A few years ago one could have imagined an unusual race with no real starting point, no fixed course or particular rules but with the sole goal to discover the violation of CP symmetry in the decay of B mesons, subatomic particles containing a beauty quark b. There were the superstars, the Michael Jordans and Tiger Woodses, as well as some outsiders and less equipped sports teams lining up for this competition. In the race for the discovery of CP violation, the superstars were high intensity electron-positron colliding beam accelerators at the Stanford Linear Accelerator Laboratory in Palo Alto and at the KEK High Energy Physics Laboratory in Japan equipped with their respective detectors called BaBar and Belle. Everybody expected a close race between both megastars, but there were other teams setting out to stir up the celebrity affair. One of these potential party crashers stealing the show from the superstars was the Collider Detector at Fermilab (CDF) collaboration, studying proton-antiproton collisions at the world’s highest energy accelerator, the Tevatron Collider at Fermilab near Chicago.

My field of research is in experimental high energy particle physics. Together with Prof. James Russ, I am a member of the large international CDF Collaboration at Fermilab. The study of B mesons is among the most exciting goals for CDF, although this was not at all anticipated when the detector was designed. The collision of a proton with an antiproton had been considered too complex an environment in which it seemed to be too hard to identify B mesons. CDF was built primarily to study the properties of the W boson, a particle mediating the weak force, and to find the top quark, which was indeed discovered in 1995. Several features in the original detector design were, however, advantageous for studies of B mesons. CDF has a large magnetic tracking volume, in which the flight paths of electrically charged particles are bent, a well-segmented calorimeter to detect electrons and to measure their energy, as well as muon chambers that allow the detection of muons even at low momentum. But it was the later installation of a silicon micro-vertex detector that made the study of B particles possible in a competitive way. This device finds the point of origin (vertex) of particles, indicating how far a B meson traveled before it decayed and thus how long it lived. The reason why CDF was even considered as a competitor in the race for CP violation mentioned above, is the fact that a few years ago CDF demonstrated its ability to measure CP violation in the B meson system with a tantalizing first indication of CP violation in B meson decays.

Before we return to the outcome of the heated race, let me first discuss the issue of why we are interested in studying the violation of fundamental symmetries in physics. After it was found in 1954 that parity P (left-right symmetry or mirror reflection of space) is strongly violated in weak interaction processes, it was also discovered that charge conjugation, C, the expected symmetry under changing all particles to antiparticles, was not a “good” symmetry of the weak interaction either. However, it was still assumed that CP, the combination of both, doing a mirror reflection and changing particles to antiparticles, was still a “good” symmetry of nature. This situation was drastically changed when CP violation was discovered in 1964 in the system of neutral K mesons, particles containing strange quarks. Today the mechanism of CP violation can be described within the Standard Model of particle physics and predicts large CP violating asymmetries for the system of neutral B mesons. Beside the fact that since 1964 CP violation had only been experimentally observed in decays of K mesons, the search for CP violation in the B quark system is of interest for another reason. As suggested by Andrei Sakharov in 1967, CP violation is probably essential for the creation of the apparent dominance of matter over antimatter in our universe. Without CP violation, all matter and antimatter would have probably annihilated after the Big Bang, leaving nothing but cosmic background radiation behind.

The Fermilab accelerator complex has been upgraded to produce an order of magnitude higher event rate in the Tevatron Collider and is running again since 2001. The CDF detector was also undergoing major upgrades, including the charged particle tracking system vital for the study of B particles. Among these improvements are a next-generation silicon micro-vertex detector and a new drift chamber filled with gas and wires used to reconstruct the trajectories of particles from B decays. Figure 1 shows the CDF detector during the installation phase.

Figure 1. Picture of the CDF detector during the upgrade phase before 2001

To cut a long story short, the heated race for CP violation was very quickly decided by the two superstars BaBar and Belle sharing the trophy for the discovery of CP violation in the B meson system. It was almost like a knockout in the first round of a boxing match. The sports giants ruled the game from the beginning and settled the issue quickly. Some might call it pure coincidence, but it is interesting to note that the value for the CP violation parameter sin 2ß= 0.79±0.44 obtained with large errors in CDF's initial attempt to measure CP violation in the B meson system is right smack on the current world average result for sin 2ß = 0.78±0.08 dominated by the measurements of the B factories.

Although this important question has been settled, the topic of quark mixing as a test of the Standard Model is of great importance. In particular, the decays of heavy quarks, such as the b quark with several possible quark transitions, offer an excellent study ground. Beside the study of CP violation, my research interests focus on the field of heavy quark decays. In particular, I am interested in particle-antiparticle oscillations. The concept of quark mixing was first developed by N. Cabibbo who, in 1963, introduced a single mixing angle to describe transitions between up, down and strange quarks. Before the discovery of the charm quark, this concept was then generalized by Kobayashi and Maskawa in 1973 to describe CP violation within the context of the Standard Model requiring the existence of a third generation of quarks. Their generalization of quark mixing for six quark flavors can be described by a 3x3 matrix. A widely used parametrization of this quark mixing matrix was developed by Carnegie Mellon’s Lincoln Wolfenstein.

It is the current and future goal of experiments studying B hadron decays to test the validity of this matrix and thus to ultimately scrutinize the Standard Model. Another crucial test beside the quest for CP violation is the observation of flavor oscillations between a B_s⁰ meson, composed of a b and an s quark, and its antiparticle B_s⁰ measuring one of the important elements of the quark mixing matrix. The physics of B_s⁰ mesons is the arena of CDF as the Tevatron is currently the only place where B_s⁰ mesons are produced. I am involved in this research and we are looking forward to see the CDF team succeed in the discovery of B_s⁰ B_s⁰ flavor oscillations in the near future (barring unexpected party crashers knocking on the door).