Lectures 2

Information about Lectures 2

Published on October 16, 2007

Author: Maitane

Source: authorstream.com

Content

Section IV:  Section IV The Standard Model TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAAAAA The Standard Model:  The Standard Model Spin ½ Fermions LEPTONS QUARKS PLUS antileptons and antiquarks. Spin 1 Bosons Mass (GeV/c2) Gluon g 0 STRONG Photon g 0 EM W and Z Bosons W, Z0 91.2/80.3 WEAK The Standard Model also predicts the existence of a spin 0 HIGGS BOSON which gives all particles their masses via its interactions. Charge (units of e) -1 0 2/3 -1/3 Theoretical Framework:  Theoretical Framework Macroscopic Microscopic Slow Classical Mechanics Quantum Mechanics Fast Special Relativity Quantum Field Theory The Standard Model is a collection of related GAUGE THEORIES which are QUANTUM FIELD THEORIES that satisfy LOCAL GAUGE INVARIANCE. ELECTROMAGNETISM: QUANTUM ELECTRODYNAMICS (QED) 1948 Feynman, Schwinger, Tomonaga (1965 Nobel Prize) ELECTROMAGNETISM: ELECTROWEAK UNIFICATION +WEAK 1968 Glashow, Weinberg, Salam (1979 Nobel Prize) STRONG: QUANTUM CHROMODYNAMICS (QCD) 1974 Politzer, Wilczek, Gross (2004 Nobel Prize) Problems with Non-Relativistic Schrodinger Equation:  Problems with Non-Relativistic Schrodinger Equation Not Lorentz Invariant! ħ =1 natural units Klein-Gordon Equation (relativistic):  Klein-Gordon Equation (relativistic) From Special Relativity: From Quantum Mechanics: Combine to give: KLEIN-GORDON EQUATION Second order in both space and time derivatives and hence Lorentz invariant. Plane wave solutions provided that allowing Negative energy solutions required to form complete set of eigenstates ANTIMATTER Antimatter:  Antimatter Feynman-Stückelberg interpretation: The negative energy solution is a negative energy particle travelling backwards in time. And then, since:  Interpret as a positive energy antiparticle travelling forwards in time. Then all solutions describe physical states with positive energy, going forward in time. Examples: e+e- annihilation pair creation All quantum numbers carried into vertex by e+, same as if viewed as outgoing e-. More particle/anti-particle examples:  More particle/anti-particle examples In first diagram, the interpretation easy as the first photon emitted has less energy than the photon it was emitted from. No need for “anti-particles” or negative energy states. In the second diagram, the emitted photon has more energy than the electron that emitted it. So can either view the top vertex as “emission of a negative energy electron travelling backwards in time” or “absorption of a positive energy positron travelling forwards in time”. Normalization :  Normalization Can show solutions of Schrodinger Equation (non-relativistic) satisfy: and solutions of Klein-Gordon Equation (relativistic) satisfy: Both are of the form: and the divergence theorem tells us that this means that D is conserved, where: If we interpret D as number of particles, and  as number of particles per unit volume, then: For plane wave of arbitrary normalization find that: Non-relativistic: particles per unit volume. Relativistic: particles per unit volume. Interaction via Particle Exchange:  Interaction via Particle Exchange Consider two particles, fixed at and , which exchange a particle of mass m. Calculate shift in energy of state i due to this process (relative to non-interacting theory) using 2nd order perturbation theory: Work in a box of volume L3, and normalize s.t. i.e. use 2E particles per box! Sum over all possible states j with different momenta. Slide10:  Consider ( transition from i to j by emission of m at ) represents a free particle with Let g be the “strength” of the emission at . Then: Similarly is the transition from j to i at and is Substituting these in, we see the shift in energy state is Original 2 particles Slide11:  Normalization of source strength “g” for relativistic situations Previously normalized wave-functions to 1 particle in a box of side L. In relativity, the box will be Lorentz contracted by a factor g i.e. particles per volume V. (Proportional to 2E particles per unit volume as we already saw a few slides back) Need to adjust “g” with energy (next year you will see “g” is “lorentz invariant” matrix element for interaction) Conventional choice: (square root as g always occurs squared) Source “appears” lorentz