Particle Physics RevisionParticle Physics Revision

The Whole LotThe Whole Lot

Announcements

Physics revision tonight.

Room C56 tomorrow morning.

Mock Exams next Wednesday.

Potential Divider revision next week.

Today’s Objectives

To revise:- Feynman Diagrams Exchange Particles

Conservation Rules Quark Composition

BaryonBaryon HadronHadron MesonMeson Pair ProductionPair Production AnnihilationAnnihilation LeptonLepton

Today’s Objectives

Explain what is meant by the term hadron.

Explain what is meant by the term lepton.

Recall the lepton numbers and baryon numbers for a variety of particles.

Cosmic Rays

High energy protons or small nuclei that travel through space from the stars.

When they enter the atmosphere, the collide with gas atoms.

This results in the creation of new particles and antiparticles with very short lifetimes and photons.

Reminder - Discovery of the Positron

Cosmic ray experiment.

Charged particles leave an ion trail as they pass through a super-cooled gas.

Causes vapour to condense to leave vapour trails – like jet plane.

Magnetic field causes charged particles to bend.

The particle bent the wrong way in a magnetic field.

Also discovered… The muon (μ-) – a negatively charge particle

with a rest mass over 200 times that of the electron

The pion (pi meson) – can be positively (π+) or negatively charged (π-) or have no charge (π0), rest mass greater than muon but less than proton.

The kaon (K meson) – which can also be positive (K+), negative (K-) or neutral (K0), rest mass greater than pion but less than proton.

Something strange going on…

Pions and kaons are both produced through the strong interaction.

However, kaons last much longer than Pions and decay through the weak interaction.

This led to kaons being referred to as strange particles we’ll talk about this later…

Hadrons vs. Leptons

Hadrons are particles and anti-particles that can interact through the strong interaction.

Examples are protons, neutrons, pions, kaons

Leptons are particles and anti-particles that do not interact through the strong interaction.

Examples are electrons, muons, neutrinos.

Can you feel the force?

Both hadrons and leptons can interact through the electromagnetic interaction if they are charged.

Hadrons can also interact through the strong interaction and leptons can also interact through the weak interaction.

Except when hadrons decay – that’s through the weak interaction (remember β-decay involves W boson).

Some quick questions...Some quick questions...

1. List these particles in the order of 1. List these particles in the order of highest mass:-highest mass:-

Electron, muon, neutron, K meson, Electron, muon, neutron, K meson, pions, protons.pions, protons.

2. What is a hadron?2. What is a hadron? 3. What is a lepton?3. What is a lepton? 4. Which quantities are conserved when 4. Which quantities are conserved when

a particle decays?a particle decays?

Classifying ParticlesClassifying Particles

Particle Physics Lesson 7Particle Physics Lesson 7

HomeworkHomework

Still no homework – need open book Still no homework – need open book papers to look over.papers to look over.

Which equation?

Known Values Value Equation

Rearranged

E λ=?

EK, f φ=?

λ f=?

v, u, a s=?

λ, m v=?

Which equation?

Known Values Value Equation

Rearranged

E λ=? E=hc/λ λ=hc/E

EK, f φ=? hf=φ+EK

φ=hf-EK

λ f=? c=fλ f=c/λ

v, u, a s=? v2=u2+2as

s=(v2-u2)/2a

λ, m v=? λ=h/mv v=h/mλ

Today’s ObjectivesToday’s Objectives

Recap yesterday’s lessonRecap yesterday’s lesson

Explain what is meant by the term baryon.Explain what is meant by the term baryon.

Explain what is meant by the term meson.Explain what is meant by the term meson.

Recall the lepton numbers and baryon numbers for a variety of particles.

Alpha and beta decayAlpha and beta decay

Rn22086

C146

AnswersAnswers

HePoRn 42

21684

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eeNC 01

147

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Accelerators

A TV tube accelerates electrons through a potential difference of about 5,000 V.

Stanford linear accelerator in California accelerates electrons through a p.d. Of 50 GeV.

The Large Hadron Collider is designed to accelerate charged particles to energies of over 7000 GeV.

Now smash them Now smash them together...together...

Total energy of particles before collision:-Total energy of particles before collision:- Rest energy + kinetic energy Rest energy + kinetic energy original original

particlesparticles

Total energy of particles after collision:-Total energy of particles after collision:- Rest energy + kinetic Rest energy + kinetic of the products of the products

Conservation of energy means that Total Conservation of energy means that Total energy of particles before collision = Total energy of particles before collision = Total energy of particles after collisionenergy of particles after collision

β− decayDecay of neutrons, which are slightly more energetic than a proton. 

Isolated protons are stable; isolated neutrons last about 10 minutes.

Some other decay modes…

K mesons decay into pi mesons, muons and antineutrinos, and antimuons and neutrinos.

Charged pi mesons decay into muons and antineutrinos, or antimuons and neutrinos.

Chargeless pion decays into high energy photons.

Some other decay modes…

Muons and antimuons decay into Muons and antimuons decay into electrons and antineutrinos, or electrons and antineutrinos, or positrons and neutrinos.positrons and neutrinos.

Note:-Note:-

The decays always obey the The decays always obey the conservation rules for energy, conservation rules for energy, momentum and charge.momentum and charge.

BaryonsBaryons

Hadrons come in two types:-Hadrons come in two types:-

Those which eventually contain Those which eventually contain protons in their decay products protons in their decay products BaryonsBaryons

Those which do not include proton in Those which do not include proton in their decay products their decay products Mesons Mesons

Particle Classification

Matter & Antimatter

Hadrons Leptons

Baryons Mesons

What question should be asked at each branch to classify the particles?

