Highlights of Contemporary Physics: Particle Physics Module Mark Pitt, 309 Robeson, pitt@vt.edu, 231-3015 Particle Physics: physics of Nature's most fundamental particles and the forces that act between them Goals: At the end of this module you will hopefully know the answer to some of the following questions: 1. Why do physicists study particle physics? 2. What is the "standard model of particle physics" and what are some examples of its successes and remarkable predictions? 3. Why is there a connection between particle physics (small scales) and cosmology (large scales)? 4. Why is such large-scale equipment required to study particle physics? 5. What is still unexplained about particle physics and what sorts of new results can we look forward to over the next ~ 10 years? 6. What are some examples of pure physics research with applications useful to society as a whole?

Highlights of Contemporary Physics: Particle Physics Module Mark Pitt, 309 Robeson, pitt@vt.edu, 231-3015 Administrative information: Writing Assignment: Write a story (or poem) with at least two elementary particles as characters. The behavior of the characters should reflect the properties of the particles. Read a Scientific American article I give you on particle physics and answer a few questions about it. Due on Tuesday December 7, 2010 in class (during last class) Test: Dec. 14, 2010, Tuesday, 10:05 AM – 12:05 PM in Robeson 210 Preparation materials for test (webpage should be ready by this Thursday Nov. 18) 4-5 page writeups for each lecture on the web Complete powerpoint version of lectures on web (but the 4-5 page writeup should contain everything you need to know Worksheet with list of concepts to study and practice problems (we will go over the practice problems at the beginning of the fifth lecture on Tuesday Dec. 7)

Composition of the Universe

Some Length Scales in Particle Physics (1 m = 1 meter) Size of typical complex molecule (collection of atoms) ~ 10-9 m Radius of the orbit of the electron in a hydrogen atom ~ 10-10 m Radius of a Pb (lead) nucleus: (contains 82 protons and 126 neutrons) ~ 10-14 m = 10 F (note: 1 F = 1 Fermi = 10-15 m) Radius of proton ~ 10-15 m = 1 F See more about length scales in the "Powers of 10" film "Radius" of quark or electron < 10-18 m = 0.001 F (ie. "pointlike" as best we can tell currently)

The Full Range of Observed Length Scales Observed length scales range from size of observable universe ~ 1026 m down to smallest sizes studied at high energy accelerators ~ 10-18 m  a 44 order of magnitude range of sizes! Observable by (aided) eye with visible light Only observable with radiation of shorter wavelength (higher energy) than light

Properties of Particles 1. Mass Recall: a) Rest energy E = m c2 b) Units of energy are 1 eV = 1 electron-volt = energy gained by an electron when it is accelerated through a 1 volt potential difference Since m = E / c2 the units for mass are: (unit of energy) / c2 = eV/c2 Note: 1 KeV = 103 eV = Kilo-eV 1 MeV = 106 eV = Mega-eV 1 GeV = 109 eV = Giga-eV  Some common masses: electron: 9.1 x 10-31 kg = 0.000511 GeV/c2 proton: 1.7 x 10-27 kg = .938 GeV/c2 Note: electron is ~2000 times lighter than the proton

Properties of Particles, continued 2. Electric charge: Electric force causes  opposite charges to attract like charges to repel electron: e- Q = -e proton: p Q = +e neutron: n Q = 0 3. Spin: Particles are "spinning" on their own internal axis BUT spin is quantized (only certain values allowed by quantum mechanics)  1/2, 3/2, 5/2, ...  fermions (half-integral spin particles)  0, 1, 2, 3, ...  bosons (integral spin particles) 4. Magnetic moment: = electric charge + spin Example: The proton is like a miniature bar magnet; when you put it in a strong magnetic field it orients itself along the magnetic field direction.  magnetic resonance imaging (MRI)

Important Constant to Know for Calculations

We will now (briefly) follow a historical path to see how we got to this set of fundamental particles and interactions

A Brief Overview of the History of Particle Physics Pre-1897: People knew about atoms, periodic table, atomic masses, etc. 1. Thomson (1897): Discovery of electron First discovery in particle physics observed with a cathode ray discharge tube similar to your television picture screen (see demonstration of this in next lecture) 2. Rutherford (1911): Discovery of atomic nucleus Rutherford experiment lead to the conclusion that there was a small positively-charged nucleus at the center of the atom Before that, people thought the atom was described by the "plum-pudding" model: positive "pudding" negative electrons ("raisins") 10-10 m

