Science Today: BaBar's Astonishing Decays
In the year following the discovery at SLAC (and simultaneously at Brookhaven National Lab) of the J/ψ in November 1974, a whole family of particles, whimsically called charmonia, appeared in experiments here and at DESY, in Hamburg, Germany. Heavy members of the family decayed into lighter members of the family, emitting photons or pions, usually. Their predilection to remain within the family was explained by their being composed of a charmed quark and its antiquark. Until the quark and antiquark annihilated, a rare occurrence, they remained charmonia. A charmed quark and its antiquark, given sufficient energy could instead become two charmed particles, but below threshold, where there wasn't sufficient energy to make charmed particles, the quark-antiquark pair persisted a long time: up to 10-20 seconds!
The discovery of charmonium states made quarks tangible. No longer could you say quarks were just a mathematical construct. Moreover, you could calculate the properties of charmonium states using techniques developed fifty years earlier to describe simple atoms. There was plenty to measure and theorists had an easy life. These were the halcyon days of particle physics.
SLAC's PEP-II asymmetric B factory was built to study the next quark, the b quark and so the energy of the machine is much higher than the energy of SPEAR, where c the quark made its appearance. However, sometimes the colliding electron or positron emits an energetic photon just before colliding with its counterpart coming from the other direction. Indeed sometimes it emits enough energy so that then the electron-positron collision has just about the same energy as there was a SPEAR and it is possible to study the charmonium particles even with PEP-II.
Much to the astonishment of particle theorists, what BaBar here at SLAC and its counterpart, Belle, at KEK, have found is that there are charmonium states that appear to contradict the simple rules we learned thirty years ago. A charmonium state much heavier than two charmed particles should decay directly to them, with little chance that it would instead remain a charmonium, with charmed quark and its antiquark. The Y(4260) announced by BaBar in June 2005 is, nevertheless, seen only by its charmonium decay: it becomes a J/ψ and two pions. It almost surely decays as well to two charmed particles (which are much harder to find than J/ψ) but it is remarkable that it can be found at all in the charmonium decay. In fact, the results from BaBar show that the Y(4260) decays more rapidly into J/ψππ than any other charmonium does, and by a lot: it is much more charmonium-like than any previous charmonium!
More recently, BaBar has observed another astonishing decay. Some charmonium state is decaying rapidly to ψ(2S)ππ. It is not completely clear yet whether this is an entirely new state or simply an additional decay of the Y(4260). In either case, this, too, shows an intensely charmonium-like character. The theorists who had been refining their high-precision calculations of charmonium spectra now have a fundamental conundrum to confront.
óBob Cahn, February 8, 2007