Welcome to week 13 of Science Friday. Last week, I covered the basics of special relativity. Today, I am going to discuss the standard model of particle physics. The history of the development of this model is extensive and would be difficult to cover in 5 science friday postings. Consequently, I will only describe the actual model, its importance, and its problems. So what is the standard model? For millennia, humans have yearned to understand what makes up the smallest constituents of the universe we reside in. The Greek philosopher Democritus coined the term “atom,” which literally means that which cannot be seen and cannot be cut. As an unfortunate turn of events, what we now know as the chemical atom is not an atom as Democritus described it. In fact, the atom can be cut into smaller parts—protons, neutrons, and electrons. But can these entities be cut as well? Over the past several decades, physicists have been slowly uncovering the answer to this very question. Their tool: massive particle accelerators that smash sub-atomic particles to generate new particles. We now know that protons and neutrons are made of particles called quarks. Protons and neutrons are just two examples of a larger classification of particle known as [b]hadrons[/b]—particles that consist of quarks. The proton and neutron have hundreds of other cousins (hadrons) that also consist of quarks but have very short half-lives. There are six types of quarks that particle physicists have identified as of today: Up Down Bottom Top Strange Charm Most of the matter we see today in the universe consists of the up and down quarks—the only quarks that make up protons and neutrons. Each of these quarks has an antiquark partner. Each quark and antiquark, in addition, is observed in three “colors.” I put colors in quotations because, at this level, color as we understand it does not exist. Physicists have (somewhat arbitrarily) used this as a descriptor of another characteristic of the quarks. What about electrons? Electrons are part of another family of particles called [b]leptons[/b]. There are six known leptons. Electron Electron Neutrino Muon Muon Neutrino Tau Tau Neutrino The muon and tau are very unstable leptons only observed in high energy interactions such as those seen in cosmic rays and particle accelerators. Just like quarks, the leptons also have antiparticle partners. Together quarks and leptons form an even larger subgroup of particles called [b]fermions[/b], that is, matter particles. There is yet another classification of particle—the bosons. Bosons are the particles responsible for mediating the four known forces of the universe. These four forces are the strong force, the weak force, gravity, and electromagnetism. The gauge bosons for all but one of the forces—gravity—have been empirically identified. The gauge bosons for the strong force are eight gluons; those for the weak force are the W plus, W minus, and Z zero bosons; and the gauge boson for the electromagnetic force is the photon. The theoretical gauge boson for gravity—the graviton—has not yet been discovered. So here we have it. The standard model. While it is somewhat elegant, it is also rather large. There seem to be too many particles. In fact, particle physicists theorize that everything we see in the universe can be distilled down to one particle. The conditions necessary to reveal this ultimate symmetry, however, were only met during a brief duration of time after the big bang. Instrumental to this understanding is the Higgs boson. The Higgs boson is responsible for giving other particles mass. It is called a scalar boson because it has no directionality associated with it. Particles that interact with the Higgs strongly, such as the weak force bosons, are rather massive, whereas particles that ignore the presence of the Higgs completely, such as the photon, are massless. The Higgs is especially crucial in our understanding of how the weak force and the electromagnetic force were unified as the electroweak force very soon after the big bang. The Higgs lays down the theoretical foundation for this theory. After the birth of the universe, the Higgs did not immediately spawn into existence; as a result, there would have been no mass for any of the particles present in this early universe, allowing for the gauge bosons of the weak force (which are massive) and the photon (which is massless) to be one force carrying particle. The intricacies of the standard model are impossible for me to explain with my current knowledge of physics. Indeed, physicists undergo years of education in both physics and mathematics before they can even start working at particle accelerators. However, it does not take a advanced knowledge of math or physics to realize that this work is at the forefront of human discovery, offering a glimpse of how the universe began, and indeed, how it might end.