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This just in from 80beats: Physical Review Letters, the king-daddy of physics journals, has published results from a group at Fermilab that give pretty convincing evidence for the existence of a new elementary particle. But before telling you why this is so dyn-o-mite, we would like to present a brief, handy, and quite entertaining Visitor’s Guide to the Particle Zoo.
A Visitor’s Guide to the Particle Zoo
There are three houses at the particle zoo. In the first house we find our friends the leptons. They come in two types (charged and neutral) and three flavors (color-coded below). These are thought to be truly fundamental in the sense that they have no smaller parts that make them up. So let’s check out the inhabitants of the strange country that is
THE LEPTON HOUSE.
(1) The electron (e-). Tiny and negative, this is your old friend from Chemistry 101. You can’t swing a dead cat without hitting a kerjllion of these. I’m not even sure why you’d want to swing a dead cat, but it is notable that about 1/2000 of the mass of any given dead cat is due to its electrons. Electrons carry one unit of negative charge, as do its two closest relatives, the muon and the tau lepton (coming up).
(2) The electron neutrino (ν_e). Tiny and electron-like, but without the charge. Like Switzerland since 1815, this one, like all neutrinos, is neutral. Which means that neutrinos — all three flavors — are exceedingly hard to detect. They just don’t interact with other members of the Zoo. Their elusive and aloof character motivated John Updike to write, with a bit of poetic hyperbole, for they do interact (weakly) and they do have mass (barely),
Neutrinos, they are very small.
They have no charge; they have no mass;
they do not interact at all.
The earth is just a silly ball
To them, through which they pass
Like dustmaids down a drafty hall.
It is true. They pass through the earth with ease, and are passing, zillions every second, through your body this very moment, just as they have every second of your little life.
(3) The mu lepton, or muon (μ). Heavier than the electron and also negatively charged, this fellow is not abundant in nature because it tends to decay *poof* into an electron (plus a couple other -ons) pretty quick-like. It is found in cosmic rays (yes, there are such things as cosmic rays) and particle accelerators. And in the Particle Zoo.
(4) The mu neutrino (ν_μ). Like the electron neutrino but more burly.
(5) The tau lepton (τ). Pretty much just like the muon, but on steroids.
(6) The tau neutrino (ν_τ). Like the mu neutrino but even more burly.
There are lots of way-cool thing about neutrinos, but the most important for this tour is that they oscillate between flavors: a muon will eventually become a tau lepton and then revert to the muon state, etcetera. This is important for remember for what comes later.
The next house holds a charming collection of subnuclear doodads called quarks. They come in six types — listed below — and three colors. So for each of the six listed below, say, the “up” quark, there are really three: the red up, the green up, and the blue up. Same for the down, and so on. [Note: the quarks are not literally red, green, and blue, which is really too bad.] Like teenagers at the mall, quarks always come in groups; one has never been observed in isolation. They also have the entertaining property of having fractional charge. The name quark was given by Murray Gell-Mann and was taken from this passage from Finnegan’s Wake by James Joyce:
Three quarks for Muster Mark!
Sure he has not got much of a bark
And sure any he has it’s all beside the mark.
What is it about leptons and poetry? The mind boggles.
In any case, there are six quarks (12 if you count anti-quarks), just as there are six leptons. Also like leptons, quarks come in three groups of two (color-coded below). Now, please do enter with me now into the dream within a dream that is
THE QUARK HOUSE.
(1) The up quark (u). This common-as-dirt quark has a charge of +2/3. This means it has a two-thirds the charge of the hard-to-avoid proton (more on protons below). It is very light.
(2) The down quark (d). Also an ultra-featherweight, this guy is also commonly found in regular stuff. But it has a charge of -1/3, one-third that of the electron. You have lots of downs in you right now this very moment. And ups, for that matter. Did you know that?
