1. High-Precision Measurement of Muon Capture on the Proton
MuCap
High-Precision Measurement of Muon Capture on the Proton
Tom Banks
APS DNP Fall Meeting
1 November 2003
University of California, Berkeley (UCB and LBNL) Petersburg Nuclear Physics Institute Paul Scherrer Institut University of Illinois, Urbana-Champaign Universite Catholique de Louvain TU Munich University of Kentucky Boston University

2. Our goal is to measure the rate Λs of muon capture on the proton
Experimental Goal
Our goal is to measure the rate Λs of muon capture on the proton
to 1% precision. This is a semileptonic weak interaction process, and (in our experiment) capture occurs predominantly from the hyperfine singlet atomic bound state.
A 1% measurement of Λs determines gp – the “weak nucleonic charged current induced pseudoscalar form factor” – to < 7% precision. gp is the least well-known of the weak nucleonic c.c. form factors, and there is significant disagreement among previous gp measurements, and theory.

3. Experimental Technique
“Lifetime” or “Disappearance” Method
For μ– in hydrogen, muon capture competes with muon decay:
This rate competition decreases the observed μ– lifetime from the vacuum lifetime, which we measure separately with μ+ :

4. Experimental Technique
“Lifetime” or “Disappearance” Method
Since our experiment can only observe the decay products e+ and e–, muon capture produces a small downward deflection of the μ– lifetime curve from the μ+ “vacuum” lifetime curve :
counts μ+
μ –
time
The capture rate is easily calculated from the measured lifetimes:

5. How Is This Experiment Superior?
High Statistics: In order to measure ΛS to 1% precision, we intend to measure both μ+ and μ– lifetimes to the level of
10 ppm, which requires recording 1010 decay events for each species. This is possible through our unique combination of detectors and analysis capabilities.
Improved Target: We use 10 bar, ultra-pure protium gas (impurities at 10–8 level, deuterium-depleted H2 to 1 ppm). This eliminates molecular formation complications, and dramatically reduces distortions in the lifetime histograms.
Our new technique should be dramatically superior to previous experiments.

6. Experimental Setup – Apparatus
entrance
scintillator
(t = 0)
Muon
Detectors
muPC1
muPC2
TPC
μ beam
The TPC is the heart of the experiment:: it allows us to perform track reconstruction, and vertex
matching between a muon and its corresponding decay electron. This significantly reduces
background. The TPC can also be used for pileup protection, and to determine low levels of
impurities in the H2 gas.
Electron
Detectors
ePC1
eSC (Hodoscope)
ePC2

7. 2003 Experimental Run
The MuCap experiment is conducted at the Paul Scherrer Institut (PSI) near Zürich, CH.
We spent March → August 2003 assembling experiment;
recorded data from September → mid-October.

8. 2003 Experimental Run
In spite of its technical complexity, MuCap is running and has achieved some excellent first physics results:
First combined assembly of muon detectors (including functional TPC), electron detectors, and high-speed DAQ.
List of limitations (may or may not want to mention all):
1. Gas purity controls are still rudimentary--no in situ montioring or active
gas cleaning. Contamination had to be estimated using analysis software.
Clean runs were achieved only after repeated refillings.
2. TPC not at full voltage, no ePC2 -> no electron tracking vectorization
3. Poor muon stopping fraction: 85% expected, best achieved was 32%.
Cause unknown
4. Efficacy of muPC1,2 called into question, particularly with regard to (3).
muPC2 was unreliable, and muPC1 was removed for second clean run.
We performed extensive systematic checks on the detectors.

9. 2003 Experimental Run
We achieved gas impurity levels of 10–7, as determined from software analysis...
... impurity capture events are
visible in the TPC data.
Most of the impurities are nitrogen. Impurity event in TPC comes from
nuclear breakup.
List of limitations (may or may not want to mention all):
1. Gas purity controls are still rudimentary--no in situ montioring or active
gas cleaning. Contamination had to be estimated using analysis software.
Clean runs were achieved only after repeated refillings.
2. TPC not at full voltage, no ePC2 -> no electron tracking vectorization
3. Poor muon stopping fraction: 85% expected, best achieved was 32%.
Cause unknown
4. Efficacy of muPC1,2 called into question, particularly with regard to (3).
muPC2 was unreliable, and muPC1 was removed for second clean run.
We also performed an intentional nitrogen-doped run (few ppm), for precision calibration.

10. 2003 Experimental Run
From September 25 → October 8 we estimate to have recorded ~ 109 good μ– TPC stops. We will spend the coming year analyzing this data and extract a 10% measurement of Λs.

11. Outlook 2004 gp TPC should reach full voltage and see decay electrons
update from Gorringe & Fearing gp
RMC
TPC should reach full voltage and see decay electrons
mCap proposed
cP-Theory
OMC Saclay
exp
theory
ePC2 will be completed, allowing for electron track vectorization
lOP (ms-1)
In situ gas cleaning and quality monitoring
1010 muon stops

12. 2003 Experimental Run – Limitations
Poor muon stopping fraction: We expected to stop ~ 80% of all incident muons in the TPC. Instead, we observed (at best) good muon stops only 32% of the time. The reasons for this discrepancy are still being explored.
Muon beam stopping distribution inside TPC
Initial beam stop profile, with all muon detectors in place. Note the large low momentum scattering in the tail of the beam. Stopping fraction was ~ 20%. z
Final beam stop profile, following the removal of muPC1– the stopping fraction improved to 32%. Note the improved Gaussian shape. This has called into question the utility of muPC1,2.