Particle Physics in the Sky

Much can be learned about hypothetical particles called axions by studying the evolution of massive stars.

This Hertzsprung-Russell (HR) diagram shows stellar populations ordered according to brightness (vertical axis) and surface temperature or color (horizontal axis). Different types of stars populate characteristic regions in this diagram and a few specific nearby stars are indicated here. The “main sequence” consists of hydrogen-burning stars like our Sun, whose brightness and color depend on stellar mass. Red giants are stars in advanced burning phases, notably the helium-burning phase. At the end of this phase they execute a well-established “blue loop” of brief contraction and re-expansion (white line with arrows). According to Friedland et al., this behavior would be suppressed by excessive emission of the hypothetical axions.

This Hertzsprung-Russell (HR) diagram shows stellar populations ordered according to brightness (vertical axis) and surface temperature or color (horizontal axis). Different types of stars populate characteristic regions in this diagram and a few specific nearby stars are indicated here. The “main sequence” consists of hydrogen-burning stars like our Sun, whose brightness and color depend on stellar mass. Red giants are stars in advanced burning phases, notably the helium-burning phase. At the end of this phase they execute a well-established “blue loop” of brief contraction and re-expansion (white line with arrows). According to Friedland et al., this behavior would be suppressed by excessive emission of the hypothetical axions.

Read more: http://physics.aps.org

Black Holes as Particle Detectors

Finding new particles usually requires high energies — that is why huge accelerators have been built, which can accelerate particles to almost the speed of light. But there are other creative ways of finding new particles: At the Vienna University of Technology, scientists presented a method to prove the existence of hypothetical “axions.” These axions could accumulate around a black hole and extract energy from it. This process could emit gravity waves, which could then be measured.

Artist’s impression of a black hole, surrounded by axions. (Credit: Image courtesy of Vienna University of Technology)

Axions are hypothetical particles with a very low mass. According to Einstein, mass is directly related to energy, and therefore very little energy is required to produce axions. “The existence of axions is not proven, but it is considered to be quite likely,” says Daniel Grumiller. Together with Gabriela Mocanu he calculated at the Vienna University of Technology (Institute for Theoretical Physics), how axions could be detected.

Astronomically Large Particles

In quantum physics, every particle is described as a wave. The wavelength corresponds to the particle’s energy. Heavy particles have small wavelengths, but the low-energy axions can have wavelengths of many kilometers. The results of Grumiller and Mocanu, based on works by Asmina Arvanitaki and Sergei Dubovsky (USA/Russia), show that axions can circle a black hole, similar to electrons circling the nucleus of an atom. Instead of the electromagnetic force, which ties the electrons and the nucleus together, it is the gravitational force which acts between the axions and the black hole.

The Boson-Cloud

However, there is a very important difference between electrons in an atom and axions around a black hole: Electrons are fermions — which means that two of them can never be in the same state. Axions on the other hand are bosons, many of them can occupy the same quantum state at the same time. They can create a “boson-cloud” surrounding the black hole. This cloud continuously sucks energy from the black hole and the number of axions in the cloud increases.

Sudden Collapse

Such a cloud is not necessarily stable. “Just like a loose pile of sand, which can suddenly slide, triggered by one single additional grain of sand, this boson cloud can suddenly collapse,” says Daniel Grumiller. The exciting thing about such a collapse is that this “bose-nova” could be measured. This event would make space and time vibrate and emit gravity waves. Detectors for gravity waves have already been developed, in 2016 they are expected to reach an accuracy at which gravity waves should be unambiguously detected. The new calculations in Vienna show that these gravity waves can not only provide us with new insights about astronomy, they can also tell us more about new kinds of particles…
Read more: www.sciencedaily.com

Axion Dark Matter and Cosmological Parameters

Blame dark matter underdog for mystery missing lithium


by David Shiga
AN UNDERDOG dark-matter particle could explain why the universe seems strangely low on lithium. If the idea holds up, it will be a boon in the hunt for dark matter, the stuff needed to account for 80 per cent of the universe’s matter.

In the universe’s first few fiery minutes, nuclear reactions forged a host of light elements, including helium, deuterium and lithium, in a process called big bang nucleosynthesis. The amounts of these elements present in the early universe, gleaned from ancient stars and primordial gas clouds, match theory, except in one respect: they contain much less of the dominant form of lithium, lithium-7, than expected. There has never been a satisfactory explanation for this.

Now help comes in the shape of hypothetical dark-matter particles called axions. These light particles were dreamed up in the 1970s as part of a theory to explain why the strong nuclear force, unlike the other forces, does not change if a particle is swapped for the antimatter counterpart of its mirror image. Axions are not the dominant theory for dark matter. That accolade goes to weakly interacting massive particles, or WIMPs. But as neither WIMPs nor axions have ever been observed, the jury is still out.

In the latest research, the underdog axions score a point. The rates of nuclear reactions that produced lithium-7 depend partly on the amount of energy that was present in the form of light. As we cannot tell how much light was there directly, we infer it from the cosmic microwave background (CMB), the echo of the big bang that emerged 380,000 years later. This is used to estimate how much lithium should be present: more light skews reaction rates and lowers expected levels of lithium.

Ozgur Erken of the University of Florida in Gainesville and colleagues suggest that something cooled photons between the synthesis of lithium and the emergence of the CMB, causing the photon energy to be underestimated, and inflating the expected amounts of lithium.

Born with very little kinetic energy, axions are a prime suspect. When their cooling power is accounted for, the predicted lithium abundance drops by half, the team calculate (Physical Review LettersDOI: 10.1103/PhysRevLett.108.061304). “We’re excited that it gives about the right correction,” says Pierre Sikivie, Erken’s colleague.

Adding in axions also creates a problem, however. Without them, CMB measurements are consistent with about four types of neutrino, close to the three types glimpsed in experiments. But if axions are present, they would skew this measurement and imply about seven neutrino types, Erken’s team calculate.  This makes Gary Steigman of Ohio State University in Columbus, who was not involved in the study, sceptical of the axion explanation for the lithium-7 anomaly
An answer should come in 2013 when much better measurements of the CMB are expected from the Planck satellite. Our best chance of glimpsing axions, meanwhile, lies in an upgraded version of an experiment called ADMX, due to start up towards the end of this year. It may also be possible to infer their existence from data from the Large Hadron Collider at CERN near Geneva in Switzerland, where they should boost the production of Higgs bosons…….
Read more:newscientist.com

The Axion Dark Matter eXperiment

Axion couplings and masses excluded at the 90% confidence level by the experiment. The main figure shows the results from the Phase I upgrade and the inset has the limits from all data by ADMX.

Dmitry Lyapustin
The Axion is a particle arising from the Peccei-Quinn solution to the strong CP problem.
Peccei-Quinn symmetry breaking in the early universe could produce a large number of axions which would still be present today, making the axion a compelling dark matter candidate.
The goal of the Axion Dark Matter eXperiment (ADMX) is to detect these relic axions through their conversion to photons in a strong magnetic field.
Results are presented from a recent ADMX data-taking, along with plans for the next phase of ADMX, which will allow the experiment to explore a significant fraction of the favored dark matter axion mass and coupling phase space.
Read more: http://arxiv.org/pdf