A Beginner's Guide to Subatomic Particles
This article will provide a complete guide to elementary subatomic particles. Elementary in this context means simple: elementary particles are not made up of smaller particles. For example, protons are made up of 3 quarks and so are not elementary particles.
The unit of mass used here is the the ecletron volt (eV). This is actually a measure of energy, but as E=mc2 tells us, mass and energy are interchangeable. For reference, 1 electron volt = 1.60217646 × 10-19 joules.
Some prefixes are also used: M for mega, and G for giga. One mega electron volt (1MeV) = 1000 electron volts. 1GeV = 1 million electron volts.
A lepton is an elementary particle and a fundamental constituent of matter.The most familiar lepton is the electron which is directly involved in nearly all chemical reactions and affects the chemical properties of atoms and molecules.
Leptons can be split into two main groups: charged leptons and neutral leptons (neutrinos). Charged leptons can combine with other particles to form various composite particles such as positronium, while neutrinos rarely interact with matter and are therefore difficult to observe.
Mass: 0.510 MeV
The electron is a particle typically found orbiting the nucleus of an atom. Subject to the electromagnetic, gravitational and weak forces, electrons are stable. Historically, it was the first particle to be discovered in 1897 by J. J. Thomson. Electrons are one of the most familiar particles to us as we encounter them in many realms of everyday life: chemical reactions, magnetism, electricity etc.
Mass: 105 MeV
Sometimes referred to as a “heavy” electron, the muon's life is a short one – a muon’s typically lives just 2.2 microseconds before it decays, usually into an electron and other particles. Discovered in 1937, muons come from cosmic radiation and are about 206 times heavier than the electron.
Mass: 1.77 GeV
The heaviest lepton, the tau, is about 3490 times the mass of the electron. Discovered in 1975 at Stanford Linear Accelerator Centre (SLAC), the tau is the only lepton that can decay into hadrons (because of its large mass). It's a short-lived particle, with a lifetime of only 3 x 10-13 seconds, and is often produced through electron-positron annihilation.
Mass: > 3 eV
The electron neutrino is a sneaky particle of practically zero mass. Like all neutrinos, it is notoriously difficult to detect. Electron neutrinos at near light speed and rarely interact with anything. Discovered in 1956, they are the result of radioactive decay. Most electron neutrinos found on Earth originate from the sun.
Mass: < 0.17 MeV
Associated with the muon, the muon neutrino is slightly heavier than the electron neutrino. There is experimental evidence that neutrinos oscillate from one type to another which means electron, muon, and tau neutrinos are all “flavours” of one particle. Muon neutrinos were discovered in 1962. They react via the weak force.
Mass: < 18 MeV
One of the most recently discovered particles, the tau neutrino was discovered in 2000 at Fermilab The tau neutrino is the heaviest of the neutrinos (35 times more massive than the electron). It is stable it can oscillate to other neutrino types or "flavours" (see muon neutrinos).
Minute Physics - What is a neutrino?
Quarks are elementary particles that form the building blocks of other particles such as baryons and mesons. Quarks combine to form composite particles called hadrons, including the familiar protons and neutrons,the particles with make up the nuclei of atoms. Due to a phenomenon known as colour confinement, quarks are never directly observed or found in isolation; they can be found only within baryons or mesons.
Mass: 1.5 – 3.3 MeV
The up quark is the smallest of the six quarks and is subject to the strong force. A proton contains two up quarks (and one down quark) so the up quark is very important for everyday life, since every atom contains at least one proton. As it comprises all the physical matter we see around us, it is, of course, stable. It was one of the first quarks to be discovered (in 1964).
Mass: 3.5 – 6.0 MeV
Like up quark, the down quark is found in everyday matter and is stable, but it is very slightly heavier. Also discovered in 1964 (by Murray Gell-Mann), quarks were first called “aces.” Both the down quark and up quark are also found in many baryons and mesons. Two down quarks and one up quark make up a neutron.
Charge: - 1/3
Mass: 70 – 130 MeV
Despite its name, there’s nothing particularly strange about the strange quark, a second-generation quark discovered by Murray Gell-Mann in 1964 when he developed the quark model. Particles containing a strange quark are assigned a “strangeness” value of -1. It is comparable in mass to the muon and has a short lifetime.
Mass: 1.16 – 1.34 GeV
The charm quark, first observed in 1974, was discovered by both Brookhaven National Lab and SLAC (Standford Linear Accelerator Centre). It is slightly lighter than the tau. Baryons and mesons made of charm quarks are called “charmed” particles.
Mass: 4.13 – 4.37 GeV
The bottom quark is about four times the mass of the proton and is easily identified in experiments. Originally named “beauty”, the bottom quark was discovered in 1977 by Leon Lederman’s group at Fermilab. It is often the decay product of the short-lived top quark.
Mass: 169 – 173 GeV
The massive and extrememly short-lived top quark is far too unstable to be found in any baryons or mesons. The last of the six quarks to be discovered, it wasn't until 1995 that physicists at Fermilab were able to observe this particle. It is the most massive observed particle and usually decays into a bottom quark via the weak interaction.
Bosons are force carriers. This means that they communicate the 4 fundamental forces of nature: electromagnetism, the nuclear strong force, the nuclear weak force, and gravity. Currently, it is not known which particle carries the force of gravity, though it is theorised that it is carried by gravitons.
This massless particle is one most people will be familiar with. The photon, better known as light, always travels at the speed of light and communicates the electromagnetic force in many forms from microwaves to gamma rays. About 1012 photons of sunlight fall on a pinhead each second. Photons display both wave and particle characteristics, a phenomena known as wave particle duality. Photons were first postulated by Albert Einstein in 1905.
The gluon is the force-carrying particle of the strong nuclear force, which holds quarks together and binds the nuclei of atoms. Discovered in 1979, it is stable, massless, and comes in 8 colour states. At extremely high temperatures, quarks and gluons fluidly mix into a quark-gluon plasma. It is theorized that gluons can interact with each other and form glueballs.
Mass: 80.398 GeV
This particle may be a weak force boson, but it's incredibly heavy! Discovered in 1983 at CERN, the W boson is best known for nuclear decay. Very massive and extremely short-lived (10-25 seconds), a W boson is heavier than an atom of iron. Unlike other bosons, it may have either positive or negative charge. The W stands for weak force.
Like the W boson, the Z boson is an extremely massive particle. It is so named because physicists at the time thought it was going to be the last particle to be discovered. It wasn't so now the “Z” stands for zero charge. Like the W, the Z is a carrier of the weak force and lives for less than a billionth of a billionth of a second. It was discovered at CERN in 1983.
Mass: 125.3 ± 0.4 (stat) ± 0.5 (sys) GeV; 126.0 ± 0.4 (stat) ± 0.4 (sys) GeV
The Higgs boson is the particle that gives mass to all other particles through the Higgs field. By extension, everything else in the universe that has mass has it because of the Higgs boson. It has been "tentatively observed" at CERN. That is to say, a new particle was observed and its properties have thus far been consistent with those predicted for the Higgs boson in the Standard Model.
If you're interested, the video below gives a nice, easy overview of what the Higgs Boson is and why it's important.
Minute Physics - The Higgs Boson
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