Elementary Particle Physics I Part 4 Manfred Jeitler WS 2008/2009

1. Elementary Particle Physics IPart 4Manfred JeitlerWS 2008/2009

2. Cabibbo angle • Strange quark is in some ways similar to Down quark • Λ0 = (uds) “heavy brother” of proton • but Strange quarks decay • into Up quarks • they are not completely separate, but somehow coupled to “first generation” • for the Weak interactions, the quarks are not quite the same as the “physical” quarks (“mass eigenstates”) • they appear to be “rotated”:

3. neutral currents • experimenting with neutrino beams • how would you make a neutrino beam? • “charged-current events”: • νμN --> μ- X, `μN --> μ+ X • “neutral-current events”: • νμN --> νμX, `μN --> `μX

4. neutral currents

5. the GIM mechanism • it was noted that all neutral-current transitions had ΔS=0 • Strange quark would transform into Up but not into Down • no “flavor-changing neutral currents” • Glashow, Iliopoulos and Maiani proposed a mechanism to explain this • now called the GIM mechanism

6. the GIM mechanism

7. completing the second generation of quarks: charm and the J/ψ particle • the GIM mechanism had proposed another quark in the 2nd generation but it seems this prediction remained unheeded by experimentalists • it was by chance that at the proton fixed-target accelerator at Brookhaven, a resonance in the cross section was observed at 3.1 GeV • “AGS” = Alternating Gradient Synchrotron, at • “BNL” = Brookhaven National Laboratory • before this had been sufficiently confirmed to allow for publication, it seems that rumors about the discovery spread to the Stanford SPEAR accelerator, an e+e- collider and allowed to look for the resonance in the appropriate place • due to the much lower background at the electron-positron collider, the resonance could be quickly confirmed there

8. completing the second generation of quarks: charm and the J/ψ particle • the new particle was understood to be a bound state of a new quark and its antiquark • due to the shared discovery, the particle got the unusual name “ J/ψ “ • Sam Ting ( ) from Brookhaven and Burt Richter from Stanford shared the Nobel prize for this discovery • the new quark was dubbed “charm”: c • the J/ψ has the quark content c`c

9. the Zweig rule probabilities are higher for decays where quark lines are connected • “less has to be done” • explanation by number of gluons to be exchanged

10. conserved quantities (quantum numbers) • mass/energy • absolutely conserved • no perpetual motion machine (alas!) • charge • absolutely conserved • upper limit: 10-19 • new quantities: • baryon number (=1/3 for any quark) • seems absolutely conserved - but maybe isn’t

11. conserved quantities (quantum numbers) • lepton numbers • Le, Lμ, Lτ • conserved? • not yet seen for charged leptons • neutrino “oscillations” seen, however! • properties that distinguish the quark “generations”: • Strangeness S: -1 for Strange, +1 for antiStrange • Charm C +1 for Charm, -1 for antiCharm • Beauty B -1 for bottom, +1 for antiBottom • (Truth T) • violated only by Weak interactions • generations live in “separate worlds” for other interactions

12. conserved quantities (quantum numbers) • isospin • conserved only by Strong interactions • violated by Electromagnetic and Weak interactions • proton and neutron described as two isospin states of the same particle (the “nucleon”) • discrete symmetries • parity, charge parity, time reversal • see later

13. the magnetic moment of the leptons • magnetic moment is proportional to angular momentum (spin) • factor of proportionality: μB = e`h / 2mc “Bohr magneton” • derived from classical argument • for quantum mechanical particles, according to Dirac equation: μ = g μB s g = “Landé g-factor” • just from Dirac equation, g=2

14. the magnetic moment of the leptons • g-factor depends on charge distribution in particle • would be 1 for a classical particle with homogeneous charge distribution • gproton = 5.59 charge distributed on outside? • composite structure of proton (quarks, gluons) • for leptons, difference from 2 due to vacuum polarization • can be developed by orders of α (fine structure constant, α ~ 1/137) • measured values: • μe = 1.001᾽159᾽652᾽1859  0.000’000’000’0038 μB • μμ = 1.001᾽165’920’80  0.000’000’000’63 eħ/2mμ

15. the magnetic moment of the leptons • very impressive correspondence between calculations and experimental results • still, for μμ there seems to be a slight discrepancy between theory and experiment • could (but need not) be “new physics” • status: PDG 2008

16. the magnetic moment of the leptons

17. g-2 muon experiment at BNL (Brookhaven, USA)