What's dark matter? Find out about the new frontiers of physics

Image: Hypothetical dark matter detection
This image of a hypothetical supersymmetry event at the Large Hadron Collider shows the transverse momentum imbalance due to dark matter particles escaping the detector (direction indicated by red arrow).

Now that physicists have found the Higgs boson, what's next? One of the most successful theories in science, the Standard Model of particle physics, seems to be complete. But what lies beyond the Standard Model? While we wait for Europe's Large Hadron Collider to start up again, there are plenty of other mysteries to explore — and the nature of dark matter looks like one of the most promising frontiers.

"We've been looking for physics beyond the Standard Model for decades now," Caltech theoretical physicist Sean Carroll said in an email. Last month, Carroll's book about the Standard Model, the Higgs boson and the Large Hadron Collider, titled "The Particle at the End of the Universe," won the Royal Society's Winton Prize. Carroll says finding the Higgs boson doesn't close the book on particle physics, but opens a new chapter of discovery.

From Higgs to WIMPs
One clue to the future plot twists lies in the mass of the Higgs boson: That mass should be much larger than it is, "by all conventional notions of naturalness," Carroll said. Current theory can accommodate a lightweight boson, but only by delicately tuning the parameters of the universe.

Image: Carroll
Sean Carroll is a theoretical physicist at Caltech.

"This tuning can be greatly ameliorated if we add appropriate new particles to the theory, as happens in supersymmetry," Carroll said. "But those particles need to have masses near the Higgs mass itself, or they don't explain the tuning."

Mysterious dark matter could be part of the explanation. It's thought to account for more than a quarter of the universe's content, but so far has only been detected by means of its gravitational effect.

"The simplest idea for dark matter is that it's a massive neutral particle that used to be in thermal equilibrium when the universe was very hot and dense, and what we have now is the leftover relic abundance," Carroll said. But no one has yet seen any conclusive evidence for that kind of particle — known as a weakly interacting massive particle, or WIMP. The LHC might detect WIMPs when it resumes its runs at higher energies in 2015. Or it might not.

"On the one hand, that's frustrating, because we like to find new particles," Carroll said. "On the other hand, it's extremely provocative, since it seems to imply that there could be something totally wrong with our cherished notions of naturalness. On the third hand, there isn't a lot of direct help from experiment about what to do concerning this provocation. So at the very least we need to think hard (which is what we should be doing anyway, and usually are)."

LUX and IceCube
Physicist Matt Strassler, a visiting scholar at Harvard and author of the "Of Particular Significance" blog, is thinking hard as well. He notes that a $10 million project in South Dakota — known as the Large Underground Xenon experiment, or LUX — recently ruled out one of the most promising possibilities for a dark matter particle.

Image: Strassler
Physicist Matt Strassler is a visiting scholar at Harvard.

It could well turn out that everything physicists thought they knew about dark matter is wrong.

"The search for dark matter will end when some type of dark matter is found (or somehow shown convincingly not to exist), not before," Strassler wrote. "The former could happen any day; the latter will not happen anytime soon."

But dark matter isn't the only game in town: Strassler is particularly excited about the detection of high-energy neutrinos from far beyond our solar system. "These neutrinos appear to be from a new, unidentified and perhaps unexpected type of source," he said.

The fact that the IceCube experiment in Antarctica is finding such neutrinos should open up a whole new frontier for astronomy. IceCube could well serve "just like any other telescope, allowing us to observe, with admittedly blurry neutrino-vision, exactly where the sky is 'bright' in high-energy neutrinos," Strassler said.

"Historically, every time we add a new type of telescope, we discover new types of objects out in space, and learn more about the objects we’ve discovered previously. So that is the hope for IceCube," he said.

What's next on the frontier of physics? Carroll and Strassler talk about dark matter, cosmic neutrinos and more on "Virtually Speaking Science," a talk show that airs at 8 p.m. ET Wednesday via Blog Talk Radio and in the Exploratorium's virtual auditorium in Second Life. Join the virtual audience, listen to the hourlong show live online or download the podcast anytime via Blog Talk Radio or iTunes. You can send in questions via Twitter, using the hashtag #askVS. And while you're at it, check out these archived shows from "Virtually Speaking Science":

Alan Boyle is's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.