See http://en.wikipedia.org/wiki/List_of_mesons. I thought quarks, being elementary particles, would be atomos (indivisible).
My further question is: how well accepted is the current fundamental particle theory? I’m not familiar with the experimental evidence, are there still debates and unexplained results?
Where are you seeing that they have fractional numbers of quarks? So far as I know, a meson by definition consists of two quarks.
In the list, for example, it lists the eta meson as (uu+dd-2ss)/sqrt(6) (ignoring the antiquark bars since I can’t type them) - they even have negative numbers of quarks? Or are these not considered real mesons?
The sqrt(6) you reference is a normalization constant, and does NOT mean that there is an irrational number of quarks. It would take a while to explain the details to you, but essentially the sqrt(6) is on the denominator because each ‘uu’ and ‘dd’ and ‘ss’ are treated like vectors, and the length of their sum is sqrt(1^2+1^2+2^2)=sqrt(6). So in order to make the length of the total vector 1 (normalized) you divide by sqrt(6) in order for the quantum probabilities to work out.
Answer : no, there are a not fractional numbers of quarks.
Your further question: the current fundamental particle theory (‘the standard model’) is very well accepted, and has passed every experimental test. That is not to say we expect it to be the ‘final theory’, but even though it may be not, it seems to be correct in the same sense that Newton’s Laws are correct despite being superseded by General Relativity.
I think that the OP is referring to the “quark content” column of the tables of mesons, which contain factors like 1/√2, 1/√6.
What you’re running into there is a phenomenon called superposition. A π[sup]±[/sup] meson can be viewed as a bound state of a down quark and an anti-up quark (or up and anti-down); if we could look at the quarks inside a π[sup]+[/sup] or π[sup]-[/sup], we’d find these two quarks 100% of the time. The π[sup]0[/sup] quark is fundamentally different: it’s a superposition of two possible states, and is written mathematically as
1/√2 * (up & anti-up) - 1/√2 * (down & anti-down).
What does this weird notation mean? Well, it’s basically saying that if we were to look inside a π[sup]0[/sup] meson, we could either expect to find a up and an anti-up quark, or a down and an anti-down quark, but (here’s the important part) not both at the same time. The probabilities of seeing each option are (by the laws of quantum mechanics) equal to the squares of those coefficients in front:
(probablility of observing up & anti-up) = (1/√2)[sup]2[/sup] = 1/2
(probablility of observing down & anti-down) = (-1/√2)[sup]2[/sup] = 1/2
The probabilities for each option in the eta meson, eta-prime meson, etc., follow the same general principle.
Oh, and to tie this back to something that non-experts might have heard of: You’ve heard about Schrödinger’s cat being “both alive and dead”, right? The way physicists would describe the state of the cat after one half-life of the radioactive atom is
1/√2 * (alive) + 1/√2 * (dead).
Just like with the quarks, Schrödinger’s cat is best described as being in a superposition of “alive” and “dead”; when we open the box and observe the cat, we’ve got a 50-50 chance of finding either of these options.
Its a mixture of states. Each state is just two quarks, but some meson can be in a superposition of different two quark states. The coefficents in front of each state give the square-root of the probablity of find it in that state and the constant (sqrt(6) in your example) just makes sure the total probability comes out to 1.
Well, there’s that neutrino bugaboo. That will probably be resolved as experimental error, but we’ll see.
There’s also a recent unexpected asymmetryin the decay of matter and anti-matter D[sup]0[/sup] Mesons. That’s still not certain enough to claim a discovery either, but at least it’s not breaking any fundamental “laws”.
Or possibly 1/√2 * (alive) - 1/√2 * (dead), depending on the details of the atomic decay used. If you think of “alive” as being a vertical line, and “dead” as being a horizontal line, the state of the cat might be described as a diagonal line, midway between vertical and horizontal. But there are two different diagonal lines, / or . Which one do we mean? Well, that depends on whether that’s a plus or minus sign.
Oh, I see, that makes sense. Thanks everyone. So there are types of particles where all the particles behave exactly the same and have the same properties, but when you look inside they contain different things? Quantum physics sure is weird.
You could even make a pion in such a way that you know it’s made from an up and an antiup, and then check that same particle later and discover that now it’s composed of a down and an antidown.
And we’re sure antiup is distinct from down and they can’t convert between the 2?
Just about the time you think you understand this completely, someone will point out that the quarks in that table are merely the “valence quarks”. A real meson (or baryon) is much more complicated than that, being filled with a sea of virtual quarks and gluons in addition to the valence quarks.
They have different charges — anti-up has charge -2/3 e, while down has charge -1/3 e. So they can’t be the same particle.
Or that it’s converted itself into an eta or an eta-prime. Quantum mechanics is indeed weird.