Wesley Smith, U. Wisconsin, January 21, Physics 301: Introduction - 1

Transcription

1 Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 1

2 Physics 301: Physics Today Prof. Wesley Smith, Undergraduate Physics Colloquium! Discussions of current research topics in physics by the scientists involved in those studies! You are encouraged to contact these researchers to find out more about, and possibly participate in their research programs. Coursework! Paper describing a particular piece of physics research being actively pursued this year.! Outline due April 7! Paper (7-10 pages) due on May 5! More information on course web page Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 2

3 Physics 301 Schedule Wesley Smith Introduction/Particle Physics Jan. 20, 2015 Matt Herndon The Higgs Boson Jan. 27, 2015 Mark Rzchowski Building new physics with atomic-level design Feb. 3, 2015 Franz Himpsel Photovoltaics Feb. 10, 2015 Albrecht Karle High Energy Neutrino Astrophysics with Ice Cube Feb. 17, 2015 Duncan Carlsmith Direct Search for Dark Matter Feb. 24, 2015 Peter Timbie Physics of the Early Universe March 3, 2015 John Kelley South Pole Ultra-high-energy neutrinos w/ara March 10, 2015 Maxim Vavilov Quantum Information March 17, 2015 Mark Saffman Atoms and Computers March 24, 2015 Clint Sprott Chaos April 7, 2015 Yang Bai Puzzles for Particle Physics April 14, 2015 Mark Eriksson Semiconductor Quantum Dot-Based Qubits April 21, 2015 Dan McCammon X-ray Astronomy April 28, 2015 Bob Joynt Superconductivity May 5, 2015 Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 3

4 Particle Physics Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 4

5 Interactions between matter particles Matter particles interact via exchange of force particles Nuclei- need a strong interaction to overcome coulomb repulsion of the protons. Gluons are the force carriers Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 5

6 Electromagnetic Force Normal electromagnetic force comes about from exchange of photons. Electromagnetic repulsion via emission of a photon electron photons Exchange of many photons allows for a smooth force (EM field)! For a very quick interaction we can see individual photon exchanges electron Probability proportional to coupling strength divided by momentum Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 6

7 Annihilation The new EM Theory has one very interesting additional feature Can rotate diagrams in any direction electron positron photon positron photon electron electron Time goes from left to right. What is an electron going backward in time? electron Antiparticles! Anti-electron or positron. This is going to be a useful way to make new particles. Also learned from studying EM force that the proton and neutron were made of smaller particles. up and down quarks. p=uud, n=udd Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 7

8 Forces and Field Particles Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 8

9 Many particles discovered Leptons Electron, neutrino, and 4 others Paired with 6 antiparticles Fundamental don t subdivide Relatively light mass Hadrons Proton, neutron, and many others Paired with antiparticles Interact through the strong force Can decay through weak force Not fundamental subdivide into quarks Two groups: Mesons made of 2 quarks intermediate mass Baryons made of 3 quarks heavier mass Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 9

10 Many particles discovered Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 10

11 Quarks Since 1969, many other experiments have been conducted to determine the underlying structure of protons/neutrons. (~GeV) All the experiments come to the same conclusion.! Protons and neutrons are composed of smaller constituents. 1x m (at most) "!Protons 2 up quarks 1 down quark (1.6 x m) "!Neutrons 1 up charge 2/3 2 down -1/3 Are there any other quarks other than UP and DOWN? Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 11

12 Three Families of Quarks Generations Note: fractionally charged particles! Charge = -1/3 Charge = +2/3 I II III d (down) u (up) Increasing mass s (strange) c (charm) b (bottom) t (top) u,d,s,... Also, each quark has a corresponding antiquark The antiquarks have opposite charge to the quarks Many hadrons possible: 3 quarks baryons, 1 quark +1 antiquark mesons Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 12

13 Leptons: Muon The muon was discovered in cosmic ray experiments (1937). It was also used in the experimental test of time dilation. We find that a muon behaves almost identical to an electron, except its mass is about 200 times more than the electron s mass. em=0.51 MeV/c2 µm=106 MeV/c2 Also neutral leptons: neutrinos Neither bind to form hadrons. Don t feel strong force Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 13

14 Three happy families! In 1975, researchers at the Stanford Linear Accelerator discovered a third charged lepton, with a mass about 3500 times that of the electron. It was named the!-lepton.! In 2000, first evidence of the! s partner, the tau-neutrino ("! ) was announced at Fermi National Accelerator Lab. Family Leptons Q = -1 Q = 0 Q = +1 Anti-Lepton Q = 0 1 e - " e 2 µ " µ 3! "! e + µ +! + " e " µ "! 3 families, just like the quarks interesting!!! Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 14

15 Conservation Law: Baryon Number Whenever a baryon is created in a reaction or a decay, an antibaryon is also created B is the Baryon Number! B = +1 for baryons! B = -1 for antibaryons! B = 0 for all other particles The sum of the baryon numbers before a reaction or a decay must equal the sum of baryon numbers after the process Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 15

16 Conservation of Lepton Number There are three conservation laws, one for each variety of lepton Law of Conservation of Electron-Lepton Number states that the sum of electron-lepton numbers before a reaction or a decay must equal the sum of the electronlepton number after the process Assigning electron-lepton numbers! L e = 1 for the electron and the electron neutrino! L e = -1 for the positron and the electron antineutrino! L e = 0 for all other particles Similarly, when a process involves muons, muon-lepton number must be conserved and when a process involves tau particles, tau-lepton numbers must be conserved! Muon- and tau-lepton numbers are assigned similarly to electron-lepton numbers Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 16

