Thursday, 18 April 2013

Dark matter remains dark - for now

CDMS detector. Courtesy of Fermilab.

For the past month and a bit, the biggest story in physics has undoubtedly been dark matter.

When Sam Ting, the charismatic Nobel Prize winning director of the AMS project, announced in February that the team who monitor his orbiting Alpha Magnetic Spectrometer, latched to the ISS, would be announcing 'significant results' in the search for dark matter, the whole physics community held its breath.  Somewhat skeptically.
The results, when they were announced on 30 March at CERN, seemed to add weight to the measurements made by PAMELA, in 2008, and were certainly of significant interest.

AMS-02 on ISS. Courtesy of CERN
But they perhaps fell short of what many were hoping for. As theoretical physicist, Katie Mack put it on 5 April in Physics Focus - somewhat bluntly, it should be said, "the AMS-02 detector on the International Space Station has not detected dark matter. It hasn't found 'indications' of dark matter, or even 'hints'. It certainly is not providing the 'best evidence yet' of dark matter's existence."

She did, however, acknowledge the important measurements the AMS-02 detector has made:
"What AMS has done is measure, to very high accuracy, the amount of antimatter the galaxy is bombarding us with. [....]. The AMS experiment detects cosmic rays - protons, electrons, and the antimatter counterparts of each, antiprotons and positrons. Before the experiment ran, we had predictions of how the matter/antimatter fraction should vary with the energy of the particles. AMS tells us our predictions were wrong. The antiprotons look about right, but there's a huge excess of high-energy positrons over what astrophysical models predict, and a bump in the electron flux at high energies. All of these results were actually seen by earlier experiments PAMELA and Fermi, but AMS confirms them to higher precision and higher energies. There's more antimatter than we thought; now we have to figure out why."

This is a concise explanation of the so-called 'positron-excess', which is one of the key indicators that dark matter - whatever it is - exists.  Mack goes on to explain, that the radio astronomers' favourite phenomenon, pulsars, are thought by many astrophysicists to be the cause of the positron excess.
"Pulsars [...] can use their extreme magnetic fields to accelerate particles and create electron-positron pairs. The fact that pulsars do this is solidly in the realm of known physics, and theoretical models can easily fit the signals seen in the cosmic ray experiments."

This is far from an accepted theory of the origin of dark matter, but it is a fascinating one nonetheless.  But where else might we look to find out about the origins of dark matter? The answer is: the mines.

Far beneath the surface of the earth, some of the most significant searches for dark matter have been underway for years. This week, one of the most notable of these, the CDMS experiment (the Cryogenic Dark Matter Search) in the Soudan mine in Minnesota, posted some extremely interesting results. Graduate student, Kevin McCarthy, reported at the American Physical Society meeting in Denver on 13 April that CDMS has found "three promising clues" of dark matter. The their silicon detectors had picked up possible signs of three weakly interacting massive particles (or 'WIMPs', as physicists call them).  Their evidence is verifiable to a level of three-sigma.

As Jason Palmer explains, particle physics has an accepted definition for a "discovery": a five-sigma level of certainty. "The number of standard deviations, or sigmas, is a measure of how unlikely it is that an experimental result is simply down to chance, in the absence of a real effect Similarly, tossing a coin and getting a number of heads in a row may just be chance, rather than a sign of a "loaded" coin The "three sigma" level represents about the same likelihood of tossing nine heads in a row. Five sigma, on the other hand, would correspond to tossing more than 21 in a row. With independent confirmation by other experiments, five-sigma findings become accepted discoveries."

So CDMS' three-sigma result falls short of this, but it most certainly counts as a 'tantalising hint'.

As science writer Valerie Jamieson notes, the CDMS dark matter signal fits with recent theories that suggest dark matter is, "not a single entity, but a 'dark sector of particles' that could include dark antimatter".

"This may be the start of a very big deal" observes dark matter theorist Dan Hooper, of Fermilab, who manage CDMS.

So whilst dark matter remains enigmatic, and most certainly dark, for now, there's some hope we may be closing in on its secrets.


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