Particle Physics

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					 Particle Physics


             James Stirling
Institute for Particle Physics Phenomenology
            University of Durham
What Is Particle Physics?
   We aim to answer the question:
     “What is the world made of?”

   Want to know the basic particles
   Want to know the forces that hold
    them together
What Are the Basic Particles?
   How far can we keep breaking matter
    into smaller pieces?




   …forever?
    Periodic Table

   Soon get to Elements
   In 1860’s Mendeleev
    arranged the elements by
    property into Periodic
    Table
   Surely too many to all be fundamental?
   Gaps in the table – predicted new elements
   Deduce an underlying structure - Atoms
    Are Atoms Fundamental?
   Around 100 years ago atoms thought to
    be “indivisible balls”
   Rutherford took large gold atoms and
    fired radioactive particles at them
   He found that most of the particles
    went straight through
   BUT occasionally some did bounce
    back…
    A New Picture of the Atom

   Atom is dense nucleus
    surrounded by cloud of
    electrons

   Now understand
    experiment
   Most of atom is empty
    space !
Is the Nucleus Fundamental?
   Do similar experiment to Rutherford
   Fire nuclei at each other…
   Find nucleus made of smaller
    particles, protons and neutrons
Is the Proton Fundamental?
   In the 1960s physicists began to
    collide protons together
   To their horror, found LOTS of
    particles – S, L, X ,…
   Is there a pattern?
   Can we deduce some underlying
    structure?
    Finding Patterns
   Like Mendeleev, group particles with
    similar properties together
   Patterns  Substructure
   In 1964 Murray Gell-Mann suggested
    that the hundreds of particles found
    could all be made of just three quarks*
   He called them up, down and strange

           *the word is from Finnegan’s Wake by James Joyce
Model of Atom Today

   Quarks and electrons
    are fundamental

   As far as we can tell
    no further
    substructure
    Powers of Ten

   For very large or very small numbers
    it is easier to use powers of ten, e.g.
   100,000,000 = 108
   0.000000000000001 = 10-15
   Note: 1 = 100
    How Small Is A Particle?

   Molecule        10-9 m
   Atom            10-10 m
   Nucleus         10-14 m
   Proton          10-15 m
   Quark           < 10-18 m
How Small Is A Particle?
    We’re Not Finished Yet!



   In 1930s beta decay was not understood –
    there was missing energy
   Pauli grudgingly introduced a new particle to
    account for this – the neutrino
   The electron and neutrino are collectively
    called leptons
Antimatter Too!
   In 1932 Anderson discovered the
    positron
   For every particle there is also an
    antiparticle
   Antimatter just like ordinary matter
    but with opposite charge
   electron negative, positron positive!
   Creating New Particles
                                  muon (-)



electron (e-)                     positron (e+)




                antimuon   (+)     E=mc2 !
    The Fundamental Particles
   There are only 12 fundamental particles
    of matter (also the antiparticles)
Discovery Timeline
    1897   electron
    1933   neutrino
    1937   muon
    1964   up, down, strange
    1974   charm
    1975   tau
    1977   bottom
    1996   top
What About Forces?
   Particles interact with each other via
    forces
   There are four types – gravity,
    electromagnetism and the strong and
    weak nuclear forces
   Each is described by the exchange of
    a force-carrying particle between the
    matter particles
The Four Forces of Nature
How Do We Know All This?
   All our experiments involve colliding
    particles
   Matter and antimatter are accelerated to
    near light-speed by an accelerator
   They are brought together inside a
    detector and collide
   They annihilate and their energy is
    released to create new particles according
    to E=mc2
    Creating New Particles
                                muon (-)



electron (e-)                   positron (e+)




                antimuon (+)
The LEP Accelerator at CERN
The LEP Accelerator at CERN
How Detectors Work
A detector does various jobs…

   Track the positions of particles
   Measure energy of particles
   Detector layered like an onion
   Each layer measures different
    things
The OPAL Detector at CERN
A View Inside
    Finding Information
   In the inner layers
    magnetic fields are used
    to find the charge and
    momentum of particles
   In the outer layers the
    energy of the particles
    are measured
A Picture From a Detector
Summary

   There are 12 fundamental matter
    particles, the Quarks and Leptons
   There are 4 forces transmitted by
    particles
   Accelerators are used to collide and
    study particles