Quarks, leptons and antiparticles

Quarks

Quarks are fundamental particles with fractional charges (in units of e):

• up (u): +2/3; down (d): -1/3; strange (s): -1/3
• charm (c): +2/3; bottom (b): -1/3; top (t): +2/3

Each quark has baryon number +1/3. Quarks are never found alone.

Hadrons: baryons and mesons

Particles made of quarks are hadrons.

• Baryons: three quarks (baryon number +1). Proton = uud (charge +1), neutron = udd (charge 0).
• Mesons: a quark and an antiquark (baryon number 0). Example: π+ = u anti-d (charge +1).

Antibaryons are made of three antiquarks.

Leptons and photons

• Leptons are fundamental and are not made of quarks: electron, muon, tau and their three neutrinos. Each has lepton number +1.
• The photon is the exchange particle of the electromagnetic force; it has no charge, no mass and is not a hadron or a lepton.

Antiparticles

Every particle has an antiparticle with the same mass (and rest energy) but opposite charge, baryon number and lepton number. The positron (e+) is the electron's antiparticle; the antiproton is anti-u anti-u anti-d with charge -1. If you know a particle's properties, you can deduce its antiparticle's.

Worked example

Find the charge and baryon number of a particle made of u, u and s quarks.

1. Charge = +2/3 + 2/3 - 1/3 = +1
2. Baryon number = 3 x 1/3 = 1, so it is a baryon

Answer: Charge +1e; baryon number 1

Check yourself

1. What is the charge, in units of e, of a particle with quark composition uud?
   Show answer

   1. +2/3 + 2/3 - 1/3 = +1.

2. What is the charge, in units of e, of a meson made of an up quark and an anti-down quark?
   Show answer

   1. Anti-down has charge +1/3, so +2/3 + 1/3 = +1.

3. What is the charge, in units of e, of a baryon made of three strange quarks?
   Show answer

   -1. 3 x (-1/3) = -1.

4. What is the quark composition of a neutron?
   • udd
   • uud
   • uds
   • u anti-d
   Show answer

   udd. Charge = +2/3 - 1/3 - 1/3 = 0.

5. Which of these particles is a lepton?
   • Proton
   • Pion
   • Muon
   • Neutron
   Show answer

   Muon. Muons are fundamental, like electrons. The others are hadrons.

6. What is a meson made of?
   • A quark and an antiquark
   • Three quarks
   • Three antiquarks
   • Two leptons
   Show answer

   A quark and an antiquark. Mesons have baryon number 1/3 - 1/3 = 0.

7. How does a positron compare with an electron?
   • Same charge, opposite mass
   • Larger mass, same charge
   • Same mass, opposite charge
   • No mass, opposite charge
   Show answer

   Same mass, opposite charge. Antiparticles have the same mass and opposite charge.

8. Which statement about the photon is correct?
   • It is a lepton, like the electron
   • It is a baryon
   • It is a meson
   • It is neither a hadron nor a lepton
   Show answer

   It is neither a hadron nor a lepton. Photons are a separate class: they are not made of quarks, so they are not hadrons, and they are not leptons.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics specification (XPH11/YPH11), Issue 3
• Pearson qualifications: International Advanced Levels Physics (2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.