About

Stephon Alexander is recognized as a National Geographic Explorer, a community of individuals committed to making the world a better place through science, exploration, education, and storytelling. Explorers are driven by curiosity and a passion for our planet, working in diverse fields such as oceanography, wildlife conservation, and technological innovation. They reveal underwater worlds, investigate ancient caves, work to end wildlife trafficking, and safeguard ecosystems for future generations. As a leader and problem solver, Alexander embodies the Explorer mindset, which emphasizes informed inquiry, responsibility, respect, and a commitment to social impact. He is part of a global community that shares values of integrity, collaboration, and ethical engagement, and he benefits from the Society’s support through project funding, professional development, and opportunities to amplify his work.

Research topics

• Physics
• Quantum mechanics
• Astrophysics
• Statistics
• Internal medicine
• Medicine

Selected publications

• Autonomous Discovery of Particle Physics Theories from Experimental Data

  arXiv (Cornell University) · 2026-03-30

  preprintOpen access1st authorCorresponding

  The search for physics beyond the Standard Model is hindered by a combinatorial explosion of possible theories. We introduce \textsc{Albert}, a neuro-symbolic artificial intelligence framework to systematically navigate this vast theory space. By encoding particle physics as a formal language, \textsc{Albert} generates tokenized sequences representing symmetries, particles, and interactions under a rule-based grammar, eliminating the hallucinations common in large language models. The reinforcement learning environment enforces first-principle theoretical constraints, computes observables with radiative corrections, and evaluates statistical likelihood via $χ^2$ analysis against experimental data. As a proof of concept, we train a 25-million-parameter transformer model using only legacy data from the Large Electron-Positron Collider, which contains no direct evidence of the top quark. Remarkably, \textsc{Albert} successfully rediscovered the Standard Model and autonomously inferred necessity and properties of the top quark, predicting its mass at $178.9\pm 5.0~\text{GeV}$, consistent with its modern measurement at the Large Hadron Collider. These results demonstrate the potential of AI-driven theory exploration as a rigorous, hallucination-free, and scalable paradigm for autonomous discovery of new physics.

  PublisherDOI

• Fermion Condensate Inflation, Dynamical Waterfall Mechanism and Primordial Black Holes

  arXiv (Cornell University) · 2026-04-23

  articleOpen access1st authorCorresponding

  Fermion condensate inflation, where inflation emerges from four-fermion interactions induced by spacetime torsion, removes the need for additional scalar fields beyond the Standard Model. In this framework, the fermion field can be decomposed into two distinguished sectors, each giving rise to bound states. After integrating out fermions, the bound fields play the roles of the inflaton and the auxiliary fields, resembling hybrid inflation with a waterfall mechanism. The inclusion of an axial chemical potential naturally introduces a mechanism to end inflation and trigger instant preheating. During the waterfall phase, the effective potential of the fermion condensate supports the formation of non-topological solitons such as Q-balls, which act as seeds of primordial black holes. This model is intrinsically connected to Chern-Simons gravity, which implies a parity-violating universe. Consequently, both the primordial black hole (PBH) dark-matter abundance and parity-violation signatures could provide observational tests of the model.

  PublisherOA PDF

• Autonomous Discovery of Particle Physics Theories from Experimental Data

  arXiv (Cornell University) · 2026-03-30

  articleOpen access1st authorCorresponding

  The search for physics beyond the Standard Model is hindered by a combinatorial explosion of possible theories. We introduce \textsc{Albert}, a neuro-symbolic artificial intelligence framework to systematically navigate this vast theory space. By encoding particle physics as a formal language, \textsc{Albert} generates tokenized sequences representing symmetries, particles, and interactions under a rule-based grammar, eliminating the hallucinations common in large language models. The reinforcement learning environment enforces first-principle theoretical constraints, computes observables with radiative corrections, and evaluates statistical likelihood via $χ^2$ analysis against experimental data. As a proof of concept, we train a 25-million-parameter transformer model using only legacy data from the Large Electron-Positron Collider, which contains no direct evidence of the top quark. Remarkably, \textsc{Albert} successfully rediscovered the Standard Model and autonomously inferred necessity and properties of the top quark, predicting its mass at $178.9\pm 5.0~\text{GeV}$, consistent with its modern measurement at the Large Hadron Collider. These results demonstrate the potential of AI-driven theory exploration as a rigorous, hallucination-free, and scalable paradigm for autonomous discovery of new physics.

