arXiv subjects

Keith A. Olive

Publications and source records attributed to Keith A. Olive.

At least 37 records · Page 2Linked to original sources

arXiv2025

How Accidental was Inflation?

Data on the cosmic microwave background (CMB) are discriminating between different models of inflation, disfavoring simple monomial potentials whilst being consistent with models whose predictions resemble those of the Starobinsky $R + R^2$ cosmological model. However, this model may suffer from theoretical problems, since it requires a large initial field value, threatening the validity of the effective field theory. This is quantified by the Swampland Distance Conjecture, which predicts the appearance of a tower of light states associated with an effective ultra-violet cutoff. This could be lower than the inflation scale for cases with an extended period of inflation, leading to an additional problem of initial conditions. No-scale supergravity models can reproduce the predictions of the Starobinsky model and accommodate the CMB data at the expense of fine-tuning of parameters at the level of $10^{-5}$. Here, we propose a solution to this problem based on an explicit realisation of the Starobinsky model in string theory, where this `deformation' parameter is calculable and takes a value of order of the one corresponding to the Starobinsky inflaton potential. Within this range, there are parameter values that accommodate more easily the combination of Planck, ACT and DESI BAO data, while also restricting the range of possible inflaton field values, thereby avoiding the swampland problem and predicting that the initial conditions for inflation compatible with the CMB data are generic.

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arXiv2025

Scalar Field Fluctuations and the Production of Dark Matter

One of the simplest possible candidates for dark matter is a stable scalar singlet beyond the Standard Model. If its mass is below the Hubble scale during inflation, long-wavelength modes of this scalar will be excited during inflation, and their subsequent evolution may lead to the correct relic density of dark matter. In this work, we provide a comprehensive analysis of the evolution of a spectator scalar. We examine three cases: (1) a non-interacting massive scalar, (2) a massive scalar with self-interactions of the form $λ_χχ^p$, and (3) a massive scalar coupled to the inflaton $ϕ$ through an interaction term of the form $σ_{n,m} ϕ^n χ^m$. In all cases, we assume minimal coupling to gravity and compare these results with the production of short-wavelength modes arising from single graviton exchange. The evolution is tracked during the reheating phase. Our findings are summarized using $(m_χ, T_{\rm RH})$ parameter planes, where $m_χ$ is the mass of the scalar field and $T_{\rm RH}$ is the reheating temperature after inflation. The non-interacting scalar is highly constrained, requiring $m_χ> 3 \times 10^{12}~\rm {GeV}$ and $ T_{\rm RH} \lesssim 7~\text{TeV}$ for an inflationary potential with a quadratic minimum. However, when self-interactions or couplings to the inflaton are included, the viable parameter space expands considerably. In these cases, sub-GeV and even sub-eV scalar masses can yield the correct relic abundance, opening new possibilities for light dark matter candidates. In all cases, we also impose additional constraints arising from the production of isocurvature fluctuations, the prevention of a secondary inflationary phase triggered by the spectator field, and the fragmentation of scalar condensates.

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arXiv2024

Aspects of Gravitational Portals and Freeze-in during Reheating

We conduct a systematic investigation of freeze-in during reheating while taking care to include both direct and indirect production of dark matter (DM) via gravitational portals and inflaton decay. Direct production of DM can occur via gravitational scattering of the inflaton, while indirect production occurs through scattering in the Standard Model radiation bath. We consider two main contributions to the radiation bath during reheating. The first, which may dominate at the onset of the reheating process, is produced via gravitational scattering of the inflaton. The second (and more standard contribution) comes from inflaton decay. We consider a broad class of DM production rates parameterized as $R_χ \propto T^{n+6}/Λ^{n+2}$, and inflaton potentials with a power-law form $V(ϕ) \propto ϕ^{k}$ about the minimum. We find the relic density produced by freeze-in for each contribution to the Standard Model bath for arbitrary $k$ and $n$, and compare these with the DM density produced gravitationally by inflaton scattering. We find that freeze-in production from the gravitationally-produced radiation bath can exceed that of the conventional decay bath and account for the observed relic density provided that $m_χ > T_{\rm RH}$, with additional $k$- and $n$-dependent constraints. For each freeze-in interaction considered, we also find $m_χ$- and $T_{\rm RH}$-dependent limits on the BSM scale, $Λ$, for which gravitational production will exceed ordinary freeze-in production.

