Wednesday, October 08, 2014

Two neutrino experimental news

NOvA running, new Daya Bay limits on sterile ones

Particle colliders at increasing energies are the "most universal" and the "most prejudice-free" way to search for new physics. New, heavier hypothetical particles of any kind (particles associated with the laws of physics at ever shorter distances) are increasingly visible, along with the effects of these or even heavier particles (all sorts of non-renormalizable operators become more visible, too) whenever the experiments are able to upgrade the energy of the colliding particles.

Physicists aren't putting all their eggs into one basket. Neutrino experiments are an important example of experiments that "ignore the high-energy frontier" described in the previous paragraph. They may be a bit cheaper but they're also looking for a "much more special" type of new physics – so one may argue that the probability of finding something new is lower. There's some sense in which the "bulk" of the U.S. experimental particle physics has been downgraded to neutrino physics.

In the recent week, The Symmetry Magazine brought us two stories about neutrino experiments.

First, the construction of NOvA is actually complete. What is NOvA? Well, NOvA stands for NuMI Off-Axis \(\nu_e\) Appearance. It didn't help you, did it?

OK, it didn't because I must explain what NuMI is. It stands for NeUtrinos at the Main Injector. That's better. In practice, NOvA is a better successor of MINOS, an experiment that waits in Minnesota for some neutrinos shot from the Fermilab in Illinois (a place that is 500 miles away). By having spelled it as NOvA, I have saved a minute even though be sure that I could write it as \({\rm NO}\nu{\rm A}\), too. ;-)

The experiment should measure the PMNS neutrino mass matrix more accurately than ever before. My understanding is that NOvA is already running. It has actually detected its first neutrinos in February 2014, 8 months before the construction was completed! ;-)

Now, the second story is that PRL has published another paper by the Daya Bay experimental team (Daya Bay is just tens of miles Northeast from the currently troubled Hong Kong):
Search for a Light Sterile Neutrino at Daya Bay (PRL)
These folks – who recently proved that the angle mixing the first and third generation neutrinos is nonzero and rather large, as approximately predicted by some string theorists (I mean those in love with the segregated non-gravitational F-theory phenomenology) – were looking for a new species of neutrinos, the sterile neutrinos that are nevertheless represented as the "fourth generation neutrinos".

This is the money graph.

These new hypothetical particles (not really needed for anything, but still plausible) can only be seen if they oscillate from-and-to the three known generations of neutrinos. The greater the "mixing angle" is, more precisely, the larger \(\sin^2 2\theta_{14}\) is, the easier it is to convert the new hypothetical neutrinos to the known one and vice versa – a process necessary for seeing the new particles.

Because the experiment hasn't seen anything, it may say that if the new neutrinos exist at all, the squared sine of the relevant (doubled) mixing angle is small enough and obeys \[

\sin^2 2\theta_{14} \lt 0.015.

\] Well, this rather strong bound is only reached for a particular "medium" mass of the new hypothetical particle, more precisely for the difference of squared masses\[

\abs{ \Delta m_{41} }^2 = | m_4^2 - m_1^2 | = 0.03\eV^2.

\] For smaller or larger values of \(\abs{ \Delta m_{41} }^2\), their limit is less constraining than \(\sin^2 2\theta_{14} \lt 0.015\), i.e. the right hand side of this inequality is a larger number. But they impose some limits for all values of \(\abs{ \Delta m_{41} }^2\) between \(10^{-0.5}\eV^2\) and \(10^{-3.5}\eV^2\). Near the very low mass differences (when the exponent is close to \(-3.5\)), the newly published Daya Bay results are really new. Near the other end of the interval, they are just excluding something that has already been excluded by other experiments – by KARMEN+LNSD and by Bugey.

Apologies for these technicalities. It seems very likely that the sterile neutrinos will not be discovered in the next 100 years. But we may be surprised. There exist models in which most of dark matter is composed of sterile neutrinos. There's nothing wrong about these models. But I would also add that there's nothing great about these models, either. Even if we found out that the dark matter is composed of sterile neutrinos, it would be a good isolated insight to know but we wouldn't learn anything deep, at least not immediately after the discovery of the sterile neutrinos. I am sure it's fair to say that the discovery of the neutralino LSP (and therefore a sign of supersymmetry) would be far more profound.

All these considerations and enthusiastic or restricted emotions should probably be known to anyone and everyone who tries to compare the importance and value of these neutrino experiments with the LHC and its successors, for example. The neutrino experiments are nice but there exist very good reasons why a smaller amount of money is going to them than to the LHC and its successors, too.


  1. {\rm NO}\nu{\rm A} is the giant lump of scintillator oil in PVC plumbing. One wonders if the very first AHA! is is instead due to poly(vinyl chloride) neutrino reactivity,;filename=TIPP_template.pdf;version=5

    The far detector should have been a mile underground tro reduce cosmic ray background. But, what the heck, NSF management is about process not product. The physicists will use a phase lock with accelerator bunches or something.

  2. "It has actually detected its first neutrinos in February 2014, 8 months before the construction was completed! ;-)"
    Haha, neutrinos seem still to be surrounded by some funny causality issues, LOL ... :-D.

    Already before I saw the comments about sterile neutrinos of DM, I wondered if they have to be significantly coupled to the other 3 generations by all means to exist ...

    Anyway, I always like such technicalities ;-)


  3. Haha, Dilaton, just to be sure, it could be a funny piece of science-fiction but the detection of neutrinos was possible because even an incomplete detector was partly usable...

    Neutrinos are notorious for weak interactions with everyone else. They are electrically neutral which means no interactions with the electromagnetic field - and very light which means very limited interactions the gravitational field. They have no color, no interactions with the strong QCD field. And they have weak interactions but they're always weak and short-range, so only decays are interactions from the weak force that may be seen well, and neutrinos don't decay.

    If you summarize it, neutrinos are almost invisible and their rare nuclear reactions in the detectors - which only feel the usual 3 flavors - are the ways to see them. The sterile neutrinos don't even have that, so the main way to make them visible is for them to oscillate into the normal 3 neutrino species, otherwise the sterile neutrinos would be almost completely invisible.

    If there were new neutrinos species that would be *completely* invisible, would they "exist"? Well, everything in our real world has mass and causes gravity, so neutrinos must always have at least some gravitational interactions. Of course, one neutrino has too weak ones, but if some sterile neutrinos were a part (or all) of dark matter, they could be visible through gravitational influences but perhaps completely invisible by all other means...

    Cheers, LM