arXiv subjects

Sarunas Verner

Publications and source records attributed to Sarunas Verner.

At least 37 records · Page 2Linked to original sources

arXiv2024

Cosmological Collider Signatures of Higgs-$R^2$ Inflation

We study the cosmological collider signatures in the Higgs-$R^2$ inflation model. We consider two distinct types of signals: one originating from the inflaton coupling to Standard Model fermions and gauge bosons, and another arising from the isocurvature mode interaction with the inflaton. In the former case, we determine that the signal magnitude is likely too small for detection by upcoming probes, primarily due to suppression by both the Planck scale and slow-roll parameters. However, we provide a detailed computation of the signal which could be potentially applicable to various Higgs inflation variants. For the isocurvature mode signals, we observe that the associated couplings remain unsuppressed when the isocurvature mode is relatively light or comparable to the inflationary scale. In this case, we study the Higgs-$R^2$ inflation parameter space that corresponds to the quasi-single-field inflation regime and find that the signal strength could be as large as $|f_{\rm NL}| > 1$, making Higgs-$R^2$ inflation a viable candidate for observation by future 21-cm surveys.

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arXiv2024

A New Window into Gravitationally Produced Scalar Dark Matter

Conventional scenarios of purely gravitationally produced dark matter with masses below the Hubble parameter at the end of inflation are in tension with Cosmic Microwave Background (CMB) constraints on the isocurvature power spectrum. We explore a more general scenario with a non-minimal coupling between the scalar dark matter field and gravity, which allows for significantly lighter scalar dark matter masses compared to minimal coupling predictions. By imposing relic abundance, isocurvature, Lyman-$α$, and Big Bang Nucleosynthesis (BBN) constraints, we show the viable parameter space for these models. Our findings demonstrate that the presence of a non-minimal coupling expands the parameter space, yielding a dark matter mass lower bound of $2 \times 10^{-4} \, \rm{eV}$.

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arXiv2024

Exploring the Truth and Beauty of Theory Landscapes with Machine Learning

Theoretical physicists describe nature by i) building a theory model and ii) determining the model parameters. The latter step involves the dual aspect of both fitting to the existing experimental data and satisfying abstract criteria like beauty, naturalness, etc. We use the Yukawa quark sector as a toy example to demonstrate how both of those tasks can be accomplished with machine learning techniques. We propose loss functions whose minimization results in true models that are also beautiful as measured by three different criteria - uniformity, sparsity, or symmetry.

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arXiv2023

The Role of Vectors in Reheating

We explore various aspects concerning the role of vector bosons during the reheating process. Generally, reheating occurs during the period of oscillations of the inflaton condensate and the evolution of the radiation bath depends on the inflaton equation of state. For oscillations about a quadratic minimum, the equation of state parameter, $w = p/ρ=0$, and the evolution of the temperature, $T(a)$ with respect to the scale factor is independent of the spin of the inflaton decay products. However, for cases when $w>0$, there is a dependence on the spin, and here we consider the evolution when the inflaton decays or scatters to vector bosons. We also investigate the gravitational production of vector bosons as potential dark matter candidates. Gravitational production predominantly occurs through the longitudinal mode. We compare these results to the gravitational production of scalars.

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arXiv2023

Seeking Truth and Beauty in Flavor Physics with Machine Learning

The discovery process of building new theoretical physics models involves the dual aspect of both fitting to the existing experimental data and satisfying abstract theorists' criteria like beauty, naturalness, etc. We design loss functions for performing both of those tasks with machine learning techniques. We use the Yukawa quark sector as a toy example to demonstrate that the optimization of these loss functions results in true and beautiful models.

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arXiv2023

Identifying the Group-Theoretic Structure of Machine-Learned Symmetries

Deep learning was recently successfully used in deriving symmetry transformations that preserve important physics quantities. Being completely agnostic, these techniques postpone the identification of the discovered symmetries to a later stage. In this letter we propose methods for examining and identifying the group-theoretic structure of such machine-learned symmetries. We design loss functions which probe the subalgebra structure either during the deep learning stage of symmetry discovery or in a subsequent post-processing stage. We illustrate the new methods with examples from the U(n) Lie group family, obtaining the respective subalgebra decompositions. As an application to particle physics, we demonstrate the identification of the residual symmetries after the spontaneous breaking of non-Abelian gauge symmetries like SU(3) and SU(5) which are commonly used in model building.

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arXiv2023

Accelerated Discovery of Machine-Learned Symmetries: Deriving the Exceptional Lie Groups G2, F4 and E6

Recent work has applied supervised deep learning to derive continuous symmetry transformations that preserve the data labels and to obtain the corresponding algebras of symmetry generators. This letter introduces two improved algorithms that significantly speed up the discovery of these symmetry transformations. The new methods are demonstrated by deriving the complete set of generators for the unitary groups U(n) and the exceptional Lie groups $G_2$, $F_4$, and $E_6$. A third post-processing algorithm renders the found generators in sparse form. We benchmark the performance improvement of the new algorithms relative to the standard approach. Given the significant complexity of the exceptional Lie groups, our results demonstrate that this machine-learning method for discovering symmetries is completely general and can be applied to a wide variety of labeled datasets.

