Download unit 5: particle physics

The Structure of Matter
The Standard Model of Elementary Particles:
Particles and Antiparticles
Composite Particles:
Examples:
In the 1950s and 1960s, hundreds of other particles were discovered, many of which are very
unstable and so are not found in ordinary matter.
Elementary Particles: particles not made out of any smaller component particles; 2 elementary
particles of the same kind are completely identical.
3 Classes of Elementary Particles:
a)
b)
c)
Antiparticles –
All antiparticles are denoted with a line above the symbol
Examples:
If a particle has zero electric charge, the antiparticle can still be distinguished because of other
quantum numbers
Example:
Some particles are their own antiparticle and must be electrically neutral
Example:
Antimatter –
What happens when antimatter comes into contact with matter?
Which is predominant in today’s universe, matter or antimatter?
Quantum numbers:
Examples:
Quarks
Quarks:
There is solid experimental evidence for the existence of 6 types or “flavours” of quarks
Quark flavour
Symbol
Electric
Charge (e)
Rest Mass
(MeV c-2)
Spin
(h/2π)
Up
Down
Strange
Charmed
Bottom
Top
Note: A particle’s “spin” is a quantum property that is analogous to, but not actually, angular
momentum (L=mvr). All known particles have spin, which must be either an integral or halfintegral multiple of the quantity h/(2π).
Bosons:
Fermions:
So all quarks are….
Hadrons –
Meson –
Baryon –
The proton is a baryon made out of….
The neutron is a baryon made out of….
1. Show that the charge of a proton is 1.
2. Show that the charge of a neutron is 0.
3. What is the quark content and the charge of an antiproton?
4. What is the quark content and the charge of an antineutron?
Baryon number (B): Baryons are assigned a quantum number
What is the baryon number for:
protons and neutrons?
antiprotons and antineutrons?
quarks?
antiquarks?
So how does B of an antiparticle compare to B of the particle?
Law of Conservation of Baryon Number: Baryon number is conserved in ALL reactions.
_
Example: n +
p  n + p + p + p
5. Show that the reaction below cannot occur; if it did, the law of conservation of baryon
number would be violated.
--
p + p  π0 + π0 + n
Leptons
Leptons: the electron and its neutrino, the muon and its neutrino, and the tau and its neutrino,
as well as all of their antiparticles
There is solid experimental evidence for the existence of 6 types of leptons
Lepton
Symbol
Electric
Charge (e)
Rest Mass
(MeV c-2)
Spin
(h/2π)
Electron
Electron
neutrino
Muon
Muon
neutrino
Tau
Tau neutrino
Because of their spin, all leptons are…..
The leptons of each family or generation are assigned a lepton number. Since there are 3
families, there are 3 lepton numbers
Lepton number (L):
Type of Leptons and Lepton numbers: electron, muon, and tau lepton with numbers, Le, Lμ, and
Lτ
The 3 kinds of lepton number are individually conserved in all reactions as is the overall Lepton
number.
Le
Electron, e
Electron neutrino, νe
Muon, μMuon neutrino, νμ
Tau, τTau neutrino, ντ
Lμ
Lτ
6. Show that all lepton numbers are conserved in the following muon decay:
_
μ  e + νe + νμ
-
-
7. Do the following reactions conserve lepton number?
a. p+  e+ + π0
b. π0  e+ + μ-
_
c. τ  π + ντ
+
+
Exchange Particles
Exchange Particles: associated with interactions or forces; includes the photon (γ), the W
and Z bosons (W± and Z0), 8 particles called gluons, and the graviton.
There is solid experimental evidence for the existence of all exchange particles except the
graviton.
Gauge bosons: particles that mediate (or transmit) the force between a pair of particles
The 4 fundamental forces have different ranges and a different boson is responsible for each
force. The mass of the boson establishes the range of the force. The bosons carry the force
between particles.
The Higgs Particle or Higgs boson: a boson-like force mediator, but does not actually mediate
any force;
explains the mass of other particles, including the W and Z bosons;
not known if this particle is elementary;
was tentatively confirmed in 2013 to be positively charged and to have zero spin.
Leptons and quarks of the standard model can be arranged into 3 families or generations.
1st generation
2nd generation
3rd generation
Leptons
e-
Quarks
u
νe
d
μ-
s
νμ
c
τ-
b
ντ
T
Conservation Laws
In all reactions, the following quantities are always conserved:
1.
2.
3.
4.
5.
6.
Strangeness (S):
Strange quark has a strangeness of:
Strange antiquark has a strangeness of:
When is strangeness conserved? When strange particles are created in a strong interaction, but it
is not conserved when they subsequently decay through the weak interaction.
This is why strange particles are always produced in pairs. If 2 particles interact to produce a
strange particle, then a strange antiparticle must also appear.
In the following reactions, determine if Q, B, L, and S are conserved.
