OCR Specification focus:
‘Use u, d, s quarks and corresponding antiquarks; apply to hadron structure.’
Quark Model Foundations
This section introduces the quark model, a framework describing how hadrons are constructed from more fundamental constituents. It outlines the essential quark types required at this level, explains the role of antiquarks, and shows how these components combine to form mesons and baryons in accordance with the simple quark model. Understanding this structure provides a basis for analysing particle interactions and classification within the Standard Model.
The Idea of Fundamental Constituents
Modern particle physics identifies quarks as some of the most basic building blocks of matter. At the OCR specification level, students study the three lightest quarks: up (u), down (d), and strange (s), along with their corresponding antiquarks.
Quark: A type of fundamental particle that combines with others to form hadrons.
Quarks cannot exist in isolation under normal conditions due to colour confinement, meaning they are always bound into larger composite particles. Although colour charge is not examined in detail at this level, it underlines why quarks are only observed inside hadrons.
Antiquarks and Their Characteristics
Each quark type has an associated antiquark, denoted with a bar: ū, d̄, s̄. Antiquarks have the same mass as their corresponding quarks but possess opposite charge and certain opposite quantum properties.
Antiquark: The antimatter counterpart of a quark, carrying opposite charge and quantum numbers.
Antiquarks are essential in forming mesons and appear in particle–antiparticle interactions. They also ensure that conservation laws such as charge, baryon number, and strangeness are satisfied in particle reactions.
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Hadron: A composite particle made of quarks or quark–antiquark pairs, bound by the strong interaction.
Baryons and Mesons: The Two Hadron Families
Hadrons fall into two major categories, distinguished by their quark content and quantum properties:
Baryons
These contain three quarks. Their key features include:
Composed of combinations of u, d, and s quarks at this level
Possess baryon number +1
Include important particles such as protons and neutrons
Strange baryons (e.g., Λ, Σ) include at least one s quark
Mesons
These contain one quark–antiquark pair. Their notable properties include:
Baryon number 0
Able to include strange quarks or antiquarks, producing kaons
Often appear in decay processes and interaction mediators
Mesons illustrate the role of antiquarks most clearly, as every meson requires an antiquark for its constitution.
Quark Flavours Used at This Level
OCR requires knowledge of the following quark flavours and their antiquarks:
Up (u)
Down (d)
Strange (s)
ū, d̄, s̄ for antiquarks
Although six quark flavours exist in the full Standard Model, only these three are necessary for the specification.
How Quarks Combine to Form Hadrons
Quark combinations follow specific structural rules. These include:
Baryons:
Three quarks
Examples: uud (proton), udd (neutron), uus, uds, dds (various strange baryons).
Diagram showing the quark structure of a proton as a baryon composed of two up quarks and one down quark. The connecting lines represent the strong interaction binding the quarks. Additional styling elements exceed OCR requirements but help visualise quark confinement. Source.
Mesons:
One quark + one antiquark
Examples: u d̄ (π⁺), d ū (π⁻), u s̄ (K⁺), s ū (K⁻).
Diagram of the positive pion (π⁺) made from an up quark and an anti-down quark. The wavy connection indicates the strong force acting between them. Extra gluon and colour-charge indicators appear but remain compatible with the required syllabus level. Source.
These structures determine the observable charge, strangeness, and interaction behaviour of the particles.
Charge Structure and Quark Composition
Each quark carries a fractional electric charge relative to the elementary charge e. Understanding these charges is essential for determining hadron charge:
u quark: +2/3 e
d quark: −1/3 e
s quark: −1/3 e
Antiquarks have the opposite charges: ū (−2/3 e), d̄ (+1/3 e), s̄ (+1/3 e)
Because the strong interaction binds quarks tightly inside hadrons, learners never observe fractional charges on isolated particles; instead, they detect the integer charges of hadrons produced from quark combinations.
Strangeness and the Role of the s Quark
The strange quark introduces an additional quantum number: strangeness, which is negative (S = −1) for s quarks and positive (S = +1) for s̄. This quantum number is:
Conserved in strong interactions
Not conserved in weak interactions
This explains the relatively long lifetimes of strange particles, such as kaons, produced in high-energy collisions. Their decay via the weak interaction gives them distinctive properties studied in particle physics.
Using the Quark Model in Classification
Students should be able to determine the internal quark structure of common hadrons and use quark composition to identify:
Particle charge
Whether a particle is a baryon or meson
Whether the particle is strange
Whether it contains an antiquark
Which interactions it can undergo, based on its quark content
The quark model provides a conceptual and predictive framework for understanding the behaviour, structure, and classification of hadrons at the level required by the OCR A-Level Physics specification.
FAQ
These three quarks are the lightest flavours, making them the ones most commonly found in the hadrons encountered in low-energy physics.
Heavier quarks (charm, bottom, and top) appear only in high-energy particle collisions and decay too rapidly to form the stable or long-lived hadrons relevant to A-Level study.
Using only u, d, and s allows the model to remain conceptually clear while still explaining the full range of hadrons encountered in typical exam contexts.
Quarks are permanently confined within hadrons due to a property of the strong interaction called confinement.
As quarks move apart, the strong force between them increases rather than decreases. This behaviour is unlike gravitational or electrostatic forces.
At large separations, attempting to separate quarks results in enough energy to create a new quark–antiquark pair, ensuring quarks always remain bound inside hadrons.
This arises from a principle called colour charge, where quarks carry one of three “colours”.
Allowable combinations must form a colour-neutral structure. This restricts stable hadrons to:
Three-quark groups (one of each colour)
Quark–antiquark pairs (colour and anti-colour)
Although colour charge is not examined in depth at OCR A-Level, it underpins why only baryons and mesons appear in the simple quark model.
The strange quark introduces strangeness, a quantum number not possessed by up or down quarks.
Strangeness helps categorise hadrons into families such as kaons and hyperons, which include one or more strange quarks.
Its presence also indicates whether the particle is likely to undergo weak decay, since strangeness is conserved in strong interactions but not in weak interactions.
Antiquarks have all quantum numbers opposite to their quark counterparts, not only charge.
This includes:
Opposite baryon number
Opposite strangeness (for s and s̄)
Opposite colour charge
These differences ensure that quark–antiquark interactions obey conservation laws, allowing mesons to form and to participate in annihilation processes when encountering their corresponding matter or antimatter partner.
Practice Questions
Question 1 (2 marks)
State the quark content of a positive pion (pi-plus) and explain why it is classified as a meson.
Mark scheme:
Correct quark content: up quark and anti-down quark (u d̄). (1 mark)
Correct explanation that mesons consist of one quark and one antiquark. (1 mark)
Question 2 (5 marks)
Hadrons are divided into two major groups: baryons and mesons.
Describe the quark structure of baryons and mesons, and explain how quark charges can be used to determine the overall charge of a hadron.
Use examples in your answer.
Mark scheme:
States that baryons consist of three quarks. (1 mark)
States that mesons consist of one quark and one antiquark. (1 mark)
States correct fractional charges of quarks (e.g., u = +2/3 e, d = −1/3 e, s = −1/3 e, antiquarks opposite). (1 mark)
Explains how the total charge is found by summing constituent quark charges. (1 mark)
Example given with correct charge reasoning (e.g., proton uud → +1 charge). (1 mark)
