Faster-than-light neutrinos, dark matter, and all that


Luca Mazzucato


Faster-than-light neutrinos, dark matter, and all that


It is always surprising to chat with Shmuel Nussinov, at the School of Physics and Astronomy of Tel Aviv University. He spends several months a year visiting US institutions such as the IAS, Princeton, Upenn, University of Maryland, and he is now visiting Chapman University. His seminars at the C. N Yang Institute are a must see and are guaranteed to leave you flabbergasted. In occasion of his recent seventieth birthday, he decided to give himself an unusual present in the form of eleven articles published on the same week…


Let us talk about the recent claim that made everybody’s jaws drop: the OPERA experiment in Gran Sasso, which collects a neutrino beam shot by the CERN facility through 700 miles of solid rock, claims to have measured faster-than-light neutrinos. Do you believe that the explanation is that neutrinos are faster than light?


Certainly not. There are several reasons to believe that their conclusions are incorrect. If neutrinos were faster-than-light as they claimed, the same anomaly would show up in a wealth of other experiments – but it does not! The simplest example stems from the physics of cosmic rays, that we can measure pretty accurately. Faster-than-light neutrinos can be rephrased as a violation of Lorentz invariance, the basic tenet of special relativity. In general, high energy pions observed from cosmic rays would almost not decay to a muon and a muon-neutrino. But if they did, assuming that the neutrinos are faster-than-light, the “bad guy” namely the muon-neutrino would carry only a tiny energy fraction, with most of the energy going to the muon instead. Which contradicts cosmic rays experiments. Another contradiction was discussed recently by Sheldon Glashow and Andrew Cohen: if we somehow have energetic muon-neutrinos that travel faster-than-light, they would decay to lower energy muon-neutrinos and electron-positron pairs. The beam would loose a great deal of energy in this way, which is in clear and strong conflict with many experiments of extremely high statistics.


What are the alternative explanations?


There is to my mind no viable alternative explanation consistent with all the data… It is probably due to a systematic error that was not accounted for. Other experiments around the world are now trying to reproduce their results, we just have to wait and see.


Let’s talk dark matter. The simplest candidate for dark matter used to be a supersymmetric particle, the neutralino. Should LHC have seen that already?


The status of dark matter searches is a field that keeps advancing fast (but not faster-than-light!). Still, it is true that if the dark matter was due to the supersymmetric particle called neutralino, it should have been seen already at LHC and also it should have been detected in the dark matter detectors underground.


What other possibilities are out there?


Maybe other rival candidates like “technicolor” or more sophisticated versions of supersymmetry, with more particles than the vanilla supersymmetry.


Suppose LHC fails to find any new particle that can account for dark matter. What should we do then, call it quits?


No way! Maybe the dark matter is just neutrons in disguise, which cannot make it to us as I pointed out in my recent seminar at Stony Brook. Imagine there are “mirror neutrons” and that the neutrons we all know and love oscillate into this new mirror species, much like neutrinos oscillate among different flavors. If the mirror particle only interaction with us is gravitational, we showed that this could account for the missing dark matter.


It seems like our best bets for new physics at LHC (supersymmetry, extra-dimensions, tehcnicolor) are not showing up. This trend was constant in the last thirty years and led us to build more and more baroque models. Are we replicating the ptolemaic system here?


You may well be right about this. But you should remember the fifteen or so other “baroque” particles that the standard model successfully predicted and were finally observed… Maybe nature is baroque itself.