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Cosmic rays and particle generation
Cosmic rays consist of high energy protons and
nuclei emitted from stars including the Sun.
The protons strike gas atoms in the upper
atmosphere and produce new short livedparticles and antiparticles.
These new particles can be detected at ground
level by cloud chambers and other detectors.
The first new particles to be detected were
muons (1937), pions (1947) and kaons (1947).
http://en.wikipedia.org/wiki/Cosmic_radiationhttp://en.wikipedia.org/wiki/Cloud_chamberhttp://en.wikipedia.org/wiki/Cloud_chamberhttp://en.wikipedia.org/wiki/Cosmic_radiation8/12/2019 As h 11b Quarks&Leptons
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AcceleratorsThese enable more controlledconditions than occur withcosmic radiation interactions.
High voltages are used toaccelerate charged particles
(electrons, positrons, protonsand antiprotons) to near lightvelocities.
The particles are made tocollide into a stationary targetor head-on with each other.
The energy produced in thesecollisions can be used tocreate other particles.
The largest linear accelerator is atStanford in California which uses a50 000 million volts (50 GV) toaccelerate electrons over adistance of 3km to an energy of 50GeV.
The largest circular accelerator isthe Large Hadron Collider (LHC)at CERNnear Geneva. This usesa circular tube of circumference27km within which protons and
antiprotons are made to makehead-on collisions at energies ofup to 7000 GeV
http://en.wikipedia.org/wiki/Particle_acceleratorhttp://en.wikipedia.org/wiki/Stanford_Linear_Accelerator_Centerhttp://en.wikipedia.org/wiki/Stanford_Linear_Accelerator_Centerhttp://en.wikipedia.org/wiki/Large_Hadron_Colliderhttp://public.web.cern.ch/Public/Welcome.htmlhttp://public.web.cern.ch/Public/Welcome.htmlhttp://en.wikipedia.org/wiki/Large_Hadron_Colliderhttp://en.wikipedia.org/wiki/Stanford_Linear_Accelerator_Centerhttp://en.wikipedia.org/wiki/Stanford_Linear_Accelerator_Centerhttp://en.wikipedia.org/wiki/Particle_accelerator8/12/2019 As h 11b Quarks&Leptons
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Hadrons and leptons
HADRONSare particles that interact through the stronginteraction
examples: protons, neutrons, pions and kaons
LEPTONSare particles that do not interact through thestrong interaction
examples: electrons, positrons, muons and neutrinos
Leptons interact through the weak interaction.
All charged particles interact through the electromagnetic interaction
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Baryons and mesons
Baryonsare hadronsthat are protons orparticles that eventuallydecay into protons.
The proton is the onlystable baryon.
Neutrons are baryonsbecause they decay intoprotons with a half-life of12 minutes. Other
baryons have muchshorter half-lives.
Baryons are also calledfermions
Mesonsare hadronsthat do not includeprotons in their decayproducts.
All mesons are unstable Examples of mesons
include pions (mesons)and kaons (K mesons)
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Pions or mesons
Pionsor mesonscan be positivelycharged (+), negatively charged (- ) oruncharged (0).
They have rest masses between themuon and proton (about 300 x theelectron).
They are all very unstable and decay bythe weak interaction.
+decays into an antimuon and a neutrino
- decays into an muon and an antineutrino0decays into two high energy photons
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Kaons or K mesons
Kaonsor K mesonscan be positively charged (K+),negatively charged (K- ) or uncharged (K0).
They have rest masses greater than pions but less than aproton (about 1000 x the electron).
They are all unstable and decay by the weak interactionfar more slowly than pions.
This relatively slow rate of decay (about 10 - 10 s) wasunexpected and led to these particles being called strangeparticles.
Kaons decay into pions, muons and neutrinos.
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Strangeness
This is a property possessed by some hadronsthat has been assigned in order to explain why
some particle interactions take place more
slowly than others or do not occur at all.
The K +mesonwas the first particle with
strangeness to be discovered and has been
assigned a strangeness number (S) of +1
Strangeness is a property, like charge, that isreversed in an antiparticle. Therefore the
K -meson has strangeness of1.
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Baryon number (B)
Conservation of baryon number
In all interactions the total baryon number is conserved.
You are expected to recal l baryon numbers in
the AS exam inat ion !
antibaryons:(e.g. antiproton)
non-baryons(e.g. mesons and leptons)
baryons:(e.g. protons and neutrons) + 1
- 1
0
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Leptons
Leptons are believed to be fundamental
particles. This means that they do not decay
into any other particles except leptons.
There are three families of leptons andantileptons. In order of increasing rest mass:
1. electrons and their neutrinos
2. muons and their neutrinos
3. tauons and their neutrinos
Only electrons and their neutrinos are stable.
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Muons (-)
These are negatively charged like electrons but
about 200 times more massive
There also exists the positively charged
antimuon (+) Muons are unstable and decay by the weak
interactioninto electrons and antineutrinos
Antimuons decay into positrons and neutrinos
Tauons are heavier versions of muons (about 20 x more massive).