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arXiv2024

The Role of the Curvaton Post-Planck

The expected improvements in the precision of inflationary physics observables including the scalar spectral index $n_{s}$ and the tensor-to-scalar ratio $r$ will reveal more than just the viability of a particular model of inflation. In the presence of a curvaton field $χ$, supposedly dead models of inflation can be resurrected as these observables are affected by curvaton perturbations. For currently successful models, improved constraints will enable us to constrain the properties of extra decaying scalar degrees of freedom produced during inflation. In this work, we demonstrate these diverse uses of a curvaton field with the most recent constraints on ($n_{s},r$) and two exemplary inflation models, the Starobinsky model, and a model of new inflation. Our analysis invokes three free parameters: the curvaton mass $m_χ$, its decay rate $Γ_χ$ the reheating temperature $T_{\rm RH}$ produced by inflaton decays. We systematically analyze possible post-inflationary era scenarios of a curvaton field. By projecting the most recent CMB data on ($n_{s},r$) into this parameter space, we can either set constraints on the curvaton parameters from successful models of inflation (so that the success is not spoiled) or determine the parameters which are able to save a model for which $n_{s}$ is predicted to be below the experimental data. We emphasize that the initial value of $\langle χ^2 \rangle \propto H^4/m_χ^2$ produced during inflation is determined from a stochastic approach and thus not a free parameter in our analysis. We also investigate the production of local non-Gaussianity $f_{NL}^{(\rm loc)}$ and apply current CMB constraints to the parameter space. Intriguingly, we find that a large value of $f_{NL}^{(\rm loc)}$ of $\mathcal{O}(1)$ can be produced for both of the two representative inflation models.

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arXiv2024

Non-universal SUSY models, $g_μ-2$, $m_H$ and dark matter

We study the anomalous magnetic moment of the muon, $g_μ- 2 \equiv 2 a_μ$, in the context of supersymmetric models beyond the CMSSM, where the unification of either the gaugino masses $M_{1,2,3}$ or sfermion and Higgs masses is relaxed, taking into account the measured mass of the Higgs boson, $m_H$, the cosmological dark matter density and the direct detection rate. We find that the model with non-unified gaugino masses can make a contribution $Δa_μ\sim 20 \times 10^{-10}$ to the anomalous magnetic moment of the muon, for example if $M_{1,2} \sim 600$ GeV and $M_3\sim 8$ TeV. The model with non-universal sfermion and Higgs masses can provide even larger $Δa_μ\sim 24 \times 10^{-10}$ if the sfermion masses for the first and the second generations are $ \sim 400 $ GeV and that of the third is $ \sim 8 $ TeV. We discuss the prospects for collider searches for supersymmetric particles in specific benchmark scenarios illustrating these possibilities, focusing in particular on the prospects for detecting the lighter smuon and the lightest neutralino.

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arXiv2024

Inflaton Production of Scalar Dark Matter through Fluctuations and Scattering

We study the effects on particle production of a Planck-suppressed coupling between the inflaton and a scalar dark matter candidate, $χ$. In the absence of this coupling, the dominant source for the relic density of $χ$ is the long wavelength modes produced from the scalar field fluctuations during inflation. In this case, there are strong constraints on the mass of the scalar and the reheating temperature after inflation from the present-day relic density of $χ$ (assuming $χ$ is stable). When a coupling $σϕ^2 χ^2$ is introduced, with $σ= {\tilde σ} m_ϕ^2/ M_P^2 \sim 10^{-10} {\tilde σ}$, where $m_ϕ$ is the inflaton mass, the allowed parameter space begins to open up considerably even for ${\tilde σ}$ as small as $\gtrsim 10^{-7}$. For ${\tilde σ} \gtrsim \frac{9}{16}$, particle production is dominated by the scattering of the inflaton condensate, either through single graviton exchange or the contact interaction between $ϕ$ and $χ$. In this regime, the range of allowed masses and reheating temperatures is maximal. For $0.004 < {\tilde σ} < 50$, constraints from isocurvature fluctuations are satisfied, and the production from parametric resonance can be neglected.

