1. Particle Physics: Status and Perspectives Part 8: The Precision Frontier
Manfred Jeitler
SS 2018

2. The Frontier

3. How to find “New Physics”
Higher energy
produce new real particles
“Energy frontier”
Higher precision
find tiny deviations in measurements
via production of virtual particles
also needs higher statistics
 “Precision frontier” or “Intensity frontier”

4. The search for Lepton Flavor Violation
are lepton numbers really absolutely conserved?
they are not for neutrinos!
should also be violated for charged leptons?
nothing seen so far
long series of experiments have been looking for the decay μe (without neutrinos)
Sindrum, Sindrum II, MEG at Paul Scherrer Institute
looking for μeγ
Switzerland
new: Mu2e at Fermilab
Chicago, USA

7. (charged lepton flavor violation)

9. Main background: muon decay in orbit (DIO)

12. The magnetic moment of the leptons
magnetic moment is proportional to angular momentum (spin)
factor of proportionality:
μB = e h / 2 me “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

13. 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 = 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
each coupling of a virtual electron-positron pair to a virtual photon introduces another factor of α
series diverges, but can be “renormalized” by redefining the (unobservable) charge and mass of “bare” lepton:
infinite minus infinite = something finite (an operation you are not usually supposed to do in elementary mathematics)
very accurate experimental measurement possible (particles circulate for long times in a magnetic field)

14. Status on muon anomalous magnetic moment

15. Calculating the muon anomalous magnetic moment

16. The discrepancy between theory and experiment
3.6 σ

17. The Brookhaven muon g-2 experiment

18. The Brookhaven muon g-2 experiment

19. The Brookhaven muon g-2 ring on its way to Fermilab

20. Distribution of electron counts versus time