MIT Physics News Spotlight
Tantalizing New Clues Into the Mysteries of Dark Matter
Dennis Overbye, New York Times
April 3, 2013
The Alpha Magnetic Spectrometer under construction in 2010 at the European
Organization for Nuclear Research in Geneva. Photo: Fred Merz/New York Times
The dark side of the universe is whispering, but scientists are still not sure what it is saying.
Samuel Ting, a professor at the Massachusetts Institute of Technology and a Nobel laureate particle physicist, said Wednesday that his $1.6 billion cosmic ray experiment on the International Space Station had found evidence of “new physical phenomena” that could represent dark matter, the mysterious stuff that serves as the gravitational foundation for galaxies and whose identification would rewrite some of the laws of physics.
The results, he said, confirmed previous reports that local interstellar space is crackling with an unexplained abundance of high energy particles, especially positrons, the antimatter version of the familiar electrons that comprise electricity and chemistry. They could be colliding particles of dark matter. Or they could be coming from previously undiscovered pulsars or other astronomical monsters, throwing off wild winds of radiation.
The tantalizing news is that even with the new data, physicists cannot tell yet which is the right answer, but they are encouraged that they soon might be able to.
“I don’t think it makes you believe it must be dark matter, nor do I think it makes you believe it cannot be,” said Neal Weiner, a particle theorist at New York University.
The good news is that the Alpha Magnetic Spectrometer, as Dr. Ting’s instrument is called, is only two years into what could be a 20-year voyage on the space station, and is working brilliantly. “Over the coming months, A.M.S. will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin,” Dr. Ting told an audience of physicists at CERN, the European Organization for Nuclear Research.
The report will be published in Physical Review Letters on Friday.
Astronomers and others from outside the collaboration responded enthusiastically.
“A.M.S. has confirmed with exquisite precision and to high energy one of the most exciting mysteries in astrophysics and particle physics,” said Justin Vandenbroucke of the University of Wisconsin and Stanford’s SLAC laboratory.
Maria Spiropulu, a Caltech particle physicist, said, “They have exquisitely small errors and they stop the plot at the cliffhanger, so we will be asking for more.”
Others, like Gregory Tarle of the University of Michigan, cautioned that uncertainties in the galactic background of radiation might make it impossible to ever get a clean answer to the source of the positrons.
Dark matter has teased and obsessed astronomers since the 1930s, when the Caltech astronomer Fritz Zwicky deduced that some invisible “missing mass” was required to supply the gravitational glue to hold clusters of galaxies together.
According to recent measurements by the Planck spacecraft, about 27 percent of the universe, by mass, is composed of some unknown form of matter unlike the atoms that make up us and everything we can see. Astronomers cannot see it, but they can detect its gravitational tug pulling the galaxies and stars around.
Figuring out what this stuff is is important for more than cosmology. The most favored candidates for its identity are as yet undiscovered particles known as WIMPs — weakly interacting massive particles — left over from the Big Bang. Such particles could drift through the Earth like wind through a screen door. Impervious to almost everything except gravity, they would form a shadow universe clumping together into invisible clouds that then attract ordinary matter. Discovering one of them could give a lift to new theories of physics, like supersymmetry, which predicts of a whole new spectrum of so-called superpartners to the particles we already know about, not to mention explicating the nature of more than a quarter of creation.
But until now, the dark matter particles have mostly eluded direct detection in the laboratory or creation at the Large Hadron Collider.
The sky could be a different story. Such WIMPs floating in the halos around galaxies would occasionally collide and annihilate one another in tiny fireballs of radiation and lighter particles, the theorists Michael Turner of the University of Chicago and Frank Wilczek of M.I.T. suggested in 1990.
The Alpha Magnetic Spectrometer is one of the most expensive, complicated and controversial experiments ever mounted in space. Built by an army of 600 scientists from 16 countries, including Italy, Germany, Russia, China and Taiwan, it took 16 years from its approval in 1995 by NASA’s administrator at the time, Dan Goldin, to get to space, on the next-to-last shuttle flight, in May 2011.
By then there were already intimations of dark matter in the heavens. Pamela, a satellite built by Italian, German, Russian and Swedish scientists, registered an excess of anti-electrons, or positrons, in space — perhaps, they said, from collisions of dark matter particles. Using data from the Fermi satellite, researchers at Stanford were also able to detect a similar excess.
Dr. Ting said his spectrometer had validated those observations with more detail and much better statistics. Among other things, as best the A.M.S. team can tell, the positrons are coming equally from all directions — a result that would favor dark matter, which should be spread evenly through the galaxy, rather than pulsars, which would be localized.
But none of the experiments has yet observed what Dr. Turner called the smoking gun of dark matter. If the signal is coming from colliding WIMPs, the theory says, the ratio of positrons to electrons will rise as the energy of the positrons and electrons rises, flattens and then drops sharply. The cutoff energy represents the mass-energy of the putative dark matter particle.
In the case of pulsars, the fraction of positrons would drop much more slowly and gradually, physicists say.
The space station spectrometer can record particles with energies as high as a trillion electron volts, Dr. Ting said, and indeed in his talk he showed individual events with positrons and electrons having energies of some 600 billion electron volts. But the data presented Wednesday ranges in energy only from a few million to 350 billion electron volts.
That data is summarized in a plot that shows the ratio of positrons to electrons rising rapidly and then ever more slowly as the energy increases. The plot runs out of data, Dr. Turner noted, just where the telltale drop should start taking place.
At CERN, Dr. Ting declined to answer questions or speculate about what happened in the empty spot at higher energies on his graph, or even when he would know. “Slowly,” he answered when asked.