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The beginning of physics IoP Physics Update 2008 Forces and fields Particles and symmetries Cosmology The Large Hadron Collider Particle detectors A Ring-Imaging Cherenkov detector A Cosmic Ray detector
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14-15th December 2008 Steve Wotton 2 Forces – A particle physicist’s view 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 forces. A consistent mathematical description is formulated by combining quantum mechanics and relativity.
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14-15th December 2008 Steve Wotton 3 Unification of forces 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 problem
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14-15th December 2008 Steve Wotton 4 Force unification – the next step Can we include gravity? We’d like to but it is HARD. Is there anything else? Yes. The weak nuclear force (beta decay). The strong nuclear force (binds protons and neutrons in nuclei). Build on the success of QED New ingredients New interactions New particles New diagrams n p e-e- e W-W- e-e- e-e- Z 0,
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14-15th December 2008 Steve Wotton 5 Particle zoo 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 objects. The chart of the nucleides A periodic table for nuclei Patterns also due to quantisation rules A complicated picture simplified…
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14-15th December 2008 Steve Wotton 6 Particle zoo 2 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 gluon. But there are problems Duplication – why? Mass hierarchy – how? Anti-matter – where? Gravity – still mysterious.
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14-15th December 2008 Steve Wotton 7 Where we are 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 problem Supersymmetry Extra dimensions We don’t know how to include gravity We don’t know the origin of symmetry breaking We do know that The Standard Model must break down at TeV energies
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14-15th December 2008 Steve Wotton 8 Higgs – the solution or the problem? LHC – the kill or cure? 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 m H (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 terms. 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 interacting).
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14-15th December 2008 Steve Wotton 9 Cosmology 1 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 -19 m = 10 17 K (Note: size of proton = 10 -15 m) LHC analogies: A time machine that recreates conditions of early universe. A microscope that sees objects smaller than can be seen with light.
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14-15th December 2008 Steve Wotton 10 Cosmology 2 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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14-15th December 2008 Steve Wotton 11 Detection techniques A discussion of techniques used in particle detectors…
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14-15th December 2008 Steve Wotton 12 Ring-Imaging Cherenkov Detectors Identifying particles using Cherenkov radiation… Cerenkov worksheet RICH animator
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14-15th December 2008 Steve Wotton 13 Cosmic Ray detection A practical demonstration of detection of high energy particles in the classroom…
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14-15th December 2008 Steve Wotton 14 Other connections 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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