Heavy Majorana neutrinos in the effective Lagrangian description: Application to hadron colliders

Abstract

We consider the effects of heavy Majorana neutrinos N with sub-TeV masses. We argue that the mere presence of these particles would be a signal of physics beyond the minimal seesaw mechanism and their interactions are, therefore, best described using an effective Lagrangian. We then consider the complete set of leading effective operators (up to dimension 6) involving the N and Standard Model fields and show that these interactions can be relatively easy to track at high-energy colliders. For example, we find that an exchange of a TeV-scale heavy vector field can yield thousands of characteristic same-sign lepton number violating ℓ + ℓ + jj events (j = light jet) at the LHC if m N < ∼ 600 GeV, which can also have a distinctive forward-backward asymmetry signal; even the Tevatron has good prospects for this signature if m N < ∼ 300 GeV.The spectacular discovery in the past decade of neutrino oscillation and its interpretation in terms of a non-vanishing neutrino mass matrix is one of the most important recent discoveries in particle physics. The m ν > ∼ O(10 −2 ) eV neutrino masses that appear in this scenario are difficult to generate naturally in the Standard Model (SM) using the Yukawa interactions; the sub-eV mass *

Photons coming from an opaque obstacle as a manifestation of heavy neutrino decays

“…This mechanism introduces right-handed sterile neutrinos that, as they do not have distinct particle and antiparticle degrees of freedom, can have a Majorana mass term leading to the tiny known masses for the standard neutrinos, as long as the Yukawa couplings between the right-handed Majorana neutrinos and the standard ones remain small. Even for the low-mass range for N considered here, the simplest type-I seesaw scenario leads to a negligible left-right neutrino mixing jU lN j 2 ∼ m ν =M N ∼ 10 −9 [7][8][9]. The mixing U lN weighs the couplings of N with the SM particles and in particular with charged leptons through the V − A interaction…”

“…We parametrize the effects of new physics by a set of effective operators O constructed with the standard model and the Majorana neutrino fields, satisfying the SUð2Þ L ⊗ Uð1Þ Y gauge symmetry [9]. The effect of these operators is suppressed by inverse powers of the new physics scale Λ, which is not necessarily related to the Majorana neutrino mass m N .…”

“…[9], the detection of Majorana neutrinos (N) would be a signal of physics beyond the minimal seesaw mechanism, and its interactions could be better described in a model-independent approach based on an effective theory. We consider a simplified scenario with only one Majorana neutrino N and negligible mixing with the ν L .…”

Search for anomalous production of prompt like-sign lepton pairs at $ \sqrt{s}=7\;\mathrm{TeV} $ with the ATLAS detector

“…It is determined for a variety of new physics models: H ±± bosons with mass between 50 GeV and 1000 GeV [48], fourth generation quarks decaying to W t with masses of 300-500 GeV, like-sign top-quark production via a contact interaction [53], and right-handed W bosons decaying to ℓ ± ℓ ± via a right-handed neutrino with m(W R ) = 800-2500 GeV [49,50]. These models are chosen as they cover a broad range of jet multiplicities and lepton p T spectra.…”

“…Pair production of doubly-charged Higgs bosons (pp → H ±± H ∓∓ ) via a virtual Z/γ * exchange is generated using Pythia for H ±± mass values between 50 GeV and 1000 GeV [48]. Production of a right-handed W boson (W R ) decaying to a charged lepton and a right-handed neutrino (N R ) [49,50] and pair production of heavy down-type fourth generation quarks (d 4 ) decaying to tW are generated using Pythia. Like-sign top-quark pair production can occur in models with flavour-changing neutral currents, e.g.…”