Presentation about

1. Particles
Classification of Particles
Hadrons and Quarks
Leptons
Thursday, 08 December 2011

2. The Stanford linear accelerator
In 1968 the Stanford accelerator shot a beam of 20 GeV electrons on a
target. The results showed clearly that the electrons were strongly
scattered by stationary protons and sometimes even bounced
backward.
q
q
q

3. The Stanford linear accelerator
Does that remind you of another famous experiment? What can be
deduced about the internal structure of the proton?
The outcomes of the experiment resemble Rutherford’s experiment that
led to the discovery of the nucleus.
The proton must be made up of sub-nuclear particles, some of which
carry negative charge. This provides proof for the existence of quarks.

4. The Structure of a Proton
These results pointed at a structure of nucleons like the proton as being
made up of three sub-nuclear particles called Quarks.
q
q
q

5. Classifying Particles
All particles can be classified in three main categories:
• Hadrons  made up of quarks. They are affected by strong forces
• Leptons  fundamental particles, i.e. they don’t have an internal
structure. In other words, they are not made up of smaller particles
and are not affected by strong forces.
• Quarks  smaller particles that combine to form hadrons. They
carry fractional charges (fractions of the charge of the electron).
So, what is everything made of?
• Quarks and Leptons

6. 1st • up (u)  +2/3
generation • down (d)  -1/3
2nd • charm (c) +2/3
generation • strange  -1/3
3rd • top (t)  +2/3
generation • bottom (b)  -1/3

7. • electron (e-)  -1
1st
generation • electron neutrino
( e)  0
• muon ( -) -1
2nd
generation • muon-neutrino
( ) 0
• tau ( -)  -1
3rd
generation • Tau-neutrino ( )
0

8. • Pions: 0=(uu) or dd
+= (ud)
Meson (1 -= (ud)
quark + 1 anti-
quark) • Kaons: 0 (ds)
+ (us)
- (us)
Baryons • Proton (uud) +1
(3 quarks) • Neutron (udd) 0

9. Lego Particles
Up quark (+2/3) Anti-up quark (+2/3)
Down quark (-1/3) Anti-Down quark (-
1/3)
Strange quark (-1/3) Anti-Strange quark (-
1/3)
Electron (-1) Positron (+1)
Neutrino (0) Anti-Neutrino (0)
Muon (-1) Anti-moun (+1)

10. Fundamental particles
A proton is made of two Up quarks and one down. Show that the sum of
the three charges gives +1.
+2/3 + 2/3 – 1/3 = +3/3 = +1
u
d u
A neutron is made of two down quarks and one up quark. Show the
neutrality of this distribution.
-1/3 – 1/3 + 2/3 = -2/3 + 2/3 = 0
d
d u

11. Lepton Number
All leptons have an additional property called Lepton Number. The
lepton number is always conserved.
Particle Lepton Number
All leptons +1
All anti-leptons -1
Hadrons
0
(baryons and mesons)

12. - Decay and lepton number
Explain why both an electron and an anti-neutrino must be formed in a
- decay.
n p e e
• The lepton number must be conserved.
• e- lepton no = +1 +1 – 1 = 0  an electron and an anti-
neutrino must be created for lepton
• e lepton no = -1 number to be conserved

13. Properties of quarks
Quarks and anti-quarks have some properties that you might not have
encountered before:
• Relative Charge  all quarks and anti-quarks carry a charge which is
a fraction of the charge of the electron, i.e. 1.6 x 10-19 C. In all
interactions charge must be conserved.
• Baryon Number  all quarks and anti-quarks have a baryon number.
The baryon number is +1/3 for quarks and -1/3 for all anti-quarks. In
all interactions baryon number must be conserved.
• Strangeness  all quarks and anti-quarks have strangeness = 0 apart
from the strange quark (strangeness = -1) and the anti-strange quark
(strangeness = +1). In all interactions involving the STRONG FORCE
strangeness must be conserved, but in weak interactions strangeness
can be conserved, or change by ±1.

