particle physics by gjjur4356


                          October 2008

Yaakov (J) Stein
Chief Scientist
RAD Data Communications
Physics ?

Physics is the search for simplicity
Aristotelian physics held that there were 4 terrestrial elements
1. earth
2. fire
3. air
4. water
All materials under the sky are combination of several elements

Aristotle (and Democritus and Epicurus) further believed
    that matter is not infinitely indivisible
i.e. that there smallest units of matter (atoms)

All Aristotelian physics was derived from pure thought
(it is commonly held that Galileo invented the idea of experiments)

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From the quantitative study of chemistry (Lavoisier)
Dalton concluded that matter is made of atoms
For example - carbon and oxygen can combine in two ways
In one the mass ratio was 3:4 in the other 3:8
From this he concluded that
• the 2 combinations were 1:1 and 2:1 in terms of atoms
• an oxygen atom is 1 1/3 times heavier than a carbon one
By careful measurement he made a list of atomic weights A
(e.g. C has atomic weight 12 and O has atomic weight 16)
But how many different atoms were there ?

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By comparing chemical characteristics of different elements
Mendeleev came up with the periodic table

Here each element has a atomic number Z (serial number)
For example
• H has Z=1 A=1
• C has Z=6 A=12
• O has Z=8 A=16
• Cu has Z=29 A=64

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Complexity - not simplicity

So we have a nice picture of elements made up of atoms
And all materials made up of elements and thus of atoms

But there are many many different kinds of atoms
This is too complex !    Physics is the search for simplicity !

Perhaps the atoms themselves are made up of simpler units ?

Unfortunately, the table is monotonic in atomic weight A
   but not linear in A
so the atoms are not made up of Z smaller particles

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    Electrons and protons

The first elementary particle discovered was the electron
    via cathode rays (Thomson), oil drops (Millikan),
    and the photoelectric effect (Hertz)

What was the connection between electrons and atoms ?
After a series of scattering experiments Rutherford
      came up with the planetary atomic model
•     the atom was mostly empty
•     at the center was a very small nucleus
•     electrons circulate around the nucleus
•     since electrons are negative and the atom neutral
      the nucleus must be positive
In later experiments Rutherford proved that the nucleus
      was made up of protons (nuclei of H atoms)
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Scattering experiments

In a scattering experiment
• particles are used as projectiles
• other particles are targets
Low energy scattering is good to measure
   the cross-sectional area of the target
For example, Rutherford bombarded thin gold foil with alpha particles
most particles go through without deflection, so nucleii are very small

High energy scattering can break up the target

Very high energy scattering can create new particles

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Weak collisions are observed by using detectors
To observe new particles created in strong collisions
   we need a new tool

In 1911 Wilson invented the cloud chamber (supercooled gas)
While looking into a glass of beer in 1952
  Glaser came up with the bubble chamber (superheated liquid)
In both, tracks are left by all charged particles
By using a magnetic field one can determine charge and mass

Today there are many sophisticated sensors
   and many Israeli specialists in this space

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Bubble chamber tracks

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Isotopes are the same element (same Z)
   but different atomic weights
So there must be something in the nucleus other than the proton
This also helped understand what kept the nucleus together
   so Rutherford invented the neutron
   which was found experimentally by Chadwick in 1932
Neutrons and protons experience a strong force
                                                        e -r/d
   when they are very close
that overcomes the electric repulsion of the protons      r2
Beta decay changes Z without changing A
   and the beta particles turn out to be electrons
So a neutron can change into a proton by ejecting an electron
   and the force responsible is called the weak force
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Let's take a short rest from matter and look into forces

4 different types of forces were known to classical physics
1.   contact
2.   gravity
3.   electric     action at a distance
4.   magnetic
Then Maxwell unified the electric and magnetic fields
Since a changing E field builds a changing B field and vice versa
   the field can build itself and travel far from sources
   the speed turns out to be the speed of light !
So the field is more fundamental than the action at a distance

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Today we speak of interactions between particles

There are four known interactions (in order of decreasing strength)
•   strong (hadrons are particles that feel the strong interaction)
•   electromagnetic (charged particles feel it)
•   weak (hadrons and leptons feel it)
•   gravitation (all particles feel it)

Theories that further unify these are called unified field theories
Everyone wants a Theory of Everything (ToE) that explains all 4

In quantum theory all interactions are mediated by bosons

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In 1932, three particles were known
• electron (negative, light)
• proton (positive, heavy)
• neutron (neutral, heavy)
In 1928, Dirac's came up with the first relativistic quantum theory
It predicted an antiparticle for each particle
In 1933 Anderson discovered a positron (antielectron)
   in a bubble chamber picture

