Abstract
Exotic self-interactions among the Standard-Model neutrinos have been proposed as a potential reason behind the tension in the expansion rate, H0, of the universe inferred from different observations. We constrain this proposal using electroweak precision observables, rare meson decays, and neutrinoless double-\{beta} decay. In contrast to previous works, we emphasize the importance of carrying out this study in a framework with full Standard-Model gauge invariance. We implement this first by working with a relevant set of Standard- Model-Effective-Field-Theory operators and subsequently by considering a UV completion in the inverse See-Saw model. We find that the scenario in which all flavors of neutrinos self-interact universally is strongly constrained, disfavoring a potential solution to the H0 problem in this case. The scenario with self-interactions only among tau neutrinos is the least constrained and can potentially be consistent with a solution to the H0 problem.
Highlights
There is a tantalizing discrepancy between the value of the Hubble constant (H0) extracted from local measurement versus the one extracted from the cosmic microwave background (CMB) data [1,2,3,4,5].Towards this end, the authors of Ref. [6] suggested to give neutrinos a new, extra strong self-coupling in the form of the dimension-six operatorLeff ∼ GνðνMνMÞðνMνMÞ: ð1ÞWe focus on the possibility that the Standard Model (SM) neutrinos are Majorana, i.e., νM are four-component Majonara fermion fields
In the previous section we considered the particlephysics constraints in conjunction with the preferred region from the CMB fit within the mostly model-independent framework of Standard Model Effective Field Theory (SMEFT)
Motivated by the approach of using neutrino selfinteractions to address the tension in the H0 measurement, we investigated the experimental constraints on this scenario
Summary
There is a tantalizing discrepancy between the value of the Hubble constant (H0) extracted from local measurement versus the one extracted from the cosmic microwave background (CMB) data [1,2,3,4,5]. Most of the studies focus on the effective neutrino-Majoron coupling in Eq (2), which violates electroweak gauge invariance This is perfectly fine as long as one focuses on the degrees of freedom well below the weak scale. We will parameterize their effect by dimension-five and six effective operators in the Standard Model Effective Field Theory (SMEFT) One such dimension-six operator contains the Majoron and induces neutrino self-interactions. The former is realized in the Type-I seesaw model while the latter in Type II In both cases, the mixing angle determines the strength of the neutrino-Majoron coupling. The case in which only tau-flavor neutrinos self-interact may still provide a solution due to the weaker constraints from particle-physics observations.
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