Particle Cards Sort the cards by mass (two column <1000

MeV/c2 on the left >1000 MeV/c2 ).

Why are their pairs of particles with the same mass?

What do you notice about relative position of leptons?

What do you notice about position of baryons?

What do you notice about position of mesons?

Some more quick Some more quick questions...questions...

What is a baryon?What is a baryon?

What is a meson?What is a meson?

What are the baryon and lepton What are the baryon and lepton numbers for the following?numbers for the following?

Proton, kaon, neutrino, pi meson, Proton, kaon, neutrino, pi meson, antimuon, neutron, anti proton, antimuon, neutron, anti proton, electron, antineutron.electron, antineutron.

Research Task some questions to research:

What is a positron? Who suggested it must exist? Who discovered it? What is a muon? When was it discovered? Who said, ‘Who ordered that?’ What is a neutrino? Who suggested it must exist? Who discovered it?

Today’s ObjectivesToday’s Objectives

To learn about lepton conservation To learn about lepton conservation rulesrules

Start practicing questions!Start practicing questions!

Leptons There are six particle-antiparticle pairs

known.  Leptons (Greek – “light thing” or “small coins”) are the smallest of the fundamental particles.  They have the following properties:

fundamental particles without structure

they interact by the weak interaction.  If they are charged, they interact by the electromagnetic interaction, but NOT the strong interaction.

Can these happen?Can these happen?

Why/Why not?Why/Why not?

Kppp

0 npK

Muon DecayMuon Decay

This happens:-This happens:-

But this doesn’t:-But this doesn’t:-

Why?Why?

ee

ee

The names of the leptons are:

Lepton Name

Lepton Symbol

Charge

Lepton Number

electron e- -1e Le = 1, Lμ= 0, Lτ = 0

electron neutrino

νe 0 Le = 1, Lμ= 0, Lτ = 0

muon μ- -1e Le= 0, Lμ = 1, Lτ = 0

muon-neutrino

νμ 0 Le= 0, Lμ = 1, Lτ = 0

tau τ- -1e Le = 0, Lμ= 0, Lτ = 1

tau neutrino ντ 0 Le = 0, Lμ= 0, Lτ = 1

Do these happen?Do these happen?

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Lepton Notes

charge and lepton number are conserved in all allowed lepton processes.

There are three categories of lepton number, Le, Lμ, and Lτ.  Each lepton has a lepton number, 0 or 1, in each category, and each antilepton has a number 0 or -1 in each category.  You need to know the lepton numbers.

Lepton Notes

Each particle has an antiparticle; for the electron, it is the positron, the muon the antimuon, and the tau, the antitau. 

We show the anti-particle either by an opposite charge (e+) or by putting a bar across the symbol.

Hadrons There are a very large number of particles that are

classified as hadrons, which are subdivided into two further classifications, the mesons, and the baryons. 

Hadrons interact by the strong, weak, and electromagnetic force.    

They are not fundamental particles but have a structure.

They have non-zero rest masses, about 1 GeV/c2 They have an associated value of charge, Q,and

baryon number B. Hadrons with zero baryon number are called

mesons; those with baryon number of 1 are called baryons. 

Mesons

These particles have a smaller rest mass than the baryons (and a lower rest mass than the tau lepton).  They have:

Zero baryon number. Short lifetimes. Antiparticles

Here are a few mesons:

Notice how short the lifetimes are of these mesons.

Name Symbol 

Q B 

Lifetime (s) Antiparticle

Pion π0 0 0 0.8 x 10-16 Itself

π+ 1 0 2.6 x 10-8 p-

Kaon K+ 1 0 1.2 x 10-8 K-

K0 0 0 8.9 x 10-11 K0

We should note the following:

Mesons have TWO quantum numbers that must be conserved in interactions.  The charge is denoted by Q, the baryon number by B.  Mesons have a baryon number of 0.

Mesons have a lepton number of 0.  This must be conserved in any interactions with leptons.

Here is a typical decay:

Notice the conservation of charge and baryon number.

Here are some more:

Show that this interaction can proceed:    

π+ μ+ + νm

Answer

Charge +1 ----> + 1 + 0  (Charge is conserved) (P)  

Baryon  0 ----> 0 + 0  (Baryon number is conserved) (P)  

Therefore it can proceed

Baryons

These are the heavyweights of particle physics, and include the familiar proton and neutron.

They are made up of three quarks They have quantum numbers such as

charge and baryon number, which must be conserved in interactions.

Let us look at the properties of the baryons:

Name Symbol

Q B 

Lifetime (s)

Antiparticle

Proton

p 1 

1 stable p

Neutron

n 0 1 898 n

Lambda

Λ0 0 1 2.6 × 10-

10

Λ0

Sigma Σ+ 1 1 0.8 × 10-

10

Σ+

Σ0 0 1 7.4 × 10-

20

Σ0

Σ- -1

1 1.5 × 10-

10

Σ-

Omega

Ω- -1

1 0.8 × 10-

10

Ω+

Typical Decays

The proton is the only stable baryon. 

All the others spontaneously decay, although the neutron within a nucleus is stable, apart from beta decay. 

The decay times are incredibly short, except the isolated neutron which takes about 8 to 10 minutes. 

Baryons decay to protons, either directly (Σ+ π+ + π0) or indirectly (W- Λ0 + K, then Λ0   π+ + π-). 

Mesons decay to photons or leptons.

Show that this decay is possible:

Λ0   π+ +   π- 

Answer

Charge 0 +1 + -1 (Charge is conserved) (P)  

Baryon  1 1 + 0 (Baryon number is conserved) (P)  

The interaction is possible.