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CMS-EXO-19-012 ; CERN-EP-2019-222
Search for high mass dijet resonances with a new background prediction method in proton-proton collisions at $\sqrt{s} = $ 13 TeV
JHEP 05 (2020) 033
Abstract: A search for narrow and broad resonances with masses greater than 1.8 TeV decaying to a pair of jets is presented. The search uses proton-proton collision data at $\sqrt{s} = $ 13 TeV collected at the LHC, corresponding to an integrated luminosity of 137 fb$^{-1}$. The background arising from standard model processes is predicted with the fit method used in previous publications and with a new method. The dijet invariant mass spectrum is well described by both data-driven methods, and no significant evidence for the production of new particles is observed. Model independent upper limits are reported on the production cross sections of narrow resonances, and broad resonances with widths up to 55% of the resonance mass. Limits are presented on the masses of narrow resonances from various models: string resonances, scalar diquarks, axigluons, colorons, excited quarks, color-octet scalars, W' and Z' bosons, Randall-Sundrum gravitons, and dark matter mediators. The limits on narrow resonances are improved by 200 to 800 GeV relative to those reported in previous CMS dijet resonance searches. The limits on dark matter mediators are presented as a function of the resonance mass and width, and on the associated coupling strength as a function of the mediator mass. These limits exclude at 95% confidence level a dark matter mediator with a mass of 1.8 TeV and width 1% of its mass or higher, up to one with a mass of 4.8 TeV and a width 45% of its mass or higher.
Figures & Tables Summary Additional Figures References CMS Publications
Figures

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Figure 1:
The pseudorapidity separation between the two wide jets for the signal and control regions. Data (black points) are compared to QCD predictions from the PYTHIA MC with detector simulation (red histogram) normalized to data. A signal from an RS graviton decaying into a $\mathrm{q} \mathrm{\bar{q}}$ pair is also shown (blue histogram) normalized to data.

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Figure 2:
The dijet mass spectra of the data and PYTHIA simulation in the signal region at low $ {| \Delta \eta |}$ (black points and red histogram), control region at middle $ {| \Delta \eta |}$ (triangles and blue histogram), and control region at high $ {| \Delta \eta |}$ (squares and magenta histogram). The simulation is normalized to data.

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Figure 3:
Three-dimensional display of the event with the second-highest dijet invariant mass of 8 TeV. The display shows the energy deposited in the electromagnetic (red) and hadronic (blue) calorimeters and the reconstructed tracks of charged particles (green). The grouping of four observed jets into two wide jets (purple) is discussed in the text.

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Figure 4:
The ratio $ {R_{\text {aux}}} $, the auxiliary transfer factor, calculated for data, PYTHIA, and POWHEG with electroweak corrections (left, upper panel). The double ratio of the same quantities in the upper left panel to $ {R_{\text {aux}}} $ from PYTHIA, along with the fit of the double ratio for data with the correction function (left, lower panel). The ratio $ {R} $, the transfer factor, calculated for data, PYTHIA, POWHEG with electroweak corrections, and corrected PYTHIA (right, upper panel). The double ratio of the same quantities in the upper right panel to $ {R} $ from PYTHIA, along with the fit of the double ratio for data with a correction function, and corrected PYTHIA using $ {\mathrm {CR}_{\text {middle}}} $ (right, lower panel). The fits in the two lower panels agree with each other within their uncertainty at 95% CL (shaded bands).

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Figure 4-a:
The ratio $ {R_{\text {aux}}} $, the auxiliary transfer factor, calculated for data, PYTHIA, and POWHEG with electroweak corrections (upper panel). The double ratio of the same quantities in the upper panel to $ {R_{\text {aux}}} $ from PYTHIA, along with the fit of the double ratio for data with the correction function (lower panel).

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Figure 4-b:
The ratio $ {R} $, the transfer factor, calculated for data, PYTHIA, POWHEG with electroweak corrections, and corrected PYTHIA (upper panel). The double ratio of the same quantities in the upper panel to $ {R} $ from PYTHIA, along with the fit of the double ratio for data with a correction function, and corrected PYTHIA using $ {\mathrm {CR}_{\text {middle}}} $ (lower panel).

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Figure 5:
Dijet mass spectrum in the signal region (points) compared to a fitted parameterization of the background (solid line) and the one obtained from the control region (green squares). The lower panel shows the difference between the data and the fitted parametrization (red, solid), and the data and the prediction obtained from the control region (green, hatched), divided by the statistical uncertainty in the data, which for the ratio method includes the statistical uncertainty in the data in the control region. Examples of predicted signals from narrow gluon-gluon, quark-gluon, and quark-quark resonances are shown (dashed coloured lines) with cross sections equal to the observed upper limits at 95% CL.

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Figure 6:
The observed 95% CL upper limits on the product of the cross section, branching fraction, and acceptance for dijet resonances decaying to quark-quark (upper left), quark-gluon (upper right), gluon-gluon (lower left), and for RS gravitons (lower right). The corresponding expected limits (dashed lines) and their variations at the one and two standard deviation levels (shaded bands) are also shown. Limits are compared to predicted cross sections for string resonances [1,2], excited quarks [4,5], axigluons [6], colorons [8], scalar diquarks [3], color-octet scalars [9], new gauge bosons W' and Z' with SM-like couplings [10], DM mediators for $ {m_{\text {DM}}} =$ 1 GeV [15,14], and RS gravitons [11]. The vertical dashed line indicates indicates the boundary between the regions where the fit method and the ratio method are used to estimate the background.

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Figure 6-a:
The observed 95% CL upper limits on the product of the cross section, branching fraction, and acceptance for dijet resonances decaying to quark-quark. The corresponding expected limits (dashed lines) and their variations at the one and two standard deviation levels (shaded bands) are also shown. Limits are compared to predicted cross sections for axigluons [6], colorons [8], scalar diquarks [3], new gauge bosons W' and Z' with SM-like couplings [10]. The vertical dashed line indicates indicates the boundary between the regions where the fit method and the ratio method are used to estimate the background.

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Figure 6-b:
The observed 95% CL upper limits on the product of the cross section, branching fraction, and acceptance for dijet resonances decaying to quark-gluon. The corresponding expected limits (dashed lines) and their variations at the one and two standard deviation levels (shaded bands) are also shown. Limits are compared to predicted cross sections for string resonances [1,2], excited quarks [4,5]. The vertical dashed line indicates indicates the boundary between the regions where the fit method and the ratio method are used to estimate the background.

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Figure 6-c:
The observed 95% CL upper limits on the product of the cross section, branching fraction, and acceptance for dijet resonances decaying to gluon-gluon. The corresponding expected limits (dashed lines) and their variations at the one and two standard deviation levels (shaded bands) are also shown. Limits are compared to predicted cross sections for color-octet scalars [9]. The vertical dashed line indicates indicates the boundary between the regions where the fit method and the ratio method are used to estimate the background.

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Figure 6-d:
The observed 95% CL upper limits on the product of the cross section, branching fraction, and acceptance for RS gravitons. The corresponding expected limits (dashed lines) and their variations at the one and two standard deviation levels (shaded bands) are also shown. Limits are compared to predicted cross sections for RS gravitons [11]. The vertical dashed line indicates indicates the boundary between the regions where the fit method and the ratio method are used to estimate the background.

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Figure 7:
The observed 95% CL upper limits on the product of the cross section, branching fraction, and acceptance for quark-quark, quark-gluon, and gluon-gluon type dijet resonances. Limits are compared to predicted cross sections for string resonances [1,2], excited quarks [4,5], axigluons [6], colorons [8], scalar diquarks [3], color-octet scalars [9], new gauge bosons W' and Z' with SM-like couplings [10], DM mediators for $ {m_{\text {DM}}} =$ 1 GeV [15,14], and RS gravitons [11]. The vertical dashed line indicates indicates the boundary between the regions where the fit method and the ratio method are used to estimate the background.

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Figure 8:
Local significance for a $\mathrm{q} \mathrm{q} $ resonance with the ratio method (blue line) and the fit method (red dashed line).

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Figure 9:
The reconstructed dijet mass spectra for a vector particle decaying to pairs of quarks are shown for a resonance mass of 2 TeV (solid histogram) and 5 TeV (dashed histogram) for various values of intrinsic width, estimated from the MadGraph {5} and PYTHIA event generators followed by the simulation of the CMS detector response.

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Figure 10:
The observed 95% CL upper limits on the product of the cross section, branching fraction, and acceptance for spin-2 resonances produced and decaying in the quark-quark (upper left) and gluon-gluon (upper right) channels, as well as for spin-1 resonances decaying in the quark-quark channel (lower), shown for various values of intrinsic width as a function of resonance mass. The vertical dashed line indicates indicates the boundary between the regions where the fit method and the ratio method are used to estimate the background.

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Figure 10-a:
The observed 95% CL upper limits on the product of the cross section, branching fraction, and acceptance for spin-2 resonances produced and decaying in the quark-quark channels, shown for various values of intrinsic width as a function of resonance mass. The vertical dashed line indicates indicates the boundary between the regions where the fit method and the ratio method are used to estimate the background.

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Figure 10-b:
The observed 95% CL upper limits on the product of the cross section, branching fraction, and acceptance for spin-2 resonances produced and decaying in the gluon-gluon channels, shown for various values of intrinsic width as a function of resonance mass. The vertical dashed line indicates indicates the boundary between the regions where the fit method and the ratio method are used to estimate the background.

