The lower panels show the ratio of the unfolded data distributions and NLO predictions.

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Figure 2-a:
Normalized $ {\chi _{\text {dijet}}} $ distribution in mass bin 4.2 $ < M_{jj} < $ 4.8 TeV. Unfolded data are compared to NLO predictions (black dotted line). The error bars represent statistical and experimental systematic uncertainties combined in quadrature. The ticks on the error bars correspond to the experimental systematic uncertainties only. Theoretical uncertainties are indicated as a gray band. Also shown are the predictions for various CI, ADD, and DM scenarios. The lower panel shows the ratio of the unfolded data distributions and NLO predictions.

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Figure 2-b:
Normalized $ {\chi _{\text {dijet}}} $ distribution in mass bin 3.6 $ < M_{jj} < $ 4.2 TeV. Unfolded data are compared to NLO predictions (black dotted line). The error bars represent statistical and experimental systematic uncertainties combined in quadrature. The ticks on the error bars correspond to the experimental systematic uncertainties only. Theoretical uncertainties are indicated as a gray band. Also shown are the predictions for various CI, ADD, and DM scenarios. The lower panel shows the ratio of the unfolded data distributions and NLO predictions.

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Figure 2-c:
Normalized $ {\chi _{\text {dijet}}} $ distribution in mass bin 3.0 $ < M_{jj} < $ 3.6 TeV. Unfolded data are compared to NLO predictions (black dotted line). The error bars represent statistical and experimental systematic uncertainties combined in quadrature. The ticks on the error bars correspond to the experimental systematic uncertainties only. Theoretical uncertainties are indicated as a gray band. Also shown are the predictions for various CI, ADD, and DM scenarios. The lower panel shows the ratio of the unfolded data distributions and NLO predictions.

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Figure 2-d:
Normalized $ {\chi _{\text {dijet}}} $ distribution in mass bin 2.4 $ < M_{jj} < $ 3.0 TeV. Unfolded data are compared to NLO predictions (black dotted line). The error bars represent statistical and experimental systematic uncertainties combined in quadrature. The ticks on the error bars correspond to the experimental systematic uncertainties only. Theoretical uncertainties are indicated as a gray band. Also shown are the predictions for various CI, ADD, and DM scenarios. The lower panel shows the ratio of the unfolded data distributions and NLO predictions.

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Figure 3:
The 95% CL upper limits on the quark coupling $ {g_{\mathrm {\mathrm{q}}}} $, as a function of mass, for an axial-vector or vector DM mediator with $ {g_{\mathrm {DM}}} = $ 1.0 and $ {m_{\mathrm {DM}}} = $ 1 GeV. The observed limits (solid), expected limits (dashed) and the variation in the expected limit at the 1 and 2 standard deviation levels (shaded bands) are shown. A dotted horizontal line shows the coupling strength for a benchmark DM mediator with $ {g_{\mathrm {{\mathrm {q}}}}} = $ 1.0. The corresponding limits on the width of the mediators are shown on the vertical axis on the right-hand side of the figure.

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Figure 4:
The 95% CL observed (red) and expected (blue) excluded regions in the plane of ${m_{\mathrm {DM}}}$ and ${M_{\text {Med}}}$, for a vector mediator (upper) and an axial-vector mediator (lower) for a DM benchmark model with $ {g_{\mathrm {DM}}} = {g_{\mathrm {\mathrm{q}}}} =$ 1.0. These are compared to constraints from the cosmological relic density of DM (gray) determined from astrophysical measurements [79], using {MadDM}. In the hatched area, DM is overabundant. The observed and expected lower bounds for ${M_{\text {Med}}}$ overlap with each other.

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Figure 4-a:
The 95% CL observed (red) and expected (blue) excluded regions in the plane of ${m_{\mathrm {DM}}}$ and ${M_{\text {Med}}}$, for a vector mediator for a DM benchmark model with $ {g_{\mathrm {DM}}} = {g_{\mathrm {\mathrm{q}}}} =$ 1.0. These are compared to constraints from the cosmological relic density of DM (gray) determined from astrophysical measurements [79], using {MadDM}. In the hatched area, DM is overabundant. The observed and expected lower bounds for ${M_{\text {Med}}}$ overlap with each other.

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Figure 4-b:
The 95% CL observed (red) and expected (blue) excluded regions in the plane of ${m_{\mathrm {DM}}}$ and ${M_{\text {Med}}}$, for a axial-vector mediator for a DM benchmark model with $ {g_{\mathrm {DM}}} = {g_{\mathrm {\mathrm{q}}}} =$ 1.0. These are compared to constraints from the cosmological relic density of DM (gray) determined from astrophysical measurements [79], using {MadDM}. In the hatched area, DM is overabundant. The observed and expected lower bounds for ${M_{\text {Med}}}$ overlap with each other.
Tables

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Table 1:
Summary of the leading experimental and theoretical uncertainties in the normalized $ {\chi _{\text {dijet}}} $ distributions, in percent. While the statistical analysis represents each uncertainty through a change in the $ {\chi _{\text {dijet}}} $ distribution correlated among all $ {\chi _{\text {dijet}}} $ bins, this table summarizes each uncertainty by a representative value to show their relative contributions. For the lowest and highest dijet mass bins, the relative shift is given for the lowest $ {\chi _{\text {dijet}}} $ bin. In the highest dijet mass bin, the dominant experimental contribution corresponds to the statistical uncertainty, while the dominant theoretical contribution is from the uncertainty in scales.

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Table 2:
Observed and expected exclusion limits at 95% CL for various CI, ADD, QBH, and DM models. The 68% ranges of expectation for the expected limit are given as well. For the DM vector mediator, couplings $ {g_{\mathrm {DM}}} =$ 1.0, $ {g_{\mathrm {\mathrm{q}}}} \geq $ 1 and a DM mass of 1 GeV are assumed and a range of masses instead of a lower limit is quoted.
Summary
A search has been presented for physics beyond the standard model, based on normalized dijet angular distributions obtained in 2016 from proton-proton collisions at the LHC. The data sample corresponds to an integrated luminosity of 35.9 fb$^{-1}$. The angular distributions, measured over a wide range of dijet invariant masses, are found to be in agreement with the predictions of perturbative quantum chromodynamics. The results are used to set 95% confidence level lower limits on the contact interaction scale for a variety of quark compositeness models, the ultraviolet cutoff in models of extra spatial dimensions, the minimum mass of quantum black holes, and the mass and couplings in dark matter models. For the first time, lower limits between 2.0 and 4.6 TeV are set on the mass of a dark matter mediator for (axial-)vector mediators, for the universal quark coupling ${g_{\mathrm{q}}} \geq $ 1. This region is not accessible through dijet resonance searches. The lower limits for the contact interaction scale $\Lambda$ range from 9.2 to 22.4 TeV. The lower limits on the ultraviolet cutoff in the Arkani-Hamed-Dimopoulos-Dvali model are in the range of 8.5-12 TeV, and are the most stringent limits available. Quantum black hole masses below 8.2 TeV are excluded in the model with six large extra spatial dimensions, and below 5.9 TeV in the Randall-Sundrum model with a single, warped extra dimension. To facilitate comparisons with the predictions of other models, the angular distributions, corrected to particle level, are published in HEPData.
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