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CMS-PAS-EXO-16-030
Search for light vector resonances decaying to quarks at $\sqrt{s}=$ 13 TeV
Abstract: We perform a search for light, narrow vector resonances decaying to quarks in the mass range from 100-300 GeV produced in association with a high transverse momentum jet using 2.7 fb$^{-1}$ of 2015 $\sqrt{s}=$ 13 TeV proton-proton collision data collected by CMS. Novel jet substructure methods and background estimation techniques are employed to search for a resonance in the jet mass distribution originating from a new particle in whose decay the quarks are merged into a single jet. We demonstrate the CMS sensitivity to vector resonances in the mass-coupling phase space not yet explored at the LHC or by any past experiments.
Figures & Tables Summary Additional Figures References CMS Publications
Figures

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Figure 1:
An example Feynman diagram of a $Z' \to q\bar{q}$ resonance production with an initial-state radiation gluon.

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Figure 2-a:
Profiled distribution of $\tau _{21}$ versus $\rho $ (a) and $\tau _{21}^\text {DDT}$ versus $\rho ^\text {DDT}$ (b) for multijet QCD MC, for different leading jet ${p_{\mathrm {T}}}$ values, which demonstrates the (de-)correlation of the variables using the DDT transformation. The region of the right distribution where $\rho ^\text {DDT} < 0$, corresponds to very small jet mass values we do not probe in this analysis, while the non-linear region where $\rho ^\text {DDT} > 4.7$ corresponds to high mass jet values where $\rho ^\text {DDT}$ acquires non-physical values for a given ${p_{\mathrm {T}}} $. Error bars denote the uncertainty on the mean for the profiled distributions.

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Figure 2-b:
Profiled distribution of $\tau _{21}$ versus $\rho $ (a) and $\tau _{21}^\text {DDT}$ versus $\rho ^\text {DDT}$ (b) for multijet QCD MC, for different leading jet ${p_{\mathrm {T}}}$ values, which demonstrates the (de-)correlation of the variables using the DDT transformation. The region of the right distribution where $\rho ^\text {DDT} < 0$, corresponds to very small jet mass values we do not probe in this analysis, while the non-linear region where $\rho ^\text {DDT} > 4.7$ corresponds to high mass jet values where $\rho ^\text {DDT}$ acquires non-physical values for a given ${p_{\mathrm {T}}} $. Error bars denote the uncertainty on the mean for the profiled distributions.

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Figure 3-a:
Data-to-simulation comparison of the leading ${p_{\mathrm {T}}}$ jet soft drop mass (a) and $\tau _{21}^\text {DDT}$ (b) after kinematic selections on the leading ${p_{\mathrm {T}}}$ jet which is dominated by QCD multijet processes (pink) with subdominant contributions from inclusive SM W, Z, and $t \bar{t}$ processes. Residual differences in data and simulation demonstrate the need for a data-driven background estimation method.

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Figure 3-b:
Data-to-simulation comparison of the leading ${p_{\mathrm {T}}}$ jet soft drop mass (a) and $\tau _{21}^\text {DDT}$ (b) after kinematic selections on the leading ${p_{\mathrm {T}}}$ jet which is dominated by QCD multijet processes (pink) with subdominant contributions from inclusive SM W, Z, and $t \bar{t}$ processes. Residual differences in data and simulation demonstrate the need for a data-driven background estimation method.

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Figure 4-a:
a: Diagram illustrating the background estimation technique which estimates the signal region (``pass") QCD background distribution from the sideband region (``fail") in the $\{ \rho ^\text {DDT}, {p_{\mathrm {T}}} \}$ phase space. The empty strip in the pass-to-fail plane is the signal region in $\rho ^\text {DDT}$ (or jet mass) for a given Z' mass. b: The transfer factor $TF(\rho ^\text {DDT}, {p_{\mathrm {T}}} )$ space, now interpolated over the signal region, for a Z' with a mass of 110 GeV as measured in data which translates ``fail" region events into the signal region.

