IB DP Physics: HL复习笔记7.3.3 Hadrons, Baryons & Mesons

Hadrons, Baryons & Mesons

  • Hadrons are the group of subatomic particles that are made up of quarks
  • These may be either a:
    • Baryon (3 quarks)
    • Meson (quark and anti-quark pair)


Hadrons may be either a baryon or a meson

  • Quarks have never been discovered on their own, always in pairs or groups of three
  • Anti-hadrons can be either
    • Anti-baryons (3 anti-quarks)
    • Anti-meson (quark and anti-quark pair)


Anti-hadrons may be either an anti-baryon or an anti-meson

  • Note that all baryons or mesons have integer (whole number) charges eg. +1e, -2e etc.
  • This means quarks in a baryon are either all quarks or all anti-quarks. Combination of quarks and anti-quarks don’t exist in a baryon
    • e.g.11.2.4-Wrong-quark-composition
  • The anti-particle of a meson is still a quark-antiquark pair. The difference being the quark becomes the anti-quark and vice versa

Worked Example

The baryon Δ++ was discovered in a particle accelerator using accelerated positive pions on hydrogen targets.Which of the following is the quark combination of this particle?Worked-Example-Baryons-Mesons-e1615291775993


Baryon Number

  • The baryon number, B, is the number of baryons in an interaction
  • B depends on whether the particle is a baryon, anti-baryon or neither
    • Baryons have a baryon number B = +1
    • Anti-baryons have a baryon number B = –1
    • Particles that are not baryons have a baryon number B = 0
  • Baryon number is a quantum number and is conserved in all interactions
    • This is one of the indicators for whether an interaction is able to occur or not


The baryon number of a particle depends if it is a baryon, anti–baryon or neither 

  • The up (u), down (d) and strange (s) quark have a baryon number of 1/3 each
  • This means that the anti–up, anti–down and anti–strange quarks have a baryon number of –1/3 each
    • Note: The baryon number of each quark is provided on the data booklet
  • The implication of this is that baryons are made up of all quarks and anti-baryons are made up of all anti-quarks
  • There are no baryons (yet discovered) that have a combination of quarks and anti-quarks eg. up, anti-down, down
  • The reason is that this would equate to a baryon number that is not a whole number (integer)

Worked Example

Show that the baryon number is conserved in β decay.


Exam Tip

  • Remembering quark combinations is useful for the exam. However, as long as you can remember the charges for each quark, it is possible to figure out the combination by making sure the combination of quarks add up to the charge of the particle (just like in the worked example)


  • Mesons are composed of a quark and an anti-quark pair
  • Mesons will have a baryon number of 0
  • They can have a charge of -1, 0 or +1
  • Types of mesons include pions, kaons, D mesons, B mesons and many other varieties
  • Pions (π–mesons) can be positive (π+), negative (π) or neutral (π0)
  • The anti–particle of the positive pion is the negative pion (and vice versa)
    • The neutral pion is its own anti–particle


Pions consist of up and down quarks, and their respective antiparticles, depending on their sign

  • Pions are the lightest mesons, making them more stable than other types of meson
  • They were originally discovered in cosmic rays and can be observed in a cloud chamber
  • The strong nuclear force keeps the protons and neutrons bound together in a nucleus and is one of the four fundamental interactions. Each of these interactions is caused by a particle exchange
  • The pion is the exchange particle of the strong nuclear force
  • This means that the strong force is transmitted between a proton and neutron by the exchange of a pion
    • Pions are said to mediate (bring about) the strong nuclear force
  • The pion created is a temporary violation of energy and mass conversation but since it is a virtual particle, it is not directly observed



  • Kaons (K–mesons) can also be positive (K+), negative (K) or neutral (K0)
  • The anti–particle of the positive kaon is the negative kaon (and vice versa)
    • The neutral kaon is its own anti–particle
  • Kaons can be produced by the strong interaction between pions and protons


Kaons consist of up, down and strange quarks, and their respective antiparticles, depending on their sign

Kaon Decay
  • Kaons are heavy and unstable and normally decay into pions
  • They are known to have unusually long lifetimes
    • This is because kaons contain a strange quark and longer lifetimes are characteristic of particles containing strange quarks
  • Kaons decay through the weak interaction
  • An example of a kaon decay would be a neutral kaon decaying into a positive pion and negative pion: