In February, the University of California, Los Angeles held its 11th Symposium on Sources and Detection of Dark Matter and Dark Energy in the Universe - "Dark Matter 2014" was aimed at discussing the latest progress in the quest to identify dark matter, the umbrella term for the unknown stuff that makes up more than a quarter of the universe yet remains a mystery.
So where does the hunt stand? Between sessions, three leading physicists at the conference spent an hour discussing the search for dark matter on several fronts.
"This conference has highlighted the progression of larger and larger experiments with remarkable advances in sensitivity," says Blas Cabrera, Professor of Physics at Stanford University and Member of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford. "This conference has highlighted the progression of larger and larger experiments with remarkable advances in sensitivity. What we’re looking for is evidence of a dark matter particle, and the leading idea for what it might be is something called a weakly interacting massive particle, or WIMP. We believe the WIMP interacts with ordinary matter only very rarely, but we have hints from a few experiments that might be evidence for WIMPs.
"Separately at this conference, we heard about improved calibrations of last fall’s results from LUX, the Large Underground Xenon detector that now leads the world in sensitivity for WIMPs above the mass of six protons – a proton being the nucleus of a single hydrogen atom. Under a standard interpretation of the data, the LUX team has ruled out a range of low-end masses for the dark matter particle, another major advance because it does not see potential detections reported by other experiments and further narrows the possibilities for how massive the WIMP might be.
"Finally, Dan [Hooper] also gave a remarkable presentation here about another effort: to indirectly detect dark matter by studying radiation coming from the center of the Milky Way galaxy. He reported the possibility of a strong dark matter signal, and I would say that was also one of the highlights of the conference because it provides us with some of the strongest evidence so far of a dark matter detection in space. Dan can explain."
Says Dan Hooper, Associate Professor in the Department of Astronomy and Astrophysics at the University of Chicago, and Senior Member of the Kavli Institute for Cosmological Physics at the university: "Four and a half years ago, I wrote my first paper on searching for evidence of dark matter at the center of the Milky Way galaxy. And now we think we have the most compelling results to date.
"What we’re looking at is actually gamma rays – the most energetic form of light – radiating from the center of the galaxy. I think that this is very likely a signal of annihilating dark matter particles. As Blas explained, we believe dark matter is made of particles, and these particles, by themselves, are expected to be stable – meaning that they don’t readily decay into other particles or forms of radiation. But at the dense core of the Milky Way galaxy, we think they collide and annihilate one another, in the process releasing huge amounts of energy in the form of gamma rays."
Dan Hooper, scientist in the Theoretical Astrophysics Group at the Fermi National Accelerator Laboratory and an associate professor in the Department of Astronomy and Astrophysics at the University of Chicago, and senior member of the Kavli Institute for Cosmological Physics at the University of Chicago. Link: Kavli Foundation.
Tim Tait, Professor of Physics and Astronomy at the University of California Irvine is involved in looking for evidence of dark matter by colliding subatomic particles at the Large Hadron Collider, or LHC, in Europe. Says Tait: "We expect that the density of dark matter particles, and therefore the intensity of the gamma-ray radiation released when they collide, should both fall as you move away from the galactic center. So, you sort of know what the profile of the signal should be, moving from the center of the galaxy outward.
"Supersymmetry proposes there are mirror particles that shadow all the known fundamental particles, and in this shadow world may lurk the dark matter particle. So, by smashing together protons in the LHC, we've tried to reveal these theoretical supersymmetric particles. So far, though, the LHC hasn't found any evidence for supersymmetry."
What would a discovery mean?
Says Cabrera: "We would have identified the dominant form of matter in the universe that seeded structure and led to galaxies, solar systems and planets, and ultimately to our Earth with intelligent life."
Source: The Kavli Foundation