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Figure 4-b:
a: Diagram illustrating the background estimation technique which estimates the signal region (``pass") QCD background distribution from the sideband region (``fail") in the $\{ \rho ^\text {DDT}, {p_{\mathrm {T}}} \}$ phase space. The empty strip in the pass-to-fail plane is the signal region in $\rho ^\text {DDT}$ (or jet mass) for a given Z' mass. b: The transfer factor $TF(\rho ^\text {DDT}, {p_{\mathrm {T}}} )$ space, now interpolated over the signal region, for a Z' with a mass of 110 GeV as measured in data which translates ``fail" region events into the signal region.

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Figure 5-a:
Distributions of merged W jets in the fail (a) and pass (b) regions of the semi-leptonic $t \bar{t}$ control region shown both for data and MC. A simultaneous fit of the two samples is performed to extract the W-tagging efficiency in both simulation (dotted blue lines) and data (solid blue lines).

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Figure 5-b:
Distributions of merged W jets in the fail (a) and pass (b) regions of the semi-leptonic $t \bar{t}$ control region shown both for data and MC. A simultaneous fit of the two samples is performed to extract the W-tagging efficiency in both simulation (dotted blue lines) and data (solid blue lines).

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Figure 6-a:
Final inputs to the statistical interpretation for a Z' mass of 135 GeV (a) and 200 GeV (b). The QCD background prediction, including uncertainties, is shown in the blue boxes while the sum of the SM processes is shown in the blue line. Contributions from the W, Z, and Z' are given as well. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown.

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Figure 6-b:
Final inputs to the statistical interpretation for a Z' mass of 135 GeV (a) and 200 GeV (b). The QCD background prediction, including uncertainties, is shown in the blue boxes while the sum of the SM processes is shown in the blue line. Contributions from the W, Z, and Z' are given as well. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown.

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Figure 7-a:
95% confidence level upper limit on the Z' production cross section compared to the theoretical cross section for a Z' with $g_B = 0.5,1$ (a) and the translation of that upper limit to a limit on $g_B$ (b). Limits from other relevant searches are also shown. Recent ATLAS results from Run 2 in [36,26] are scaled to the coupling $g_B$.

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Figure 7-b:
95% confidence level upper limit on the Z' production cross section compared to the theoretical cross section for a Z' with $g_B = 0.5,1$ (a) and the translation of that upper limit to a limit on $g_B$ (b). Limits from other relevant searches are also shown. Recent ATLAS results from Run 2 in [36,26] are scaled to the coupling $g_B$.
Tables

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Table 1:
Summary of the systematic uncertainties for signal and background. Symbol $^\ddagger $ denotes this is only applied to Z' signal. Symbol $^\dagger $ denotes a shape uncertainty on the peaking SM W/Z and Z' signal shape. Symbol $^\triangle $ denotes EWK uncertainties are also included for the SM W/Z processes.
Summary
We present the first limits on a search for a light Z' boson decaying to a quark-antiquark pair in the mass range from 100-300 GeV with the CMS detector. We do not observe any excess above the SM prediction and set limits on the Z' coupling to quarks, $g_B$, as a function of the Z' mass. Our limits are the most stringent in the mass range less than 300 GeV. For masses below 140 GeV, they are the only limits set.
Additional Figures

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Additional Figure 1:
Trigger efficiency as a function of the leading jet $p_T$ and soft drop mass measured in using the data.

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Additional Figure 2:
Profiled distribution of $\tau _{21}$ versus $\rho ^\text {DDT}$ for multijet QCD MC, for different leading jet ${p_{\mathrm {T}}}$ values, which demonstrates the correlation of the variables using the DDT transformation. The region where $\rho ^\text {DDT} < 0$, corresponds to very small jet mass values we do not probe in this analysis, while the non-linear region where $\rho ^\text {DDT} > $ 4.7 corresponds to high mass jet values where $\rho ^\text {DDT}$ acquires non-physical values for a given ${p_{\mathrm {T}}} $. Error bars denote the uncertainty on the mean for the profiled distributions.

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Additional Figure 3:
Signal mass distributions for various simulated Z' masses probed in this analysis after applying the final analysis selection.

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Additional Figure 4:
The 95% confidence level upper limit on the coupling $g_B$ as a function of the Z' mass. Limits from other relevant searches are also shown. Recent ATLAS results from Run 2 in [ATLAS-CONF-2016-029,ATLAS-CONF-2016-030] are scaled to the coupling $g_B$.
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Compact Muon Solenoid
LHC, CERN