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CMS-EXO-17-001 ; CERN-EP-2017-235
Search for low mass vector resonances decaying into quark-antiquark pairs in proton-proton collisions at $\sqrt{s} = $ 13 TeV
JHEP 01 (2018) 097
Abstract: A search for narrow vector resonances decaying into quark-antiquark pairs is presented. The analysis is based on data collected in proton-proton collisions at $\sqrt{s} = $ 13 TeV with the CMS detector at the LHC, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The hypothetical resonance is produced with sufficiently high transverse momentum that its decay products are merged into a single jet with two-prong substructure. A signal would be identified as a peak over a smoothly falling background in the distribution of the invariant mass of the jet, using novel jet substructure techniques. No evidence for such a resonance is observed within the mass range of 50-300 GeV. Upper limits at 95% confidence level are set on the production cross section, and presented in a mass-coupling parameter space. The limits further constrain simplified models of dark matter production involving a mediator interacting between quarks and dark matter particles through a vector or axial-vector current. In the framework of these models, the results are the most sensitive to date, extending for the first time the search region to masses below 100 GeV.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Distributions of data (points) and simulated backgrounds (histograms), of the leading $ {p_{\mathrm {T}}} $ jet soft-drop mass after the jet kinematic selection. Dashed lines illustrate the signal contribution for different Z' boson masses. The multijet processes (QCD) dominate the background component, with subdominant contributions from inclusive SM W, Z, and ${\mathrm{t} {}\mathrm{\bar{t}}} $ and single top quark processes. The QCD simulation is corrected by an overall factor of 0.74 to match the data yield.

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Figure 2:
The distribution of $ X_{ 5-\text{percent} } $ used to define the $N_2^{1,\text {DDT}}$ variable, corresponding to the 5% quantile of the $N_2^{1}$ distribution in simulated multijet events. The distribution is shown as a function of the jet $\rho $ and $ {p_{\mathrm {T}}} $ and smoothed using a kNN approach [64]. The $N_2^{1}$ distribution is mostly insensitive to the jet $\rho $ and $ {p_{\mathrm {T}}} $ in the kinematic phase space considered for this analysis ($-5.5 < \rho < -2.0$). Residual correlations in simulation are corrected by applying a decorrelation procedure that yields the $N_2^{1,\text {DDT}}$ variable.

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Figure 3:
Distributions of data (points) and simulated backgrounds (histograms), of the $N_2^{1,\text {DDT}}$ variable for the leading $ {p_{\mathrm {T}}} $ jet after the kinematic selection. Dashed lines illustrate the signal contribution for different Z' boson masses. The multijet processes (QCD) dominate the background component, with subdominant contributions from inclusive SM W, Z, and ${\mathrm{t} {}\mathrm{\bar{t}}} $ and single top quark processes. The QCD simulation is corrected by an overall factor of 0.74 to match the data yield.

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Figure 4:
A schematic of the background estimation method. The pass-to-fail ratio, $R_{\rm p/f}(\rho (m_\text {SD}, {p_{\mathrm {T}}}))$, is defined from the events passing and failing the $N_2^\text {1,DDT}$ selection. The variable $N_2^\text {1,DDT}$ is constructed so that, for simulated multijet events, $R_{\rm p/f}$ is constant (left). To account for residual differences between data and simulation, $R_{\rm p/f}$ is extracted by performing a two-dimensional fit to data in ($ \rho, {p_{\mathrm {T}}} $) space (right).

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Figure 4-a:
A schematic of the background estimation method. The pass-to-fail ratio, $R_{\rm p/f}(\rho (m_\text {SD}, {p_{\mathrm {T}}}))$, is defined from the events passing and failing the $N_2^\text {1,DDT}$ selection. The variable $N_2^\text {1,DDT}$ is constructed so that, for simulated multijet events, $R_{\rm p/f}$ is constant.

