arXiv subjects

Markus Rummel

Publications and source records attributed to Markus Rummel.

At least 19 recordsLinked to original sources

arXiv2021

Extremal learning: extremizing the output of a neural network in regression problems

Neural networks allow us to model complex relationships between variables. We show how to efficiently find extrema of a trained neural network in regression problems. Finding the extremizing input of an approximated model is formulated as the training of an additional neural network with a loss function that minimizes when the extremizing input is achieved. We further show how to incorporate additional constraints on the input vector such as limiting the extrapolation of the extremizing input vector from the original training data set. An instructional example of this approach using TensorFlow is included.

cs.LG↗

arXiv2020

Nuclear Predictions for $H$ Spectroscopy without Nuclear Errors

Nuclear-structure effects often provide an irreducible theory error that prevents using precision atomic measurements to test fundamental theory. We apply newly developed effective field theory tools to Hydrogen atoms, and use them to show that (to the accuracy of present measurements) all nuclear finite-size effects (e.g. the charge radius, Friar moments, nuclear polarizabilities, recoil corrections, Zemach moments {\it etc.}) only enter into atomic energies through exactly two parameters, independent of any nuclear-modelling uncertainties. Since precise measurements are available for more than two atomic levels in Hydrogen, this observation allows the use of precision atomic measurements to eliminate the theory error associated with nuclear matrix elements. We apply this reasoning to the seven atomic measurements whose experimental accuracy is smaller than 10 kHz to provide predictions for nuclear-size effects whose theoretical accuracy is not subject to nuclear-modelling uncertainties and so are much smaller than 1 kHz. Furthermore, the accuracy of these predictions can improve as atomic measurements improve, allowing precision fundamental tests to become possible well below the 'irreducible' error floor of nuclear theory.

hep-ph↗

arXiv2019

Constraining Fundamental Physics with the Event Horizon Telescope

We show how Event Horizon Telescope (EHT) observations of the supermassive object at the center of M87 can constrain deviations from General Relativity (GR) in a relatively model-independent way. We focus on the class of theories whose deviations from GR modify black holes into alternative compact objects whose properties approach those of an ordinary black hole sufficiently far from the would-be event horizon. We examine this class for two reasons: ($i$) they tend to reproduce black-hole expectations for astrophysical accretion disks (and so do not undermine the evidence linking black holes to active galactic nuclei); ($ii$) they lend themselves to a robust effective-field-theory treatment that expands in powers of $\ell/r$, where $\ell$ is the fundamental length scale that sets the distance over which deviations from GR are significant and $r$ is a measure of distance from the would-be horizon. At leading order the observational impact of these types of theories arise as modifications to the transmission and reflection coefficients of modes as they approach the horizon. We show how EHT observations can constrain this reflection coefficient, assuming only that the deviations from GR are small enough to be treated perturbatively. Our preliminary analysis indicates that such reflection coefficients can already be constrained to be less than of order 10\% (corresponding to $\ell \lesssim 100 \mu m$), and so can rule out some benchmark cases used when seeking black-hole echoes. The precise bounds depend on the black hole spin, as well as on detailed properties of the reflection coefficient (such as its dependence on angular direction).

gr-qc↗

arXiv2018

Improving Statistical Sensitivity of X-ray Searches for Axion-Like Particles

X-ray observations of bright AGNs in or behind galaxy clusters offer unique capabilities to constrain axion-like particles (ALPs). Existing analysis technique rely on measurements of the global goodness-of-fit. We develop a new analysis methodology that improves the statistical sensitivity to ALP-photon oscillations by isolating the characteristic quasi-sinusoidal modulations induced by ALPs. This involves analysing residuals in wavelength space allowing the Fourier structure to be made manifest as well as a machine learning approach. For telescopes with microcalorimeter resolution, simulations suggest these methods give an additional factor of two in sensitivity to ALPs compared to previous approaches.

astro-ph.CO↗

arXiv2018

Effective Field Theory of Black Hole Echoes

Gravitational wave `echoes' during black-hole merging events have been advocated as possible signals of modifications to gravity in the strong-field (but semiclassical) regime. In these proposals the observable effect comes entirely from the appearance of nonzero reflection probability at the horizon, which vanishes for a standard black hole. We show how to apply EFT reasoning to these arguments, using and extending earlier work for localized systems that relates choices of boundary condition to the action for the physics responsible for these boundary conditions. EFT reasoning applied to this action argues that linear `Robin' boundary conditions dominate at low energies, and we determine the relationship between the corresponding effective coupling (whose value is the one relevant low-energy prediction of particular modifications to General Relativity for these systems) and the phenomenologically measurable near-horizon reflection coefficient. Because this connection involves only near-horizon physics it is comparatively simple to establish, and we do so for perturbations in both the Schwarzschild geometry (which is the one most often studied theoretically) and the Kerr geometry (which is the one of observational interest for post-merger ring down). In passing we identify the renormalization-group evolution of the effective couplings as a function of a regularization distance from the horizon, that enforces how physics does not depend on the precise position where the boundary conditions are imposed. We show that the perfect-absorber/perfect-emitter boundary conditions of General Relativity correspond to the only fixed points of this evolution. Nontrivial running of all other RG evolution reflects how modifications to gravity necessarily introduce new physics near the horizon.

