High Energy Physics

1. Faculty of science High Energy Physics Winthrop Professor Ian McArthur and Adjunct/Professor Jackie Davidson

2. AIM: To explore nature on the smallest length scales we can achieve 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) anti-quarks anti-leptons three groups(or generations) of quarks and leptons lowest energy/mass generation • Fundamental bosons: ‘mediators of interactions between particles’ • g, W±, Z0, gluons, graviton? • Higgs boson Current status (10-20 m) Using and verifying the ‘Standard Model

3. discovered 1995 periodic table of the Standard Model discovered 2012

4. How do we know this? high energy scattering experiments in accelerators

5. How do we know this? Most of what we know about the world around us is as a result ofscattering experiments

6. very complicated!

7. compact muon solenoid detector in the Large Hadron Collider

8. 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).

10. What we identify as the fundamental constituents of matter depends on the length scale:

11. But visible matter makes up only about 4% of the energy/mass of the Universe. (more on this later…) 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

12. 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 …

13. Weak interaction Radioactive decay Not a binding interaction, but does involve transfer of energy and momentum.

14. What is an interaction? • No interaction • No exchange of energy and momentum • Interaction • Exchange of energy and momentum

15. Classical (or macroscopic) description of interactions Electromagnetic interaction Gravitational interaction Charged particle interacts with electric field of another charged particle. The field transfers energy and momentum.

16. Quantum description of interactions • 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 This classical description breaks down on microscopic distance scales and needs to be replaced by a quantum description.

17. 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.

18. … Back to quantum electrodynamics This is the quantum analogue of the electric field (no net energy loss). Charged particles can emit and absorb virtual photons. Virtual photos can only exist for a short time, so there are two possibilities: The virtual photon is reabsorbed by the same charged particle.

19. 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.

20. 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 = 1.000 000 000 Theory with virtual photons: g/2 = 1.001 159 652 41 (±20) 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:

21. A charged particle can also emit real photons if it is accelerated. These can exist indefinitely and propagate off to infinity (electromagnetic radiation).

22. 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).

23. Quantum theory of the weak interaction (proton) lepton number conservation baryon number conservation charge conservation All interactions conserve: energy/momentum, charge, baryon and lepton number. (neutron) The weak interaction is mediated by exchange of virtual W± and Z0 bosons. eg beta decay

24. Quantum theory of the weak interaction weak scattering interaction p Z-boson has mass (short-ranged interactions) Z0 ne p ne Similar to electromagnetic scattering g photon is massless (infinite-ranged interactions) Unification of electromagnetic and weak interactions theory

25. 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)

26. 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.