  PublisherOA PDF

• Detecting the <i>π</i> -axiverse through parametric resonance

  Journal of Cosmology and Astroparticle Physics · 2026-03-01

  articleOpen access1st authorCorresponding

  Abstract Axions are a leading dark matter candidate. In this work, we study the detectability of a multi-axion-like model, dubbed the π -axiverse, that is distinguishable from the string axiverse. The dark matter candidates are N 2 -1 pseudo-Nambu-Goto modes (pion- and kaon-like states) stemming from spontaneous breaking of a global SU( N ) flavor symmetry. The low energy theory includes N -1 axionic couplings with additional couplings to the Standard Model photon kinetic energy, reminiscent of the string theory dilaton-photon coupling. We explore the parametric resonance of photons interacting with such a dark sector. Axions are well known to form macroscopic solitonic-like objects (axion stars), which experience instabilities due to overdensities stemming from mergers or accretion processes. The instabilities produce high-intensity bursts of radiation via parametric resonance that may be detected at observatories such as MeerKAT, the Square Kilometre Array (SKA), and the next generation Very Large Array (ngVLA). Using numerical methods, we systematically explore the multi-dimensional parameter space of the π -axiverse to search for regions where such signals are detectable, which generically differ from single axion models. We identify regions of the parameter space where MeerKAT, SKA, and ngVLA can resolve such signals, assessing the potential of transient searches to constrain the model. Our results provide a significant step forward in understanding the phenomenology and indirect detection of multi-axion-dilaton dark matter.

  PublisherDOI

• Fermion Condensate Inflation, Dynamical Waterfall Mechanism and Primordial Black Holes

  arXiv (Cornell University) · 2026-04-23

  preprintOpen access1st authorCorresponding

  Fermion condensate inflation, where inflation emerges from four-fermion interactions induced by spacetime torsion, removes the need for additional scalar fields beyond the Standard Model. In this framework, the fermion field can be decomposed into two distinguished sectors, each giving rise to bound states. After integrating out fermions, the bound fields play the roles of the inflaton and the auxiliary fields, resembling hybrid inflation with a waterfall mechanism. The inclusion of an axial chemical potential naturally introduces a mechanism to end inflation and trigger instant preheating. During the waterfall phase, the effective potential of the fermion condensate supports the formation of non-topological solitons such as Q-balls, which act as seeds of primordial black holes. This model is intrinsically connected to Chern-Simons gravity, which implies a parity-violating universe. Consequently, both the primordial black hole (PBH) dark-matter abundance and parity-violation signatures could provide observational tests of the model.

  PublisherDOI

• Kinetically Coupled Dark Matter Condensates

  ArXiv.org · 2025-06-09

  preprintOpen accessSenior author

  Dark matter consisting of ultralight bosons can form a macroscopic Bose-Einstein condensate with distinctive observational signatures. While this possibility has been extensively studied for axions and axion-like particles $-$ pseudoscalars with masses protected by shift symmetry $-$ realistic models from string theory and other higher-dimensional theories predict more complex structures. Here we investigate a two-field generalization where an axion couples to a moduli field through its kinetic term, representing the phase and radial modes of a complex scalar field. We demonstrate that when this system forms a gravitationally bound Bose-Einstein condensate, the kinetic coupling produces dramatic modifications to cosmological evolution compared to the canonical single-field case. Most notably, the axion Jeans scale becomes dynamically dependent on the moduli field's evolution, fundamentally altering structure formation. By mapping existing observational constraints from canonical axion models to our two-field scenario, we identify regions of parameter space that are already excluded by current observations. In particular, consistency with observations requires that the moduli field must take on small field values, $χ/M_{\rm pl} \ll 1$, throughout most of cosmic history for this class of axions to remain a viable description of all dark matter.

