The Y(4260): Fun with Onia
May 6, 2006
Since everyone’s doing it, I’m going to make an attempt at a “generally-accessible” post about a physics topic of interest to me. The particle I’m going to talk about is very obscure, even in the particle physics world, so we’ll build up to it …
CESR, the accelerator that feeds the CLEO experiment, collides electrons and positrons at precisely calibrated energies, and from time to time they will annihilate and produce a quark-antiquark pair. The quark produced has to be lighter than half the center of mass energy, since you have to make a pair. At the energies we are running at now, we can make the up, down, strange, and charm quarks. The first three aren’t all so interesting for us (other experiments study them better), so we concentrate on charm-anticharm production.
One tends to have this image of antimatter as being on a single-minded mission to seek out the nearest bit of matter and annihilate with it, but that’s not really true. The quark and antiquark will only annihilate each other if, in a sense, they wind up at the same place at the same time. The way we produce them, they essentially go into orbit around each other, and this tends to keep them apart. Quantum mechanics means the orbits are fuzzy, and in any amount of time there’s some probability that they’ll meet up (or else the pairing would be stable!), but that behavior is “suppressed.” When this happens for charm-anticharm, the orbiting system’s lifetime, if it can only decay through annihilation, is on the order of 10-20 seconds — an eternity, particle-wise.
States of this kind, where matter and antimatter orbit each other, get the suffix “-onium”: an electron-positron system is positronium, a bottom-antibottom system is bottomonium, and our case, charm-anticharm, is charmonium. (-Onia is the accepted plural…) Quantum mechanics dictates that certain orbits, and only certain orbits, are possible, and we refer to each one as a separate kind of particle (we can do this, even though they are composite systems, the same way we can call an atom a particle). The most famous particle of this family is called the J/ψ, for politico-historical reasons. It is the second lightest charmonium particle, and the lightest that can be easily made at our kind of accelerator. Other charmonium particles get names involving ψ, to remind us that they’re in this family.
When the orbiting pair have enough energy, the system can fall apart another way. The strong interaction, which binds the quarks, doesn’t itself care what type of quark is involved, and is perfectly happy to bind a charm quark to a lighter antiquark, a combination called (for historical reasons) a D meson. When the charm-anticharm system is heavy enough, what usually happens is a light quark pair (down-antidown, or whatever) will be created out of the stew of gluons keeping the charm and anticharm in orbit. Since this can happen anywhere, it is much more likely to happen than the two original quarks meeting up and annihilating. The charm and anticharm then pair up with the lighter quarks, and the charmonium state basically falls apart. When this kind of decay can happen, it is by far preferred over the annihilation, and it leads to the heavy states living much shorter lives (as we pass the threshold, the lifetimes get shorter by a factor of a thousand).
There’s one extra decay mechanism that has to be mentioned. A heavy charmonium state can change to a lighter one by changing the charm-anticharm orbit and dumping the energy somewhere. Depending on the details, it can emit light (exactly like the way light is produced in a neon light, for example), or it can put the energy into gluons, which after some rearranging and quark-popping show up as bound states of light quarks; in particular decays involving two pions are very popular. The particle ψ(3686) (the number indicates the mass in megaelectronvolts) is too light to decay by producing D mesons, but is heavier than the J/ψ, and its primary decay process produces a J/ψ and two pions. Its lifetime is roughly a third that of the J/ψ — the extra decay possibility doesn’t change things much in this case. Charmonium states that can decay to D mesons can decay this way too, but again they prefer not to because the D channel is faster, and so the probability is very low.
So the overall picture here is charmonia heavy enough (above 3730 MeV, give or take) like to decay to D mesons, if possible, and have short lives; charmonia lighter than that decay by annihilation or emission of photons or pions, and last for a long while. Until last year, all the data we had agreed with this prediction.
In July 2005, our colleagues at the BaBar experiment reported finding a new particle, christened the Y(4260) (4260 again for the mass, Y for “we don’t know what this is”), decaying to J/ψ and two pions. This transition would normally be taken as prima facie evidence that this was in the charmonium family. Now, the Y(4260) is heavier than DD threshold, which means it should really want to decay to D pairs; normally, if we could see the J/ψ π π transition, we would expect to see D pairs at rate orders of magnitude larger. Strangely enough, though, the Y(4260) sits in a dip in the production rate for D pairs; for that reason nobody had ever suspected there might be a charmonium state there.
So the Y(4260) decays very strangely. But it doesn’t stop there. We understand the force binding the charm-anticharm system pretty well (we think), at least for conventional charmonium states, and we can predict what masses the different particles should have. We thought we knew what data objects corresponded to what theoretical objects, and the Y(4260) was an unwanted interloper (it is rather a “who ordered that?” situation). Even after rethinking the assignments from the theory, the Y(4260) just doesn’t fit in nicely, and although attempts have been made to explain it as a conventional (albeit very weird) charmonium state, these aren’t terribly well regarded; the prevailing attitude is that the Y(4260) is not your everyday particle. There’s the exciting possibility that it is a “hybrid,” a long-predicted but not-yet-seen type of particle, where a gluon is actually a permanent part of the makeup of the particle, instead of just an ephemeral messenger keeping the quarks bound together.
So the Y(4260) is exciting. How is CLEO involved? BaBar’s result implied that the accelerator could actually tune to right around 4260 MeV and produce the Y(4260) directly. Since there was only the one BaBar paper, the result needed confirmation, and we were in a good position to provide it. So we ran there, and in a paper published a few days ago in Physical Review Letters confirmed the BaBar discovery and added some new information of our own. We searched for decays that BaBar did not have enough data to go after; we definitely saw one and had evidence for another, and in fact what we did see has already ruled out at least one model of what the Y(4260) could be. The hybrid explanation is still alive…
And the investigations continue!