Quarks are fundamental particles that contribute most of the mass in the universe. They never appear on their own but always in groups of three or more. Each quark has an electrical charge, a baryon number, and a strange number. The symbol of a quark is q.
The baryon number and the types of quarks
The baryon number indicates if you are dealing with a particle or an antiparticle. See the following table showing the different types of quarks.
Table 1. Types of quarks: symbols, charge, Baryon numbers and strange numbers. |
|---|
Particle | Symbol | Electrical charge | Baryon number | Strange number |
Up | u | + ⅔ | + ⅓ | 0 |
Down | d | -⅓ | + ⅓ | 0 |
Strange | s | -⅓ | + ⅓ | -1 |
Charm | c | + ⅔ | + ⅓ | 0 |
Top | t | + ⅔ | + ⅓ | 0 |
Bottom | b | -⅓ | + ⅓ | 0 |
For every quark in this table, there is an antiquark. Antiquarks possess the opposite charge, baryon number, and strange number, but the same mass.
Figure 1. Quarks are the particles that make up the hadrons, which carry almost all the mass in the universe, such as protons and neutrons. Source: Manuel R. Camacho, Study Smarter. What is the physics behind quarks?
Quarks are involved in several physical processes. They make up the mass of the universe, as they are the elemental particles that make up the protons and neutrons and give them an electrical charge. Quarks also form special hadrons such as the Pion plus and Kaon plus. And quarks are present in beta decay, which is a form of radiation.
Hadron charge and quarks
Protons and neutrons both consist of three quarks whose symbol is qqq. The combination of up and down quarks tells you which kind of particle you are dealing with. To know which particle a quark makes, you need to add three quarks in such a way that you obtain a fundamental charge of 1 for a proton or 0 for a neutron, as in the following examples.
Proton
As protons have an elemental charge of 1, the quark charges that compose the proton must be 1. To have three quarks and a charge value of 1, you must have two up quarks and one down quark.
\(proton = udu\)
Adding the total charge of the three quarks gives you 1.
\(proton \space charge = \frac{2}{3} - \frac{1}{3} + \frac{2}{3} = 1\)
The total fundamental charge indicates that you are dealing with a proton. Protons and neutrons are also known as baryons, which consist of normal matter. Adding their baryon numbers must give you 1.
\(proton = \frac{1}{3} + \frac{1}{3} + \frac{1}{3} = 1\)
A baryon number equal to 1 tells you that you are dealing with a baryon consisting of normal matter.
Neutron
As neutrons have a fundamental charge of 0, the quark charges that compose the proton must be 0. To have three quarks and no charge, you must have two down quarks and one up quark.
\(neutron = dud\)
Adding all the charges, you get 0.
\(neutron \space charge = -\frac{1}{3} + \frac{2}{3} - \frac{1}{3} = 0\)
The total fundamental charge indicates that you are dealing with a neutron. Neutrons and protons are also known as baryons, which consist of normal matter. Adding their baryon numbers must give you 1.
\(neutron = \frac{1}{3} + \frac{1}{3} + \frac{1}{3} = 1\)
A baryon number equal to 1 tells you that you are dealing with a baryon consisting of normal matter.
Pion plus and kaon plus hadrons
Quarks can combine themselves with an antiquark, creating a matter-antimatter duo, as in the case of the pion plus and kaon plus hadrons.
- Pion plus: a combination of an up quark that has a charge of + ⅔ and an anti-down quark with a charge of + ⅓ and thus a total charge of 1.
- Kaon plus: a combination of an up quark that has a charge of + ⅔ and a strange antiquark with a charge of + ⅓ and thus a total charge of 1.
The pion plus and kaon plus quarks have a baryon number of 0, indicating that they are a combination of matter and antimatter.
Quarks and beta decay
If a nucleus has too many neutrons or protons, a process called beta decay can begin. Beta decay transforms a proton into a neutron or a neutron into a proton. Protons consist of two up quarks and one down quark (udu), while neutrons consist of two down and one up quark (dud).
In the case of a neutron to proton conversion, one down quark must convert itself into an up quark. This conversion includes the release of an electron, which takes away the negative charge, and an antineutrino, as shown below:
\(^1_0{n} \rightarrow ^1_1{p} + ^0_{-1}{e} + \bar{v_\varepsilon}\)
You can observe the conservation in the equation. The neutron has a baryon number of 1 in the upper corner and 0 as its fundamental charge in the bottom corner.
The result of the decay must be a proton with a charge of 1 and an electron with a charge of -1. In this process, an antineutrino is emitted as well.
\(\text{charge conservation = neutron charge = proton charge + electron charges}\)
Weak interaction and quarks
The process that converts a neutron into a proton is called the weak interaction process. There are four weak interaction processes, as listed below.
The neutron to proton process: a neutron converts into a proton and releases an electron and an antineutrino.
\(^1_0{n} \rightarrow ^1_1{p} + ^0_{-1}{e} + \bar{v_\varepsilon}\)
The proton to neutron process: a proton converts into a neutron and releases a positron and a neutrino.
\(^1_1{p} \rightarrow ^1_0{n} + ^0_{-1}{e} + \bar{v_\varepsilon}\)
Electron capture: an atomic nucleus captures an electron, and the proton absorbs the electron. This reaction releases a neutron and a neutrino.
\(^1_1{p} + ^0_{-1}{e} \rightarrow ^1_0{n} + \bar{v_\varepsilon}\)
Proton and electron collision: a proton collides with an electron, which reaction releases a neutron and a neutrino.
\(^1_1{p} + ^0_{-1}e \rightarrow ^1_0{n} + \bar{v_\varepsilon}\)
In the four processes, a W+ or W- boson particle acts as a carrier of the energy.
Figure 2. Electron capture is one of the processes where the weak interaction takes place and one particle is converted into another. Here, we see the capture of an electron by a nucleus, which turns a positive electron into a neutron and releases a positron. Source: Manuel R. Camacho, StudySmarter.
Feynman diagram and quarks
The Feynman diagram is a way to show the interaction between particles as they emit or absorb energy while creating other particles. Let us consider the example of the beta decay of a neutron into a proton, as shown below:
\(^1_0{n} \rightarrow ^1_1{p} + ^0_{-1}{e} + \bar{v_\varepsilon}\)
The Feynman diagram for this is:
Figure 3. Feynman diagram for the beta decay of a proton. Source: Manuel R. Camacho, StudySmarter.
Strange quarks and the strange number
High energy photons such as gamma rays can collide with particles, emitting other particles and radiation. In the earth’s atmosphere, they inject energy into molecules of air, creating strange quarks. However, the particles created do not separate themselves into smaller particles as quickly as scientists expected. This effect was explained by a new property called strangeness, which is indicated by the strange number. Strange numbers only change during weak force interactions.
Quark Physics - Key takeaways
- Matter as we know it consists of quarks, hadrons that are the neutron, and protons made of positive quarks called up and down quarks.
- Positive quarks have a charge of + ⅔ and - ⅓. When three are added together into a neutron or proton, the respective combination is either 0 or 1.
- There are also other particles, such as the pion plus and the kaon plus, which consist of a combination of quarks and antiquarks. In contrast to neutrons and protons, they only have two quarks rather than three.
Explanations 
Exams 
Magazine 
