1. 23 February 2011Modern Physics III Lecture 71Modern Physics III Lecture 61 Modern Physics for Frommies III A Universe of Leptons, Quarks and Bosons; the Standard Model of Elementary Particles Lecture 7 Fromm Institute for Lifelong Learning, University of San Francisco

2. 23 February 2011Modern Physics III Lecture 72 Agenda Administrative Matters Higgs Redux Electroweak Unification The Current Paradigm

3. 23 February 2011Modern Physics III Lecture 73 Administrative Matters Full schedule of colloquia is posted on the Wiki and should be posted in Fromm Hall. Next colloquium is in March A list of popular books pertaining to Elementary Particle Physics is posted on the Wiki. It was been updated recently A sort of Glossary or at least a listing of particle types with examples has been posted on the Wiki. There are additions to the slides posted before last week’s lecture. These have been re posted as ModPhys III Lecture 6 4 R1.pdf and ModPhys III Lecture 6.ppt Please give some thought as to what you would like me to teach next time. Give me feed back at next week’s meeting. A mixture of Modern Physics stuff: Atomic and molecular physics, nuclear physics, solid state physics, etc. Cosmology Repeat starting with Relativity again

4. 23 February 2011Modern Physics III Lecture 74 Higgs Redux There is no straightforward way to put massive field quanta into gauge theories. Invariance under local transformations of the group of gauge symmetries → introduction of gauge bosons Lorentz invariance  massive gauge bosons require mc 2 terms. Unfortunately, this destroys gauge invariance. Simple cheating (ignoring mass terms until the end) doesn’t work. Renormalizability is destroyed. Screening and “effective” mass: photon (  ) analogy  in a conductor has short range →  appears to acquire effective mass due to screening by conduction electrons. Saying that we can treat  in conductor as massive is not to say that the  really does become massive, this mass is not a property of  itself but derives from its environment.

5. 23 February 2011Modern Physics III Lecture 75 Fabrication or modeling tool with no physical basis, but if the whole universe were conductive (plasma) then  s would appear to be massive. Can we do something like this for W and Z? Doesn’t affect  Affects W  and Z 0  not coupled to electric charge must couple to weak isospin charge. Some possibilities: Matter field associated with Matter field associated with known quarks and leptons Force field associated with known gauge bosons Some entirely different field For now, let’s just call it the Higgs field. We still have to be careful not to destroy gauge symmetry even though we are calling our masses “effective”. This is tricky!

6. 23 February 2011Modern Physics III Lecture 76 Hidden symmetry: Symmetry there but hidden by an arbitrary or capricious choice. Remember the freezing teenagers of last lecture. We can introduce the mass terms along with additional compensating terms that restore the hidden symmetry using the all pervading Higgs field. Introduce a new doublet, the Higgs doublet. Invariant under local SU(2) transforms Arbitrarily, pick say  lower as something we can’t perceive but which fills the universe’s vacuum, exerting a “drag force’ on anything that reacts with it. This gives the W and the Z apparent masses The symmetry of the Higgs doublet is hidden by our choice but is still there

7. 23 February 2011Modern Physics III Lecture 77 We seem to suggest that universe is not symmetric w.r.t. SU(2) transformations that swap the 2 doublet fields, The swapped universe, with  upper providing the pervasive drag force is manifestly different. The symmetry appears to be broken, but really it’s not. The W and Z still interact just as readily with  upper as they did with  lower. It’s just that in the swapped universe the the all pervading background is provided by  upper. For some reason the universe chose to pick one of the two cases. The choice was arbitrary but had to be made. Choice → physical state which hides the true underlying symmetry Higgs potential makes the choice So, we have  lower giving masses to W and Z. What is the rôle of  upper ?

8. 23 February 2011Modern Physics III Lecture 78 Recall the 1 st generation quark doublet Non zero value for u at a point in space-time  1 or more u quarks there Likewise for d At most points u = d = 0 but they are ≠ 0 inside an atomic nucleus  lower ≠ 0  quanta of Higgs field, Higgs bosons The Higgs doublet fields are complex so doublet represents 4 fields rather than 2.

9. 23 February 2011Modern Physics III Lecture 79 The representation of a massive vector particle is different than that of a massless one. 2 more degrees of freedom for m ≠ 0 from no longer forbidden transverse polarization. When W and Z pick up effective mass from pervasive Higgs field background, they need to incorporate more field components into their description. W  each take one of the 2 charged (upper) components. Z 0 takes 1 of the lower (neutral). The leftover neutral gives the uniform, pervasive, ≠ 0 background that causes the W and Z to apparently have mass. This final leftover can be excited to deviate from the uniform background  presense of Higgs boson(s).

10. 23 February 2011Modern Physics III Lecture 710 The Higgs boson is an electrically neutral, spin-0 (scalar) particle. Only fundamental scalar particle ( quarks and leptons are spin-1/2, gauge bosons are spin 1 or 2). New kinds of weak interactions at energies high enough for H to be excited. Cosmological implications The nature of “mass”. Are all masses due to Higgs or Higgs like mechanisms. Is mass, one of the most basic and common sense attributes of a physical object, just a sham?

