arXiv subjects

Ahmed Ayad

Publications and source records attributed to Ahmed Ayad.

11 recordsLinked to original sources

arXiv2026

Axion-Sourced Gravitational Waves in $\mathrm{B\!-\!L}$ Hybrid Inflation

Minimal supersymmetric $\mathrm{B\!-\!L}$ hybrid inflation predicts a negligible vacuum tensor-to-scalar ratio $r_{\rm vac}\sim 10^{-8}$ due to its extremely flat potential. In this letter, we show that adding a spectator pseudoscalar axion-like field coupled to the $\rm {U}(1)_{\mathrm{B\!-\!L}}$ gauge sector via a Chern-Simons term circumvents this suppression. The rolling axion triggers a tachyonic instability for one gauge field helicity, exponentially amplifying gauge fluctuations. These sourced modes generate a stochastic gravitational-wave background with tensor power spectrum ${\cal P}_{T}^{\rm src}\propto e^{4πξ}/ξ^{6}$, where $ξ= α\dotϕ / (2 f_a H)$ is the gauge-field instability parameter. For $ξ\sim3.3$--$3.6$, the tensor-to-scalar ratio reaches $r\sim10^{-3}$, within the sensitivity of LiteBIRD. For $ξ\sim3.6$, the gravitational-wave spectrum develops a peaked shape that enters the LISA sensitivity band (peaking at frequencies around $10^{-3}$ Hz) with an amplitude $h^2Ω_{\rm GW}\sim2.5\times10^{-13}$. The mechanism predicts chiral gravitational waves, a broken consistency relation $r=16ε$, and a distinctive $r$--$Ω_{\rm GW}$ correlation that could be tested by future CMB and interferometer observations.

astro-ph.CO↗

arXiv2025

Impact of adiabatic temperature fluctuations on the power spectrum of axion density perturbations

Axions and axion-like particles (ALPs) have gained substantial attention as potential candidates for cold dark matter. The ALP field can exhibit fluctuations stemming from initial conditions. These initial field fluctuations hold the potential to give rise to gravitationally bound configurations known as axion miniclusters (AMC). While this proposition is widely accepted in the post-inflationary Peccei-Quinn symmetry-breaking scenario, uncertainties persist regarding the pre-inflationary scenario, where the effects of the initial field fluctuations may be suppressed by inflation. In this study, we investigate the influence of adiabatic temperature fluctuations of the primordial plasma on the evolution of axion density perturbations and their power spectrum, aiming to explore the possibility of AMC formation in the pre-inflationary scenario. Our analysis reveals that the impact of adiabatic temperature fluctuations becomes significant when $f_{\rm a} / H_{\rm inf} \gtrsim 1.25 \times 10^4$ and surpasses that of quantum fluctuations by up to five orders of magnitude on large scales. This emphasizes the possibility of also forming AMC within the pre-inflationary scenario. Consequently, a detection of AMC would not reliably differentiate between the pre- and post-inflationary origin of axions.

astro-ph.CO↗

arXiv2022

Phenomenological Aspects of Axion-Like Particles in Cosmology and Astrophysics

Cosmology and particle physics are closer today than ever before, with several searches underway at the interface between cosmology, particle physics, and field theory. The mystery of dark matter (DM) is one of the greatest common unsolved problems between these fields. It is established now based on many astrophysical and cosmological observations that only a small fraction of the total matter content of the universe is made of baryonic matter, while the vast majority is constituted by dark matter. However, the nature of such a component is still unknown. One theoretically well-motivated approach to understanding the nature of dark matter would be through looking for light pseudo-scalar candidates for dark matter such as axions and axion-like particles (ALPs). Axions are hypothetical elementary particles resulting from the Peccei-Quinn (PQ) solution to the strong CP (charge-parity) problem in quantum chromodynamics (QCD). Furthermore, many theoretically well-motivated extensions to the standard model of particle physics (SMPP) predicted the existence of more pseudo-scalar particles similar to the QCD axion and called ALPs. Axions and ALPs are characterized by their coupling with two photons. While the coupling parameter for axions is related to the axion mass, there is no direct relation between the coupling parameter and the mass of ALPs. Nevertheless, it is expected that ALPs share the same phenomenology of axions. In the past years, axions and ALPs regained popularity and slowly became one of the most appealing candidates that possibly contribute to the dark matter density of the universe. In this thesis, we focus on studying the phenomenology of axions and ALPs interactions with photons to constrain some of their properties.