(3) The charm quark (c). [Note: I am not making these names up. They are official as they can be.] The charm is heavier than the up but has the same charge (+2/3). Like the three quarks that round out this display, it likes to decay quickly *poof* into other particles so you don’t find it much in nature, but it is common at big accelerator labs like Fermilab and CERN.
(4) The strange quark (s). Like the charm, the strange is medium-sized by quark standards. Unlike the charm, it has a charge of -1/3. Also unlike the charm, it has three eyes. That’s so very strange.
(6) The bottom quark (b). This guy follows the down and the strange by having a charge of -1/3. Like the charm, strange, and top, the bottom is uncomfortable with being itself and so decays quickly into other particles.
Now then, all the particles we have visited so far are the stuff that makes up all the matter we know of. BUT WAIT! you say — oh, we can hear you now — how about protons, huh? How about neutrons? You heard about these in science class, sure you did. Welp, I’ll tell you about them. Any given proton is made of up of two up quarks (for a total charge of 4/3) and one down quark (charge of -1/3) for a total of three quarks (with total charge 4/3 – 1/3 = 1). So these wildly famous -ons are not fundamental particles. There are also a whole bunch of other -ons, made up of quarks. Some, like the proton and neutron, are made up of three quarks and are termed baryons. Others are made up of two quarks and are called mesons. (Meanwhile, baryons and mesons make up a group called hadrons. But we digress.)
Now for something really cool: there is such a thing as antimatter. It is exists in real actual nature and not just in Star Trek reruns. To be precise: Each of the particles listed above has a corresponding antiparticle: the electron has the positron, the muon has the anti-muon, the up quark has the anti-up, the blue strange has the blue anti-strange, etc. There are even anti-protons, made of two anti-up quarks and one anti-down quark, and anti-neutrons, made of two anti-down quarks and one anti-up quark. What makes these different, for our money, is that they carry charges opposite those of their corresponding “normal” particle. So the positron is basically a positively-charged electron, an anti-up quark has a charge of -2/3, etc. Antiparticles also differ in something called “helicity,” but that’s for another day. But this is for now: There is even (get this!) anti-hydrogen, which is the only anti-atom physicists have made in the lab. It, of course, consists of one positron bound to one anti-proton. Now that’s butt-kickin’ fine.
So the leptons and the quarks make up stuff: the air; the fizz on Coca-cola; your head; hot dogs; birds’ feet; the Great State of North Dakota; asteroid 624 Hektor; galaxies far, far away; alien eyeballs. Now, what else could there possibly be for particles to do? Nothing, you’d think, but you’d be wrong there. Particles also carry forces. For example, the electromagnetic force, the weak nuclear force, the strong nuclear force, and — in all likelihood — the gravitational force (but not The Force, sorry, Luke). These are the “four fundamental forces of nature” and are all mediated by fundamental particles. (The “force of friction” in all its manifestations is, at root, electromagnetic.)
The force carriers are the fundamental members of a group called… wait for it… the vector bosons. There are only a few such fundamental vector bosons, much, I’m sure, to your relief. So let us now venture through the veritable wonderland that is
THE VECTOR BOSON HOUSE.
(1) The photon (γ). This is your basic old-school light particle. It has no charge and moves fast as all get-out. Although the photon constitutes what we humans colloquially call “light,” it is so much more: radio waves, infrared radiation, ultraviolet light, X-rays, and gamma rays are all just photons of different wavelengths. The photon is also responsible for carrying the electromagnetic force. So when you get shocked by touching a doorknob, when kids’ hair stands on end at the bottom of a plastic slide during dry weather, when you get hit by lightning, when you use a compass or put your favorite picture on your fridge, you have this zippy little character to thank. It passes only between particles that carry electric charge.
(2) The W+, W-, and Z^0 bosons. These dudes are all found in the same enclosure because they all carry the same force: the weak nuclear one. This is the least well-known force among the general human public. It is responsible for something called beta decay and for the tiny number of interactions in which neutrinos take part.
