arXiv subjects

Brett Altschul

Publications and source records attributed to Brett Altschul.

At least 19 recordsLinked to original sources

arXiv2026

Gravitational Wave Parity-Violating Strain from Pulsar Glitches in Chern-Simons Modified Gravity

We investigate gravitational-wave birefringence from pulsar glitches in dynamical Chern-Simons gravity, a parity-violating extension of general relativity motivated by string theory and quantum gravity. Using the Hartle-Thorne slow-rotation formalism to model the neutron star background and a perturbative treatment of the Chern-Simons coupling $\alpha$, we derive the modified Regge-Wheeler equation governing axial gravitational perturbations and compute the resulting polarization-dependent phase shift. The parity-violating interaction produces a fractional asymmetry between the right- and left-handed circular polarizations that scales as $\alpha^2$, grows linearly with frequency and rotation rate, and falls as the fourth power of the stellar radius. For a constant-density interior model we obtain an analytic matching solution for the background scalar field, and show that the interior curvature factor vanishes identically--the interior solution being conformally flat--so that the scalar dipole is sourced entirely in the vacuum exterior; a centrally condensed equation of state can only increase the predicted signal. We further extend the analysis to a two-fluid model incorporating differential rotation between the neutron superfluid and the charged component; the resulting correction is bounded by the fractional glitch amplitude itself, independently of the equation of state, and is negligible for typical pulsar parameters. Expressed through the dimensionless quantity $\zeta \propto \alpha M/R_\star^{3}$, the fractional polarization asymmetry for millisecond pulsars reaches $\sim 10^{-3}$ at kilohertz frequencies at the boundary of perturbative validity--four orders of magnitude above the sensitivity of current ground-based detectors to strain asymmetries. However, realistic bounds are limited by the signal-to-noise ratio required to resolve a ratio of gravitational wave strains.

gr-qc↗

arXiv2025

Primordial Gravitational Wave Birefringence in a de Sitter Background with Chern-Simons Coupling

In this work, we investigate tensor perturbations in a de Sitter background within the framework of Chern-Simons modified gravity. We introduce transverse-traceless perturbations and analyze how the Chern-Simons Cotton tensor induces parity-violating modifications to gravitational wave propagation, while the Pontryagin density vanishes at linear order. Using a mode decomposition of the scalar background field, we derive the sub- and super-horizon limits of the wave equations and uncover chiral corrections in the dispersion relations of tensor modes. The resulting birefringence exhibits both amplitude and velocity components, alternating with the phase of the scalar field. Particular solutions sourced by the scalar background show helicity-dependent amplification and a characteristic scaling of the radiated flux that reduces smoothly to the Minkowski limit. The accumulated phase difference between right- and left-handed modes grows quadratically inside the horizon and becomes frozen outside, leaving a permanent parity-violating imprint in the primordial tensor spectrum. Finally, by promoting the Chern-Simons field to a massive dark matter candidate, we demonstrate how its mass-dependent dynamics connect gravitational birefringence to axion-like dark matter phenomenology.

gr-qc↗

arXiv2025

First Order Axial Perturbation of the Reissner-Nordstr\"{o}m Metric in a Possible Parity-Violating Gravity Background

We study axial perturbations of Reissner-Nordstr\"{o}m black holes within the general framework of parity-violating modified gravity theories. We derive the governing equations for a class of frame-dragging perturbations, focusing on the symmetry structure and radial dependence of the perturbed metric component, describing its behavior across three distinct regions: near the singularity ($r \rightarrow 0$), between the inner and outer Reissner-Nordstr\"{o}m horizons ($r_-< r< r_+$), and in the asymptotic exterior regime ($r \rightarrow \infty$). Using a combination of analytical and numerical methods, we analyze the solutions for varying black hole charge-to-mass ratios ($Q/M$) and angular momentum parameters ($l$). Key findings include the suppression of perturbations by the electromagnetic field for higher $Q/M$; the emergence of radial resonance-like behavior for specific $l$ values; and a high degree of symmetry for solutions in the extremal limit ($Q/M \sim 1$), attributed to the AdS$_2 \times S^2$ near-horizon geometry. The WKB approximation is employed to study the high-$l$ regime, revealing quantized radial resonance modes and singular behavior in the extremal limit. Additionally, we explore the role of boundary conditions and the possibility of a Chern-Simons field $\Theta$ as the source of the parity violation, showing that consistency and the behavior of the perturbations under time reversal demand a constant field (and thus no actually observable Chern-Simons effects) at leading order. These results provide a basis for further analysis of the stability and dynamical properties of charged black holes in parity-violating theories, with potential experimental signatures in gravitational wave observations.