hep-ph↗

arXiv2022

Potential of the MeerKAT telescope to detect the stimulated decay of axion-like particles

A prominent aspect of the hunt for cold dark matter is looking for light scalar candidates such as axion-like particles (ALPs). The coupling between ALPs and photons allows for the spontaneous decay of ALPs into pairs of photons. It has been previously shown that stimulated ALP decay rates can become significant on cosmic time scales. Furthermore, it has been claimed, in several recent works, that ALPs can gravitationally thermalize and form macroscopic condensates. Consequently, the photon occupation number of ambient populations (like the cosmic microwave background) can receive Bose enhancement in dense ALP clumps and grows exponentially. For cold dark matter ALPs, this can lead to radio emissions produced from this process and could be observed by the forthcoming radio telescopes. In this work, we investigate the detectability of such a radio signature from some astrophysical targets using the MeerKAT radio telescopes. The results show that the MeerKAT telescope is able to probe the ALPs parameter space with limits reaching the current level of the CAST experiment and the potential level of the IAXO experiment with an arcminute visibility taper.

astro-ph.HE↗

arXiv2022

Axion-like particle searches with MeerKAT and SKA

In the past few years, the search for axion-like particles (ALPs) has grown significantly due to their potential to account for the total abundance of the cold dark matter (CDM) in the universe. The coupling between ALPs and photons allows the spontaneous decay of ALPs into pairs of photons. For ALPs condensed in CDM halos around galaxies, the stimulated decay of ALPs is also possible. In this work, we examine the detectability of the radio emissions produced from this process with forthcoming radio telescopes such as the Square Kilometer Array (SKA) and MeerKAT. Our results, using recent more realistic sensitivity estimates, show that previous non-observation upper-limits projected for the SKA were highly optimistic, with the limits from dwarf galaxy observations being weakened by an order of magnitude at least. Notably, our results also depend far more strongly on ALP mass than previously, due to the inclusion of frequency dependent degradation effects. We show that the strongest potential environment to probe ALPs is nearby radio galaxies (due to the strong photon enhancement factor). In addition, with the use of a visibility taper, ALPs in the mass range of $4.96 \times 10^{-7} \ \text{--} \ 1.04 \times 10^{-4} \ {\rm eV}$ would have non-observation upper limits on the ALP-photon $g_{aγ}$ in the range of $1.83 \times 10^{-12} \ \text{--} \ 7.69 \times 10^{-10} \ {\rm GeV}^{-1}$ with SKA. MeerKAT can only produce limits similar to the CAST experiment within 50 hours of observation. Finally, we demonstrate that magnetic conversion of CDM ALPs to photons, in galactic magnetic fields, is highly sub-dominant, even to spontaneous decay.

astro-ph.HE↗

arXiv2021

NuCLS: A scalable crowdsourcing, deep learning approach and dataset for nucleus classification, localization and segmentation

High-resolution mapping of cells and tissue structures provides a foundation for developing interpretable machine-learning models for computational pathology. Deep learning algorithms can provide accurate mappings given large numbers of labeled instances for training and validation. Generating adequate volume of quality labels has emerged as a critical barrier in computational pathology given the time and effort required from pathologists. In this paper we describe an approach for engaging crowds of medical students and pathologists that was used to produce a dataset of over 220,000 annotations of cell nuclei in breast cancers. We show how suggested annotations generated by a weak algorithm can improve the accuracy of annotations generated by non-experts and can yield useful data for training segmentation algorithms without laborious manual tracing. We systematically examine interrater agreement and describe modifications to the MaskRCNN model to improve cell mapping. We also describe a technique we call Decision Tree Approximation of Learned Embeddings (DTALE) that leverages nucleus segmentations and morphologic features to improve the transparency of nucleus classification models. The annotation data produced in this study are freely available for algorithm development and benchmarking at: https://sites.google.com/view/nucls.

eess.IV↗

arXiv2020

Potential of SKA to Detect CDM ALPs with Radio Astronomy

Axion-like particles (ALPs) are light pseudo-scalar particles predicted in many theoretically well-motivated extensions to the standard model of particle physics (SM). The search for cold dark matter (CDM) ALPs has gained tremendous ground over the last few years. Essentially ALPs are characterized by their coupling with two photons which allows ALPs to decay into two photons. In this work, we explore the potential of the Square Kilometer Array (SKA) to detect CDM ALPs with radio astronomy in an attempt to detect an observational signature of ALPs conversion onto photons in astrophysical field.

