Sterile neutrino

Neutrinos are weird and hard to fathom sometimes. There is a subtle but important difference in how they are detected, due to their extremely low interaction rate with more 'ordinary' matter.

With say a high energy proton plowing through a cloud or bubble chamber ( pre solid-state detector days ) you'd see a track formed by successive collisions along some line ( or evident curve if a strong enough magnetic field present ). This track is actually a string of distinct expanding voids formed in the chamber's working fluid - just below a phase change ( near 'boiling' ), so local addition of energy by the proton causes expansion from a collision vertex - suitably photographed with the right timing delay and triggers. Too early after the proton's passage and the voids haven't grown to viewable size, too late and they get too big thus showing a 'thick' track. Think of a track as like a string of closely spaced beads. So a given track represents very many collisions, or put another way the interaction 'cross section' for protons with normal matter is quite high and thus very short distances separate collisions in the usual density of materials used.

Not so with neutrinos. It's like the order of a light year between collision vertices in say lead. As we don't have detectors of that dimension, or larger, we aren't ever going to see a given neutrino bounce off anything more than once in a typical detector. So they have an extraordinarily low 'cross section' for collisions. Cross section is a calculated quantity that reflects the probability of some interaction. It has the units of area - amusingly called 'barns' - and you can think of it like some target to hit. If the target is small then it's hard to hit, and vice versa. Can you hit a barn door with a neutrino ? ;-)

Anyhow neutrino detection isn't by tracks per se, but by deduction at single interaction vertices. So if a neutrino is 'born' in the Sun, say, that's one vertex ( that we never see of course ) and the other may be in a detector that we build. We find evidence with some events where momentum and/or energy doesn't balance sufficiently before and after collision points well enough, and we attribute the residual to the neutrino. Thus the neutrino 'appears' as a correction to keep our accounting neat. We like the general conservation laws, but that's OK to use that in this way. In a sense we have nominated a special account type, the neutrino ledger, for energy and momentum transport between places in the Universe. There's no neutrino 'track' visible in cloud chamber or bubble chamber photos ( solid state devices haven't really changed this aspect of neutrino detection, while improving many other measurement modes ).

So what's all the fuss with neutrino types, sterile or otherwise? Well you have to make some assumptions ( all are quite reasonable ) and work on large sets of data. So if I do neutrino detections at different distances from some known source ( like the Sun or an artificial nuclear reactor ) I'll find a waxing and a waning of the numbers in a rhythmic pattern. I deduce that the neutrino 'flavour' is changing with distance, as different detector setups ( one may be more sensitive to electron interactions, others with muons ) are used. What's generally been found found is that the total amount of neutrinos reduces in an acceptable way with distance from source ( like inverse square as you'd expect with an expanding shell of area ), but which type of lepton interaction ( electron/muon/tau ) is dominant at any given distance is what fluctuates.

The really queer bit is pure quantum mechanics. So just to annoy and confuse you, there is flavour mixing in addition to the flavour changing with distance as described above. While not absolutely necessary as an explanatory mechanism, it turns out the results of these type of studies make the simplest sense ( Occam if you like ) if one posits that : what is experimentally deemed to be an electron neutrino, or a muon neutrino, or a tau neutrino actually are each superpositions or mixtures of other 'base' types. These base types ( either three or four of them, you could label them as I, II, II and maybe IV if you like ) are not separately detected, but only mixtures thereof are. And these mixtures ( of I, II, III or IV ) are what we see as electron-type, muon-type, tau-type or sterile-type.

[ So it's a bit like like being told that 'Neopolitan' ice cream - you know, where strawberry ( yum ), chocolate and vanilla flavours are in distinct adjacent blocks/areas of a tub - are really each a superposition of three other ice cream types that you've never tasted separately. Nor will you ever know what I, II, III taste like alone. ]

The sterile neutrino is, by definition, not interacting ( even at the incredibly low cross section of neutrinos generally ) at all. So how can one say it exists if it doesn't interact? What's the accounting trick here? To a point this now depends on who you ask .... traditionally the idea has been that maybe we haven't accounted for all neutrinos by adding up the 'visible'/deducible electron/muon/tau types. Maybe a fourth, sterile type ( yet another mixture of I, II, II and now IV - as there has to be as many 'hidden'/base types as the number of distinct mixtures ) exists. So it's never detected per se, but the other types morph into/out-of it none the less. Sounds like the reserve bench for a footy game, there may be 18 guys out on the field ( Aussie Rules here ) playing at any one moment, but you can still swap to and from a pool of two extras on the sidelines. Ditto for basketball etc. I think/remember the key parameters being in the decay modes of certain things via neutrino production. The rates of decay depend on how many choices/pathways of doing that, which in turn depends on how many neutrino flavours ( 3 or 4 ) that you have.

Cheers, Mike.

I don't want to nit-pick, but it seems it was Rabi speaking of the muon. At least that is what THE INTERNET says.

Michael

I think I.I.Rabi was the first to say that. When the particle 'zoo' started to appear with the 50's and 60's 'atom smasher' machines, and confusion reigned as new particles were just everywhere, many others copied the joke/comment.

He is perhaps less well known for his standard comment as a researcher in the Radiation Laboratory ( MIT, USA ) during WWII, upon seeing a new device or weapons design : "How many Germans will it kill?". Chilling to read now, but that was then. Different times.

Cheers, Mike.