The Next Big Physics Machines Are Neutrino Detectors
Now that the Large Hadron Collider has apparently found the Higgs Boson, a sort of malaise has set over some of the scientists behind the project. What do you do next with a 17-mile underground ring? And will there be a successoran even larger collider of hadrons?
Fear not, lovers of big physics machines. While we await that answer, multiple new mega-projects are coming in the form of sprawling neutrino detectors, according to neutrino scientists who spoke at this weekend's meeting of the American Association for the Advancement of Science in Boston. China's building them, Japan is vying for an even larger one that it already has, and the U.S.'s Fermilab, outside Chicago, hopes to start firing neutrinos toward a detector 800 miles away near Mount Rushmore.
Neutrinos are the second smallest known particle (six orders of magnitude smaller than an electron!). The sun creates them. Supernovas create them. The Earth gives off its own. Physicists are fond of reminding people that billions of these ghostly particles pass through your body all the time. But therein lies the problem: Neutrinos interact with ordinary matter so little that they are impossible to detect directly.
That is why you need such big machines. To detect neutrinos, physicists frequently have used a big tank of water (or a mass of ice) with a batch of photomultipliers. Most neutrinos pass right through the water without interacting with anything, but every now and then one strikes the nucleus of an atom. The particle that's then released travels faster than light (within the water medium, where the speed of light is slightly slower than normalthis is how the process doesn't violate the laws of physics.) It creates what's called Cherenkov radiation, which is how scientists know a neutrino came through. They can also start to figure out its energy level and the direction it came from.
So neutrino detectors have always been big. Japan's Super-Kamiokande, a super-sized version of the original Kamiokande where important discoveries of neutrino oscillation have been made, uses a tank holding 50,000 tons of pure water. An experiment called IceCube sends "strings" of neutrino detectors thousands of feet deep into the Antarctic ice. Many more are already in operation.
But detecting the lowest-energy neutrinos, and unraveling some of the ongoing mysteries of these chargeless, nearly massless ghost particles, requires even larger detectors than we have today. So a new slate of detectors are coming. Japan seeks to build a Hyper-Kamiokande, which, as the name suggests, would grow larger than the first twothe detector would be 250 meters long, says Chang Kee Jung of State University of New York. LENA in Europe and Daya Bay II in China also hope to join the race to get bigger.
America's big neutrino hope is the proposed Long Baseline Neutrino Experiment: Scientists at Fermilab would fire a neutrino beam across Illinois and Iowa to a detector in the Homestake gold mine in South Dakota, hoping to observe neutrinos and anti-neutrinos in detail, according to Fermilab's Sam Zeller who spoke at AAAS. Fermilab is also working on the NOvA project, which would fire toward a detector in northern Minnesota.