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| CMS-PAS-EXO-17-028 | ||
| Search for heavy Majorana neutrinos in the same-sign dilepton channel in proton-proton collisions at $\sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| March 2018 | ||
| Abstract: A search is performed for a heavy Majorana neutrino (N) decaying into a W boson and a lepton using the CMS detector at the LHC. A signature of two same-sign leptons and at least one jet is searched for using data collected during 2016 in proton-proton collisions at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The data are found to be consistent with the expected standard model background. Upper limits are set, in the context of a Type I seesaw mechanism, on the cross section times branching fraction for production of a heavy Majorana neutrino in the mass range between 20 and 1600 GeV. The results are interpreted as limits on $|V_{\mathrm{e} \mathrm{N}}|^2$, $|V_{\mu \mathrm{N}}|^2$ and, $|V_{\mathrm{e} \mathrm{N}} V_{\mu \mathrm{N}}^*|^2 / (|V_{\mathrm{e} \mathrm{N}}|^2 + |V_{\mu \mathrm{N}}|^2)$, where $V_{\ell \mathrm{N}}$ is the matrix element describing the mixing of the heavy neutrino with the standard model neutrino of flavor $\ell = \mu,\,e$. In the mass range considered, the upper limits range between 2.3 $ \times 10^{-5}$ and unity. These limits are the most restrictive direct limits for heavy Majorana neutrino masses above 430 GeV. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, JHEP 01 (2019) 122. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Current limits on $ {| {V_{\ell {\mathrm {N}}}} |}^2$ for $ {\mathrm {e}}$ (left) and $\mu $ (right), as well as the expected sensitivity from future experiments. Taken from Ref. [25]. |
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Figure 1-a:
Current limits on $ {| {V_{\ell {\mathrm {N}}}} |}^2$ for $ {\mathrm {e}}$ (left) and $\mu $ (right), as well as the expected sensitivity from future experiments. Taken from Ref. [25]. |
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Figure 1-b:
Current limits on $ {| {V_{\ell {\mathrm {N}}}} |}^2$ for $ {\mathrm {e}}$ (left) and $\mu $ (right), as well as the expected sensitivity from future experiments. Taken from Ref. [25]. |
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Figure 2:
Diagram representing a resonance production of a Majorana neutrino ($ {\mathrm {N}} $), via Drell-Yan (DY) $s$-channel process and its decay into same-sign leptons and two quarks. The charge-conjugate diagram results in a $\ell ^- \ell ^- \mathrm{ q \bar{q} }\prime $ final state. Taken from Ref. [35]. |
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Figure 3:
Diagram for photon-initiated processes for $ {\mathrm {q}}\gamma \to {\mathrm {N}} \ell ^{\pm} {\mathrm {q}}^{'}$. Taken from Ref. [35]. |
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Figure 4:
Kinematic distributions for the low-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
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Figure 4-a:
Kinematic distributions for the low-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
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Figure 4-b:
Kinematic distributions for the low-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
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Figure 4-c:
Kinematic distributions for the low-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
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Figure 4-d:
Kinematic distributions for the low-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
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Figure 4-e:
Kinematic distributions for the low-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
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Figure 4-f:
Kinematic distributions for the low-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
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Figure 5:
Kinematic distributions for the high-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
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Figure 5-a:
Kinematic distributions for the high-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
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Figure 5-b:
Kinematic distributions for the high-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
png pdf |
Figure 5-c:
Kinematic distributions for the high-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
png pdf |
Figure 5-d:
Kinematic distributions for the high-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
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Figure 5-e:
Kinematic distributions for the high-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
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Figure 5-f:
