arXiv subjects

Paul M. Saffin

Publications and source records attributed to Paul M. Saffin.

At least 55 records · Page 3Linked to original sources

arXiv2018

Rotating kinky braneworlds

Cylindrical braneworlds have been used in the literature as a convenient way to resolve co-dimension-two branes. They are prevented from collapsing by a massless worldvolume field with non-trivial winding, but here we discuss another way of preventing collapse, which is to rotate the brane. We use a simple microscopic field theory model of a domain wall with a condensate for which rotation is a necessity, not just a nice added extra. This is due to a splitting instability, whereby the effective potential trapping the condensate is not strong enough to hold it on the defect in the presence of winding without charge. We use analytic defect solutions in the field theory (kinky vortons) to construct a thin-wall braneworld model by including gravitational dynamics, and we allow for the rotation required by the microscopic theory. We then discuss the impact rotation has on the bulk and brane geometry, thereby providing an anchor for further cosmological investigations. Our setup naturally leads to worldvolume fields living at slightly different radii, and we speculate on the consequences of this in regard to the fermion mass-hierarchy.

hep-th↗

arXiv2018

Simulations of Cold Electroweak Baryogenesis: Dependence on the source of CP-violation

We compute the baryon asymmetry created in a tachyonic electroweak symmetry breaking transition, focusing on the dependence on the source of effective CP-violation. Earlier simulations of Cold Electroweak Baryogenesis have almost exclusively considered a very specific CP-violating term explicitly biasing Chern-Simons number. We compare four different dimension six, scalar-gauge CP-violating terms, involving both the Higgs field and another dynamical scalar coupled to SU(2) or U(1) gauge fields. We find that for sensible values of parameters, all implementations can generate a baryon asymmetry consistent with observations, showing that baryogenesis is a generic outcome of a fast tachyonic electroweak transition.

hep-ph↗

arXiv2017

Simulations of Cold Electroweak Baryogenesis: Quench from portal coupling to new singlet field

We compute the baryon asymmetry generated from Cold Electroweak Baryogenesis, when a dynamical Beyond-the-Standard-Model scalar singlet field triggers the spinodal transition. Using a simple potential for this additional field, we match the speed of the quench to earlier simulations with a "by-hand" mass flip. We find that for the parameter subspace most similar to a by-hand transition, the final baryon asymmetry shows a similar dependence on quench time and is of the same magnitude. For more general parameter choices the Higgs-singlet dynamics can be very complicated, resulting in an enhancement of the final baryon asymmetry. Our results validate and generalise results of simulations in the literature and open up the Cold Electroweak Baryogenesis scenario to further model building.

hep-ph↗

arXiv2017

Recrudescence of massive fermion production by oscillons

In this letter we bring together the physics of preheating, following a period of inflation, and the dynamics of non-topological solitons, namely oscillons. We show that the oscillating condensate that makes up an oscillon can be an efficient engine for producing heavy fermions, just as a homogeneous condensate is known for doing the same. This then allows heavy fermions to be produced when the energy scale of the Universe has dropped below the scale naturally associated to the fermions.

hep-th↗

arXiv2017

Simulations of Cold Electroweak Baryogenesis: Hypercharge U(1) and the creation of helical magnetic fields

We perform numerical simulations of Cold Electroweak Baryogenesis, including for the first time in the Bosonic sector the full electroweak gauge group SU(2)$\times$U(1) and CP-violation. We find that the maximum generated baryon asymmetry is reduced by a factor of three relative to the SU(2)-only model, but that the quench time dependence is very similar. In addition, we compute the magnitude of the helical magnetic fields, and find that it is proportional to the strength of CP-violation and dependent on quench time, but is not proportional to the magnitude of the baryon asymmetry as proposed in the literature. Astrophysical signatures of primordial magnetic helicity can therefore not in general be used as evidence that electroweak baryogenesis has taken place.

hep-ph↗

arXiv2017

Simulations of "Tunnelling of the 3rd Kind"

We consider the phenomenon of "tunnelling of the 3rd kind", whereby a magnetic field may traverse a classically impenetrable barrier by pair creation of unimpeded quantum fermions. These propagate through the barrier and generate a magnetic field on the other side. We study this numerically using quantum fermions coupled to a classical Higgs-gauge system, where we set up a magnetic field outside a box shielded by two superconducting barriers. We examine the magnitude of the internal magnetic field, and find agreement with existing perturbative results within a factor of two.

hep-ph↗

arXiv2017

Simulations of Cold Electroweak Baryogenesis: Finding the optimal quench time

We revisit the numerical computation of the baryon asymmetry from Cold Electroweak Baryogenesis given the physical Higgs mass. We investigate the dependence of the asymmetry on the speed at which electroweak symmetry breaking takes place. The maximum asymmetry does not occur for arbitrarily fast quenches, but at quench times of about $τ_q\simeq 16\, m_H^{-1}$, with no asymmetry created for quenches slower than $τ_q>30\, m_H^{-1}$. Curiously, we also find that the overall sign of the asymmetry depends on the quench time.