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Figure 4-b:
A schematic of the background estimation method. The pass-to-fail ratio, $R_{\rm p/f}(\rho (m_\text {SD}, {p_{\mathrm {T}}}))$, is defined from the events passing and failing the $N_2^\text {1,DDT}$ selection. To account for residual differences between data and simulation, $R_{\rm p/f}$ is extracted by performing a two-dimensional fit to data in ($ \rho, {p_{\mathrm {T}}} $) space.

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Figure 5:
Soft-drop jet mass distributions that pass (left) and fail (right) the $N_{2}^\text {1,DDT}$ selection in the semileptonic ${\mathrm{t} {}\mathrm{\bar{t}}} $ sample. Results of fits to data and simulation are shown.

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Figure 5-a:
Soft-drop jet mass distributions that pass the $N_{2}^\text {1,DDT}$ selection in the semileptonic ${\mathrm{t} {}\mathrm{\bar{t}}} $ sample. Results of fits to data and simulation are shown.

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Figure 5-b:
Soft-drop jet mass distributions that fail the $N_{2}^\text {1,DDT}$ selection in the semileptonic ${\mathrm{t} {}\mathrm{\bar{t}}} $ sample. Results of fits to data and simulation are shown.

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Figure 6:
Soft-drop jet mass distribution for the different ${p_{\mathrm {T}}}$ ranges of the fit from 500 to 1000 GeV. Data are shown as black points. The multijet background prediction, including uncertainties, is shown by the shaded bands. Contributions from the W and Z boson, and top quark background processes, and a hypothetical Z' boson signal at a mass of 135 GeV are also indicated. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown. The scale on the x-axis differs for each ${p_{\mathrm {T}}}$ range due to the kinematic selection on $\rho $.

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Figure 6-a:
Soft-drop jet mass distribution for the ${p_{\mathrm {T}}}$ range of the fit from 500 to 600 GeV. Data are shown as black points. The multijet background prediction, including uncertainties, is shown by the shaded bands. Contributions from the W and Z boson, and top quark background processes, and a hypothetical Z' boson signal at a mass of 135 GeV are also indicated. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown.

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Figure 6-b:
Soft-drop jet mass distribution for the ${p_{\mathrm {T}}}$ range of the fit from 600 to 700 GeV. Data are shown as black points. The multijet background prediction, including uncertainties, is shown by the shaded bands. Contributions from the W and Z boson, and top quark background processes, and a hypothetical Z' boson signal at a mass of 135 GeV are also indicated. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown.

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Figure 6-c:
Soft-drop jet mass distribution for the ${p_{\mathrm {T}}}$ range of the fit from 700 to 800 GeV. Data are shown as black points. The multijet background prediction, including uncertainties, is shown by the shaded bands. Contributions from the W and Z boson, and top quark background processes, and a hypothetical Z' boson signal at a mass of 135 GeV are also indicated. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown.

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Figure 6-d:
Soft-drop jet mass distribution for the ${p_{\mathrm {T}}}$ range of the fit from 800 to 900 GeV. Data are shown as black points. The multijet background prediction, including uncertainties, is shown by the shaded bands. Contributions from the W and Z boson, and top quark background processes, and a hypothetical Z' boson signal at a mass of 135 GeV are also indicated. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown.

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Figure 6-e:
Soft-drop jet mass distribution for the ${p_{\mathrm {T}}}$ range of the fit from 900 to 1000 GeV. Data are shown as black points. The multijet background prediction, including uncertainties, is shown by the shaded bands. Contributions from the W and Z boson, and top quark background processes, and a hypothetical Z' boson signal at a mass of 135 GeV are also indicated. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown.

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Figure 7:
The 95% CL upper limits on the Z' boson production cross section compared to theoretical cross sections (left) and on the quark coupling $g_{\rm q^{\prime}}$ as a function of resonance mass for a leptophobic Z' resonance that only couples to quarks (right). The observed limits (solid), expected limits (dashed) and their variation at the 1 and 2 standard deviation levels (shaded bands) are shown. Limits from other relevant searches and an indirect constraint on a potential Z' signal from the SM Z boson width [72] are also shown.

