Rare Decays of the B Meson: Particle Physics and the Question of Matter vs. Antimatter
Since the dawn of time, people have looked up at the stars and wondered: “How did we get here?” In order to answer that question, scientists must look backwards in time to the birth of the universe itself. While cosmologists turn their eyes to the stars, particle physicists can turn back the clock from right here on earth. In huge accelerators, we collide particles at high energies; the higher the energy of the collision, the further back in time we can go.
The Big Bang theory states that the universe was formed from pure energy. Experimental physics has shown that a symmetry in the laws of nature causes energy to form matter and antimatter in equal proportions. This is a direct result of conservation laws, which require the amount of energy, matter, and charge in the universe to remain constant. From everyday experience, however, we know that the world around us is made out of matter. Were this not the case, we could accidentally touch something made out of antimatter; our hand and the object could then annihilate, returning to a form of pure energy. Clearly this does not occur. As a result, we know that something must have happened in those first few nanoseconds after the Big Bang. The universe broke the matter-antimatter symmetry. The question remains as to how.
In 1964, physicists discovered that a fundamental symmetry, believed to be conserved, was actually weakly broken in the decay of the short-lived particle named the K0 meson. This discovery led to the theory that matter and antimatter were not created in equal proportions during the Big Bang. The laws of nature seem to slightly prefer matter over antimatter. To better understand the symmetry-breaking that leads to this preference, physicists turned their attention to an unstable particle called the B meson. The BaBar detector at the Stanford Linear Accelerator Center turned on in 1999 to study the decays of this particle. In the heart of BaBar, high-energy collisions of electrons with positrons (the antimatter counterpart of the electron) create B mesons. A better understanding of how the B meson decays and how often it decays into certain types of particles, will help confirm or refute theories of matter-antimatter asymmetry.
At Harvard, I studied rare decays of the B meson into a final state consisting of two low-mass particles. Every particle called a “meson” is composed of two quarks (a quark and an anti-quark, to be precise). The B meson is no exception to that rule, though one of its quarks is heavy and the other is light. The heavy quark is extremely unstable, causing the B meson to decay rapidly. Usually the heavy quark in the B meson decays into another variety of heavy quark. Occasionally, however, the B meson’s heavy quark decays into a light quark; the probability that this type of decay will occur is small. Both decays are mediated by the weak force, which is the only force that allows quarks to change type (what physicists refer to as “flavor”). The lower probability to decay from heavy to light quarks is referred to as CKM suppression. The name arises because the relevant entry in the Cabbibo-Kobayashi-Maskawa matrix, which determines the probability of a decay through the weak interaction, is small.
In the decays I studied, the process through which the heavy quark decays also creates a light quark and anti-quark. These two new quarks combine with the original light quark from the B meson, and the quark which recently decayed, to form two new mesons. These final-state particles now contain only light quarks and are, themselves, light. As a result of CKM suppression in the decay, out of every million B mesons, only a few decay into these types of two-particle final states.
The rarity of these decays into light particles makes the processes difficult to study. Because of this, current measurements are dominated by large errors, and many decays, which should theoretically be possible, have not even been observed. In my research, I measured several rare decay rates more accurately, and discovered new decays that had only been anticipated theoretically.
To search for these rare processes, I first processed data from the BaBar detector to remove as much background as possible, without sacrificing many signal events. I then performed unbinned, multivariate maximum likelihood fits to the remaining data. This mathematical technique allowed me to discriminate between my rare signal and the prolific background events from other processes. An example of such a background event would be when an electron-positron collision does not produce a B meson, but creates many light particles instead, including the final-state particles I want the B meson to decay into.
With vast computer programs describing the physics behind particle decays and the interaction of the decay products with the detector, my colleagues and I simulated the data recorded by the BaBar detector. This process of creating “fake data” is called Monte Carlo simulation because it involves the generation of many random numbers.
With Monte Carlo simulations, we created both signal and background events. Signal events, based on theoretical calculations, allowed me to describe what my signal would look like, were it to occur in nature. With simulated background events, I could determine the efficiency of my background-removal process, and ensure that no background events were misinterpreted as signal. By carefully using both simulated and real data (from samples where no signal events were possible) I thoroughly validated the fitting procedure before looking for real signal events in the full sample of real data. This analysis method avoids possible experimental bias in the measurements.
In my thesis research, I observed four new decay processes for the first time, improved the measurement of one that had previously been observed, and set an upper limit for how often another process could occur. These and related measurements will be used to validate or rule out numerous theories which attempt to explain what could have caused the matter predominance in the universe.
Although the BaBar detector has finished collecting data, construction of next-generation B-physics detectors is underway in Italy and Japan. For although the question “Why is the universe made out of matter?” is simple, the answer is not. Despite decades of particle physics research, much work still lies ahead of us in the quest to understand what happened in the first few nanoseconds after the Big Bang.
Corry L. Lee 