hep-ph↗

arXiv2016

Highly Excited Strings I: Generating Function

This is the first of a series of detailed papers on string amplitudes with highly excited strings (HES). In the present paper we construct a generating function for string amplitudes with generic HES vertex operators using a fixed-loop momentum formalism. We generalise the proof of the chiral splitting theorem of D'Hoker and Phong to string amplitudes with arbitrary HES vertex operators (with generic KK and winding charges, polarisation tensors and oscillators) in general toroidal compactifications $\mathcal{E}=\mathbb{R}^{D-1,1}\times \mathbb{T}^{D_{\rm cr}-D}$ (with generic constant Kähler and complex structure target space moduli, background Kaluza-Klein (KK) gauge fields and torsion). We adopt a novel approach that does not rely on a "reverse engineering" method to make explicit the loop momenta, thus avoiding a certain ambiguity pointed out in a recent paper by Sen, while also keeping the genus of the worldsheet generic. This approach will also be useful in discussions of quantum gravity and in particular in relation to black holes in string theory, non-locality and breakdown of local effective field theory, as well as in discussions of cosmic superstrings and their phenomenological relevance. We also discuss the manifestation of wave/particle (or rather wave/string) duality in string theory.

hep-th↗

arXiv2016

Disformally self-tuning gravity

We extend a previous self-tuning analysis of the most general scalar-tensor theory of gravity in four dimensions with second order field equations by considering a generalized coupling to the matter sector. Through allowing a disformal coupling to matter we are able to extend the Fab Four model and construct a new class of theories that are able to tune away the cosmological constant on Friedmann-Lemaitre-Robertson-Walker backgrounds.

hep-th↗

arXiv2015

The Parametrized Post-Newtonian-Vainshteinian Formalism

Light degrees of freedom that modify gravity on cosmological scales must be "screened" on solar system scales in order to be compatible with data. The Vainshtein mechanism achieves this through a breakdown of classical perturbation theory, as large interactions involving new degrees of freedom become important below the so-called Vainshtein radius. We begin to develop an extension of the Parameterized Post-Newtonian (PPN) formalism that is able to handle Vainshteinian corrections. We argue that theories with a unique Vainshtein scale must be expanded using two small parameters. In this Parameterized Post-Newtonian-Vainshteinian (PPNV) expansion, the primary expansion parameter that controls the PPN order is, as usual, the velocity $v$. The secondary expansion parameter, $α$, controls the strength of the Vainshteinian correction and is a theory-specific combination of the Schwarzschild radius and the Vainshtein radius of the source that is independent of its mass. We present the general framework and apply it to Cubic Galileon theory both inside and outside the Vainshtein radius. The PPNV framework can be used to determine the compatibility of such theories with solar system and other strong-field data.

gr-qc↗

arXiv2015

Cold Baryogenesis from first principles in the Two-Higgs Doublet model with Fermions

We present a first-principles numerical computation of the baryon asymmetry in electroweak-scale baryogenesis. For the scenario of Cold Baryogenesis, we consider a one fermion-family reduced CP-violating two Higgs-doublet model, including a classical SU(2)-gauge/two-Higgs sector coupled to one quantum left-handed fermion doublet and two right-handed singlets. Separately, the C(CP) breaking of the two-Higgs potential and the C and P breaking of the gauge-fermion interactions do not provide a baryon asymmetry. Only when combined does baryogenesis occur. Through large-scale computer simulations, we compute the asymmetry for one particularly favourable scalar potential. The numerical signal is at the boundary of what is numerically discernible with the available computer resources, but we tentatively find an asymmetry of $|η|\leq 3.5\times 10^{-7}$.

hep-ph↗

arXiv2014

Charge-Swapping Q-balls

Q-balls are non-topological solitonic solutions to a wide class of field theories that possess global symmetries. Here we show that in these same theories there also exists a tower of novel composite Q-ball solutions where, within one composite Q-ball, positive and negative charges co-exist and swap at a frequency lower than the natural frequency of an individual Q-ball. These charge-swapping Q-balls are constructed by assembling Q-balls and anti-Q-balls tightly such that their nonlinear cores overlap. We explain why charge-swapping Q-balls can form and why they swap charges.

hep-th↗

arXiv2014

Oscillon Lifetime in the Presence of Quantum Fluctuations

We consider the stability of oscillons in 2+1 space-time dimensions, in the presence of quantum fluctuations. Taking the oscillon to be the inhomogeneous mean field of a self-interacting quantum scalar field, we compare its classical evolution to the evolution in the presence of quantum fluctuations. The evolution of these and their back reaction onto the mean field is implemented through the inhomogeneous Hartree approximation, in turn computed as a statistical ensemble of field realizations. We find that although the lifetime of the oscillon is dramatically reduced compared to the classical limit, the regions of longevity are similar in the space of Gaussian initial configurations.

