We study an extension of the Standard Model with electroweak scale right-handed singlet fermions $N$ that induces neutrino masses, plus a generic new physics sector at a higher scale $Λ$. The latter is parametrized in terms of effective operators in the language of the $ν$SMEFT. We study its phenomenology considering operators up to $d=6$, where additional production and decay modes for $N$ are present in addition to those arising from the mixing with the active neutrinos. We focus on the production with four-Fermi operators and we identify the most relevant additional decay modes to be $N\to νγ$ and $N\to 3f$. We assess the sensitivity of future Higgs factories on the $ν$SMEFT in regions of the parameter space where the new states decay promptly, displaced or are stable on detector lengths. We show that new physics scale up to $5-60\;$TeV can be explored, depending on the collider considered.
The dark axion portal is a recently introduced portal between the standard model and the dark sector. It connects both the dark photon and the axion (or axion-like particle) to the photon simultaneously through an anomaly triangle. While the vector portal and the axion portal have been popular venues to search for the dark photon and axion, respectively, the new portal provides new detection channels if they coexist. The dark axion portal is not a result of the simple combination of the two portals, and its value is not determined by the other portal values; it should be tested independently. …
We consider an extension of the Standard Model with two singlet leptons, with masses in the electroweak range, that induce neutrino masses via the see-saw mechanism, plus a generic new physics sector at a higher scale, $Λ$. We apply the minimal flavor violation (MFV) principle to the corresponding Effective Field Theory ($ν$SMEFT) valid at energy scales $E \ll Λ$. We identify the irreducible sources of lepton flavor and lepton number violation at the renormalizable level, and apply the MFV ansätz to derive the scaling of the Wilson coefficients of the $ν$SMEFT operators up to dimension six. We…
We investigate the prospects of searching dark sector models via exotic Z-boson decay at future $e^+ e^-$ colliders with Giga Z and Tera Z options. Four general categories of dark sector models: Higgs portal dark matter, vector portal dark matter, inelastic dark matter and axion-like particles, are considered. Focusing on channels motivated by the dark sector models, we carry out a model independent study of the sensitivities of Z-factories in probing exotic decays. The limits on branching ratios of the exotic Z decay are typically $\mathcal{O} (10^{-6} - 10^{-8.5}) $ for the Giga Z and $\math…
We consider an extension of the Standard Model with two right-handed singlet fermions with mass at the electroweak scale that induce neutrino masses, plus a generic new physics sector at a higher scale $Λ$. We focus on the effective operators of lowest dimension $d=5$, which induce new production and decay modes for the singlet fermions. We assess the sensitivity of future Higgs Factories, such as FCC-ee, CLIC-380, ILC and CEPC, to the coefficients of these operators for various center of mass energies. We show that future lepton colliders can test the cut-off of the theory up to $Λ\simeq 500 …
We study displaced vertex signatures of long-lived particles (LLPs) from exotic Higgs decays in the context of a Higgs-portal model and a neutral-naturalness model at the CEPC and FCC-ee. Such two models feature two representative mass ranges for LLPs, which show very different behavior in their decay signatures. The Higgs-portal model contains a very light sub-GeV scalar boson stemming from a singlet scalar field appended to the Standard Model. Such a light scalar LLP decays into a pair of muons or pions, giving rise to a distinctive signature of collimated muon-jet or pion-jet, thanks to the…
We introduce a dark neutrino sector which respects a hidden $U(1)^\prime$ gauge symmetry, subsequently broken by the vacuum expectation value of a dark scalar. The model is a self-consistent realisation of an extended hidden sector that communicates with the SM only via the three renormalizable portals, namely neutrino, vector and scalar mixing. The interplay between portal couplings leads to several novel signatures in heavy neutrino, dark photon, and dark scalar searches, typically characterised by multi-leptons plus missing energy and displaced vertices. A striking signature arises in kaon …
It is now a firmly established fact that all family lepton numbers are violated in Nature. In this paper we discuss the implications of this observation for future searches for rare tau decays in the supersymmetric see-saw model. Using the two loop renormalization group evolution of the soft terms and the Yukawa couplings we show that there exists a lower bound on the rate of the rare process mu --> e gamma of the form BR(mu --> e gamma) > C BR(tau --> mu gamma) BR(tau --> e gamma), where C is a constant that depends on supersymmetric parameters. Our only assumption is the absence of cancellat…
Inflation occurring at energy densities less than (10$^{14}$ GeV)$^4$ produces tensor perturbations too small to be measured by cosmological surveys. However, we show that it is possible to probe low scale inflation by measuring the mass of the inflaton at low energy experiments. Detection prospects and cosmological constraints are determined for low scale quartic hilltop models of inflation paired with a curvaton field, which imprints the spectrum of scalar perturbations observed in large scale structure and on the cosmic microwave background. With cosmological constraints applied, low scale …
