Isabelle John

Research

The aim of my research is to understand and model the various astrophysical processes in our Galaxy. I especially focus on simulating the processes governing cosmic rays, from their production at various types of sources, their propagation through the Galactic environment, and their detection at Earth.

My current projects focus on understanding the effects of the Sun on the local cosmic-ray flux. Due to the solar winds and their change in intensity throughout the cycles of solar activity, called solar modulation, they become suppressed at low energies. I also study the contribution to the local flux of cosmic-ray electrons and positrons from nearby pulsars, which are rapidly rotating neutron stars that are able to accelerate electrons and positrons to energies of hundreds of TeV.

Understanding precisely the astrophysical origin of the cosmic-ray spectra measured at Earth is vital when searching for new physics processes, such as dark matter, whose annihilation or decay may produce part of the local cosmic-ray flux.

Besides cosmic rays, I also use observations of stars to determine if their evolution and dynamics is affected by dark matter. For this, I especially focus on the stars closest to the Galactic Center, where the dark matter densities are extremely high.

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List of Publications
Thesis Projects

Collaborators:

List of Publications

You can also find a full list of my publications on iNSPIRE .

First Observations of Solar Halo Gamma Rays Over a Full Solar Cycle

Tim Linden, Jung-Tsung Li, Bei Zhou, Isabelle John, Milena Crnogorčević, Annika H. G. Peter, John F. Beacom

Submitted to Journal.

arXiv:2505.04625

Using 15 years of Fermi-LAT data, we detect the solar halo, a bright emission of gamma rays between 31.6 MeV and 100 GeV and extending out to 45 degrees around the Sun. These gamma rays are produced by the inverse-Compton scattering of highly-energetic cosmic-ray electrons and positrons with sunlight. The morphology of the solar halo is shaped by the activity of the Sun, called solar modulation, and changes over time. Our analysis provides results on the strength and time-variation of solar modulation near the Sun, showing remarkable consistency with complementary measurements of cosmic rays taken near Earth.

Every Nearby Energetic Pulsar Is Surrounded by a Region of Inhibited Diffusion

Isabelle John & Tim Linden

Submitted to Journal.

arXiv:2503.17442

Recently, the H.E.S.S. telescope has released the cosmic-ray electron-plus-positron flux up to 40 TeV, which follows a smooth and steeply falling powerlaw above 1 TeV. This is in tension with simple pulsar models that predict a hard flux at 10 TeV. We find 20 pulsars that would individually overproduce the H.E.S.S. data and show that they must be surrounded by a region of inhibited diffusion, which causes the e+e- produced by the pulsar to lose energy already in the vicinity of the pulsar. This reduces the high-energy flux at Earth, consistent with H.E.S.S. data. We conclude that every nearby energetic pulsar must be surrounded by such a region of inhibited diffusion, and is detectable in radio, x-ray or gamma-ray searches.

Strong Constraints on Dark Photon and Scalar Dark Matter Decay from INTEGRAL and AMS-02

Thong T. Q. Nguyen, Isabelle John, Tim Linden & Tim M. P. Tait

Submitted to Journal.

arXiv:2412.00180

Dark matter particles may decay into Standard Model particles, leaving distinct signatures in the cosmic-ray and photon fluxes. We consider the decay of dark photon and scalar dark matter particles, and, compared to previous analyses, implement a more realistic scenario that includes the full decay width to all Standard Model particles. Using the INTEGRAL x-ray and AMS-02 cosmic-ray positron, we derive constraints on the lifetime of dark photon and scalar dark matter particles over a wide mass range that strongly exceed previous results.

Dark Branches of Immortal Stars at the Galactic Center

Isabelle John, Rebecca K. Leane & Tim Linden

Accepted by PRD.

arXiv:2405.12267

The capture and subsequent annihilation of dark matter in stellar cores can act as a power source to stars alternatively to nuclear fusion, which slows down or even halts stellar evolution, granting stars immortality. We establish the dark mater sequence, a new branch of stars on the Hertzprung-Russell diagram populated by dark-matter powered stars. Using this dark main sequence, we can determine the presence of dark matter burning in star clusters.

Dark Matter Scattering Constraints from Observations of Stars Surrounding Sgr A*

Isabelle John, Rebecca K. Leane & Tim Linden

Published in: Phys.Rev.D 109 (2024) 12, 123041

arXiv:2311.16228

The stars around Sgr A*, called the S-cluster, face extremely high dark matter densities, making the capture and subsequent annihilation of dark matter in these stars extremely efficient, which changes them significantly: (1) stars can be prevented from forming, (2) stars can be partially powered by dark matter, slowing their evolution and making them appear younger, and (3) dark matter can disrupt stars that migrate towards the Galactic Center, moving them off the main sequence. Based on these scenarios, we constrain dark matter using recent measurements of S-cluster stars.

Accurate Inverse-Compton Models Strongly Enhance Leptophilic Dark Matter Signals

Isabelle John & Tim Linden

Published in: Phys.Rev.D 108 (2023) 10, 103022

arXiv:2304.07317

The annihilation of TeV-scale dark matter into electron-positron (e+e-) pairs produces a sharp cutoff in the local cosmic-ray e+e- spectrum at an energy near the dark matter mass. We show that this sharp signal is greatly enhanced when energy losses from inverse-Compton scattering are precisely modelled as a stochastic process instead of using the commonly used continuous approximation. This increases the detectability of heavy dark matter annihilating to e+e− final states.

