Abstract
If neutrinos are Dirac particles and, as suggested by the so far null LHC results, any new physics lies at energies well above the electroweak scale, the Standard Model effective field theory has to be extended with operators involving the right-handed neutrinos. In this paper, we study this effective field theory and set constraints on the different dimension-six interactions. To that aim, we use LHC searches for associated production of light (and tau) leptons with missing energy, monojet searches, as well as pion and tau decays. Our bounds are generally above the TeV for order one couplings. One particular exception is given by operators involving top quarks. These provide new signals in top decays not yet studied at colliders. Thus, we also design an LHC analysis to explore these signatures in the toverline{t} production. Our results are also valid if the right-handed neutrinos are Majorana and long-lived.
Highlights
RH neutrino N [7]. lL and qL stand for the left-handed lepton and quark doublets, respectively
If neutrinos are Dirac particles and, as suggested by the so far null LHC results, any new physics lies at energies well above the electroweak scale, the Standard Model effective field theory has to be extended with operators involving the right-handed neutrinos
The interaction rate for producing RH neutrinos out of the thermal bath behaves as Γ ∼ T 5/Λ4. The latter are in thermal equilibrium at the time of the Big Bang Nucleosynthesis (BBN) provided Γ TB2BN/mP, with mP being the Planck mass and TBBN ∼ MeV; namely if Λ 200 GeV ∼ v. (There is no experimental evidence of the pre BBN era — some models even require reheating right before T ∼ MeV [8, 9] — so there are no actual cosmological constraints on the νSMEFT if N decouples before BBN.)
Summary
The basis of dimension-six operators containing N and respecting lepton and baryon numbers is shown in table 1. We focus mostly on observables sensitive to the four-fermion operators in the classes RRRR, LLRR and LRRL, where L (R) denotes left (right) handed fermions. These are typically more relevant because they can be generated at tree level in UV completions of the SM. In the Majorana case they must be suppressed in order for N to be long-lived; otherwise it would decay rather promptly into two body final states [14, 15] In such case, the collider signals are very different from the ones considered in this article and related works [16,17,18]; they will be presented elsewhere.
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