17 Strange Particles Some particles discovered in the 1950 s were found to exhibit unusual properties in their production and decay and were given the name strange particles! These include K,!, ". Peculiar features include! Always produced in pairs! Although produced by the strong interaction, they do not decay into particles that interact via the strong interaction, but instead into particles that interact via weak interactions! They decay much more slowly than particles decaying via strong interactions To explain these unusual properties, a new law, conservation of strangeness, was introduced! Also needed a new quantum number, S The Law of Conservation of Strangeness states that the sum of strangeness numbers before a reaction or a decay must equal the sum of the strangeness numbers after the process Strong and electromagnetic interactions obey the law of conservation of strangeness, but the weak interactions do not Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 17

18 Charge u Electric charge = +2/3 e Electric charge = -1 What does it really mean for a particle to have electric charge? It means the particle has an attribute which allows it to talk to (or couple to ) the photon, the mediator of the electromagnetic interaction. The strength of the interaction depends on the amount of charge. Which of these might you expect experiences a larger electrical repulsion? u e u e Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 18

19 Weak Force Proposed a massive force-carrying particle for the weak force P! F 2! e 4 /(p 2 +M B2 ) 2!!!!!!! Coupling strength: Same as EM force p momentum of the W or Z bosons If the mass of the W boson was large compared to the momentum the probability of a weak interaction could be very low! Same formula for electric and weak forces: Put in M B large for the W boson Put in M B =0 for the photon W - Unified the forces! M W = 80GeV Later seen directly n d u d e - " e u u d p Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 19

20 The Forces Revisited E M Weak Strong Gravity Couples to: Particles with electric charge Weak charge: quarks and leptons Color charge: quarks All particles with mass Example Attraction between protons and electrons Nuclear beta decay and nuclear fission Holds protons and neutrons together the nucleus Only attractive Quanta: Force Carrier Photon W and Z Boson Gluon Graviton Mass 0 80 and 91 GeV 0 0 Strength in an Atom F = 2.3x10-8 N Decays can take thousands of years F = 2.3x10 2 N F = 2.3x10-47 N Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 20

21 Fermions and Bosons A fermion is any particle that has an odd half-integer (like 1/2, 3/2, and so forth) spin. Quarks and leptons, as well as most composite particles, like protons and neutrons, are fermions. A consequence of the odd half-integer spin is that fermions obey the Pauli Exclusion Principle and therefore cannot co-exist in the same state at same location at the same time. Bosons are those particles which have an integer spin (0, 1, 2...). All the force carrier particles are bosons, as are those composite particles with an even number of fermion particles (like mesons). *Graviton has spin 2 Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 21

22 The Standard Model What is the Standard Model?! Explains the hundreds of common particles: atoms - protons, neutrons and electrons! Explains the interactions between them Basic building blocks! 6 quarks: up, down! 6 leptons: electrons! Bosons: force carrier particles All common matter particles are composites of the quarks and leptons and interact by exchange of the bosons Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 22

23 Origin of mass - the Higgs mechanism Simplest theory all particles are massless!! A field pervades the universe Particles interacting with this field acquire mass the stronger the interaction the larger the mass The field is a quantum field the quantum is the Higgs boson Finding the Higgs particle establishes the presence of the field Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 23

24 Higgs Mechanism Which of these falls more slowly?! An unopened parachute! Fully opened parachute The greater the interaction with the medium (air) the lower the falling speed. Higgs field permeates all space, particles interact with differing strengths with the Higgs field. The higher the interaction the larger the mass of the particle. The simplest theory with Higgs fields results in a new self-interacting particle: the Higgs boson, which itself has a mass but, theory can t predict its mass. Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 24

25 Further investigations: The Problems of Standard Model Higgs self energy corrections! Higgs couples to itself! However, this coupling becomes infinite!! Contributions ot this from fermions (leptons and quarks) and vector bosons (W, Z) come with opposite sign! SM particles can mitigate to about 1 TeV energy scale! However, new physics should show up at! few TeV Super-symmetry! What if there are equal number of fermions and bosons in the real theory at high masses (~few TeV)?! Many new fundamental scalar and fermionic fields # must be massive to fit observations! But, this could solve the problem of Higgs divergences # Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 25

26 Supersymmetry A new physics theory which doubles known particles again but the new particles have very large mass LHC may be able to produce them Dark matter candidates Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 26

27 Outstanding Mysteries Dark Matter Gravitational lensing The Bullet Cluster (1E ). Two galaxies colliding. Red shows concentration of visible matter. Blue shows dark matter inferred by gravitational lensing. What is dark matter composed of?!supersymmetric particles perhaps? The lightest supersymmetric particle predicted by theory has all the right properties! Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 27

28 Dark Matter & Energy? The calculated makeup of the Universe We only understand 4.6% of it after 100s of years of trying!! Don t know what Dark Matter is Don t know what Dark Energy is but SOMETHING is accelerating the expansion of our Universe Supersymmetry? The Neutralino particle? A new force field particle, like the Higgs, Wesley Smith, U. Wisconsin, January 21, 2014 Physics 301: Introduction - 28