Rutherford Experiment   source Au (gold) foil detector = alpha particle = 4He Au = gold nucleus (2 protons, 2 neutrons) (79 protons, 118 neutrons) Calculated: typical deflection angle for  particle interacting with ONE Au plum pudding atom is  ~ 0.01o In typical thin gold foil there are lots of atoms, so the  particle passes within the atomic radius (~10-10 m) of many Au atoms In fact, expect ~ 10000 = 104 scatterings Statistical theory says: average total scattering angle ~

Rutherford Experiment, continued For large (  > 90o) scattering ALL of the individual scatterings must be in the same direction like flipping “heads” 10000 times in a row! Probability of  > 90o : probability of flipping heads 10000 times in a row: = (1/2)10000 ~ 10-3000 Even with 1012  particles/second incident on target it would take ~ 102981 years to observe 1 event! For comparison, universe ~ 1010 years old BUT Rutherford, Geiger, and Marsden (1909) found Probability ( > 90o) ~ 10-4 Conclusion: The positive charge of the atom is concentrated in a small nucleus at the center. In Rutherford’s own words: The proton was explicity “discovered” in 1919 by Rutherford:  + 14N  p + 17O

A Brief Overview of the History of Particle Physics, cont. 3. Antiparticles (1928): Dirac Equation Relativistic version of the quantum mechanical Schroedinger equation Dirac equation predicts the existence of antiparticles Antiparticle: same mass, but opposite charge and magnetic moment Example: e- : electron (Q = -e) and e+ : positron (Q = +e) are antiparticles Positron discovered in 1933 by Anderson matter + antimatter  energy e- + e+   +  p + p   +   = photon 4. Neutron (1932): Neutron: electric charge = 0, mass ~ mass of proton discovered by Chadwick in 1932 At this point (1933), all constituents of everyday matter (e-, p, n) were known. Why did people continue on doing this research after that?

A Brief Overview of the History of Particle Physics, cont. 5. Create new particles (~1920's - 1950's): Use cosmic rays from outer space Example: Iron (Fe) nucleus with kinetic energy 5000 GeV bombarding atoms in the upper atmosphere  This high energy particle interacts with nuclei in the upper atmosphere to create new, short-lived (10-6  10-23 seconds) Examples of new particles observed during this period: Muons: + - mass ~ .105 GeV/c2 Pions: + - mass ~ .140 GeV/c2 both ~ 1/10 of the mass of the proton

Example of a Cosmic Ray "Shower" of Particles 5000 GeV Iron nucleus ~ 200 new particles created (mostly pions = + - ) nuclear photographic emulsion

A Brief Overview of the History of Particle Physics, cont. 6. 1950’s: Advent of particle accelerators (Berkeley, Brookhaven) Accelerate particles (mostly electrons or protons) to high energies Collide with targets (like protons in hydrogen atoms) use particle detectors to observe NEW particles  The new particles created are short-lived (10-6  10-23 seconds)  Hundreds of new particles observed (see Particle Data Book) At this point, some organization was needed to this “particle zoo.” It was believed that nature couldn’t be so complicated at its most “fundamental level.”

A Brief Overview of the History of Particle Physics, cont. 7. Organization arrives in 1964: Quark model (Murray Gell- Mann) up u : q = charge= +2/3 e down d : -1/3 e strange s : -1/3 e  Prediction of fractionally charged particles which compose all hadrons (of which the proton and neutron are the simplest examples)  Gell-Mann’s theory could explain all known hadrons at that time Hadrons Baryons Mesons 3 quark objects 2 quark objects Proton (u u d) 2/3 + 2/3 – 1/3 = +1 + (u d ): 2/3 + 1/3 = +1 Neutron (d d u) -1/3 – 1/3 + 2/3 = 0

A Brief Overview of the History of Particle Physics, cont. 8. Further quark discoveries: 1974: charm (c) quark mass = 1.5 GeV/c2 1977: bottom (b) quark mass = 4.7 GeV/c2 1996: top (t) quark mass = 175 GeV/c2 Current picture of fundamental “matter constituents”: 3 families of leptons + 3 families of quarks

How do these fundamental particles interact with each other? Current picture of fundamental “matter constituents”: 3 families of leptons + 3 families of quarks How do these fundamental particles interact with each other?  Through the four fundamental forces

Classical Description of Forces Particles feel a force “field” emanating from other particles Examples: M1 M2 R Q1 Q2 R

Modern Description of Forces “Quantum Field Theory”  Forces between particles are caused by the exchange of discrete particles (called exchange particles or exchange bosons) The type of exchange particle depends on the force examples: electromagnetic force: photon () m = 0 weak force: Z boson: mZ = 90 GeV/c2 BUT doesn’t this violate energy conservation?