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Types of Radiation: Modern Particle, Charge, Mass, Range,
- Alpha: Helium nucleus, +2e, 3730 (heavy), very short
- Beta: electron, -e, .511 (light), moderate
- Gamma & X-Rays: photon, 0, 0, very large
Radioactivity-Discovery/Understanding/Point Physicists toward
Roentgen's observation of X-rays in 1985, physicists intensified study of invisible phenomena. Becquerel in 1986 discovered natural radioactivity by observing x-ray emission from Uranium salts-indicates non-conservation of energy. Curies showed many elements are radioactie. Solved energy conservation in 1919 Rutherford/Soddy elements are transmuting in process of radioactive decay b/c nucleus is not fundamental-p&ns.
Alpha Rays to Study Atomic Structure
Discovery of Split Nucleus
- 1) Alphas were projectiles in scattering experiments performed to study the structure of matter. they are useful because of their large mass and high charge-perturb nucleus strongly.
- 2) Rutherford 1917-1919 by bombarding Nitrogen gas kept ina small chamber with alpha particles. observed radiation leaving his chamber w/ longer range than alpha and like hydrogen nuclei (p). chips of Ni nuclie knowcked off. nuclei not fundamental.
Thomsons discovery of e-/Discovery of Nucleus-Understanding components of atom
- 1) Thomson's discovery of the electron and its presence in all matter showed that the atom had to have a positiev component to balance the negative component.
- 2) Changed view of atom to one where all positive charge and mass is in a small region surrounded by negative e-. set stage for Bohr's planetary model. Gold foil.
Pauli's exclusion Principle
- 1) QM gave way to describe observable features of atoms-spectrum of radiation they emit.
- 2) PEP-no 2 e- can share all same quantum numbers, allowed all chemical properties of elements to be explained. distribution of e- in orbitas of an atom determines how that atom interacts chemically w/ other atoms. could explain all distribution, predict chemical prop.
Discovery of components of nucleus
- Proton: particle was known since discovery of hydrogen and ionization. Rutherford named.
- Neutron: Chadwick, 1932. He showed that it could not be a gamma ray, by measuring scattering from p, He, Ni. unless gamma had same mass as p. after Chadwick, all in place. Neg cloud of e- whose charge was balanced by same # of p's innucles w/ neutrons to give observed mass of atom.
Two Processes in Atomic/Sub
Quantities conserved/Not conserved
- Decay: A-->B+C+..
- Scattering A+B-->C+D+
Conserved: electric charge, relativistic energy/momentum, total angular momentum, parity (EM/Strong), Baryon/Lepton #
Not: Mass, Parity (weak), Meson #
Lifetime vs. Decay Time
- -a particle that has a lifetime is unstable and will decay to other particles
- -decay time refers to specific period of time between production and decay of a single unstable particle. each unstable particle has a different decay time.
- -for each type of particle decay, there is a fixed prblity that the decay will occur in a small time interval.
- -in a collection of a large number of unstable particles, the number remaining decreases with time. the avg of this exponential distribution is the lifetime of the particle.
- -decay time is a property of a single unstable partice, lifetime is average dcay time of group of the same type of particles.
Cross-section related to scattering
Cross sections are a measure of the probability of a scattering process occurring. Shooting bullets randomly at a target. Target is large, probability of hitting is large, etc. Cross-sections, like lifetimes, are related to the strength of the force involved in the interaction. Strong forces lead to short lifetimes and large cross-sections.
Gold Foil: Back-Scattering/Using Alpha Particles
- 1) If the atom had been made up of e- in a "pudding" of positive charge, there would not have been enough mass anywhere to cause heavy alpha particles to rebound.
- 2) Problem with using x-rays or electrons is that they would be scattered by the e- in the atom, alphas are unaffected by light electrons and scatter off nucleus.
Same number of protons/electrons?
Why did people expect Neutral particles?
- 1) If they were uneven, the atom would be a charged object and would be strongly attracted/repelled by other atoms and forming matter would be hard.
- 2) The masses of Z protons and Z electrons in nucleus did not add up to the correct mass. Couldn't add more protons, b/c it would change the charge. Had to make up the mass with neutral particles.
Main conceptual difficulty that nucleus has p/n in a region 10^-14 m?
Neutron more difficult to observe than a proton/alpha particle?
- 1) The problem was how to hold all the positive charge from the Z protons together. Positive charges repel. So what was keeping them together.
- 2) Neutrons are only produced by nuclear interactions, can't be made chemically like protons, not common in naturally occuring radioactive substances like alphas. To see neutron-->fire projectile at nuclues. It does not interact EM with matter, leave tracks in a cloud chamber, not bend by B field. It's unstable-lifetime of 15 minutes.
Feynman Diagram about interaction of two electrons
Violates UP, why is it possible?
- 1) Photon is messenger particle that transmits force between two electrons. carrying away some of one electron's energy when it is emitted and transferring the energy to the other e- when it is absorbed.
- 2) Energy and momentum conservation are violated. If energy E is observable, it must occur over a long enough period of time. If it's less than t>h(2pi E) then can't be observed. Non-observation of energy comes from our picture of the process, since we can't observe that picture, we can cheat conservation law.
What should spin of pion be?
Why is it not mesotron?
- charge: negative, neutral, positive
- -Mesotrons did not interact with matter in the way in which Yukawa's particle should have. The negative version of pion should have been absorbed more quickly than the + version. Both charges of mesotron could sail through large amounts of material, and lifetime was too long.
structure of Standard Model, which particles are fundamental and why?
The Standard Model is the theory of EM, Weak and Strong Forces, an describes the interaction of fundamental particles in exchanges mediated by these forces. Fundamental particles have no constituents, indivisible, no substructre. quarks, leptons, bosons. 6 quarks, 6 leptons in 3 generations. Bosons are force carriers.
Parity Inverted Process
Probability or Decay rates
Which process will happen more often
- 1) Flip p--> -p but s stays the same.
- 2) If weak force conserved partiy the two processes would occur with the same probability. Since parity conservation is violated in weak decays, they will be at different rates.
- 3) Parity Inverted will never happen, parity is maximally violated in weak interactions.
Use of Particle Accelerators led to discovery of new particles 1950's
-necessary to use high-energy beams of charged particles?
-no cosmic rays?
- High energies are required because most new particles are heavy objects. Producing them requires lots of energy to be converted into the new particle's mass. Neutral could be used, but difficult to get to accelerate to high energies.
- 2) cosmic rays occur rarely.
Modern Particle Detector
- Muon Detectors, Magnetized Iron, Hadronic Calorimeter, EM Calorimeter, Solenoid Magnet, Tracking Detectors, Beam Pipe
- -Tracking in a B-field measures charge, momentum, direction, by recording position of ionizaiton as particle passes thru solid/gas
- -Calorimetry measures energy by detecting interactions as particles lose energy while going thru material
- -Muon detectors measure muons by tracking detectors placed outside detector seeing muons not absorbed by material calorimeters
- -Data Acquisition selects events for later analysis by high speed electronics and computers.
Forces, No. of Conservation, Charge, Spin, Components/Color, Examples of Lepton, Meson, Baryon
- Lepton: W,EM--N(e), N(u), N(t)--0 +/-1--1/2--fundamental, no color--electron, muon, neutrino
- Meson: W, EM, S--none--0, +/-1--0,1,2--q/anti-q, color/anti-color--pion, kaon,
- Baryon: W, EM, S--N(baryon)--0,+-1, +-2--q1q2q3, 3 different colors, anti-qs w/ anti-color for anti-baryon--proton, neutron, lambda
Any constituents of Particles above?
Quarks in mesons and baryons are bound together by strong force so they are also gluons present in these forces. Mesons and baryons also contain virtual quarks and anti-quarks. Leptons are also surrounded by a "fog" of virtual particles.
Pauli Exclusion Principle applied to delta++baryon (uuu) suggest that quarks had to change colors?
the delta++ is a spin 3/2 particle. the spins of it's u-quarks must point in same direction, three identical spin 1/2 particles in same quantum state must be confined to small region. violates PEP, so must assign a new quantum number to the quarks, if all 3 have different value of quantum number, they can cohabit. Had to invent 3 colors. Colors are properties of particle that allows it to experience strong force.
Main conceptual difficulty of Quark theory? Strong force explain it?
