- Title: Constraints and tests of the OPERA superluminal neutrinos
- Authors: Xiao-Jun Bi, Peng-Fei Yin, Zhao-Huan Yu, and Qiang Yuan
- Institution: Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
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.
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.