[Paper Review] Search for Long-Lived Heavy Neutral Leptons with Lepton Flavour Conserving or Violating Decays to a Jet and a Charged Lepton

Hayrapetyan, Aram, Aram Hayrapetyan+2337 more|arXiv (Cornell University)|Jan 1, 2023

Particle physics theoretical and experimental studiesPhysics and Astronomy75 references1 citations

TL;DR

This study presents a search for long-lived heavy neutral leptons (HNLs) in proton-proton collisions at 13 TeV using 138 fb⁻¹ of data from the CMS experiment at CERN. It employs a deep neural network jet tagger to identify displaced jets from HNL decays into a lepton and quarks, probing all three lepton generations simultaneously. The analysis sets the most stringent limits to date, excluding |VµN|² > 5 × 10⁻⁷ (4 × 10⁻⁷) for Dirac (Majorana) HNLs with a 10 GeV mass at 95% confidence level, corresponding to proper decay lengths of 17 mm (10 mm).

ABSTRACT

A search for long-lived heavy neutral leptons (HNLs) is presented, which considers the hadronic final state and coupling scenarios involving all three lepton generations in the 2-20 GeV HNL mass range for the first time. Events comprising two leptons (electrons or muons) and jets are analyzed in a data sample of proton-proton collisions, recorded with the CMS experiment at the CERN LHC at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. A novel jet tagger, based on a deep neural network, has been developed to identify jets from an HNL decay using various features of the jet and its constituent particles. The network output can be used as a powerful discriminating tool to probe a broad range of HNL lifetimes and masses. Contributions from background processes are determined from data. No excess of events in data over the expected background is observed. Upper limits on the HNL production cross section are derived as functions of the HNL mass and the three coupling strengths $V_{\ell\mathrm{N}}$ to each lepton generation $\ell$ and presented as exclusion limits in the coupling-mass plane, as lower limits on the HNL lifetime, and on the HNL mass. In this search, the most stringent limit on the coupling strength is obtained for pure muon coupling scenarios; values of $\lvert V_{\mu\mathrm{N}} vert^{2}\gt $ 5 (4)$ imes$10$^{-7}$ are excluded for Dirac (Majorana) HNLs with a mass of 10 GeV at a confidence level of 95% that correspond to proper decay lengths of 17 (10) mm.

Motivation & Objective

• To search for long-lived heavy neutral leptons (HNLs) in the 2–20 GeV mass range with lifetimes up to 10⁴ mm.
• To probe HNL couplings to all three lepton generations (e, μ, τ) simultaneously, including both lepton-flavour-conserving and violating scenarios.
• To develop and apply a novel deep neural network-based jet tagger to identify displaced jets from HNL decays.
• To set exclusion limits on HNL production cross sections, coupling strengths, and proper decay lengths using data-driven background estimation.
• To improve sensitivity to HNLs beyond previous searches that assumed exclusive coupling to a single lepton generation.

Proposed method

• A deep neural network is trained to identify jets originating from HNL decays using kinematic and substructure features of jets and their constituents.
• The network uses input variables such as jet mass, track multiplicity, impact parameters, and secondary vertex properties to distinguish displaced HNL decay jets from prompt jets.
• The analysis selects events with two leptons (e, μ) and jets, with one lepton prompt and one displaced, consistent with HNL decay.
• Background contributions are estimated directly from data using control regions, minimizing reliance on simulation.
• Exclusion limits are derived using the CLs method, accounting for uncertainties in signal and background modeling.
• The search spans the full coupling space of VℓN to e, μ, and τ, enabling sensitivity to both Dirac and Majorana HNLs.

Experimental results

Research questions

• RQ1 What are the most stringent exclusion limits on HNL couplings to muons (VμN) in the 2–20 GeV mass range, considering long-lived HNLs with displaced decays?
• RQ2 How effective is a deep neural network jet tagger in identifying displaced jets from HNL decays compared to traditional methods?
• RQ3 What are the sensitivity limits on HNL proper decay lengths (cτ₀) for both Dirac and Majorana HNLs across the full coupling space?
• RQ4 How do the exclusion limits change when allowing for mixed couplings to multiple lepton generations, rather than assuming exclusive coupling to one?
• RQ5 What is the impact of including both lepton-flavour-conserving and lepton-flavour-violating decay modes on the overall sensitivity of the search?

Key findings

• No significant excess of events over the expected background is observed in the data.
• For a 10 GeV Dirac HNL, |VµN|² > 5 × 10⁻⁷ is excluded at 95% confidence level, corresponding to a proper decay length of 17 mm.
• For a 10 GeV Majorana HNL, |VµN|² > 4 × 10⁻⁷ is excluded at 95% confidence level, corresponding to a proper decay length of 10 mm.
• The search sets the most stringent limits to date on HNL couplings in the 2–20 GeV mass range, particularly for pure muon coupling scenarios.
• The deep neural network jet tagger significantly enhances sensitivity to displaced jets, enabling probing of long-lived HNLs with cτ₀ up to 10⁴ mm.
• Exclusion limits are presented as functions of HNL mass and coupling strength, covering all three lepton generations and both Dirac and Majorana HNL cases.