In an article published in the prestigious academic journal Physical Review Letters, UGR researchers demarcate the properties of one of the particles which potentially make up dark matter: axions. The article, published by Adrian Ayala and his thesis director Inmaculada Domínguez, could help determine the very nature of dark matter, one of the most puzzling enigmas yet to be solved in physics. The gravitational effects of the universe indicate that this dark matter accounts for over 80% of its total mass.

The research is a clear demonstration of the fruitful collaborations currently taking place between particle physicists and astrophysicists. Indeed, such collaborative projects have given rise to the birth of a new branch of science: astroparticle physics.

In order to carry out the project, researchers essentially employed the stars as particle physics laboratories. Due to the high temperatures found in the internal stellar structure, the photons can turn into axions which, in turn, escape to the exterior of the stellar structure, carrying energy with them. This energy loss can have consequences, whether observable or not, in some phases of stellar evolution.

“In our research, we have carried out numerical simulations of a star’s evolution since its birth right up until it uses up all the hydrogen and then the helium in its interior, including the processes that produce axions,” Ayala, one of the main researchers involved, indicates.

The results indicate that the axion emissions can significantly reduce the time involved in the central combustion of the helium - the so called Horizontal Branch (HB) phase. The energy transported away by the axions is compensated for by energy produced by nuclear combustion, which leads to a higher helium consumption rate and thus, to a more rapid evolution. A high axion production rate implies a shorter HB phase and a lower probability of observing HB stars.

Thanks to recent breakthroughs in the study of globular clusters, the scientists were able to compare the results of the numerical simulations conducted during this project with the data, as Inmaculada Domínguez explains; “By comparing the number of stars observed in the HB phase with the number of stars in phases which are not influenced by axions (such as the Red Giant Branch or RGB), we have calculated the maximum axion emission rate.”

The production of axions depends on the constant coupling of axions and photons. The researchers point out that “We have established a maximum limit for this constant, which is the most accurate limit established both theoretically and experimentally to-date”.

The group explains that the degree of accuracy involved in determining the fixed value of the photon-axion coupling “critically depends on the degree of accuracy in determining the initial helium contents of the star’s in the globular cluster.”

Both Adrian Ayala and Inmaculada Domínguez form part of the research group “FQM-292 Stellar Evolution and Nucleosynthesis”. Other researchers involved in this project also included Maurizio Giannotti (Barry University, USA), Alessandro Mirizzi (Deutsches Elektronen-Synchrotron, DESY, Germany) and Oscar Straniero (National Astrophysics Institute, INAF-Astronomic Observatory in Teramo, Italy)

Bibliography:

Revisiting the Bound on Axion-Photon Coupling from Globular Clusters

Adrián Ayala, Inma Domínguez, Maurizio Giannotti, Alessandro Mirizzi and Oscar Straniero

Phys. Rev. Lett. 113, 191302 – November 2014

The article can be downloaded here:

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.191302

Contact:

Adrián Ayala

Theoretical Physics and Cosmology Department, University of Granada

Email: aayala@ugr.es

Telephone: +34 958 249 062