Violation Of Unitarity Research Articles

Symmetry in Neutrino Oscillation in Matter: New Picture and the νSM–Non-Unitarity Interplay

We update and summarize the present status of our understanding of the reparametrization symmetry with an i↔j state exchange in neutrino oscillation in matter. We introduce a systematic method called “Symmetry Finder” (SF) to uncover such symmetries, demonstrate its efficient hunting capability, and examine their characteristic features. Apparently they have a local nature: the 1–2 and 1–3 state exchange symmetries exist at around the solar and atmospheric resonances, respectively, with the level-crossing states exchanged. However, this view is not supported, to date, in the globally valid Denton et al. (DMP) perturbation theory, which possesses the 1–2, but not the 1–3, exchange symmetry. This is probably due to our lack of understanding, and we find a clue for a larger symmetry structure than we know of. In the latter part of this article, we introduce non-unitarity, or unitarity violation (UV), into the νSM neutrino paradigm, a low-energy description of beyond νSM new physics at a high (or low) scale. Based on the analyses of UV extended versions of the atmospheric resonance and the DMP perturbation theories, we argue that the reparametrization symmetry has a diagnostic capability for the theory with the νSM and UV sectors. Speculation is given on the topological nature of the identity, which determines the transformation property of the UV α parameters.

Bounds on anomalous quartic WWZγ couplings in e – p collisions at the FCC-he

Non-Abelian structure of the standard model (SM) predicts the self-interactions of gauge bosons triple gauge couplings (TGC) and quartic gauge couplings (QGC). On the other hand, it is also important to determine the deviations from SM via anomalous triple gauge couplings and anomalous quartic gauge couplings (aQGC) to test the nature of the SM and to see the effects of new physics arising from the beyond the standard model. In this study, we focus on the e − p → jZ γ ν e → j(l + l −)γ ν e process to examine the WWZ γ aQGC at the Future Circular Collider-hadron electron with center-of-mass energy TeV using a model-independent way in the effective theory approach. A Cut-based method is applied to analyse the signal and relevant SM background. Using the total cross-sections, we constrained the anomalous f T,i=0,1,2,5,6,7/Λ4 couplings arising from dimension-8 operators at 95% confidence level for the dilepton decay of the Z-boson. The results are composed of integrated luminosities of and 3 ab −1 under the systematic uncertainties of 0%, 5% and 10%. In the calculations, we use the form factor to eliminate the effects of the violation of unitarity. With this, we obtained the limits for the energy scale of the form factor Λ = 1 TeV. In addition to all this, we use the advantages of ep collisions for analysing the process with much cleaner environment and lower background effects compared with the pp collisions. The obtained sensitivities on f T, i=0, 1, 2, 5, 6, 7/Λ4 are comparable with the experimental results and related phenomenological studies in the literature.

Perturbative unitarity and NEC violation in genesis cosmology

Explorations of the violation of null energy condition (NEC) in cosmology could enrich our understanding of the very early universe and the related gravity theories. Although a fully stable NEC violation can be realized in the “beyond Horndeski” theory, it remains an open question whether a violation of the NEC is allowed by some fundamental properties of UV-complete theories or the consistency requirements of effective field theory (EFT). We investigate the tree-level perturbative unitarity for stable NEC violations in the contexts of both Galileon and “beyond Horndeski” genesis cosmology, in which the universe is asymptotically Minkowskian in the past. We find that the constraints of perturbative unitarity imply that we may need some unknown new physics below the cut-off scale of the EFT other than that represented by the “beyond Horndeski” operators.

Unitarity in multi-Higgs production using the Schwinger–Dyson equation

In spontaneously broken scalar theories, perturbative and semi-classical calculations show that multi-particle production grows rapidly with the number of scalar particles at sufficiently high energies. According to recent computations, for λn ≫ 1, where n is the multiplicity and λ is the self-coupling, the amplitude grows exponentially with the energy, resulting in a divergent propagator and predicting the violation of perturbative unitarity. In this paper, in the scalar theory the self-consistent Schwinger–Dyson equation is solved in the spectral representation and the transition rate is calculated. We find an amplitude growing quadratically with the energy which leads to an asymptotically decreasing propagator.

Beta-decay implications for the W -boson mass anomaly

We point out the necessity to consider $\beta$-decay observables in resolutions of the $W$-boson anomaly in the Standard Model Effective Field Theory that go beyond pure oblique corrections. We demonstrate that present global analyses that explain the $W$-boson mass anomaly predict a large, percent-level, violation of first-row CKM unitarity. We investigate what solutions to the $W$-boson mass anomaly survive after including $\beta$-decay constraints.

Phase transitions as a manifestation of spontaneous unitarity violation

Spontaneous symmetry breaking is well understood under equilibrium conditions as a consequence of the singularity of the thermodynamic limit. How a single global orientation of the order parameter dynamically emerges from an initially symmetric state during a phase transition, however, is not captured by this paradigm. Here, we present a series of symmetry arguments suggesting that singling out a global choice for the ordered state is in fact forbidden under unitary time evolution, even in the presence of an environment and infinitesimal symmetry breaking perturbations. We thus argue that the observation of phase transitions in our everyday world presents a manifestation of the unitarity of quantum dynamics itself being spontaneously broken. We argue that this agrees with the observation that Schrödinger’s time dependent equation is rendered unstable for macroscopic objects owing to the same singular thermodynamic limit that affects equilibrium configurations.

Entanglement between two gravitating universes

We study two disjoint universes in an entangled pure state. When only one universe contains gravity, the path integral for the nth Rényi entropy includes a wormhole between the n copies of the gravitating universe, leading to a standard ‘island formula’ for entanglement entropy consistent with unitarity of quantum information. When both universes contain gravity, gravitational corrections to this configuration lead to a violation of unitarity. However, the path integral is now dominated by a novel wormhole with 2n boundaries connecting replica copies of both universes. The analytic continuation of this contribution involves a quotient by replica symmetry, giving a cylinder connecting the two universes. When entanglement is large, this configuration has an effective description as a ‘swap wormhole’, a geometry in which the boundaries of the two universes are glued together by a ‘swaperator’. This description allows precise computation of a generalized entropy-like formula for entanglement entropy. The quantum extremal surface computing the entropy lives on the Lorentzian continuation of the cylinder/swap wormhole, which has a connected Cauchy slice stretching between the universes—a realization of the ER = EPR idea. The new wormhole restores unitarity of quantum information.

New physics searches in a low threshold scintillating argon bubble chamber measuring coherent elastic neutrino-nucleus scattering in reactors

The sensitivity to New Physics of a low threshold scintillating argon bubble chamber measuring coherent elastic neutrino-nucleus scattering in reactors is reported. Namely, light scalar mediators, sterile neutrino oscillations, unitarity violation, and non-standard interactions are studied. The results indicate that this detector could be able to set stronger constraints than current limits set by the recent COHERENT measurements. Considering the best scenario, a 100 kg detector located 30 m from a 2000 MW$_{th}$ reactor, a sterile neutrino search would cover most of the space parameter allowed from the reactor anti-neutrino anomaly fit. Unitarity violation studies could set constraints on $\alpha_{11}$ more stringent than the current oscillation experiments fit. A low threshold argon detector with very low backgrounds has the potential to explore New Physics in different scenarios and set competitive constraints.

Tree-level unitarity in Higgs inflation in the metric and the Palatini formulation

We calculate the tree-level amplitudes for electrically neutral 2 → 2 scattering for the Standard Model Higgs doublet non-minimally coupled to the Ricci scalar. We consider both the metric and the Palatini formulation of gravity. We find the partial wave unitarity limit for a general background field value. Our results are in agreement with previous work. In the electroweak vacuum, tree-level unitarity is violated at ∼ MPl/ξ in the metric formulation, and at sim {M}_{mathrm{P}1}/sqrt{xi } in the Palatini formulation. In the inflationary large field background, the unitarity limit is at sim {M}_{mathrm{P}1}/sqrt{xi } in both formulations. We compare the unitarity violation energy to scales relevant during inflation. We also calculate a direct collider limit on ξ in the Palatini formulation, ξ < 2.5 × 1031.

New oscillation and scattering constraints on the tau row matrix elements without assuming unitarity

The tau neutrino is the least well measured particle in the Standard Model. Most notably, the tau neutrino row of the lepton mixing matrix is quite poorly constrained when unitarity is not assumed. In this paper, we identify data sets involving tau neutrinos that improve our understanding of the tau neutrino part of the mixing matrix, in particular ντ appearance in atmospheric neutrinos. We present new results on the elements of the tau row leveraging existing constraints on the electron and muon rows for the cases of unitarity violation, with and without kinematically accessible steriles. We also show the expected sensitivity due to upcoming experiments and demonstrate that the tau neutrino row precision may be comparable to the muon neutrino row in a careful combined fit.

Toward diagnosing neutrino non-unitarity through CP phase correlations

Abstract We discuss correlations between the neutrino-mass-embedded Standard Model CP phase δ and the phases that originate from new physics which cause neutrino-sector unitarity violation (UV) at low energies. This study aims to provide one of the building blocks for machinery to diagnose non-unitarity, our ultimate goal. We extend the perturbation theory of neutrino oscillation in matter proposed by Denton et al. (DMP) to include the UV effect expressed by the α parametrization. By analyzing the DMP-UV perturbation theory to first order, we are able to draw a complete picture of the δ–UV phase correlations in the whole kinematical region covered by terrestrial neutrino experiments. Two regions exist with characteristically different patterns of the correlations: (i) the chiral-type $[e^{- i \delta } \alpha _{\mu e}, \, e^{ - i \delta } \alpha _{\tau e}, \, \alpha _{\tau \mu }]$ (Particle Data Group convention) correlation in the entire high-energy region $\vert \rho E \vert \gtrsim 6 \, (\text{g/cm}^3)$ GeV, and (ii) (blobs of the α parameters)–e±iδ correlation anywhere else. Some relevant aspects for the measurement of the UV parameters, such as the necessity of determining all the αβγ elements at once, are also pointed out. Subject Index: B52, B54

Time orientability and particle production from universal horizons

We discuss particle production in spacetimes endowed with a universal horizon in Einstein--Aether and Horava gravity. We argue that continuity and differentiability of the lapse function require the orientation of the foliation in the interior of the horizon to be reversed with respect to the exterior one. Unless this is allowed, interaction of gravitating scalar fields with the universal horizon leads to unitarity violations in the quantum theory. This property is responsible for particle production by the universal horizon, as we show by computing explicitly its Hawking temperature for all stationary and spherically symmetric spacetimes. We particularize our result to known analytic solutions, including those compatible with observational constraints.

A Phenomenological Approach to Multi-Higgs Production at High Energy

We tackle the issue of the factorial growth in the amplitudes of multi-Higgs production at high energy by developing a phenomenological approach based on the Higgs splitting functions and Sudakov factors. We utilize the method of generating functionals to define several jet observables for the Higgs sector. Our results suggest that pure Higgs splittings should retain a good Ultraviolet (UV) behavior in contrast to the common picture represented by the breakdown of perturbation theory and the violation of unitarity due to the high multiplicity of particles produced at or near threshold, which is found in scalar theories. We thus argue that the issue of the factorial growth in the amplitude of multi-Higgs production is probably associated with applying perturbation theory in a regime where it is no longer valid and with the n\(\lambda\) \(\to\) \(\infty\) limit, as opposed to being a sign of new physics.

Unitarity bounds on effective field theories at the LHC

Effective Field Theory (EFT) extensions of the Standard Model are tools to compute observables (e.g. cross sections with partonic center-of-mass energy sqrt{hat{s}} ) as a systematically improvable expansion suppressed by a new physics scale M. If one is interested in EFT predictions in the parameter space where M < sqrt{hat{s}} , concerns of self-consistency emerge, which can manifest as a violation of perturbative partial-wave unitarity. However, when we search for the effects of an EFT at a hadron collider with center-of-mass energy sqrt{s} using an inclusive strategy, we typically do not have access to the event-by-event value of sqrt{hat{s}} . This motivates the need for a formalism that incorporates parton distribution functions into the perturbative partial-wave unitarity analysis. Developing such a framework and initiating an exploration of its implications is the goal of this work. Our approach opens up a potentially valid region of the EFT parameter space where M ≪ sqrt{s} . We provide evidence that there exist valid EFTs in this parameter space. The perturbative unitarity bounds are sensitive to the details of a given search, an effect we investigate by varying kinematic cuts.

The strong couplings of massive Yang-Mills theory

We study the massive Yang-Mills theory in which the mass term is added by hand. The standard perturbative approach suggests that the massless limit of this theory is not smooth. We confirm that this issue is related to the existence of additional degrees of freedom, which are absent in the massless theory. Nevertheless, we show that the longitudinal modes become strongly coupled at the Vainshtein scale, which coincides with the scale of the unitarity violation. Beyond this scale, they decouple from the remaining degrees of freedom, and the massless theory is restored up to small corrections. From here, it follows that the apparent discontinuity in the massless limit is only an artefact of the perturbation theory. The massless limit of massive Yang-Mills theory is smooth, as originally proposed in [21].

