High Energy Physics Advisory Panel's Subpanel on Vision for the Future of High-Energy Physics
High Energy Physics
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High Energy Physics WINTHROP PROFESSOR IAN MCARTHUR and ADJUNCT/PROFESSOR JACKIE DAVIDSON FACULTY OF SCIENCE
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Faculty of science. High Energy Physics. Winthrop Professor Ian McArthur and Adjunct/Professor Jackie Davidson. AIM: To explore nature on the smallest length scales we can achieve. Standard Model has two kinds of fundamental particles with different functions: - PowerPoint PPT Presentation
Transcript of High Energy Physics
High Energy Physics
WINTHROP PROFESSOR IAN MCARTHURand ADJUNCT/PROFESSOR JACKIE DAVIDSON
FACULTY OF SCIENCE
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
Standard Model has two kinds of fundamental particles with different functions:
Fundamental fermions: ‘building blocks of matter’• quarks ( u, d, c, s, t, b)• leptons ( e-, ne, m-, nm, t-, nt)
AIM: To explore nature on the smallest length scales we can achieve
Current status (10-20 m)Using and verifying the ‘Standard Model
anti-quarks
anti-leptons
three groups (or generations) of quarks and leptonslowest energy/mass generation
Fundamental bosons: ‘mediators of interactions between particles’• g, W±, Z0, gluons, graviton? • Higgs boson
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
periodic table of the Standard
Model
discovered 1995
discovered 2012
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
How do we know this?high energy scattering experiments in accelerators
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
Most of what we know about the world around us is as a result ofscattering experiments
How do we know this?
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
very complicated!
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
compact muon solenoid detector in the Large Hadron Collider
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
Why ‘high energy’ physics?
A particle of momentum p can exhibit wavelike phenomena characterised by a wavelength:
A probe particle cannot resolve structures smaller than its wavelength.
To probe structures on a length scale l, probe particles are needed with de Broglie wavelength λ ≪ l.
Particle accelerators accelerate particles to large p(ie small l).
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
optical microscope
electron microscope
Stanfordmark III
accelerator
Stanfordtwo-mile
accelerator
‘light’ used visible light electrons electrons electrons
energy 2 eV 50 000 eV 1 GeV 20 GeV
wavelength 10-5 cm 10-9 cm 10-13 cm 10-14 cm
smallest object seen
millionsof atoms
thousandsof atoms
atomicnucleus
particles inside nucleus
typical objectof study
living cell virus atomic nucleus proton
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
What we identify as the fundamental constituents of matter depends on the length scale:
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
Most of the visible matter in the universe consists of up quarks, down quarks and electrons in various bound states.
Up quark (u) charge +⅔ Down quark (d) charge -⅓
But visible matter makes up only about 4% of the energy/mass of the Universe. (more on this later…)
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
What is it that holds fundamental fermions in these bound states?It is the fundamental interactions.
Strong interaction binds u and d quarks to make protons and neutrons.
‘Residual’ strong interaction binds protons and neutrons into nuclei.
Electromagnetic interaction binds electrons and nuclei to form atoms.
EM interaction (in residual form) binds atoms to form molecules and
crystals.
Gravitational interaction binds matter to form stars, galaxies …
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
Weak interaction Radioactive decay
Not a binding interaction, but does involve transfer of energy and momentum.
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
What is an interaction?
No interaction
• No exchange of energy and momentum
Interaction
• Exchange of energy and momentum
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
Electromagnetic interaction Gravitational interaction
Classical (or macroscopic) description of interactions
Charged particle interacts with electric field of another charged particle.
The field transfers energy and momentum.
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
Quantum description of interactions
This classical description breaks down on microscopic distance scales and needs to be replaced by a quantum description.
quantum electrodynamics (QED) 1940-1950
much simpler – no fields, only particles
energy and momentum transfer in an electromagnetic interaction occurs via virtual photon exchange.
Feynman diagram
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
What is a ‘virtual particle’?
For a classical particle, energy and momentum are related:
Non-relativistic:
Relativistic:
Quantum mechanics says a particle of momentum p can have an energy E which is different from the energy Eclassical we would expect it to have classically.
If then the particle is called a virtual particle and can exist
only for a time ΔT such that
If E = Eclassical then the particle is called a real particle and can exist indefinitely.
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
… Back to quantum electrodynamics
Charged particles can emit and absorb virtual photons.
Virtual photos can only exist for a short time, so there are two possibilities:
1. The virtual photon is reabsorbed by the same charged particle.
This is the quantum analogue of the electric field (no net energy loss).
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
2. The virtual photon can be absorbed by another charged particle.
This transfer of energy and momentum is an interaction.
This is the quantum version of a particle interacting with the electric field of another particle.
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
If a particle has zero charge it cannot emit or absorb virtual photons, so does not participate in electromagnetic interactions.
The potential energy of a pair of charged particles due to virtual photon exchange is:
Coulomb interaction
Can we believe this?Lande g-factor relates magnetic moment of electron to its spin.
Experimental result: g/2 = 1.001 159 652 38 (±29)Theory without virtual photons: g/2 = 1.000 000 000Theory with virtual photons: g/2 = 1.001 159 652 41 (±20)
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
A charged particle can also emit real photons if it is accelerated. These can exist indefinitely and propagate off to infinity (electromagnetic radiation).
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
How can virtual particle exchange give rise to both attractive and repulsive forces?
Virtual particle exchange between a pair of particles gives rise to a potential energy V(r) for the pair of particles depending on their separation, r.
Particles move in directions which minimise their potential energy.
If V(r) increases with increasing separation, particles will move closer together (an attractive force).
If V(r) decreases with increasing separation, particles will move apart (a repulsive force).
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
Quantum theory of the weak interaction
The weak interaction is mediated by exchange of virtual W± and Z0 bosons.
eg beta decay
(neutron)
(proton)
charge conservation
lepton number conservation
All interactions conserve: energy/momentum, charge, baryon and lepton number.
baryon number conservation
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
Quantum theory of the weak interaction
p
p
ne
neZ0
weak scattering interaction
Similar to electromagnetic scatteringg
photon is massless(infinite-ranged interactions)
Z-boson has mass(short-ranged interactions)
Unification of electromagnetic and weak interactions theory
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
Quantum theory of the strong interaction
Quantum chromodynamics (QCD) 1980s
The strong interaction binds quarks into protons and neutrons.
The strong interaction is mediated by exchange of virtual particles called gluons.
The interaction is short-range, 10-15 m (approximate proton diameter)
The University of Western Australia© Ian McArthur and Jackie Davidson, 2012
There’s more to do:
The Standard Model has been very successful at predicting scattering interactions observed in high energy accelerators.• The latest success was prediction and measurement of the Higgs boson. • Classical theory would not have predicted experimental results to date.
However: The Standard Model does not give a quantum theory of gravity.
• In accelerators this is not a problem as gravity is a very weak force on an atomic scale.
• In black holes and in Big Bang theory, where energies are extremely high, quantum gravity cannot be ignored.
Visible matter makes up only 4.6% of energy/mass of the Universe.• 24% of energy/mass of the Universe is in ‘cold dark matter’, which is currently
unknown.• 71.4% of energy/mass of the Universe is in ‘dark energy’, which is currently not
understood. Work continues in supersymmetry theory, string theory, the quest to unify, simplify
and understand the constants of nature, and energy hierarchy of observed particles.
