The Standard Model of Particle Physics

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The Standard Model of Particle Physics

• Topics
• Classically known particles
• Cosmic forces
• The Heisenberg Uncertainty Principle
• Forces mediated by virtual particles
• The Particle Zoo
• The Standard Model.

Motivation What are the forces of the
Universe? What are the particles of the Universe?

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Basic atomic particles

• Baryons (high mass particles, spin ½)
• Proton positive charge, m1.007 u
• Neutron zero charge, m1.008 u.
• Lepton (low mass particle, spin ½)
• Electron negative charge, m0.000548 u
• Boson (spin1)
• Photon the particle of energy.
• Example
• 13C 6 protons, 7 neutrons (13-67), 6
  electrons.
• This system was beautiful, simple, and complete.
• This explained all the elements, all of matter,
  all of energy, for many years.

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Also, there are four forces

• Electromagnetismclassic knowledge
• Known since ancient times.
• Originally viewed as two separate force fields
  electricity and magnetism.
• Unified by Maxwells laws.
• Electromagnetism affects particles that have
  positive or negative charge, such as protons and
  electrons.
• Preview facts (spoilers!)
• Maxwell discovered that photons are related to
  electromagnetism, but it goes beyond thatit will
  be seen that photons are carriers for the
  electromagnetic force.

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Also, there are four forces

• Gravityclassic knowledge
• Known since ancient times.
• Originally described by Newtons law of gravity,
  acting as an instantaneous force.
• Gravity was long known to affect all particles
  that have mass.
• An enormous source of frustration to Einstein, in
  that it violated Special Relativity (it exceeded
  the speed of light).
• Extremely weak, but since there are no negative
  gravity charges, this force adds up over
  distance.
• Preview facts (spoilers!)
• Einstein ultimately expanded gravitys influence,
  to say that gravity affected energy, such as
  photons, the same way it affects matter.

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Also, there are four forces

• Strong forceclassic knowledge
• Quantified in 1934.
• Only important within atoms, affecting
  protons/neutrons.
• Drops off exponentially.
• Changes in energy stored in strong force releases
  energy.
• Preview facts (spoilers!)
• Affects all hadrons (i.e., quark-matter).
• Really, just a side-effect of the strong
  interaction (which is also known as the color
  force).

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Also, there are four forces

• Weak forceclassic knowledge
• Quantified in 1930s.
• Best known for ß- decay.
• n ? p e- ?e
• 3H ? 3He e- ?e
• (12.3 year half-life)
• Weakest of the forces, except for gravity.
• Preview facts (spoilers!)
• To be united with the electromagnetic force.
• Affects hadrons and leptons, including neutrinos.

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Heisenberg uncertainty principle

• To understand forces of the Universe, we will
  have to digress slightly.
• Recall the de Broglie wavelength for an electron
  
• ?h/p ? ?ph
• With some work (Heisenberg, 1927), this can be
  used to derive something remarkable, specifically
  the uncertainty principle
• ?x ?p h/2 where h h/2p
• This means that, on a ultra-microscopic level,
  there is a limit on knowledge accuracy. In order
  to identify the position (?x) of an object very
  well, you will lose accuracy on how well you can
  know its momentum (?p).

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Uncertainty principle

• This is not a comment on measuring technologyit
  is not something you can circumvent with better
  equipmentit is a limitation imposed by the
  physics of the Universe. These elements of the
  Universe are notas deterministic as Einstein
  would have liked.
• What happens if you try to defy the Uncertainty
  Principle?
• The Observer Effect appears, effectively
  enforcing the Uncertainty Principle
• Example Suppose you are studying a moving
  electron, and wish to defy the uncertainty
  principle. You plan to measure its momentum and
  position as accurately as possible.
• To learn its position, you bombard it with
  high-energy photons. (High-energy small ? ?
  accurate locations). Such high-energy photons
  will disturb the electrons momentum.
• ? Good position information, bad momentum
  information
• You cant have your cake, and eat it too!

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Uncertainty principle

• The uncertainty principle can be written two ways
  
• ?x ?p h/2 AND ?E ?t h/2
• The second version says that, in a similar way,
  there is a limitation to how precisely the
  Universe enforcesover the small period of time
  (?t)fluctuations in energy (?E) a particle can
  have.
• Written this way, the uncertainty principle leads
  us to extremely profound consequences
• The total energy of a subatomic particle can vary
  wildly, in violation of the law of conservation
  of energy, just as long as the variation happens
  over a very small time!!!
• In quantum physics, the Universe no longer plays
  by what we consider to be the rules the Universe
  breaks the rules if it can get away with it
  without being caught!

