1. High Precision Measurement of Muon Capture on the Proton
MuCap
High Precision Measurement of Muon Capture on the Proton
Tom Banks
Weak Interactions Group
UC Berkeley Physics
LBL NSD
Monday Morning Meeting
30 August 2004

2. Purpose of the Experiment
Our goal is to measure the rate Λs for muon capture on the proton
to 1% precision. Muon capture is a semileptonic weak interaction process, and in our experiment capture occurs predominantly from the hyperfine singlet atomic bound state.

3. Experimental Technique
For μ– in hydrogen gas, muon capture competes with muon decay:
However, the muon capture channel is not available for μ+, so positive muons can only disappear via decay:

4. Experimental Technique
“Lifetime” or “Disappearance” Method
MuCap only observes decay electrons. Therefore, due to muon capture, we see a small downward deflection of the μ– decay curve from the μ+ “vacuum” decay curve (i.e. the μ– appears to have a shorter lifetime):
log(counts) μ+
μ –
time
The capture rate is easily calculated from the measured muon lifetimes:

5. Standard Model Theory
At the elementary particle level, the weak interaction is fundamentally V–A. g W νμ d u μ–
gVud

6. Standard Model Theory n νμ W p μ–
However, muon capture involves nucleons rather than isolated quarks. The strongly-interacting substructure of the proton and neutron complicates the weak interaction physics. These complicating effects are encapsulated in the nucleonic charged-current’s four “induced form factors”: n νμ q W p μ–

7. Standard Model Theory
With the interaction Lagrangian in hand, we can calculate the capture rate using S-matrix formalism:
where the matrix element is
This gives an expression relating the muon capture rate Λ to the induced form factors. Muon capture is a means of probing hadronic structure: a 1% measurement of Λs determines the induced pseudoscalar form factor gp to < 7% precision.

8. Motivation gV = 0.9755(5) gM = 3.5821(25) gA = 1.245(3)
gp has long been the least well-known of the weak nucleonic charged-
current form factors.
gV = (5)
values and q2-dependence are known from EM form factors via CVC.
gM = (25)
value known from β–decay; q2-dep. is known from neutrino scattering.
gA = 1.245(3)
gP = ? (8.26, 8.7, 10.6, 12.2 …)

9. Motivation
No overlap between
theory/RMC/OMC
exp

10. How Is MuCap Better?
Improved Target: We use 10 bar, ultra-pure protium gas (impurities at 10–8 level, deuterium-depleted H2 to 1 ppm). This dramatically reduces molecular formation complications and related distortions in the lifetime histograms.
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 (especially our active target) and analysis capabilities.
Our new technique should be dramatically superior to previous experiments.

11. Experimental Setup – Facilities
Location
Paul Scherrer Institute (PSI),
Switzerland
Muon Source
• PSI accelerator (ring cyclotron)
generates 590 MeV proton beam
(v ~ 0.8c)
• protons crash into graphite target
and produce pions
• pions decay to muons
Muon Beam Properties
• Particles: μ+ or μ–
• Momentum ~ MeV/c
• rate ~ 50 kHz

12. Experimental Setup – Apparatus
muSC
(t = 0)
muPC1
Muon
Detectors
muPC2
TPC
μ beam
• eDetectors cover 75% of 4π
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.
• μ,e detectors are decoupled to prevent cross-talk
Electron
Detectors
ePC1
eSC (Hodoscope)
ePC2

13. Experimental Setup – TPC
• The time projection chamber (TPC), is our active gas-filled target. It operates in proportional mode and records anode and cathode signals from entrance muons and decay electrons.
• The TPC is the heart of the experiment: it allows us to identify good muon stops and perform vertex matching between a muon and its corresponding decay electron. The TPC can also be used for pileup and wall-stop protection, and to determine low levels of impurities in the H2 gas.

14. 2003 Commissioning Run
Assembly:
March → August
Data-Taking:
September → mid-October.
First “complete” setup since MuCap inception in 1997.

15. ↓ 2003 Commissioning Run Partial setup Muon Detectors
muSC
(t = 0)
muPC1
Partial setup ↓
Muon and electron track vector reconstruction not possible
Vertex matching not possible (no local pileup protection)
Muon
Detectors
muPC2
TPC (partial)
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

16. 2003 Commissioning Run
In spite of numerous complications, we achieved some excellent first 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.

17. 2003 Commissioning Run
We achieved gas impurity levels of 10–7, as determined from real-time software analysis of impurity events, and post-run chromatography analysis.
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.

18. 2003 Commissioning Run
From September 25 → October 8 we estimate to have recorded ~ 109 good μ– TPC stops. I have spent 2004 analyzing this data, originally hoping to extract a 10% measurement of Λs.

19. 2003 Commissioning Run gp 2003 data error spreads OMC Saclay
RMC gp
cP-Theory
proposed
statistics
systematics (D2 )
exp
theory
OMC
Saclay
lOP (ms-1)

20. This year we hope to fill in all of the missing pieces...
Upcoming 2004 Run
This year we hope to fill in all of the missing pieces...

21. 2004: Improvements
new beampipe + muon detector assembly for accurate tracking and enhanced muon stopping fraction
second electron wire chamber (ePC2) is operational for electron tracking

22. 2004: Improvements
New on-site gas cleaning, recirculation, and quality
monitoring systems are ready

23. Greater than 109 statistics
Outlook 2004
Higher TPC voltage
Faster, better DAQ
Greater than 109 statistics
Setup and testing
Data-taking
Analysis...

24. Collaborating Institutions Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia Paul Scherrer Institute (PSI), Villigen, Switzerland University of California, Berkeley (UCB and LBNL), USA University of Illinois, Urbana-Champaign (UIUC), USA Universite Catholique de Louvain, Belgium TU Munich, Garching, Germany University of Kentucky, USA Boston University, USA
The MuCap experiment is supported in part by the United States Department of Energy and the National Science Foundation.