A Tour of the Standard Model of Elementary Particles and Fields

Transcription

1 A Tour of the Standard Model of Elementary Particles and Fields What Do We Know About the Fundamental Structure of Nature and How Do We Know It? Dr. Michael G. Strauss The University of Oklahoma Elementary Particle Physics a.k.a. High Energy Physcs What are the fundamental particles and forces in nature, and how do they work?

2 What is the Fundamental Structure of Nature? This uestion has been pondered for over 2500 years Ancient Greece (followers of Thales) Air Earth Water Fire Ancient Greece (Democritus) Indivisible particles (ατοµοσ) What are the Fundamental Forces of Nature? At the turn of the century, two forces were known Gravity Electromagnetism If you know the most fundamental particles of nature and the most fundamental forces of nature, you can construct a complete and accurate theory of the natural universe. (At least in theory)

3 The Fundamental Particles and Forces in the Universe (at the turn of the new century) Particles Quarks Leptons Forces Gravity Electromagnetic Force Weak Nuclear Force Strong Nuclear Force Electroweak The Standard Model: A Theory of Everything (except gravity) The Fundamental Particles: (Fermions) six uarks (and antiuarks) u c t d s b Charge = +2/3e Charge = -1/3 e six leptons (and antileptons) e - µ - τ - ν e ν µ ν τ The Fundamental Forces: (Bosons) Strong force: 8 gluons Weak force: W +, W -, Z 0 Electromagnetic force: γ Higgs Boson: H (plus a lot of Nobel Prize winning math)

4 Nomenclature Anti-particle: The partner particle to ordinary matter. Every particle has an antiparticle. The charge and uantum numbers of the antiparticle are opposite that of the particle, and the mass is the same. Hadron: Any particle made up of uarks and/or antiuarks. Baryon: Any particle made up of three uarks (Antibaryons are made up of three antiuarks) Mesons: Any particle made up of a uark and an antiuark. Selected Hadrons Baryons Mesons p: uud π + : ud n: udd π 0 : uu Λ: uds π : ud Ω: sss K + : us Λ c :udc D 0 : cu p: uud Properties of hadrons can be explained from the properties of their constituents. (e.g. Proton electric charge is (+2/3 +2/3-1/3)e = +e; Neutron electric charge is (+2/3-1/3-1/3)e = 0) Most of the visible matter in the universe is made up of up and down uarks and electrons.

5 Quarks Are Always Bound as Hadrons There are three types of strong nuclear charge which can attract uarks. (In contrast, there is one type of electromagnetic charge that is either plus or minus. ) These strong charges are called color. red, anti-red green, anti-green blue, anti-blue Quarks are always found in nature bound together as colorless objects. This is called confinement. Three primary colors make a colorless object (baryons) A color and anti-color make a colorless object (mesons) Quantum Chromodynamics (QCD) describes uark and gluon interactions How Do We Know the Fundamental Structure of Anything? (How Do You Know How Your Car Works?) Be taught by someone who already knows Take it apart (or look inside) Put it together

6 Earnest Rutherford s 1911 Experiment Pudding Plum Pudding The Results Early Evidence for Quarks (late 1960 s) (Looking Inside the Proton) Incoming electron (e - ) Proton (p) Deep Inelastic Scattering

7 The Wave Nature of Matter The de Broglie Wavelength λ = h/p h = J s p = mv (momentum) In order to see an object, the wavelength of the probe must be smaller than the object being observed. But How Do You Put Protons Together? E = m 0 c 2 E 2 = m 02 c 4 E 2 = m 02 c 4 + c 2 p 2 Answer: Mass is a form of energy. If I can concentrate enough energy at any point, I can create any particle(s) with mass.