We present a realistic, simple and natural model of strongly-interacting dark matter based on the neutrino-portal paradigm. The strong interactions at small velocities are generated by the exchange of dark photons, and produce the observed core-like DM distribution in galactic centers; this effect could be spoiled by the formation of DM bound states (also due to dark-photon effects), which we avoid by requiring the DM candidates to be light, with masses below O(10 GeV). The mixing of the dark photon with the Z and ordinary photon is strongly suppressed by introducing a softly-broken discrete s…
The Standard Model with an added Higgs portal interaction and no explicit mass terms is a classically scale-invariant theory. In this case the scale of electroweak symmetry breaking can be induced radiatively by the Coleman-Weinberg mechanism operational in a hidden sector, and then transmitted to the Standard Model through the Higgs portal. The smallness of the generated values for the Higgs vev and mass, compared to the UV cutoff of our classically scale-invariant effective theory, is naturally explained by this mechanism. We show how these classically conformal models can generate the baryo…
We study the viability of pseudo Nambu-Goldstone bosons (Majorons) arising in see-saw models as dark matter candidates. Interestingly the stability of the Majoron as dark matter is related to the scale that sets the see-saw and leptogenesis mechanisms, while its annihilation and scattering cross section off nuclei can be set through the Higgs portal. For $\mathcal{O}(GeV) - \mathcal{O}(TeV)$ Majorons, we compute observables such as the abundance, scattering cross section, Higgs invisible decay width, and emission lines and compare with current data in order to outline the excluded versus still…
We explore the sensitivity of flavour changing b -> s transitions to a (sub-)GeV hidden sector with generic couplings to the Standard Model through the Higgs, vector and axion portals. The underlying two-body decays of B mesons, B -> X_s S and B0 -> SS, where S denotes a generic new GeV-scale particle, may significantly enhance the yield of monochromatic lepton pairs in the final state via prompt decays of S to a dilepton pair. Existing measurements of the charged lepton spectrum in neutral-current semileptonic B decays provide bounds on the parameters of the light sector that are significantl…
It is often said that asymmetric dark matter is light compared to typical weakly interacting massive particles. Here we point out a simple scheme with a neutrino portal and $\mathcal{O}(60 \text{ GeV})$ asymmetric dark matter which may be ''added'' to any standard baryogenesis scenario. The dark sector contains a copy of the Standard Model gauge group, as well as (at least) one matter family, Higgs, and right-handed neutrino. After baryogenesis, some lepton asymmetry is transferred to the dark sector through the neutrino portal where dark sphalerons convert it into a dark baryon asymmetry. Dar…
Confining hidden sectors are an attractive possibility for physics beyond the Standard Model (SM). They are especially motivated by neutral naturalness theories, which reconcile the lightness of the Higgs with the strong constraints on colored top partners. We study hidden QCD with one light quark flavor, coupled to the SM via effective operators suppressed by the mass $M$ of new electroweak-charged particles. This effective field theory is inspired by a new tripled top model of supersymmetric neutral naturalness. The hidden sector is accessed primarily via the $Z$ and Higgs portals, which als…
We investigate the sensitivity to long-lived dark photons produced in Higgstrahlung events via the Higgs portal, $H \rightarrow γ_{D} γ_{D}$, with the Silicon Detector (SiD) at the International Linear Collider (ILC). These events provide a useful benchmark for tracking and vertex detector performance with long-lived particle decays at displaced vertices. The ILC is one of several Higgs factories proposed by the international community to study the properties of the Higgs boson at high precision.
We point out a novel role for the Standard Model neutrino in dark matter phenomenology where the exchange of neutrinos generates a long-range potential between dark matter particles. The resulting dark matter self interaction could be sufficiently strong to impact small-scale structure formation, without the need of any dark force carrier. This is a generic feature of theories where dark matter couples to the visible sector through the neutrino portal. It is highly testable with improved decay rate measurements at future $Z$, Higgs, and $τ$ factories, as well as precision cosmology.
The Higgs sector in neutral naturalness models provides a portal to the hidden sectors, and thus measurements of Higgs couplings at current and future colliders play a central role in constraining the parameter space of the model. We investigate a class of neutral naturalness models, in which the Higgs boson is a pseudo-Goldstone boson from the universal SO(N)/SO(N-1) coset structure. Integrating out the radial mode from the spontaneous global symmetry breaking, we obtain various dimension-six operators in the Standard Model effective field theory, and calculate the low energy Higgs effective …
We currently live in the age of the CKM paradigm. The $3\times 3$ matrix that links $(d,\ s,\ b)$ quarks to $(u,\ c,\ t)$ in the charged current weak interaction, being complex and nominally with 18 parameters, can be accounted for by just 3 rotation angles and one $CP$ violating (CPV) phase, with unitarity and the CKM phases triumphantly tested at the B factories. But the CKM picture is unsatisfactory and has too many parameters. The main aim of Flavor Physics and $CP$ violation (FPCP) studies is the pursuit to uncover New Physics beyond the Standard Model (SM). Two highlights of LHC Run~1 pe…
