TeV messenger for cosmology and fundamental physics – CosmoTeV

The observations of distant gamma-ray allow to have access to powerful sources, with large magnetic fields, high energies and important propagation distances. One key point of the CosmoTeV project is that these extragalactic gamma-ray observations give access to fundamental subjects:

- transparency of the universe and axions: axions are particles predicted in beyond standard model theories explaining CP conservation in strong interactions. The existence of axion like particles could modify the optical depth of the TeV photons. Such a scenario might explain the observation of blazars up to redshitfs of 0.6.

- turbulent magnetic fields and axions: If axion-like particles exist, turbulent magnetic fields in TeV photon emission regions could induce effects in the source energy spectra. The observation of TeV blazars can lead to constraints the coupling of axion-like particles to photons by searching for irregularity in their energy spectra.

- large-scale magnetic fields: TeV photons from distant sources can be destroyed by pair creation on the extragalactic background light. Observations of TeV blazars can test the fact that these particles induce an electromagnetic cascade. Some characteristics of the cascade depend on the ambient magnetic field. These observations therefore allow constraining the strength of inter-galactic magnetic fields.

- photon time-of-flight and Lorentz invariance: TeV blazars are variable on short time scales, of the order of one minute.This feature can be used to search for an energy dependent speed of light.

Up to now, the obtain results deal with axion-like particles:

- data from the HESS telescope array have been used to constrain their coupling to photons. Competitive constraints have been obtained with the observation of the blazar PKS 2155-304. The results are published in Physical Review D.

- A phenomenological study of the modification of the universe transparency to photons in the presence of axion-like particles has been performed. A new observable has been proposed, related to the anisotropy of the anomalous transparency. The perspective for the future gamma-ray observatory CTA have been studied. This study is accepted for publication in Journal of Cosmology and Astroparticle Physics.

These results have been presented in international conferences (ICRC 2013 and PATRAS 2013).

- the results on the search for axion-like particles through spectral irregularities have to be generalized for a larger sample of sources, in particular in anticipation of the CTA project.

- data from the blazar PG1553 observed by HESS will be used to constrain a possible violation of Lorentz invariance.

- A work on the phenomenology aiming at defining the observables related to large scale magnetic fields have to be done.

- «Constraints on axionlike particles with H.E.S.S. from the irregularity of the PKS 2155-304 energy spectrum«, HESS Collaboration, accepted for publication in Physical Review D (arXiv:1311.3148)

- «Anisotropy test of the axion-like particle universe opacity problem: a case for the Cherenkov Telescope Array«, Denis Wouters & Pierre Brun, accepted for publication in Journal of Cosmology and Astroparticle Physics (arXiv:1309.6752).

Extragalactic TeV gamma ray sources carry potentially very useful information for probing basic physics. The goal of this project is to bring together all necessary ingredients to exploit these data the best. The axes of the proposal are the study of the cosmological standard model, the search for new light bosons (axion-like particles, ALPs) and the search for Lorentz invariance breaking. In all cases, specific properties in the sources energy spectra are sought. TeV photons allow probing the extragalactic background light, which they interact with. This is how the cosmological model will be tested (metric, cosmological parameters). In case of an absence of detection of exotic effects (axions, Lorentz invariance breakdown), it will be possible to constraint particles physics models predicting ALPs and to set lower limits on the energy scale up to which Lorentz invariance holds. The IRFU team is traditionally involved in the search for new physics, in particular the search for non-baryonic dark matter. This project aims at extending the activity of the group to other fundamental physics domains within astroparticle physics. We propose to join forces with the Polish group of R. Moderski, who is specialized in the modeling of conventional TeV gamma ray emissions. His PhD student A. Barnacka is already in joint supervision PhD thesis together with the IRFU group in Saclay. The Warsaw Nicolaus Copernicus Center appears as associated lab, which receives no funding from the ANR. Predictions for the intrinsic spectrum of the source and the magnetic field configuration will be used to make theoretical predictions. Those will be tested with the HESS, Fermi and HESS II data and then used to estimate the sensitivity of future gamma ray observatories to these effects. A key point of this project is that it aggregates different types of knowledge that can be used for other analyses as by-products. It is the merging of two teams specialized respectively in the modeling of TeV emissions and in the search for new physics with gamma-ray astronomy that is the important point here. So in a second phase, we propose to study other subjects, for example constraining the properties of the propagation of TeV photons in the intergalactic medium (large-scale relic magnetic fields, ultra-high energy cosmic rays, gravitational lensing, etc). Other even more challenging subjects may be studied if the previous analyses are successful, such as the use of gamma ray bursts. It is foreseen to organize two workshops dedicated to the search for new phenomena with TeV sources. This project enlarges the topics in which the IRFU team is traditionally involved. The proposal is held by a team of young physicists whose collaboration will naturally sustain when the project ends thanks to the starting of next generation observatories and the wealth of subjects raised. ANR funding will allow hiring a 3 year post-doc and increase significantly the work capacity of the team. The funding will also cover travels and the organization of dedicated workshops.