New Collider Designs Could Reveal Signals of Matter-Antimatter Asymmetry Origins

Scientists are investigating the potential for discovering physics beyond the Standard Model through the search for heavy neutrinos at future colliders. Benjamin Fuks (Laboratoire de Physique Théorique et Hautes Energies), Jonathan Kriewald and Miha Nemevšek (Jožef Stefan Institute), along with Fabrizio Nesti et al., present a detailed analysis of displaced signals arising from heavy neutrino production within the framework of the Left-Right symmetric model. Their research is significant because it demonstrates how future electron-positron colliders, such as the proposed FCC-ee, could extend the reach of searches for left-right symmetry breaking scales far beyond the capabilities of the Large Hadron Collider. By comprehensively examining various production and decay channels and developing a realistic vertexing algorithm, the team establishes the potential for identifying long-lived particles and mapping out the parameter space of this compelling theoretical framework.

This work presents an extensive analysis of displaced signals arising from heavy neutrino production, considering a comprehensive range of production channels and decay modes.

Researchers have developed realistic estimates of signal yields and selection efficiencies using a dedicated vertexing algorithm designed to reconstruct the decay positions and four-momenta of these long-lived particles. The study focuses on channels mediated by W and WR bosons, scalar mixing, and gauge/scalar boson fusion, all linked to the underlying mechanism of neutrino mass generation.
Detailed calculations of total production rates and differential distributions allow for differentiation between these channels and provide analytical estimates of signal strength. A key achievement is the development of a vertexing algorithm capable of accurately establishing the displaced decay positions of heavy neutrinos, enabling a reliable reconstruction of their four-momenta.

This reconstruction is crucial for determining realistic parameter space reaches within the Left-Right symmetric model across various channels. By meticulously modelling these signatures and accounting for detector effects, scientists have established sensitivity estimates for different production and decay pathways.

These estimates reveal that future colliders could uncover evidence of heavy neutrinos and provide insights into the origin of neutrino masses, potentially confirming the existence of physics beyond the Standard Model. The findings highlight the potential of future lepton colliders to explore fundamental questions in particle physics and push the boundaries of our understanding of the universe.

Displaced vertex reconstruction and heavy neutrino searches with the IDEA detector

A new graph-based vertexing algorithm forms the core of this research, enabling full kinematical reconstruction of displaced long-lived particle decays. This refined treatment of displaced vertices surpasses previous studies relying on the Delphes 3 and FCCAnalyses software stack, which utilised vertex fitting code originally developed for searches for heavy neutrinos with muon final states and dark scalar decays.

The study investigates heavy neutrino production at future electron-positron colliders within the Left-Right symmetric model, considering channels mediated by both vector bosons and scalar interactions. Researchers employed this new vertexing framework to fully exploit the tracking capabilities of the proposed IDEA detector, demonstrating its performance and facilitating the extraction of physical parameters across multiple production channels.

Detailed Monte Carlo simulations validated analytical calculations of momentum variables, lepton flavours, transverse displacement, and angular distributions, allowing differentiation between various production mechanisms. Total production rates and differential distributions were derived to estimate signal yields and assess the potential for channel separation.

The work focuses on identifying displaced heavy neutrinos, potentially accompanied by prompt activity and forward leptons, to probe the parameter space of the Left-Right symmetric model. Realistic estimates of selection efficiencies were obtained by establishing displaced decay positions and reconstructing the four-momenta of long-lived particles.

The updated model file, mlrsm-1.2, publicly available via FeynRules, was used throughout the analysis to ensure consistency and accuracy. Parameters such as the right-handed gauge boson mass, MWR, were explored in the multi-TeV range, with the scalar triplet mass, m∆, typically constrained to below 160 GeV due to the WW threshold.

Sensitivity to the heavy neutrino mass, mN, extends up to approximately half the collider’s centre-of-mass energy, although detector geometry and tracking volume ultimately limit reconstruction efficiency for highly displaced events. Despite these limitations, the expected reach of the FCC-ee exceeds current LHC searches for relatively light neutrinos with masses around 10 GeV.

Heavy neutrino and scalar production mechanisms in electron-positron collisions at TeV energies

Cross sections for heavy neutrino and scalar production in electron-positron collisions reach 10 picobarns at centre-of-mass energies of 1 TeV for a benchmark scenario with a heavy WR mass of 7 TeV and heavy neutrino mass of 30 GeV. These calculations encompass various production channels, including those mediated by Z, ZLR, and WR bosons, as well as scalar mixing and gauge/scalar boson fusion.

Total production rates and differential distributions were derived to differentiate these channels and estimate signal yields analytically. For the production of heavy neutrinos via gauge boson exchange, the cross section for electron-positron collisions producing a heavy neutrino pair via s-channel Z exchange is approximately 4παw c2w v2 e + a2 e √s (s −M2)2 + Γ2M2 αwa2 N 6c2w √sβ3 Ns.

At the Z pole (√s ∼MZ), this s-channel Z exchange dominates heavy neutrino pair production. Moving away from the pole, WR exchange becomes dominant, with cross sections reaching comparable levels at very high energies, specifically √s ≳2 TeV. Boost factors for heavy neutrino production are calculated as γNN = √s 2mN for pair production and γNν = s + m2 N 2√s mN for associated Nν production, directly influencing the displacement of decay products.

The normalised differential cross section for production angles exhibits a universal form of 1 σ dσ dcθ = 3 8 1 + c2 θ, independent of √s and mN. Transverse momentum spectra for s-channel vector exchange are well approximated by 1 σ dσ dpT = 1 pmax T 3x(2 −x2) 4 √ 1 −x2, where x = pT /pmax T and pmax T = √s β/2.

Angular and transverse momentum distributions were analysed for a scenario with MWR = 7 TeV, mN = 30 GeV, and m∆= 75 GeV at centre-of-mass energies of 240 GeV and 1 TeV. Associated Nν production, dominated by t- and u-channel W-boson exchanges, displays a markedly different angular behaviour, described by the normalised distribution 1 σ dσ dcθ = M2W M2W + sβ2 1 2M2W + sβ2(1 −cθ) 2 + 1 2M2W + sβ2(1 + cθ) 2. These kinematic features are crucial for reconstructing the full four-momenta of long-lived particles and determining parameter space reaches in the Left-Right symmetric model.

Lepton collider potential for probing TeV-scale left-right symmetry

Scientists have extensively investigated the production of heavy neutrinos within the framework of the Left-Right symmetric model, focusing on future electron-positron colliders. Their work encompasses a comprehensive analysis of various production channels, including those mediated by gauge bosons and scalars, and considers the implications for understanding the origin of neutrino mass.

Detailed calculations of production rates and distributions were performed to distinguish between these channels and estimate signal yields. The study acknowledges limitations related to the complexity of modelling detector effects and the reliance on specific assumptions within the Left-Right symmetric model. Future research could focus on refining the vertexing algorithms and exploring the impact of different background processes to further improve sensitivity and validate these findings.