arXiv subjects

Ian B. Whittingham

Publications and source records attributed to Ian B. Whittingham.

13 recordsLinked to original sources

arXiv2026

On gravitational baryogenesis in inflationary $f(R)$ Cosmologies

The present study investigates the feasibility of generating the observed baryon-antibaryon asymmetry of the Universe $η$ by gravitational baryogenesis during the inflationary phase of two $f(R)$ modified theories of gravity: the widely used Starobinsky $f(R)=R+R^{2}/M^{2}$ model, and the recently proposed power-law model $f(R)=c_{1}R^{2+k/4}+c_{2}R+c_{3}$ of Odintsov and Oikonomou (2025). The present investigation is undertaken in the Einstein frame. The motion of the scalaron is studied for the slow roll inflationary era using analytic approximate solutions. Some of the expressions for the Starobinsky model were obtained first by Motohashi and Nishizawa (2012) but the expressions for the power-law model of Odintsov and Oikonomou and its generalization are new. Although the values of $η$ calculated for both models are quite close to the observed value, they are based upon several assumptions that qualify these results. Asymmetries generated before the end of inflation must be diluted, making agreement with observation difficult to achieve unless baryogenesis occurs at the end of the inflation era. The calculations also assume rapid thermalization of the plasma of the gravitationally produced particles, and that the interaction rate of the baryon-violating processes is greater than the expansion rate of the Universe until decoupling. The present calculations strongly violate these assumptions and would require significant variation of input parameters, which is excluded by CMB observations, to obtain agreement. A complete treatment of thermalization of the gravitationally produced plasma, baryon-number violating freeze-out, and entropy dilution is required to establish whether or not gravitational baryogenesis during slow-roll inflation is a viable mechanism for producing the observed baryon asymmetry of the Universe for these inflationary $f(R)$ cosmologies.

astro-ph.CO↗

arXiv2021

Scattering of low energy neutrinos and antineutrinos by atomic electrons

Studies of neutrino mixing and oscillations, solar neutrinos as background in dark matter searches involving electron detection, detection of sterile neutrino warm dark matter, and of possible electromagnetic properties of neutrinos, have generated interest in the low energy O(10 keV) scattering of electron neutrinos and antineutrinos by atomic electrons where the binding of the atomic electron cannot be ignored. Of particular interest is the ionization of atoms by neutrinos and antineutrinos. Most existing calculations are based on modifications of the free electron differential cross section, which destroy the relationship between the neutrino helicities and the orbital and spin angular momenta of the atomic electrons. The present calculations maintain the full collision dynamics by formulating the scattering in configuration space using the Bound Interaction Picture, rather than the usual formulation in the Interaction Picture in momentum space as appropriate to scattering by free electrons. Energy spectra of ionization electrons produced by scattering of neutrinos and antineutrinos with energies of 5, 10, 20, and 30 keV by hydrogen, helium and neon have been calculated using Dirac central field eigenfunctions and are presented as ratios to the spectra for scattering by free electrons. Binding effects increase strongly with atomic number, are largest for low neutrino energy and, for each neutrino energy, greatest at the high electron energy end of the spectrum. The most extreme effects of binding are for 5 keV scattering by Ne where the ratios are less than 0.1. The energy spectra are calculated for both a Coulombic final electron state and a free final electron state. The results indicate that the binding effects from the continuum state of the final electron are significant and can be comparable to those arising from the bound initial electron state.

hep-ph↗

arXiv2016

Soft symmetry improvement of two particle irreducible effective actions

Two particle irreducible effective actions (2PIEAs) are valuable non-perturbative techniques in quantum field theory; however, finite truncations of them violate the Ward identities (WIs) of theories with spontaneously broken symmetries. The symmetry improvement (SI) method of Pilaftsis and Teresi attempts to overcome this by imposing the WIs as constraints on the solution; however the method suffers from the non-existence of solutions in linear response theory and in certain truncations in equilibrium. Motivated by this, we introduce a new method called soft symmetry improvement (SSI) which relaxes the constraint. Violations of WIs are allowed but punished in a least-squares implementation of the symmetry improvement idea. A new parameter $ξ$ controls the strength of the constraint. The method interpolates between the unimproved ($ξ\to \infty$) and SI ($ξ\to 0$) cases and the hope is that practically useful solutions can be found for finite $ξ$. We study the SSI-2PIEA for a scalar O(N) model in the Hartree-Fock approximation. We find that the method is IR sensitive: the system must be formulated in finite volume $V$ and temperature $T=β^{-1}$ and the $Vβ\to \infty$ limit taken carefully. Three distinct limits exist. Two are equivalent to the unimproved 2PIEA and SI-2PIEA respectively, and the third is a new limit where the WI is satisfied but the phase transition is strongly first order and solutions can fail to exist depending on $ξ$. Further, these limits are disconnected from each other; there is no smooth way to interpolate from one to another. These results suggest that any potential advantages of SSI methods, and indeed any application of (S)SI methods out of equilibrium, must occur in finite volume.