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Figure 7-a:
The 95% CL upper limits on the Z' boson production cross section compared to theoretical cross sections as a function of resonance mass for a leptophobic Z' resonance that only couples to quarks. The observed limits (solid), expected limits (dashed) and their variation at the 1 and 2 standard deviation levels (shaded bands) are shown. Limits from other relevant searches and an indirect constraint on a potential Z' signal from the SM Z boson width [72] are also shown.

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Figure 7-b:
The 95% CL upper limits on the quark coupling $g_{\rm q^{\prime}}$ as a function of resonance mass for a leptophobic Z' resonance that only couples to quarks. The observed limits (solid), expected limits (dashed) and their variation at the 1 and 2 standard deviation levels (shaded bands) are shown. Limits from other relevant searches and an indirect constraint on a potential Z' signal from the SM Z boson width [72] are also shown.

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Figure 8:
The 95% CL observed (solid red) and expected (dashed red) excluded regions in the plane of dark matter particle mass ($m_{\rm DM}$) vs. mediator mass ($m_{\rm Med}$), for vector mediators. A branching fraction of 100% is assumed for a leptophobic vector mediator decaying to dijets. The exclusion is computed for a quark coupling choice $g_{q} = $ 0.25 and for a dark matter coupling $g_{\rm DM} = $ 1. The excluded regions from the dijet resolved analysis (blue dot dashed lines) using early 2016 data [35] are also shown. Results are compared to constraints from the cosmological relic density of DM (light gray) determined from astrophysical measurements [74,75] and MadDM version 2.0.6 [76,77] as described in Ref. [78].

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Figure 9:
Distributions of data (points) and simulated backgrounds (histograms) of the leading ${p_{\mathrm {T}}}$ jet $N_2^{1}$ (top left) and $\rho $ (top right) observables, after the kinematic selection. The soft-drop jet mass distributions for the passing (bottom left) and failing (bottom right) region, defined by the $N_2^{1,\text {DDT}}$ selection, are also shown. The decorrelation ensures that the shape of the multijet mass distribution in both regions is unaffected by the $N_2^{1,\text {DDT}}$ selection for different ${p_{\mathrm {T}}}$ ranges. Dashed lines illustrate the signal contribution for different Z' boson masses. The multijet processes (QCD) dominate the background component, with subdominant contributions from inclusive SM W, Z, and ${\mathrm{t} {}\mathrm{\bar{t}}}$ and single top quark processes. The QCD simulation is scaled by an overall factor of 0.74 to match the data yield. Residual differences between data and simulation demonstrate the need for a background estimation method based on control samples in data.

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Figure 9-a:
Distributions of data (points) and simulated backgrounds (histograms) of the leading ${p_{\mathrm {T}}}$ jet $N_2^{1}$, after the kinematic selection. Dashed lines illustrate the signal contribution for different Z' boson masses. The multijet processes (QCD) dominate the background component, with subdominant contributions from inclusive SM W, Z, and ${\mathrm{t} {}\mathrm{\bar{t}}}$ and single top quark processes. The QCD simulation is scaled by an overall factor of 0.74 to match the data yield. Residual differences between data and simulation demonstrate the need for a background estimation method based on control samples in data.

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Figure 9-b:
Distributions of data (points) and simulated backgrounds (histograms) of the $\rho $ observable, after the kinematic selection. Dashed lines illustrate the signal contribution for different Z' boson masses. The multijet processes (QCD) dominate the background component, with subdominant contributions from inclusive SM W, Z, and ${\mathrm{t} {}\mathrm{\bar{t}}}$ and single top quark processes. The QCD simulation is scaled by an overall factor of 0.74 to match the data yield. Residual differences between data and simulation demonstrate the need for a background estimation method based on control samples in data.

