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.

A Look at the Top Quark

All the known matter in the universe is composed of two classes of particles: quarks and leptons. There are six types, or flavors, of quarks and six types of leptons. The figure to the left shows these fundamental particles. Three of the leptons, the electron (e), the muon (μ), and the tau lepton (τ) have an electrical charge that is a factor of –1 that of a proton, and three of the leptons, called the electron neutrino (ν_e), the muon neutrino (ν_μ), and the tau neutrino (ν_τ) have zero electrical charge. Quarks are named (in order of increasing mass) up (u), down (d), strange (s), charm (c), bottom (b), and top (t). The up, charm, and top quark have a charge that is +2/3 that of a proton, and the down, strange, and bottom quarks have a charge that is –1/3 that of a proton.

Therefore, in an atom composed of a nucleus surrounded by electrons, the electrons are fundamental particles, which means they are not composed of anything smaller as far as we know. But the nucleus is composed of neutrons and protons, which are themselves composed of quarks. At a very basic level a proton is made up two up quarks and a down quark with electric charge +2/3 + 2/3 – 1/3 = 1 while a neutron is made up of one up quark and two down quarks with an electric charge of +2/3 – 1/3 – 1/3 = 0. The two quarks and two leptons in the first column in the figure are called the first generation of particles, the second column is the second generation of particles, and the third column is the third generation of particles. Most of all the matter we know of is made of the first generation of particles since atoms are made of neutrons and protons and electrons with the neutrons and protons made of up and down quarks.

A New Particle Discovered at CERN

About a week ago, an experiment at CERN announced the discovery of a new particle, the Ξ_cc⁺⁺ (pronounced Ksigh-see-see-plus-plus). Many people who have read about this discovery have asked me about its significance and if I was involved. So, this post will deviate from the usual discussion of the relationship between Christianity and science and focus on the discovery of the Ξ_cc⁺⁺ with a few additional observations in my conclusion.