1. LBL Comet-Mu2E Workshop January 24, 2009 Mu-e Conversion Backgrounds and Sensitivities– from proposal to measurement Doug Bryman "Wishing does not make a poor man rich." Arabian ProverbArabian Proverb

2. LFV ExperimentsLimit Reached Goal; (Result/Goal) “Comments” Badertscher et al. 1982 µ->e 7x10 -11 TRIUMF TPC Ahmad et al. 1987 µ->e 4.6x10 -12 2x10 -12 (2) Data collection took 5x as long as originally guessed (1 month!) SINDRUM II Bertl et al. 2006 µ->e 7x10 -13 Au 4.3x10 -12 (6x10 -13 ) Ti “10 -14” (1987) -> 3x10 -14(1993) “engineering” Ti (>60) Flux lower by 10; pion suppression device didn’t work; unanticipated high electron bkg.; shorter running. MEGA Ahmed et al. 2002 µ->eγ 1.2x10 -11 0.9->4x10 -13 “engineering” (133-35) Death by a thousand blows to acceptance N.B.: Every one of these experiments was carefully reviewed by experts!

3. MEGA at LAMPF

4. SINDRUM II PSI Proposed 10 8 stops; (muE1) beam was only 10 7 Designed “PMC” to kill pions; simulated; swamped unexpectedly by electrons; solenoid took years longer to obtain. Eventually went to very low momentum (50 MeV/c) killing pions by range; pion background persisted. Final result obtained in a couple of months; group had dispersed….” could have done better”….

5. MEG PSI Proposal goal (1999): <2x10 -14 in 2.2x10 7 s. Engineering…lost factor 17 Current goal (2008): <1.7x10 -13 in 4.5x10 7 s. “ Goals “ 2008 (1999) MeasuredSimulated Gamma energy %4.5 – 5.0(1.4) Gamma Timing (ns)0.15(0.1) Gamma Position (mm)4.5 – 9.0(1.7) Gamma Efficiency (%)>40(70) e + Timing (ns)0.1 e + Momentum (%)0.8 (0.3) e + Angle (mrad)10.5 (10) e + Efficiency (%)65 (95) Muon decay Point (mm)2.1 Muon Rate (10 8 /s)0.3 (1.0) Running Time (weeks)100 (49) Single Event Sens (10 -13 )0.5 (0.094) Accidental Rate (10 -13 )0.1 – 0.3 # Accidental Events0.2 - 0.5 90% CL Limit (10 -13 )1.7 (0.2)

6. COMET?----------------Mu2E? 0.047 0.07x0.6=0.42 0.41 0.34 s.e.s: 2.3 x 10 -17 * * * * * Kaons?

7. Sensitivity/Background Estimate Questions What are the uncertainties and risk factors in the background, acceptance estimates? What processes are missing? How are the backgrounds to be measured? How is a blind analysis to be done? What would make a believable signal?

8. Optimistic resolutions (DIO) – contamination? Optimistic acceptances – extra losses due to cuts Missing background sources due to high energy production… e.g. more electrons (from where?); previous experiments used low energy sources. Combinations of cosmics and beam-related effects? Fill in your own…. How to lose a factor 10 (100)?

9. Advance Background-related Measurements? Cosmic rays – could be done in a test setup Extinction – could be done in advance Radiative pion capture -> 100 MeV electrons? Neutrons? Pbars? …

10. The bifurcation method uses two uncorrelated high-rejection cuts to estimate each background A B C D CUT1 CUT2 The signal region A is not directly examined Measure B: Invert CUT1 and apply CUT2  Measure C+D: Invert CUT2  Estimate A: A = BC/D  Measure D: Invert CUT2 and apply CUT1 Background estimate = A! Digression: E787/E949 Blind Analysis and Background Estimate Methodology

11. Background estimates are performed using data different from those used to develop cuts Entire Data Set Cut Development Background Estimates 1/3 2/3 Compare background estimates Estimate and then measure the backgrounds near the signal region to verify estimates.

12. Conclusions/Recommendations Perform risk analysis to get best estimates of ultimate sensitivity Devise measurements to test assumptions Make Conservative claims – then do better! The 2005 HEPAP review of MECO stated that a minimal goal of achieving sensitivity significantly less 10 -15 than was essential to justify the considerable effort and expense.