CMS-EXO-20-011

CMS-EXO-20-011 ; CERN-EP-2022-181
Search for a heavy composite Majorana neutrino in events with dilepton signatures from proton-proton collisions at $\sqrt{s} =$ 13 TeV
CMS Collaboration
6 October 2022
Phys. Lett. B 843 (2023) 137803
Abstract: Results are presented of a search for a heavy Majorana neutrino $\mathrm{N}_{\ell}$ decaying into two same-flavor leptons $\ell$ (electrons or muons) and a quark-pair jet. A model is considered in which the $\mathrm{N}_{\ell}$ is an excited neutrino in a compositeness scenario. The analysis is performed using a sample of proton-proton collisions at $\sqrt{s} =$ 13 TeV recorded by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 138 fb $^{-1}$ . The data are found to be in agreement with the standard model prediction. For the process in which the $\mathrm{N}_{\ell}$ is produced in association with a lepton, followed by the decay of the $\mathrm{N}_{\ell}$ to a same-flavor lepton and a quark pair, an upper limit at 95% confidence level on the product of the cross section and branching fraction is obtained as a function of the $\mathrm{N}_{\ell}$ mass $m_{\mathrm{N}_{\ell}}$ and the compositeness scale $\Lambda$ . For this model the data exclude the existence of $\mathrm{N}_{\mathrm{e}}$ ( $\mathrm{N}_{\mu}$ ) for $m_{\mathrm{N}_{\ell}}$ below 6.0 (6.1) TeV, at the limit where $m_{\mathrm{N}_{\ell}}$ is equal to $\Lambda$ . For $m_{\mathrm{N}_{\ell}} \approx$ 1 TeV, values of $\Lambda$ less than 20 (23) TeV are excluded. These results represent a considerable improvement in sensitivity, covering a larger parameter space than previous searches in pp collisions at 13 TeV.
Links: e-print arXiv:2210.03082 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The fermion interaction as a sum of gauge (center) and contact (right) contributions.
png pdf	Figure 2: Feynman diagrams for the decay of a heavy composite Majorana neutrino to $\ell\mathrm{q}\overline{\mathrm{q}}^\prime$ .
png pdf	Figure 3: Distribution of $m(\ell\ell\,\mathrm{J})$ in the DY-enriched CR for the electron (upper left) and muon (upper right) flavors, and of $m(\mathrm{e}\mu \mathrm{J})$ in the top-quark-enriched CR (lower). Data points are overlaid on the post-fit background (stacked histograms). The overflow is included in the last bin. The middle panels show ratios of the data to the pre-fit background prediction and post-fit background yield as red open squares and blue points, respectively. The gray band in the middle panels indicates the systematic component of the post-fit uncertainty. The lower panels show the distributions of the pulls, defined in the text.
png pdf	Figure 3-a: Distribution of $m(\ell\ell\,\mathrm{J})$ in the DY-enriched CR for the electron flavor. Data points are overlaid on the post-fit background (stacked histograms). The overflow is included in the last bin. The middle panel shows ratios of the data to the pre-fit background prediction and post-fit background yield as red open squares and blue points, respectively. The gray band in the middle panel indicates the systematic component of the post-fit uncertainty. The lower panel shows the distributions of the pulls, defined in the text.
png pdf	Figure 3-b: Distribution of $m(\ell\ell\,\mathrm{J})$ in the DY-enriched CR for the muon flavor. Data points are overlaid on the post-fit background (stacked histograms). The overflow is included in the last bin. The middle panel shows ratios of the data to the pre-fit background prediction and post-fit background yield as red open squares and blue points, respectively. The gray band in the middle panel indicates the systematic component of the post-fit uncertainty. The lower panel shows the distributions of the pulls, defined in the text.
png pdf	Figure 3-c: Distribution of $m(\mathrm{e}\mu \mathrm{J})$ in the top-quark-enriched CR. Data points are overlaid on the post-fit background (stacked histograms). The overflow is included in the last bin. The middle panel shows ratios of the data to the pre-fit background prediction and post-fit background yield as red open squares and blue points, respectively. The gray band in the middle panel indicates the systematic component of the post-fit uncertainty. The lower panel shows the distributions of the pulls, defined in the text.
png pdf	Figure 4: Distributions of $m(\ell\ell\,\mathrm{J})$ for the data, and the post-fit backgrounds (stacked histograms), in the SRs of the $\mathrm{e}\mathrm{e}\mathrm{q}\overline{\mathrm{q}}^\prime$ (left) and the $\mu\mu\mathrm{q}\overline{\mathrm{q}}^\prime$ (right) channels. The template for one signal hypothesis is shown overlaid as a yellow solid line. The overflow is included in the last bin. The middle panels show ratios of the data to the pre-fit background prediction and post-fit background yield as red open squares and blue points, respectively. The gray band in the middle panels indicates the systematic component of the post-fit uncertainty. The lower panels show the distributions of the pulls, defined in the text.
