Neutrinos

There was a neutrino named Bright

The Economist, Sep 23rd 2011, 10:15 by J.P.   http://www.economist.com/blogs/babbage/2011/09/neutrinos

Who travelled faster than light

NEUTRINOS possess a seemingly endless capacity to discombobulate. First the elusive particles, which theorists believe to be as abundant in the universe as photons, but which almost never interact with anything, turned out to have mass. That discovery, made at Japan's Super-Kamiokande detector in 1998, flew in the face of the Standard Model, a 40-year-old rulebook of particle physics which predicted they ought to be massless (and which has since been tweaked to accommodate the result). Now researchers at CERN, the world's main particle-physics laboratory, report that their neutrinos appear to confound what is, if anything, an even bigger theoretical colossus: Albert Einstein's special theory of relativity. They did it by apparently travelling faster than the speed of light.

Physicists from OPERA, one of the experiments at CERN, send beams of neutrinos from the organisation's headquarters on the outskirts of Geneva, through the Earth's crust to an underground laboratory 730km away underneath Gran Sasso, a mountain in the Apennines. They use fancy kit like high-precision GPS and atomic clocks to measure the distance the neutrinos travel to within 20cm and their time of flight to within ten nanoseconds (billionths of a second). The neutrinos in question appear to be reaching the detector 60 nanoseconds faster than light would take to cover the same distance. That translates to a speed 0.002% higher than the 299,792,458 metres per second at which light zaps through a vacuum.

The result, published in arXiv, an online database, is based on data from 15,000 neutrinos detected at Gran Sasso over three years. If it holds up it would be the first chink in what has until now been the impenetrable armour of special relativity, a theory which has been tested—and confirmed—time and again since its publication in 1905. The theory states that as an object speeds up, time slows down until it stops altogether on hitting the speed of light. Anything going faster than light would, in other words, be moving backwards in time.

A violation of special relativity that affects only neutrinos would be very weird indeed. To confuse matters further, observations of neutrinos emitted by a supernova observed in 1987 established that the particles travel at just below the speed of light through the vacuum of space to a precision four orders of magnitude better than the OPERA claim. That means that the OPERA neutrinos would have to be interacting with matter in some bizarre way that violates special relativity.

The odds, it must be admitted, are that a mistake has been made somewhere in the long chain of timing measurements required to compare the moment when neutrinos are created at CERN by smashing a beam of protons into a target, and their detection in Gran Sasso, though OPERA's researchers have done their best to account for all possible instrumental quirks. What makes the result slightly less than incredible is that an experiment in America, called MINOS, detected a similar anomaly in 2007. MINOS's researchers dismissed that result as a mismeasurement. Now, though, the experiment has ten times more data than it did four years ago, as well as ideas about how to make the necessary calculations more accurate. (A proposed upgrade called MINOS+, which could start collecting data in 2013, might be able to determine the flight time to within one nanosecond.)

Physicists working on another neutrino experiment in Japan, known as T2K, are holding a meeting next week and the OPERA result will be high on the agenda. The effect may be too small to spot in the data recorded before T2K was damaged by the earthquake in March. Moreover, T2K's detector is located just 295km from the neutrino source, so the effect would be just 25 nanoseconds, if it were real. T2K hopes to start taking data again in 2012. 

If the Japanese and American experiments do see the same strange result, it would be the greatest revolution in physics since, well, special relativity burst onto the scene. And it would be fair to say of a neutrino what a wag once quipped about a lady named Bright: that it went away, in a relative way, and came back on the previous night.