Scalar Self-interactions Research Articles

Reissner-Nordström dyonic black holes with gauged scalar hair

For gauged scalar fields minimally coupled to Einstein-Maxwell theory, the Mayo-Bekenstein no-hair theorem can be circumvented when including appropriate scalar self-interactions, allowing static, electrically charged black holes to be endowed with (Abelian) gauged scalar hair. Here we show these spherically symmetric solutions can be extended to include a magnetic charge in a model with scalar multiplets. The resulting dyonic configurations share most of the properties of the electrically charged solutions, in particular satisfying the same resonance condition, with the existence of a mass gap with respect to the bald Reissner-Nordström dyonic black holes. A distinctive feature, however, is that no solitonic limit exists for a non-zero magnetic charge.

Scale symmetry breaking and generation of mass at quantum critical points

We study an asymptotically free theory of N relativistic Dirac fermions and a real scalar field coupled by Yukawa and scalar self-interactions in three dimensions using functional renormalisation. In the limit of many fermion flavours, the cubic scalar coupling becomes exactly marginal due to quantum fluctuations, leading to a line of strongly-coupled infrared fixed points. Fermion mass can be generated through a quantum phase transition even if chiral symmetry is absent. The line of fixed points terminates at a critical endpoint due to the loss of vacuum stability. Exactly at the endpoint, scale symmetry is broken spontaneously, leading to the generation of fermion mass. Intriguingly, the absence of chiral symmetry is a prerequisite for the spontaneous generation of fermion mass, and not a consequence thereof. We also highlight close similarities between Gross-Neveu and Gross-Neveu-Yukawa theories at and away from critical points, and establish the large-N equivalence of their functional RG flows and quantum effective actions. Further implications including for conformal field theories are indicated.

Axio-Chameleons: a novel string-friendly multi-field screening mechanism

Scalar-tensor theories with the shift symmetries required by light scalars are well-explored modifications to GR. For these, two-derivative scalar self-interactions usually dominate at low energies and interestingly compete with the two-derivative metric interactions of GR itself. Although much effort has been invested in single scalars (on grounds of simplicity) these happen to have no two-derivative interactions, requiring such models to explore higher-derivative interactions (that usually would be less important at low-energies). This suggests multiple-scalar sigma models as well-motivated candidates for finding new phenomena in tests of gravity. We identify a new multi-field screening mechanism appropriate for two light scalar fields (an axion and a Brans-Dicke style dilaton) that relies on their mutual two-derivative interactions. We show how very weak axion-matter couplings can introduce axion gradients that can reduce the apparent coupling of the Brans-Dicke scalar to macroscopic matter sources. We further identify a relaxation mechanism that allows this reduction to be amplified to a suppression by the ratio of the axion gradient's length scale to the source's radius (similar in size to the suppression found in Chameleon models). Unlike some screening mechanisms our proposal is technically natural and works deep within the regime of control of the low-energy EFT. It uses only ingredients that commonly appear in the low-energy limit of string vacua and so is likely to have wider applications to models that admit UV completions. We briefly discuss phenomenological implications and challenges for this scenario, which suggests re-examination of decay loss bounds and the value of equivalence-principle tests for different-sized objects.

New boundary conditions in Einstein-scalar gravity in three dimensions

We analyze the backreaction of a class of scalar field self-interactions with the possibility of evolving from an AdS vacuum to a fixed point where the scalar field potential vanishes. Exact solutions which interpolate between these regions, ranging from stationary black hole to dynamical spacetimes are constructed. Their surface charges are finite but non-integrable. We study the properties of these charges on the solutions. In particular, we show that the integrable part of the charges provides a realization of the conformal algebra by means of a modification of the Dirac bracket proposed by Barnich and Troessaert. The latter construction allows for a field dependent central extension, whose value tends to the Brown-Henneaux central charge at late times.

Core-collapse supernova constraint on the origin of sterile neutrino dark matter via neutrino self-interactions

Novel neutrino self-interaction can open up viable parameter space for the relic abundance of sterile-neutrino dark matter (SνDM). In this work, we constrain the relic target using core-collapse supernova which features the same fundamental process and a similar environment to the early universe era when SνDM is dominantly produced. We present a detailed calculation of the effects of a massive scalar mediated neutrino self-interaction on the supernova cooling rate, including the derivation of the thermal potential in the presence of non-zero chemical potentials from plasma species. Our results demonstrate that the supernova cooling argument can cover the neutrino self-interaction parameter space that complements terrestrial and cosmological probes.

Lorentz symmetry violating Lifshitz-type field theories

In this paper, we discuss the ultraviolet sector of (3 + 1)-dimensional Lifshitz-type anisotropic higher derivative scalar, fermion and gauge field theories, with anisotropy exponent [Formula: see text] and with explicit breaking of Lorentz symmetry. By discarding from the action of all momentum-dependent vertex operators, which is essential to avoid phenomenologically unacceptable deformations of the light cone, we find that renormalizable scalar self-interaction and Yukawa-like couplings are, in general, asymptotically free. However, the requirement of cancelling momentum-dependent vertex operators is incompatible with gauge symmetry and, therefore, for this kind of theories, gauge symmetry as well as Lorentz symmetry are recovered only as emergent properties below some energy scale [Formula: see text], that must be constrained from experiments. The quantum corrections to the scalar mass and their impact on the hierarchy problem are also analyzed.

Probing modified gravity with integrated Sachs-Wolfe CMB and galaxy cross-correlations

We use the cross-correlation power spectrum of the integrated Sachs-Wolfe (ISW) effect in the cosmic microwave background (CMB) temperature anisotropy and galaxy fluctuations to probe the physics of late-time cosmic acceleration. For this purpose, we focus on three models of dark energy that belong to a sub-class of Horndeski theories with the speed of gravity equivalent to that of light: Galileon Ghost Condensate (GGC), Generalized Cubic Covariant Galileon (GCCG), and K-mouflage. In the GGC and GCCG models, the existence of cubic-order scalar self-interactions allows a possibility for realizing negative ISW-galaxy cross-correlations, while the K-mouflage model predicts a positive correlation similar to the Λ-cold-dark-matter (ΛCDM) model. In our analysis, we fix the parameters of each model to their best-fit values derived from a baseline likelihood analysis with observational data from CMB, baryon acoustic oscillations, and supernovae type Ia. Then we fit those best-fit models to the ISW-galaxy cross-correlation power spectrum extracted from a collection of photometric redshift surveys. We find that both GGC and GCCG best-fit models degrade the fit to the ISW-galaxy cross-correlation data compared to ΛCDM best-fit model. This is attributed to the fact that, for their best-fit values constrained from the baseline likelihood, the cubic-order scalar self-interaction gives rise to suppressed ISW tails relative to ΛCDM. The K-mouflage best-fit model is largely degenerate with the ΛCDM best-fit model and has a positively correlated ISW-galaxy power close to that of ΛCDM.

Inflation with Sterile Scalar Coupled to Massive Fermions and to Gravity

In the recent paper [3] it was shown that the consistency of the quantum theory of a sterile scalar coupled to massive fermions requires the inclusion of odd-power terms in the potential of scalar self-interaction. One of the most important examples of a sterile scalar is the inflaton, that is typically a real scalar field which does not belong to representations of particle physics gauge groups, such as $SU(2)$. Here we explore the effects of the odd-power terms in the inflaton potential on main observables, such as the scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$, in the case of a strong, non-minimal coupling of the inflaton to gravity.

Kaon–baryon coupling schemes and kaon condensation in hyperon-mixed matter

Abstract The possible coexistence of kaon condensation and hyperons in highly dense matter [the (Y + K) phase] is investigated on the basis of the relativistic mean-field theory combined with the effective chiral Lagrangian. Two coupling schemes for the s-wave kaon–baryon interaction are compared regarding the onset density of kaon condensation in hyperon-mixed matter and the equation of state for the developed (Y + K) phase. One is the contact interaction scheme related to the nonlinear effective chiral Lagrangian. The other is the meson exchange scheme, where the interaction vertices between the kaon field and baryons are described by an exchange of mesons (σ, σ* mesons for scalar coupling, and ω, ρ, ϕ mesons for vector coupling). It is shown that in the meson exchange scheme, the contribution from the nonlinear scalar self-interaction gives rise to a repulsive effect for the kaon effective energy, pushing up the onset density of kaon condensation as compared with the contact interaction scheme. In general, the difference in kaon–baryon dynamics between the contact interaction scheme and the meson exchange scheme relies on the specific forms of the nonlinear self-interacting meson terms. They generate many-baryon forces through the equations of motion for the meson mean fields. However, they should have a definite effect on the ground state properties of nuclear matter only around the saturation density. It is shown that the nonlinear self-interacting term is not relevant to repulsive energy leading to stiffening of the equation of state at high densities, and that it cannot be compensated with a large attractive energy due to the appearance of the (Y + K) phase in the case of the contact interaction scheme. We also discuss what effects are necessary in the contact interaction scheme to make the equation of state with (Y + K) phase stiff enough to be consistent with recent observations of massive neutron stars.

