Physics › Particles › Quarks and antiquarks

Quarks and antiquarks

Three quarks, up, down and strange, build every hadron on the specification. Baryons are made of three quarks, mesons of a quark bound to an antiquark, and beta-minus decay is a single down quark changing into an up quark.

Year 12AQA 3.2.1.6CIE 11.2.1, 11.2.2, 11.2.3, 11.2.4OCR A 6.4.2(e), 6.4.2(f), 6.4.2(g), 6.4.2(i), 6.4.2(k), 6.4.2(l)EDEXCEL 141

Builds on Classification of particles.

IN THIS TOPIC

  • Use the booklet's table of charge, baryon number and strangeness for the u, d and s quarks and their antiquarks.
  • Build the proton, neutron, antiproton, antineutron, pions and kaons from quarks.
  • Check any composition by adding thirds, for charge, baryon number and strangeness alike.
  • Describe the decay of the neutron in terms of its quarks.
  • Take any decay apart at quark level, naming the quark that changed flavour and the W boson that carried the charge away.

COMMON MISCONCEPTION

Protons and neutrons are fundamental.

The quark table

Every hadron AQA tests is built from three quark flavours. An up quark carries charge +⅔ e, baryon number +⅓ and strangeness 0. A down carries −⅓ e, +⅓ and 0. A strange carries −⅓ e, +⅓ and strangeness −1, the flavour the quantum number is named after. Each has an antiquark, written with a bar (u, d, s) and every sign flipped. The full table comes printed in the data booklet, so the examinable skill is building with it.

Three flavours is the whole requirement on every board but one, and where you should stop unless you sit CIE. CIE names all six flavours, on the grounds that a quark is a fundamental particle and the family is complete at six: up, down, strange, charm, top and bottom. The extra three only repeat the pattern already on the page, charm and top carrying +⅔ e and bottom −⅓ e, with each antiquark taking the opposite charge, and CIE asks for no other property of them whatever. Edexcel adds a single historical note, that the symmetry of the model predicted the top quark before anyone found one.

Baryons: three quarks

The proton is uud and the neutron is udd: the quark charges add to plus one and zero exactlyprotonu+2/3u+2/3d−1/3total charge +1neutronu+2/3d−1/3d−1/3total charge 0same recipe book, one swapped ingredient
FIG. 1The proton is uud, charge ⅔ + ⅔ − ⅓ = +1. The neutron is udd, charge 0. One swapped quark apart.

A baryon is three quarks, so its baryon number is three thirds, which comes to +1 as it must. The proton is uud and the neutron is udd, their charges falling straight out of the fractions. Antibaryons take the mirror recipes, the antiproton being uud and the antineutron udd, with charges −1 and 0 and baryon number −1 each.

Mesons: a quark and an antiquark

The specified mesons: each is one quark and one antiquark, and the charges and strangeness follow from the pairπ⁺udcharge +1S = 0π⁻ducharge −1S = 0K⁺uscharge +1S = +1K⁰dscharge 0S = +1the amber circle is always the antiquark
FIG. 2The specified mesons as pairs. π⁺ is u with anti-d, π⁻ is d with anti-u, K⁺ is u with anti-s, and K⁰ is d with anti-s.

A meson is a quark bound to an antiquark, so its baryon number is ⅓ − ⅓ = 0, exactly as the classification demanded. The pion family holds no strange quarks at all, so every pion has S = 0. Each kaon contains one strange or anti-strange quark, and that single quark is where its strangeness of ±1 lives. A K containing s has S = +1, and one containing s has S = −1. Only u, d and s and their antiquarks build the hadrons anyone examines. Even on CIE the heavier three never have to be assembled into anything, so the rest of the family stays off the paper.

WORKED EXAMPLE

Building a kaon from the table

The K+ has quark content us. Verify its charge and strangeness from the quark table.

Take charge first. The u contributes +⅔ and the s, being an anti-strange, contributes +⅓, giving +1 and matching the kaon's label.

Now strangeness. The u carries none, and the s carries S = +1, the strange quark itself being the one with S = −1. That gives S = +1.

Baryon number finishes the check at ⅓ − ⅓ = 0, as any meson must. Every hadron question is this same assembly job, run with the booklet's table open in front of you.

GUIDED PRACTICE

The antiproton, assembled

Write the antiproton's quark content and verify its charge and baryon number.

Show the working

Mirror the proton's uud, and the antiproton is uud.

Charge comes to −⅔ − ⅔ + ⅓ = −1, and baryon number to three antiquarks at −⅓ each, so −1. Every sign flipped, exactly as the mirror demands.

INDEPENDENT PRACTICE

A mystery baryon

A particle has charge −1, baryon number +1 and strangeness 0, and contains no strange quarks. Deduce a possible quark content from u and d alone.

Show the working

Baryon number +1 means three quarks. With u at +⅔ and d at −⅓, the only trio that reaches −1 is ddd, since −⅓ −⅓ −⅓ = −1.

Three identical down quarks make a real particle, the Δ−. That deduction ran entirely on bookkeeping numbers, and particles of this kind were predicted on paper before anyone saw one.

The neutron's decay, at last explained

Neutron decay at the quark level: one down quark becomes an up quark, and the W minus delivers the electron and antineutrinouddneutron: udduudproton: uudW⁻e⁻+νthe change of quark character that beta-minus decay is
FIG. 3Neutron decay at quark level: udd becomes uud as one down quark changes to an up, the W minus delivering the electron and antineutrino.