gr-qc↗

arXiv2025

Renormalization Group Running of the Parity Operator in Lorentz-Violating Quantum Field Theory

In conventional relativistic quantum field theory, the discrete operators $\textbf{C}$, $\textbf{P}$, and $\textbf{T}$ are matrix operators with no renormalization scale dependence. However, in a Lorentz-violating theory with a fermion $f^{\mu}$ term in the action, these operators may acquire nontrivial renormalization group behavior. Because the $f^{\mu}$ term may actually be exchanged in the action for an equivalent $c^{\nu\mu}$ term, the scale dependence depends explicitly on the renormalization scheme, even at one-loop order. The scheme dependence means it is always possible to set the scale dependence parameter $1-X$ to zero, but for analyses of some high-energy electron-photon processes, using a scheme with $X=0-$and thus definite scale dependences for $\textbf{C}$, $\textbf{P}$, and $\textbf{T}-$may nonetheless be more convenient.

hep-th↗

arXiv2025

Dalitz Plot Kinematics for a Lorentz-Violating Three-Body Decay

Rates for particle interaction processes and decays will be modified in a Lorentz-violating quantum field theory, because of changes to the particle kinematics$-$particularly through the modified dispersion relations affecting the outgoing particle phase space. We outline here these changes to the rates for three-particle decays. Considering a process with a constant scattering amplitude (not directly modified by the Lorentz violation), we calculate leading order corrections to the kinematics for a decay into three identical spinless particles whose propagation is affected by a $c_{\mu\nu}$-type symmetric tensor background. We examine the angular distribution of the daughter particles and describe the shape of the corresponding Dalitz plot outlining the kinematically allowed region, according to two toy models for the $c_{\mu\nu}$ textures. Precision measurements of the boundaries of this region could be used to constrain Lorentz violation coefficients for the particles involved in processes such as $\eta\rightarrow 3\pi^{0}$.

hep-ph↗

arXiv2024

Bound States and Particle Production by Breather-Type Background Field Configurations

We investigate the interaction of fermion fields with oscillating domain walls, inspired by breather-type solutions of the sine-Gordon equation, a nonlinear system of fundamental importance. Our study focuses on the fermionic bound states and particle production induced by a time-dependent scalar background field. The fermions couple to two domain walls undergoing harmonic motion, and we explore the resulting dynamics of the fermionic wave functions. We demonstrate that while fermions initially form bound states around the domain walls, the energy provided by the oscillatory motion of the scalar field induces an outward flux of fermions and antifermions, leading to particle production and eventual flux propagation toward spatial infinity. Through numerical simulations, we observe that the fermion density exhibits quasiperiodic behavior, with partial recurrences of the bound state configurations after each oscillation period. However, the fermion wave functions do not remain localized, and over time, the density decreases as more particles escape the vicinity of the domain walls. Our results highlight that the sine-Gordon-like breather background, when coupled non-supersymmetrically to fermions, does not preserve integrability or stability, with the oscillations driving a continuous energy transfer into the fermionic modes. This study sheds light on the challenges of maintaining steady-state fermion solutions in time-dependent topological backgrounds and offers insights into particle production mechanisms in nonlinear dynamical systems with oscillating solitons.

hep-th↗

arXiv2024

Radiation from an Oscillating Dipole in the Presence of Photon-Sector CPT and Lorentz Violation

We examine one of the standard loci for studying electromagnetic wave emission -- the radiation from an oscillating electric dipole -- in a model in which the electromagnetic sector is modified to include novel CPT- and Lorentz-violating propagation effects involving a preferred axial vector background. We evaluate the vacuum-birefringent radiation fields, including nonperturbative terms where appropriate. In general, the energy-momentum carried by the fields in this model is known to have a complicated nonperturbative structure, which cannot be captured by naive power series expansions in the components of the preferred background vector. However, we nevertheless find that at the lowest nontrivial orders, there are actually no modifications to the Larmor expressions for the energy-momentum emission.

