Blind Measurements and Precision Muon Physics David Hertzog University of Washington
A word about the Evolution of Precision Time "our future discoveries must be looked for in the sixth place of decimals." MuLan It follows that every means which facilitates accuracy in measurement is a possible factor in a future discovery, and this will, I trust, be a sufficient excuse for bringing to your notice the various methods and results which form the subject matter of these lectures. - Albert Abraham Michelson- MuCap New g-2 MuSun
An unusual subatomic particle Mass ~ 207 m e : heavy electron Lifetime ~2.2 ms is long: beams, probes m - can form 1-electron hydrogen-like atoms m - p, m - d, m - A Muonium: m - e + Primary decay m + e + n e n m (parity violation) Easily polarized: p + m + n m n p + m + Typically 42,000 times more sensitive to new physics quantum loops compared to electrons ~(m m /m e ) 2 Lepton number conservation: No m e conversion p m -
First: The case for going blind Taste tests Medical Trials This is not new stuff So, why is it novel in physics?
Precision measurements have a checkered history. You stop analyzing when you agree with the current world average
As we know
World avg dt m /t m is 18 ppm, but is it right? Lessons from History m+ Neutron Lifetime Precision vs Accuracy 10 ±1 ppm? Goal of MuLan is 1 ppm. But, how did we get this far??
First the blinding story from our Muon g-2 Experiment e Momentum Spin μ -10 a Expt. =11659208.0 6.3 10 0.54 ppm
The stakes are high In discovery physics 3.6 s standard deviations) is not enough. That s called a hint of something Note: 3 s is 99.7% probability of being right, so go figure The Standard Model Our Experiment The Fear is BIAS and Errors
We measure (1) Precession frequency TIME (2) Muon distribution (3) Magnetic field map B g 2 1 2 3
BNL Storage Ring Quads
Fit is sophisticated, but looks like this N(t) = N 0 e -t/t [1+Acos(w a t + f)] Billions of events so getting a good fit is difficult but mandatory Magnetic Field uncertainty is purely systematic -- alignment -- absolute calibrations -- db/dx at measurement -- probe signal quality Field measured in terms of proton NMR in SAME magnet
A double-blind analysis culture Field and Precession are frequency measurements which employ clocks Offsets are added to intermediate analysis results that are shared with others Nobody knows the offsets from the field and the precession analyses Nobody can divide them to obtain a physics result All consistency plots, fits, etc, can be performed without bias When analyses are stable, reveal secret offsets, divide, and publish Magnetic Field Secret Offsets Secret Offsets My Team My Team Data Production BU Yale One year, we published two days after the unblinding.
Examples of Self Consistency of Measurements 14 Individual analyses independent and consistent Bennett et al, PRD 73, 072003 (2006) Different techniques (same data)
Let s do it again even better
New Muon Campus at Fermilab
But we have to move the ring from New York to Chicago 17
Requires rather special engineering! Go the long way, but hope to avoid hurricane season ust passes through Illinois tollway by a few inches
The task has begun: BNL to FNAL
Muon Lifetime Weak Interaction Strength (Now I will describe our most recent measurement) G F Muon decay is a pure weak process determines G m, often called G F
Paul Scherrer Institut Villigen, Switzerland msr Neutrons SINQ 2.2 ma cyclotron, 590 MeV protons Swiss Light Source
PSI: a 1.3 MW facility with many secondary muon beams. Example: pe3 beamline at PSI MuSun MuCap MEG MuLan PiBeta + new p/m area Lamb shift below
t m + determined Fermi Constant to unprecedented precision t m + needed for reference lifetime for precision muon capture experiments MuCap g P MuLan Motivation G F a M Z 9 ppm 0.37 ppb 23 ppm 0.6 ppm Capture rate from lifetime difference m and m + MuSun L 1A Is lifetime in bound muonium the same as the free lifetime?
G F & t m precision has improved by ~4 orders of magnitude over 60 years. Achieved!
Number (log scale) The experimental concept Real data Kicker On e + Measurement Period 170 of these time Fill Period
The complete detector has 30 active houses, with 170 tile pairs e + 80 pe/mip
In beam, backed off
MuLan collected two datasets, each containing 10 12 muon decays Ferromagnetic Target, 2006 Quartz Target, 2007 Two (very different) data sets Different blinded clock frequencies used Revealed only after all analyses of both data sets completed Most systematic errors are common
Easy to blind this kind of measurement Everything runs off a super-high precision, stable oscillator Adjust it to give 450 MHz but make a secret offset (of the oscillator) by ±200 ppm of central value [ done by someone outside collaboration ] Our analysis seconds are not real seconds but it doesn t matter until we are done Two different data sets with vastly different conditions Each has its own offset When we think we are ready, map BOTH to yet another blind space to see if we have internal consistency If we do, we can proceed to unblinding to real seconds space Two data sets agreed to better than 1 ppm so we are okay 29
Final Errors and Numbers ppm units Effect 2006 2007 Comment Kicker extinction stability 0.20 0.07 Voltage measurements of plates Upstream muon stops 0.10 0.10 Upper limit from measurements Overall gain stability: 0.25 0.25 MPV vs time in fill; includes: Short time; after a pulse MPVs in next fill & laser studies Long time; during full fill Different by PMT type Electronic ped fluctuation Bench-test supported Unseen small pulses Uncorrected pileup effect gain Timing stability 0.12 0.12 Laser with external reference ctr. Pileup correction 0.20 0.20 Extrapolation to zero ADT Residual polarization 0.10 0.20 Long relax; quartz spin cancelation Clock stability 0.03 0.03 Calibration and measurement Total Systematic 0.42 0.42 Highly correlated for 2006/2007 Total Statistical 1.14 1.68 t(r06) = 2 196 979.9 ± 2.5 ± 0.9 ps t(r07) = 2 196 981.2 ± 3.7 ± 0.9 ps t(combined) = 2 196 980.3 ± 2.2 ps (1.0 ppm) Dt(R07 R06) = 1.3 ps
Lifetime history FAST The most precise particle or nuclear or (we believe) atomic lifetime ever measured New G F G F (MuLan) = 1.166 378 8(7) x 10-5 GeV -2 (0.6 ppm)
Some recent Press
MuLan at PSI 2004 2007 2006
Conclusions Blind measurements can be very satisfying since you can t go back They can be very frustrating because you can t go back They can also make you take MUCH longer before you think you have really checked everything