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| CMS-EXO-17-012 ; CERN-EP-2018-006 | ||
| Search for heavy neutral leptons in events with three charged leptons in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 8 February 2018 | ||
| Phys. Rev. Lett. 120 (2018) 221801 | ||
| Abstract: A search for a heavy neutral lepton N of Majorana nature decaying into a W boson and a charged lepton is performed using the CMS detector at the LHC. The targeted signature consists of three prompt charged leptons in any flavor combination of electrons and muons. The data were collected in proton-proton collisions at a center-of-mass energy of 13 TeV, with an integrated luminosity of 35.9 fb$^{-1}$. The search is performed in the N mass range between 1 GeV and 1.2 TeV. The data are found to be consistent with the expected standard model background. Upper limits are set on the values of $| {{V^{}_{\mathrm{e} \mathrm{N} }} } |^2$ and $| {{V^{}_{\mu \mathrm{N} }} } |^2$, where ${V^{}_{\ell \mathrm{N} }} $ is the matrix element describing the mixing of N with the standard model neutrino of flavor $\ell$. These are the first direct limits for N masses above 500 GeV and the first limits obtained at a hadron collider for N masses below 40 GeV. | ||
| Links: e-print arXiv:1802.02965 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Observed and expected event yields as a function of $ {M_{2\ell \text {OS}}^{\text{min}}} $ and $ {M_\text {T}} $ for events with at least two electrons (upper), and with at least two muons (lower). The contribution of each background source is shown. The first 8 bins of each figure correspond to the low-mass region, while the rest display the high-mass region. |
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Figure 1-a:
Observed and expected event yields as a function of $ {M_{2\ell \text {OS}}^{\text{min}}} $ and $ {M_\text {T}} $ for events with at least two electrons. The contribution of each background source is shown. The first 8 bins of the figure correspond to the low-mass region, while the rest display the high-mass region. |
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Figure 1-b:
Observed and expected event yields as a function of $ {M_{2\ell \text {OS}}^{\text{min}}} $ and $ {M_\text {T}} $ for events with at least two muons. The contribution of each background source is shown. The first 8 bins of the figure correspond to the low-mass region, while the rest display the high-mass region. |
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Figure 2:
Exclusion region at 95% CL in the $ | {V^{}_{{\mathrm {e}} {\mathrm {N}}}} |^2$ vs. $ {m_{{\mathrm {N}}}} $ (left) and $ | {V^{}_{\mu {\mathrm {N}}}} |^2$ vs. $ {m_{{\mathrm {N}}}} $ (right) planes. 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, while the dotted black curve is the observed limit in the approximation of prompt N decays. Also shown are the best upper limits at 95% CL from other collider searches in L3 [36], DELPHI [33], ATLAS [23], and CMS [22]. |
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Figure 2-a:
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, while the dotted black curve is the observed limit in the approximation of prompt N decays. Also shown are the best upper limits at 95% CL from other collider searches in L3 [36], DELPHI [33], ATLAS [23], and CMS [22]. |
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Figure 2-b:
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, while the dotted black curve is the observed limit in the approximation of prompt N decays. Also shown are the best upper limits at 95% CL from other collider searches in L3 [36], DELPHI [33], ATLAS [23], and CMS [22]. |
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Figure A1:
Expanded version of Fig.1, showing the observed and expected event yields in the low-mass region with $ {p_{\mathrm {T}}} ^\text {leading} < $ 30 GeV, for events with at least 2 electrons (left) and 2 muons (right). Contributions from various possible signals are shown for comparison. |
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Figure A1-a:
Expanded version of Fig.1, showing the observed and expected event yields in the low-mass region with $ {p_{\mathrm {T}}} ^\text {leading} < $ 30 GeV, for events with at least 2 electrons. Contributions from various possible signals are shown for comparison. |
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Figure A1-b:
Expanded version of Fig.1, showing the observed and expected event yields in the low-mass region with $ {p_{\mathrm {T}}} ^\text {leading} < $ 30 GeV, for events with at least 2 muons. Contributions from various possible signals are shown for comparison. |
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Figure A2:
Expanded version of Fig.1, showing the observed and expected event yields in the low-mass region with 30 GeV $ \leq {p_{\mathrm {T}}} ^\text {leading} < $ 55 GeV, for events with at least 2 electrons (left) and 2 muons (right). Contributions from various possible signals are shown for comparison. |
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Figure A2-a:
Expanded version of Fig.1, showing the observed and expected event yields in the low-mass region with 30 GeV $ \leq {p_{\mathrm {T}}} ^\text {leading} < $ 55 GeV, for events with at least 2 electrons. Contributions from various possible signals are shown for comparison. |
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Figure A2-b:
Expanded version of Fig.1, showing the observed and expected event yields in the low-mass region with 30 GeV $ \leq {p_{\mathrm {T}}} ^\text {leading} < $ 55 GeV, for events with at least 2 muons. Contributions from various possible signals are shown for comparison. |
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Figure A3:
Expanded version of Fig.1, showing the observed and expected event yields in the high-mass region without an OSSF pair, for events with at least 2 electrons (left) and 2 muons (right). Contributions from various possible signals are shown for comparison. |
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Figure A3-a:
Expanded version of Fig.1, showing the observed and expected event yields in the high-mass region without an OSSF pair, for events with at least 2 electrons. Contributions from various possible signals are shown for comparison. |
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Figure A3-b:
Expanded version of Fig.1, showing the observed and expected event yields in the high-mass region without an OSSF pair, for events with at least 2 muons. Contributions from various possible signals are shown for comparison. |
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Figure A4:
Expanded version of Fig.1, showing the observed and expected event yields in the high-mass region with an OSSF pair, for events with at least 2 electrons (left) and 2 muons (right). Contributions from various possible signals are shown for comparison. |
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Figure A4-a:
Expanded version of Fig.1, showing the observed and expected event yields in the high-mass region with an OSSF pair, for events with at least 2 electrons. Contributions from various possible signals are shown for comparison. |
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Figure A4-b:
Expanded version of Fig.1, showing the observed and expected event yields in the high-mass region with an OSSF pair, for events with at least 2 muons. Contributions from various possible signals are shown for comparison. |
| Tables | |
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Table A1:
Observed (expected) event yields in the low-mass search region. The uncertainties contain both the statistical and systematic components. |
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Table A2:
Observed (expected) event yields in the high-mass search region for events with no OSSF lepton pair. The uncertainties contain both the statistical and systematic components. |
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Table A3:
Observed (expected) event yields in the high-mass search region for events with an OSSF lepton pair. The uncertainties contain both the statistical and systematic components. |
| Summary |
|
In summary, a search has been performed for a heavy neutral lepton N of Majorana nature produced in the decays of a W boson, with subsequent prompt decays of N to $\mathrm{W}\ell$, where the vector boson decays to $\ell\nu$. The event signature consists of three charged leptons in any combination of electrons and muons. No statistically significant excess of events over the expected standard model background is observed. Upper limits at 95% confidence level are set on the mixing parameters $| V_{\mathrm{eN}} |^2$ and $| V_{\mu\mathrm{N}} |^2$, ranging between 1.2$ \times 10^{-5} $ and 1.8 for N masses in the range 1 GeV $ < m_{\mathrm{N}} < $ 1.2 TeV. These results surpass those obtained in previous searches carried out by the ATLAS [23] and CMS [22,24] Collaborations, and are the first direct limits for $m_{\mathrm{N}}> $ 500 GeV. This search also provides the first probes for low masses ($m_{\mathrm{N}}< $ 40 GeV) at the LHC, improving on the limits set previously by the L3 [29] and DELPHI [33] Collaborations. For N masses below 3 GeV, the most stringent limits to date are obtained from the beam-dump experiments: CHARM [26,31], BEBC [25], FMMF [32], and NuTeV [34]. |
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