hep-ph↗

arXiv2013

The cosmology of the Fab-Four

We have recently proposed a novel self tuning mechanism to alleviate the famous cosmological constant problem, based on the general scalar tensor theory proposed by Horndeski. The self-tuning model ends up consisting of four geometric terms in the action, with each term containing a free potential function of the scalar field; the four together being labeled as the Fab-Four. In this paper we begin the important task of deriving the cosmology associated with the Fab-Four Lagrangian. Performing a phase plane analysis of the system we are able to obtain a number of fixed points for the system, with some remarkable new solutions emerging from the trade-off between the various potentials. As well as obtaining inflationary solutions we also find conventional radiation/matter-like solutions, but in regimes where the energy density is dominated by a cosmological constant, and where we do not have any explicit forms of radiation or matter. Stability conditions for matter solutions are obtained and we show how it is possible for there to exist an extended period of `matter domination' opening up the possibility that we can generate cosmological structures, and recover a consistent cosmology even in the presence of a large cosmological constant.

hep-th↗

arXiv2013

Ensemble fermions for electroweak dynamics and the fermion preheating temperature

We refine the implementation of ensemble fermions for the electroweak sector of the Standard Model, introduced previously. We consider the behavior of different observables as the size of the ensemble is increased and show that the dynamics converges for ensemble sizes small enough that simulations of the entire electroweak sector become numerically tractable. We apply the method to the computation of the effective preheating temperature during a fast electroweak transition, relevant for Cold Electroweak Baryogenesis. We find that this temperature is never below 20 GeV, and this in combination with the early results convincingly rules out Standard Model CP-violation as the origin of the baryon asymmetry of the Universe.

hep-ph↗

arXiv2013

Gravitational Waves from Oscillon Preheating

Oscillons are long-lived, localized excitations of nonlinear scalar fields which may be copiously produced during preheating after inflation, leading to a possible oscillon-dominated phase in the early Universe. For example, this can happen after axion monodromy inflation, on which we run our simulations. We investigate the stochastic gravitational wave background associated with an oscillon-dominated phase. An isolated oscillon is spherically symmetric and does not radiate gravitational waves, and we show that the flux of gravitational radiation generated between oscillons is also small. However, a significant stochastic gravitational wave background may be generated during preheating itself (i.e, when oscillons are forming), and in this case the characteristic size of the oscillons is imprinted on the gravitational wave power spectrum, which has multiple, distinct peaks.

astro-ph.CO↗

arXiv2013

Adiabatic and Isocurvature Perturbation Projections in Multi-Field Inflation

Current data are in good agreement with the predictions of single field inflation. However, the hemispherical asymmetry seen in the cosmic microwave background data, may hint at a potential problem. Generalizing to multi-field models may provide one possible explanation. A useful way of modeling perturbations in multi-field inflation is to investigate the projection of the perturbation along and perpendicular to the background fields' trajectory. These correspond to the adiabatic and isocurvature perturbations. However, it is important to note that in general there are no corresponding adiabatic and isocurvature fields. The purpose of this article is to highlight the distinction between a field redefinition and a perturbation projection. We provide a detailed derivation of the evolution of the isocurvature perturbation to show that no assumption of an adiabatic or isocurvature field is needed. We also show how this evolution equation is consistent with the field covariant evolution equations for the isocurvature perturbation in the flat field space limit.

hep-th↗

arXiv2013

Duality and Decay of Macroscopic F-Strings

We study the decay of fundamental string loops of arbitrary size L/min(n,m)>>\sqrt{α'}, labelled by (n,m;λ_n,\barλ_m), where n,m correspond to left- and right-mover harmonics and λ_n,\barλ_m to polarisation tensors, and find that a description in terms of the recent coherent vertex operator construction of Hindmarsh and Skliros is computationally very efficient. We primarily show that the decay rates and mass shifts of vertex operators (n,m;λ_n,\barλ_m) and their "duals" (n,m;λ_n,\barλ_m^*) are equal to leading order in the string coupling, implying for instance that decay rates of epicycloids equal those of hypocycloids. We then compute the power and decay rates associated to massless IR radiation for the trajectory (1,1;λ_1,\barλ_1), and find that it is precisely reproduced by the low energy effective theory of Dabholkar and Harvey. Guided by this correspondence, we conjecture the result for arbitrary trajectories (n,m;λ_n,\barλ_m) and discover a curious relation between gravitational and axion plus dilaton radiation. It is now possible to start exploring string evolution in regimes where a low energy effective description is less useful, such as in the vicinity of cusps.

hep-th↗