Particle physics
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The Universe is composed of matter particles such as protons, neutrons, and electrons.
All matter particles have corresponding antimatter particles, which are identical in most properties but have the opposite charge:
- If a matter particle is positively charged, its antimatter counterpart is negatively charged, and vice versa.
- If a matter particle has zero charge, its antiparticle also has zero charge.
Common matter–antimatter particle pairs are shown in the figure below.
For particles other than electrons, their antiparticle counterpart has the same name, named with the prefix “anti-” and is symbolised by the same letter with a bar above it (e.g. for antiproton).
Be careful not to confuse neutral particles, like the neutron and neutrino, with their antiparticles.
Antiparticles are written with a bar over the symbol, so do not rely solely on the charge to identify a particle or antiparticle.
Antimatter particles have the same mass as their corresponding matter particles.
Charged antimatter particles have an equal and opposite charge to their corresponding matter particles.
The electron and positron have the same mass but an opposite charge:
The proton and antiproton also have the same mass but an opposite charge.
The rest mass–energy of a particle is the energy equivalent to the mass of the particle when it is at rest.
It can be calculated using Einstein’s famous equation:
Where:
- is the rest mass–energy,
- is the particle’s rest mass, and
- is the speed of light
Rest mass–energy is important for calculations related to particle interactions, in which mass can be converted to energy.
Rest mass–energy has SI units of joules (J) but is often expressed in for convenience.
The table below provides the mass in and the rest mass–energy in for a proton, neutron, electron and neutrino.
The rest mass–energy for the neutrino is which is on the scale of magnitudes smaller than the other particles in the above table.
Hadrons are particles composed of fundamental particles called quarks.
Fundamental particles have no internal structure: they cannot be divided into smaller constituents.
Quarks do not exist in isolation, but only in pairs or in groups of three (or higher for exotic particles) to form hadrons:
- Hadrons experience the strong nuclear force.
- Hadrons decay by the weak nuclear force and also experience the electromagnetic force if charged.
- There are two classes of hadrons: baryons and mesons.
It is useful to know that exotic hadrons consisting of four or more quarks have been observed in particle colliders, but they are extremely unstable and short-lived.
Baryons are hadrons composed of three quarks.
Baryon number is always conserved in particle interactions:
- All baryons have a baryon number of
- Antibaryons have a baryon number of
Examples of baryons include protons and neutrons:
- Proton: A baryon with a baryon number of composed of two up quarks and one down quark
- Neutron: A baryon with a baryon number of composed of one up quark and two down quarks
All quarks have a baryon number of while all antiquarks have a baryon number of
An up quark, has a charge of and a down quark, has a charge of Their antiquarks have opposite charges.
Antiprotons and antineutrons are antibaryons. Each quark in its corresponding baryon is replaced by an antiquark.
Mesons are hadrons composed of a quark–antiquark pair. They are short-lived particles that often appear when high-energy collisions briefly create new matter.
Mesons have a baryon number of
Examples of mesons include:
- Pions: Common mesons that come in both charged and neutral forms.
- Kaons: Another class of mesons, which includes charged and neutral variants.
Pions are mesons made up from a combination of up, down, anti-up and anti-down quarks.
Some examples of pions are shown below.
Charged pions are made up of one up and one anti-down quark (positive) or one down and one anti-up quark (negative).
Neutral pions consist of an up anti-up pair or a down anti-down pair, where the relative charges cancel out.
The pion acts as the exchange particle of the strong nuclear force.
Kaons are mesons containing either a strange or an anti-strange quark.
Strange quarks have a charge of
Some examples of kaons are shown below.
Charged kaons include either an up or anti-up quark, with their overall relative charge reflecting the charge of that quark.
Neutral kaons are those which include a down or anti-down quark.
Kaons will quickly decay into pions
Leptons are fundamental particles.
Unlike hadrons, leptons are not composed of quarks, meaning they are not affected by the strong nuclear force.
