GENEVA – Now that light has been cast on a new particle that looks pretty darn like the mythical Higgs boson, it’s time for Geneva’s European Organization for Nuclear Research (CERN) to start a considerably more obscure quest: the ever elusive "dark matter."
The international particle physics laboratory released its first results last week – results obtained thanks to the famous particle accelerator, the Large Hadron Collider (LHC), but also thanks to the Alpha Magnetic Spectrometer (AMS) installed in May 2011 on the International Space Station (ISS). The initial results were revealed in the presence of the crew that put the AMS into orbit – that of the space shuttle Endeavour.
This project is part of a series of experiments whose goal is to identify the nature of the oh-so-enigmatic substance that fills about a quarter of the Universe.
In 1933, Fritz Zwicky realized that there was definitely something wrong with the sky. As he was trying to estimate the mass of the cluster of galaxies known as Coma Berenices, the Swiss astronomer noticed that said galaxies were rotating too fast around the center of the cluster, and should therefore be ejected – much like a child off a fast spinning carousel.
Unless – he assumed – there is something hidden in the cluster, a kind of heavy and stable additional matter, capable of using gravity to keep these "spiraling" galaxies together. The concept of dark matter was born. Now the only thing left to do was to find out what on earth this dark matter was made of.
Soon enough, several theories involving some of the most exotic ingredients known to scientists (including neutrinos) were suggested. To no avail. Clearly the dark matter, accountable for 22% of the universe -- five times more than the total of visible matter contained in all the galaxies -- was composed of "something else" of an unknown nature.
Identifying the galaxy’s WIMPs
The latest explanation involves the very fashionable WIMPs, short for Weakly Interacting Massive Particles. These corpuscles weigh 10 to 10,000 times more than a proton, which means that if our galaxy is bathed in dark matter, identifying these WIMPs using advanced detectors should be possible. Various teams around the world have built a plethora of instruments (DAMA and Xenon in Italy, CoGeNT and CDMS in the U.S., Picasso in Canada, Zeplin in England, Edelweiss in France), each one working with an extremely sensitive crystal that spots any new corpuscular intrusion.
However, scientists know that their task is not an easy one: First of all, they believe that WIMPs could interact with their crystals about only once a year; then the "noise pollution" caused by cosmic rays and natural radioactive decay makes things even more complicated; so far, the experiments have led to conflicting results. A failure? "Quite the opposite: using better detectors, scientists have managed to narrow down the field of research," Professor Jules Gascon, a member of the French Edelweiss team, told the monthly French science magazine La Recherche. "The next generation of instruments will reduce it even more."
Maybe the problem lies somewhere else? Are they looking in the wrong place? According to research by the European Southern Observatory (ESO), dark matter is not homogeneously distributed in our galaxy. Some theories postulate that dark matter is present in such small quantities, in a 13,000 light-years radius around the sun, that it’s nearly impossible to spot...
Researchers at the University of Michigan wrote in the international scientific journal Nature on July 12, that they thought the mysterious entity was lurking in filaments connecting clusters of galaxies. The astrophysicists came to this conclusion after studying two such clusters, named Abell 222 and Abell 223, located some 2.4 billion light-years away from Earth.
Other scientists therefore prefer to track down WIMPs indirectly, looking for clues of their existence. During the Big Bang, the elusive particles -- just like all the other particles -- were created together with their antimatter doppelganger: electrons were born with positrons, antiprotons with protons, and, similarly, with every WIMP an anti-WIMP appeared, just as invisible as its double.
What matters is that when these opposites meet, they annihilate each other, thus generating particles -- but this time, very tangible particles -- of matter and antimatter. These are the ones that physicists are stalking with the Alpha Magnetic Spectrometer, built partly at the CERN and partly at the University of Geneva, and installed on the ISS. "We’re trying for example to discover whether there are sources of anomalous positrons in the universe," says Swiss physicist Martin Pohl.
Meanwhile at the CERN, researchers not only track down traces of the elusive dark matter – they actually try to recreate particles, using the LHC! And physicists are all the more motivated since they apparently managed to get their hands on the Higgs boson. "This particle is the keystone of the Standard Model, that is, the most comprehensive theory to date," says CERN Director Rolf-Dieter Heuer. "In the coming months, we are going to study the boson’s characteristics thoroughly. But if what we find doesn’t correspond with our predictions, this means that there could be an even more general theory, which could include dark matter."
This even more general theory has a name: super-symmetry -- SUSY for short. It rests on the assumption that every particle comes with a mirror particle – the latter being much more massive; which means that for example, electrons are accompanied by their heavyweight counterparts called "selectrons". These super-particles (hence selectrons’ inital "s") may be the secret behind dark matter. But physicists think that if none have been observed so far, it’s probably because today’s instruments are not powerful enough to create them.
"We also know for a fact," says Rolf-Dieter Heuer, "that super-particles decay rapidly into lighter super-particles and into particles that we may be able to detect in order to reconstruct this kind of funneling effect," – the whole process being basically comparable to a set of Matryoshka dolls. "The thing is, the last of these nested super-particles cannot decay, it remains stable, and can be seen." Has it been seen? "Not yet, but perhaps we haven’t looked at our data well enough..."
And although finding dark matter probably won’t happen overnight, cheer up: things might get easier in 2015. At the end of 2012 the LHC will be shut down for maintenance and upgrades for up to two years. After that, physicists hope that they’ll be able to work with twice as much energy as today, and thus generate particles with an increasingly high mass. Suffice it to say that the constituents of dark matter, if they exist, should start looking for a good hiding place... "It will take time and patience," warns Rolf-Dieter Heuer, "but it’s obvious that with the LHC and the AMS, chasing the mysterious dark matter has become one of today’s major scientific challenges. "
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Photo – NASA Goddard Photo and Video