hep-th↗

arXiv2023

Scaling Monte-Carlo-Based Inference on Antibody and TCR Repertoires

Previously, it has been shown that maximum-entropy models of immune-repertoire sequence can be used to determine a person's vaccination status. However, this approach has the drawback of requiring a computationally intensive method to compute each model's partition function ($Z$), the normalization constant required for calculating the probability that the model will generate a given sequence. Specifically, the method required generating approximately $10^{10}$ sequences via Monte-Carlo simulations for each model. This is impractical for large numbers of models. Here we propose an alternative method that requires estimating $Z$ this way for only a few models: it then uses these expensive estimates to estimate $Z$ more efficiently for the remaining models. We demonstrate that this new method enables the generation of accurate estimates for 27 models using only three expensive estimates, thereby reducing the computational cost by an order of magnitude. Importantly, this gain in efficiency is achieved with only minimal impact on classification accuracy. Thus, this new method enables larger-scale investigations in computational immunology and represents a useful contribution to energy-based modeling more generally.

q-bio.QM↗

arXiv2023

Exiting Inflation with a Smooth Scale Factor

The expectation that the physical expansion of space occurs smoothly may be expressed mathematically as a requirement for continuity in the time derivative of the metric scale factor of the Friedmann-Robertson-Walker cosmology. We explore the consequences of imposing such a smoothness requirement, examining the forms of possible interpolating functions between the end of inflation and subsequent radiation- or matter-dominated eras, using a straightforward geometric model of the interpolating behavior. We quantify the magnitude of the cusp found in a direct transition from the end of slow roll inflation to the subsequent era, analyze the validity several smooth interpolator candidates, and investigate equation-of-state and thermodynamic constraints. We find an order-of-magnitude increase in the size of the universe at the end of the transition to a single-component radiation or matter era. We also evaluate the interpolating functions in terms of the standard theory of preheating and determine the effect on the number of bosons produced.

gr-qc↗

arXiv2023

Creation of Bound Half-Fermion Pairs by Solitons

In the presence of topologically nontrivial bosonic field configurations, the fermion number operator may take on fractional eigenvalues, because of the existence of zero-energy fermion modes. The simplest examples of this occur in 1+1 dimensions, with zero modes attached to kink-type solitons. In the presence of a kink-antikink pair, the two associated zero modes bifurcate into positive and negative energy levels with energies $\pm ge^{-g\Delta}$, in terms of the Yukawa coupling $g\ll 1$ and the distance $\Delta$ between the kink and antikink centers. When the kink and antikink are moving, it seems that there could be Landau-Zener-like transitions between these two fermionic modes, which would be interpretable as the creation or annihilation of fermion-antifermion pairs; however, with only two solitons in relative motion, this does not occur. If a third solitary wave is introduced farther away to perturb the kink-antikink system, a movement of the faraway kink can induce transitions between the discrete fermion modes bound to the solitons. These state changes can be interpreted globally as creation or destruction of a novel type of pair: a half-fermion and a half-antifermion. The production of the half-integral pairs will dominate over other particle production channels as long as the solitary waves remain well separated, so that there is a manifold of discrete fermion states whose energies are either zero or exponentially close to zero.

hep-th↗

arXiv2022

Aspects of the Equivalence Between the $f^μ$ and $c^{νμ}$ Terms in Lorentz-Violating Quantum Field Theory

It is known that in Lorentz-violating effective field theory, there is a classical equivalence between certain coefficients ($c$ and $f$), in spite of the fact that the operators the two types of coefficients describe appear to have opposite behaviors under $\textbf{CPT}$. This paper is a continuation of previous work extending this equivalence to the quantum level: generalizing the explicit spinorial point transformations that interconvert the $c$ and $f$ terms; demonstrating that the transformations do not give rise to any additional anomaly terms as the quantum level; and giving explicit prescriptions for modifying the $\textbf{C}$, $\textbf{P}$, and $\textbf{T}$ operators in the $f$ theory, so that they correspond to the correct interchanges of physical particle states.

hep-th↗

arXiv2022

Single-Particle Quantum Mechanics of the Free Klein-Gordon Equation with Lorentz Violation

In spite of its problems with interactions, the first-quantized Klein-Gordon equation is a satisfactory theory of free spinless particles. Moreover, the usual theory may be extended to describe Lorentz-violating behavior, of the same types that exist can in second-quantized scalar field theories. However, because the construction of the theory requires a restriction to positive-energy modes, the Hilbert space inner product and the position operator depend explicitly on the form of the Lorentz violation.

hep-th↗

arXiv2022

Renormalization Scheme Dependence of $β$-Functions In Lorentz-Violating Quantum Field Theory