You are not expected to know about these in AS physics
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Lepton and antileptons
increasing mass
leptons
electron e - muon - tauon -
electron
neutrino e
muon
neutrino
tauon
neutrino
antileptons
positron e + antimuon + antitauon +
electron
antineutrino e
muon
antineutrino
tauon
antineutrino
leptons
anti-leptons
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Lepton number (L)
Conservation of lepton number
In all interactions the total lepton number isconserved.
Lepton numbers are given in the AS examination
leptons: (electrons, muons and theirneutrinos)
+ 1
antileptons: (positrons, antimuons
and their antineutrinos)
- 1
non-leptons (e.g. hadrons and photons) 0
leptons:(electrons, muons andtheir neutrinos)
antileptons:(positrons, antimuons
and their antineutrinos)
non-leptons(e.g. hadrons andphotons)
+ 1
- 1
0
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Answers:
particle matter orantimatter hadron orlepton
electromagnetic
interaction ? baryon, mesonor neither
proton matter hadron yes baryon
positron antimatter lepton yes neither
K0 matter hadron no meson
antineutrino antimatter lepton no neither
muon matter lepton yes neither
neutron matter hadron no baryon
Complete:
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Particle overview
matter and antimatter
hadrons leptons
baryons mesons
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Interaction conservation rules
All interactions must conserve:
ENERGY
CHARGE LEPTON NUMBER & TYPE
BARYON NUMBER
and with strong interactions only:STRANGENESS
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Energy and matter
Accelerators such as the Large Hadron Collider increase the kineticenergy of particles involved in collisions.
The total energy of the particles before the collision is equal to the rest
plus kinetic energies of the particles.
Likewise the total energy of the particles present after the collision.
Using the conservation of energy:
rest energy = total energy + kinetic energy
of the products before of the products
The smaller the final kinetic energy the better. This allows more energy
to go into particle creation. Head-on collisions such as those with the
LHC given the minimum final kinetic energy.
http://en.wikipedia.org/wiki/Large_Hadron_Colliderhttp://en.wikipedia.org/wiki/Large_Hadron_Collider8/12/2019 As h 11b Quarks&Leptons
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Question
A proton and an antiproton, both with 1.5 GeV of kineticenergy, make a head-on collision.
If the collision only produces K mesons and photons what isthe maximum number of K mesons that could be produced?
[Take the rest energy of a proton = 938 MeV;
K meson = 490 MeV]
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Lepton type conservation
The total electron or muon lepton numbers must also
be conserved in any interaction.
Example:
The total lepton number on both sides is the same = +1
However, in terms of muons and muon antineutrinos the
number changes from +1 to1
In terms of electrons and electron neutrinos the numberchanges from 0 to +2
Therefore this interaction is not permitted
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Allowed interactions
Beta-minus decayLepton number before
= 0
Lepton number after
= 0 + 11 = 0
Muon decay
Lepton number before
= + 1Lepton number after
= + 11 + 1 = + 1
n p + e-+ ve
- e- + ve+ v
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Questions
1. Write an equation for beta-plus decay:
2. Write an equation for +decay:
3. Can the interaction shown below occur?
Yes, lepton numbers: +1 +0 = 0 +1
and it also conserves electron lepton type number
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Sigma particlesThese are all baryons (B= +1) with a strangeness, S of -1.
sigma-plus +
charge = + 1 (equals proton charge)
sigma-zero 0
charge = 0
sigma-minus
charge = - 1
(Note: This is NOTthe antiparticle of +)
Like all baryons they eventually decay into protons for example: n + -, after which the neutron decays into a proton
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Conservation of strangeness
Strangeness is always
conserved in all strong
interactions (no leptons
involved).
Strangeness is not
always conserved in
weak interactions.
Strangeness conserved:
S: 0 + 0 -1 + 1
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Can the following interactions occur ?
-
+ n K
-
+ 0
1.
2.
3.
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Further interactions to check
4.
5.
6. e- + e+ p + p + ve + ve
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Quarks
Baryons and mesons are not fundamentalparticles of matter (unlike leptons) but are
composed of smaller particles called
quarks. Quarks feel the STRONG force (unlike
leptons).
Quarks are never found in isolation butalways in pairs or triplets.
http://en.wikipedia.org/wiki/Quarkshttp://en.wikipedia.org/wiki/Quarks8/12/2019 As h 11b Quarks&Leptons
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The quark model of hadrons
BARYONSConsist of three quarks:
ANTIBARYONS
Consist of three antiquarks:
MESONS
Consist of a quark-antiquark pair:
q q q
q q q
q q
This is called the Standard Model.
http://en.wikipedia.org/wiki/Standard_Modelhttp://en.wikipedia.org/wiki/Standard_Model8/12/2019 As h 11b Quarks&Leptons
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Types of quark
There are six types (flavours) of quarks: up (u);down (d);charm (c); strange (s);top (t) andbottom (b)
Charm and top quarks are increasingly moremassive versions of the up quark.