Probing the Ellis-Bronnikov wormhole geometry with a scalar field: Clouds, waves and Q-balls

The Ellis-Bronnikov solution provides a simple toy model for the study of various aspects of wormhole physics. In this work we solve the Klein-Gordon equation in this background and find an exact solution in terms of Heun's function. This may describe ‘scalar clouds’ (i.e. localized, particle-like configuration) or scalar waves. However, in the former case, the radial derivative of the scalar field is discontinuous at the wormhole's throat (except for the spherical case). This pathology is absent for a suitable scalar field self-interaction, and we provide evidence for the existence of spherically symmetric and spinning Q-balls in a Ellis-Bronnikov wormhole background.

Effective potential of scalar-tensor gravity with quartic self-interaction of scalar field

One-loop effective potential of scalar-tensor gravity with a quartic scalar field self-interaction is evaluated up to first post-Minkowskian order. The potential develops an instability in the strong field regime which is expected from an effective theory. Depending on model parameters the instability region can be exponentially far in a strong field region. Possible applications of the model for inflationary scenarios are highlighted. It is shown that the model can enter the slow-roll regime with a certain set of parameters.

Scalar-tensor theories within Asymptotic Safety

Asymptotic Safety provides an elegant mechanism for obtaining a consistent high-energy completion of gravity and gravity-matter systems. Following the initial idea by Steven Weinberg, the construction builds on an interacting fixed point of the theories renormalization group (RG) flow. In this work we use the Wetterich equation for the effective average action to investigate the RG flow of gravity supplemented by a real scalar field. We give a non-perturbative proof that the subspace of interactions respecting the global shift-symmetry of the scalar kinetic term is closed under RG transformations. Subsequently, we compute the beta functions in an approximation comprising the Einstein-Hilbert action supplemented by the shift-symmetric quartic scalar self-interaction and the two lowest order shift-symmetric interactions coupling scalar-bilinears to the spacetime curvature. The computation utilizes the background field method with an arbitrary background, demonstrating that the results are manifestly background independent. Our beta functions exhibit an interacting fixed point suitable for Asymptotic Safety, where all matter interactions are non-vanishing. The presence of this fixed point is rooted in the interplay of the matter couplings which our work tracks for the first time. The relation of our findings with previous results in the literature is discussed in detail and we conclude with a brief outlook on potential phenomenological applications.

Scalar self-interaction in the spacetime of a cosmic dispiration

In the present paper we investigate the classical self-interaction associated with a charged scalar point particle placed at rest in the gravitational field of a linear topological defect known in literature as cosmic dispiration, a combination of a cosmic string with a screw dislocation. We found exact expressions for the Green's function and, consequently, for the self-energy by considering both massive and massless scalar point particles. We present numerical graphs and analyze the nature of the self-interactions in the cosmic dispiration spacetime itself and in the cases of pure screw dislocation and cosmic string spacetimes.

Remarks on the Dirac Kepler/Coulomb problem with a space-dependent mass term

We investigate the Dirac Kepler/Coulomb (KC) problem in -dimensions where the charged particle (an electron) is simultaneously affected by a position-dependent effective mass term. In polar coordinates, we examine two instances: (i) the quantum dynamics of an electron moving in the presence of both a Coulomb potential and a Coulomb-like scalar potential. This problem turns to be exactly solvable; (ii) the planar dynamics of a self-interacting electron in a Coulomb potential where nonlinearities are introduced via a scalar self-interaction, known as the Soler model. We obtain the approximate (nonlinear) ground-state energy levels together with their respective eigenfunctions. Thus, within the same framework, we present two different relativistic quantum-mechanical models which can be further explored.

Chiral quark-meson coupling models for finite nuclei and their (e,e′p) reactions

The chiral quark-meson coupling (CQMC) models are applied to revisiting static properties of spherical nuclei in comparison with the results from the conventional quantum hadrodynamics (QHD)-like models as well as the available experimental data. They are also used to describe the nuclei off which the electrons scatter to understand their dynamic properties. After calibrating the model parameters at equilibrium nuclear matter density, binding energies, charge radii, single-particle spectra, and density distributions of the nuclei are analyzed and compared. The nonlinear scalar self-interaction in each model is also discussed in consideration of the reproduction of nuclear saturation properties.

Effects of nonstandard neutrino self-interactions and magnetic moment on collective Majorana neutrino oscillations

We derive the effective Hamiltonian describing collective oscillations of Majorana neutrinos with a transition magnetic moment, allowing for the presence of scalar and pseudoscalar nonstandard neutrino self-interactions (NSSIs). Using this Hamiltonian, we analyze new flavor instability channels of collective oscillations in a core-collapse supernova environment that open up in the presence of a small but nonzero neutrino magnetic moment. It turns out that, contrary to certain claims in the literature, within the minimally extended Standard Model (i.e., without NSSIs), no new instabilities arise within the linear order, nor do they produce any observable signatures in the neutrino flavor-energy spectra, at least for magnetic moments up to $10^{-15}~\mu_{\text{B}}$ and quite realistic fields of the order of $10^{12}~$Gauss. On the other hand, in the presence of NSSIs, new fast and slow instabilities mixing neutrinos and antineutrinos appear, which show up in the spectra even for tiny magnetic moments of the order of $10^{-24}~\mu_{\text{B}}$, leading to considerable distortions of the spectra and nonstandard spectral splits. We study sensitivity of collective oscillations to these, NSSI-induced instabilities in detail and discuss the observability of the NSSI couplings triggering them.