hep-th↗

arXiv2016

Linear Response Theory for Symmetry Improved Two Particle Irreducible Effective Actions

We investigate the linear response of an O(N) scalar quantum field theory subject to external perturbations using the symmetry improved two particle irreducible effective action formalism [A. Pilaftsis and D. Teresi, Nucl. Phys. B874, 594 (2013)]. Despite satisfactory equilibrium behavior, we find a number of unphysical effects at the linear response level. Goldstone boson field fluctuations are over-determined, with the only consistent solution being to set the fluctuations and their driving sources to zero, except for momentum modes where the Higgs and Goldstone self-energies obey a particular relationship. Also Higgs field fluctuations propagate masslessly, despite the Higgs propagator having the correct mass. These pathologies are independent of any truncation of the effective action and still exist even if we relax the over-determining Ward identities, so long as the constraint is formulated O(N)-covariantly. We discuss possible reasons for the apparent incompatibility of the constraints and linear response approximation and possible ways forward.

hep-th↗

arXiv2015

Dark matter relic density in scalar-tensor gravity revisited

We revisit the calculation of dark matter relic abundances in scalar-tensor gravity using a generic form $A(φ_*) = e^{βφ_*^2/2}$ for the coupling between the scalar field $φ_*$ and the metric, for which detailed Big Bang Nucleosynthesis constraints are available. We find that BBN constraints restrict the modified expansion rate in these models to be almost degenerate with the standard expansion history at the time of dark matter decoupling. In this case the maximum level of enhancement of the dark matter relic density was found to be a factor of $\sim 3$, several orders of magnitude below that found in previous investigations.

astro-ph.CO↗

arXiv2015

Long-range states in excited ultracold 3He-4He dimers

Long-range bound states of the excited heteronuclear 3He*--4He* system that dissociate to either 3He(1s2s 3S_1) + 4He(1s2p 3P_j) or 3He(1s2p 3P_j) + 4He(1s2s 3S_1), where j=0, 1, 2, are investigated using both single-channel and multichannel calculations in order to analyse the effects of Coriolis and non-adiabatic couplings. The multichannel calculations predict two groups of resonances above the lowest asymptotic energy. One of these groups dissociates to an atomic pair with the 2p excitation on the fermionic atom and the other dissociates to two asymptotes which correspond to the 2p excitation on either atom. Many of these resonances could be identified with levels in the single-channel calculation although the differences in energies were large. The total parity was found to have a significant influence on the ability to make these identifications. No purely bound states were found, although several resonances with line widths smaller than 1 MHz were obtained.

physics.atm-clus↗

arXiv2015

Symmetry improvement of 3PI effective actions for O(N) scalar field theory

[Abridged] n-Particle Irreducible Effective Actions ($n$PIEA) are a powerful tool for extracting non-perturbative and non-equilibrium physics from quantum field theories. Unfortunately, practical truncations of $n$PIEA can unphysically violate symmetries. Pilaftsis and Teresi (PT) addressed this by introducing a "symmetry improvement" scheme in the context of the 2PIEA for an O(2) scalar theory, ensuring that the Goldstone boson is massless in the broken symmetry phase [A. Pilaftsis and D. Teresi, Nuc.Phys. B 874, 2 (2013), pp. 594--619]. We extend this by introducing a symmetry improved 3PIEA for O(N) theories, for which the basic variables are the 1-, 2- and 3-point correlation functions. This requires the imposition of a Ward identity involving the 3-point function. The method leads to an infinity of physically distinct schemes, though an analogue of d'Alembert's principle is used to single out a unique scheme. The standard equivalence hierarchy of $n$PIEA no longer holds with symmetry improvement and we investigate the difference between the symmetry improved 3PIEA and 2PIEA. We present renormalized equations of motion and counter-terms for 2 and 3 loop truncations of the effective action, leaving their numerical solution to future work. We solve the Hartree-Fock approximation and find that our method achieves a middle ground between the unimproved 2PIEA and PT methods. The phase transition predicted by our method is weakly first order and the Goldstone theorem is satisfied. We also show that, in contrast to PT, the symmetry improved 3PIEA at 2 loops does not predict the correct Higgs decay rate, but does at 3 loops. These results suggest that symmetry improvement should not be applied to $n$PIEA truncated to $<n$ loops. We also show that symmetry improvement is compatible with the Coleman-Mermin-Wagner theorem, a check on the consistency of the formalism.