Kinematic distributions for the high-mass signal region after all the selections are applied, except for the final optimization requirements in SR1 (left) and SR2 (right). The figures show the background predictions from prompt same-sign leptons (Prompt), misidentified leptons (Misid. lepton), mismeasured charge (Mismeas. charge) together with the number of events observed in data after combining the events in the $ {\mathrm {e}} {\mathrm {e}}$, $\mu \mu $, and $ {\mathrm {e}}\mu $ channels. |
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Figure 6:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the low-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 6-a:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the low-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 6-b:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the low-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 6-c:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the low-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 6-d:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the low-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 6-e:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the low-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 6-f:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the low-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 7:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the high-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 7-a:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the high-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 7-b:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the high-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 7-c:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the high-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 7-d:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the high-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 7-e:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the high-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 7-f:
Observed event yields and estimated backgrounds after the application of all selections for each Majorana neutrino mass signal region for the high-mass SR1 (left) and SR2 (right). The background predictions from prompt same-sign leptons (Prompt bkgd.), misidentified leptons (Misid. bkgd.), and mismeasured charge (Charge mismeas. bkgd.) are included together with the number of events observed in data. The uncertainties shown are the combined quadrature of statistical and systematic uncertainties. |
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Figure 8:
Exclusion region at 95% CL in the $ {| {V_{{\mathrm {e}} {\mathrm {N}}}} |}^2$ vs. $ {m_\mathrm {N}} $ plane. The dashed black curve is the expected upper limit, with one and two standard-deviation bands shown in dark green and light yellow, respectively. The solid black curve is the observed upper limit. Also shown are the upper limits from other direct searches: DELPHI [26], L3 [27,28], ATLAS [32], and the upper limits from the CMS $\sqrt {s} = $ 8 TeV 2012 data [31] and the trilepton analysis [36] based on the same 2016 data set as used in this analysis. |
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Figure 9:
Exclusion region at 95% CL in the $ {| {V_{\mu {\mathrm {N}}}} |}^2$ vs. $ {m_\mathrm {N}} $ plane. The dashed black curve is the expected upper limit, with one and two standard-deviation bands shown in dark green and light yellow, respectively. The solid black curve is the observed upper limit. Also shown are the upper limits from other direct searches: DELPHI [26], L3 [27,28], ATLAS [32], and the upper limits from the CMS $\sqrt {s} = $ 8 TeV 2012 data [31] and the trilepton analysis [36] based on the same 2016 data set as used in this analysis. |
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Figure 10:
Exclusion region at 95% CL in the $ {| {V_{{\mathrm {e}} {\mathrm {N}}}} {V^{*}_{\mu {\mathrm {N}}}} |}^2 / ({| {V_{{\mathrm {e}} {\mathrm {N}}}} |}^2 + {| {V_{\mu {\mathrm {N}}}} |}^2)$ vs. $ {m_\mathrm {N}} $ plane. The dashed black curve is the expected upper limit, with one and two standard-deviation bands shown in dark green and light yellow, respectively. The solid black curve is the observed upper limit. Also shown are the upper limits from the CMS $\sqrt {s} = $ 8 TeV 2012 data [31]. |
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Figure 11:
Comparison of the observed 95% CL exclusion regions for $| {V_{{\mathrm {e}} {\mathrm {N}}}} |^2$, $| {V_{\mu {\mathrm {N}}}} |^2$, and $ {| {V_{{\mathrm {e}} {\mathrm {N}}}} {V^{*}_{\mu {\mathrm {N}}}} |}^2 / (| {V_{{\mathrm {e}} {\mathrm {N}}}} |^2 + | {V_{\mu {\mathrm {N}}}} |^2)$. |
| Tables | |
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Table 1:
Selection requirements for the low- and high-mass search regions. |
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Table 2:
Observed event yields and estimated backgrounds in the control regions. The uncertainty in the background yield is the sum in quadrature of the statistical and systematic uncertainties. |
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Table 3:
Summary of the relative systematic uncertainties in heavy Majorana neutrino signal yields and in the background from prompt same-sign leptons, both estimated from simulation. The relative systematic uncertainties assigned to the misidentified-lepton and mismeasured-charge backgrounds estimated from control regions in data are also shown. The uncertainties are given for the low-mass (high-mass) selections. The range given for each systematic uncertainty source covers the variation across the mass range. |
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Table 4:
Summary of contributions to the systematic uncertainty related to the prompt same-sign lepton, misidentified-lepton, and mismeasured-charge backgrounds to the total background uncertainty. The numbers are for the SR1 (SR2) for the case of ${m_\mathrm {N}} = $ 50 and 500 GeV. |
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Table 5:
Observed event yields and estimated backgrounds after the application of all selections, except for the final optimization. The background predictions from prompt same-sign lepton (Prompt bkgd.), misidentified leptons (Misid. bkgd.), mismeasured charge (Charge mismeas. bkgd.) and the total background (Total bkgd.) are shown together with the number of events observed in data. The uncertainties shown are the statistical and systematic components, respectively. The "-'' indicates that the background is considered negligible. |
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Table 6:
Number of expected signal events after all the selections are applied. The matrix element $|{\mathrm V_{\ell N}}|$ is equal to $1 \times 10^{-4}$, $1 \times 10^{-2}$, and 1 for $ {m_\mathrm {N}} = $ 50 GeV, $ {m_\mathrm {N}} = $ 200 GeV, and $ {m_\mathrm {N}} = $ 1000 GeV respectively. |
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Table 7:
Selection requirements on discriminating variables determined by the optimization for each Majorana neutrino mass point. The last column shows the overall signal acceptance for $s$-channel. The uncertainties shown are statistical only. |
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Table 8:
Selection requirements on discriminating variables determined by the optimization for each Majorana neutrino mass point in the high-mass SR1. The last column shows the overall signal acceptance for $s$-channel [Numbers in bracket for for $t$-channel]. The uncertainties shown are statistical only. The "-'' indicates that no selection requirement is made. |
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Table 9:
Selection requirements on discriminating variables determined by the optimization for each Majorana neutrino mass point in the high-mass SR2. The last column shows the overall signal acceptance for $s$-channel [Numbers in bracket for for $t$-channel]. The uncertainties shown are statistical only. The "-'' indicates that no selection requirement is made. |
| Summary |
| A search for heavy Majorana neutrinos in the final states with same-sign dileptons and jet(s) has been performed in proton-proton collisions at a center-of-mass energy of 13 TeV, using a data set corresponding to an integrated luminosity of 35.9 fb$^{-1}$. No significant excess of events compared to the expected standard model background prediction is observed. Upper limits at 95% CL are set on the mixing matrix element between standard model neutrinos and heavy Majorana neutrino ($ |{{V_{\ell \mathrm{N}}} }$), as a function of heavy Majorana neutrino mass, ${m_\mathrm{N}} $. The analysis improved on previous searches by adding single-jet events into the signal region, which improved sensitivities for the lowest and and highest mass regions. For ${m_\mathrm{N}} = $ 40 GeV the observed (expected) limits are $ |{{V_{\mathrm{e} \mathrm{N}}} }|^{2} < 9.5 (8.0) \times 10^{-5}$, $ |{{V_{\mu \mathrm{N}}} }|^{2} < 2.0 (2.4) \times 10^{-5}$, and $ |{{V_{\mathrm{e} \mathrm{N}}} {V^{*}_{\mu \mathrm{N}}} }^2 / ( |{{V_{\mathrm{e} \mathrm{N}}} }^2 + |{{V_{\mu \mathrm{N}}} }^2) < 2.74 (2.84) \times 10^{-5}$. While for ${m_\mathrm{N}} = $ 1000 GeV the limits are $ |{{V_{\mathrm{e} \mathrm{N}}} }|^{2} < 0.43 (0.32)$, $ |{{V_{\mu \mathrm{N}}} }|^{2} < 0.27 (0.16)$, and $ |{{V_{\mathrm{e} \mathrm{N}}} {V^{*}_{\mu \mathrm{N}}} }^2 / ( |{{V_{\mathrm{e} \mathrm{N}}} }^2 + |{{V_{\mu \mathrm{N}}} }^2) < 0.14 (0.13)$. The search is sensitive to masses of heavy neutrinos up to 1600 GeV. The limits on the mixing matrix elements were placed up to 1430 GeV for the $ |{{V_{\mu \mathrm{N}}} }|^{2}$, 1240 GeV for $ |{{V_{\mathrm{e} \mathrm{N}}} }|^{2}$, and 1600 GeV for $ |{{V_{\mathrm{e} \mathrm{N}}} {V^{*}_{\mu \mathrm{N}}} }^2 / ( |{{V_{\mathrm{e} \mathrm{N}}} }^2 + |{{V_{\mu \mathrm{N}}} }^2)$. These limits are the most restrictive direct limits for heavy Majorana neutrino masses above 430 GeV. These are the first limits on the heavy neutrino mixing parameters with the standard model neutrinos for Majorana neutrino masses above 1200 GeV. |
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