Neutrino mass and lepton flavor violation in A4-based left–right symmetric model with linear seesaw

We explore an [Formula: see text] flavor-based left–right symmetric model where neutrino masses and mixing are explained through linear seesaw mechanism. Taking [Formula: see text] flavor symmetry and left–right gauge symmetry together and adding a low-scale seesaw mechanism gives the advantage of (a) getting diagonal structures for both charged lepton mass matrix and Dirac neutrino mass matrix, (b) simpler relations among neutrino observables in the model and (c) large light-heavy neutrino mixing which gives dominant contributions to various lepton flavor violating decays like [Formula: see text], [Formula: see text], [Formula: see text]. By saturating the experimental bounds on these decays we derive constraints on input model parameters and study analytically as well as numerically the correlation among model parameters and their dependence on experimentally determined neutrino parameters like [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text]. We also study CP-violation via Jarlskog invariants and show extra contributions to CP-violating effects that the model generates due to unitarity violation in the neutrino sector.

ππ scattering in partially-quenched twisted-mass chiral perturbation theory

We study pion-pion scattering in partially-quenched twisted-mass lattice QCD using chiral perturbation theory. The specific partially-quenched setup corresponds to that used in numerical lattice QCD calculations of the $I=0$ scattering length. We study the discretization errors proportional to $a^2$, with $a$ the lattice spacing, and the errors that arise due to the use of L\"uscher's two-particle quantization condition in a theory that is not unitary. We argue that the former can be as large as $\sim 100\%$, but explain how they can be systematically subtracted using a calculation of the $I=2$ scattering amplitude in the same partially-quenched framework. We estimate the error from the violation of unitarity to be $\sim 25\%$, and argue that this error will be difficult to reduce in practice.

Z mediated flavor changing neutral currents with a fourth vectorlike family

We discuss $Z$ mediated flavor changing neutral currents within a model where the hierarchical quark and lepton masses are explained via a fourth vectorlike family, together with a scalar sector consisting of two Higgs doublets augmented by a gauge singlet scalar field that spontaneously breaks an extra global $U(1{)}^{\ensuremath{'}}$ symmetry. The $Z$ mediated flavor violating interactions arise from the mixings between the SM fermions and the vectorlike fermions, where the mixing is discussed in an analytic approximation and also exactly numerically. We first discuss charged lepton flavor violating (CLFV) $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma}$, $\ensuremath{\tau}\ensuremath{\rightarrow}3\ensuremath{\mu}$ and $Z\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau}$ decays and find that they cannot significantly constrain the masses of charged vectorlike leptons. However, the 790 GeV mass bound arising from collider searches on vectorlike lepton doublets can set further constraints on the model parameter space. We also consider rare $t\ensuremath{\rightarrow}cZ$ decays as well as unitarity violation in the CKM mixing in order to constrain the quark sector of the model under consideration.

Unitarity violation and the geometry of Higgs EFTs

We derive the scale of unitarity violation from the geometry of Effective Field Theory (EFT) extensions of the Standard Model Higgs sector. The high-energy behavior of amplitudes with more than four scalar legs depends on derivatives of geometric invariants with respect to the physical Higgs field h, such that higher-point amplitudes begin to reconstruct the scalar manifold away from our vacuum. In theories whose low-energy limit can be described by the Higgs EFT (HEFT) but not the Standard Model EFT (SMEFT), non-analyticities in the vicinity of our vacuum limit the radius of convergence of geometric invariants, leading to unitarity violation at energies below 4πv. Our results unify approaches to the HEFT/SMEFT dichotomy based on unitarity, analyticity, and geometry, and more broadly illustrate the sense in which observables probe the geometry of an EFT. Along the way, we provide novel basis-independent results for Goldstone/Higgs boson scattering amplitudes expressed in terms of geometric covariant quantities.

Non-unitary leptonic flavor mixing and CP violation in neutrino-antineutrino oscillations

If massive neutrinos are Majorana particles, then the lepton number should be violated in nature and neutrino-antineutrino oscillations να↔ν‾β (for α,β=e,μ,τ) will definitely take place. In the present paper, we study the properties of CP violation in neutrino-antineutrino oscillations with the non-unitary leptonic flavor mixing matrix, which is actually a natural prediction in the canonical seesaw model due to the mixing between light and heavy Majorana neutrinos. The oscillation probabilities P(να→ν‾β) and P(ν‾α→νβ) are derived, and the CP asymmetries Aαβ≡[P(να→ν‾β)−P(ν‾α→νβ)]/[P(να→ν‾β)+P(ν‾α→νβ)] are also calculated. Taking into account current experimental bounds on the leptonic unitarity violation, we show that the CP asymmetries induced by the non-unitary mixing parameters can significantly deviate from those in the limit of a unitary leptonic flavor mixing.