Effective quantum field theories that allow for the possibility of Lorentz symmetry violation can sometimes also include redundancies of description in their Lagrangians. Explicit calculations in a Lorentz-violating generalization of Yukawa theory show that when this kind of redundancy exists, different renormalization schemes may lead to different expressions for the renormalization group $β$-functions, even at only one-loop order. However, the renormalization group scaling of physically observable quantities appears not to share this kind of scheme dependence.

hep-th↗

arXiv2022

Cold Atoms in Space: Community Workshop Summary and Proposed Road-Map

We summarize the discussions at a virtual Community Workshop on Cold Atoms in Space concerning the status of cold atom technologies, the prospective scientific and societal opportunities offered by their deployment in space, and the developments needed before cold atoms could be operated in space. The cold atom technologies discussed include atomic clocks, quantum gravimeters and accelerometers, and atom interferometers. Prospective applications include metrology, geodesy and measurement of terrestrial mass change due to, e.g., climate change, and fundamental science experiments such as tests of the equivalence principle, searches for dark matter, measurements of gravitational waves and tests of quantum mechanics. We review the current status of cold atom technologies and outline the requirements for their space qualification, including the development paths and the corresponding technical milestones, and identifying possible pathfinder missions to pave the way for missions to exploit the full potential of cold atoms in space. Finally, we present a first draft of a possible road-map for achieving these goals, that we propose for discussion by the interested cold atom, Earth Observation, fundamental physics and other prospective scientific user communities, together with ESA and national space and research funding agencies.

astro-ph.IM↗

arXiv2021

Improved Bounds on Lorentz Symmetry Violation From High-Energy Astrophysical Sources

Observations of the synchrotron and inverse Compton emissions from ultrarelativistic electrons in astrophysical sources can reveal a great deal about the energy-momentum relations of those electrons. They can thus be used to place bounds on the possibility of Lorentz violation in the electron sector. Recent $γ$-ray telescope data allow the Lorentz-violating electron $c^{νμ}$ parameters to be constrained extremely well, so that all bounds are at the level of $7\times 10^{-16}$ or better.

hep-ph↗

arXiv2020

Problems with Lorentz Violation Originating From a Cosmologically Varying Pseudoscalar Field

Lorentz- and CPT-violating models of electrodynamics with Chern-Simons terms are typically plagued by various sorts of instabilities. However, when the Chern-Simons term arises from a slow time variation in a pseudoscalar field with an axion-like electromagnetic coupling, the total energy of the theory is bounded below. We examine the behavior of such a theory, finding that in a systematic power series expansion of the magnetic and pseudoscalar fields, singularities appear in the field profiles. Some of the questionable behavior can be cured by taking a fully nonperturbative approach, but other problematic terms remain. This may be an indication that Cerenkov-like radiation will automatically carry away energy from a moving charge, preventing a charge from moving with uniform velocity over extended distances.

hep-th↗

arXiv2020

Separability of the Planar $1/ρ^{2}$ Potential In Multiple Coordinate Systems

With a number of special Hamiltonians, solutions of the Schrödinger equation may be found by separation of variables in more than one coordinate system. The class of potentials involved includes a number of important examples, including the isotropic harmonic oscillator and the Coulomb potential. Multiply separable Hamiltonians exhibit a number of interesting features, including "accidental" degeneracies in their bound state spectra and often classical bound state orbits that always close. We examine another potential, for which the Schrödinger equation is separable in both cylindrical and parabolic coordinates: a $z$-independent $V\propto 1/ρ^{2}=1/(x^{2}+y^{2})$ in three dimensions. All the persistent, bound classical orbits in this potential close, because all other orbits with negative energies fall to the center at $ρ=0$. When separated in parabolic coordinates, the Schrödinger equation splits into three individual equations, two of which are equivalent to the radial equation in a Coulomb potential---one equation with an attractive potential, the other with an equally strong repulsive potential.

quant-ph↗

arXiv2020

Top Hadrons in Lorentz-Violating Field Theory

If there is Lorentz symmetry violation in the $t$ quark sector of the standard model, changes to particles' dispersion relations might allow for the existence of stable top-flavored hadrons. Observations of the survival of high-energy $γ$-rays over astrophysical distances can be used to place one-sided constraints on certain linear combinations of Lorentz violation coefficients in the $t$ sector at the $\sim 10^{-4}$ level of precision.

hep-ph↗