PU The beginning of physics IoP Physics Update 2008

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					The beginning of physics

                    IoP Physics Update 2008

                       Forces and fields
                   Particles and symmetries
                  The Large Hadron Collider
                       Particle detectors
              A Ring-Imaging Cherenkov detector
                    A Cosmic Ray detector
Forces – A particle physicist’s view                       Steve Wotton

     Familiar electric, magnetic and
      gravitational forces are described by
      separate theories
     Classical picture
          Action at a distance – two bodies feel a
           mutually attractive or repulsive force even
           though separated by large distances
          Introduce abstract concept of a field – the
           strength and direction of the force felt by a
           test body is uniquely defined at every
           point in space
          Strength of force from an idealised point
           body decreases according to an inverse
           square law.
          We draw pictures of the fields as if they
           had a physical existence (a prejudice
           reinforced by e.g. iron-filings aligning
           along magnetic field lines)
     Quantum field theory view
          Forces are due to the exchange of (virtual)
           particles that carry momentum
          Strength of the force is determined by the
           coupling to the force carrying particle. The
           strength is (very) different for the known
          A consistent mathematical description is
           formulated by combining quantum
           mechanics and relativity.

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Unification of forces                             Steve Wotton

     Electricity and magnetism once
      considered separate phenomena.
     Now unified and enshrined in
      Maxwell’s equations.
        Electric and magnetic forces are
         merely different manifestations of the
         same underlying mechanism.
        By changing our viewpoint, an electric
         field can become a magnetic field (and
         vice versa). Relativity at work.
     Add an extra ingredient – Quantum
      Mechanics – we get Quantum
      Electrodynamics. The photon
      (quantum of light) is the force
      carrying particle.
        A very, very, very, very, very, very,
         very, very, well-tested theory.
        Feynman expressed the calculations
         in pictures (Feynman diagrams)
        Each diagram represents a
         mathematical term in the solution to a

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Force unification – the next step                                        Steve Wotton

   Can we include gravity?                                              p

        We’d like to but it is HARD.        n

   Is there anything else?
                                                           W-                e-
        Yes.
             The weak nuclear force (beta
             The strong nuclear force                                       ne
              (binds protons and neutrons
              in nuclei).
   Build on the success of QED                       e-

   New ingredients
        New interactions                                       Z0 , g

        New particles
        New diagrams

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Particle zoo                                     Steve Wotton

     The quest for the elements
        A search for order
     Earth, air, fire and water
     The chemical elements
        Periodic table
        Patterns are due to a set of
         quantisation rules that must be
         followed when adding more
         electrons to an atom.
        A complicated picture (many
         elements with different properties)
         simplified by applying a set of rules
         to build elements from a small
         number of more fundamental
     The chart of the nucleides
        A periodic table for nuclei
        Patterns also due to quantisation
        A complicated picture simplified…

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Particle zoo 2                                  Steve Wotton

   Many different particles can be
    created in the lab.
   A complicated picture but we can
    discern patterns.
   Must be due to an underlying
    theory that combines a smaller
    number of more fundamental
    particles using a set of rules.
   The fundamental particles
        All ordinary matter made of up
         quark, down quark, electrons and
         electron neutrinos.
        All forces (except gravity) mediated
         by photon, Z boson, W boson and
   But there are problems
          Duplication – why?
          Mass hierarchy – how?
          Anti-matter – where?
          Gravity – still mysterious.
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Where we are                                          Steve Wotton

     The Standard Model.
        Describes all known particles and their
         electroweak and strong interactions.
        No significant deviations from SM observed
         to date.
        Observed differences between forces due to
         non-exact symmetry.
        Possibly the best physical theory in the
         history of physics.
     But…
        We are still waiting for the Higgs boson
        We don’t understand the origin of mass
        We don’t know how to solve the hierarchy
             Supersymmetry
             Extra dimensions
        We don’t know how to include gravity
        We don’t know the origin of symmetry
        We do know that The Standard Model must
         break down at TeV energies

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Higgs – the solution or the problem? LHC – the kill or cure?   Steve Wotton

     If the Higgs particle is not found in the
      mass range accessible to the LHC…
        Breakdown of Standard Model.
        Violation of unitarity in WW scattering for
         large mH (sum of probabilities cannot
         exceed 1).
     If the Higgs particle is found at the LHC…
        Low mass (compared to Planck mass)
         implies a convenient cancellation of large
        Or there must be new physics.
     Supersymmetry is a favourite candidate
      for new physics (symmetry is good, more
      is better) but...
        Requires new particles to exist (Who
         ordered that?, Rabi).
        Properties must explain why they haven’t
         been observed already (heavy, or weakly

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Cosmology 1                                                                 Steve Wotton

   Looking back in time
        Universe contains fixed amount of energy (mass and radiation)
        Space is expanding, universe is cooling.
        kT = hc/λ = eV relates temperature (T), length (λ), accelerating potential
         (V) through fundamental constants k, h, c, e.
        E.g 14TeV = 10-19m = 1017K (Note: size of proton = 10-15m)

                                                            LHC analogies:
                                                              A time machine
                                                              that recreates
                                                              conditions of
                                                              early universe.
                                                              A microscope
                                                              that sees objects
                                                              smaller than can
                                                              be seen with

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Cosmology 2                                     Steve Wotton

   The LHC will recreate the conditions
    of the early Universe.
   Provides evidence of the processes
    that are assumed to operate.
   May explain:
        Dark matter.
        Matter-antimatter asymmetry.
        Mechanisms driving evolution of
         early universe.
        The origin of mass.
        The unification of all known forces.

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Detection techniques                                         Steve Wotton

   A discussion of techniques used in particle detectors…

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Ring-Imaging Cherenkov Detectors   Steve Wotton

   Identifying particles using
    Cherenkov radiation…
        Cerenkov worksheet
        RICH animator

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Cosmic Ray detection            Steve Wotton

   A practical demonstration
    of detection of high
    energy particles in the

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Other connections                                             Steve Wotton

   Medical imaging
        Photomultiplier tubes
        MRI
        Radio-isotopes production in accelerators
   Proton cancer therapy
   Security scanning
          Image Intensifiers
          X-ray
          Gamma-ray
          Cosmic-ray
   Non-destructive testing
        Cosmic rays or neutrinos (“X-raying” the Pyramids)
        Fragile/valuable objects
               Paintings
               Sculptures
               Wine
               Archaeological remains
        Engineering

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