lecture ii modern physics. atomic particles atoms are made of protons, neutrons and electrons ...
Post on 21-Dec-2015
221 Views
Preview:
TRANSCRIPT
Atomic Particles
Atoms are made of protons, neutrons and electrons
99.999999999999% of the atom is empty space Electrons have locations
described by probability functions
Nuclei have protons and neutrons
nucleus
mp = 1836 me
Leptons
An electron is the most common example of a lepton – particles which appear pointlike
Neutrinos are also leptons There are 3 generations of leptons, each has
a massive particle and an associated neutrino Each lepton also has an anti-lepton (for
example the electron and positron) Heavier leptons decay into lighter leptons
plus neutrinos (but lepton number must be conserved in these decays)
Types of Leptons
Lepton Charge
Mass (GeV/c2)
Electron neutrino
0 0
Electron -1 0.000511
Muon neutrino
0 0
Muon -1 0.106
Tau neutrino
0 0
Tau -1 175
Quarks
Experiments have shown that protons and neutrons are made of smaller particles
We call them “quarks”, a phrase coined by Murray Gellman after James Joyce’s “three quarks for Muster Mark”
Every quark has an anti-quark
Modern picture of atom
Types of Quarks
Flavor Charge Mass (GeV/c2)
Up 2/3 0.003
Down -1/3 0.006
Charm 2/3 1.3
Strange -1/3 0.1
Top 2/3 175
Bottom -1/3 4.3
Quarks come in three generations
All normal matter is made of the lightest 2 quarks
Combining Quarks
Particles made of quarks are called hadrons
3 quarks can combine to make a baryon (examples are protons and neutrons)
A quark and an anti-quark can combine to make a meson (examples are pions and kaons)
proton
meson
Fractional quark electromagnetic charges add to integers in all hadrons
Color charges
Each quark has a color charge and each anti-quark has an anti-color charge
Particles made of quarks are color neutral, either R+B+G or color + anti-color
Quarks are continually changing their colors – they are not one color
Atomic Forces
Electrons are bound to nucleus by Coulomb (electromagnetic) force
Protons in nucleus are held together by residual strong nuclear force
Neutrons can beta-decay into protons by weak nuclear force, emitting an electron and an anti-neutrino
F = k q1 q2
r2
n = p + e +
Protons and neutrons are made up of quarks bound together by gluons.
Like charges repel, so why does the positive charge within a proton not cause the proton toexplode?
The (Coulomb) repulsion isdefeated by a new force:The STRONG force.
Fundamental Forces
Gravity and the electromagnetic forces both have infinite range but gravity is 1036 times weaker at a given distance
The strong and weak forces are both short range forces (<10-14 m)
The weak force is 108 times weaker than the strong force within a nucleus
The Uncertainty Principle Classical physics
Measurement uncertainty is due to limitations of the measurement apparatus
There is no limit in principle to how accurate a measurement can be made
Quantum Mechanics There is a fundamental limit to the accuracy of a
measurement determined by the Heisenburg uncertainty principle
If a measurement of position is made with precision x and a simultaneous measurement of linear momentum is made with precision p, then the product of the two uncertainties can never be less than h/2
xx p
Force Carriers
Each force has a particle which carries the force and is unaffected by it
Photons carry the electromagnetic force between charged particles
Gluons carry the strong force between color charged quarks
Force Carriers
Separating two quarks creates more quarks as energy from the color-force field increases until it is enough to form 2 new quarks
Weak force is carried by W and Z particles; heavier quarks and leptons decay into lighter ones by changing flavor
Forces are mediated by particles
Photons mediate electric and magnetic forces. (Faraday and Ampère demonstrated that electric and magnetic forces were different manifestations of the same “electromagnetic” force.)
ee
ee
There is also the weak force
It is responsible for the process by which two protons “fuse” together in the core of the sun.
It is “carried” by the W and Z particles.
Neutrons transform to protons via beta decay. It is a result of the weakforce.
eenppp
Gravity is the only other force.
It so weak as to be negligible in particle physics experiments.
Einstein’s “General Theory of Relativity” superseded Newton’s Theory of Gravity in 1915.
An “ultimate” theory should explain how gravitons mediate gravity…….?
Unifying Forces
Weak and electromagnetic forces have been unified into the “electroweak” force They have equal strength at 10-18 m Weak force is so much weaker at larger distances
because the W and Z particles are massive and the photon is massless
Attempts to unify the strong force with the electroweak force are called “Grand Unified Theories”
There is no accepted GUT at present
Gravity
Gravity may be carried by the graviton – it has not yet been detected
Gravity is not relevant on the sub-atomic scale because it is so weak
Scientists are trying to find a “Theory of Everything” which can connect General Relativity (the current theory of gravity) to the other 3 forces
There is no accepted Theory of Everything (TOE) at present
Electric Magnetic
ElectromagneticWeak
StrongElectroweak
Gravity
Theory of Everything?
Standard Model
Ampere, Faraday, Maxwell
Glashow, Salam, Weinberg
The Standard Model
The weak and electromagnetic forces were unified by Glashow, Weinberg &Salam. Electroweak force
GWS also explained how to incorporateQCD, the model of the strong force.
Their model defines the laws for all known interactions except gravity.
Spin
Spin is a purely quantum mechanical property which can be measured and which must be conserved in particle interactions
Particles with half-integer spin are “fermions” Particles with integer spin are “bosons”
* Graviton has spin 2
Quantum numbers
Electric charge (fractional for quarks, integer for everything else)
Spin (half-integer or integer) Color charge (overall neutral in particles) Flavor (type of quark) Lepton family number (electron, muon or tau) Fermions obey the Pauli exclusion principle –
no 2 fermions in the same atom can have identical quantum numbers
Bosons do not obey the Pauli principle
Standard Model
6 quarks (and 6 anti-quarks) 6 leptons (and 6 anti-leptons) 4 forces Force carriers (, W+, W-, Zo, 8 gluons, graviton)
Some questions
Do free quarks exist? Did they ever? Why do we observe matter and almost no antimatter if
we believe there is a symmetry between the two in the universe?
Why can't the Standard Model predict a particle's mass? Are quarks and leptons actually fundamental, or made
up of even more fundamental particles? Why are there exactly three generations of quarks and
leptons? How does gravity fit into all of this?
top related