It was an object with a charge that was not an integer multiple of the electron charge, e. Quarks have charges 1/3, 2/3 of e. Strong force, which binds quarks together, increases strength the farther apart the quarks get. Never free particles, because it would take infinite force to tear them away. Another consequence is that color-change can never be observed. objects are color neutral. their quarks are color+anti-color (mesons), 3 different colors (baryons) 3 different anti-colors (anti-baryons)
Spontaneous Symmetry Breaking, why is an essential element of Standard Model
SSB is a mechanism whose most important role is to provide mass to the fundamental particles of Standard Model. The first observation is the observation that fermions and especially the electroweak boson have mass, even though underlying condition in QED is for bosons to be massless. Mass is acquired through a new field called the Higgs field, which permeates all apace. Fermions and bosons move through this field, their interaction with the field is the mechanism by which they acquire mass. How people acquire language.
Charge, Mass, Spin and discovery of Fundamental Particles
- Leptons (all spin 1/2) :
- -e-neutrino: 0,0, 1933 proposed by Pauli and Fermi, 1955 at Hanford by Reines and Cowan. Inverse beta decay using neutrinos
- -electron: -e, .511 MeV, 1897 by JJ Thomson, q/m using motion in E-and B-fields.
- -mu-neutrino: 0,0 11962 Lederman, Schwartz, and Steinberger, created muon-neutrinos from pion decay.
- -muon: -e, 105 MeV, 1937 at Caltech by Anderson and Neddemeyer, penetrating tracks in cloud chamber from cosmic rays
- tau neutrino: 0,0 2000 Fermilab by Donut collaboration
- tau: -e, 1777 MeV, 1975 SLAC by Perl
Charge, Mass, Discovery of QUARKS
- spin 1/2, 3 colors each
- -up (u): +2/3e. .001-.005 GeV, 1964 proposed by Gell-Mann, 1966 by Friedman, Kendall, Taylor
- -down (u): -1/3e, .003-.009 GeV, same as u
- -charm (c): +2/3e, 1.2-1.4 GeV, observed at high-mass resonance with a width too narrow for u,d,s.
- -strange (s):-1/3e, .08-.17 GeV, same as u
- -top (t): +2/3e, 174 GeV, observed in proton-antiproton collisions
- - bottom (b): -1/3 e, 4-4.5 GeV, observed a high-mass resonance with a width too narrow to be explained by u,d,s,c.
- photon: Electromagnetic, 0, 0, 1905 photoelectric effect by Einstein, Planck's explanation for blackbody radiation
- W: Weak 10^-5, +/-e, 80.4 GeV, 1983 CERN, proton-antiproton collisions
- Z: Weak 10^-5, 0, 91.2 GeV. same.
- Gluon: Strong,1, 0, 0, Observation of 3-jet events in e+e- collisions at 30 GeV
EW SYMMETRIC BREAKING spin 0 particle
Higgs, 0, 114-200 GeV, 1967 integral part of electroweak theory of Glashow, Weinberg, Salam, not yet observed
Experimental Results that are at odds with Standard Model?
- -Higgs boson has not yet been observed.
- -Neutrinos have mass and oscillate between flavors, in current SM their mass is zero.
- -Observed composition of the universe is unknown, Dark Matter/Dark Energy are required to explain many astrophysical phenomena in a coherent way, but these things aren't in SM.
Main Conceptual Problems with Standard Model
- -SM does not explain any of the experimental problems.
- -19 parameters are required by the model.
- -SM gives no reason why there are 3 families of leptons and quarks.
- -Electro-weak symmetry breaking has no reason.
- -Values of Higgs mass is very unstable due to self-energy corrections. cured with 16 decimal places. Naturalness problem.
- -scale of electro-weak interactions is very different from the natural scale of gravity, important when discussing quantum effects. Hiearchy Problem.
- -SM predicts that universe is made of equal parts matter and anti-matter.
- -force of Gravity cannot be incorporated into mathematical structure of SM.
theory that extends SM
Supersymmetry addresses naturalness probem (Higgs mass is unstable due to self-energy corrections, cured with parameter arbitrarily set to 16 decimal places). Theory postulates the existence of super partners for all known SM particles. super-partners are differ from SM counterparts by having spins that are different by 1/2 a unit. the super-partners cancel the SM interactions that cause Higgs mass to be very high. the supersymmetric theories predict existence of particles that have the properties of dark matter.