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arXiv2024

Inflaton decay in No-Scale Supergravity and Starobinsky-like models

We consider the decay of the inflaton in Starobinsky-like models arising from either an $R+R^2$ theory of gravity or $N=1$ no-scale supergravity models. If Standard Model matter is simply introduced to the $R + R^2$ theory, the inflaton (which appears when the theory is conformally transformed to the Einstein frame) couples to matter predominantly in Standard Model Higgs kinetic terms. This will typically lead to a reheating temperature of $\sim 3 \times 10^9$~GeV. However, if the Standard Model Higgs is conformally coupled to curvature, the decay rate may be suppressed and vanishes for a conformal coupling $ξ= 1/6$. Nevertheless, inflaton decays through the conformal anomaly leading to a reheating temperature of order $10^8$~GeV. The Starobinsky potential may also arise in no-scale supergravity. In this case, the inflaton decays if there is a direct coupling of the inflaton to matter in the superpotential or to gauge fields through the gauge kinetic function. We also discuss the relation between the theories and demonstrate the correspondence between the no-scale models and the conformally coupled $R+R^2$ theory (with $ξ= 1/6$).

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arXiv2024

Bare mass effects on the reheating process after inflation

We consider the effects of a bare mass term for the inflaton, when the inflationary potential takes the form $V(ϕ)= λϕ^k$ about its minimum with $k \ge 4$. We concentrate on $k=4$, but discuss general cases as well. Further, we assume $λϕ_{\rm end}^2 \gg m_ϕ^2$, where $ϕ_{\rm end}$ is the inflaton field value when the inflationary expansion ends. We show that the presence of a mass term (which may be present due to radiative corrections or supersymmetry breaking) can significantly alter the reheating process, as the equation of state of the inflaton condensate changes from $w_ϕ=\frac{1}{3}$ to $w_ϕ=0$ when $λϕ^2$ drops below $m_ϕ^2$. We show that for a mass $m_ϕ\gtrsim T_{\rm RH}/250$, the mass term will dominate at reheating. We compute the effects on the reheating temperature for cases where reheating is due to inflaton decay (to fermions, scalars, or vectors) or to inflaton scattering (to scalars or vectors). For scattering to scalars and in the absence of a decay, we derive a strong upper limit to the inflaton bare mass $m_ϕ< 350~{\rm MeV} (T_{\rm RH}/10^{10}~{\rm GeV})^{3/5}$, as there is always a residual inflaton background which acts as cold dark matter. We also consider the effect of the bare mass term on the fragmentation of the inflaton condensate.

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arXiv2024

Minimal Production of Prompt Gravitational Waves during Reheating

The inflationary reheating phase begins when accelerated expansion ends. As all Standard Model particles are coupled to gravity, gravitational interactions will lead to particle production. This includes the thermal bath, dark matter and gravitational radiation. Here, we compute the spectrum of gravitational waves from the inflatoncondensate during the initial phase of reheating. As particular examples of inflation, we consider the Starobinsky model and T-models, all of which are in good phenomenological agreement with CMB anisotropy measurements. The T-models are distinguished by the shape of the potential about its minimum and can be approximated by $V \sim ϕ^k$, where $ϕ$ is the inflaton. Interestingly, the shape of the gravitational wave spectrum (when observed) can be used to distinguish among the models considered. As we show, the Starobinsky model and T-models with $k=2$, provide very different spectra when compared to models with $k=4$ or $k>4$. Observation of multiple harmonics in the spectrum can be interpreted as a direct measurement of the inflaton mass. Furthermore, the cutoff in frequency can be used to determine the reheating temperature.

