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Quanta, Quarks, Qubits - University of Sydney ... Quarks and the Colour Force QCD, Quantum Chromodynamics, the Colour Force, was proposed in the 1970s to explain how quarks stick together

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  • Quanta, Quarks, Qubits A few thoughts on 100 years of ‘modern’ physics’

  • Quanta

    Blackbody radiation and the ‘ultraviolet catastrophe’ — classical physics does not agree with the observed world

    Planck’s idea:

    • atoms are harmonic oscillators that only absorb and emit energy in ‘bundles’ of hν.

    • Bolzmann distribution of energies Nn = N0 e(-nhν/kT)

  • What does it mean?

    • Mathematical trick? Planck probably thought so, certainly thought light was classical

    • Quantum nature of physical reality? Einstein in 1905 — photoelectric effect

  • Early ‘modern’ physics

    Syllabus talks about:

    • Heisenberg and Pauli, Fermi and his piles, Pauli and the neutrino

    • Investigations: Hydrogen spectrum, cloud chamber

  • Other investigations

    • Thanks to Mark Butler, Gosford HS • Model the size of an atom • Moderators in nuclear reactors • Flame spectra • Planck’s constant with LEDs • Bragg, diffraction and interference • nuclear physics with match-sticks: plotting half-life, chain


    • nuclear physics with ping-pong balls or scrunched-up paper

  • Quanta: Quarks • So where are we now? • Everything we experience* can be described

    very well** as matter (atoms) and forces (EM, gravity)

    • If we look hard, we see other stuff: nuclear physics (decaying atoms, radiation), new matter (weird, short-lived particle ‘zoo’)

    * except for emotions, consciousness, dreams ...

    ** except for * and a few things we’ll get to in a minute

  • The Standard Model

    • Explains everything*** in the universe in terms of a small number of ideas

    • 12 particles of matter — quarks and leptons (electrons, neutrinos, ...)

    • 2 fundamental forces (electro-weak force and strong ‘colour’ force)

    *** gravity and some fringe observations not included

  • Matter: Crystals/solids/gases/liquids — molecules — atoms — electrons,

    protons and neutrons

    Forces: Ignoring gravity, the only force we ‘feel’ is Electromagnetism. QED, quantum electrodynamics — quantum theory of light

    interacting with electrically-charged particles (electrons, protons …)




    QED: forces are treated as exchanges of photons

  • Our greatest invention

    photon electron


    QED: Quantum Electrodynamics Theory & Experiment agree to 13 decimal places

    Almost everything you ever experience is QED

  • Problems Helium nucleus has two positively-charged protons.

    Why don’t the protons fly apart due to Coulomb force?

    Some heavy atoms are unstable — they ‘decay’ spontaneously, releasing beta-radiation (electrons) and changing into different atoms.

    Somehow a neutron becomes a proton by ‘ejecting’ an electron … how does this happen?

    Protons and neutrons don’t seem to be simple ‘point particles’ or little spheres – they have structure.

    What could it be?

    Strange new particles appear when you bang electrons or protons together at high energies.

    What are they, and why do they disappear so quickly?

  • Protons and neutrons have structure

    Colliding high-speed electrons with atoms uncovered the nucleus

    Higher energy collisions showed the nucleus has structure — protons and neutrons

    Even higher energy collisions show that Ps and Ns have structure — but what could it be?

    Around the same time, many new particles were showing up — too many ‘fundamental’ particles (similar

    situation to the periodic table a century before)

    Models developed based on new particles called quarks — explained the masses and charges of P, N and

    some of the new particles

    What held the quarks together? What was the theory?

  • Quarks and the Colour Force

    There are six ‘flavours’ of quark: up, down, strange, charm, beauty and truth (bottom and top)

    Protons and neutrons are uud and udd

    The other four quarks appear (briefly) in accelerator experiments: the D is an up + anti-charm, the Σ+ is a strange and two ups

    Quarks always combine in threes or as a quark-antiquark pair, because they feel a new kind of force …

  • Quarks and the Colour Force QCD, Quantum Chromodynamics, the Colour Force, was

    proposed in the 1970s to explain how quarks stick together.

    Quarks have a new kind of charge — ‘colour’ — that comes in three types: ‘red’, ‘blue’ and ‘green’

    In EM, + and – make an electrically neutral object (atom)

    In QCD, r, g and b make a ‘colour neutral’ object (baryon)

    Also, colour + anti-colour make a neutral object (meson)

    The HADRONS:

  • Leptons: The Other Particle Family Hadrons (protons, neutrons …) are made of quarks

    What about electrons – what are they?

    Particle accelerator experiments: some new particles appear that are NOT made of quarks

    μ (muon): charge –1, no colour, m ~ 200 x electron τ (tau): charge –1, no colour, m ~ 3500 x electron

    Other than mass, exactly the same — three ‘generations’ of electron?

    1931: Wolfgang Pauli predicted a new particle — the neutrino — because the energy didn’t add up in some radioactive decays:

    decays to become:

    Unstable Tritium

    3H1 Stable 3He2


    Neutrino — must exist to account for electron’s energy


  • Leptons There is a kind of neutrino associated with each ‘electron-type’

    lepton: an e-neutrino, μ-neutrino and τ-neutrino No electric or colour charge — don’t feel the EM or QCD forces. So

    what do they feel? Must be some other force …

    The Weak Force — quarks and leptons can change type by exchanging new force-carrier particles, the W and Z






  • The List of ‘fundamental’ particles

    That’s it — aside from Gravity, that’s everything in the universe …

  • Some unanswered questions Unification and GUTs

    Electricity, magnetism parts of same underlying thing — electromagnetism (Maxwell, 1868)

    EM, weak force parts of same underlying thing — ‘electroweak’ force (Glashow–Weignberg–Salam, 1970s)

    Can this be continued? Are electroweak force and QCD part of some underlying Grand Unified Theory? What about gravity? Is there

    just one force — a SUPERFORCE — to describe everything?

    Why mass?

    There are many ‘fundamental’ particles, all with very different masses — why? Why do they have masses at all?

    A bit like the periodic table and the 1960s particle zoo … is there another layer to the onion?

  • Some unanswered questions

    Before top quark was found, physicists found good mathematical reasons for it to exist — three generations make the SM work

    Also very good mathematical arguments and experimental results to expect no more than three generations.

    Why would nature decide to create three copies of itself in this way?

    2002: Neutrinos can change type — and they have mass! That’s not Standard Model physics.

    Why three generations of particles?

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