Weak Interactions. The Theory of GLASHOW, SALAM and WEINBERG

Transcription

1 Weak Interactions The Theory of GLASHOW, SALAM and WEINBERG ~ (Nobel 1979) Theory of the unified weak and electromagnetic interaction, transmitted by exchange of intermediate vector bosons mass generated by Higgs field A. Geiser, Particle Physics 1

2 Discovery of the W and Z (1983) To produce the heavy W and Z bosons (m ~ GeV) need high energy collider! : conversion of SPS proton accelerator at CERN into proton-antiproton collider challenge: make antiproton beam! success! -> first W and Z produced 1982/83 (Nobel 1984) Z 0 -> e + e - UA1 Carlo Rubbia Simon van der Meer A. Geiser, Particle Physics 2

3 Z production at LHC Now millions of events yesterday s signal is today s background and tomorrow s calibration A. Geiser, Particle Physics 3

4 Three Boson LEP W/Z bosons carry electroweak charge (like colour for gluons) -> measure rate of W pair production at LEP II only ν exchange No ZWW vertex A. Geiser, Particle Physics 4

5 Electroweak Physics at HERA Neutral Current (NC) interactions e Charged Current (CC) interactions e A. Geiser, Particle Physics 5

6 Weak interactions are "left-handed" lefthanded electrons interact (CC) e - e - righthanded electrons do not! e - e + cross section linearly proportional to polarization ± e p e σ = (1 ± Pe) σ polcc ± p unpolcc left polarization right A. Geiser, Particle Physics 6

7 Electroweak Unification NC CC M W A. Geiser, Particle Physics 7

8 The Quest for Unification of Forces Electroweak Unification LHC Grand Unified Theories? Superstring Theories? Maxwell s equations HERA electric strong Big Bang magnetic weak gravity A. Geiser, Particle Physics 8

9 α s running and Grand Unification with SUSY (see later):? A. Geiser, Particle Physics 9

10 Antimatter relativistic Schrödinger equation (Dirac equation) two solutions: one with positive, one with negative energy Dirac: interpret negative solution as 1932 antielectrons (positrons) found in conversion of energy into matter C.D.Anderson (Nobel 1936) 1995 antihydrogen consisting of antiprotons and positrons produced at CERN P.A.M. Dirac (Nobel 1933) In principle: antiworld can be built from antimatter In practice: produced only in accelerators and in cosmic rays A. Geiser, Particle Physics 10

11 Pair Production e.g. γ e + + e when radiation A. Geiser, Particle Physics 11

12 Annihilation e + + e 2hf radiation A. Geiser, Particle Physics 12

13 The Matter Antimatter Puzzle Why does the Universe look like this not that? As far as we can see in universe, no large-scale antimatter. -> need CP violation! A. Geiser, Particle Physics 13

14 The Matter Antimatter Puzzle -> particles, anti-particles and photons in thermal equilibrium colliding, annihilating, being re-created etc. Slight difference in fundamental interactions between matter and antimatter ( CP violation )? -> matter slightly more likely to survive Ratio of baryons (e.g. p, n) to photons today tells us about this asymmetry - it is about 1: A. Geiser, Particle Physics 14

15 CP symmetry graphics: M.C. Escher Parity (reflection) Charge Conjugation (black white) C P Like weak interaction, symmetric under CP (at first sight!) Can there be small deviations from this symmetry? A. Geiser, Particle Physics 15

16 CP violation in B meson decays + e e First B decays Second B decays (B 0 ) t Decay length ~ 1/4 mm t 2 c J/ψ c d s K 0 s 1 (or ) d b B 0 B 0 d d Asymmetry ( t ) = B 0 - B 0 B 0 + B 0 Simply count decays as function of t! A. Geiser, Particle Physics 16

17 Example: measurement from BaBar at SLAC (also Belle at KEK) CP violation in B meson decays B and anti-b are indeed different (also found earlier for K decays: ) James W. Cronin Val L. Fitch data taking stopped (Nobel 1980) Belle/Super-Belle continuing (DESY!) M. Kobayashi T. Maskawa (Nobel 2008) A. Geiser, Particle Physics 17

18 The Mystery of Mass upu d down c charm s strange t top. ν e e-neutrino e electron. ν µ µ-neutrino µ muon. b bottom ν τ τ-neutrino A. Geiser, Particle Physics 18 τtau

19 The Mass (BEH) Mechanism P. Higgs et al. ( ,71) Brout, Englert, Guralnik, Hagen, Kibble, many subvariants which is right? Peter Higgs François Englert (Nobel 2013) source: vixra blog A. Geiser, Particle Physics 19

20 Fermion Mass from Higgs field? very brilliant scientist (fermion) works with speed of light! -> massless room = vacuum people = Higgs vacuum expectation value A. Geiser, Particle Physics 20

21 Fermion Mass from Higgs field? scientist becomes famous! enters room with people A. Geiser, Particle Physics 21

22 Fermion Mass from Higgs field? people cluster around him hamper his movement/working speed -> he becomes massive! A. Geiser, Particle Physics 22

23 Neutrino oscillations: neutrinos are massive! not long enough to oscillate Cosmic ray Probability (ν µ remains ν µ ) Downgoing SNO SK P ~ m 2 Upgoing long enough to oscillate L(km) for 1GeV neutrinos 1998 Takaaki Kajita Arthur McDonald A. Geiser, Particle Physics (Nobel 2015) 23

24 What do we know about Neutrino mass? < 2.2 ev > 1 mev (taken from R. B. Patterson, Ann Rev Nucl Part Sci 65 (2015) ) are the masses of Dirac type (generated by Higgs)? or of Majorana type (ν s are their own antiparticles, masses have non-standard Model origin)? CP violation? A. Geiser, Particle Physics 24

25 The quest for the top quark Electroweak precision measurements at LEP/CERN sensitive to top quark mass and Higgs mass (indirect effects)... to the heaviest -> M t ~ 170 GeV A. Geiser, Particle Physics 25

26 The Tevatron (Fermilab) data taking ended in 2011 analysis still ongoing A. Geiser, Particle Physics 26

27 Top quark discovery (Fermilab 1995) Top quark actually found where expected! Tevatron at Fermilab (CDF + D0) measured mass value: (PDG16) M top = ± 0.9 GeV A. Geiser, Particle Physics 27