Beta-minus decay, met two lessons ago as “a neutron becomes a proton”, is one line of quark chemistry. One down quark becomes an up quark, so udd turns into uud, with a W− carrying away the charge difference before becoming the electron and the electron antineutrino. The other two quarks watch.

Any decay, taken apart the same way

The neutron has no monopoly on that method, and OCR names the general version as an outcome of its own. Write the quark content under every particle on both sides of the arrow, then look for the single quark that changed flavour. Everything else is spectators, and a W boson carrying off whatever charge the change released.

Run it on the kaon's decay into pions, promised a lesson ago. A K− is su, and it goes to a π− and a π0. The strange quark becomes an up quark, emitting a W− exactly as the neutron's down quark did. That W− becomes a d with an u, which is the π−, while the kaon's own u spectates throughout and pairs with the new up quark to make the π0.

One line of quark chemistry has now accounted for the strangeness rule. All of a kaon's strangeness lives in that one strange quark, so destroying it takes S from −1 to 0, and changing a quark's flavour is something only the weak interaction can do. The strong interaction leaves every flavour count exactly as it found it, which is why it can create strange particles only in cancelling pairs and can never take one apart. What the last lesson asserted about strangeness turns out to be a statement about what one quark is allowed to become.

INDEPENDENT PRACTICE

A heavier strange particle

A Λ0 baryon has quark content uds, and it decays to a proton and a π−. Identify the quark that changes flavour, name the interaction, and say what happens to strangeness.

Show the working

Line up the contents. The proton is uud and the π− is du, so the lambda's u and d are spectators and the quark that moved is the s, which became a u. The W− it emitted became the d and the u of the pion.

Strangeness runs −1 → 0, a change of one, so this is the weak interaction and nothing else. The lambda's comparatively long life says the same thing. The method never changes: find the quark that moved, and the interaction names itself.

ASSESSMENT FOCUS

  • Learn the recipes cold. Proton uud, neutron udd, antiproton uud, antineutron udd. Mesons are a quark with an antiquark, pions at strangeness 0 and kaons at ±1. The quark properties table itself is printed in the booklet, so spend that exam second reading it.
  • Verify any composition by adding thirds. Charges must sum to a whole number of e, and a trio of quarks must give B = +1.
  • CIE alone wants all six flavours by name and by charge: up, down, strange, charm, top and bottom, with charm and top at +⅔ e and bottom at −⅓ e. Nothing further about the heavy three is asked. On AQA, OCR and Edexcel, three flavours is the whole answer, so do not volunteer six and lose the thread.
  • Give neutron decay at quark level when the paper is on this topic. One d becomes a u, a W− leaves, and it becomes e− with an electron antineutrino. Writing “n → p” alone here undersells what you know.
  • Antiquarks flip every sign, charge and baryon number and strangeness together. Flipping the charge alone is the classic table-reading slip.
  • OCR alone asks for decays beyond beta at quark level, and the method is always the same: write the quark content under every particle, point at the one that changed flavour, and name the W that carried the charge off. For a kaon that is s becoming u, with the W− turning into a pion and strangeness dropping by one.

CHECK YOURSELF

Using quark charges, show that the proton's charge is +1 and the neutron's is 0, then write the neutron's decay at quark level.

Show a hint

Add thirds, then change exactly one flavour.

Show the answer

Proton, uud, gives ⅔ + ⅔ − ⅓ = +1. Neutron, udd, gives ⅔ − ⅓ − ⅓ = 0. Both come out as whole charges, as every hadron must.

For the decay, one d becomes a u, so udd → uud + e− + an electron antineutrino, with the W− mediating.

Check the vertex in thirds. The d starts at −1 third and the u ends at +2 thirds, so the W must carry off −3 thirds, one whole negative charge. It balances, and the neutron's decay is one quark changing flavour.

Baryons are three quarks. Mesons are a quark with an antiquark.

Antiquarks flip every sign.

The neutron's decay is one d becoming a u.

WORKBOOK

Printable practice for this topic: original exam-style questions with room to work, and a fully worked answer book. Free to use; please do not redistribute or sell.

Question workbook

PDF · 58 KB · print edition · white background

Question workbook

PDF · 58 KB · screen edition · navy

Answer book

PDF · 56 KB · fully worked · navy
19 questions on this topicAnswer them one at a time and mark yourself against the mark scheme.Practise this topic

Or read them with their mark schemes on the quarks and antiquarks questions page.

6 flashcards on this topicDefinitions, off-sheet equations and a spot-the-error card, scheduled by spaced repetition in your browser.Revise with flashcards

CHECK YOUR PROGRESS

Rate how confident you feel with each objective for this lesson. Ratings are saved in this browser, on this device, unless you sign in.

  • Use the booklet's table of charge, baryon number and strangeness for the u, d and s quarks and their antiquarks.
  • Build the proton, neutron, antiproton, antineutron, pions and kaons from quarks.
  • Check any composition by adding thirds, for charge, baryon number and strangeness alike.
  • Describe the decay of the neutron in terms of its quarks.
  • Take any decay apart at quark level, naming the quark that changed flavour and the W boson that carried the charge away.

Open the full revision checklist to track your progress across the whole unit.