8 Creating Matter Step 1: Collide a particle with an antiparticle Step 2: They will annihiliate each other to create a force carrying boson (e.g. photon, Z, W +/- ) Step 3: That boson will decay to create any fundamental particle and antiparticle with mass less than or eual to the total energy, (as long as certain other features are conserved) Creating Matter Step 1: Accelerate two particles towards each other e - e + Step 2: Let them collide and annihiliate each other to create energy Step 3: That energy can create any particle and its antiparticle with mass less than or eual to the total energy

9 Feynman Diagram of Annihilation e + any fundamental particle e.g. µ - Space e - Photon or Z0 the corresponding antiparticle e.g. µ + Time Feynman Diagram of Coloumb Scattering Exchange Particles e + e + Space Photon e - e - Time

10 Neutron Decay and the Weak Force e - Space Neutron d d u W - ν e u d u Proton Time Question: The neutron has a mass of about 1 GeV/c 2 and the W has a mass of about 84 GeV/c 2. How is energy conserved in neutron decay? Answer: During the very brief period of time that the W exists, energy is not conserved!...how can this be? Heisenberg s Uncertainty Principle: E t h/2π mc 2 (d/c) h/2π mc 2 hc/2πd d h/2πmc So if d < h/2πmc a virtual particle can be produced.

11 Creating Hadrons 1. Quarks created from initial annihilation 2. Strong nuclear force acts like a rubber band 3. Eventually the rubber band breaks creating new uarks Production of Hadrons e + meson meson Space e - Photon or Z0 meson meson Time

12 Space valence uarks u u What is the Structure of a Proton? gluons virtual sea uarks d Time Heisenberg Uncertainty Principle E t h/4π h = J s E = m 0 c 2

13 Accelerating and Colliding Particles To accelerate a particle, the particle must be charged and stable: p, p, e -, e + Protons/antiprotons are more massive than electrons, so it is easier to produce higher energy collisions. Electrons/positrons are fundamental particles, so their collisions do not produce as many superfluous particles. Fermilab, USA: pp, 1800 GeV SLAC, USA: e - e +, 92 GeV LEP, France: e - e +, 160 GeV HERA, Germany: p,e + KEK, Japan: e - e +, 50 GeV CLEO, USA: e - e +, 12 GeV BES, China: e - e +, 4 GeV LHC, France: pp, 16,000 GeV (to be completed 2005) Discovery of the Top Quark Fermilab, USA 1) Produce top uarks in proton-antiproton collisions 2) Electronically scan through data to look for top uark signature events. 3) Generate simulated top uark events and simulated background events on computers. 4) Compare data with simulation to look for more signal events than expected background events.

14 High Energy Physics Future Goals 1) How is the mass of particles generated? Where is the Higgs Boson? 2) Is there a more fundamental theory than the Standard Model? Something must exist at an energy below 1 TeV. a) What about compositeness? Are the uarks and leptons made of something more fundamental? (Technicolor) b) How are all three fundamental forces combined into a single force? (GUTs) c) Are there a whole set of undiscovered particles? (supersymmetry) d) What unexpected surprises await us? Comments about the Higgs [the Higgs is] the toilet of the Standard Model; every house must have one; but no one likes to talk about it. [the Higgs is] the rug of ignorance under which the problems of the Standard Model have been swept. -David Kestenbaum [the Higgs] will smash open the Standard Model. - Chris Hill a single Higgs is just dumb. It doesn t explain anything. -Chris Hill

15 The Standard Model and Its Limits Physicists look on the Standard Model with a mixture of reverence and frustration. Since they have put it together, they have always known that it is incomplete... There must be a larger, more elegant theory. - David Kestenbaum Strong The Unification of the Forces Interaction Strength EM Weak Gravity Electroweak GUT TOE 100 GeV GeV GeV Energy

16 High Energy Physics and Cosmology Big GUTs Hadrons Nuclei galaxies Now bang formed formed Time s s 10-6 s 10 s s s Temperature K K K K 10 3 K 3 K Energy TeV TeV 1 GeV 1 MeV 0.3 ev ev Current accelerators probe at about 0.2 TeV s Benefits of High Energy Physics Answers fundamental uestions about the structure of the universe that man has pondered for millenia. Leads to future technology. Technological advances can only be made when the underlying physical principles are understood. e.g. Electricity, Semi-conductors, Superconductors Spin-off applications result from technologies developed to accelerate, collide and detect particles CT scans, Proton Therapy, World Wide Web Economic benefits (30% annual return on investment). Develops an educated work force.

17 Summary and Conclusions The Standard Model has been experimentally verified. It stands on firm ground with other great classification schemes of science like the periodic table. The discovery of the top uark leaves only one piece of the Standard Model missing, the Higgs Boson. High energy physics brings new knowledge, current technologies to society, and possible future applications. The next decade should be extremely exciting as we probe the structure of nature in the 1 TeV energy range. Hopefully, the resilient Standard Model will break.