Skip to main content

Research Repository

Advanced Search

Outputs (69)

Boson Star Superradiance (2024)
Journal Article
Gao, H., Saffin, P. M., Wang, Y., Xie, Q., & Zhou, S. (in press). Boson Star Superradiance. Science China Physics, Mechanics and Astronomy,

Recently, it has been realized that in some systems internal space rotation can induce energy amplification for scattered waves, similar to rotation in real space. In particularly, it has been shown that energy extraction is possible for a Q-ball, a... Read More about Boson Star Superradiance.

Quantum corrected Q-ball dynamics (2024)
Journal Article
Xie, Q., Saffin, P. M., Tranberg, A., & Zhou, S. (2024). Quantum corrected Q-ball dynamics. Journal of High Energy Physics, 2024(1), Article 165. https://doi.org/10.1007/jhep01%282024%29165

The physics of individual Q-balls and interactions between multiple Q-balls are well-studied in classical numerical simulations. Interesting properties and phenomena have been discovered, involving stability, forces, collisions and swapping of charge... Read More about Quantum corrected Q-ball dynamics.

Clockwork cosmology (2023)
Journal Article
Wood, K., Saffin, P. M., & Avgoustidis, A. (2023). Clockwork cosmology. Journal of Cosmology and Astroparticle Physics, 2023(07), Article 062. https://doi.org/10.1088/1475-7516/2023/07/062

The higher order generalisation of the clockwork mechanism to gravitational interactions provides a means to generate an exponentially suppressed coupling to matter from a fundamental theory of multiple interacting gravitons, without introducing larg... Read More about Clockwork cosmology.

Vertex functions and their flow equations from the 2PI effective action (2022)
Journal Article
Millington, P., & Saffin, P. M. (2022). Vertex functions and their flow equations from the 2PI effective action. Journal of Physics A: Mathematical and Theoretical, 55(43), Article 435402. https://doi.org/10.1088/1751-8121/ac99ae

By exploiting the convexity of the two-particle-irreducible effective action, we describe a procedure for extracting n-point vertex functions. This procedure is developed within the context of a zero-dimensional ‘quantum field theory’ and subsequentl... Read More about Vertex functions and their flow equations from the 2PI effective action.

Optimisation of Thimble simulations and quantum dynamics of multiple fields in real time (2022)
Journal Article
Woodward, S., Saffin, P. M., Mou, Z. G., & Tranberg, A. (2022). Optimisation of Thimble simulations and quantum dynamics of multiple fields in real time. Journal of High Energy Physics, 2022(10), Article 82. https://doi.org/10.1007/JHEP10%282022%29082

We apply the Generalised Thimble approach to the computation of exact path integrals and correlators in real-time quantum field theory. We first investigate the details of the numerical implementation and ways of optimizing the algorithm. We subseque... Read More about Optimisation of Thimble simulations and quantum dynamics of multiple fields in real time.

Charge-swapping Q-balls in a logarithmic potential and Affleck-Dine condensate fragmentation (2022)
Journal Article
Hou, S. Y., Saffin, P. M., Xie, Q. X., & Zhou, S. Y. (2022). Charge-swapping Q-balls in a logarithmic potential and Affleck-Dine condensate fragmentation. Journal of High Energy Physics, 2022(7), Article 60. https://doi.org/10.1007/JHEP07%282022%29060

We study charge-swapping Q-balls, a kind of composite Q-ball where positive and negative charges co-exist and swap with time, in models with a logarithmic potential that arises naturally in supersymmetric extensions of the Standard Model. We show tha... Read More about Charge-swapping Q-balls in a logarithmic potential and Affleck-Dine condensate fragmentation.

Gravitational collapse of quantum fields and Choptuik scaling (2022)
Journal Article
Berczi, B., Saffin, P. M., & Zhou, S. (2022). Gravitational collapse of quantum fields and Choptuik scaling. Journal of High Energy Physics, 2022(2), Article 183. https://doi.org/10.1007/jhep02%282022%29183

Gravitational collapse into a black hole has been extensively studied with classical sources. We develop a new formalism to simulate quantum fields forming a black hole. By choosing a convenient coherent state, this formalism taps into well-establish... Read More about Gravitational collapse of quantum fields and Choptuik scaling.

Benchmarking regulator-sourced 2PI and average 1PI flow equations in zero dimensions (2021)
Journal Article
Millington, P., & Saffin, P. M. (2021). Benchmarking regulator-sourced 2PI and average 1PI flow equations in zero dimensions. Journal of Physics A: Mathematical and Theoretical, 54(46), Article 465401. https://doi.org/10.1088/1751-8121/ac2d00

We elucidate the regulator-sourced 2PI and average 1PI approaches for deriving exact flow equations in the case of a zero dimensional quantum field theory, wherein the scale dependence of the usual renormalisation group evolution is replaced by a sim... Read More about Benchmarking regulator-sourced 2PI and average 1PI flow equations in zero dimensions.

Erratum: Alternative flow equation for the functional renormalization group [Phys. Rev. D 100, 101702(R) (2019)] (2021)
Journal Article
Alexander, E., Millington, P., Nursey, J., & Saffin, P. M. (2021). Erratum: Alternative flow equation for the functional renormalization group [Phys. Rev. D 100, 101702(R) (2019)]. Physical Review D, 104(6), Article 069906. https://doi.org/10.1103/physrevd.104.069906

We correct the functional identity reported in our paper for the inverse two-point function, as obtained from the 2PI effective action. The original conclusions are otherwise unaffected. The 2PI effective action is convex with respect to the variable... Read More about Erratum: Alternative flow equation for the functional renormalization group [Phys. Rev. D 100, 101702(R) (2019)].

Gravitational collapse with quantum fields (2021)
Journal Article
Berczi, B., Saffin, P. M., & Zhou, S. (2021). Gravitational collapse with quantum fields. Physical Review D, 104(4), Article L041703. https://doi.org/10.1103/PhysRevD.104.L041703

Gravitational collapse into a black hole has been extensively studied with classical sources. We develop a new formalism to simulate quantum fields forming a black hole. This formalism utilizes well-established techniques used for classical collapse... Read More about Gravitational collapse with quantum fields.

Charge-Swapping Q-balls and Their Lifetimes (2021)
Journal Article
Xie, Q., Saffin, P. M., & Zhou, S. (2021). Charge-Swapping Q-balls and Their Lifetimes. Journal of High Energy Physics, 2021(7), Article 62. https://doi.org/10.1007/jhep07%282021%29062

For scalar theories accommodating spherically symmetric Q-balls, there are also towers of quasi-stable composite Q-balls, called charge swapping Q-balls (CSQs). We investigate the properties, particularly the lifetimes, of these long-lived CSQs in 2+... Read More about Charge-Swapping Q-balls and Their Lifetimes.

Dipole radiation and beyond from axion stars in electromagnetic fields (2021)
Journal Article
Amin, M. A., Long, A. J., Mou, Z., & Saffin, P. M. (2021). Dipole radiation and beyond from axion stars in electromagnetic fields. Journal of High Energy Physics, 2021(6), Article 182. https://doi.org/10.1007/jhep06%282021%29182

We investigate the production of photons from coherently oscillating, spatially localized clumps of axionic fields (oscillons and axion stars) in the presence of external electromagnetic fields. We delineate different qualitative behaviour of the pho... Read More about Dipole radiation and beyond from axion stars in electromagnetic fields.

Deconstructing higher order clockwork gravity (2021)
Journal Article
Avgoustidis, A., Niedermann, F., Padilla, A., & Saffin, P. M. (2021). Deconstructing higher order clockwork gravity. Physical Review D, 103(12), Article 124007. https://doi.org/10.1103/physrevd.103.124007

We consider the higher order clockwork theory of gravitational interactions, whereby a number of gravitons are coupled together with TeV strength, but nevertheless generate a Planck scale coupling to matter without the need for a dilaton. It is shown... Read More about Deconstructing higher order clockwork gravity.

Statistics on Lefschetz thimbles: Bell/Leggett-Garg inequalities and the classical-statistical approximation (2021)
Journal Article
Millington, P., Mou, Z., Saffin, P. M., & Tranberg, A. (2021). Statistics on Lefschetz thimbles: Bell/Leggett-Garg inequalities and the classical-statistical approximation. Journal of High Energy Physics, 2021(3), Article 77. https://doi.org/10.1007/jhep03%282021%29077

Inspired by Lefschetz thimble theory, we treat Quantum Field Theory as a statistical theory with a complex Probability Distribution Function (PDF). Such complex-valued PDFs permit the violation of Bell-type inequalities, which cannot be violated by a... Read More about Statistics on Lefschetz thimbles: Bell/Leggett-Garg inequalities and the classical-statistical approximation.

Simulations of a bubble wall interacting with an electroweak plasma (2021)
Journal Article
Mou, Z., Saffin, P. M., & Tranberg, A. (2021). Simulations of a bubble wall interacting with an electroweak plasma. Journal of High Energy Physics, 2021(2), Article 189. https://doi.org/10.1007/jhep02%282021%29189

We perform large-scale real-time simulations of a bubble wall sweeping through an out-of-equilibrium plasma. The scenario we have in mind is the electroweak phase transition, which may be first order in extensions of the Standard Model, and produce s... Read More about Simulations of a bubble wall interacting with an electroweak plasma.

