Signatures of cosmic rays and new fundamental particles in the Very high energy sky

Project Information

VerSi

Grant agreement ID: 101062808

DOI

10.3030/101062808

Project closed

EC signature date 1 July 2022

Start date 1 December 2022

End date 30 November 2024

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 195 914,88

Coordinated by

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 1 - VerSi (Signatures of cosmic rays and new fundamental particles in the Very high energy sky)

Reporting period: 2022-12-01 to 2024-11-30

Summary of the context and overall objectives of the project

The purpose of my research is to uncover the properties of cosmic rays and new fundamental particles by exploiting their Signatures in Very high energy radiations. The most fascinating open questions in our understanding of Nature, such as dark matter or the strong Charge-Parity (CP) problem suggest the presence of fundamental interactions Beyond the Standard Model of particle physics (BSM), and could be the next cosmic-ray-driven discovery. For example, the leading hypothesis is that dark matter is made of new particles, among which Weakly Interacting Massive Particles (WIMPs) are the most prominent candidates. New light particles, such as the axion and axion-like particles (ALPs) are often predicted in BSM theories solving e.g. the CP problem and, by interacting with photons, can leave an imprint on the spectra of cosmic accelerators. After acceleration, cosmic radiation propagates in the interstellar environment, bringing messages of its properties, including the presence or absence of BSM phenomena.

Recently, tantalising anomalies have thrilled the community as potential hints for new physics phenomena. My project objective is to shed new light on them by exploiting current and forthcoming observations of the very high energy sky with innovative techniques. A long-standing excess of gamma-rays at GeV energies is measured towards the Galactic Center (GCE) with data from the Large Area Telescope (LAT) onboard the Fermi satellite, and could be the very first signature of particle dark matter in our Galaxy. Cosmic ray accelerators, such as millisecond pulsars (MSPs) could also explain the signal, but numerous modeling uncertainties prevent us from firmly assessing their contribution to the excess. MSPs are rapidly rotating neutron stars in which the period is decreased to milliseconds, and could accelerate particles up to TeV energies, producing high energy photons. In addition, an excess of positrons with respect to the flux produced by spallation of cosmic hadrons has been established with PAMELA and AMS-02 data. Although this could be explained by dark matter, recent multi-wavelength observations of halos of GeV-TeV photons around Galactic pulsars demonstrated that these objects could indeed be accelerators of cosmic positrons, producing halos of photon emissions when positrons interact with the interstellar medium. Finally, by inspecting the gamma-ray spectrum of various Galactic sources, hints for a modulation coming from photon-ALPs interactions have been found that are in tension with other, independent bounds.

From one side, my project objective is to discover and characterise the multiwavelength emission around Galactic cosmic ray accelerators such as pulsars and MSPs using photons recorded at different wavelengths, and to build comprehensive, phenomenological modelling to tailor their searches in observational data. From the other side, we aim at investigating the properties of possible new fundamental particles by exploiting observations of very high-energy photons, and to robustly characterize the backgrounds for these searches.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

My work developed in two main, complementary working packages, one focusing on cosmic ray research, and the other one on the interpretation of the Fermi GCE excess and BSM searches.
Regarding the cosmic ray research, I have successfully produced templates to tailor new searches of emission from middle-aged pulsar halos from X-ray to multi-TeV energies, focusing on the Geminga pulsar halo and on another candidate. I have used the results of these investigations to robustly characterize the pulsar’s halo properties, namely the ambient magnetic field, which for both sources is suggested to be of the order of few micro Gauss. Contextually, I have successfully delivered a new, state-of the art modeling of the secondary positron emission coming from spallation of primary cosmic rays, and a detailed inspection of the Galactic pulsars included in catalogs, identifying a shortlist of sources that could contribute significantly to the positron flux at Earth. This is crucial to further inform and direct future multi-wavelength observations. In addition, I have identified specific classes of MSPs that could provide a suitable environment for particle acceleration, and build phenomenological models to explain and predict their gamma ray emission in the GeV to TeV domain. This work is still in progress, and will again deliver templates to tailor searches for TeV emissions with current and future observatories.

As for the second working package, I have carefully characterized the properties of the gamma-ray emission towards the inner Galaxy at energies larger than 10 GeV, where a possible MSPs contribution can be dominating the GCE. By using publicly available Fermi-LAT data, the significance, morphology and energy spectrum of the GCE was robustly derived, and the flux distribution of faint photon sources in the inner Galaxy was measured. In the figure below, the energy spectrum of the Galactic Center excess detected in Fermi-LAT data is illustrated in linear scale to highlight the high energy tail at energies larger than 10 GeV. Model interpretations assuming dark matter annihilations or millisecond pulsar prompt (purple dotted) plus inverse Compton emission (dashed) are overlaid for comparison. The high-energy tail of the excess emission, robustly detected thanks to our work, supports the attempt of explaining, at least partially, the excess in terms of a population of point-like sources, likely corresponding to millisecond pulsars. The consequences for the dark matter interpretation of the excess are in progress, together with the finalization of a framework to predict the gamma ray signals expected for populations of MSPs in our Galaxy.
The discovery of a possible first hint for a gamma-ray emission coming from the Sagittarius dwarf spheroidal galaxy, and thus possibly from MSPs in this system inspired us to carefully evaluate the robustness of this claim, also in light of MSPs population models. As for the ALPs, I have investigated ALPs-photon signals using Fermi-LAT data by using anomaly detection techniques borrowed from machine learning, and simulated datasets.

The results of this work have been published or will be submitted soon in major peer-reviewed journals and are publicly accessible through the arXiv preprint repository. Moreover, the dissemination to the scientific community proceeded through talks and posters at main international conferences in the field, as well as in various workshops and invited seminars.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our first comprehensive multi-wavelength investigation of pulsar halos paves the way for future modelings and observations of these astrophysical objects and similar targets using the same strategies, and constrains crucial physical parameters such as the magnetic field, which are fundamental to further develop our theoretical models. The delivered state-of-the-art modeling for the cosmic ray positrons coming from spallation of primary cosmic rays permitted to precisely identify the space left for primary astrophysical sources and for BSM signals. Our framework for the emission of positrons coming from detected pulsars in catalogs explains the data observed from AMS-02 with high accuracy, and defines a shortlist of the most important sources to be further inspected, thus impacting future observations of such objects.
Concerning the interpretation of the debated GCE, our robust characterization of the inner Galaxy in gamma rays at energies larger than 10 GeV with innovative techniques has permitted further corroboration for a partial, astrophysical origin of the excess. Our results will guide future theoretical interpretations of the excess. The phenomenological model for the emission of high energy particles and photons from MSPs we have built and that we will continue to refine with the forthcoming observational data will be used as reference for these endeavours. The non-confirmation of the claimed gamma ray emission from MSPs in the Sagittarius dwarf galaxy strongly impacts the understanding of the emission of gamma rays from MSPs in such objects.
Finally, our innovative work on the ALPs-photon signatures using anomaly detection methods from machine learning is expected to impact future searches of such signals with forthcoming TeV telescopes, possibly becoming a reference technique.
The frameworks and collaboration set up during the project period offer further exciting opportunities for future impact in the understanding of the signatures of cosmic rays and new fundamental particles in the very high energy sky.

The Galactic Center excess spectrum obtained in our work compared to different theoretical models.