Leptons experience the following forces:
- Weak nuclear force
- Gravitational force
- Electromagnetic force (if charged)
There are a total of six leptons, categorised into three different flavours (types):
- Electron
- Muon
- Tauon
There are three uncharged leptons, one for each flavour:
- Electron neutrino
- Muon neutrino
- Tauon neutrino
Neutrinos are the most abundant leptons in the Universe and possess no charge and negligible mass. They are formed during particle interactions that also involve charged leptons.
It is useful to note that physicists use the term “flavour” because it’s a lighthearted label for the different “varieties” of quarks and leptons. It does not imply any literal taste.
There are three charged leptons with charge one for each of the three flavours: the electron, muon and tauon.
The three charged leptons have almost identical properties, but the muon and tauon particles have much greater masses than the electron:
It is important to note that is the unified atomic mass unit, equal to
Muons and tauons are extremely unstable, and will quickly decay into other particles:
- The muon will decay into an electron and an electron antineutrino and a muon neutrino.
- Tauons are massive enough to decay into a variety of particles, including baryons such as protons or neutrons.
Anti-leptons have the same mass and opposite charge to their corresponding leptons.
For example, the electron has a charge of and the positron has a charge of
For the six leptons, there are six corresponding anti-leptons as shown in the table.
There are three different types of lepton number that are separately conserved in particle interactions:
- Electron lepton number
- for electrons and electron neutrinos.
- for positrons and electron antineutrinos.
- Muon lepton number
- for muons and muon neutrinos.
- for antimuons and electron antineutrinos.
- Tauon lepton number
- for tauons and tauon neutrinos.
- for antitauons and tauon antineutrinos.
Hadrons are subatomic particles composed of smaller particles called quarks. These hadrons can be categorised into two main types:
- Baryons
- Made up of three quarks
- Examples of these include protons and neutrons
- Mesons
- A quark–antiquark pair.
- Examples of these include pions and kaons.
In an anti-hadron, each quark is swapped for its antiquark compared to the corresponding hadron.
The simple quark model of hadrons includes three types of quarks (along with their charges):
- Up
- Down
- Strange
Their corresponding antiquarks are:
- Anti-up
- Anti-down
- Anti-strange
All baryons and mesons possess integer charges (whole number values like etc.).
A mixture of quarks and antiquarks is not possible in baryons: combining quarks with antiquarks in a group of three would lead to a non-integer charge.
Mesons are a quark–antiquark pair, but some combinations would lead to a non-integer charge, so they are not allowed.
For example, there is no meson consisting of a pair since this would lead to a charge of
Protons and neutrons are not fundamental particles; they are composite particles made up of quarks.
Both protons and neutrons are baryons, which are composed of three quarks:
- Protons are composed of two up quarks and one down quark Its quark structure is
- Neutrons are composed of two down quarks and one up quark Its quark structure is
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The charge of a particle is the sum of the charges of its quarks.
Up quark
Down quark
To find the charge of a proton, we identify the quark composition:
Total charge calculation:
The proton has a charge of
Similarly, to find the charge of a neutron, we know the quark composition is:
Total charge calculation:
The neutron is neutral and has no net charge.
Be careful not to confuse the quark compositions for protons and neutrons.
The decay of particles can be explained using the quark model.
The weak nuclear force is one of the four fundamental forces of nature, alongside gravity, electromagnetism, and the strong nuclear force.
It is responsible for radioactive decay processes that involve the transformation of quarks. It allows quarks to change their type or ‘flavour’ from one to another.
The decay of hadrons often involves the transformation of one quark into another and the emission of leptons (electrons, positrons, and neutrinos).
Quark transformations occur during various types of particle interactions, such as beta-minus and beta-plus decay.
Each type of decay involves the conversion of one type of quark into another.
Charge, energy, and momentum must be conserved during these interactions.
We can determine the number of unknown particles in a decay equation by balancing charge on both sides of the equation.
The steps to balance a quark transformation equation are:
1. Identify the initial quark composition: determine the initial quarks involved in the transformation, noting their respective charges.