Classical decay rates of oscillons (2020)
Journal Article
Zhang, H., Amin, M. A., Copeland, E. J., Saffin, P. M., & Lozanov, K. D. (2020). Classical decay rates of oscillons. Journal of Cosmology and Astroparticle Physics, 2020(07), 055-055. https://doi.org/10.1088/1475-7516/2020/07/055

Oscillons are extremely long-lived, spatially-localized field configurations in real-valued scalar field theories that slowly lose energy via radiation of scalar waves. Before their eventual demise, oscillons can pass through (one or more) exceptiona... Read More about Classical decay rates of oscillons.

Black holes in self-tuning cubic Horndeski cosmology (2020)
Journal Article
Emond, W. T., Lehébel, A., & Saffin, P. M. (2020). Black holes in self-tuning cubic Horndeski cosmology. Physical Review D, 101(8), Article 084008. https://doi.org/10.1103/physrevd.101.084008

Observations of neutron star mergers in the late Universe have given significant restrictions to the class of viable scalar-tensor theories. In this paper we construct black holes within the “self-tuning” class of this restricted set, whereby the bar... Read More about Black holes in self-tuning cubic Horndeski cosmology.

Alternative flow equation for the functional renormalization group (2019)
Journal Article
Alexander, E., Millington, P., Nursey, J., & Saffin, P. M. (2019). Alternative flow equation for the functional renormalization group. Physical Review D, 100(10), Article 101702(R). https://doi.org/10.1103/physrevd.100.101702

We derive an alternative to the Wetterich-Morris-Ellwanger equation by means of the two-particle irreducible (2PI) effective action, exploiting the method of external sources due to Garbrecht and Millington. The latter allows the two-point source of... Read More about Alternative flow equation for the functional renormalization group.

Quantum tunnelling, real-time dynamics and Picard-Lefschetz thimbles (2019)
Journal Article
Mou, Z., Saffin, P. M., & Tranberg, A. (2019). Quantum tunnelling, real-time dynamics and Picard-Lefschetz thimbles. Journal of High Energy Physics, 2019(11), https://doi.org/10.1007/jhep11%282019%29135

We follow up the work, where in light of the Picard-Lefschetz thimble approach, we split up the real-time path integral into two parts: the initial density matrix part which can be represented via an ensemble of initial conditions, and the dynamic pa... Read More about Quantum tunnelling, real-time dynamics and Picard-Lefschetz thimbles.

Visualising quantum effective action calculations in zero dimensions (2019)
Journal Article
Millington, P., & Saffin, P. M. (2019). Visualising quantum effective action calculations in zero dimensions. Journal of Physics A: Mathematical and Theoretical, 52(40), Article 405401. https://doi.org/10.1088/1751-8121/ab37e6

© 2019 IOP Publishing Ltd. We present an explicit treatment of the two-particle-irreducible (2PI) effective action for a zero-dimensional quantum field theory. The advantage of this simple playground is that we are required to deal only with function... Read More about Visualising quantum effective action calculations in zero dimensions.

Real-time quantum dynamics, path integrals and the method of thimbles (2019)
Journal Article
Mou, Z., Saffin, P. M., Tranberg, A., & Woodward, S. (2019). Real-time quantum dynamics, path integrals and the method of thimbles. Journal of High Energy Physics, 2019(6), https://doi.org/10.1007/jhep06%282019%29094

Direct numerical evaluation of the real-time path integral has a well-known sign problem that makes convergence exponentially slow. One promising remedy is to use Picard-Lefschetz theory to flow the domain of the field variables into the complex plan... Read More about Real-time quantum dynamics, path integrals and the method of thimbles.

Well-tempered cosmology (2019)
Journal Article
Emond, W. T., Li, C., Saffin, P. M., & Zhou, S. (2019). Well-tempered cosmology. Journal of Cosmology and Astroparticle Physics, 2019(05), https://doi.org/10.1088/1475-7516/2019/05/038

We examine an approach to cosmology, known as Well-Tempering, that allows for a de Sitter phase whose expansion is independent of the cosmological constant. Starting from a generic scalar-tensor theory compatible with the recent gravitational wave ob... Read More about Well-tempered cosmology.

Dark Energy after GW170817 Revisited (2019)
Journal Article
Copeland, E. J., Kopp, M., Padilla, A., Saffin, P. M., & Skordis, C. (2019). Dark Energy after GW170817 Revisited. Physical Review Letters, 122(6), Article 061301. https://doi.org/10.1103/physrevlett.122.061301

We revisit the status of scalar-tensor theories with applications to dark energy in the aftermath of the gravitational wave signal GW170817 and its optical counterpart GRB170817A. At the level of the cosmological background, we identify a class of th... Read More about Dark Energy after GW170817 Revisited.

Higher order clockwork gravity (2018)
Journal Article
Niedermann, F., Padilla, A., & Saffin, P. M. (2018). Higher order clockwork gravity. Physical Review D, 98(10), Article 104014. https://doi.org/10.1103/physrevd.98.104014

We present a higher order generalisation of the clockwork mechanism starting from an underlying non-linear multigravity theory with a single scale and nearest neighbour ghost-free interactions. Without introducing any hierarchies in the underlying po... Read More about Higher order clockwork gravity.

Boltzmann equations for preheating (2018)
Journal Article
Emond, W. T., Millington, P., & Saffin, P. M. (2018). Boltzmann equations for preheating. Journal of Cosmology and Astroparticle Physics, 2018(09), doi:10.1088/1475-7516/2018/09/041

We derive quantum Boltzmann equations for preheating by means of the density matrix formalism, which account for both the non-adiabatic particle production and the leading collisional processes between the produced particles. In so doing, we illustra... Read More about Boltzmann equations for preheating.

Rotating kinky braneworlds (2018)
Journal Article
Niedermann, F., & Saffin, P. M. (2018). Rotating kinky braneworlds. Journal of High Energy Physics, 2018(7), Article 183. https://doi.org/10.1007/jhep07%282018%29183

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 preventin... Read More about Rotating kinky braneworlds.

Simulations of cold electroweak baryogenesis: dependence on the source of CP-violation (2018)
Journal Article
Mou, Z., Saffin, P. M., & Tranberg, A. (2018). Simulations of cold electroweak baryogenesis: dependence on the source of CP-violation. Journal of High Energy Physics, 2018(5), Article 197. https://doi.org/10.1007/JHEP05%282018%29197

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 consider... Read More about Simulations of cold electroweak baryogenesis: dependence on the source of CP-violation.

Simulations of Cold Electroweak Baryogenesis: quench from portal coupling to new singlet field (2018)
Journal Article
Mou, Z., Saffin, P. M., & Tranberg, A. (2018). Simulations of Cold Electroweak Baryogenesis: quench from portal coupling to new singlet field. Journal of High Energy Physics, 2018(1), Article 103. https://doi.org/10.1007/JHEP01%282018%29103

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... Read More about Simulations of Cold Electroweak Baryogenesis: quench from portal coupling to new singlet field.

Recrudescence of massive fermion production by oscillons (2017)
Journal Article
Saffin, P. M. (in press). Recrudescence of massive fermion production by oscillons. Journal of High Energy Physics, 2017(7), Article 126. https://doi.org/10.1007/JHEP07%282017%29126

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 h... Read More about Recrudescence of massive fermion production by oscillons.

Simulations of “tunnelling of the 3rd kind” (2017)
Journal Article
Mou, Z., Saffin, P. M., Tognarelli, P., & Tranberg, A. (2017). Simulations of “tunnelling of the 3rd kind”. Journal of High Energy Physics, 2017(7), https://doi.org/10.1007/JHEP07%282017%29015

We consider the phenomenon of ``tunnelling of the 3rd kind" \cite{third}, 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 magn... Read More about Simulations of “tunnelling of the 3rd kind”.

Simulations of Cold Electroweak Baryogenesis: finding the optimal quench time (2017)
Journal Article
Mou, Z., Saffin, P. M., & Tranberg, A. (2017). Simulations of Cold Electroweak Baryogenesis: finding the optimal quench time. Journal of High Energy Physics, 2017(7), https://doi.org/10.1007/JHEP07%282017%29010

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 maximu... Read More about Simulations of Cold Electroweak Baryogenesis: finding the optimal quench time.

Simulations of Cold Electroweak Baryogenesis: hypercharge U(1) and the creation of helical magnetic fields (2017)
Journal Article
Mou, Z., Saffin, P. M., & Tranberg, A. (2017). Simulations of Cold Electroweak Baryogenesis: hypercharge U(1) and the creation of helical magnetic fields. Journal of High Energy Physics, 2017(6), https://doi.org/10.1007/JHEP06%282017%29075

We perform numerical simulations of Cold Electroweak Baryogenesis, including for the first time in the Bosonic sector the full electroweak gauge group SU(2)×U(1) and CP-violation. We find that the maximum generated baryon asymmetry is reduced by a fa... Read More about Simulations of Cold Electroweak Baryogenesis: hypercharge U(1) and the creation of helical magnetic fields.

Highly excited strings I: Generating function (2017)
Journal Article
Skliros, D. P., Copeland, E. J., & Saffin, P. M. (in press). Highly excited strings I: Generating function. Nuclear Physics B, https://doi.org/10.1016/j.nuclphysb.2016.12.022

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 forma... Read More about Highly excited strings I: Generating function.

