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Register a new userThe Neutrino Option
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Ilaria Brivio
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The Neutrino Option is a scenario where the Higgs mass is generated at the same time as neutrino masses in the type-I seesaw model. This framework provides a dynamical origin for the scalar potential of the Standard Model and suggests a new approach to the hierarchy problem. Here we review the preliminary analysis of Ref. [1], that showed the viability of this scenario, as well as the improved study of Ref. [2], that led to a better identification of the region of the parameter space where the Neutrino Option can be realized. We find that experimental constraints from both Higgs and neutrino physics can be accommodated introducing 2 heavy Majorana neutrinos with mass $M_1simeq M_2sim 0.5 - 10$ PeV and Yukawa couplings to the lepton doublet of order $ 10^{-4}-10^{-2}$, assuming that at the scale $M$ the classical Higgs potential is approximately conformal, with a quartic Higgs coupling $lambda_0sim 0.01-0.05$. Specifying the light neutrino mass ordering, the ratio $M_2/M_1$ or a given value of the top quark mass identifies narrower ranges for all the parameters. Although no further signature of the Neutrino Option is generally predicted at the currently accessible energy scales, conformal UV completions have been proposed, that could be tested e.g. via detection of gravitational waves. Leptogenesis can also be successfully realized in this scenario, that intriguingly ties together the breaking of the conformal and electroweak symmetries with the violation of lepton number.
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We examine the compatibility between the Neutrino Option, in which the electroweak scale is generated by PeV mass type I seesaw Majorana neutrinos, and leptogenesis. We find the Neutrino Option is consistent with resonant leptogenesis. Working within the minimal seesaw scenario with two heavy Majorana neutrinos $N_{1,2}$, which form a pseudo-Dirac pair, we explore the viable parameter space. We find that the Neutrino Option and successful leptogenesis are compatible in the cases of a neutrino mass spectrum with normal (inverted) ordering for $1.2 times 10^6 < M text{ (GeV)} < 8.8 times 10^6$ ($2.4 times 10^6 < M text{ (GeV)} < 7.4 times 10^6$), with $M = (M_1 + M_2)/2$ and $M_{1,2}$ the masses of $N_{1,2}$. Successful leptogenesis requires that $Delta M/M equiv (M_2 - M_1)/M sim 10^{-8}$. We further show that leptogenesis can produce the baryon asymmetry of the Universe within the Neutrino Option scenario when the requisite CP violation in leptogenesis is provided exclusively by the Dirac or Majorana low energy CP violation phases of the PMNS matrix.
It was recently proposed that the electroweak hierarchy problem is absent if the generation of the Higgs potential stems exclusively from quantum effects of heavy right-handed neutrinos which can also generate active neutrino masses via the type-I seesaw mechanism. Hence, in this framework dubbed the neutrino option, the tree-level scalar potential is assumed to vanish at high energies. Such a scenario therefore lends itself particularly well to be embedded in a classically scale-invariant theory. In this paper we perform a survey of models featuring conformal symmetry at the high scale. We find that the minimal framework compatible with the neutrino option requires the Standard Model to be extended by two real scalar singlet fields in addition to right-handed neutrinos. The spontaneous breaking of scale invariance, which induces the dynamical generation of Majorana masses for the right-handed neutrinos, is triggered by renormalization group effects. We identify the parameter space of the model for which a phenomenologically viable Higgs potential and neutrino masses are generated, and for which all coupling constants remain in the perturbative regime up to the Planck scale.
The LHC data on the elastic scattering indicate that the forward slope increase is not consistent with the contributions of the simple Regge poles only with the linear Regge trajectories. The dynamics might be associated with unitarization in the direct channel of reaction. We discuss the problems of the Regge model and provide a respective illustration of the direct-channel option.
We investigate the possibility that a minimal realization of left-right supersymmetry can be reachable at a high-energy upgrade of the LHC, expected to operate at a center-of-mass energy of 27 TeV. This minimal scenario has a relatively light $SU(2)_R$ doubly-charged Higgs boson, which could decay dominantly into tau-lepton pairs. We explore the associated signals comprised of at least three hadronically-decaying taus, or with at least two hadronic taus and one same-sign-same-flavor charged lepton pair. Our analysis shows that the former signature is challenging to use for getting handles on the signal due to the large corresponding background, and that the latter one can lead to a handful of new physics events in an almost background-free environment. We however find that a signal comprised of three hadronically-decaying tau leptons is likely to be observed at a low luminosity of proton-proton collisions at a 27 TeV upgrade of the LHC.
The historical discovery of neutrino oscillations using solar and atmospheric neutrinos, and subsequent accelerator and reactor studies, has brought neutrino physics to the precision era. We note that CP effects in oscillation phenomena could be difficult to extract in the presence of unitarity violation. As a result upcoming dedicated leptonic CP violation studies should take into account the non-unitarity of the lepton mixing matrix. Restricting non-unitarity will shed light on the seesaw scale, and thereby guide us towards the new physics responsible for neutrino mass generation.
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