A quantitative study on helicity inversion in Majorana neutrino decays at the LHC

Abstract

We report an analytical and numerical investigation into the impact of helicity inversion in LHC processes that do not conserve lepton number $(L)$. As a case study, we focus on the production and decay of Majorana neutrinos $(N)$ through on- and off-shell $W$ bosons in the Phenomenological Type I Seesaw model. Using the Monte Carlo event generator \texttt{MadGraph5\_aMC@NLO} in conjunction with the \texttt{HeavyN} model libraries, we perform exact matrix element (ME) computations without the narrow width approximation. Despite helicity inversion appearing explicitly in MEs, we report the absence of helicity suppression of $L$-violating collider observables for $1\to 4$ and $2\to 4$ processes that are dominated by resonant $N$ production. We attribute this incongruity to the different scalings of 4-momenta and squared 4-momenta in MEs and squared MEs, with exact cancelations occurring in the latter when $N$ goes on-shell in the small-width limit. In off-shell regimes, total suppression / enhancement of $L$ violation can emerge. Implications for other neutrino mass models are discussed.

16 pages, 4 figures (9 eps files). Published version. v2 includes significantly extended discussions / results on asymmetries in off-shell limits. Happy January!

References in corpus (15)

• The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations
• Parton distributions for the LHC
• LHAPDF6: parton density access in the LHC precision era
• The fate of hints: updated global analysis of three-flavor neutrino oscillations
• FeynRules - Feynman rules made easy
• Automatic spin-entangled decays of heavy resonances in Monte Carlo simulations
• Right-Handed Neutrinos at LHC and the Mechanism of Neutrino Mass Generation
• Distinguishing seesaw models at LHC with multi-lepton signals
• Left-Right Symmetry: from LHC to Neutrinoless Double Beta Decay
• Heavy Majorana Neutrinos from $W\gamma$ Fusion at Hadron Colliders
• Narrow-width approximation accuracy
• Lepton Number Violation and W' Chiral Couplings at the LHC
• Narrow-width approximation limitations
• Majorana neutrinos and lepton-number-violating signals in top-quark and W-boson rare decays
• A threshold-improved narrow-width approximation for BSM physics

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