In the core of an atom lies the nucleus, composed of protons and neutrons. If we look closer, however, the proton and neutron are made of simpler entities, called quarks. Quarks can pair up in threes to form baryons (such as a proton) or in twos to form mesons (made of a quark and an anti-quark). The mesons produced at accelerators or by energetic cosmic ray interactions in the atmosphere provide ideal testing grounds for our ideas about quantum mechanics and particle interactions. Studies of these mesons have led to numerous discoveries, including the discovery that more quarks exist than just the two required for the proton and neutron. In fact, we have found six species of quarks that can be grouped into three families. Each family has a pair of quarks with similar nuclear and electromagnetic interactions to those in the proton and neutron, but with different masses. The quarks can also make transitions within families through the weak force, first observed in radioactive decay of some nuclei. However, the quark identities are mixed up! The three weakly interacting families involve quantum mechanical mixtures of the quarks that form the mesons, proton, and neutron. This is described through a mixing matrix whose parameters have been measured. The mixed-up quarks within the mesons, combined with the interactions between the quarks, leads to quantum mechanical tunneling between a meson and its anti-particle meson (also called mixing). This provides a sensitive probe of the tunneling interactions. If we start with a meson initially, it really can behave later in time as an anti-meson with a probability dependent on the underlying physics.
The phenomena sketched above have provided the motivation for a number of novel accelerators and detectors designed to study mixing. The BaBar detector at SLAC and the Belle detector at KEK in Japan have collected an enormous amount of data on the meson-mixing phenomenon. The rarest of the mixing processes, involving mesons containing the charmed quark, has just been seen for the first time. The tunneling contribution through the weak force is smallest for this meson-system, which makes it a particularly good place to look for new physics contributions. The mixing details are not yet known with high precision but are already leading to papers that try to ferret out or constrain details of the possible underlying physics. Both the BaBar and Belle collaborations will use their increasing statistics over the next few years to measure the characteristics of this tunneling process with much better detail.
The meson mixing dynamics and the quark mixing matrix bring together some of the most important physics we are after. They are linked to the presently unknown physics that gives quarks their masses and also the physics that generates an asymmetry between the behavior of particles and their anti-particles. This physics was crucial in shaping characteristics of the universe in its early history. Understanding this physics remains a major goal of not only the BaBar and Belle collaborations, but also of the higher-energy accelerators such as the Large Hadron Collider at CERN.
—Abe Seiden, March 22, 2007
Above image: Abe Seiden.