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arXiv2023

Scalar Dark Matter Production from Preheating and Structure Formation Constraints

We investigate the out-of-equilibrium production of scalar dark matter (DM) from the inflaton condensate during inflation and reheating. We assume that this scalar couples only to the inflaton via a direct quartic coupling and is minimally coupled to gravity. We consider all possible production regimes: purely gravitational, weak direct coupling (perturbative), and strong direct coupling (non-perturbative). For each regime, we use different approaches to determine the dark matter phase space distribution and the corresponding relic abundance. For the purely gravitational regime, scalar dark matter quanta are copiously excited during inflation resulting in an infrared (IR) dominated distribution function and a relic abundance which overcloses the universe for a reheating temperature $T_\text{reh}>34 ~\text{GeV}$. A non-vanishing direct coupling induces an effective DM mass and suppresses the large IR modes in favor of ultraviolet (UV) modes and a minimal scalar abundance is generated when the interference between the direct and gravitational couplings is maximal. For large direct couplings, backreaction on the inflaton condensate is accounted for by using the Hartree approximation and lattice simulation techniques. Since scalar DM candidates can behave as non-cold dark matter, we estimate the impact of such species on the matter power spectrum and derive the corresponding constraints from the Lyman-$α$ measurements. We find that they correspond to a lower bound on the DM mass of $\gtrsim 3\times 10^{-4} \, \rm{eV}$ for purely gravitational production, and $\gtrsim 20 \, \rm {eV}$ for direct coupling production. We discuss the implications of these results.

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arXiv2023

Isocurvature Constraints on Scalar Dark Matter Production from the Inflaton

We investigate the production of a spectator scalar dark matter field that is directly coupled to the inflaton during inflation and reheating. We consider two specific inflationary potentials, namely the Starobinsky and T-model of inflation, which satisfy the constraints on the scalar tilt, $n_s$, and tensor-to-scalar ratio, $r$, measured by the Planck satellite. Excitation of light scalar dark matter during inflation may result in large isocurvature perturbations, which can be avoided by inducing a sizable effective dark matter mass during the inflationary phase. For purely gravitational production, the Planck isocurvature constraints require the dark matter mass to be larger than the Hubble scale at horizon exit, with $m_χ \gtrsim 0.5H_*$. For small bare dark matter masses $m_χ \ll H_*$, these constraints translate into a lower bound on the dark matter coupling to the inflaton. We argue that these constraints can be applied to a wide class of single-field slow-roll inflation models. We also derive isocurvature, dark matter abundance, and Lyman-$α$ constraints on the direct coupling and bare dark matter mass.

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arXiv2023

Testing the Scalar Weak Gravity Conjecture in No-scale Supergravity

We explore possible extensions of the Weak Gravity Conjecture (WGC) to scalar field theories. To avoid charged black hole remnants, the WGC requires the existence of a particle with a mass $m < g q M_P$, with charge $q$ and U(1) gauge coupling $g$, allowing the decay to shed the black hole charge. Although there is no obvious problem that arises in the absence of a U(1) charge, it has been postulated that gravity must remain the weakest force even when extended to scalar interactions. Quantifying this conjecture may be done by comparing scalar and gravitational amplitudes, or as we advocate here by comparing scattering cross sections. In theories with non-trivial field space geometries, by working out examples with perturbation theory around arbitrary field values and performing tadpole resummations, we argue that the conjecture must be applied only at the extrema of the scalar potential (when expressed in locally canonical coordinates). We consider several toy models in the context of no-scale supergravity and also consider examples of inflationary models.

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arXiv2023

Supergravity Scattering Amplitudes

Supergravity theories with non-minimal Kähler potentials are characterized by a non-trivial field space manifold with corresponding non-trivial kinetic terms. The scattering amplitudes in these theories can be calculated at fixed background field values by making a field redefinition to Riemann normal coordinates. Because of the Kähler structure of supergravity, a more compact method for calculating amplitudes is obtained by a redefinition to Kähler normal coordinates. We compare both methods and calculate the explicit transformations and amplitudes for several examples in the context of no-scale supergravity with one and two chiral superfields. We show that in all cases the equivalence of the scattering amplitudes using either Riemann normal or Kähler normal coordinates is possible only at extremal points of the scalar potential.

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arXiv2023

Discovering Sparse Representations of Lie Groups with Machine Learning

Recent work has used deep learning to derive symmetry transformations, which preserve conserved quantities, and to obtain the corresponding algebras of generators. In this letter, we extend this technique to derive sparse representations of arbitrary Lie algebras. We show that our method reproduces the canonical (sparse) representations of the generators of the Lorentz group, as well as the $U(n)$ and $SU(n)$ families of Lie groups. This approach is completely general and can be used to find the infinitesimal generators for any Lie group.