Disformally self-tuning gravity (2016)
Journal Article
Emond, W. T., & Saffin, P. M. (in press). Disformally self-tuning gravity. Journal of High Energy Physics, 2016(3), https://doi.org/10.1007/JHEP03%282016%29161

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 ma... Read More about Disformally self-tuning gravity.

The Parametrized Post-Newtonian-Vainshteinian formalism (2015)
Journal Article
Avilez-Lopez, A., Padilla, A., Saffin, P. M., & Skordis, C. (2015). The Parametrized Post-Newtonian-Vainshteinian formalism. Journal of Cosmology and Astroparticle Physics, 2015(6), Article 044. https://doi.org/10.1088/1475-7516/2015/06/044

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 i... Read More about The Parametrized Post-Newtonian-Vainshteinian formalism.

Cold Baryogenesis from first principles in the two-Higgs doublet model with fermions (2015)
Journal Article
Mou, Z., Saffin, P. M., & Tranberg, A. (2015). Cold Baryogenesis from first principles in the two-Higgs doublet model with fermions. Journal of High Energy Physics, 2015, https://doi.org/10.1007/JHEP06%282015%29163

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... Read More about Cold Baryogenesis from first principles in the two-Higgs doublet model with fermions.

Unitarity and the Vainshtein mechanism (2015)
Journal Article
Kaloper, N., Padilla, A., Saffin, P. M., & Stefanyszyn, D. (2015). Unitarity and the Vainshtein mechanism. Physical Review D, 91(4), https://doi.org/10.1103/PhysRevD.91.045017

We investigate low energy limits of massive gauge theories that feature the Vainshtein mechanism, focussing on the effects of the UV modes that are integrated out. It turns out that the Goldstone sectors are significantly influenced by the effects fr... Read More about Unitarity and the Vainshtein mechanism.

Charge-swapping q balls (2014)
Journal Article
Copeland, E. J., Saffin, P. M., & Zhou, S. Y. (2014). Charge-swapping q balls. Physical Review Letters, 113(23), https://doi.org/10.1103/PhysRevLett.113.231603

© 2014 American Physical Society. Q balls are nontopological 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... Read More about Charge-swapping q balls.

Oscillon lifetime in the presence of quantum fluctuations (2014)
Journal Article
Saffin, P. M., Tognarelli, P., & Tranberg, A. (2014). Oscillon lifetime in the presence of quantum fluctuations. Journal of High Energy Physics, 2014(8), Article 125. https://doi.org/10.1007/JHEP08%282014%29125

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... Read More about Oscillon lifetime in the presence of quantum fluctuations.

Ensemble fermions for electroweak dynamics and the fermion preheating temperature (2013)
Journal Article
Mou, Z., Tranberg, A., & Saffin, P. M. (2013). Ensemble fermions for electroweak dynamics and the fermion preheating temperature. Journal of High Energy Physics, 2013(11), https://doi.org/10.1007/JHEP11%282013%29097

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... Read More about Ensemble fermions for electroweak dynamics and the fermion preheating temperature.

Gravitational waves from oscillon preheating (2013)
Journal Article
Zhou, S., Copeland, E. J., Easther, R., Finkel, H., Mou, Z., & Saffin, P. M. (2013). Gravitational waves from oscillon preheating. Journal of High Energy Physics, 2013, Article 26. https://doi.org/10.1007/JHEP10%282013%29026

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 axi... Read More about Gravitational waves from oscillon preheating.

Adiabatic and isocurvature perturbation projections in multi-field inflation (2013)
Journal Article
Gordon, C., & Saffin, P. M. (2013). Adiabatic and isocurvature perturbation projections in multi-field inflation. Journal of Cosmology and Astroparticle Physics, 2013(8), Article 021. https://doi.org/10.1088/1475-7516/2013/08/021

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... Read More about Adiabatic and isocurvature perturbation projections in multi-field inflation.

Duality and decay of macroscopic F-strings (2013)
Journal Article
Skliros, D. P., Copeland, E. J., & Saffin, P. M. (2013). Duality and decay of macroscopic F-strings. Physical Review Letters, 111(4), Article 041601. https://doi.org/10.1103/PhysRevLett.111.041601

We study the decay of fundamental string loops of arbitrary size L/min(n,m)≫√α′, labeled by (n, m; λn, ¯λm), where n, m correspond to left- and right-mover harmonics and λn, ¯λm to polarization tensors, and find that a description in terms of the rec... Read More about Duality and decay of macroscopic F-strings.

The covariance of multi-field perturbations, pseudo-susy and fNL (2012)
Journal Article
Saffin, P. M. (2012). The covariance of multi-field perturbations, pseudo-susy and fNL. Journal of Cosmology and Astroparticle Physics, 2012(9), Article 002. https://doi.org/10.1088/1475-7516/2012/09/002

We reconsider cosmological perturbation theory for multi-component scalars, enforcing covariance in field-space, and ensuring that phyical observations are independent of field re-definitions. We use the formalism to clarify some issues in the litera... Read More about The covariance of multi-field perturbations, pseudo-susy and fNL.

Classical duals, Legendre transforms and the Vainshtein mechanism (2012)
Journal Article
Padilla, A., & Saffin, P. M. (2012). Classical duals, Legendre transforms and the Vainshtein mechanism. Journal of High Energy Physics, 2012(7), Article 122. https://doi.org/10.1007/JHEP07%282012%29122

We show how to generalize the classical duals found by Gabadadze {et al} to a very large class of self-interacting theories. This enables one to adopt a perturbative description beyond the scale at which classical perturbation theory breaks down in t... Read More about Classical duals, Legendre transforms and the Vainshtein mechanism.

Self-tuning and the derivation of a class of scalar-tensor theories (2012)
Journal Article
Charmousis, C., Copeland, E. J., Padilla, A., & Saffin, P. M. (2012). Self-tuning and the derivation of a class of scalar-tensor theories. Physical Review D, 85(10), Article 104040. https://doi.org/10.1103/PhysRevD.85.104040

We have recently proposed a special class of scalar tensor theories known as the Fab Four. These arose from attempts to analyse the cosmological constant problem within the context of Horndeski's most general scalar tensor theory. The Fab Four togeth... Read More about Self-tuning and the derivation of a class of scalar-tensor theories.

Dynamical simulations of electroweak baryogenesis with fermions (2012)
Journal Article
Saffin, P. M., & Tranberg, A. (2012). Dynamical simulations of electroweak baryogenesis with fermions. Journal of High Energy Physics, 2012(2), Article 102. https://doi.org/10.1007/JHEP02%282012%29102

We perform real-time numerical lattice simulations of a one-family version of the Standard Model. We model the quantum fermions using the ensemble method and treat the bosonic scalar and non-abelian gauge fields classically. Our main interest is elec... Read More about Dynamical simulations of electroweak baryogenesis with fermions.

General second-order scalar-tensor theory and self-tuning (2012)
Journal Article
Charmousis, C., Copeland, E. J., Saffin, P. M., & Padilla, A. (2012). General second-order scalar-tensor theory and self-tuning. Physical Review Letters, 108(5), Article 051101. https://doi.org/10.1103/PhysRevLett.108.051101

Starting from the most general scalar-tensor theory with second order field equations in four dimensions, we establish the unique action that will allow for the existence of a consistent self-tuning mechanism on FLRW backgrounds, and show how it can... Read More about General second-order scalar-tensor theory and self-tuning.

Reducing heterotic M-theory to five dimensional supergravity on a manifold with boundary (2011)
Journal Article
Moss, I. G., Omotani, J. T., & Saffin, P. M. (2011). Reducing heterotic M-theory to five dimensional supergravity on a manifold with boundary. Journal of High Energy Physics, 2011(10), https://doi.org/10.1007/JHEP10%282011%29088

This paper constructs the reduction of heterotic M-theory in eleven dimensions to a supergravity model on a manifold with boundary in five dimensions using a Calabi-Yau three-fold. New results are presented for the boundary terms in the action and fo... Read More about Reducing heterotic M-theory to five dimensional supergravity on a manifold with boundary.

Colliding branes and big crunches (2011)
Journal Article
Omotani, J., Saffin, P. M., & Louko, J. (2011). Colliding branes and big crunches. Physical Review D, 84(6), Article 063526. https://doi.org/10.1103/PhysRevD.84.063526

We examine the global structure of colliding domain walls in AdS spacetime and come to the conclusion that singularities forming from such collisions are of the big-crunch type rather than that of a black brane.

Real-time fermions for baryogenesis simulations (2011)
Journal Article
Saffin, P. M., & Tranberg, A. (2011). Real-time fermions for baryogenesis simulations. Journal of High Energy Physics, 2011(7), https://doi.org/10.1007/JHEP07%282011%29066

We study how to numerically simulate quantum fermions out of thermal equilibrium, in the context of electroweak baryogenesis. We find that by combining the lattice implementation of Aarts and Smit [1] with the "low cost" fermions of Borsanyi and Hind... Read More about Real-time fermions for baryogenesis simulations.