Pulsars Do Not Produce Sharp Features in the Cosmic-Ray Electron and Positron Spectra

Isabelle John & Tim Linden

Published in: Phys.Rev.D 107 (2023) 10, 103021

arXiv:2206.04699

Pulsars produce high energy electrons and positrons and dominate the local cosmic-ray positron flux at high energies. Standard pulsar models predict a sharp cutoff due to the rapid energy losses that the e+e- experience during propagation. We show that this sharp cutoff does not exist — it is erroneously introduced by the continuous approximation of inverse-Compton energy losses, which does not account for the stochastic and catastrophic nature of energy losses. Instead, pulsar features are much smoother. Intriguingly, this establishes dark matter as the only mechanism able of producing sharp spectral features, making the cosmic ray positrons a promising avenue for dark matter searches.

Cosmic-Ray Positrons Strongly Constrain Leptophilic Dark Matter

Isabelle John & Tim Linden

Published in: JCAP 12 (2021) 007

arXiv:2107.10261

The cosmic-ray positron flux is one of the most powerful probes of dark matter annihilation. We use the extremely precise positron measurements by AMS-02 to search for spectral features consistent with leptophilic dark matter signals. Finding no significant signals, we derive strong constraints on the dark matter annihilation cross section in the mass range of 5 GeV to 1 TeV, that improve on previous constraints.

Recommendations of the LHC Dark Matter Working Group: Comparing LHC searches for heavy mediators of dark matter production in visible and invisible decay channels

Andreas Albert, Mihailo Backovic, Antonio Boveia, Oliver Buchmueller, Giorgio Busoni, Albert De Roeck, Caterina Doglioni, Tristan DuPree, Malcolm Fairbairn, Marie-Helene Genest, Stefania Gori, Giuliano Gustavino, Kristian Hahn, Ulrich Haisch, Philip C. Harris, Dan Hayden, Valerio Ippolito, Isabelle John, Felix Kahlhoefer, Suchita Kulkarni, Greg Landsberg, Steven Lowette, Kentarou Mawatari, Antonio Riotto, William Shepherd, Tim M.P. Tait, Emma Tolley, Patrick Tunney, Bryan Zaldivar & Markus Zinser

Published in: Phys.Dark Univ. 26 (2019) 100377

arXiv:1703.05703

Weakly-coupled TeV-scale particles may mediate the interactions between normal matter and dark matter. If so, the LHC would produce dark matter through these mediators, leading to the familiar "mono-X" search signatures, but the mediators would also produce signals without missing momentum via the same vertices involved in their production. This document from the LHC Dark Matter Working Group suggests how to compare searches for these two types of signals in case of vector and axial-vector mediators, based on a workshop that took place on September 19/20, 2016 and subsequent discussions. These suggestions include how to extend the spin-1 mediated simplified models already in widespread use to include lepton couplings. This document also provides analytic calculations of the relic density in the simplified models and reports an issue that arose when ATLAS and CMS first began to use preliminary numerical calculations of the dark matter relic density in these models.

Thesis Projects

In September 2024, I received my PhD from Stockholm University in Theoretical Physics, with focus on Theoretical and Computational Astroparticle Physics. Previously, I received a bachelor's and master's degree in Physics from Lund University. I also worked on a project in Beam Physics as a technical student at CERN.

PhD Thesis

How The Positron Became Cool: A Study of Cosmic-Ray Positrons from Pulsars and Dark Matter

PhD Advisor: Tim Linden

Available at: DiVA Stockholm University

Created by powerful pulsars, mysterious dark matter annihilation or cosmic-ray interactions, positrons travel through the Galaxy and undergo immense energy losses, which shapes their spectrum measured at Earth. This is the story of how the positron became cool, in one way or another.

Report

Space Charge and Intrabeam Scattering Effects for Lead-Ions and Oxygen-Ions in the LHC Injector Chain.

Supervisor: Hannes Bartosik (CERN)

Available at: CERN Document Server

The Large Hadron Collider (LHC) can accelerate and collide particle beams at high energies that enable us to study the fundamental laws of particle physics and nature. Typically, the LHC is run with proton beams as well as lead beams. A prospective run with an oxygen beam is considered as the oxygen nuclei are intermediate in size between protons and lead nuclei. I investigate the feasibility of such an oxygen beam considering effects from space charge and intrabeam scattering in the LHC injection chain.

Master Thesis

Study of Dark Matter Models in Astrophysics and Particle Physics

Supervisors: Caterina Doglioni (Lund University) & Antonio Boveia (Ohio State University)

Available at: Lund University Libraries

Freezing-in, freezing-out — I calculate the dark matter relic density for a new vector mediator considering freeze-in and freeze-out production mechanisms, for various sets of coupling constants to quarks, leptons and dark matter particles, in the parameter ranges testable at the LHC.
Detect or constrain — I compare constraints from LHC searches and direct detection experiments for these dark matter models, and investigate how the astrophysical uncertainties on the local dark matter density and velocity distribution can affect direct detection limits.

Bachelor Thesis

Summarising Constraints On Dark Matter At The Large Hadron Collider

Supervisor: Caterina Doglioni (Lund University)

Available at: Lund University Libraries

Considering a new type of mediator that couples to both Standard Model and dark matter particles, I develop signal templates for various mediator decay channels that can be probed at the LHC.

Research

Graph View of My Research

List of Publications

Thesis Projects