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arXiv2024

Limits on Non-Relativistic Matter During Big-Bang Nucleosynthesis

Big-bang nucleosynthesis (BBN) probes the cosmic mass-energy density at temperatures $\sim 10$ MeV to $\sim 100$ keV. Here, we consider the effect of a cosmic matter-like species that is non-relativistic and pressureless during BBN. Such a component must decay; doing so during BBN can alter the baryon-to-photon ratio, $η$, and the effective number of neutrino species. We use light element abundances and the cosmic microwave background (CMB) constraints on $η$ and $N_ν$ to place constraints on such a matter component. We find that electromagnetic decays heat the photons relative to neutrinos, and thus dilute the effective number of relativistic species to $N_{\rm eff} < 3$ for the case of three Standard Model neutrino species. Intriguingly, likelihood results based on {\em Planck} CMB data alone find $N_ν = 2.800 \pm 0.294$, and when combined with standard BBN and the observations of D and \he4 give $N_ν = 2.898 \pm 0.141$. While both results are consistent with the Standard Model, we find that a nonzero abundance of electromagnetically decaying matter gives a better fit to these results. Our best-fit results are for a matter species that decays entirely electromagnetically with a lifetime $τ_X = 0.89 \ \rm sec$ and pre-decay density that is a fraction $ξ= (ρ_X/ρ_{\rm rad})|_{10 \ \rm MeV} = 0.0026$ of the radiation energy density at 10 MeV; similarly good fits are found over a range where $ξτ_X^{1/2}$ is constant. On the other hand, decaying matter often spoils the BBN+CMB concordance, and we present limits in the $(τ_X,ξ)$ plane for both electromagnetic and invisible decays. For dark (invisible) decays, standard BBN (i.e. $ξ=0$) supplies the best fit. We end with a brief discussion of the impact of future measurements including CMB-S4.

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arXiv2023

Effects of Fragmentation on Post-Inflationary Reheating

We consider the effects of fragmentation on the post-inflationary epoch of reheating. In simple single field models of inflation, an inflaton condensate undergoes an oscillatory phase once inflationary expansion ends. The equation of state of the condensate depends on the shape of the scalar potential, $V(ϕ)$, about its minimum. Assuming $V(ϕ) \sim ϕ^k$, the equation of state parameter is given by $w = P_ϕ/ρ_ϕ= (k-2)/(k+2)$. The evolution of condensate and the reheating process depend on $k$. For $k \ge 4$, inflaton self-interactions may lead to the fragmentation of the condensate and alter the reheating process. Indeed, these self-interactions lead to the production of a massless gas of inflaton particles as $w$ relaxes to 1/3. If reheating occurs before fragmentation, the effects of fragmentation are harmless. We find, however, that the effects of fragmentation depend sensitively to the specific reheating process. Reheating through the decays to fermions is largely excluded since perturbative couplings would imply that fragmentation occurs before reheating and in fact could prevent reheating from completion. Reheating through the decays to boson is relatively unaffected by fragmentation and reheating through scatterings results in a lower reheating temperature.

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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

Electroweak Loop Contributions to the Direct Detection of Wino Dark Matter

Electroweak loop corrections to the matrix elements for the spin-independent scattering of cold dark matter particles on nuclei are generally small, typically below the uncertainty in the local density of cold dark matter. However, as shown in this paper, there are instances in which the electroweak loop corrections are relatively large, and change significantly the spin-independent dark matter scattering rate. An important example occurs when the dark matter particle is a wino, e.g., in anomaly-mediated supersymmetry breaking (AMSB) and pure gravity mediation (PGM) models. We find that the one-loop electroweak corrections to the spin-independent wino LSP scattering cross section generally interfere constructively with the tree-level contribution for AMSB models with negative Higgsino mixing, $μ< 0$, and in PGM-like models for both signs of $μ$, lifting the cross section out of the neutrino fog and into a range that is potentially detectable in the next generation of direct searches for cold dark matter scattering.