14. Properties of quarks
Baryon
Name Symbol Charge Strangeness
Number
up u +2/3 +1/3 0
Quarks
down d -1/3 +1/3 0
strange s -1/3 +1/3 -1
Anti-quarks
anti-up u -2/3 -1/3 0
anti-down d +1/3 -1/3 0
anti-strange s +1/3 -1/3 +1

15. Mesons and Quarks
A K+ meson is made up of an up quark and an anti-strange quark. Work
out the relative charge, baryon number and strangeness of this
particle.
• Charge +2/3 + 1/3 = +3/3 = +1
• Baryon no  +1/3 – 1/3 = 0
• Strangeness  0 + 1 = +1
s
u

16. Mesons and Quarks
A - meson is made up of an anti-up quark and a down quark. Work out
the relative charge, baryon number and strangeness of this particle.
• Charge -2/3 - 1/3 = -3/3 = -1
• Baryon no  +1/3 – 1/3 = 0
• Strangeness  0 – 0 = 0
u
d

17. Change of Quarks in - Decay
In a - decay one quark in the neutron changes character (flavour) to
form a proton. Complete the diagram with the correct quarks in the
proton.
proton
After
u du
e- e
W
u dd Before
neutron
A down quark in the neutron changes into an up quark, emitting an
electron and an anti-neutrino.

18. Stable and Unstable Baryons
In a - decay one quark in the neutron changes character (flavour) to
form a proton. But why does that happen?
• The proton is the only stable Baryon.
• All other Baryons eventually decay into a proton.
• So, it is not surprising that in “nature” the neutron decays into a
proton releasing an electron and an anti-neutrino.

19. Change of Quarks in + Decay
In a - decay one quark in the neutron changes character (flavour) to
form a proton. Complete the diagram with the correct quarks in the
proton and the neutron.
neutron After
u dd
e+ e
W
u du Before
proton
An up quark in the neutron changes into an down quark, emitting a
positron and a neutrino.

20. Conservation Laws
In all particle interactions these conservation laws apply and must be
fulfilled for the interaction to happen:
• Conservation of Charge  In all interactions charge must be
conserved. So, Sum of Charges before = Sum of Charges after
• Conservation of Baryon Number  In all interactions baryon
number must be conserved.
• Conservation of Strangeness  In all interactions involving the
STRONG FORCE strangeness must be conserved, but in weak
interactions strangeness can be conserved, or change by ±1.
• Conservation of Lepton Number  In all interactions the lepton
number must be conserved.

21. Conservation Laws
Applying the conservation laws, show whether the following
interactions are possible or not.
Answer n p e e
Answer e e
Answer n p e e
Answer n e

22. Answer
n p e e
This reaction can occur because:
• Charge is conserved  Before: 0 After: +1 – 1 + 0 = 0
• Baryon no is conserved  Before: +1 After: +1 + 0 + 0 = +1
• Lepton no is conserved  Before: 0 After: 0 + 1 - 1 = 0
• Strangeness is conserved  Before: 0 After: 0

23. Answer
e e
This reaction can occur because:
• Charge is conserved  Before: 0 After: -1 + 1 = 0
• Baryon no is conserved  Before: 0 After: 0 + 0 = 0
• Lepton no is conserved  Before: 0 After: +1 - 1 = 0
• Strangeness is conserved  Before: 0 After: 0

24. Answer
n p e e
This reaction cannot occur because:
• Charge is conserved  Before: 0 After: +1 - 1 + 0 = 0
• Baryon no is conserved  Before: 0 After: 0 + 0 + 0 = 0
• Strangeness is conserved  Before: 0 After: 0
• But, Lepton no is not conserved  Before: 0 After: +1 + 1 = 2

25. Answer
n e
This reaction cannot occur because:
• Charge is conserved  Before: 0 After: -1 + 2/3 + 1/3 = 0
• But, Baryon not is conserved  Before: 1 After: 0 + 0 = 0
• Strangeness is changed by +1 conserved  Before: 0 After: +1
• But, Lepton no is not conserved  Before: 0 After: +1 + 0 = +1