This PowerPoint details an introduction to particle physics and includes several examples explaining the conservation of quantum numbers.

1. Elementary Particles
by Bryan Kim

2. Elementary Particles

3. Elementary Particles - Overview
› Elementary particles are believed to be the smallest
particles which make up matter and energy.
– i.e. Elementary particles do not have internal structure.
› Elementary particles can be categorized as fermions or
bosons, which are then subcategorized.
› There are 12 fundamental fermions and 4 fundamental
bosons.
– Fermions subcategorize to quarks and leptons.
– The four bosons are gluons, Higgs boson, gravitons, and
electroweak bosons.

4. The Standard Model of
Particle Physics

5. The Standard Model of Particle Physics

6. The Standard Model of Particle Physics
› The Standard Model illustrates the elementary particles
as previously stated.
› The Standard Model depicts the theory which explains the
exchange between elementary particles and their
consequences.
› Note that every fermion has an antifermion.
– Each fermion and antifermion have opposing charges and their
respective quantum numbers
› e.g. An up has charge 2/3e and baryon number 1/3, while an antiup has
charge -2/3e and baryon number -1/3.

7. Quarks

8. Quarks
› As seen in the Standard Model, there are six types of quarks
(and six corresponding antiquarks).
› Three quarks combined forms a baryon.
– e.g. Protons and neutrons, both baryons, are formed of, respectively,
two ups and one down, and one up and two downs.
› Proton = uud, neutron = udd.
› Referring to the Standard Model, up has a charge of 2/3e and down has a
charge of -1/3e, which corresponds to the charges of a proton and neutron:
+1 and 0.
› When a quark and its antiquark meet, both particles are
annihilated into energy.
– A quark and antiquark pair can also create a meson – e.g. an up and
an antidown can create a meson
called pion (u𝑑).

9. Leptons

10. Leptons
› The other six fundamental fermions are called leptons, as
seen in the Standard Model.
› Just like baryons, leptons have a corresponding quantum
number called lepton number.
– Leptons have a lepton number of +1 and antileptons have a
lepton number of -1
– Lepton number differs by baryon numbers in that the generation
of the lepton number is significant.
› e.g. An electron has a lepton number of +1I and an antimuon has a lepton
number of -1II and so on.

11. Quantum Number
Conservation

12. Baryon Number Conservation
› As stated previously, a quark has a baryon number of 1/3
and an antiquark has a baryon number of -1/3.
– e.g. A proton is uud (two ups and one down), which means it has
a baryon number of 1. An antiproton is 𝑢𝑢𝑑 (two antiups and
one antidown), which means it has a baryon number of -1.
› Baryon number is conserved in all reactions.

13. Strangeness Number Conservation
› For baryons, there is another conserved quantum number
called the strangeness number.
– The strangeness number of a baryon is calculated by (number of
antistrange quarks – number of strange quarks).
– In this way, an strange has a strangeness number of -1 and an
antistrange has a strangeness number of +1.
› Strangeness number is conserved in all reactions.

14. Lepton Number Conservation
› As previously stated, lepton numbers must be conserved
by generation.
– Electrons and electron neutrinos are generation I.
– Muons and muon neutrinos are generation II.
– Taus and tau neutrinos are generation III.
› Lepton number is conserved in all reactions.

15. Feasibility of Particle
Reactions

16. Feasibility of Particle Reactions
› The feasibility of particle reactions can be determined by
examining the conservation of baryon numbers,
strangeness numbers, lepton numbers by generation, and
charge.
› It may be useful to keep track of quantum numbers and
charge with a table as shown below.
Reaction
Baryon #
Strangeness #
Lepton #
Charge

17. Feasibility of Particle Reactions
› In this example, a proton turns into a neutron, a positron,
and an electron neutrino.
– This reaction is known as beta plus (ß+) decay.
› This reaction is feasible, as all quantum numbers such as
baryon number, strangeness number, and lepton number
are conserved as well as the charge.
Reaction p  n e+ ve
Baryon # +1 = +1 0 0
Strangeness # 0 = 0 0 0
Lepton # 0 = 0 -1I +1I
Charge +1 = 0 +1 0

18. Feasibility of Particle Reactions
› In this example, a muon turns into a muon neutrino, an
electron and an electron antineutrino.
› This reaction is feasible, as all quantum numbers such as
baryon number, strangeness number, and lepton number
are conserved as well as the charge.
– Note that the lepton numbers are conserved by generation.
Reaction µ  vµ e 𝒗e
Baryon # 0 = 0 0 0
Strangeness # 0 = 0 0 0
Lepton # +1II = +1II +1I -1I
Charge -1 = 0 -1 0

19. Feasibility of Particle Reactions
› In this example, a neutron turns into a proton, an
electron, and an muon antineutrino.
› This reaction is not feasible, since, although baryon
number, strangeness number, and charge are conserved,
lepton number is not.
– Note the difference in lepton generation – I and II.
Reaction n  p e 𝒗µ
Baryon # 1 = 1 0 0
Strangeness # 0 = 0 0 0
Lepton # 0 = 0 +1I -1II
Charge 0 = +1 -1 0

20. Thanks!