  PublisherOA PDFDOI

• Perturbations in pseudo-Nambu-Goldstone Higgs inflation

  Journal of Cosmology and Astroparticle Physics · 2025-10-01

  article1st authorCorresponding

  Abstract Pseudo-Nambu-Goldstone (pNG) Higgs Inflation is a novel approach to relate the Higgs boson and its interaction with Electroweak gauge bosons with cosmic inflation, with the potential of solving both the fine-tuning issues in the Higgs mass and inflationary potentials. In this work, we present a linear perturbation analysis of the minimal implementation of pNG Higgs inflation using the symmetry coset SU(5)/SO(5). Similar to Chromo-natural inflation, this model exhibits a period of instability in the tensor modes that exponentially enhance left-handed gravitational waves. Thus, large Chern-Simons couplings β ≳ 6 × 10 8 and decay constants f ≳ 1 × 10 18 GeV are required to suppress the tensor-to-scalar ratio r to be compatible with the cosmic microwave background (CMB) measurement. These large couplings also cause an overproduction of the scalar modes, making the minimal construction of pNG Higgs inflation disfavored by CMB observations. However, this tension could potentially be relieved by considering multi-field inflation. The pNG Higgs construction naturally contains multiple scalar fields via the interplay of spontaneously broken global and gauge symmetries. The rich structure enables a broad range of multi-field inflation, and we conclude by briefly discussing this possibility and future work.

  PublisherDOI

• Birefringence in fermion-attenuated gravitational wave power spectrum

  Physics Letters B · 2025-04-29 · 1 citations

  articleOpen access

  Within the framework of Chern-Simons gravity, a theory that dynamically violates parity, we analyze the power spectrum of gravitational waves in light of the damping effect due to the free streaming relativistic neutrinos and dark fermions. The power spectrum is expressed terms of right- and left-handed polarizations, and the evolution of the gravitational waves is studied numerically. Birefringence is explicitly shown in the power spectrum, though the difference in the amplitudes is small. Specific features of peaks and dips appear gravitational wave power spectrum mirroring chiral gravitational wave mediated parametric resonance during reheating. Our result represents a useful tool to test Chern-Simons gravity and enables to constrain mechanisms of inflation and reheating related to this theory. We predict a falsifiable pattern of observable peaks and dips in the chiral independent gravitational power spectrum, eventually observable in next space-borne gravitational interferometers, including LISA, Taiji and Tianqin.

  PublisherDOI

• Erratum: Kalb-Ramond field and gravitational parity violation [Phys. Rev. D <b>110</b> , 044067 (2024)]

  Physical review. D/Physical review. D. · 2025-11-13

  articleSenior author
  PublisherDOI

• Higgs Inflation and the Electroweak Gauge Sector

  Fortschritte der Physik · 2025-03-01 · 1 citations

  articleOpen access1st authorCorresponding

  Abstract We introduce a method that allows the Higgs to be the inflaton. The Higgs is considered as a pseudo‐Nambu‐Goldstone (pNG) boson of a global coset symmetry , which is spontaneously breaks at an energy scale . A suitable Chern−Simons (CS) interaction is given to it, with representing the dimensionless CS coupling strength and an decay constant. As a result, slow‐roll inflation occurs via ‐induced friction down a steep sinusoidal potential. To obey electroweak symmetry, the lowest‐order CS interaction is required to be quadratic in the Higgs, with the coupling strength . Higher‐order interaction terms keep the full Lagrangian nearly invariant under the approximate pNG shift symmetry. Employing the simplest symmetry coset , ‐folds of inflation occur when . Successfully explaining inflation necessitates small values of the decay constant, ; this in turn requires large , which is ruled out by electric dipole measurements. Although the electroweak hierarchy problem while achieving successful inflation, the real benefit is found in providing a different path to identifying the Higgs as the inflaton, outside the standard modified‐gravity framework.

  PublisherOA PDFDOI

Recent grants

• CAREER: Confronting fundamental theories with observations in cosmology

  NSF · $377k · 2008–2013

Frequent coauthors

• Evan McDonough

  89 shared

• Antonino Marcianò

  Fudan University

  53 shared

• Leah Jenks

  33 shared

• Michael W. Toomey

  Massachusetts Institute of Technology

  31 shared

• David N. Spergel

  30 shared

• Nicolás Yunes

  University of Illinois Urbana-Champaign

  29 shared

• Lee Smolin

  Perimeter Institute

  28 shared

• Robert Sims

  24 shared

Education

• B.S.

  Haverford College

  1993

• Ph.D.

  Brown University

  2000

Awards & honors

• NSF CAREER AWARD
• APS E. Bouchet Award
• AAAS John Wesley Powell Memorial Award
• Trinidad and Tobago, NIHERST National Medal of Science