Multi-Galileons, solitons, and Derrick’s theorem (2011)
Journal Article
Padilla, A., Saffin, P. M., & Zhou, S. (2011). Multi-Galileons, solitons, and Derrick’s theorem. Physical Review D, 83(4), https://doi.org/10.1103/PhysRevD.83.045009

The field theory Galilean symmetry, which was introduced in the context of modified gravity, gives a neat way to construct Lorentz-covariant theories of a scalar field, such that the equations of motion contain at most second-order derivatives. Here... Read More about Multi-Galileons, solitons, and Derrick’s theorem.

Bi-galileon theory II: phenomenology (2011)
Journal Article
Padilla, A., Saffin, P. M., & Zhou, S. (2011). Bi-galileon theory II: phenomenology. Journal of High Energy Physics, 2011(1), https://doi.org/10.1007/JHEP01%282011%29099

We continue to introduce bi-galileon theory, the generalisation of the single galileon model introduced by Nicolis et al. The theory contains two coupled scalar fields and is described by a Lagrangian that is invariant under Galilean shifts in those... Read More about Bi-galileon theory II: phenomenology.

Bi-galileon theory I: motivation and formulation (2010)
Journal Article
Padilla, A., Saffin, P. M., & Zhou, S. (2010). Bi-galileon theory I: motivation and formulation. Journal of High Energy Physics, 2010(12), https://doi.org/10.1007/JHEP12%282010%29031

We introduce bi-galileon theory, the generalisation of the single galileon model introduced by Nicolis et al. The theory contains two coupled scalar fields and is described by a Lagrangian that is invariant under Galilean shifts in those fields. This... Read More about Bi-galileon theory I: motivation and formulation.

The gravitational dynamics of warped throats (2010)
Journal Article
Butcher, N. A., & Saffin, P. M. (in press). The gravitational dynamics of warped throats. Journal of High Energy Physics, 2010(6), https://doi.org/10.1007/JHEP06%282010%29044

We investigate the time evolution due to gravitational dynamics of a particular spacetime commonly used in brane-cosmology and string compactifications, namely the Klebanov-Strassler geometry, which is achieved by adding a perturbation to the momentu... Read More about The gravitational dynamics of warped throats.

Evolution and stability of cosmic string loops with Y-junctions (2009)
Journal Article
Bevis, N., Copeland, E. J., Martin, P., Niz, G., Pourtsidou, A., Saffin, P. M., & Steer, D. (2009). Evolution and stability of cosmic string loops with Y-junctions. Physical Review D, 80(12), https://doi.org/10.1103/PhysRevD.80.125030

We study the evolution of non-periodic cosmic string loops containing Y-junctions, such as may form during the evolution of a network of (p,q) cosmic superstrings. We set up and solve the Nambu-Goto equations of motion for a loop with junctions, focu... Read More about Evolution and stability of cosmic string loops with Y-junctions.

Strong coupling in Ho?ava gravity (2009)
Journal Article
Charmousis, C., Niz, G., Padilla, A., & Saffin, P. M. (2009). Strong coupling in Ho?ava gravity. Journal of High Energy Physics, 2009(8), https://doi.org/10.1088/1126-6708/2009/08/070

By studying perturbations about the vacuum, we show that Horava gravity suffers from two different strong coupling problems, extending all the way into the deep infra-red. The first of these is associated with the principle of detailed balance and ex... Read More about Strong coupling in Ho?ava gravity.

Cosmological constraints on f(G) dark energy models (2009)
Journal Article
Zhou, S., Copeland, E. J., & Saffin, P. M. (2009). Cosmological constraints on f(G) dark energy models. Journal of Cosmology and Astroparticle Physics, 2009(7), https://doi.org/10.1088/1475-7516/2009/07/009

Modified gravity theories with the Gauss-Bonnet term G=R2?4R??R??+R????R???? have recently gained a lot of attention as a possible explanation of dark energy. We perform a thorough phase space analysis on the so-called f(G) models, where f(G) is some... Read More about Cosmological constraints on f(G) dark energy models.

The evolution of conifolds (2008)
Journal Article
Butcher, N. A., & Saffin, P. M. (2008). The evolution of conifolds. Journal of High Energy Physics, 2008(12), https://doi.org/10.1088/1126-6708/2008/12/012

We simulate the gravitational dynamics of the conifold geometries (resolved and deformed) involved in the description of certain compact spacetimes. As the cycles of the conifold collapse towards a singular geometry we find that a horizon develops, s... Read More about The evolution of conifolds.

Some stationary properties of a Q-ball in arbitrary space dimensions (2008)
Journal Article
Tsumagari, M. I., Copeland, E. J., & Saffin, P. M. (2008). Some stationary properties of a Q-ball in arbitrary space dimensions. Physical Review D, 78(6), https://doi.org/10.1103/PhysRevD.78.065021

Introducing new physically motivated ansaetze, we explore both analytically and numerically the classical and absolute stabilities of a single Q-ball in an arbitrary number of spatial dimensions D, working in both the thin and thick wall limits.