hep-ph↗

arXiv2020

Normalization of the Ground State of the Supersymmetric Harmonic Oscillator

Supersymmetry plays a main role in all current thinking about superstring theory. Indeed, many remarkable properties of string theory have been explained using supersymmetry as a tool. So far, there has been no unbroken supersymmetry observed in nature, and if nature is described by supersymmetry, it must be broken. Supersymmetry may be broken spontaneously at any order of perturbation theory or dynamically due to nonperturbative effects. To examine the methods of supersymmetry breaking, special attention is given to discuss the normalization of the ground state of the supersymmetric harmonic oscillator. This study explains that perturbation theory gives incorrect results for both the ground-state wave function and the energy spectrum and it fails to give an explanation to the supersymmetry breaking.

physics.gen-ph↗

arXiv2020

Probing a cosmic axion-like particle background within the jets of active galactic nuclei

Axions or more generally axion-like particles (ALPs) are pseudo-scalar particles predicted by many extensions of the Standard Model of particle physics (SM) and considered as viable candidates for dark matter (DM) in the universe. If they really exist in nature, they are expected to couple with photons in the presence of an external electromagnetic field through a form of the Primakoff effect. In addition, many string theory models of the early universe motivate the existence of a homogeneous Cosmic ALP Background (CAB) with 0.1-1 keV energies analogous to the Cosmic Microwave Background (CMB), arising via the decay of string theory moduli in the very early universe. The coupling between the CAB ALPs traveling in cosmic magnetic fields and photons allows ALPs to oscillate into photons and vice versa. In this work, we test the CAB model that is put forward to explain the soft X-ray excess in the Coma cluster due to CAB ALPs conversion into photons using the M87 jet environment. Then we demonstrate the potential of the active galactic nuclei (AGNs) jet environment to probe low-mass ALP models, and to potentially constrain the model proposed to better explain the Coma cluster soft X-ray excess. We find that the overall X-ray emission for the M87 AGN requires an ALP-photon coupling $g_{aγ}$ in the range of $\sim7.50\times 10^{-15}-6.56\times 10^{-14}$ GeV$^{-1}$ for ALP masses $m_a \lesssim 10^{-13}$ eV as long as the M87 jet is misaligned by less than about 20 degrees from the line of sight. These values are up to an order of magnitude smaller than the current best fit value on $g_{aγ}\sim 2\times 10^{-13}$ GeV$^{-1}$ obtained in soft X-ray excess CAB model for the Coma cluster. Our results cast doubt on the current limits of the largest allowed value of $g_{aγ}$ and suggest a new constraint that $g_{aγ}\lesssim 6.56\times 10^{-14}$ GeV$^{-1}$ when a CAB is assumed.

astro-ph.HE↗

arXiv2019

Phenomenology of axion-like particles coupling with photons in the jets of active galactic nuclei

astro-ph.HE↗

arXiv2019

Supersymmetric Quantum Mechanics and Path Integrals

Supersymmetry plays a main role in all current thinking about superstring theory. Indeed, many remarkable properties of string theory have been explained using supersymmetry as a tool. In this dissertation, we review the basic formulation of supersymmetric quantum mechanics starting with introducing the concepts of supercharges and superalgebra. We show that, if there is a supersymmetric state, it is the zero-energy ground state. If such a state exists, the supersymmetry is unbroken otherwise it is broken. So far, there has been no unbroken supersymmetry observed in nature, and if nature is described by supersymmetry, it must be broken. In fact, supersymmetry may be broken spontaneously at any order of perturbation theory, or dynamically due to non-perturbative effects. The goal of this dissertation is to study the methods of supersymmetry breaking. For this purpose, special attention is given to discuss the normalization of the ground state of the supersymmetric harmonic oscillator. Then we explain that perturbation theory gives us incorrect results for both the ground state wave function as well as the energy spectrum and it fails to give an explanation to the supersymmetry breaking. Later in the dissertation, a review of the uses of instantons in quantum mechanics is given. In particular, instantons are used to compute the tunneling effects within the path integral approach to quantum mechanics. As a result, we give evidence that the instantons, which are a non-perturbative effect in quantum mechanics and can not be seen in perturbation theory, leads to calculate the corrections to the ground state energy and provides a possible explanation for the supersymmetry breaking.

hep-th↗