# Standard Model

### Definition

The Standard Model is a theory of the elementary particles, the most fundamental particles which cannot be broken down into other particles and are the building blocks of the universe. Elementary particles fall under one of two categories: fermions and bosons. There are multiple different interpretations and depictions of the Standard Model with slight variations.

### Fermions

Fermions are particles which make up matter. Elementary fermions can be subdivided into two categories, quarks and leptons. Fermions have odd half-integer spins, e.g. $\frac{1}{2}$, $\frac{3}{2}$, $\frac{5}{2}$ - with most common fermions having a spin of $\frac{1}{2}$.

Not all fermions are elementary!

Baryons are an example of composite particles which are also fermions. This therefore means both protons and neutrons are fermions.

#### Quarks

Quarks are dairy products fermions which experience the strong nuclear force - the force which holds hadrons together. There are six quarks: the up, charm and top quarks with a charge of $\frac{2}{3}$, and the down, strange and bottom quarks with a charge of $-\frac{1}{3}$. Up and down quarks are the most stable and make up protons and neutrons, and top and bottom quarks are the most massive and also the most unstable - usually decaying into other elementary particles in a split second. Each quark has an antiparticle counterpart with the opposite charge, i.e. $-\frac{2}{3}$ for the antiup, anticharm and antitop particles and $\frac{1}{3}$ for the antidown, antistrange and antibottom particles.

#### Leptons

Leptons are fermions which, conversely to quarks, do not experience the strong nuclear force. There are six leptons: the electron, muon and tau particles, each with a charge of $-1$, and their associated neutrinos. Each charged lepton also has an antiparticle counterpart with a charge of $1$: the positron, antimuon and antitau respectively. The neutrinos also have antineutrino counterparts, which while not having a charge, have a different type of charge negated - called the lepton number.

##### Neutrinos

Neutrinos are leptons without a charge, which were first theorised to explain a loss of energy in beta decay.

Below is an example of neutron decay, where a neutron turns into a proton, an electron and an electron antineutrino:

Initially, physicists observed that neutrons decayed into protons and electrons, which conserves mass and charge. However, the total energy held by the proton and electron was observed to be less than the energy of the neutron. According to the law of conservation of energy this isn't possible, and that energy has to go somewhere - leading to the existence of neutrinos first being theorised.

### Bosons

Bosons are force-mediating particles, and have integer spins. All elementary bosons are gauge bosons, with the exception of the Higgs boson, a scalar boson which is so special that it is the only boson in its category. Other bosons, such as mesons, are composite particles.

#### Gauge Bosons

Gauge bosons are elementary bosons which act as force carriers between elementary fermions and cannot be directly detected by themselves. Each fundamental force corresponds to a type of gauge boson.

Gauge Boson Force Effect
Photon Electrostatic Attracts and repels charged particles
Gluon Strong nuclear Holds together nucleons
W and Z bosons Weak nuclear Causes nuclear decay and fusion

Gravity also has a corresponding theoretical boson called the graviton, however it is often excluded because it conflicts with Einstein's theory of general relativity.

#### Higgs Boson

The Higgs boson is the only fundamental scalar boson, and it acts between matter and the Higgs field - a field of energy which gives matter the property of mass. It has zero spin and no charge, and it is famously difficult to observe - its existence has only been proven in around 2011-2013 using the Large Hadron Collider. The Higgs boson is incredibly unstable, decaying so quickly that its decay time couldn't be directly measured.

Hadrons are composite subatomic particles made of two or more quarks, held together by the strong nuclear force. They can be subdivided into two categories, baryons and mesons.

The sum of charges of the quarks in a hadron determines the overall charge of the hadron. Hadrons must have an integer charge.

#### Baryons

Baryons are hadrons which are made up of an odd number of quarks, usually three. Baryons are considered composite fermions as they have odd half-integer spins.

Protons and neutrons are two common examples of baryons. Protons are formed from two up quarks and a down quark, resulting in a charge of $1$, and neutrons are formed from an up quark and two down quarks, resulting in a charge of $0$.

#### Mesons

Mesons are hadrons which are made up of an even number of quarks, usually a pairing of a quark and an antiquark. Unlike baryons, all mesons are very unstable, with the longest-lived mesons lasting for only a few hundredths of a microsecond. Larger mesons decay to other less massive mesons, which ultimately decay into stable electrons, neutrinos and photons.

Mesons are also considered composite bosons as they have an integer spin, and can act as force carriers under certain circumstances (e.g. pions can act as force carriers between nucleons).

### Antimatter

Antimatter has the same properties of regular matter, but the opposite charge.

When matter comes into contact with antimatter, they annihilate - turning into a lot of pure energy in the form of photons. This process can also happen in reverse.

The amount of mass and energy converted can be calculated using Albert Einstein's famous formula, $E=m{c}^{2}$.