Transcript particle physics

Particle
Physics
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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Atoms
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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Chemistry
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
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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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Sensors
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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Nuclei
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
when they are very close
that overcomes the electric repulsion of the protons
e -r/d
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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Forces
Let's take a short rest from matter and look into forces
4 different types of forces were known to classical physics
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contact
gravity
electric
magnetic
action at a distance
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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Interactions
Today we speak of interactions between particles
There are four known interactions (in order of decreasing strength)
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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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Antiparticles
In
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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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Photon
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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Neutrinos
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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Muons
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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Pions
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 :
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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 :
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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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Quarks
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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