Some of the most likely components of dark matter are axions. Scientists have suggested that these particles can be formed in large numbers in supernova explosions and immediately turn into gamma quanta.
Axions and dark matter in the Universe
The search for the universe’s dark matter could end as early as tomorrow — if we’re lucky enough to see a supernova. The nature of dark matter has eluded astronomers for 90 years, since they realized that 85% of the matter in the Universe is not visible in our telescopes. The most likely candidate for dark matter today is the axion, a light particle that researchers are desperately trying to find around the world.
Astrophysicists at the University of California, Berkeley, say an axion can be detected within seconds of detecting gamma rays from a supernova explosion. These particles, if they exist, should be produced in large numbers within the first 10 seconds after the collapse of the core of a massive luminary. These axions generate high-energy gamma rays in the intense magnetic field of the star.
Such a detection today is only possible if the single gamma-ray telescope in orbit, the Fermi Space Telescope, is pointed in the direction of the supernova at the time of its explosion. Given the telescope’s field of view, that’s about a one in 10 chance.
Detection of axions in gamma rays
Nevertheless, a single gamma-ray detection would allow an accurate determination of the mass of an axion, in particular the so-called QCD axion, over a huge range of theoretical masses, including the mass ranges now being explored in experiments on Earth. Lack of detection, however, would exclude a wide range of potential masses for the axion and render most current searches for dark matter irrelevant.
However, for gamma rays to be bright enough to be detected, the supernova should be nearby, within our Galaxy or one of its satellite galaxies, and nearby stars explode on average only every few decades. The last close supernova explosion occurred in 1987 in the Large Magellanic Cloud, one of the Milky Way’s satellites. At that time, the no longer-existing Solar Maximum Mission gamma-ray telescope was pointed toward the supernova, but it was not sensitive enough to detect the predicted gamma-ray intensity, according to an analysis by a team at the University of California, Berkeley.
Waiting for supernova
“If we were to see a supernova, like supernova 1987A, with a modern gamma-ray telescope, we would be able to detect or rule out this QCD axion, this most interesting axion, across much of its parameter space—essentially the entire parameter space that cannot be probed in the laboratory, and much of the parameter space that can be probed in the laboratory, too,” said Benjamin Safdi, associate professor of physics at the University of California, Berkeley, a senior research scientist.
However, researchers are concerned that when the long-awaited supernova explodes in the nearby universe, we won’t be ready to see the gamma rays produced by axions. Scientists are now in touch with colleagues who build gamma-ray telescopes to assess the feasibility of launching one or a whole fleet of such telescopes to cover 100% of the sky 24/7 and make sure they catch any gamma-ray burst in time. They even proposed a name for their constellation of gamma-ray satellites covering the entire sky – GALactic AXion Instrument for Supernova, or GALAXIS.
QCD axions and their physical properties
The search for dark matter initially focused on faint, massive compact halo objects (MACHOs) that are theoretically scattered throughout the galaxy and cosmos, but when they failed to materialize, physicists began looking for elementary particles that are theoretically all around us and should be detectable in Earth’s laboratories. These weakly interacting massive particles (WIMPs) have also not been detected.
Currently, the best candidate for dark matter is the axion, a particle that fits well into the standard model of physics and solves several other prominent mysteries in elementary particle physics. Axions also drop neatly out of string theory, a hypothesis about the basic geometry of the Universe, and may be able to unify gravity, which explains interaction at cosmic scales, with the theory of quantum mechanics.
The most strongly axion candidate, named the QCD axion (after the prevailing theory of strong interaction, quantum chromodynamics), theoretically interacts with all matter, albeit weakly, through the four forces of nature: gravity, electromagnetism, the strong interaction that holds atoms together, and the weak interaction that explains the disintegration of atoms.
One result of this is that in a strong magnetic field, the axion should occasionally turn into an electromagnetic wave or photon. The axion is markedly different from another light, weakly interacting particle, the neutrino, which interacts only through gravity and weak interaction and completely ignores the electromagnetic force.
Laboratory bench experiments such as the ALPHA Consortium (Axion Longitudinal Plasma HAloscope), DMradio and ABRACADABRA, involving researchers at the University of California, Berkeley, utilize compact cavities that, like a tuning fork, resonate and amplify the weak electromagnetic field or photon generated when a low-mass axion is transformed in the presence of a strong magnetic field.
Axion detection model
In addition, astrophysicists have proposed searching for axions formed inside neutron stars immediately after the collapse of a supernova core, as in 1987A. However, so far they have focused mainly on the detection of gamma rays from the slow conversion of these axions into photons in the magnetic fields of galaxies. Safdi and his colleagues realized that this process was not very efficient at producing gamma rays, or at least not efficient enough to detect them from Earth.
Instead, they investigated the production of gamma rays by axions in strong magnetic fields around the same star that generated the axions. This process, as shown by supercomputer simulations, very effectively creates a gamma ray burst that depends on the axion mass, and this burst should occur simultaneously with a neutrino burst from inside the hot neutron star. However, this burst of axions lasts only 10 seconds after the formation of the neutron star, after which the production rate drops dramatically — although it is still several hours before the outer layers of the star explode.
Two years ago, Safdi and his colleagues set the best upper limit on the QCD axion mass at about 16 million electron volts, about 32 times smaller than the mass of an electron. This was based on the cooling rate of neutron stars, which would cool faster if axions were formed along with neutrinos inside these hot, compact bodies.
In this paper, a team from the University of California, Berkeley, not only describes the production of gamma rays after the collapse of a neutron star’s core, but also uses the non-detection of gamma rays from supernova 1987A to set the best constraints on the mass of axion-like particles, which differ from QCD axions in that they don’t interact using strong forces.
Provided by phys.org
