The charming case of X(3872) (APS April 2008)
Most of the media attention in particle physics goes to the high-energy frontier. The big news stories tend to be discovery of new particles like the top quark and the likely discovery of the Higgs at the Large Hadron Collider.
However, there is a lot of lower-energy particle physics research going on and the whole field is getting very interesting. At the APS meeting this morning, Eric Swanson of the University of Pittsburgh gave a nice summary of what he calls "the new charmonia." These are particles observed in collider experiments that have an energy suggesting they are contain charm and anti-charm quarks.
Swanson showed a table of particles that have been observed where he rated a dozen particles in terms of robustness of results and of interest to him. Sitting in the corner of extremely robust and very high interest is the particle called the X(3872). The 3872 refers to the energy of the particle which is measured as 3871.2 MeV and the X refers to some other technical properties of the particle but also indicates that it isn't yet well understood.
The X(3872) is established as a real signal without any reasonable doubt. But what is it? The energy of the particle and the ways in which it decays suggest a few possibilities. It could be a tetraquark: a particle consisting of four quarks. It could also be a signal that doesn't indicate an actual stable particle as such but the energy at which certain particle processes take place. Perhaps the most interesting case, and the one that Swanson favors is that this is a "molecule" of D0 and anti-D0 mesons. (A D0 is a combination of a charm and an anti-up quark.) These two particles would be bound together just like two hydrogen atoms can bind together to make a gaseous hydrogen molecule.
If that is true it would be first known case of this kind of matter. It opens up a whole host of interesting ideas about the chemistry of subatomic particles instead of a chemistry of atoms. It also really pushes physicists into a deeper understanding of their theories of quarks and the strong force that operates between them. The idea of particles being "merely" constituted of quarks is known to be an approximation that works at low energies, so there is bound to be a lot new to learn about how this matter behaves.
Swanson provocatively asked, "The constituent quark model must fail somewhere. Have we seen it?"
See all posts from the American Physical Society April 2008 conference here.