Where in space are the faster-than-light neutrinos?

Carmen Sandiego, capable of eluding detection by all but the most persistent gumshoes, might be a superluminal neutrino as well as a super-villain. (Image: Carmen Sandiego, Broderbund.)

If there is one particle that breaks the laws of physics and runs fast enough to get away with it, it is the superluminal neutrino.

Several weeks ago, the OPERA experiment announced that they had measured neutrinos travelling faster than the speed of light.  The neutrinos, which traveled from CERN to the Gran Sasso Laboratory, arrived at the detector 60 nanoseconds earlier than light (with statistical errors of 6.9 ns and systematic errors of 7.4 ns).  According to Einstein’s Theory of Special Relativity, this behavior is criminal.  While the level of skepticism in the scientific community that neutrinos could behave so badly remains high, the possibility of a real detection opens the door to new physics.

The authors consider one such possibility of new physics: the Coleman and Glashow formulation of Lorentz-Invaraiant Violation (LIV) neutrinos.  Although the name is a mouthful, the science is clear.  The energy E, mass m and momentum p of these neutrinos are related by


where the right-most term is a mass-like perturbation called the “effective mass.”  Although the effective mass in the equation above goes as the momentum squared, other power indices are plausible.

Several other observations constrain this violation.  FERMILAB79 and MINOS measured neutrino speeds comparable to those of OPERA.  Supernova 1987A emitted a swarm of neutrinos all traveling within 10^-9 of the speed of light.  However, particle physics itself places more stringent constraints on the LIV model than any of the above detections.  If neutrinos do violate Lorentz invariance, then pion, muon and three-body decays above certain energies are forbidden.  As a result, we would expect to see a cutoff in the spectrum of neutrinos above a critical energy.  An atmospheric neutrino detector called IceCube has measured neutrinos as energetic as 400 TeV from about 1 kiloparsec away, which limits LIV to < 8 x 10^-13.

Energy spectrum as a function of neutrino energy. The dotted and solid vertical lines indicate the energy cutoffs for different amounts of Lorentz invariance violation. The detection of 400 TeV neutrinos (IceCube) limits the LIV parameter to < 8 x 10^-13. (Bi et al. 2011.)

But what about energies higher than 400 TeV?  The authors hope that the neutrinos from supernova remnants and pions produced via GZK processes will create ultra-high energy neutrinos much more energetic than 400 TeV.  Additionally, the large distances ( > 1 kiloparsec) these neutrinos travel would allow for particularly sensitive measurements of their speeds.  With the full detector array activated and longer exposure times, IceCube could detect such neutrinos.  Their detection would constrain the LIV parameter to an even smaller number, further reconciling neutrino speeds with Special Relativity and suspending their apparent criminal activites.

About Lauren Weiss

A Planet Hunter and midnight playwright, Lauren is a graduate student at UC Berkeley. She works with Geoff Marcy to characterize exoplanets. After graduating from Harvard, Lauren received her MPhil degree from Cambridge, where she hosted an astronomy podcast called the Astropod (http://www.ast.cam.ac.uk/astropod/) in 2011. Her greatest desire for the coming era of astronomy is that we will find Yoda on another planet.


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