A particle named Y(4260) has been identified by the BaBar experiment, an international collaboration of researchers formed by the Italian physicists of the National Institute for Nuclear Physics (Infn) with its seat in Stanford (California).
Y(4260) could provide useful hints about the character of the strong force that holds atomic nuclei together. The discovery was announced during the international symposium “Lepton Photon” in Uppsala in Sweden. “At first sight Y(4260) seems to be what we call a charmonic state, that is to say a particle made up of the combination of a charm quark and of its equivalent antiparticle: an anticharm quark,” explains Marcello Giorgi, Infn researcher, professor of Physics at Pisa University.
Physicists have known for some time that for each particle, an antiparticle exists. The electron’s antiparticle, for instance, is the positron – effectively an electron with a positive electric charge. During the 1950s, however, physicists discovered that many particles were composed of smaller fundamental particles and their corresponding antiparticles. The first case was that of positronium, formed by the combination of an electron and a positron. The first charmonium, that is to say a particle made up of a charm quark and anti-charm, was later discovered at Brookhaven and at the Stanford Linear Accelerator (SLAC), by Samuel Ting and Burton Richter, respectively.
Ultimately, it was realized that charmoni are a real family of similar particles, but with a different mass. Nobody had been able to observe Y(4260) until now, not only because it is hard to make in current particle accelerators but also because it is extremely unstable.
Compound particles could be key to explaining fundamental particles. Quarks and the corresponding anti-quarks, for instance, might be held together through several mechanisms. In order to understand the so-called strong force physicists must understand these mechanisms. The strong force holds together quarks of different types to form neutrons and protons, but also holds neutrons and protons together within atomic nuclei.
The most surprising aspect of Y(4260) is that some of its properties are unusual for a charmonium particle. This makes the physicists think that this particle might be something much more exotic: a complex of D mesons, perhaps, or a state made up of four quarks. Strange indeed, but charming nevertheless.
Rewrite by David Bradley Science Writer