A New Particle Lurking?

In 1995 when I was a post-doctoral researcher at the Stanford Linear Accelerator Center (SLAC) in California, the top quark was discovered by two experiments at Fermilab near Chicago. This was an exciting event, for the top quark was the sixth and last quark predicted by the standard model of particle physics and had been searched for since 1977 when the bottom quark was discovered. (Quarks come in pairs so when the bottom quark was discovered, which was the fifth known quark at the time, a sixth quark was predicted to also exist. For a brief discussion of quarks and the standard model of physics see a previous post about the top quark.)

A physicist who was collaborating on one of the experiments that discovered the top quark at Fermilab came to give a lecture at SLAC to explain the details of this significant discovery to a packed auditorium. When the presentation ended I was walking out of the auditorium with a graduate student and I remarked, "Wow, that was great. They discovered the top quark." The student replied, "No it wasn't great. It looks exactly what we expected to find." In that moment, I realized the graduate student was correct and had taught me a lesson about scientific inquiry. In general, the most significant discoveries are not those that are predicted based on what we already know, but rather those that are completely unexpected. Though the discovery of the top quark was extremely important in the field of particle physics, it was indeed, exactly what we had expected to find.

The Dark Side

In the 1920's a number of experiments observed that certain nuclei would decay by emitting a "beta" particle in a process called beta decay. Although the beta particle itself was shown to simply be an electron, the process of beta decay exhibited some strange and unexplained properties. In particular, the decay products did not conserve energy, momentum, or angular momentum. Up until that time, all known physical processes had conserved these three quantities. That is, the amount of energy, momentum, and angular momentum at the end of any process was exactly the same as the amount at the beginning of the process. However, beta decay seemed to violate these well known conservation laws. A number of explanations were proposed including the possibility that these conservation laws were not absolute. However, one explanation seemed much more simple and elegant. The physicist Wolfgang Pauli proposed that another particle was also emitted during beta decay along with the electron, but this other particle was nearly impossible to detect. For all practical purposes it was invisible to any experimental detectors of the time. Now it may seem crazy to postulate an unknown, nearly undetectable particle simply to preserve well-established laws of physics, but consider the genius of this idea. By simply proposing the addition of one unknown entity all of the problems with beta decay could be solved. Enrico Fermi named the unknown particle a "neutrino," or "little neutron" in Italian. It took about 25 years to experimentally confirm the existence of this particle, but eventually the neutrino was discovered in 1956. The simple but elegant introduction of a nearly undetectable neutral matter particle was the solution to multiple problems.

Today we have a similar situation to that of the 1920's. When we observe the cosmos we find that there are a number of problems that seem to violate well established laws of physics. Einstein's theory of general relativity, which describes how gravity works, is a remarkably successful theory with tremendous predictive power, but when we try to use that theory to explain certain observed effects, the theory doesn't quite work. For instance, when we watch how fast the stars in galaxies rotate about the galactic center we find that the outer part of the galaxy doesn't obey Einstein's theory if we assume that we can actually see all of the matter in the galaxy. (See the graph at the end of this blog post.) We also find that the large scale distribution of galaxies throughout the universe has some problems if we assume that we can observe all of the matter that is there. (See the opening figure of this blog post.) In addition, we know that gravity can actually bend the path of light, but the amount of bending we observe is much greater than what we would expect using the theory of general relativity and the amount of visible matter.