Elementary Particles (2024 Edition) Quiz

Answer:

First, a bit of history that places the incorrect options in context. A molecule is the smallest piece of a compound (a substance made up of two or more different elements) that has the chemical properties of that compound. It is made up of atoms, which were given their name (meaning indivisible) in the 19th century because they were once thought to be the elementary particles of matter. Near the end of that century, the electron was discovered, so it was clear that there was stuff inside of the atoms, and its structure started to be explored. The discovery that most of an atom's mass was located in a small central region, dubbed the nucleus, led to further investigation, and the discovery that the nucleus contained two different types of particle, protons and neutrons, led to them being considered the basic building blocks of matter. But, big but, there were a lot of unanswered questions, and continued experimentation led to the discovery of even more particles, many more, and the development of a new explanation. (Although in the 21st century this is still basically the model taught in schools, because it can be explained in a relatively straightforward manner.)

Just as molecules are made up of a combination of atoms, and atoms are an assembly of protons, neutrons and electrons, the Standard Model now describes protons and neutrons as hadrons, a type of particle that is made up of two or more quarks held together. The six types of quark are described as having different flavor: U or up, D or down, C or charm, S or strange, T or top, and B or bottom. The words are not their official designation, the letters are, and some prefer to use alternative names starting with the designated letter, but the words make nice mnemonics.

Quarks also have a property described as color charge, or just color. Each one is designated red, blue or green, arbitrary designations reflecting the three primary colors of visible light. For the forces holding two or more quarks together in a hadron, there must be a white color charge on the hadron. If there are only two quarks (in the class of particle called a meson), they must be a quark and its own anti-quark to cancel. For three to combine, there must be one of each color present, to add up to white.

Because protons and neutrons have an odd number of quarks (three), they belong to the subgroup of hadrons called baryons. Protons have two Up quarks (charge +2/3 that of an electron) and one Down quark (charge -1/3e), giving an overall charge of +1. Neutrons have one Up and two Down quarks, giving an overall charge of 0. The quarks in each particle are held together by gluons.

The twelve fundamental particles which are fermions (meaning they have spin values of 1/2, a statement I just want you to accept without asking for an explanation, as it doesn't mean they actually spin around, just that they can be described by Fermi-Dirac statistics, which is not really much of an explanation until you go do a lot more mathematical study). They can be grouped into what are called three generations, each of which has two quarks, a charged lepton and a neutral lepton. The two quarks in each group have charges of +2/3 and -1/3, but the masses of higher generation quarks are larger than those of the first generation, and they tend to decay. (This model no longer considers that an elementary particle exists forever in its given condition, but that is another story.) Unless you are working with cosmic rays or particle accelerators, the only baryons you are likely to meet are the proton and the neutron.

The most familiar charged lepton is the electron, which, along with the electron neutrino and the U and D quarks is called the first generation of particles. The electron neutrino is the most common neutrino by such a margin that it is usually just called the neutrino (also a product of the fact that it was the first kind of neutrino discovered, and long thought to be the only type). While an electron is charged, the neutrino (as its name suggests) is neutral. To keep the number of answers at 15, the electron neutrino was listed as neutrino, and the muon and tau neutrinos not included in the list for this quiz.

The second generation of elementary particles are the C and S quarks, the muon and the muon neutrino. The third generation contains the T and B quarks, the tau particle, and the tau neutrino.

In addition to the twelve fermions, there are five known bosons, fundamental particles with integer spin values. (What this means, if you are interested, is that they obey Bose-Einstein statistics and do not obey the Pauli exclusion principle.) The four with spin 1 act as force carriers, and are called gauge bosons: the gluon (of which there are eight types) mentioned earlier holds quarks together; the photon is the wave-particle of light responsible for electromagnetic interactions; the W and Z bosons mediate the interactions of neutrinos. W got its name from the weak force they explain, Z because it was held to be the last piece of the puzzle.

The last elementary particle to be verified was the Higgs boson, with spin 0. Its existence was predicted in 1964, as a way to explain why some particles had mass when that didn't fit in with theoretical calculations. Its discovery in 2012 at the Large Hadron Collider at CERN led to the 2013 Nobel Prize in Physics for Peter Higgs and François Englert, two of those responsible for the almost-simultaneous 1964 papers predicting its existence.

With this addition, the Standard Model has been able to explain everything except gravity, which is currently understood via Einstein's Theory of General Relativity, an independent model. And the fact that the universe's expansion is increasing. Or why neutrinos spontaneously change into neutrinos of a different group. For that matter, why do they have any mass at all? And why does the world seem to contain primarily the kind of particles we call matter, as opposed to those we call anti-matter, which look to be equally viable?