11. 23 February 2011Modern Physics III Lecture 711 Electroweak Unification Steven Weinberg, 20 November 1967, Physical Review Letters Not Steven Weinberg Speculative but seemingly self-consistent model of interactions of e - and their corresponding partner e

12. 23 February 2011Modern Physics III Lecture 712 Gauge theory based on local SU(2) invariance Also incorporated a U(1) gauge symmetry like that of QED, The telling aspect of this model is that the the single boson associated with the U(1) symmetry is not quite the  of QED. Call it B 0. The properties of the minimal eB 0 interaction vertex do not quite reproduce those of the QED vertex. B0B0 ≠  In addition, SU(2) generates 3 field quanta. 2 of them are identified as the W  and Weinberg predicted, with no experimental basis, that the 3 rd was the electrically neutral W 0. Neutral currents not observed until 1973

13. 23 February 2011Modern Physics III Lecture 713 Apart from the difference in masses, U(1)’s B 0 and SU(2)’s W 0 are actually quite similar. For any fundamental process one could never tell which was responsible for the interaction. B0B0 W0W0 OR All you observe is 2 electrons bouncing off each other Possibly the exchanged quantum is really a mixture of the two  could be a mixture of B 0 and W 0 Likewise, Z 0 could be another mixture of B 0 and W 0

14. 23 February 2011Modern Physics III Lecture 714 The Glashow-Salam-Weinberg model (GSW) is that of a single interconnected electroweak force having 2 separate facets U(1) and SU(2) lacked Higgs Essentially identical to Weinberg but independent Nobel prize in physics 1979

15. 23 February 2011Modern Physics III Lecture 715 The properties of the  are well known, thanks to QED. GSW model allows calculation of the electroweak mixing (Weinberg) angle. or The effects of the so called neutral currents were not seen until 1973. Time for some experimental history.

16. 23 February 2011Modern Physics III Lecture 716 To cleanly study WI one needs a beam of neutrinos. J. Steinberger, M. Schwartz, L. Lederman and others designed and built such a beam ca. 1960. p beam from AGS Be target Focusing and sweeping  etc.  →   m Steel wall 13.5 m thick Detector Al plate spark chambers 10 T Neutral currents not thought up until 1967 but the above people did show that there are two types of neutrinos, e and . Nobel Prize in Physics 1988 Mostly

17. 23 February 2011Modern Physics III Lecture 717 In 1973, the Gargamelle 1 bubble chamber (12 m 3 of liquid freon) at CERN, exposed to such a beam, saw the first neutral current event 1. From the works of François Rabelais. Gargamelle, the giantess, was Gargantua’s mother  e → e

18. 23 February 2011Modern Physics III Lecture 718 Ca. 1978. C. Prescott, V. Hughes et al. Series of polarized electron scattering experiments at SLAC looking for parity violaton. Scatter polarized electrons of opposite helicities ffrom LH 2 and LD 2 targets  Z0Z0 A EM A NC Interference:

19. 23 February 2011Modern Physics III Lecture 719

20. 23 February 2011Modern Physics III Lecture 720 Finally the W  and the Z 0 were directly observed 1976: Carlo Rubbia suggests running the CERN SPS as a proton – antiproton collider in hopes of attaining sufficient energy. Simon van der Meer figures out how to make the accelerator do this, stochastic cooling Collider starts running in 1981 1983: The 100 member WA1 collaboration sees the W and the Z. 1984 Nobel Prize in Physics Carlo Rubbia Simon van der Meer

21. 23 February 2011Modern Physics III Lecture 721 Parity and Other Violations Consider the reflection of a movie in a mirror Physical laws in the reflected scene appear to be identical to those in the direct scene. Conclusions: (1) Laws of physics invariant under reflection. (2) There is no experiment that can distinuish our universe from the one in the mirror. Mirror reflection a.k.a. parity inversion is a discrete symmetry operation. Symmetry  group which leaves physical quantity invariant or symmetrical “discrete”  all or nothing

22. 23 February 2011Modern Physics III Lecture 722 Noether’s theorem: Symmetry  conserved quantity, parity, with 2 and only 2 possible values, “even” and “odd”. Any system, with say odd parity, obeying laws that are parity invariant, as familiar laws are, remains odd, unless there’s something fishy going on. Mid 1950’s:  puzzle ( these are mesons,  is not the  lepton) Same mass and lifetime but, Physicists uncomfortable Allowing a parity violating process would be an easy fix. Flies in the face of physical intuition. Rotational invariance, why not reflection. Never the less, physicists began thinking about it.

23. 23 February 2011Modern Physics III Lecture 723  lifetime (10 -8 sec), long by particle physics standards  WI 1956: T, D. Lee and C. N. Yang looked carefully at WI, No evidebce one way or the other re parity invariance Spin, Helicity and Parity: Consider e - (s=1/2) moving with momentum p Apply right and left hand rules, s = 1/2 is RH helicity, s = -1/2 is LH. Now, reflect in a mirror spin Direct Reflected Observer Parity inversion changes the handedness of spinning particles Observer RHLH

24. 23 February 2011Modern Physics III Lecture 724 A set of particles is invariant under a parity invariant interaction  process must not change if the handedness of every particle is changed OR Characteristics of a process change when all spins are flipped  parity is not conserved. C. S. (“Madame”) Wu at Columbia and E. Ambler at NBS Nucleons align so net nuclear spin = 5 ħ Using a strong magnetic field and extremely low temperature it was possible to polarize the 60 Co sample, i.e most of the nuclear magnetic moments aligned with the field.

25. 23 February 2011Modern Physics III Lecture 725 C. S. Wu 1912 - 1997

26. 23 February 2011Modern Physics III Lecture 726 T. D. Lee and C. N. Yang Nobel Prize 1957 The Wu experiment showed WI have a strong penchant for LH particles How strong is this preference? 1957: R. Garwin et al. looked at  decay at the Nevis cyclotron

27. 23 February 2011Modern Physics III Lecture 727  stopped in carbon absorber. Measure angle of e - emission w.r.t  flight direction Measured ratio: 1/2  10 % WI has only LH currents. Note, antiparticles are RH Note that the universe is matter rather than antimatter. Parity violation is maximal for WI  is now known as

28. 23 February 2011Modern Physics III Lecture 728 time