## Tuesday, August 09, 2016 ... /////

### IceCube rules out the sterile neutrino model for LSND

All babies are being killed and embryos are being aborted these days.

ATLAS and CMS at the LHC have basically ruled out all theories predicting new particle physics phenomena for the first 10/fb of the $\sqrt{s}=13\TeV$ data.

Meanwhile, South Dakota-based LUX has improved the limits on the WIMP dark matter cross section by a factor of four: dark matter, if it exists, is harder to be directly detected than previously thought. (Dark matter may also be composed of LIGO-style black holes in which case we will hopefully not play with it here on Earth.) I must mention that almost simultaneously, a China-based experiment PandaX-II has basically matched the results of LUX. See a comparison of the two charts.

Lots of kind hypothetical new particles and processes were killed or postponed. What happened to the evil ones? They were killed or postponed, too. This also applies to sterile neutrinos, a particular family of beasts that are totally plausible but not beloved by people like me.

IceCube is based on the 86-string theory.

At the South Pole, The IceCube Neutrino Observatory was designed to detect ${\rm TeV}$-scale high-energy cosmic muon neutrinos. Those get converted to muons and IceCube is particularly sensitive to those.

In 1996, the LSND collaboration in Los Alamos announced the evidence for some kind of novel neutrino oscillations, $\bar\nu_\mu\to \bar\nu_e$, that differed from the more established solar and atmospheric neutrino oscillations.

The LSND anomaly was controversial, a point often repeated by my Rutgers graduate instructor of practical particle physics, Glennys Farrar. In 1998, I was working on a term paper on neutrinos and this "duty" has approximately doubled my knowledge of neutrino physics.

While the oscillation between the muon and electron flavors was originally proposed by LSND, physicists quickly realized that the wavelengths of the oscillations needed to explain the LSND observation disagree with the parameters (squared mass differences) obtained from the other, more well-known oscillations.

The most popular attitude among the pro-LSND phenomenologists was to postulate another flavor of the neutrinos but one that has no associated charged lepton – because we haven't seen a fourth charged lepton species – the sterile neutrino. The word "sterile" means that it is impotent to breed a charged lepton through interactions involving a virtual $W$-boson.

IceCube drilling tower and hose reel in 2009.

You may pick the simplest model of this form and claim that the observed oscillation was actually changing muon neutrinos into the new, sterile neutrinos. That gives you a well-defined model that may be tested. And it just happens that IceCube has tested it. IceCube wasn't designed to test sterile neutrinos – instead, just those ordinary ones in the cosmic rays. But it just turns out that due to some luck in the numbers, IceCube is sensitive exactly to these hypothetical sterile neutrinos proposed to explain the LSND anomaly.

Well, in May 2016, IceCube released a preprint

Searches for Sterile Neutrinos with the IceCube Detector
which just appeared in PRL and where they excluded some piece of the parameter space of theories with sterile neutrinos. In particular, the best point fitted to explain the LSND anomaly was ruled out at some 99% confidence level. The disappearance of the muon neutrinos could have been seen but it wasn't.

Because of the publication in PRL, the story was covered in two semi-technical articles published by the Symmetry Magazine and Physics.APS.org. Some paragraphs in these articles go beyond my text. I also liked the factoid that one banana (the most popular radioactive fruit) emits some 10 neutrinos a second.

This negative result doesn't say what is the true explanation of the LSND anomaly. Obviously, one possible simple explanation is that the LSND members were intoxicated by LSD – there's some linguistic evidence supporting this hypothesis. Am I the first one to have noticed? :-) I don't want to claim authoritatively that the result was wrong. LSND made another claim in 2001 and Fermilab's MiniBooNE published a potentially related anomaly a decade later. Add a 2011 neutrino anomaly from reactors and a 1994 anomaly from radioactive sources.

So there is some potential evidence in favor of new neutrino species. On the other hand, there are also lots of experiments with negative results, often almost directly contradicting the "positive" claims above.

In the theoretical picture, I believe that sterile neutrinos are basically unmotivated – both in quantum field theory and string theory – but sterile neutrinos may be particles that may have other names, e.g. "modulinos", superpartners of the scalar moduli in string theory, or something else. So while I am basically unexcited by the proposal to trust the sterile neutrinos without further explanations – it is an ugly theory – I am very far from being able to prove a no-go theorem of any sort. They're plausible and their discovery would be a game-changer of course.

However, at this moment, the "negative" results – exclusions – seem to be the kings of the day.