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Uncertainty principle and virtual particles

• EVEN WEIRDER ?E ?t h/2
• Recall that energy and matter are
  interchangeable
• E mc2 ? ?E ?mc2 ? (?mc2)?t
  h/2.
• Particles and their anti-matter twins can pop
  into existence, out of nothing, in a perfect
  vacuum, just as long as they recombine in a short
  time ?t.
• The tinier the particles, the longer they can
  last.
• The Universe, on a microscopic level, is seething
  with an infinite sea of virtual particle pairs
  and virtual photons popping into existence, then
  disappearing as they recombine back into nothing.
• Created and annihilated, created and
  annihilatedwhat a waste of time Richard
  Feynman

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Forces mediated by virtual particles

• So how do forces really work?
• Two particles interact by constantly exchanging
  particles. This steady stream of particles is
  what expresses the force. The particles are said
  to mediate the force.
• The particles are virtual particles, created out
  of nothing.
• Example Electromagnetic force
• Two electrons are separated by a small distance.
  They constantly produce virtual photons in all
  directions.
• The virtual photons that reach the partner
  particle are absorbed, thus transmitting
  information about the emitting particle.
• If the photons miss the partner particle (by
  being sent in the wrong direction), that is not
  a problem because they were virtual particles,
  and were not sent in the first place after all!
• Photons are massless, so the range of this force
  is infinite.
• Does this disturb you? It should!

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Back to matter probing subatomia

• How accelerators work
• Charged particles are deflected by magnetic
  fields.
• A suitable arrangement of electromagnets can
  force particles into circular paths.
• Electric fields accelerate the particles.
• Recall the relativistic energy E2 (pc)2
  (mc2)2
• Energies can be driven to values exceeding rest
  masses of other particles.
• BLAM! They can transform into these other
  particles via E mc2
• Accelerators are rated in power 1 eV (1.6 10-19
  J).
• 14 TeV is the energy of a 1 gram object falling
  0.2 mm in 1g.
• More powerful accelerators ?
• more energy ?
• creation of more massive particles.

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What they have discovered

• Neutrinos zero charge, essentially massless
  predicted in 1931 discovered in 1956 found to
  be hyper-relativistic in 2011?
• Muons negative charge, 0.1 u discovered in
  1936.
• ?-mesons neutral, positive, negative charge, 0.1
  u discovered in 1947.
• K-mesons (kaons) neutral, positive, negative
  charge, 0.53 u discovered in 1947.
• ?-baryons 1.2 u, neutral discovered in 1947.
• Xi-baryons 1.3 u, neutral discovered in 1964.
• J/? meson, t, upsilon meson, gluon, W and Z
  meson, and others followed.
• Current list of mesons
• Current list of baryons
• Surely, as this continued, physicists concluded
  the so-called elementary particles must in fact
  be composite articles.
• What are the real core particles?

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The Standard Model of particle physics

• The Standard Model of particle physics was
  developed by Sheldon Glashow (1960) Steven
  Weinberg and Abdus Salam (1967). It treats
  electromagnetism, strong, and weak interactions,
  but not gravity. Major successes include
  correctly predicting the mass of W and Z bosons.
  Even so, it has flaws, and is clearly not the
  final solution.
• Charge Family 1 Family 2 Family 3
• Quarks 2/3 up (u) charm (c) top (t)
• -1/3 down (d) strange (s) bottom (b)
• Leptons -1 electron (e-) muon (µ-) tau (t-)
• 0 electron neutrino (?e) muon neutrino (?µ)
  tau neutrino (?t)
• Family 1 includes the familiar forms of matter
  protons, neutrons, electrons, neutrinos.

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Quarks

• Proposed by Gell-Mann and Zweig in 1964 first
  observed in 1968 by deep inelastic scattering
  experiments which probed the interior structure
  in protons and neutrons, and found three
  mass-globs inside these baryons.
• Therefore their subatomic compositions are
• Proton u u dCharge (2/3)
  (2/3) (-1/3) 1
• Neutron u d dCharge (2/3)
  (-1/3) (-1/3) 0
• Leptons (neutrinos and electrons) are still
  considered elementary.
• The presence of mesons (which are unstable, and
  were found to contain two quarks), ultimately
  demanded the introduction of four more quarks.