contracted to particles of high energy, and effective coupling must decrease to compensate. The presence of the E is important. The presence of the 2 is just convention. The absence of the m (in gamma) is just a convention to keep g dimensionless here. Rest frame 1 particle per V Lab. frame 1 particle per V/g Beginning to put it together:  Beginning to put it together Different states j have different momenta . Therefore sum is actually an integral over all momenta: And so energy change: Final throes:  Final throes The integral can be done by choosing the z-axis along . Then and so Write this integral as one half of the integral from - to +, which can be done by residues giving Appendix D Slide14:  Can also exchange particle from 2 to 1: Get the same result: Total shift in energy due to particle exchange is ATTRACTIVE force between two particles which decreases exponentially with range r. Yukawa Potential:  Yukawa Potential YUKAWA POTENTIAL Characteristic range = 1/m (Compton wavelength of exchanged particle) For m0, infinite range Yukawa potential with m = 139 MeV/c2 gives good description of long range interaction between two nucleons and was the basis for the prediction of the existence of the pion. Hideki Yukawa 1949 Nobel Prize Scattering from the Yukawa Potential:  Scattering from the Yukawa Potential Consider elastic scattering (no energy transfer) Born Approximation Yukawa Potential The integral can be done by choosing the z-axis along , then and For elastic scattering, and exchanged massive particle is highly “virtual” q2 is invariant “VIRTUAL MASS” Virtual Particles:  Forces arise due to the exchange of unobservable VIRTUAL particles. The mass of the virtual particle, q2, is given by and is not the physical mass m, i.e. it is OFF MASS-SHELL. The mass of a virtual particle can by +ve, -ve or imaginary. A virtual particle which is off-mass shell by amount Dm can only exist for time and range If q2 = m2, then the particle is real and can be observed. Virtual Particles ħ =c=1 natural units Slide18:  For virtual particle exchange, expect a contribution to the matrix element of where Qualitatively: the propagator is inversely proportional to how far the particle is off-shell. The further off-shell, the smaller the probability of producing such a virtual state. For m  0 ; e.g. single g exchange q2 0, very low energy transfer EM scattering COUPLING CONSTANT STRENGTH OF INTERACTION PHYSICAL (On-shell) mass VIRTUAL (Off-shell) mass PROPAGATOR Feynman Diagrams:  Feynman Diagrams Results of calculations based on a single process in Time-Ordered Perturbation Theory (sometimes called old-fashioned, OFPT) depend on the reference frame. The sum of all time orderings is not frame dependent and provides the basis for our relativistic theory of Quantum Mechanics. The sum of all time orderings are represented by FEYNMAN DIAGRAMS. Space + = FEYNMAN DIAGRAM Slide20:  Feynman diagrams represent a term in the perturbation theory expansion of the matrix element for an interaction. Normally, a matrix element contains an infinite number of Feynman diagrams. But each vertex gives a factor of g, so if g is small (i.e. the perturbation is small) only need the first few. Example: QED Total amplitude Total rate Fermi’s Golden Rule Anatomy of Feynman Diagrams:  Feynman devised a pictorial method for evaluating matrix elements for the interactions between fundamental particles in a few simple rules. We shall use Feynman diagrams extensively throughout this course. Represent particles (and antiparticles): and their interaction point (vertex) with a “ “. Each vertex gives a factor of the coupling constant, g. Anatomy of Feynman Diagrams Slide22:  External Lines (visible particles) Internal lines (propagators) Each propagator gives a factor of Slide23:  Examples: WEAK Section V:  Section V QED QED:  QED QUANTUM ELECTRODYNAMICS is the gauge theory of electromagnetic interactions. Consider a non-relativistic charged particle in an EM field: given in term of vector and scalar potentials Maxwell’s Equations Change in state of e- requires change in field  Interaction via virtual g emission Slide26:  Schrodinger equation is invariant under the gauge transformation  LOCAL GAUGE INVARIANCE LOCAL GAUGE INVARIANCE requires a physical GAUGE FIELD (photon) and completely specifies the form of the interaction between the particle and