A Strong Electroweak Phase Transition from the Inflaton Field

We study a singlet scalar extension of the Standard Model. The singlet scalar is coupled non-minimally to gravity and assumed to drive inflation, and also couple sufficiently strongly with the SM Higgs field in order to provide for a strong first order electroweak phase transition. Requiring the model to describe inflation successfully, be compatible with the LHC data, and yield a strong first order electroweak phase transition, we identify the regions of the parameter space where the model is viable. We also include a singlet fermion with scalar coupling to the singlet scalar to probe the sensitivity of the constraints on additional degrees of freedom and their couplings in the singlet sector. We also comment on the general feasibility of these fields to act as dark matter.Comment: 16 pages, 3 figures; minor changes to match the published versio

Phase transition and gravitational wave phenomenology of scalar conformal extensions of the Standard Model

Thermal corrections in classically conformal models typically induce a strong first-order electroweak phase transition, thereby resulting in a stochastic gravitational wave background that could be detectable at gravitational wave observatories. After reviewing the basics of classically conformal scenarios, in this paper we investigate the phase transition dynamics in a thermal environment and the related gravitational wave phenomenology within the framework of scalar conformal extensions of the Standard Model. We find that minimal extensions involving only one additional scalar field struggle to reproduce the correct phase transition dynamics once thermal corrections are accounted for. Next-to-minimal models, instead, yield the desired electroweak symmetry breaking and typically result in a very strong gravitational wave signal.Comment: 9 pages and 7 figures. Minor changes to match the published versio

Baryogenesis in the two doublet and inert singlet extension of the Standard Model

We investigate an extension of the Standard Model containing two Higgs doublets and a singlet scalar field (2HDSM). We show that the model can have a strongly first-order phase transition and give rise to the observed baryon asymmetry of the Universe, consistent with all experimental constraints. In particular, the constraints from the electron and neutron electric dipole moments are less constraining here than in pure two-Higgs-doublet model (2HDM). The two-step, first-order transition in 2HDSM, induced by the singlet field, may lead to strong supercooling and low nucleation temperatures in comparison with the critical temperature, $T_n \ll T_c$, which can significantly alter the usual phase-transition pattern in 2HD models with $T_n \approx T_c$. Furthermore, the singlet field can be the dark matter particle. However, in models with a strong first-order transition its abundance is typically but a thousandth of the observed dark matter abundance.Comment: 25 pages, 8 figures; minor changes to match the published versio

Formation and Evolution of Primordial Black Hole Binaries in the Early Universe

The abundance of primordial black holes (PBHs) in the mass range $0.1 - 10^3 M_\odot$ can potentially be tested by gravitational wave observations due to the large merger rate of PBH binaries formed in the early universe. To put the estimates of the latter on a firmer footing, we first derive analytical PBH merger rate for general PBH mass functions while imposing a minimal initial comoving distance between the binary and the PBH nearest to it, in order to pick only initial configurations where the binary would not get disrupted. We then study the formation and evolution of PBH binaries before recombination by performing N-body simulations. We find that the analytical estimate based on the tidally perturbed 2-body system strongly overestimates the present merger rate when PBHs comprise all dark matter, as most initial binaries are disrupted by the surrounding PBHs. This is mostly due to the formation of compact N-body systems at matter-radiation equality. However, if PBHs make up a small fraction of the dark matter, $f_{\rm PBH} \lesssim 10\%$, these estimates become more reliable. In that case, the merger rate observed by LIGO imposes the strongest constraint on the PBH abundance in the mass range $2 - 160 M_\odot$. Finally, we argue that, even if most initial PBH binaries are perturbed, the present BH-BH merger rate of binaries formed in the early universe is larger than $\mathcal{O}(10)\,{\rm Gpc}^{-3} {\rm yr}^{-1}\, f_{\rm PBH}^3$Comment: 32pages, 12 figures, typos corrected, references added, figures updated, matches version published in JCA

Observational Properties of Feebly Coupled Dark Matter

We show that decoupled hidden sectors can have observational consequences. As a representative model example, we study dark matter production in the Higgs portal model with one real singlet scalar $s$ coupled to the Standard Model Higgs via $\lambda_{\rm hs}\Phi^\dagger\Phi s^2$ and demonstrate how the combination of non-observation of cosmological isocurvature perturbations and astrophysical limits on dark matter self-interactions imply stringent bounds on the magnitude of the scalar self-coupling $\lambda_{\rm s}s^4$. For example, for dark matter mass $m_{\rm s}=10$ MeV and Hubble scale during cosmic inflation $H_*=10^{12}$ GeV, we find $10^{-4}\lesssim \lambda_{\rm s}\lesssim 0.2$.Comment: 4 pages, 1 figure. Prepared for the proceedings of the ICHEP2016 conference, 3-10 August 2016, Chicago, United State

Detecting circular polarisation in the stochastic gravitational-wave background from a first-order cosmological phase transition

We discuss the observability of circular polarisation of the stochastic gravitational-wave background (SGWB) generated by helical turbulence following a first-order cosmological phase transition, using a model that incorporates the effects of both direct and inverse energy cascades. We explore the strength of the gravitational-wave signal and the dependence of its polarisation on the helicity fraction, $\zeta_*$, the strength of the transition, $\alpha$, the bubble size, $R_*$, and the temperature, $T_*$, at which the transition finishes. We calculate the prospective signal-to-noise ratios of the SGWB strength and polarisation signals in the LISA experiment, exploring the parameter space in a way that is minimally sensitive to the underlying particle physics model. We find that discovery of SGWB polarisation is generally more challenging than measuring the total SGWB signal, but would be possible for appropriately strong transitions with large bubble sizes and a substantial polarisation fraction.Comment: 31 pages, 8 Figure