Saturday, February 27, 2016

Messy nuclear physics more likely than sterile neutrinos

I was asked about my views concerning the repeatedly reported neutrino anomalies.

The anomalies have been around from mid 2014 but the recent wave of penetration of this gospel was probably ignited on February 12th by the Symmetry Magazine jointly published by SLAC and Fermilab.

A dozen of other sources that noticed the anomalies includes ArsTechnica UK, Physics World, Science News, and "Too Many Anti Neutrinos Evidence", a blog post elsewhere.

The Daya Bay reactor (which is 35 miles from Hong Kong) has measured some neutrino flux and they saw a rather strong, 4-sigma excess of neutrinos around \(5\MeV\). The same bump was previously seen by Double Chooz (France) and RENO (Korea) experiments as 3+ sigma excesses in the same energy range.

I have virtually no doubt that the deviation from the model is real and not a statistical fluke. The combination of 4, 3, 3-sigma exceeses is almost 6 sigma. But I don't see how this excess immediately supports a bold hypothesis about the fourth neutrino flavor – a sterile neutrino is usually discussed.

Don't get me wrong. It would be cool if a new neutrino species were found and a sterile neutrino would even be "somewhat qualitatively different" from the neutrinos we know. But a wishful thinking is not valid evidence.

I find it much more likely that the theoretical prediction is neglecting some important effect – perhaps some interference of neutrinos resulting from different points of a nucleus. This may affect both the production of neutrinos – they're neutrinos coming from some diverse messy reactions in a nuclear reactor – as well as the detection side – all three experiments use an isotope of gadolinium and a liquid scintillator to detect the ghostly particles.

The neglected interference effect looks particularly sensible to me because the Daya Bay experiment also sees a deficit of the neutrino flux in the remainder of the interval between \(1\MeV\) and \(7\MeV\). So what is added in \(4-6\MeV\) is subtracted in the intervals \(1-4\MeV\) and \(6-7\MeV\).

Maybe we will see in the future.

If there were new neutrinos species, it would have important technical consequences for the model building in particle physics. But I would surely disagree with a Nobel prize for some deviation from a messy nuclear model. These models are complicated and not particularly well tested and such deviations are bound to occur.

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