Stars offer unique laboratories for particle physics. They can be sensitive to minute interactions of neutrinos, as well as to other, hypothetical weakly interacting particles. The origin of this sensitivity lies in the mechanism of stellar evolution: it can be sped up, or even qualitatively changed, by processes that drain energy from stellar interiors. New particle physics often provides such processes. In fact, sometimes, astrophysical bounds turn out to be more stringent than anything done in the lab.
For example, we investigated a class of extra-dimensional models, in which the photon is localized on our brane by gravity only. These models predict that an off-shell photon, created as an intermediate state, could escape from our brane into the bulk with a small rate. An example of such a process is provided by orthopositronium annihilation, which motivated a round of precision experiments. In out paper , we pointed out that, in this framework, a photon in plasma should also have a small branching ratio into extra dimension. Plasmons in stellar interiors are thus expected to decay, carrying away energy. We analyzed the resulting rates of energy loss in globular cluster stars and for the core-collapse supernovae. Our bounds on the model parameter exceed the possible reach of orthopositronium experiments by many orders of magnitude.
Next, we explored the sensitivity of massive stars to neutrino magnetic moments . We found that, unlike in solar-mass stars, which had been traditionally studied for this purpose, in massive stars the additional cooling due to the neutrino magnetic moment can qualitatively change the evolution. Specifically, neutrino magnetic moments can result in a new type of supernova in which a partial carbon-oxygen core explodes within a massive star.
Most recently, we have been studying astrophysical bounds on the axion-photon coupling. This coupling creates additional mechanism of energy loss that is most effective for the conditions of helium burning. Early in 2013, we published a letter  pointing out that the axion energy losses lead to dramatic effects in massive stars. In particular, the duration of the blue loop evolutionary phase is shortened, or the loop is completely eliminated. On the other hand, the existence of the blue loop is well established observationally in more than one way, in particular, because Cepheid variable stars exist. Therefore, we were able to obtain a bound on the axion-photon coupling that at present exceeds direct laboratory bounds from experiments such as CAST at CERN. This work has been selected for an APS Viewpoint highlight . Further work is in progress to strengthen the bound, by taking account of astrophysical uncertainties.
- A. Friedland and M. Giannotti, Astrophysical bounds on photons escaping into extra dimensions, Phys. Rev. Lett. 100, 031602 (2008).
- A. Heger, A. Friedland, M. Giannotti and V. Cirigliano, The Impact of Neutrino Magnetic Moments on the Evolution of Massive Stars, Astrophys. J. 696, 608 (2009).
- A. Friedland, M. Giannotti and M. Wise, Constraining the Axion-Photon Coupling with Massive Stars, Phys. Rev. Lett. 110, 061101 (2013)
- G. Raffelt, Viewpoint: Particle Physics in the Sky, Physics 6, 14 (2013).