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Figure 10-c:
The observed 95% CL upper limits on the product of the cross section, branching fraction, and acceptance for spin-1 resonances decaying in the quark-quark channel, shown for various values of intrinsic width as a function of resonance mass. The vertical dashed line indicates indicates the boundary between the regions where the fit method and the ratio method are used to estimate the background.

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Figure 11:
The 95% CL upper limits on the universal quark coupling $ {g_\mathrm{q}} $ as a function of resonance mass for a vector mediator of interactions between quarks and DM particles (left), and between quarks only (right). The dashed horizontal lines on the right plot show the coupling strength for which the cross section for dijet production in this leptophobic Z' model is the same as for a DM mediator for $ {g_\mathrm{q}} =$ 0.25. The right vertical axis shows the natural width of the mediator divided by its mass. The expected limits (dashed lines) and their variation at the one and two standard deviation levels (shaded bands) are also shown.

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Figure 11-a:
The 95% CL upper limits on the universal quark coupling $ {g_\mathrm{q}} $ as a function of resonance mass for a vector mediator of interactions between quarks and DM particles. The right vertical axis shows the natural width of the mediator divided by its mass. The expected limits (dashed lines) and their variation at the one and two standard deviation levels (shaded bands) are also shown.

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Figure 11-b:
The 95% CL upper limits on the universal quark coupling $ {g_\mathrm{q}} $ as a function of resonance mass for a vector mediator of interactions between quarks. The dashed horizontal lines show the coupling strength for which the cross section for dijet production in this leptophobic Z' model is the same as for a DM mediator for $ {g_\mathrm{q}} =$ 0.25. The right vertical axis shows the natural width of the mediator divided by its mass. The expected limits (dashed lines) and their variation at the one and two standard deviation levels (shaded bands) are also shown.
Tables

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Table 1:
Observed and expected mass limits at 95% CL from this analysis. The listed models are excluded between 1.8 TeV and the indicated mass limit by this analysis. The SM-like Z' resonance is also excluded within the mass interval between 3.1 and 3.3 TeV.
Summary
A search for resonances decaying into a pair of jets has been performed using proton-proton collision data at $\sqrt{s}=13$ TeV corresponding to an integrated luminosity of 137 fb$^{-1}$. The dijet mass spectra are observed to be smoothly falling distributions of events with typically two-jet topology, although one unusual event with a four-jet topology was found at high mass. The background is predicted using two methods. The fit method uses an empirical functional form to fit the background in the signal region, defined by requiring the pseudorapidity separation of two jets in dijet $|{\Delta\eta}| < $1.1, while the ratio method uses two control regions at higher values of $|{\Delta\eta}|$ to predict the background in the signal region. The ratio method is a new background prediction method, which is independent of and complementary to the fit method. No evidence for resonant particle production is observed. Generic upper limits are presented on the product of the cross section, the branching fraction, and the acceptance for narrow and broad quark-quark, quark-gluon, and gluon-gluon resonances. The limits are applied to various models of new resonances and yield the following 95% confidence level lower limits on the resonance masses: 7.9 TeV for string resonances, 7.5 TeV for scalar diquarks, 6.6 TeV for axigluons and colorons, 6.3 TeV for excited quarks, 3.7 TeV for color-octet scalars, 3.6 TeV for W' bosons with SM-like couplings, 2.9 TeV and between 3.1 and 3.3 TeV for Z' bosons with SM-like couplings, 2.6 TeV for Randall-Sundrum gravitons, and 2.8 TeV for dark matter (DM) mediators. With this search, limits on narrow resonances are improved by 200 to 800 GeV relative to those reported in previous CMS dijet resonance searches. Limits are also presented for spin-2 resonances with intrinsic widths as large as 30% of the resonance mass, and spin-1 resonances with intrinsic widths as large as 55% of the resonance mass. These limits are used to improve and extend the exclusions of a DM mediator to larger values of the resonance mass and coupling to quarks. In the search for broad resonances, the ratio method provides significantly enhanced sensitivity compared to the fit method, resulting in the exclusion at 95% confidence level of a DM mediator with mass less than 4.8 TeV for a width equal to 45% of the mass, which corresponds to a coupling to quarks ${g_\mathrm{q}} =$ 0.9.
Additional Figures

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Additional Figure 1:
Ratio of expected limits between the fit method and the ratio method as a function of resonance mass for broad spin-1 resonances decaying to a pair of quarks, and for different resonance widths.

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Additional Figure 2:
Ratio of expected limits between the fit method and the ratio method as a function of resonance mass for broad spin-2 resonances decaying to a pair of gluons, and for different resonance widths.

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Additional Figure 3:
Three dimensional display of the event with the highest dijet mass at 8.4 TeV. The display shows the energy deposited in the electromagnetic (red) and hadronic (blue) calorimeters and the reconstructed tracks of charged particles (green).
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