1. Rare and Forbidden Decays in the Standard Model
Elizabeth Mullin
2008 SULI Presentation

2. Motivation
Some extensions of the Standard Model can enhance rare/forbidden decay rates
SUSY
multiple Higgs doublets
extra dimensions
Three possible outcomes
find something and confirm new theories
don’t find anything and set an upper limit
don’t find anything and reject new theories

3. Decays Looking for D0→l+l- (decay probability) Rare Decays:
D0→e+e - ( )
D0→μ+μ ( )
Forbidden Decay:
D0→e±μ∓ ( 0 )

4. Previous Analysis Completed in 2004
160 million D0 events from BaBar runs 1, 2, & 3
90% confidence level upper limits:
1⋅ D0→e+e –
5⋅ D0→μ+μ -
3⋅ D0→e±μ∓
Improvements:
more data
look at additional variables

5. Background Primary sources
combinatoric – random decays that look like signal
misidentified particles – e.g. pion identified as a muon
Use simulated decays (Monte Carlo) to analyze expected background and signal
generic MC to look at the background
signal MC to look at the expected signal

6. Background Reduction background reduction vs. signal efficiency
particle ID requirements
cuts on different variables
momentum for the D0, electrons, muons
angle between the decay daughters
detector hits

7. New Variables Missing transverse momentum
reduce background from decays into leptons and neutrinos l+ υ l+ l-
signal s w+
background D0 c
neutrinos aren’t detected, so a high net momentum means that something is missing

8. missing transverse momentum
signal Monte Carlo
background Monte Carlo
D0→e+e-

9. New Variables Helicity angle
angle between the D0 direction and decay daughter direction in the D0 rest frame
D0 rest frame
lab frame
center of mass frame D0 l+ l-
vcm l-
D0 direction l+ θh D0 l+ l-

10. helicity angle
signal Monte Carlo
background Monte Carlo
D0→e+e-

11. Future Work Bremsstrahlung recovery for electrons
energy lost to radiation can be recovered and added back into the electron energy to reduce the tail in the low mass sideband
Optimize cuts on variables
maximize signal
minimize background

12. References
Chunhui Chen. Search for the flavor-changing neutral current and lepton-flavor violation decays of D0→l+l-. Physics Review Letters, vol (2004).