png pdf	Figure 4-a: Distribution of $m(\ell\ell\,\mathrm{J})$ for the data, and the post-fit backgrounds (stacked histograms), in the SRs of the $\mathrm{e}\mathrm{e}\mathrm{q}\overline{\mathrm{q}}^\prime$ channel. The template for one signal hypothesis is shown overlaid as a yellow solid line. The overflow is included in the last bin. The middle panel shows ratios of the data to the pre-fit background prediction and post-fit background yield as red open squares and blue points, respectively. The gray band in the middle panel indicates the systematic component of the post-fit uncertainty. The lower panel shows the distributions of the pulls, defined in the text.
png pdf	Figure 4-b: Distribution of $m(\ell\ell\,\mathrm{J})$ for the data, and the post-fit backgrounds (stacked histograms), in the SRs of the $\mu\mu\mathrm{q}\overline{\mathrm{q}}^\prime$ channel. The template for one signal hypothesis is shown overlaid as a yellow solid line. The overflow is included in the last bin. The middle panel shows ratios of the data to the pre-fit background prediction and post-fit background yield as red open squares and blue points, respectively. The gray band in the middle panel indicates the systematic component of the post-fit uncertainty. The lower panel shows the distributions of the pulls, defined in the text.
png pdf	Figure 5: Expected (black dashed lines with green dark and yellow light bands) and observed (solid blue lines) limits on the product of cross section and branching fraction for the $\mathrm{e}\mathrm{e}\mathrm{q}\overline{\mathrm{q}}^\prime$ (left) and $\mu\mu\mathrm{q}\overline{\mathrm{q}}^\prime$ (right) channels. The uncertainty bands account for the post-fit statistical and systematic uncertainty. The magenta dot-dashed lines denote the model cross sections for the benchmark scale parameter $\Lambda =$ 13 TeV.
png pdf	Figure 5-a: Expected (black dashed lines with green dark and yellow light bands) and observed (solid blue lines) limits on the product of cross section and branching fraction for the $\mathrm{e}\mathrm{e}\mathrm{q}\overline{\mathrm{q}}^\prime$ channel. The uncertainty band accounts for the post-fit statistical and systematic uncertainty. The magenta dot-dashed line denotes the model cross sections for the benchmark scale parameter $\Lambda =$ 13 TeV.
png pdf	Figure 5-b: Expected (black dashed lines with green dark and yellow light bands) and observed (solid blue lines) limits on the product of cross section and branching fraction for the $\mu\mu\mathrm{q}\overline{\mathrm{q}}^\prime$ channel. The uncertainty band accounts for the post-fit statistical and systematic uncertainty. The magenta dot-dashed line denotes the model cross sections for the benchmark scale parameter $\Lambda =$ 13 TeV.
png pdf	Figure 6: Expected (black dashed lines with green dark and yellow light bands) and observed (solid blue lines) limits in the ( $m_{\mathrm{N}_{\ell}}$ , $\Lambda$ ) plane of the composite model for the $\mathrm{e}\mathrm{e}\mathrm{q}\overline{\mathrm{q}}^\prime$ (left) and $\mu\mu\mathrm{q}\overline{\mathrm{q}}^\prime$ (right) channels. The gray shading indicates the region where $m_{\mathrm{N}_{\ell}}$ would exceed $\Lambda$ , the EFT scale parameter, and the three solid magenta lines in the lower part of the plots represent the fraction of the signal-model phase space that satisfies the unitarity condition in the EFT approximation.
png pdf	Figure 6-a: Expected (black dashed lines with green dark and yellow light bands) and observed (solid blue lines) limits in the ( $m_{\mathrm{N}_{\ell}}$ , $\Lambda$ ) plane of the composite model for the $\mathrm{e}\mathrm{e}\mathrm{q}\overline{\mathrm{q}}^\prime$ channel. The gray shading indicates the region where $m_{\mathrm{N}_{\ell}}$ would exceed $\Lambda$ , the EFT scale parameter, and the three solid magenta lines in the lower part of the plot represent the fraction of the signal-model phase space that satisfies the unitarity condition in the EFT approximation.
png pdf	Figure 6-b: Expected (black dashed lines with green dark and yellow light bands) and observed (solid blue lines) limits in the ( $m_{\mathrm{N}_{\ell}}$ , $\Lambda$ ) plane of the composite model for the $\mu\mu\mathrm{q}\overline{\mathrm{q}}^\prime$ channel. The gray shading indicates the region where $m_{\mathrm{N}_{\ell}}$ would exceed $\Lambda$ , the EFT scale parameter, and the three solid magenta lines in the lower part of the plot represent the fraction of the signal-model phase space that satisfies the unitarity condition in the EFT approximation.