So we need to add
• positron
• antiproton
• antineutron
This is a nice simple picture !
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In 1923 Einstein predicted
  that electromagnetic fields were made up of photons
Later relativistic quantum theories showed him to be correct
The photon was the first boson discovered

Photons have no mass, and thus travel at the speed of light
Photons have no charge and are their own antiparticles
But photons do have energy

The frequency of EM radiation is related to the photon energy
  through the fundamental relation E = h u

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Quantum numbers

According to quantum theory
  all elementary particles have certain characteristics
These include its mass, charge, and spin
Later new quantum numbers needed to be added
In interactions, characteristics are ruled by conservation laws

Table of particles we know so far :

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Fermions and Bosons

Classical particles obey Maxwell-Boltzmann statistics
  but quantum particles are indistinguishable
In quantum mechanics particles are described by a field 
The probability of finding a particle is ||2
Indistinguishability means |(1) F(2)|2 = | F(1) (2)|2
which can either mean
 (1) F(2) = F(1) (2) Bose-Einstein statistics (bosons)
 (1) F(2) = - F(1) (2) Fermi-Dirac statistics (fermions)
Note that two Fermions can't be in the same state (Pauli principle)
Spin-statistics theorem -
 fermions have half integral spin
 bosons have integral spin

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Enrico Fermi observed that in beta decay
   not all the expected energy was in the emitted electron
It was later more directly observed
He concluded that some other particle took some of the energy
  and called it the neutrino (small neutral particle)
The neutrino is almost massless
  and only reacts via the weak interaction
And we also need an antineutrino !

Later it was discovered that there are different types of neutrino

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While observing byproducts of cosmic radiation in 1936
  Anderson observed a very heavy electron (mass about 100 MeV)
Since its mass was between
• the light electron (lepton = light) and
• the proton (baryon = heavy)
he called it a meson
But today that name is used for other particles
  and we call this negatively charge particle the muon
  or more precisely the mu minus
  and the muon is known to be a lepton not a meson
Its antiparticle is the mu plus

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Yukawa's theory of the strong force predicts a boson
  with intermediate mass - the meson

At first the muon was thought to be that particle
  but it turned out to be a fermion
  and not to participate in the strong force

In 1947 the pi meson (or simply pion) was discovered
  with mass about 140 MeV

There are three types - pi zero, pi plus, and pi minus

Later other mesons were predicted and discovered - K and eta

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    So what Fermions do we have ?
Leptons :
•   electron, positron
•   electron neutrino, electron antineutrino
•   mu minus, mu plus
•   muon neutrino, muon antineutrino
•   tau minus, tau plus
•   tau neutrino, tau antineutrino
Mesons :
• pi zero, pi plus, pi minus
• kay zero, antikay zero, kay plus, kay minus
• eta
Baryons :
•   proton, antiproton
•   neutron, antineutron
•   lambda, antilambda
•   sigma zero, sigma plus, sigma minus and their three anti-s
•   xi zero, antixi zero, xi minus, antixi plus
•   omega minus, antiomega plus

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 So what Bosons do we have ?

Gauge bosons :
• photon (charge 0) - electromagnetic interaction
• gluon (g) (charge 0) - strong interaction
• W (charge -1) and antiW (charge +1) - weak interaction
• Z (charge 0) - weak interaction
• graviton (?) - gravity

Higgs boson - in electroweak theory creates mass
And many more are unconfirmed as yet …
• X
        grand unified theories
• Y
• W-prime, Z-prime, …

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The eight-fold way

The Fermion picture is no longer simple
In the early 1960s, Gellmann and Neeman (independently)
  observed new symmetries that connected baryons/mesons

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This observation led to a new picture, called the standard model
In the standard model, baryons and mesons are composite
Quarks com in 6 flavors -
  up, down, charm, strange, top, and bottom
There are thus 6 particles and 6 antiparticles (all are spin ½)

Due to color confinement, quarks never exist as free particles
Instead, they form hadrons - particles that feel the strong interaction
• baryons are made up of 3 quarks
• mesons are made of one quark and one antiquark
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 Color confinement

Quarks can be either red, green, or blue
Antiquarks can be either antired, antigreen, or antiblue
Only combinations with resulting color white attract
Hadrons are made up of quarks
  such that the resulting color is zero
  and the resulting charge is always an integer
The model explains all the properties of the baryons and mesons
For example,
• proton = u u d (charge +1)
• neutron = u d d (charge 0)
• lambda = u d s (charge 0)
• pi-plus = u anti-d (charge +1)
• kay zero = d anti-s (charge 0)

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A simple picture again !

6 quark types (u d c s t b)
6 lepton type (e e-neutrino mu mu-neutrino tau tau-neutrino)
4 gauge boson types (photon gluon Z W)
  and maybe one Higgs !

Detector from the
LHC (Geneva)

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