Our understanding of the structure of matter has progressed rapidly in the past 150 years. Electrons, bound by the electromagnetic force (via photons) to dense nuclei, dance expertly under our control in lasers, microelectronics and other devices. Protons and neutrons form each nucleus despite the mutual repulsion of the positively charged protons. This hinted at a new type of interaction, the strong force. Not only does this force bind the nucleus together, but we now view each proton or neutron as an assembly of three quarks. These groups of ‘up’ and ‘down’ quarks are also held together by the same strong force (via gluons).

A new layer of sub-atomic alchemy begins to emerge once we consider neutron decay. Inside the neutron an
up quark changes into a down quark, transforming it into a proton, and an electron and an antineutrino (an electrically uncharged cousin of the electron) are emitted. Once again, this is a hint of a new force at work. Neutron decay being rather slow, this was christened the ‘weak force.’

We have now introduced all of the particles necessary to build most of the known universe. But nature has strangely supplied us with two extra copies of the basic quarks (up, down) and leptons (electron, neutrino); see Figure 1. These second and third ‘generations’ are distinguished by each being heaver than the preceding one. They are only produced when enough extra energy is available, for example in cosmic-ray interactions, the early universe, or at accelerators. They decay into lighter particles, also via the weak interaction. For example, the third-generation bottom quark most frequently decays into a second-generation charm quark.

Figure 1. The three generations of quarks and leptons. (copyright 2000, Particle Data Group of lawrence Berkeley National Laboratory)	Figure 2. An illustration of all the possible interactions to the W boson. The six boldest lines indicate couplings of nominal strength, while the other pairs denote successively weaker couplings.

The feeble weak interactions would be lost among the more copious strong and electromagnetic interactions except that they alone can change the type, or ‘flavor,’ of quarks. Pairs of heavy bottom and anti-bottom quarks can be created or destroyed by any of the three interactions. But once the pair separates, the heavy bottom quark can pair with a light anti-quark in a bound state known as a B meson and survives until the weak force causes it to decay. Such is the case for bottom quark pairs produced at CLEO.

The CLEO collaboration has been a major player in B physics for more than 20 years. Most bottom quarks decay to charm quarks by emitting a W boson, the carrier of the weak force, but at a surprisingly slow rate. The strength with which the weak interaction couples to pairs of quarks varies and is the one other known distinction (in addition to mass) among the generations. The coupling to quarks varies over two orders of magnitude. The different interactions of the W boson are illustrated in Figure 2. There is currently no deep understanding of the origin of the pattern. By studying B meson decays, the collaboration made some of the best measurements of certain weak interactions among quarks.

Many mysteries in particle physics today are connected to the weak interaction. The weak force is apparently the sole source of certain symmetry violations: parity (mirror inversion), charge conjugation (matter-antimatter) and even small violations of the combination of these operations (CP), which is believed to be equivalent to time-reversal symmetry violations. As a result of the potential to study CP violation, B meson physics became so interesting that two new accelerators and detectors were recently constructed to study it in the U.S. and in Japan. (Other experiments also study B physics; see Manfred Paulini’s article in this Newsletter.) With these new experiments taking data, CLEO users have decided to move on to new pastures with the CLEO-c project, which will begin in 2003.

CLEO-c researchers will concentrate on the physics of charm quark decays and studies of low-energy hadron spectroscopy. We will make precise measurements of weak interactions involving charm quarks. In the spectroscopy arena, we will use certain gluon-rich particle decays to search for as yet unseen but theoretically expected states made up either entirely or in large part of gluons (see Colin Morningstar’s article in the 2001 newsletter). The program will allow for crucial tests of Lattice QCD (computer-based calculations of how the strong force works). Understanding how strong interactions affect our measurements with Lattice QCD is crucial for precision studies of weak interactions.

Since the other key distinction among the three sets is their mass, one is also led to study the origins of mass in particle physics. Amazingly, this is now possible within a fairly solid theoretical framework. Several Carnegie Mellon faculty members are part of a large international collaboration involving the CMS detector for use at the Large Hadron Collider at CERN in Geneva, Switzerland. Data taking will begin in 2007 and a major goal is observation of the Higgs boson, thought to be a key player in creating the mass of all elementary particles. But that is a story for another article.

Links for more information:
http://www.lns.cornell.edu/public/CLEO/
http://particleadventure.org/particleadventure/