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arXiv2023

Deep Learning Symmetries and Their Lie Groups, Algebras, and Subalgebras from First Principles

We design a deep-learning algorithm for the discovery and identification of the continuous group of symmetries present in a labeled dataset. We use fully connected neural networks to model the symmetry transformations and the corresponding generators. We construct loss functions that ensure that the applied transformations are symmetries and that the corresponding set of generators forms a closed (sub)algebra. Our procedure is validated with several examples illustrating different types of conserved quantities preserved by symmetry. In the process of deriving the full set of symmetries, we analyze the complete subgroup structure of the rotation groups $SO(2)$, $SO(3)$, and $SO(4)$, and of the Lorentz group $SO(1,3)$. Other examples include squeeze mapping, piecewise discontinuous labels, and $SO(10)$, demonstrating that our method is completely general, with many possible applications in physics and data science. Our study also opens the door for using a machine learning approach in the mathematical study of Lie groups and their properties.

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arXiv2022

Gravitational Portals with Non-Minimal Couplings

We consider the effects of non-minimal couplings to curvature of the form $ξ_S S^2 R$, for three types of scalars: the Higgs boson, the inflaton, and a scalar dark matter candidate. We compute the abundance of dark matter produced by these non-minimal couplings to gravity and compare to similar results with minimal couplings. We also compute the contribution to the radiation bath during reheating. The main effect is a potential augmentation of the maximum temperature during reheating. A model independent limit of $\mathcal{O}(10^{12})$ GeV is obtained. For couplings $ξ_S \gtrsim \mathcal{O}(1)$, these dominate over minimal gravitational interactions.

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arXiv2022

Gravitational portals in the early Universe

We consider the production of matter and radiation during reheating after inflation, restricting our attention solely to gravitational interactions. Processes considered are the exchange of a graviton, $h_{μν}$, involved in the scattering of the inflaton or particles in the newly created radiation bath. In particular, we consider the gravitational production of dark matter (scalar or fermionic) from the thermal bath as well as from scattering of the inflaton condensate. We also consider the gravitational production of radiation from inflaton scattering. In the latter case, we also derive a lower bound on the maximal temperature of order of $10^{12}$ GeV for a typical $α-$attractor scenario from $ϕϕ\rightarrow h_{μν} \rightarrow$ Standard Model fields (dominated by the production of Higgs bosons). This lower gravitational bound becomes the effective maximal temperature for reheating temperatures, $T_{\rm{RH}} \lesssim 10^9$ GeV. The processes we consider are all minimal in the sense that they are present in any non-minimal extension of the Standard Model theory based on Einstein gravity and can not be neglected. We compare each of these processes to determine their relative importance in the production of both radiation and dark matter.

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arXiv2022

BICEP/Keck Constraints on Attractor Models of Inflation and Reheating

Recent BICEP/Keck data on the cosmic microwave background, in combination with previous WMAP and Planck data, impose strong new constraints on the tilt in the scalar perturbation spectrum, $n_s$, as well as the tensor-to-scalar ratio, $r$. These constrain the number of e-folds of inflation, $N_*$, the magnitude of the inflaton coupling to matter, $y$, and the reheating temperature, $T_{\rm reh}$, which we evaluate in attractor models of inflation as formulated in no-scale supergravity. The 68% C.L. region of $(n_s, r)$ favours large values of $N_*, y$ and $T_{\rm reh}$ that are constrained by the production of gravitinos and supersymmetric dark matter.

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arXiv2021

Freeze-in from Preheating

We consider the production of dark matter during the process of reheating after inflation. The relic density of dark matter from freeze-in depends on both the energy density and energy distribution of the inflaton scattering or decay products composing the radiation bath. We compare the perturbative and non-perturbative calculations of the energy density in radiation. We also consider the (likely) possibility that the final state scalar products are unstable. Assuming either thermal or non-thermal energy distribution functions, we compare the resulting relic density based on these different approaches. We show that the present-day cold dark matter density can be obtained through freeze-in from preheating for a large range of dark matter masses.

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arXiv2021

On the Realization of WIMPflation

We consider models for inflation with a stable inflaton. Reheating is achieved through scattering processes such as $ϕϕ\to h h$, where $h$ is the Standard Model Higgs boson. We consider the reheating process in detail and show that for a relatively large coupling (needed for the late annihilations of the inflaton during freeze-out), reheating is almost instantaneous leading to a relatively high reheating temperature. The process $ϕϕ\leftrightarrow h h$ brings the inflaton back into equilibrium, leading to a well studied scalar singlet dark matter candidate and Higgs portal model. We argue that such models can be derived from no-scale supergravity.

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