Yaakov (J) Stein

1
ParticlePhysics
October 2008

• Yaakov (J) Stein
• Chief Scientist
• RAD Data Communications

2
Physics ?

• Physics is the search for simplicity
• Aristotelian physics held that there were 4
  terrestrial elements
• earth
• fire
• air
• 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)

3
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 34 in the other 38
• From this he concluded that
• the 2 combinations were 11 and 21 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 ?

4
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 Z1 A1
• C has Z6 A12
• O has Z8 A16
• Cu has Z29 A64

5
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

6
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

8
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
• 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

11
Forces

• Let's take a short rest from matter and look into
  forces
• 4 different types of forces were known to
  classical physics
• contact
• gravity
• electric
• 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

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)
• 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 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 !

14
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

18
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
• 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
• Y
• W-prime, Z-prime,

grand unified theories
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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)