Strange and bottom quarks are increasinglymore massive versions of the down quark.
There are also six corresponding antiquarks(anti-up, anti-down etc..).
Only knowledge of the up, down and strangequarks is required for AS physics.
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Properties of quarkssymbol relative
mass
charge
proton = 1
baryon
number
strangeness
up u 1 + + 0
down d 2 - + 0
strange s 40 - + -1
charm c 600 + + 0
top t 90 000 + + 0
bot tom b 2000 - + 0
1
2
40
+
- +
+
+
-
0
0
- 1
up u
down d
strange s
charm c 600 + + 0
bot tom b 2000 - + 0
top t 90 000 + + 0
Antiquarks have the same mass but opposite
charge, baryon number and strangeness.
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Protons and neutrons
Protons consist of two up and
one down quark (uud)
Charge = + + - = +1
Baryon number = + + + = +1
Neutrons consist of one up and
two down quarks (udd)
Charge = + - - = 0
Baryon number = + + + = +1
u
d
u
u
d
d
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Antiprotons and antineutrons
Antiprotons consist of two upantiquarks and one downantiquark (uud)
Charge = - - + = -1
Baryon number = - - - = -1
Antineutrons consist of one upantiquark and two downantiquarks (udd)
Charge = - + + = 0
Baryon number = - - - = -1
u u
d
u
d
d
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Sigma particle questionInsert the missing information below:
sigma-plus +
Quarks: uus
Charge = + + - = + 1
B = + + + = + 1
S = 0 + 01 =1
sigma-zero 0
Quarks: uds
Charge = + - - = 0
B = + + + = + 1
S = 0 + 01 =1
sigma-minus
Quarks: ddsCharge = - - - = 1
B = + + + = + 1
S = 0 + 01 =1
anti-sigma-plus +
Quarks: u u s
Charge = - - + = 1
B = - - - = 1
S = 0 + 0 + 1 = + 1
anti-sigma-zero 0
Quarks: u d s
Charge = - + + = 0
B = - - - = 1
S = 0 + 0 + 1 = + 1
anti-sigma-minus
Quarks: d d sCharge = + + + = + 1
B = - - - = 1
S = 0 + 0 + 1 = + 1
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Charged mesons
These are made up
of a quark and an
antiquark pair made
of up and downquarks and
antiquarks
+= ud
charge = + + = +1
B = + - = 0
-= ud
charge = - - = -1
B = - + = 0
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Uncharged mesons
These are made upof a quark and anantiquark pair of up,down or strangequarks
0
= uucharge = + - = 0
B = + - = 0
S = 0 + 0 = 0
OR 0= dd
charge = - + = 0
B = + - = 0
S = 0 + 0 = 0
OR 0= ss
charge = - + = 0B = + - = 0
S = -1 + 1 = 0
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K mesons
These combinations a strange quark along withan up or down quark
normal particles:
K+= us (S = 0 + 1 = +1)
K0= ds (S = 0 + 1 = +1)
antiparticles:
K-= su (S = - 1 + 0 = - 1)K0= sd (S = - 1 + 0 = - 1)
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Meson summary diagram
(du)
+
(ud)
K +
(us)
K
(su)
K 0
(ds)
K 0
(sd)
0
(uu, dd or ss)
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Answers
name symbol quarks charge B S
proton p u u d + 1 + 1 0
neutron n u d d 0 + 1 0
pion
plus
+ u d + 1 0 0
omegaminus
- s s s - 1 + 1 - 3
neutral
lambda
0 u d s 0 + 1 - 1
Complete:
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Quark interactions
Quarks undergo both strongand weakinteractions.
In stronginteractions the numbers and types
of quark are conserved.In weakinteractions the overall number of
quarks can change and individual quarks
can change from one type to another.
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Example of a STRONG quark interaction:
Quark annihilation
In quark terms the interaction is:
An up quark and an up antiquark annihilate each other in a strong forceinteraction producing two pions with the release of radiation.
ENERGY: mass-energy is converted into kinetic energy and photons
CHARGE: + 11 = + 11 conserved
LEPTON NUMBER: strong interactionno leptons involvedBARYON NUMBER: + 11 = 0 + 0 conserved
STRANGNESS NUMBER: 0 + 0 = 0 + 0 conserved
This reaction can occur.
u u d + u u d u d + u d
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Examples of a WEAK quark interaction:1. Beta-minus decay
With beta-minus decay a
neutron (udd) changes into
a proton (uud).
This can be represented on
a Feynman diagram as a
down quark changing into
an up quark.
Note: An early version of the text
book shows, incorrectly, an electron-
neutrino.
d u + -+ e
d
u-
ve
time
W-
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Strong or weak interaction ?
Do any of the following occur ?
1) leptons involved
2) total strangeness changes
3) quarks change type
4) W+or W-exchange particle
NO
YES
STRONGINTERACTION
e.g.: p + p ++ -
when a proton undergoes
annihilation with anantiproton to produce two
pions
WEAK INTERACTIONExample: Beta decay: n p + e -+ e