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Quarks and forces

• In quantum mechanics, particles interact with
  each other by fields, but also interactions can
  be thought of as the continual exchange of
  mediating particles.
• Electromagnetism
• We have seen that the mediating particles are
  virtual photons.
• The weak interaction
• Mediated by the exchange of Z, W, and W- bosons.
• The strong interaction
• Mediated by the exchange of gluons.
• As a detailed example, lets look at how quarks
  interact with each other

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Quark color

• Quarks have a special, additional quantum
  characteristics called color.
• Any quark can assume any color.
• The colors are called red, green, and blue.
• There are three anti-colors anti-red,
  anti-green, and anti-blue.
• No Net Color is Allowed
• Quarks combine so that there is no net color in
  the resulting particle.
• In particles that consist of three quarks (such
  as protons, neutrons), the three quarks must
  either be (red, green, and blue), or they must be
  (anti-red, anti-green, and anti-blue).
• Particles such as mesons that consist of only two
  quarks contain a color-anticolor pair (red
  anti-red), (green anti-green), (blue
  anti-blue).

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Quark color as a force quantum chromodynamics

• Quarks can change color by exchanging gluons.
• A gluon consists of a packet of color and an
  anticolor.
• Consider a particle containing a blue (1), red
  (2), and green (3) quark.
• The blue quark (1) emits a gluon that contains
  blue color and anti-red.The emission of the blue
  makes the quark greyThe emission of the
  anti-red turns the quark (1) red.
• The gluon is absorbed by the red quark (2).The
  absorption of the anti-red part turns the quark
  greyThe absorption of the blue turns the
  quark (2) blue.
• This color force holds the proton, neutron, or
  meson together.
• A relatively weak echo of this force from one
  proton (or neutron) can affect nearby protons (or
  neutrons). This is the origin of the strong
  nuclear force!

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Separating quarks

• An interesting characteristic of the color force
  is that it does not drop off with distance.
• Suppose you tried to pull a quark out of a
  proton
• You would have to pull so hard, for so far, that
  the energy required would be equivalent to the
  rest-energy of three quarks, in a new atom.
• If you managed to pull a quark out of an atom,
  youd discover you simply had two new atoms, each
  with three quarks, and that you had pointlessly
  expended a great deal of energy.
• You cannot isolate quarks.

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Standard Model summary

• Fermions
• 6 quarks up, down, charm, strange, top, bottom
• Hadrons (quark-matter) baryons (p, n, etc.) (3
  quarks) and mesons (2 quarks)
• Leptons electrons, muons, tau, and three
  neutrinos
• Bosons (force particles)
• Photon (electromagnetic)
• W, W-, Z boson (weak interaction)
• Gluons (color)
• Three families (or generations) of particles
• Family 1 contains all the particles encountered
  in everyday matter
• Family 1 particles up, down (proton, neutron),
  electron, electron neutrino
• Families 2, 3 include bizarre, unstable particles
• Forces
• Electroweak (electromagnetic and weak)
• Strong force (a side-effect of the color force)
• Gravity (not yet incorporated into the theory)
  from massless bosons gravitons

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The Higgs boson

• This particle is predicted by the Standard Model.
• The Higgs boson has spin 0 (all other bosons
  are spin1)
• It creates the Higgs field
• Particles passing through the Higgs field feel a
  kind of drag.
• This drag is what we call mass.
• Even the Higgs boson feels this drag (and hence
  has mass).
• The Standard Model predicts its mass to be
  probably 85-215 protons. In 2012, two separate
  experiments at the LHC detected a particle with a
  mass of 134 protons uncertainly about 5s.
• One problem with the Standard Model is that it is
  not constrained to obey charge-parity
  symmetryyet it does. One way out of this bind
  is that there could be yet ANOTHER weirdo
  particle called an axion More on this, later.

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Appendix Ancient concepts of the elements

• Aristotle (Greece 384-322 BC) defined the
  elements to be air, fire, water, earth, and
  quintessence (the immutable material of the
  cosmos)
• Ancient China saw a single underlying form of
  energy that could appear in one of five
  different, inter-changing forms earth, fire,
  water, metal, wood.
• Babylonia saw earth, sea, sky, wind.
• Modern elements
• Lavoisier (1789) 33 elements
• Berzelius (1818) 49 elements
• Mendeleev (1869) 66 elements
• (1919) 72 elements
• (1955) 101 elements
• (2014) 118 elements (most massive several
  not officially approved)

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