2. Identify the final quark composition: determine the final quark composition after the transformation, again noting their respective charges.
3. Set up the equation: The equation should represent the initial quark composition transforming into the final composition. For example:
4. Calculate total charge:
5. Check for charge conservation: Ensure that the total initial charge equals the total final charge:
- In beta-minus decay ( decay), a neutron decays into a proton This occurs because one of the down quarks in the neutron transforms into an up quark via the weak nuclear force.
- In beta-plus decay ( decay), a proton decays into a neutron This occurs because one of the up quarks in the proton transforms into a down quark
Decays involving the strange quark also occur through the weak nuclear force. The strange quark can transform into an up quark or a down quark.
For example, a kaon has a quark composition It can decay into a pion and a neutrino–antineutrino pair via the weak nuclear force.
The following conservation laws apply to particle decays:
- Charge conservation: the total charge before and after a decay must be the same.
- Lepton number conservation: the total lepton number is conserved (within a family of leptons).
- Baryon number conservation: the total baryon number remains unchanged during a decay.
In beta-minus decay ( decay), a neutron within the nucleus of an atom decays into a proton.
During this process, the neutron emits a high-energy electron (beta-minus particle, and an anti-electron neutrino
The equation for beta-minus decay is:
In beta-minus decay ( decay), a parent nucleus transforms into a different element, known as the daughter nucleus.
- The proton number increases by one: a neutron has turned into a proton, so the element has gained an extra proton.
- The nucleon number (mass number) remains the same since a neutron decays to a proton, so the total number of protons and neutrons stays constant.
For a parent nucleus with proton number and mass number the daughter nucleus is given by:
The decay equation can be checked by confirming that mass and charge are conserved:
- On the left-hand side of the arrow, the total mass is and the charge is
- On the right-hand side, we have a mass of (as beta-minus particles and anti-electron neutrinos have no mass) and a charge of
Thus, no charge or mass has been created or destroyed in this decay.
In beta-minus decay ( decay), a neutron inside the nucleus is transformed into a proton, emitting an electron and an anti-electron neutrino, This can be further explained at the subatomic level.
At the subatomic level, particles like protons and neutrons are composed of quarks:
- A neutron consists of two down quarks and one up quark
- A proton consists of two up quarks and one down quark
The beta-minus decay process can be understood in terms of quark transformation:
- During the decay, one of the down quarks in the neutron is converted into an up quark
- This conversion changes the neutron into a proton while emitting an electron and an anti-electron neutrino.
The equation for beta-minus decay is:
At the quark level, this can be written as:
In beta-plus decay ( decay), a proton within the nucleus is converted into a neutron.
Memory tool: beta-plus decay means a proton changes.
During this process, the proton emits a high-energy positron (a beta-plus particle, ) and an electron neutrino,
The general equation for beta-plus decay is:
During beta-plus decay ( decay):
- The proton number decreases by one: since a proton has been converted into a neutron, the atom has lost a proton.
- The nucleon number (mass number) remains the same: no nucleons are lost or gained, so the total number of protons and neutrons stays constant.
This may be written in a general form for an unknown parent nucleus with the proton number and mass number
Checking mass and charge conservation:
- On the left-hand side of the arrow, we have a mass of and a charge of (each proton has a charge of ).
- On the right-hand side, we have a mass of (as beta-plus particles and electron neutrinos have no mass) and a charge of
- Thus, no charge or mass has been created or destroyed in this decay.
In beta-plus decay ( decay), a proton inside a nucleus is converted into a neutron, accompanied by the emission of a positron ) and an electron neutrino This can be further explained at the subatomic level.
Beta-plus decay involves a transformation in the quark composition of the proton.
- A proton consists of two up quarks and one down quark
- A neutron consists of one up quark and two down quarks
During beta-plus decay, one of the up quarks in the proton converts into a down quark, causing the proton to change into a neutron.
The quark-level equation for this decay can be written as:
The overall equation for beta-plus decay is: