1 / 16

PH 301

PH 301. Dr. Cecilia Vogel Lecture 19. Review. quarks and leptons matter and antimatter. conservation laws hadrons, baryons, mesons flavor and color. Outline. Conservation Laws. Energy momentum – in CM frame, p=0 angular momentum (spin) lepton #, which = # leptons-#antileptons

dillon-chavez
Télécharger la présentation

PH 301

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. PH 301 Dr. Cecilia Vogel Lecture 19

  2. Review • quarks and leptons • matter and antimatter • conservation laws • hadrons, baryons, mesons • flavor and color Outline

  3. Conservation Laws • Energy • momentum – in CM frame, p=0 • angular momentum (spin) • lepton #, which = # leptons-#antileptons • each “generation” • baryon #, which = (#quarks-#antiquarks)/3 • electric charge • color charge • always neutral –don’t worry • flavor is conserved, except in weak interactions • often accompanied by neutrino

  4. PAL - Conservation Check all 8 Conservation Lawsfor the neutron-decay reaction shown below:

  5. Lepton # Conservation • lepton # is conserved • # electron leptons - # elect. antileptons = constant • # mu leptons - # mu antileptons = constant • # tau leptons - # tau antileptons = constant • example • t - e- + nt +ne • electron lepton number initial =0 • electron lepton number final = 1-1 = 0 • conserved • tau lepton number initial = 1 • tau lepton number final = 1 • conserved

  6. Baryons • baryon is • any particle made of 3 quarks • such as p and n • antibaryon made of 3 antiquarks • such as p = uud • meson is • any particle made of quarks and antiquark • such as p+ = ud • which has charge =? (add quark & antiquark charges) • pentaquark is • any particle made of 4 quarks and an antiquark

  7. Baryon # Conservation • # quarks - # antiquarks = constant • Usually OK to use • # baryons - # antibaryons = constant • so long as no pentaquarks involved • example: a proton can’t turn into a positron • initial baryon # = 1 • final baryon # = 0 • not conserved — won’t happen!

  8. Momentum Conservation • pi =pf • for simplicity, always consider the center of mass frame for the initial particles • pi =0 • then pf =0 • example: e+ + e-  photons • can it just be one photon? NO • pf=h/l is not =0, p not conserved • Can it be two photons? YES • yes — if have same l and move in opposite dir • pf = h/l - h/l = 0 (conserved)

  9. Energy Conservation • Ei = Ef • mass is NOT conserved • can be turned to and from other forms of energy • example: e+ + e-  photons • If e- and e+ are approx at rest when they annihilate, what are the wavelengths of the photons? • Ei = (0.511MeV)+(0.511MeV) • Ef = 2hc/l. • l = 2.4 pm

  10. Angular Momentum Conservation • Angular momentum is conserved • must include spin • example: e+ + e-  g + g • spin of e is ½, spin of g is 1 (see table) • so ½ + ½ = 1 + 1??? • no • angular momentum must be added as vectors

  11. Angular Momentum Addition • angular momentum must be added as vectors • classical ang mom vectors can add to anything • from the difference between the two ang mom’s • to the sum • quantum ang mom vectors can add to anything • from the difference between the two ang mom’s • to the sum • in integer steps

  12. Angular Momentum Addition • quantum ang mom vectors can add to anything • from the difference between the two ang mom’s • to the sum, in integer steps • example: e+ + e-  g + g • ½ and ½ can add to zero • 1 and 1can also add to zero • so angular momentum can be conserved, 0=0 • other examples: • ½ and ½ can add to 0 or 1 • 1 and 1 can add to 0, 1, or 2 • ½ and 1 can add to ½ or 3/2

  13. Electric Charge Conservation • Electric charge is conserved • example: neutron decays into a proton • baryon # would be conserved (1=1) • but charge would not be conserved (0≠1) • so another particle produced = electron • but now electron lepton # not conserved (0≠1) • what can we add that will fix lepton # • without messing up baryon # or charge? • answer: an uncharged antilepton • All conserved in this:

  14. Flavor Conservation • Flavor is conserved • well, not really • in • a down quark turns into an up quark • quark flavor not conserved • Flavor is conserved • except in weak interactions • identified by the appearance of neutrinos

  15. Color Conservation • Color is conserved • what is color? • quarks have color charge • red or green or blue • baryons have white or neutral color charge • one quark each • red + green + blue = white

  16. Color Conservation • anti quarks have color charge • antired (cyan) or antigreen (magenta) or antiblue (yellow) • mesons have white or neutral color charge • one quark color, one quark anticolor • e.g. blue + antiblue = blue + yellow = white • Color is conserved • because it’s always neutral

More Related