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| CMS-PAS-EXO-17-001 | ||
| Search for light vector resonances decaying to a quark pair produced in association with a jet in proton-proton collisions at $\sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| May 2017 | ||
| Abstract: A search for narrow vector resonances decaying to a quark-antiquark pair is presented. The search is based on events collected in $\sqrt{s}= $ 13 TeV proton-proton collisions with the CMS detector at the LHC. The data sample, collected in 2016, corresponds to an integrated luminosity of 35.9 fb$^{-1}$. The hypothetical resonance is produced with sufficiently high transverse momentum that the decay products of the resonance are merged into a single jet. The resulting experimental signature is a single massive jet with two-prong substructure produced in association with a jet from initial-state radiation. Signal is identified as an enhancement over background processes in the distribution of the invariant mass of the jet. No evidence for such resonance is observed within the targeted mass range from 50-300 GeV. Upper limits at a 95% confidence level are set on the production cross-section of leptophobic vector resonances. Results are presented in a mass-coupling phase space and are the most sensitive to date, extending previous limits below 100 GeV. The limits also constrain simplified models of dark matter, with a leptophobic mediator interacting between quarks and dark matter particles through a vector or axial-vector current. | ||
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CDS record (PDF) ;
inSPIRE record ;
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These preliminary results are superseded in this paper, JHEP 01 (2018) 097. The superseded preliminary plots can be found here. |
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| Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
$N_2^{1,\text {DDT}}$ transformation map built using a k-Nearest neighbor (kNN) approach and shown as a function of the jet $\rho $ and ${p_{\mathrm {T}}} $. The map corresponds to the 5% quantile of the $N_2^{1}$ distribution in simulated QCD multijet events. 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 ($-5.5 < \rho < 2$) and further decorrelated yielding the $N_2^{1,\text {DDT}}$ variable. |
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Figure 2:
Data to simulation comparison of the (a) leading ${p_{\mathrm {T}}}$ jet soft drop mass and (b) $N_2^{1,\text {DDT}}$ variables, after kinematic selections on the leading ${p_{\mathrm {T}}}$ jet. Dashed lines illustrate the signal contribution for different Z' masses. The multijet processes (QCD) dominate the background component, with subdominant contributions from inclusive SM W, Z, and ${\mathrm{ t } \mathrm{ \bar{t} } }$ processes. Residual differences in data and simulation demonstrate the need for a data-driven background estimation method. |
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Figure 2-a:
Data to simulation comparison of the leading ${p_{\mathrm {T}}}$ jet soft drop mass. Dashed lines illustrate the signal contribution for different Z' masses. The multijet processes (QCD) dominate the background component, with subdominant contributions from inclusive SM W, Z, and ${\mathrm{ t } \mathrm{ \bar{t} } }$ processes. |
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Figure 2-b:
Data to simulation comparison of the $N_2^{1,\text {DDT}}$ variables, after kinematic selections on the leading ${p_{\mathrm {T}}}$ jet. Dashed lines illustrate the signal contribution for different Z' masses. The multijet processes (QCD) dominate the background component, with subdominant contributions from inclusive SM W, Z, and ${\mathrm{ t } \mathrm{ \bar{t} } }$ processes. |
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Figure 3:
A schematic of the background estimation method. The pass-to-fail ratio, translating from failing to passing regions after applying a $N_2^\text {1,DDT}$ selection, is extracted by performing a two-dimensional fit in ($ \rho , {p_{\mathrm {T}}} $) space. |
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Figure 4:
Soft-drop jet mass distribution 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 4-a:
Soft-drop jet mass distribution 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 4-b:
Soft-drop jet mass distribution 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 5:
Soft drop jet mass distribution for the different ${p_{\mathrm {T}}}$ categories of the fit from 500-1000 GeV. Data are shown as the black points. The QCD background prediction, including uncertainties, is shown in the gray boxes. Contributions from the W, Z, and a hypothetical Z' signal at a mass of 135 GeV are indicated as well. 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 ${p_{\mathrm {T}}}$ category due to the kinematic selection on $\rho $. |
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Figure 5-a:
Soft drop jet mass distribution for the 500-600 GeV ${p_{\mathrm {T}}}$ fit category. Data are shown as the black points. The QCD background prediction, including uncertainties, is shown in the gray boxes. Contributions from the W, Z, and a hypothetical Z' signal at a mass of 135 GeV are indicated as well. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown. |
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Figure 5-b:
Soft drop jet mass distribution for the 600-700 GeV ${p_{\mathrm {T}}}$ fit category. Data are shown as the black points. The QCD background prediction, including uncertainties, is shown in the gray boxes. Contributions from the W, Z, and a hypothetical Z' signal at a mass of 135 GeV are indicated as well. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown. |
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Figure 5-c:
Soft drop jet mass distribution for the 700-800 GeV ${p_{\mathrm {T}}}$ fit category. Data are shown as the black points. The QCD background prediction, including uncertainties, is shown in the gray boxes. Contributions from the W, Z, and a hypothetical Z' signal at a mass of 135 GeV are indicated as well. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown. |
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Figure 5-d:
Soft drop jet mass distribution for the 800-900 GeV ${p_{\mathrm {T}}}$ fit category. Data are shown as the black points. The QCD background prediction, including uncertainties, is shown in the gray boxes. Contributions from the W, Z, and a hypothetical Z' signal at a mass of 135 GeV are indicated as well. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown. |
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Figure 5-e:
Soft drop jet mass distribution for the 900-1000 GeV ${p_{\mathrm {T}}}$ fit category. Data are shown as the black points. The QCD background prediction, including uncertainties, is shown in the gray boxes. Contributions from the W, Z, and a hypothetical Z' signal at a mass of 135 GeV are indicated 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:
(a) 95% CL upper limits on the Z' production cross section compared to the theoretical cross section and (b) translation of the upper limits to limits on $g_q$ as a function of the Z' mass. Limits from other relevant searches are also shown. An indirect constraint on a potential Z' signal from the SM Z boson width [68] is also shown. |
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Figure 6-a:
95% CL upper limits on the Z' production cross section compared to the theoretical cross section. |
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Figure 6-b:
Translation of the upper limits to limits on $g_q$ as a function of the Z' mass. Limits from other relevant searches are also shown. An indirect constraint on a potential Z' signal from the SM Z boson width [68] is also shown. |
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Figure 7:
The p-value as a function of Z' mass. The maximum local p-value, at 115 GeV , is $1.72\times 10^{-3}$ and the global p-value corresponds to 0.0138. |
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Figure 8:
The 95% CL observed (red) excluded regions in the plane of dark matter mass ($m_{DM}$) vs. mediator mass ($m_{Z'}$), for vector mediators. The exclusion is computed with quark coupling choice $g_q = $ 0.25 and for a dark matter coupling $g_{DM} = $ 1. The excluded regions from the dijet resolved analysis using 2016 data [36] are shown in blue. |
| Tables | |
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Table 1:
Summary of the systematic uncertainties for signal and subdominant background processes and their relative size. Symbol $^\dagger $ denotes a shape uncertainty on the peaking SM W/Z and Z' signal shape. Symbol $^\triangle $ denotes uncertainties decorrelated per ${p_{\mathrm {T}}}$ bin between 500-1000 GeV, for the SM W/Z and Z' processes. |
| Summary |
| In summary, we present a search for a light Z' boson decaying to a quark-antiquark pair and reconstructed as a single jet at $\sqrt{s} = $ 13 TeV using a data sample corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Novel substructure techniques are employed to identify the Z' jets. The signal is then extracted on top of a falling QCD soft drop mass distribution (including contributions from W, Z, and top background processes) using an entirely data-driven QCD background prediction. We observe a modest excess near 115 GeV with the largest local significance of 2.9$\sigma$ and global significance of 2.2$\sigma$. We set 95% CL upper limits on the Z' coupling to quarks, $g_q$, as a function of the Z' mass. We present limits for the first time in the mass range from 50-100 GeV with the CMS detector. We exclude coupling values of $g_q > 0.25$ over the Z' mass range from 50 to 300 GeV with strong constraints below Z' masses of 200 GeV. |
| Additional Figures | |
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Additional Figure 1:
Data to simulation comparison of the $N_2^{1}$ observable of the leading ${p_{\mathrm {T}}}$ jet, after kinematic selections. Dashed lines illustrate the signal contribution for different Z' masses. The multijet processes (QCD) dominate the background component, with subdominant contributions from inclusive SM W,Z, and ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ processes. Residual differences in data and simulation demonstrate the need for a data-driven background estimation method. |
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Additional Figure 2:
Soft drop jet mass distribution for all ${p_{\mathrm {T}}}$ categories of the fit from 500-1000 GeV. Data are shown as the black points. The QCD background prediction, including uncertainties, is shown in the gray boxes. Contributions from the W, Z, and a hypothetical Z$^{\prime }$ signal at a mass of 135 GeV are indicated as well. In the bottom panel, the ratio of the data to the background prediction, including uncertainties, is shown. Only events in the $\rho $ range $-5.5 < \rho < -2$ are considered. |
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