Tables
png pdf	Table 1: The impact of each systematic uncertainty on the signal strength $\mu$ as extracted from the ML fit, for the $\mathrm{N}_{\ell}$ signal point with $m_{\mathrm{N}_{\ell}} =$ 0.5 TeV and $\Lambda =$ 13 TeV. Upper and lower uncertainties are given, for both electron and muon channels.

Summary

A search is reported for a heavy composite Majorana neutrino

$\mathrm{N}_{\ell}$ , where the flavor

$\ell$ corresponds to an electron or muon, that appears in composite fermion models. In the specific model considered, the

$\mathrm{N}_{\ell}$ is produced in association with a lepton and subsequently decays into a same-flavor lepton plus two quarks, leading to a signature with two same-flavor leptons and at least one large-radius jet. The analysis is performed using a sample of proton-proton collisions at

$\sqrt{s} =$ 13 TeV recorded by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 138 fb

$^{-1}$ . The data are found to be in agreement with the standard model expectations. In the context of an effective field theory with compositeness scale parameter

$\Lambda$ , an upper limit at 95% CL is established on

$\sigma(\mathrm{p}\mathrm{p}\to\ell\mathrm{N}_{\ell})\mathcal{B}(\mathrm{N}_{\ell}\to\ell\mathrm{q}\overline{\mathrm{q}}^\prime)$ as a function of

$\Lambda$ and the

$\mathrm{N}_{\ell}$ mass

$m_{\mathrm{N}_{\ell}}$ . Masses less than 6.0 (6.1) TeV are excluded for

$\ell =\mathrm{e}\,(\mu)$ , at the limit

$m_{\mathrm{N}_{\ell}} = \Lambda$ . For

$m_{\mathrm{N}_{\ell}} \approx$ 1 TeV, values of

$\Lambda$ less than 20 (23) TeV are excluded. The present search represents a considerable improvement in sensitivity, covering a larger parameter space than previous searches in pp collisions at 13 TeV.

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