Apr 6 2013
Scientists have been trying already for years to resolve the issue whether dark matter really exists, and if so what are properties of its constituent particles.
The AMS-02 detector on board of the International Space Station (source: CERN)
Physicists expected that results of the most recent measurements of anti-matter in cosmic rays arriving to Earth conducted with the help of the AMS-02 detector on board of the International Space Station (ISS) would help to resolve the issue. However, the presented yesterday results are still ambiguous.
High-energy antimatter particles do not arrive to Earth predominantly from any selected direction. That may suggest that we are observing effects of annihilation of particles within a cloud of dark matter ("halo") evenly surrounding our galaxy. "It may mean that dark matter consists of particles of very exotic properties, which are very difficult to simulate using known, well-grounded theories, in particular the super-symmetry theory”, comments Professor Leszek Roszkowski from NCBJ. He adds that the published data by no means exclude an alternative possibility: pulsars as sources of superfluous anti-particles. Professor Roszkowski’s team have been studying various aspects of the super-symmetry theory and dark matter for more than 20 years.
Super-symmetry is one of the more popular theories that go beyond modern physics of elementary particles. It assumes that every known particle is accompanied by super-partner of a greater mass. However, such supper-symmetric partners have been experimentally found neither in Cosmos nor in any accelerator-based experiment so far run on Earth. Dark matter remains a completely obscure component of the Universe. We can only indirectly conclude from observations of trajectories of stars within galaxies and galaxies in galaxy clusters that such thing must exist. Both stars and galaxies move faster than the calculations show. The effect might be explained assuming that the galaxies and/or galaxy clusters are more massive that we think they are on the basis of what we see. This invisible component that interacts with common matter only by gravitational forces was dubbed “dark matter”. According to the most recent measurements performed by the Planck European satellite, typical matter of which the visible cosmos (including people) is built amounts to merely 4.9% of the entire Universe matter/energy. Dark matter share is much larger, it reaches 26.8%. The rest is dark energy responsible for ever accelerating rate of Universe expansion.
Computer simulations and astronomical observations suggest that while visible matter inside galaxies usually takes the shape of some disks with spiral arms, dark matter surrounds such disks as uniform, almost spherical haloes. If that is the case, dark matter particles must collide inside such galactic haloes. Theoretical predictions indicate that such collisions should produce particle-anti-particle pairs, e.g. electron-positron or proton-antiproton. Besides, particles and anti-particles produced in dark matter collisions should have characteristically high energies.
"Cosmos is full of protons and electrons, but there are not so many positrons and antiprotons, particularly very high energetic ones. If really observed, such particles would be a substantial argument in favour of the hypothesis that dark matter does indeed exist” – explains Professor Roszkowski.
Measurements of intensity and energy of positrons arriving to Earth from cosmos were started several years ago with the help of the PAMELA detector on board of the Resurs-DK Russian satellite. The obtained results (later confirmed by data acquired during the FermiLAT experiment) indicated a significant surplus of high energy positrons. It was a big surprise for all involved scientists.
"Quite unusual properties of dark matter particles would have to be postulated to explain origin of those positrons in terms of dark matter. The colliding particles would have to produce positrons but not antiprotons. Dark matter particles are not described so in any of the popular theories of modern physics. Additionally, existence of a region in which dark matter density is by several orders of magnitude higher than within the halo would have to be postulated somewhere in the halo vicinity” – said Professor Roszkowski.
Therefore interpretation of data on beams of high-energy positrons in terms of dark matter was treated pretty sceptically by many physicists. Alternative suggestions to explain positron surplus by influence of a pulsar or several pulsars appeared soon. Pulsars are quickly rotating neutron stars. High energy anti-particles – just like those detected by the PAMELA detector – could have been produced in pulsars’ neighbourhood.
This controversy was supposed to be resolved by new data acquired by the AMS-02 (Alpha Magnetic Spectrometer) instrument. 56 institutions from 16 countries contributed to the $1.5 billion project to develop this 8.5 ton mass detector of elementary particles. AMS-02 has been operated outside ISS since May 2011. It precisely monitors cosmic rays allowing to analyse energies of arriving particles and to register time-resolved beam intensities. Because of its exceptional sensitivity, AMS-02 has been proclaimed “Hubble telescope for cosmic rays” in the scientists community.
Preliminary AMS-02 data published yesterday show that high energy antiparticles are arriving to Earth from all directions. It seems that the antiparticles have been produced in dark matter annihilation.
"The problem is that the collected data can still be interpreted in terms of pulsars. As a matter of fact a pulsar should be a point-like source of positrons. However, positrons – as charged particles – must interact with magnetic fields within galaxies, and their initially directional beams may easily diffuse into omni-directional streams. Data published yesterday do not allow to exclude that possibility” – points out Professor Roszkowski. It is also not clear whether intensity of positron beams suddenly drops at even higher energies; such an observation would be an argument in favour of the dark matter hypothesis.
AMS-02 is efficiently monitoring high energy positrons in cosmic rays. However, other (possibly even more accurate) data are indispensable, including data from other experiments. First data acquired in the LUX experiment are scheduled later this year. New generation cosmic ray detectors, including the so-called single-tone detectors are to be operational by 2017. Professor Roszkowski’s team has just published scientific paper in which they point out that taking some realistic assumptions the coming 5 years should be enough to acquire wealth of data sufficient to resolve the dark matter issue.
"Single-tone detectors will be milestones in dark matter research. If they will not succeed, requirements for new dark matter experiments will for a long time exceed capabilities of modern physics. However, we believe they will, since it is very likely. At the same time the LHC accelerator is to resume by 2015 acquiring new data to conduct search for 'new physics', including dark matter. So, we are facing very interesting years ahead of us” – concludes Professor Roszkowski.