A new paper that appeared on the arXiv preprint repository reports the observation of a new particle with a mass of 4.43 GeV by the BELLE collaboration, an experiment studying heavy quarks in an attempt to understand the origin of CP violation.
The importance of this discovery is that this particle does not appear to be predicted by theory, and thus might be evidence of "new physics", such as particles and interactions beyond the Standard Model of particle physics.
But before jumping up and down, screaming "I always knew it, the Standard Model had to be wrong", let's take a deep breath. The experimentalists themselves don't make any claim that their observation invalidates the Standard Model, and that for a number of excellent reasons.
The first is that the predictions of the Standard Model for bound states of quarks are anything but straightforward. The reason for this is that QCD, the part of the Standard Model that describes the interactions of quarks, is very complicated to solve due to the fact that the gluons, the particles that mediate the interactions between quarks, interact with themselves (this is to be contrasted with electrodynamics, where the photons do not interact with themselves, but only couple to charged particles, making the theory much easier to solve).
As a consequence of this, predictions for the masses of hadrons, as particles made up of quarks and gluons are called, come either from models that approximate QCD with something simpler (and whose failure in making a correct prediction hence says little about QCD itself) or from lattice simulations of QCD. The latter are very demanding in terms of time and computing resources, and the lack of a prediction of any given particle state can easily be due to the particular channel in which it can appear not having been studied well enough on the lattice (particularly since some of these channels are very difficult to study on the lattice).
So a more thorough theoretical investigation may well show that the state observed by the experimentalists is in fact predicted by QCD. The second is that short-lived heavy particles like this one are not observed directly, the way that electrons or muons are. These kinds of particles have such a short lifetime that they never get to leave a track in a particle detector. Instead, they are observed as bumps in the energy distribution of the decay products of other particles. Loosely speaking, when a particle A can decay into particles B, C and D via a process like A -> X+B, X -> C+D, then the distribution of the energies of particles C and D will be different from that of a direct decay A -> B+C+D, keeping a signature of the properties of particle X.
So when one plots the number of decays observed at each energy of the particle pair C+D, there will be a bump in the histogram around the mass of particle X. By locating such bumps and fitting them against a Breit-Wigner form, one can then find the mass and decay rate of particle X. However, the decays of subatomic particles are fundamentally stochastic processes, and there is always the possibility of a bump occurring by random chance.
Those kinds of false bumps will go away as further observations are added ("better statistics", as physicists call it), whereas genuine particle bumps will become more prominent. Experimentalists generally take great care to publish only results that are statistically very unlikely to have arisen by chance, but the devil is a squirrel, as the Germans say, and results that seemed certain have gone away (e.g. the pentaquark).
So one has to wait for future confirmation. In spite of these cautions, the observation of heavy quark states whose properties don't mesh with the Standard Model is currently the best avenue for the discovery of new physics -- at least until the LHC turns on.
