Mysterious dark matter observed in cosmos mainly as gravitational shadows has been inferred for a long time. Perhaps it is composed of neutralinos, particles predicted by super-symmetric theories of the matter. Estimations performed by a team of physicists from NCBJ in Świerk suggest that new generation detectors (just in the commissioning phase) will help to definitively resolve the neutralino existence question within two coming years.

Our Universe contains almost six times more dark matter than barion matter of which our visible world is built. It is quite mysterious component of the cosmos – no single elementary particle of the dark matter has been detected to this day. How­ever, neutralino, a particle predicted by some new physics super-symmetry theories is a good candidate. A team of theoreticians from NCBJ in Świerk led by Professor Leszek Roszkowski has presented new predictions of properties of hypothetical neutralinos as particles of the dark matter. The main conclusion is that the detector currently under construction – and perhaps even the currently ope­ra­ted detectors – should be able to finally confirm or finally exclude the hypothesis in the coming two years. The results were presented during the COSMO-15 Conference that last week attracted to Warsaw almost 250 elementary particle physicists, astrophysicists, and cosmologists from the entire world.

Grounds on which dark matter existence is postulated include results of analysis of movements of stars in galaxies and movements of galaxies in galaxy clusters. These objects move faster than expected, so they must be influenced by some stronger gravitational fields than those exerted by visible masses in the vicinity (galaxy nucleus, neighbouring galaxies). Dark matter is very difficult to observe since practically it does not interact with electromagnetic radiation.

In spite of many years of investigations we still do not know what particles the dark matter is composed of. Until recently neutrinos were considered. Neutrino elementary particles interact with regular matter by means of weak nuclear forces rather than by electromagnetic forces. The former forces are really weak: its is estimated that lead target capable to stop a beam of neutrinos would have to be several light years thick. Initially it seemed that some suitably large collections of neutrinos (provided that neutrinos have any mass at all, which is not certain) might play the role of the dark matter and effectively influence movements of stars and galaxies.

„However, with time it has become clear that even if neutrinos could interact gravitationally, they would not be able to make up the postulated masses of dark matter. Now we are approaching an investigations phase which should reliably confirm or exclude another group of dark matter candidate particles” – said Professor Roszkowski.

Hypothetic Weakly Interacting Massive Particles (WIMPs) are promising candidate particles. To be invisible the particles should not interact by means of electromagnetic forces with the visible matter. Initially WIMP mass was estimated to a few proton masses, but predictions of several newer theoretical models are concentrated rather in the range of a few hundred proton masses, some are even higher. Axions are other probable candidate particles. Contrary to WIMPs their mass is extremely small, 10¹⁵ times lower than proton mass. They might interact with the visible matter at least 10¹⁸ times weaker than neutrinos do, which make them extremely difficult to detect.

A few other candidates are also emerging from the super-symmetry theories. Such theories predict that every elementary particle predicted by the Standard Model has its more massive counterpart (hence the super-symmetry). Very massive super-particles may be unstable and decay to some less massive super-particles; however, the least massive super-particle should be stable. In some theories such super-particles interact with the visible matter very weakly, therefore they are a good candidate for the dark matter elementary particles. Such particles are referred to as neutralinos. The super-symmetric description may be extended to other cases producing other candidates, like hypothetic gravitinos (gravitons super-symmetric counter-partners), hypothetic axinos (axions super-symmetric counter-partners) etc.

„Neutralinos, axinos and gravitinos are all excellent candidates capable to explain the dark matter mystery. However, neutralino is somewhat exceptional within that group. Our team’s results suggest that if it exists, it should be detected by new generation detectors” – explains Professor Roszkowski.

The most recent experimental results taken by NCBJ scientists into account when estimating the most probable neutralino mass include the Higgs boson mass determined experimentally in 2012, the fact that no new particles have been found within energy range penetrable by LHC, as well as some rare meson decays in which super-symmetric particles are supposedly involved.

„The possibility that the lightest super-symmetric particle (i.e. neutralino) has mass located in a relatively narrow range centred around one thousand proton masses became the most natural one after the discovery of the Higgs boson. Our estimation is surprisingly precise: the so-far explored range covered as much as 20-30 orders of magnitude” – said Professor Roszkowski.

Neutralino of a mass around one thousand proton masses would mean that super-symmetric particles are probably too heavy to be possibly produced by LHC. Simultaneously, the entire range of possible masses and interactions that hypothetical neutralino almost entirely is covered by capabilities of the currently commissioned one-tonne XENON-1T detector in Gran Sasso close to Rome.

„We will soon see whether neutralino of a mass around one thousand proton masses is just a convincing hypothesis, or else a physical fact. We have recently shown that the particle mass might be larger (slightly manipulating the state of Universe just after the Big Bang). But even then new experiments will be capable to discover or exclude the particle. The two coming years are going to be a very exiting period for our team” – smiles Professor Roszkowski.

The COSMO conference has been organized since 1997 (venue of this year event is in Warsaw). Conference participants discuss key issues of cosmology, including nature of the dark matter/dark energy, origin and evolution of large-scale cosmic structures, the earliest stages of Universe evolution just after the Big Bang. Outstanding astrophysicists, elementary particle physicists and cosmologists who attended past COSMO events include Stephen Hawking (University of Cambrigde) and Andrei Linde (Stanford University). Both these scientists are still members of the Conference Supervisory Committee, chaired from the beginning by Professor Roszkowski. Later editions of the Conference were organized in many leading research centres of the world, including CERN (Geneva), Chicago, Cambridge and Tokyo.