(3) The gluon (g). Put this in your pipe and smoke it: A helium nucleus is composed of two neutrons and two protons, all stuck together nice and tight and secure, right? Sure. You probably learned this back in high school. There is a problem here, though, a problem your high-school chem teacher may not have told you about. Can you find it?… Sure you can, but I’ll tell you anyway: How do those two protons stay together in such a tight bundle? Ever think of that? I mean, unlike charges attract and like charges repel, right? And these two protons (of like charge) are nearly in contact with each other, so that’s weird, okay? This is where our friend the gluon comes into play.
The gluon carries the strong nuclear force. This force works between quarks (which make up protons and neutrons, remember?), is always attractive, and is, well, really strong. So the strong force, mediated by our new friend the gluon, holds the nucleus together despite the push-apart effect of the electromagnetic force between the two protons. Nifty, huh? Hell yes it is.
Fine. If you’ve read this far, I should mention a few “theoretical” -ons which we’re pretty sure exist out there in the world but which have not yet been trapped. First there is the Higgs boson, and this one’s a real Big Deal. They’re hoping to capture it at the CERN’s Large Hadron Collider. The so-called Standard Model, which for our purposes is everything I’ve told you so far, predicts that the Higgs should exist and that it has the highly amusing property of investing stuff with mass. After that there is the graviton, the boson that carries the gravitational force. Then we have the tachyon, which is a faster-than-light particle which shows up in some so-called quantum field and string theories. It probably doesn’t actually exist, because then we would have to allow for the breaking of causality, which means (in principle) that you could die before you were born. So everything gets all higgledy-piggledy once we allow tachyons into the universe. Good for science fiction, not good for science. Finally, there is whatever crazy particle (or particles, or whatever) in the world make(s) up dark matter, which so far we have never directly observed but which accounts for about 85% (!) of the mass of the whole universe. Which provides a nice segue into the point of this whole thing.
Introducing — perhaps — the oddest of birds, the sterile neutrino
This possible new particle is the sterile neutrino. It is an unfortunate name, to be sure. But it is apt, because this guy, unlike other particles, interacts ONLY through gravity. It does not interact through the electromagnetic, the strong nuclear, or, unlike its neutrino compatriots, the weak nuclear forces. The new data from Fermilab, which confirm and build upon some older and highly inconclusive data taken at Los Alamos National Laboratory, indicate that there should be four kinds of anti-neutrinos, not three. This is determined by observing the rate of anti-neutrino oscillations, which turns out to be faster than expected. But this faster rate does not obtain in the land of “normal” neutrinos, where three are all we get. So what? Three things:
(1) The sterile neutrino is not predicted by the Standard Model. Therefore it may point to some undreamed-of new physics, or something else major-big. This is evident in the fact that the nice balance of the lepton species — 6 and 6, each consisting of three pairings — will be broken. A mess will be made of this nice balance, which is very interesting.
(2) The sterile neutrino may help solve the problem of dark matter. If this discovery holds up then these sterile neutrinos would be an ideal candidate for solving this issue, which is a world-class physics problem, and one of the leading questions in the world of science.
(3) The sterile neutrino may help to solve the problem of why the universe is made almost entirely of matter, and is not an equal matter-antimatter mix. This obvious asymmetry is another of today’s major physics & cosmology questions.
So if the sterile neutrino is real, some problems will be solved, and some problems will be created. But: These data may all turn out to be bogus. Random noise. One must remember this in science as in everyday life: What appears to be the case may not, in fact, be the case. But the nice thing about science is that, ofttimes, the probability of bogusness can be calculated. In this case, it is 3%, which may not sound like much but is really quite a lot in scienceland. More work, to be sure, will be done to see how low this probability can be pushed, or if it can be pushed at all.
So that’s your science fix for the day. I hope you enjoyed reading it as much as enjoyed writing it. In the meantime, I have a question for Julie the Particle Zookeeper: If the sterile neutrino proves to be for real, what color will it be? How will it be made to look sterile? Inquiring minds want to know.
Until next time, keep it plugged in and tuned to psnt.net.