J4 Leptons and th e standard model Particle Physics J4 Objectives State the three family structure of quarks and leptons in the standard model State the lepton number of the leptons in each family ID: 330706
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Slide1
Particle Physics
J4 Leptons and th
e standard modelSlide2
Particle Physics
J4 Objectives
State the three family structure of quarks and leptons in the standard model
State the lepton number of the leptons in each family
Solve problems involving conservation laws in particle reactions
Evaluate the significance of the Higgs particle (boson)Slide3
Probing deep into matter
Quarks Leptons and standard model
Quarks + Leptons = The standard model
It consists of three families where each family appears to be a copy of the one before it and heavier.
A number of decays which appear possible do not occur
Energy, charge, momentum and angular momentum… fineIt is for this reason that conservation of lepton number was introduced where each family has it’s own Lepton number and each antiparticle the opposite.Slide4
Leptons
Leptons
e
n
e
Quarks
d
u
m
n
m
t
n
t
s
c
b
t
Family 1 (light)
Family 2 (heavy)
Family 3 (v. heavy)
Leptons interact via the weak and electromagnetic force not the strong force!Slide5
Particle Physics
Leptons
In interactions
Family
lepton number is conserved
Which of these reactions conserve lepton number ?ne + m-
e
-
+
n
m
n
e
+ n p +
m
-
Slide6
Particle Physics
A
muon
decays in 2.2 x 10
-6
seconds(weak force decay)Muons can replace electrons in atoms!Tau particle (1974) decays in 3.3 x 10-13 secondsTau mass 3.5 million x electron mass and has only been seen in labs!We don’t know why we have these particles, their discovery came as a surprise!Slide7
Probing deep into matter
Conservation laws
In all reactions the following quantities are ALWAYS conserved!
Energy
Momentum
Angular momentumElectric ChargeBaryon NumberColourLepton NumberSlide8
Probing deep into matter
The Higgs Particle –
The easily confused should look away now
Neutral, spin zero key part of the standard model.
Estimated mass approximately 200
GeV/c2Both fermilab and the LHC are looking for it, nothing yetIt is linked to massPhotons, W and Z (electroweak bosons) should have zero mass according to the mathematically (symmetry) theory.The Higgs mechanism allows the W and Z to have mass (many others reached a similar conclusion)Slide9
Probing deep into matter
The Higgs Particle
It required the introduction of a new particle which, when it interacts with the particles in the standard model, leads to mass
It is the quantum of a Higgs field (like a photon and an electromagnetic field)
In the later case zero field leads to zero energy, in the former there is no zero value in the lowest energy state!
Think of a ball being pulled through a liquid, effectively the mass increases