Transmission of an inhomogeneous state via resonant tunnelling (2008)
Journal Article
Saffin, P. M., Padilla, A., & Copeland, E. J. (2008). Transmission of an inhomogeneous state via resonant tunnelling. Journal of High Energy Physics, 2008(9), https://doi.org/10.1088/1126-6708/2008/09/055

We recently investigated the nature of resonant tunnelling in standard scalar Quantum Field Theory, uncovering the conditions required for resonance. It was shown that whereas the homogeneous false vacuum may decay via bubble nucleation, it may not d... Read More about Transmission of an inhomogeneous state via resonant tunnelling.

Cosmic string Y-junctions: a comparison between field theoretic and Nambu-Goto dynamics (2008)
Journal Article
Bevis, N., & Saffin, P. M. (2008). Cosmic string Y-junctions: a comparison between field theoretic and Nambu-Goto dynamics. Physical Review D, 78(2), https://doi.org/10.1103/PhysRevD.78.023503

We explore the formation of cosmic string Y-junctions when strings of two different types collide, which has recently become important since string theory can yield cosmic strings of distinct types. Using a model containing two types of local U(1) st... Read More about Cosmic string Y-junctions: a comparison between field theoretic and Nambu-Goto dynamics.

The fermion spectrum in braneworld collisions (2007)
Journal Article
Saffin, P. M., & Tranberg, A. (2007). The fermion spectrum in braneworld collisions. Journal of High Energy Physics, 2007(12), https://doi.org/10.1088/1126-6708/2007/12/053

In braneworld collisions fermions originally localised on one brane can be transferred to another brane, or to a space-time boundary. By modelling branes as scalar field kinks we bounce them off boundaries and study resulting effects according to a b... Read More about The fermion spectrum in braneworld collisions.

Singular manifolds, topology change and the dynamics of compactification (2007)
Journal Article
Butcher, N. A., & Saffin, P. M. (2007). Singular manifolds, topology change and the dynamics of compactification. Journal of High Energy Physics, 2007(11), https://doi.org/10.1088/1126-6708/2007/11/062

We investigate the dynamics of the geometric transitions associated to compactified spacetimes. By including the dynamics of gravity we are able to follow the evolution of collapsing cycles as they attempt to undergo a topology changing transition. R... Read More about Singular manifolds, topology change and the dynamics of compactification.

Particle transfer in braneworld collisions (2007)
Journal Article
Saffin, P. M., & Tranberg, A. (2007). Particle transfer in braneworld collisions. Journal of High Energy Physics, 2007(8), https://doi.org/10.1088/1126-6708/2007/08/072

We study the behaviour of fermions localized on moving kinks as these collide with either antikinks or spacetime boundaries. We numerically solve for the evolution of the scalar kinks and the bound (i.e. localized) fermion modes, and calculate the nu... Read More about Particle transfer in braneworld collisions.

Oscillons and quasi-breathers in D+1 dimensions (2007)
Journal Article
Saffin, P. M., & Tranberg, A. (2007). Oscillons and quasi-breathers in D+1 dimensions. Journal of High Energy Physics, 2007(1), https://doi.org/10.1088/1126-6708/2007/01/030

We study oscillons in D+1 space-time dimensions using a spherically symmetric ansatz. From Gaussian initial conditions, these evolve by emitting radiation, approaching ``quasi-breathers'', near-periodic solutions to the equations of motion. Using a t... Read More about Oscillons and quasi-breathers in D+1 dimensions.

Scaling solutions and geodesics in moduli space (2006)
Journal Article
Karthauser, J., & Saffin, P. M. (2006). Scaling solutions and geodesics in moduli space. Classical and Quantum Gravity, 23(14), https://doi.org/10.1088/0264-9381/23/14/004

In this paper we consider cosmological scaling solutions in general relativity coupled to scalar fields with a non-trivial moduli space metric. We discover that the scaling property of the cosmology is synonymous with the scalar fields tracing out a... Read More about Scaling solutions and geodesics in moduli space.

The Dynamics of coset dimensional reduction (2006)
Journal Article
Karthauser, J. L., & Saffin, P. M. (2006). The Dynamics of coset dimensional reduction. Physical Review D, 73(8), https://doi.org/10.1103/PhysRevD.73.084027

The evolution of multiple scalar fields in cosmology has been much studied, particularly when the potential is formed from a series of exponentials. For a certain subclass of such systems it is possible to get `assisted` behaviour, where the presence... Read More about The Dynamics of coset dimensional reduction.