gr-qc↗

arXiv2018

Failure of Perturbation Theory Near Horizons: the Rindler Example

Persistent puzzles to do with information loss for black holes have stimulated critical reassessment of the domain of validity of semiclassical EFT reasoning in curved spacetimes, particularly in the presence of horizons. We argue here that perturbative predictions about evolution for very long times near a horizon are subject to problems of secular growth - i.e. powers of small couplings come systematically together with growing functions of time. Such growth signals a breakdown of naive perturbative calculations of late-time behaviour, regardless of how small ambient curvatures might be. Similar issues of secular growth also arise in cosmology, and we build evidence for the case that such effects should be generic for gravitational fields. In particular, inferences using free fields coupled only to background metrics can be misleading at very late times due to the implicit assumption they make of perturbation theory when neglecting other interactions. Using the Rindler horizon as an example we show how this secular growth parallels similar phenomena for thermal systems, and how it can be resummed to allow late-time inferences to be drawn more robustly. Some comments are made about the appearance of an IR/UV interplay in this calculation, as well as on the possible relevance of our calculations to predictions near black-hole horizons.

hep-th↗

arXiv2017

Constraints on Axion-Like Particles from X-ray Observations of NGC1275

Axion-like particles (ALPs) can induce localised oscillatory modulations in the spectra of photon sources passing through astrophysical magnetic fields. Ultra-deep Chandra observations of the Perseus cluster contain over $5 \times 10^5$ counts from the AGN of the central cluster galaxy NGC1275, and represent a dataset of extraordinary quality for ALP searches. We use this dataset to search for X-ray spectral irregularities from the AGN. The absence of irregularities at the O(30%) level allows us to place leading constraints on the ALP-photon mixing parameter $g_{aγγ} \lesssim 1.4 - 4.0 \times 10^{-12} {\rm GeV}^{-1}$ for $m_a \lesssim 10^{-12}$ eV, depending on assumptions on the magnetic field realisation along the line of sight.

astro-ph.HE↗

arXiv2017

Reduced Theoretical Error for QED Tests with 4He+ Spectroscopy

We apply point-particle effective field theory (PPEFT) to electronic and muonic 4He+ ions, and use it to identify linear combinations of spectroscopic measurements for which the theoretical uncertainties are much smaller than for any particular energy levels. The error is reduced because these combinations are independent of all short-range physics effects up to a given order in the expansion in the small parameters R/a_B and(Z alpha) (where R and a_B are the ion's nuclear and Bohr radii). In particular, the theory error is not limited by the precision with which nuclear matrix elements can be computed, or compromised by the existence of any novel short-range interactions, should these exist. These combinations of 4He+ measurements therefore provide particularly precise tests of QED. The restriction to 4He+ arises because our analysis assumes a spherically symmetric nucleus, but the argument used is more general and extendable to both nuclei with spin, and to higher orders in R/a_B and (Z alpha).

hep-ph↗

arXiv2017

The 3.5 keV Line from Stringy Axions

An interesting result in particle astrophysics is the recent detection of an unexplained 3.5 keV line from galaxy clusters. A promising model, which can explain the morphology of the signal and its non-observation in dwarf spheroidal galaxies, involves a 7 keV dark matter particle decaying into a pair of ultra-light axions that convert into photons in the magnetic field of the clusters. Given that light axions emerge naturally in 4D string vacua, in this paper we present a microscopic realisation of this model within the framework of type IIB flux compactifications. Dark matter is a local closed string axion which develops a tiny mass due to subdominant poly-instanton corrections to the superpotential and couples via kinetic mixing to an almost massless open string axion living on a D3-brane at a singularity. The interaction of this ultra-light axion with photons is induced by U(1) kinetic mixing. After describing the Calabi-Yau geometry and the brane set-up, we discuss in depth moduli stabilisation, the resulting mass spectrum and the strength of all relevant couplings.

hep-th↗

arXiv2017

The Proton Radius Problem and Point-Particle Effective Field Theory

We argue that the proton's charge-radius contributes differently to shifts of Hydrogen-like energy levels than naively expected due to an incorrect choice for the boundary condition at the proton's position in standard calculations. In particular we show how to obtain the correct boundary condition, which depends on the charge radius itself in a predictable way. We argue this difference in boundary conditions only matters when they are imposed at a radius $r=ε< Z α/m$ where $m$ is the orbiting-particle mass, because only then is the particle relativistic at these distances. The boundary condition difference is therefore important for ordinary Hydrogen while not for muonic Hydrogen. The boundary condition can be interpreted in terms of a second type of nuclear moment, and a prediction is made for the proton-radius energy shift as a function of charge-radius, $r_p$, this second nuclear moment, $h$, and orbiting particle mass, $m$. The observed difference between electronic and muonic contributions to the Lamb shift is accounted for with $r_p \simeq 0.87$ fm similar to its traditional value, and $2mh$ of order a few fm.

hep-ph↗

arXiv2017

Point-Particle Effective Field Theory III: Relativistic Fermions and the Dirac Equation

We formulate point-particle effective field theory (PPEFT) for relativistic spin-half fermions interacting with a massive, charged finite-sized source using a first-quantized effective field theory for the heavy compact object and a second-quantized language for the lighter fermion with which it interacts. This description shows how to determine the near-source boundary condition for the Dirac field in terms of the relevant physical properties of the source, and reduces to the standard choices in the limit of a point source. Using a first-quantized effective description is appropriate when the compact object is sufficiently heavy, and is simpler than (though equivalent to) the effective theory that treats the compact source in a second-quantized way. As an application we use the PPEFT to parameterize the leading energy shift for the bound energy levels due to finite-sized source effects in a model-independent way, allowing these effects to be fit in precision measurements. Besides capturing finite-source-size effects, the PPEFT treatment also efficiently captures how other short-distance source interactions can shift bound-state energy levels, such as due to vacuum polarization (through the Uehling potential) or strong interactions for Coulomb bound states of hadrons, or any hypothetical new short-range forces sourced by nuclei.

hep-ph↗

arXiv2017

Point-Particle Effective Field Theory II: Relativistic Effects and Coulomb/Inverse-Square Competition

We apply point-particle effective field theory (PPEFT) to compute the leading shifts due to finite-size source effects in the Coulomb bound energy levels of a relativistic spinless charged particle. This is the analogue for spinless electrons of the contribution of the charge-radius of the source to these levels, and we disagree with standard calculations in several ways. Most notably we find there are two effective interactions with the same dimension that contribute to leading order in the nuclear size. One is the standard charge-radius contribution, while the other is a contact interaction whose leading contribution to $δE$ arises linearly in the small length scale, $ε$, characterizing the finite-size effects, and is suppressed by $(Zα)^5$. We argue that standard calculations miss the contributions of this second operator because they err in their choice of boundary conditions at the source for the wave-function of the orbiting particle. PPEFT predicts how this boundary condition depends on the source's charge radius, as well as on the orbiting particle's mass. Its contribution turns out to be crucial if the charge radius satisfies $ε\lesssim (Zα)^2 a_B$, with $a_B$ the Bohr radius, since then relativistic effects become important. We show how the problem is equivalent to solving the Schrödinger equation with competing Coulomb, inverse-square and delta-function potentials, which we solve explicitly. A similar enhancement is not predicted for the hyperfine structure, due to its spin-dependence. We show how the charge-radius effectively runs due to classical renormalization effects, and why the resulting RG flow is central to predicting the size of the energy shifts. We discuss how this flow is relevant to systems having much larger-than-geometric cross sections, and the possible relevance to catalysis of reactions through scattering with monopoles.

hep-ph↗

arXiv2017

Constraints on Axion-Like Particles from Non-Observation of Spectral Modulations for X-ray Point Sources

We extend previous searches for X-ray spectral modulations induced by ALP-photon conversion to a variety of new sources, all consisting of quasars or AGNs located in or behind galaxy clusters. We consider a total of seven new sources, with data drawn from the Chandra archive. In all cases the spectrum is well fit by an absorbed power-law with no evidence for spectral modulations, allowing constraints to be placed on the ALP-photon coupling parameter $g_{aγγ}$. Two sources are particularly good: the Seyfert galaxy 2E3140 in A1795 and the AGN NGC3862 within the cluster A1367, leading to 95\% bounds for light ALPs ($m_a \lesssim 10^{-12} {\rm eV}$) of $g_{aγγ} \lesssim 1.5 \times 10^{-12} {\rm GeV}^{-1}$ and $g_{aγγ} \lesssim 2.4 \times 10^{-12} {\rm GeV}^{-1}$ respectively.