field. Photons (all gauge bosons) are intrinsically massless (though gauge bosons of the Weak Force evade this requirement by “symmetry breaking”) Charge is conserved – the charge q which interacts with the field must not change in space or time. DEMAND that QED be a GAUGE THEORY Appendix E The Electromagnetic Vertex:  The Electromagnetic Vertex All electromagnetic interactions can be described by the photon propagator and the EM vertex: The coupling constant, g, is proportional to the fermion charge. Energy, momentum, angular momentum and charge always conserved. QED vertex NEVER changes particle type or flavour i.e. but not or QED vertex always conserves PARITY g STANDARD MODEL ELECTROMAGNETIC VERTEX +antiparticles Pure QED Processes:  Pure QED Processes Compton Scattering (ge-ge-) Bremsstrahlung (e-e-g) Pair Production (g e+e-) The processes e-e-g and g e+e-cannot occur for real e, g due to energy, momentum conservation. g g e- e- e- Ze Slide29:  e+e- Annihilation p0 Decay J/+- The coupling strength determines “order of magnitude” of the matrix element. For particles interacting/decaying via EM interaction: typical values for cross-sections/lifetimes sem ~ 10-2 mb tem ~ 10-20 s Scattering in QED:  Examples: Calculate the “spin-less” cross-sections for the two processes: (1) (2) Fermi’s Golden rule and Born Approximation (see page 55): For both processes write the SAME matrix element is the strength of the interaction. measures the probability that the photon carries 4-momentum i.e. smaller probability for higher mass. Scattering in QED Electron-proton scattering Electron-positron annihilation (1) “Spin-less” e-p Scattering:  (1) “Spin-less” e-p Scattering q2 is the four-momentum transfer: Neglecting electron mass: i.e. and Therefore for ELASTIC scattering e e RUTHERFORD SCATTERING Discovery of Quarks:  Discovery of Quarks Virtual g carries 4-momentum Large q  Large , small Large , large High q wave-function oscillates rapidly in space and time  probes short distances and short time. q2 small q2 increases q2 large Rutherford Scattering Excited states Elastic scattering from quarks in proton l<< size of proton q2 > 1 (GeV)2 E = 8 GeV Expected Rutherford scattering (2) “Spin-less” e+e- Annihilation:  (2) “Spin-less” e+e- Annihilation Same formula, but different 4-momentum transfer: Assuming we are in the centre-of-mass system Integrating gives total cross-section: Slide34:  This is not quite correct, because we have neglected spin. The actual cross-section (using the Dirac equation) is Example: Cross-section at GeV (i.e. 11 GeV electrons colliding with 11 GeV positrons) The Drell-Yan Process:  The Drell-Yan Process Can also annihilate as in the Drell-Yan process. Example: See example sheet 1 (Question 11) Experimental Tests of QED:  Experimental Tests of QED QED is an extremely successful theory tested to very high precision. Example: Magnetic moments of e,  : For a point-like spin ½ particle: Dirac Equation However, higher order terms introduce an anomalous magnetic moment i.e. g not quite 2. O(a) O(a4) 12672 diagrams O(1)     Slide37:  O(a3) Agreement at the level of 1 in 108. QED provides a remarkable precise description of the electromagnetic interaction ! Experiment Theory Higher Orders:  Higher Orders So far only considered lowest order term in the perturbation series. Higher order terms also contribute Second order suppressed by a2 relative to first order. Provided a is small, i.e. perturbation is small, lowest order dominates. +     +     Lowest Order Second Order Third Order Running of :  Running of  specifies the strength of the interaction between an electron and a photon. BUT a is NOT a constant. Consider an electric charge in a dielectric medium. Charge Q appears screened by a halo of +ve charges. Only see full value of charge Q at small distance. Consider a free electron. The same effect can happen due to quantum fluctuations that lead to a cloud of virtual e+e- pairs The vacuum acts like a dielectric medium The virtual e+e- pairs are polarised At large distances the bare electron charge is screened. At shorter distances, screening effect reduced and see a larger effective charge i.e. a. Slide40:  Measure a(q2) from etc a increases with increasing q2 (i.e. closer to the bare charge) At q2=0: a=1/137 At q2=(100 GeV)2: a=1/128

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