Best constraint on mass of photons, using observations of super-massive black holes

The research findings appear in the September issue of Physical Review Letters. "Black hole bombs and photon mass bounds" is co-authored by Emanuele Berti, UM assistant professor of physics and astronomy, along with fellow researchers Paolo Pani, Vitor Cardoso, Leonardo Gualtieri and Akihiro Ishibashi.

This paper details how the scientists, who work in Portugal, Italy, Japan and the U.S., found a way to use astrophysical observations to test a fundamental aspect of the Standard Model -- namely, that photons have no mass -- better than anyone before.

"The test works like this: if photons had a mass, they would trigger an instability that would spin down all black holes in the universe," Berti said. "But astronomers tell us that the gigantic, super-massive black holes at galactic centers are spinning, so this instability cannot be too strong.

"The mass of the photon, if it has a mass at all, must be extremely tiny."

"Ultralight photons with nonzero mass would produce a 'black hole bomb': a strong instability that would extract energy from the black hole very quickly," said Pani, the paper's lead author. "The very existence of such particles is constrained by the observation of spinning black holes. With this technique, we have succeeded in constraining the mass of the photon to unprecedented levels: the mass must be one hundred billion of billions times smaller than the present constraint on the neutrino mass, which is about two electron-volts."

The results of this study can be used to investigate the existence of new particles, such as those possibly contributing to the dark matter that is the subject of a search using the Large Hadron Collider at CERN in Geneva. CERN is the site where the breakthrough discovery of the Higgs boson was reported earlier this year.

"That discovery filled one of the most important gaps in our understanding of the standard model of particle physics, because it explains how particles get their mass," Gualtieri said. "However, not all particles have mass. Physics makes progress by testing every nook and cranny of our commonly accepted theories. So, if we believe that a particle has no mass, we'd better test this idea with precise experiments.

"Observations of super-massive black holes may provide new insights which are not accessible in laboratory experiments. This would certainly be exciting. Perhaps these new frontiers in astrophysics will give us a clearer understanding of the microscopic universe."

"Paolo, Vitor, Leonardo and I are all part of an IRSES Network on 'Numerical Relativity and High-Energy Physics' funded by the European Union," Berti said. "Paolo presented a talk on this work at the first meeting of our network that was held in Aveiro, Portugal in July. This network will be used in the next four years to strengthen our collaboration even further."

Pani, who received the Fubini Prize from the Italian National Institute of Nuclear Physics for the best Ph.D. thesis nationwide in 2011, is a post-doctoral researcher at Instituto Superior Técnico in Lisbon, Portugal, supported by a European Marie Curie Fellowship.

"Paolo started working with us when he visited Ole Miss in 2007," Berti said. "We have been working together on this particular project since January 2012, and we have co-authored nine papers so far."

A post-doctorate researcher at UM before returning to his native Portugal, Cardoso is a professor at Instituto Superior Tecnico, where his group is supported by a prestigious European Research Council Starting Grant. Cardoso and Berti have published 37 papers together over the past decade.

"Gualtieri and I were both Ph.D. students under the supervision of Valeria Ferrari in Rome, Italy," Berti said. "We have also been collaborating for more than a decade. Leonardo is now a research professor ('ricercatore') in Rome."

Ishibashi works at the KEK Theory Center and at the Department of Physics of Kinki University in Japan, where physicists at the center are studying in great depth phenomena similar to the one described in the PRL paper.

This study was funded, in part, by National Science Foundation Grant No. PHY-0900735 and by CAREER Grant No. PHY-1055103.