Search for heavy neutral leptons in events with three charged leptons in proton-proton collisions at $\sqrt{s} =$ 13 TeV

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 $|\mathrm{V}\mathrm{eN}|^2$ and $|\mathrm{V}{\mu\mathrm{N}}|^2$, 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.

• The paper establishes upper limits on heavy neutral lepton couplings using trilepton event analysis in 13 TeV proton-proton collisions.
• It employs kinematic selections and data-driven background methods to effectively distinguish HNL signals from Standard Model processes.
• The study probes a wide mass range, refining the parameter space for seesaw and νMSM models and advancing previous collider limits.

Search for Heavy Neutral Leptons in Proton-Proton Collisions at 13 TeV

The paper presents an investigation conducted using the CMS detector at the LHC to search for heavy neutral leptons (HNLs) of Majorana nature, specifically focusing on their potential decay into a charged lepton and a $\PW$ boson. This study seeks to explore regions of parameter space relevant to neutrino masses and mixing, a key aspect of theories extending the standard model (SM), such as the seesaw mechanism and the neutrino minimal standard model ($\nu$MSM).

The search focuses on events with three prompt charged leptons, exploiting the unique signature of such events to differentiate between SM background processes and potential signals of HNLs. The analysis leverages $\Pp\Pp$ collision data with a center-of-mass energy of 13 TeV and an integrated luminosity of 35.9\fbinv. The mass range probed extends from 1 GeV to 1.2 TeV, allowing the study to address previously unexplored territory, particularly for HNL masses above 500 GeV and below 40 GeV.

Methodology and Signal Discrimination

The analysis is characterized by two distinct search regions, categorized by the HNL mass relative to the $\PW$ boson mass—targeting low ($< m_\PW$) and high ($> m_\PW$) mass domains. The trilepton signal is a primary focus, while systematic reductions in the SM backgrounds are achieved through kinematic constraints and specific selection criteria. These criteria include considerations of missing transverse momentum ($\ptmiss$), invariant mass variables, and lepton $\pt$ thresholds.

Background estimation leverages both data-driven methods and Monte Carlo simulations, incorporating various SM processes such as top quark pair production, $W\gamma^*$, and diboson events. Systematic uncertainties impacting the analysis were thoroughly considered, involving both theoretical uncertainties from simulation and experimental factors including detector efficiency and luminosity measurements.

Results and Implications

The findings indicate no significant excess beyond the expected SM background, leading to the establishment of upper limits on the coupling parameters $\abs{V^{}^2}$ and $\abs{V^{}^2}$ at a 95% confidence level. These constraints on the mixing parameters showcase an advancement over previously established limits, significantly contributing to the understanding of possible HNL mass and mixing scenarios.

The results also highlight the constraints on potential models extending the SM, providing valuable insights that rule out or limit the parameter space for HNLs predicted by the $\nu$MSM, and other seesaw-type extensions. The constraint areas span neutrino mass regions that were previously accessible only by non-collider experiments, underscoring the importance of collider-based probes in exploring the properties and possible existence of HNLs.

Future Directions

The continuation and expansion of such searches can provide critical insights into the nature of neutrino masses and the potential for physics beyond the SM. Future directions could involve exploiting higher integrated luminosities available in forthcoming runs of the LHC, employing advanced detector technologies, and using refined statistical analyses to enhance sensitivity to HNL signatures. These efforts are vital for confirming or refuting theoretical models that propose extensions to the SM, thereby contributing to a more comprehensive understanding of fundamental particle interactions and the wider cosmos.