astro-ph.HE↗

arXiv2016

Consistency of Hitomi, XMM-Newton and Chandra 3.5 keV data from Perseus

\emph{Hitomi} observations of Perseus with the Soft X-ray Spectrometer (SXS) provide a high-resolution look at the 3.5 keV feature reported by multiple groups in the Perseus cluster. The \emph{Hitomi} spectrum -- which involves the sum of diffuse cluster emission and the point-like central Active Galactic Nucleus (AGN) -- does not show any excess at $E \sim 3.5 {\rm keV}$, giving an apparent inconsistency with previous observations of excess diffuse emission. We point out that 2009 \emph{Chandra} data reveals a strong dip in the AGN spectrum at $E = (3.54 \pm 0.02) {\rm keV}$ (cluster frame) -- the identical energy to the diffuse excess observed by \emph{XMM-Newton}. Scaling this dip to the 2016 AGN luminosity and adding it to the diffuse \emph{XMM-Newton} excess, this predicts an overall dip in the SXS field of view of $(-5.9 \pm 4.4) \times 10^{-6} \, {\rm ph} \, {\rm cm}^{-2} \, {\rm s}^{-1}$ at $E= 3.54$ keV -- a precise match to the \emph{Hitomi} data when broadened by the dark matter virial velocity. We describe models of Fluorescent Dark Matter that can reproduce this physics, in which dark matter absorbs and then re-emits 3.5 keV photons emitted from the central AGN.

astro-ph.HE↗

arXiv2015

De Sitter vacua from a D-term generated racetrack potential in hypersurface Calabi-Yau compactifications

In arXiv:1407.7580 a mechanism to fix the closed string moduli in a de Sitter minimum was proposed: a D-term potential generates a linear relation between the volumes of two rigid divisors which in turn produces at lower energies a race-track potential with de Sitter minima at exponentially large volume. In this paper, we systematically search for implementations of this mechanism among all toric Calabi-Yau hypersurfaces with $h^{1,1}\leq 4$ from the Kreuzer-Skarke list. For these, topological data can be computed explicitly allowing us to find the subset of three-folds which have two rigid toric divisors that do not intersect each other and that are orthogonal to $h^{1,1}-2$ independent four-cycles. These manifolds allow to find D7-brane configurations compatible with the de Sitter uplift mechanism and we find an abundance of consistent choices of D7-brane fluxes inducing D-terms leading to a de Sitter minimum. Finally, we work out a couple of models in detail, checking the global consistency conditions and computing the value of the potential at the minimum.

hep-th↗

arXiv2015

Accidental Kähler Moduli Inflation

We study a model of accidental inflation in type IIB string theory where inflation occurs near the inflection point of a small Kähler modulus. A racetrack structure helps to alleviate the known concern that string-loop corrections may spoil Kähler Moduli Inflation unless having a significant suppression via the string coupling or a special brane setup. Also, the hierarchy of gauge group ranks required for the separation between moduli stabilization and inflationary dynamics is relaxed. The relaxation becomes more significant when we use the recently proposed D-term generated racetrack model.

hep-th↗

arXiv2014

Observational consistency and future predictions for a 3.5 keV ALP to photon line

Motivated by the possibility of explaining the 3.5 keV line through dark matter decaying to axion-like particles that subsequently convert to photons, we study ALP-photon conversion for sightlines passing within 50 pc of the galactic centre. Conversion depends on the galactic centre magnetic field which is highly uncertain. For fields at low or mid-range of observational estimates (10--100 $μ$G), no observable signal is possible. For fields at the high range of observational estimates (a pervasive poloidal mG field over the central 150 pc) it is possible to generate sufficient signal to explain recent observations of a 3.5 keV line in the galactic centre. In this scenario, the galactic centre line signal comes predominantly from the region with $z > 20$ pc, reconciling the results from the Chandra and XMM-Newton X-ray telescopes. The dark matter to ALP to photon scenario also naturally predicts the non-observation of the 3.5 keV line in stacked galaxy spectra. We further explore predictions for the line flux in galaxies and suggest a set of galaxies that is optimised for observing the 3.5 keV line in this model.

hep-ph↗

arXiv2014

De Sitter Vacua from a D-term Generated Racetrack Uplift

We propose an uplift mechanism using a structure of multi-Kähler moduli dependence in the F-term potential of type IIB string theory compactifications. This mechanism requires a D-term condition that fixes one modulus to be proportional to another modulus, resulting in a trivial D-term potential. De Sitter minima are realized along with an enhancement of the volume in the Large Volume Scenario and no additional suppression of the uplift term such as warping is required. We further show the possibility to realize the uplift mechanism in the presence of more Kähler moduli such that we expect the uplift mechanism to work in many other compactifications.

hep-th↗