The imitation game: Proca stars that can mimic the Schwarzschild shadow

Can a dynamically robust bosonic star (BS) produce an (effective) shadow that mimics that of a black hole (BH)? We focus on models of spherical BSs with free scalar or vector fields, as well as with polynomial or axionic self-interacting fields. The BH shadow is linked to the existence of light rings (LRs). For free bosonic fields, yielding mini-BSs, it is known that these stars can become ultra-compact — i.e., possess LRs — but only for perturbatively unstable solutions. We show this remains the case even when different self-interactions are considered.However, an effective shadow can arise in a different way: if BSs reproduce the existence of an innermost stable circular orbit (ISCO) for timelike geodesics (located at r ISCO = 6M for a Schwarzschild BH of mass M), the accretion flow morphology around BHs is mimicked and an effective shadow arises in an astrophysical environment. Even though spherical BSs may accommodate stable timelike circular orbits all the way down to their centre, we show the angular velocity Ω along such orbits may have a maximum away from the origin, at R Ω; this scale was recently observed to mimic the BH's ISCO in some scenarios of accretion flow. Then: (i) for free scalar fields or with quartic self-interactions, R Ω ≠ 0 only for perturbatively unstable BSs; (ii) for higher scalar self-interactions, e.g. axionic, R Ω ≠ 0 is possible for perturbatively stable BSs, but no solution with R Ω = 6M was found in the parameter space explored; (iii) but for free vector fields, yielding Proca stars, perturbatively stable solutions with R Ω ≠ 0 exist, and indeed R Ω = 6M for a particular solution. Thus, dynamically robust spherical Proca stars succeed in the imitation game: they can mimic the shadow of a (near-)equilibrium Schwarzschild BH with the same M, in an astrophysical environment, despite the absence of a LR, at least under some observation conditions, as we confirm by explicitly comparing the lensing of such Proca stars and Schwarzschild BHs.

Stability of rotating scalar boson stars with nonlinear interactions

We study the stability of rotating scalar boson stars, comparing those made from a simple massive complex scalar (referred to as mini boson stars), to those with several different types of nonlinear interactions. To that end, we numerically evolve the nonlinear Einstein-Klein-Gordon equations in 3D, beginning with stationary boson star solutions. We show that the linear, non-axisymmetric instability found in mini boson stars with azimuthal number $m=1$ persists across the entire parameter space for these stars, though the timescale diverges in the Newtonian limit. Therefore, any boson star with $m=1$ that is sufficiently far into the non-relativistic regime, where the leading order mass term dominates, will be unstable, independent of the nonlinear scalar self-interactions. However, we do find regions of $m=1$ boson star parameter space where adding nonlinear interactions to the scalar potential quenches the non-axisymmetric instability, both on the non-relativistic, and the relativistic branches of solutions. We also consider select boson stars with $m=2$, finding instability in all cases. For the cases exhibiting instability, we follow the nonlinear development, finding a range of dynamics including fragmentation into multiple unbound non-rotating stars, and formation of binary black holes. Finally, we comment on the relationship between stability and criteria based on the rotating boson star's frequency in relation to that of a spherical boson star or the existence of a co-rotation point. The boson stars that we find not to exhibit instability when evolved for many dynamical times include rapidly rotating cases where the compactness is comparable to that of a black hole or neutron star.

Three Discontinuous Galerkin Methods for One- and Two-Dimensional Nonlinear Dirac Equations with a Scalar Self-Interaction

Three Discontinuous Galerkin Methods for One- and Two-Dimensional Nonlinear Dirac Equations with a Scalar Self-Interaction

Two interacting scalars system in curved spacetime \u2014 vacuum stability from the curved spacetime Effective Field Theory (cEFT) perspective

In this article we investigated the influence of the gravity mediated higher dimensional operators on the issue of vacuum stability in a model containing two interacting scalar fields. As a framework we used the curved spacetime Effective Field Theory (cEFT) applied to the aforementioned system in which one of the scalars is heavy. After integrating out the heavy scalar we used the standard Euclidean approach to the obtained cEFT. Apart from analyzing the influence of standard operators like the non-minimal coupling to gravity and the dimension six contribution to the scalar field potential, we also investigated the rarely discussed dimension six contribution to the kinetic term and the new gravity mediated contribution to the scalar quartic self-interaction.