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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

The CMSSM Survives Planck, the LHC, LUX-ZEPLIN, Fermi-LAT, H.E.S.S. and IceCube

We revisit the viability of the CMSSM, searching for regions of parameter space that yield a neutralino dark matter density compatible with Planck measurements, as well as LHC constraints including sparticle searches and the mass of the Higgs boson, recent direct limits on spin-independent and -dependent dark matter scattering from the LUX-ZEPLIN (LZ) experiment, the indirect constraints from Fermi-LAT and H.E.S.S. on dark matter annihilations to photons in dwarf spheroidal galaxies and the Galactic Centre, and the IceCube limits on muons from annihilations to neutrinos in the Sun. For representative values of $\tan β$ and $A_0$ we map in detail the Planck-compatible strips in CMSSM parameter planes, which exhibit multiple distinctive features for large $\tan β$, $A_0 = 0$ and $μ> 0$, and identify portions of the strips that survive all the phenomenological constraints. We find that the most powerful constraint is that from $m_h$, followed by the LZ limit on spin-independent scattering, whereas sparticle searches at the LHC and indirect dark matter searches are less restrictive. Most of the surviving CMSSM parameter space features a Higgsino-like dark matter particle with a mass $\sim 1000-1100$ GeV, which could best be probed with future direct searches for dark matter scattering.

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arXiv2023

The Neutron Mean Life and Big Bang Nucleosynthesis

We explore the effect of neutron lifetime and its uncertainty on standard big-bang nucleosynthesis (BBN). BBN describes the cosmic production of the light nuclides $^1{\rm H}$, ${\rm D}$, $^3{\rm H}$+$^3{\rm He}$, $^4{\rm He}$, and $^7{\rm Li}$+$^7{\rm Be}$ in the first minutes of cosmic time. The neutron mean life $τ_n$ has two roles in modern BBN calculations: (1) it normalizes the matrix element for weak $n \leftrightarrow p$ interconversions, and (2) it sets the rate of free neutron decay after the weak interactions freeze out. We review the history of the interplay between $τ_n$ measurements and BBN, and present a study of the sensitivity of the light element abundances to the modern neutron lifetime measurements. We find that $τ_n$ uncertainties dominate the predicted $^4{\rm He}$ error budget, but these theory errors remain smaller than the uncertainties in $^4{\rm He}$ observations, even with the dispersion in recent neutron lifetime measurements. For the other light-element predictions, $τ_n$ contributes negligibly to their error budget. Turning the problem around, we combine present BBN and cosmic microwave background (CMB) determinations of the cosmic baryon density to $\textit{predict}$ a "cosmologically preferred" mean life of $τ_{n}({\rm BBN+CMB}) = 870 \pm 16 \ \rm sec$, which is consistent with experimental mean life determinations. We go on to show that if future astronomical and cosmological helium observations can reach an uncertainty of $σ_{\rm obs}(Y_p) = 0.001$ in the $^4{\rm He}$ mass fraction $Y_p$, this could begin to discriminate between the mean life determinations.

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arXiv2023

Quantifying Limits on CP Violating Phases from EDMs in Supersymmetry

We revisit the calculation of the electron, neutron, and proton electric dipole moments (EDMs) in the constrained minimal supersymmetric standard model (CMSSM). The relatively large mass of the Higgs boson, $m_H \simeq 125$ GeV coupled with the (as yet) lack of discovery of any supersymmetric particle at the LHC, has pushed the supersymmetry breaking scale to several TeV or higher. Though one might expect this decoupling to have relaxed completely any bounds on the two CP violating phases in the CMSSM ($θ_μ$ and $θ_A$), the impressive experimental improvements in the limits on the EDMs (particularly the electron EDM) still allow us to set constraints of order $(0.01 - 0.1)π$ on $θ_A$ and $(0.001 - 0.1)π$ on $θ_μ$. We also discuss the impact of future improvements in the experimental limits on supersymmetric 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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