MP466 Particle Physics Lecturer: Jon-Ivar Skullerud, - - PowerPoint PPT Presentation

MP466 Particle Physics

Lecturer: Jon-Ivar Skullerud, jonivar@thphys.nuim.ie Room 1.7c, Science Bldg Lectures: Wed 9–10, Hall C, Arts Bldg Fri 11–12, Hall B, Arts Bldg Tutorials: Mon 14–15, Hall B, Arts Bldg (TBC)

Outline of course

Basic concepts: particles, forces and cross-sections Symmetries and conservation laws Strong interactions: isospin, SU(3), the quark model, QCD Relativistic quantum mechanics: the Dirac equation Weak interactions

Recommended books

• M. Thomson, Modern Particle Physics, (2013) Cambridge University

Press

• A. Bettini, Introduction to Elementary Particle Physics, 2nd Edition

(2014) Cambridge University Press Online course material is at http://www.thphys.nuim.ie/Notes/MP466/

Assessment

Continuous assessment (5 assignments) 20% Exam 80% Assignments are compulsory!

What is a particle?

Common prejudices:

1 Particles are localisable 2 Particles carry energy and momentum 3 Particles have definite masses 4 Particles have definite charges, spins and other quantum numbers

Heisenberg’s uncertainty relation gives conflict between the first two ∆x∆px ≥ 2

Mathematical definitions

A particle is an excitation of a quantum field A particle is an irreducible representation of the Poincar´ e group

Classification of particles

Bosons and fermions

If 1 and 2 are identical particles, then ψ(1, 2) = ±ψ(2, 1) +: bosons −: fermions Spin–statistics theorem: boson/fermion ← → intrinsic angular momentum bosons: S = n fermions: S = (n + 1

2)

Stable and unstable

Free particle wavefunction Ψ( r, t) = Nei(

p· r−Et)/

We see that |Ψ(0, t)|2 = constant — stable particle Some particles are unstable: |Ψ(0, t)|2 = N0e−t/τ This corresponds to a complex energy

The four forces

1 Electromagnetism — affects everything with (electric) charge ◮ Responsible for all of chemistry and material properties ◮ Strength α =

e2 4πǫc ≈ 1 137

2 Strong force — affects everything with colour charge ◮ Responsible for keeping quarks together in protons and neutrons ◮ Responsible for keeping nuclei together ◮ Strength αs = g 2

s

4π ∼ 0.3

3 Weak force — affects all known particles ◮ Responsible for some radioactive processes, eg β decay ◮ Strength αW = g 2

W

4π ∼ 0.034

4 Gravity — affects everything with energy/momentum ◮ Responsible for keeping planets, stars, galaxies together ◮ Responsible for expansion of the universe ◮ No quantum theory of gravity known ◮ Negligible on (sub)atomic scale

In quantum description, all fields have quantised excitations: γ, g, W ±, Z 0 — all spin 1 (hypothetical graviton would be spin 2)

Matter particles

Elementary matter constituents

Leptons e, µ, τ, νe, νµ, ντ Quarks u, d, s, c, b, t Elementary particles are described as point particles (mathematical points)

Hadrons

Bound states of quarks (and gluons) Mesons: bosons — π, ρ, K, η, J/ψ, D, B, . . . Baryons: fermions n, p, Σ, Ξ, Λ, Ω, . . . Hadrons have a size ∼ 1 fm (10−15m)

Higgs field

The Higgs field and its associate particle does not fit neatly into matter–force classification Spin 0 (scalar boson) The field as a nonzero expectation value in the vacuum (vev) Coupling of vev to elementary particles behaves like a mass for those particles (Brout–Englert–Higgs mechanism)

Energy scales