Particle Physics¶
… is a field of fundamental physics our research belongs to. You will also hear people calling it high energy physics or hep in short. What is our universe made of? Why is there matter around us? Particle physics is a field of physics that tries to answer these questions by studying the elementary (or fundamental) particles, the smallest and irreducible constituents of the universe, and interactions between those particles.
Most of you have probably heard of atoms and molecules. For example, water is made of H2O molecules. H2O is made of two hydrogen atoms and one oxygen atom. So, you were told that matter around you is made of various atoms. Atoms, in turn, consist of two elements: a positively charged nucleus, the “core” of an atom, and negatively charged electrons that surround the core. Using our solar system for an analogy, a nucleus is like the Sun and electrons are like the planets. Furthermore, a nucleus consists of two type of particles: protons (positively charged) and neutrons (electrically neutral).
Is that it? For the electron, yes. Electrons are fundamental particles. However you can in turn break down protons and neutrons into quarks. And that is where our knowledge stops. As far as we know, quarks are irreducible particles. A proton is made of two up quarks and one down quark. A neutron is made of one up quark and two down quarks. An up quark carries an electric charge 2/3, and a down quark -1/3. This makes a proton +1 electric charge while a neutron electrically neutral.
Elementary particles¶
There are total of six types of quarks known today. An electron is one of six leptons, another category of fundamental particles. Quarks and leptons are the constituents of matter particles we encounter in our everyday life. Furthermore, there are particles that are responsible for interactions among quarks and leptons. They are responsible for quarks forming nucleons (protons and neutrons), a nucleus and electrons forming an atom, and all atomic and molecular interactions in chemistry and beyond. These are called gauge bosons in the “periodic table of particle physics” shown below. What differentites quarks from leptons is related to how they interact with peers, and will be discussed in the next.
Fundamental interactions¶
We know there are (at least) four fundamental interactions in the nature, distinguished by the mechanism to propagate the force in charge. You probably have heard that the gravitational force is what keeps you bounded to the Earth and keeps you from flying like a bird (unfortunately). It also keeps planets of our solar system orbiting around the Sun (pretty important). Gravity is one of the four fundamental forces (interactions). The other one that we have all encountered is the electromagnetic (EM) force. Just like how the gravity is responsible for an Earth orbitting around the Sun, the EM force is responsible for an electron orbitting around a nucleus, hence the formation of atoms. Chemical rections of atoms and molecules are the result of exchanging electrons, and thus also goverened by the EM interactions. What bounds quarks into a proton or a neutron is called the strong force. It is also responsible for binding protons and neutrons into a nucleus, hence the formation of nuclei. Finally, what initiates nuclear fission and fusion processes is the weak force. Without the weak force, the Sun cannot burn and we would not have existed in the way we are present today. Each type of interaction is mediated by a particular type of gauge bosons, shown in the table above. Photons, or light particles, mediate the EM force. Gluons are responsible for the strong interactions. Three weak bosons, W+, W-, and neutral Z boson, are responsible for mediating the weak force.
So which quarks and leptons participate in these interactions? All leptons and quarks participate in the weak interactions. All quarks participate in the strong interactions while the leptons don’t. All electrically charged particles, including all quarks and charged leptons (electron, muon, and tau), participate in EM interactions. So, the neutral leptons, aka neutrinos, do not participate in the strong nor the EM interactions, making them the least interacting particles in our periodic table.