hep-th↗

arXiv2014

Dark matter relic density in Gauss-Bonnet braneworld cosmology

The relic density of symmetric and asymmetric dark matter in a Gauss-Bonnet (GB) modified Randall-Sundrum (RS) type II braneworld cosmology is investigated. The existing study of symmetric dark matter in a GB braneworld (Okada and Okada, 2009) found that the expansion rate was reduced compared to that in standard General Relativity (GR), thereby delaying particle freeze-out and resulting in relic abundances which are suppressed by up to $\mathcal{O}(10^{-2})$. This is in direct contrast to the behaviour observed in RS braneworlds where the expansion rate is enhanced and the final relic abundance boosted. However, this finding that relic abundances are suppressed in a GB braneworld is based upon a highly contrived situation in which the GB era evolves directly into a standard GR era, rather than passing through a RS era as is the general situation. This collapse of the RS era requires equating the mass scale $m_α$ of the GB modification and the mass scale $m_σ$ of the brane tension. However, if the GB contribution is to be considered as the lowest order correction from string theory to the RS action, we would expect $m_α > m_σ$. We investigate the effect upon the relic abundance of choosing more realistic values for the ratio $\mathcal{R}_{m} \equiv m_α/m_σ$ and find that the relic abundance can be either enhanced or suppressed by more than two orders of magnitude. However, suppression only occurs for a small range of parameter choices and, overwhelmingly, the predominant situation is that of enhancement as we recover the usual Randall-Sundrum type behaviour in the limit $\mathcal{R}_m \gg 1$. We use the latest observational bound $Ω_{DM}h^2 = 0.1187 \pm 0.0017$ to constrain the various model parameters and briefly discuss the implications for direct/indirect dark matter detection experiments as well as dark matter particle models.

astro-ph.CO↗

arXiv2014

Asymmetric dark matter in braneworld cosmology

We investigate the effect of a braneworld expansion era on the relic density of asymmetric dark matter. We find that the enhanced expansion rate in the early universe predicted by the Randall-Sundrum II (RSII) model leads to earlier particle freeze-out and an enhanced relic density. This effect has been observed previously by Okada and Seto (2004) for symmetric dark matter models and here we extend their results to the case of asymmetric dark matter. We also discuss the enhanced asymmetric annihilation rate in the braneworld scenario and its implications for indirect detection experiments.

astro-ph.CO↗

arXiv2011

Photoassociation spectroscopy of ultracold metastable ^3He dimers

The bound states of the fermionic ^3He(2 ^3S_1)+ ^3He(2 ^3P_j)system, where j=0,1,2, are investigated using the recently available ab initio short-range ${}^{1,3,5}Σ^{+}_{g,u}$ and ${}^{1,3,5}Π_{g,u}$ potentials computed by Deguilhem et al. (J. Phys. B: At. Mol. Opt. Phys., 2009, 42, 015102). Single-channel and multichannel calculations have been undertaken in order to investigate the effects of Coriolis and non-adiabatic couplings. The possible experimental observability of the theoretical levels is assessed using criteria based upon the short-range character of each level and their coupling to metastable ground states. Purely long-range levels have been identified and 30 short-range levels near five asymptotes are suggested for experimental investigation.

physics.atm-clus↗

arXiv2010

Photoassociation spectra and the validity of the dipole approximation for weakly bound dimers

Photoassociation (PA) of ultracold metastable helium to the 2s2p manifold is theoretically investigated using a non-perturbative close-coupled treatment in which the laser coupling is evaluated without assuming the dipole approximation. The results are compared with our previous study [Cocks and Whittingham, Phys. Rev. A 80, 023417 (2009)] that makes use of the dipole approximation. The approximation is found to strongly affect the PA spectra because the photoassociated levels are weakly bound, and a similar impact is predicted to occur in other systems of a weakly bound nature. The inclusion or not of the approximation does not affect the resonance positions or widths, however significant differences are observed in the background of the spectra and the maximum laser intensity at which resonances are discernable. Couplings not satisfying the dipole selection rule |J-1| <= J' <= |J+1| do not lead to observable resonances.

physics.atom-ph↗

arXiv2007

Autoionization of spin-polarized metastable helium in tight anisotropic harmonic traps

Spin-dipole mediated interactions between tightly confined metastable helium atoms couple the spin-polarized quintet ${}^{5}Σ^{+}_{g}$ state to the singlet ${}^{1}Σ^{+}_{g}$ state from which autoionization is highly probable, resulting in finite lifetimes for the trap eigenstates. We extend our earlier study on spherically symmetric harmonic traps to the interesting cases of axially symmetric anisotropic harmonic traps and report results for the lowest 10 states in "cigar-like" and "pancake-like" traps with average frequencies of 100 kHz and 1 MHz. We find that there is a significant suppression of ionization, and subsequent increase in lifetimes, at trap aspect ratios $A=p/q$, where $p$ and $q$ are integers, for those states that are degenerate in the absence of collisions or spin-dipole interactions.

physics.atom-ph↗

arXiv2006

Spin-dipole induced lifetime of the least-bound quintet sigma state of He(2S)+He(2S)

The properties of the least-bound vibrational level (v=14) of the quintet sigma state formed during the ultracold collision of two spin-polarized metastable helium atoms are crucial to studies of photoassociation spectroscopy of metastable helium. We report a calculation of the autoionization lifetime of this state induced by spin-dipole coupling of the quintet sigma state to the singlet sigma state from which Penning and associative ionization processes are highly probable. We find a lifetime of about 150 microseconds, significantly larger than the recent experimental estimates of (4-5) microseconds.

physics.atom-ph↗