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Figure 9-c:
The soft-drop jet mass distribution for the passing region, defined by the $N_2^{1,\text {DDT}}$ selection, is shown. The decorrelation ensures that the shape of the multijet mass distribution is unaffected by the $N_2^{1,\text {DDT}}$ selection for different ${p_{\mathrm {T}}}$ ranges. Dashed lines illustrate the signal contribution for different Z' boson masses. The multijet processes (QCD) dominate the background component, with subdominant contributions from inclusive SM W, Z, and ${\mathrm{t} {}\mathrm{\bar{t}}}$ and single top quark processes. The QCD simulation is scaled by an overall factor of 0.74 to match the data yield. Residual differences between data and simulation demonstrate the need for a background estimation method based on control samples in data.

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Figure 9-d:
The soft-drop jet mass distribution for the failing region, defined by the $N_2^{1,\text {DDT}}$ selection, is shown. The decorrelation ensures that the shape of the multijet mass distribution is unaffected by the $N_2^{1,\text {DDT}}$ selection for different ${p_{\mathrm {T}}}$ ranges. Dashed lines illustrate the signal contribution for different Z' boson masses. The multijet processes (QCD) dominate the background component, with subdominant contributions from inclusive SM W, Z, and ${\mathrm{t} {}\mathrm{\bar{t}}}$ and single top quark processes. The QCD simulation is scaled by an overall factor of 0.74 to match the data yield. Residual differences between data and simulation demonstrate the need for a background estimation method based on control samples in data.

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Figure 10:
Soft-drop jet mass distribution for the passing region and combined ${p_{\mathrm {T}}}$ categories. The multijet background prediction in the passing region is obtained using the failing region and the pass-fail ratio $R_{\rm p/f}(m_\text {SD}, {p_{\mathrm {T}}})$. Data are shown as black points. The multijet background prediction, including uncertainties, is shown by the shaded bands. Contributions from the W and Z boson, and top quark background processes, and a hypothetical Z' boson signal at a mass of 135 GeV are also indicated. The features at 45, 185, 220 and 255 GeV in the $m_{\rm SD}$ distribution are due to the kinematic selection on $\rho $, which affects each ${p_{\mathrm {T}}} $ category differently. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown.

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Figure 11:
The observed p-value, obtained from the fit to data, as a function of the Z' boson mass. The maximum local observed $\text {p}$-value, at 115 GeV, is 1.72$ \times 10^{-3}$ and corresponds to 2.9 standard deviations from the background-only expectation, and the global p-value, calculated over the probed mass range, corresponds to 0.0138 and 2.2 standard deviations.
Tables

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Table 1:
Summary of the systematic uncertainties for signal and background processes and their relative size. The symbol $^\triangle $ denotes uncertainties decorrelated per $ {p_{\mathrm {T}}} $ bin in the 500-1000 GeV range. The symbol $^\dagger $ denotes a shape uncertainty in the peaking SM W and Z boson backgrounds and Z' boson signal shape. A long dash ($ --- $) indicates that the uncertainty does not apply.
Summary
A search for a vector resonance (Z') decaying into a quark-antiquark pair and reconstructed as a single jet has been presented, using a data set comprising proton-proton collisions at $\sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Novel substructure techniques are employed to identify a jet containing a Z' boson candidate over a smoothly falling soft-drop jet mass distribution in data. No significant excess above the SM prediction is observed, and 95% confidence level upper limits are set on the Z' boson coupling to quarks, $g_{\rm q^{\prime}}$, as a function of the Z' boson mass. Coupling values of $g_{\rm q^{\prime}} > 0.25$ are excluded over the Z' mass range from 50 to 300 GeV, with strong constraints for masses less than 200 GeV. The results obtained for masses from 50 to 100 GeV represent the first direct limits to be published in this range. Limits are set on a simplified model of dark matter mediators that only couple to quarks and dark matter particles, excluding vector mediators with masses between 50 and 300 GeV, and using a universal quark coupling $g_{\rm q} = $ 0.25 and a dark matter coupling $g_{\rm DM} = $ 1.0.
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