_
_
8. uud + ud  ds + uds
_
_
_
9. ds  ud + ud
_
10. uds  ud + uud
Interactions and exchange particles
Basic interaction vertices:
There are 4 fundamental forces or interactions in nature.
Interaction
Interaction acts on
Exchange Particle(s)
Relative Strength
Electromagnetic
Weak (nuclear)
Strong (nuclear)
Gravitational
The electromagnetic and weak nuclear interactions have been combined to form the electroweak
interaction.
How does the range of the exchange force relate to the mass of the exchange particle?
The shorter the range of the exchange force, the more massive the exchange particle.
The exchange particles for gravitation and the EM interaction both have infinite range, so must have zero
rest masses. Meanwhile the weak interaction has the heaviest boson because its range is the shortest. The
strong interaction has an exchange particle of intermediate mass.
At a fundamental level, particle physics views an interaction between 2 elementary particles in
terms of interaction vertices.
All interaction vertices should be read from left to right. The left hand side represents BEFORE
and the right hand side represents AFTER.
Usually the time axis goes to the right and the space or position axis goes upwards, but they can
be reversed.
Because arrows on antiparticles are drawn in the opposite direction to those on particles, we
sometimes say that these antiparticles travel backwards in time. This is just an expression! All
particles/antiparticles move forwards in time.
The electromagnetic interaction is the exchange of a virtual photon between charged particles.
The exchanged photon is not observable. This interaction vertex is show on the left below.
By rotating the arms of the vertices, the following interaction possibilities are generated.
Electric charge, baryon number, and lepton number are conserved at an interaction vertex.
The total Q, B, and L, going into a vertex must equal the total Q, B, and L leaving the vertex.
Feynman Diagrams
Feynman diagrams: pictorial representations or “spacetime diagrams” of particle interactions
that uses interaction vertices in order to build up possible physical processes.
Examples: below left is electron-electron scattering (the exchange of a virtual photon in the
interaction between electrons), below right is Higgs decay to two photons via top loop;
In this Feynman diagram below, an electron and positron destroy each other, producing a virtual
photon which becomes a quark-antiquark pair. Then one radiates a gluon.
Other examples of Feynman diagrams for basic interaction vertices.
To build a Feynman diagram,` the following are needed:
1)
2)
3)
Examples: Draw the Feynman diagram for the following processes:
11. e- + e+  e- + e+
12. e- + e+  γ + γ
13. beta minus decay in which a neutron decays into a proton, an electron, and a neutrino
14. positive beta (positron) decay: p  n + e- + νe
Quark Confinement (or confinement of colour):
Why is this so?
Suppose you wanted to remove a quark from inside a meson. The force between the quark and the
antiquark is constant no matter what their separation is. Therefore, the total energy needed to separate the
quark from the antiquark gets larger and larger as the separation increases. To free the quark completely
would require an infinite amount of energy, and so it is impossible. If you insisted on providing more and
more energy in the hope of isolating the quark, all that would happen would be the production of a
meson-antimeson pair and not free quarks.
Table of some baryons
Particle
Symbol
Proton
Neutron
Delta
Delta
Delta
Delta
Lambda
Sigma
Sigma
Sigma
Xi
Xi
Omega
p
n
Δ++
Δ+
Δ0
ΔΛ0
Σ+
Σ0
ΣΞ0
ΞΩ-
Quark
Content
uud
ddu
uuu
uud
udd
ddd
uds
uus
uds
dds
uss
dss
sss
Mass
MeV/c2
938.3
939.6
1232
1232
1232
1232
1115.7
1189.4
1192.5
1197.4
1315
1321
1672
Mean
lifetime (s)
Stable
885.7±0.8
6×10-24
6×10-24
6×10-24
6×10-24
2.60×10-10
0.8×10-10
6×10-20
1.5×10-10
2.9×10-10
1.6×10-10
0.82×10-10
Decays to
Unobserved
p + e- + νe
π+ + p
π+ + n or π0 + p
π0 + n or π- + p
π- + n
π- + p or πo + n
π0 + p or π+ + n
Λ0 + γ
π- + n
Λ0 + π0
Λ0 + πΛ0 + K- or Ξ0 + π-
Table of some mesons
Particle
Symbol
Charged Pion
Neutral Pion
Charged Kaon
Neutral Kaon
Eta
Eta Prime
π+
π0
K+
K0
η
η'
Antiparticle
π−
Self
K−
K0
Self
Self
Quark
Content
ud
uu - dd
ds
uu + dd - 2ss
uu + dd + ss
Mass
MeV/c2
139.6
135.0
493.7
497.7
547.8
957.6
Mean
lifetime (s)
2.60×10-8
0.84×10-16
1.24×10-8
5×10-19
3×10-21
Principal
decays
μ+ + νμ
2γ
μ+ + νμ or π+ + π0