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In the UK we don't study particle physics in any sort of depth, even at A level (the last two years of school), although it is studied in the US (I believe). Me being a UK physics A level student, I got interested in this area and started poking around on the Internet. However, having never studied this before, I found that most information was far to complicated, so I figured the only way to get to grips with it is to write about it. And that's what I've done here. Don't worry, I wrote this in super-simple terms, because it was my first time as well.
So sit back, grab a Coke and have a read!
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With the 20th Century came the realisation that the atom was not the basic building block of everything- it is in fact made up of other particles. These particles, called elementary particles, are what atoms were once classed as. Elementary particles are those particles that are not known to have a substructure- that is, they are themselves not made up of anything. There are dozens of elementary particles known today, but there are also many more hypothetical particles that are still being searched for, the Higgs Boson, for example.
All elementary particles can be categorised according to the spin-statistics theorem, which is an area of quantum mechanics. The spin of a particle is its angular momentum (rotational or circular momentum). Angular momentum is part of a particle’s overall momentum that is due to its spinning. Spin-statistics theorem says that all particles have a spin which is either an integer (a whole number) or a half-integer (a number which is divisible by 0.5). Particles with integer spin are called bosons, and particles with half-integer spin are called fermions.
Bosons are fermions are differentiated not only by their spin, but by other properties called quantum statistics. Bosons are governed by Bose-Einstein statistics, which among other things, states that bosons of the same energy can occupy the same quantum state. In other words, multiple identical bosons can exist in the same place in space. Bosons are thus associated with transmitting interactions (carrying forces- gravity, magnetism, etc.) Fermions, on the other hand, are controlled by Fermi-Dirac statistics which says that two identical fermions may not occupy the same quantum state (this is called the Pauli Exclusion Principle). Thus, fermions are thought to be the constituents of matter.
According to the Standard Model of particle physics there are four bosons: the photon, the W boson, the Z boson, and the gluon. Additionally, the Higgs boson is predicted by the electroweak theory, and the graviton is part of quantum field theory. These six particles combined are the carriers of the four fundamental forces. The photon is the carrier of electromagnetism; and the graviton carries gravitation, hence its name. Strong interaction, or the strong nuclear force, is said to be the (indirect) product of the gluon. This is the force that holds the nuclei of atoms together. W and Z bosons are the particles that mediate weak interaction, or the weak nuclear force, which is what drives radioactive decay of atoms.
The Standard Model is also comprised of twelve fermions. All twelve of these fermions have a spin of ½, although any half-integer spin is possible. Additionally, all known fermions are classified as Dirac fermions which mean that they have their own antiparticle (for example, the antiparticle of the electron is the positron). Fermions are classified according to whether they undergo strong interaction. If a particle is susceptible to strong interaction it is called a quark (or an antiquark); if it is not, it’s called a lepton (or an antilepton). However, both quarks and leptons are affected by electromagnetic, gravitation and weak nuclear forces.
There are six quarks grouped into three generations: up quark and down quark (first generation), charm quark and strange quark (second generation), and top quark and bottom quark (third generation). However, due to a phenomenon called confinement, quarks are never found singularly- instead they collect together in groups of two or three to form hadrons. Because hadrons are composed of elementary particles, they cannot be elementary particles themselves- they are composite particles.
There are two sub-sets of hadrons: baryons and mesons. A baryon is composed of quarks (with no antiquarks). There are two types of baryon. The most common baryons are the proton (two up quarks, one down quark) and neutron (one up quark, two down quarks). Collectively, protons and neutrons are called nucleons. The second type, the hyperon, is composed of one of more strange quarks. Additionally, a recent discovery has confirmed the existence of a third baryon- the pentaquark, which is made up of four quarks and one antiquark.
Mesons are comprised of multiples of quark-antiquark pairs. Ordinary mesons, such as the pion and kaon (from which the residual strong force that holds nucleons together originates), are made up of a quark and an antiquark. Other types of mesons include the tetraquark (two quarks, two antiquarks) and the glueball which is several gluons collected together. Hybrids of quark-antiquark pairs and gluons also exist.
Because all fermions (and thus quarks) have half-integer spins, when a quark-antiquark pair comes together as a meson, an integer spin is created. Thus mesons are called composite bosons (because bosons have integer spins). And so in baryons where there is an odd number of quarks, a half-integer spin still prevails, and so baryons are called composite fermions.
Like quarks, there are also six leptons: electron, electron neutrino, muon, muon neutrino, tauon and tauon neutrino. Leptons are divided equally up into three generations. The first generation, electronic leptons, comprise of electrons and electron neutrinos. The second generation, muonic leptons, comprise of muons and muon neutrinos. The third and final generation called tauonic leptons is made up of tauon and tauon neutrinos.
Yes, I know this is a sudden stop, but this isn't an essay, so I haven't done a "conclusion".
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