Precision Muon Physics Group
K.D. Chitwood, P. Debevec, S. Clayton, D. Hertzog,
P. Kammel, B. Kiburg, R. McNabb, F. Mulhauser, C.
C. Polly, A. Sharp, D. Webber
muon capture on proton
- + p  + n
L to 1 %
muon capture on deuteron
- + d  + n +n
L to 1 %
muon decay
+  e++e+
t+ to 1 ppm
Nucleon form factors,
chiral symmetry of QCD
 Cap experiment
gP to < 7% (3%)
Basic EW two nucleon reaction,
calibrate v-d reactions
D project
Fermi Coupling Constant
 Lan experiment
GF to < 1ppm
Scientific case
QCD is the theory of strong interactions
• non Abelian Gauge theory, running coupling constant,
• Precision tests at high energies
• strong coupling, confinement at “everyday” energies
Theoretical approaches to low energy domain
• QCD inspired models
• Lattice QCD
Quark
handedness
conserved
• Effective field theories (chiral perturbation theory)
systematic expansion around chiral limit
“spontaneous broken” symmetry governs dynamics
mass generation
accurate QCD prediction and QCD foundation for models
model independent predictions of important reactions
Cap
- + p  + n
nucleon level
elementary level
u
Ja
d
p
n
W
W
-
Ja

Ja = <d| ga (1- g5) |u>

-
QCD
Ja = <n| Va - Aa |p>
Va= gV(q2) ga
+ igM(q2)/2M sab qb
Aa= gA(q2) ga g5 + gP(q2) qa/m g5
•
fundamental and least known weak nucleon FF
•
solid theoretical prediction at 2-3% levelp
•
basic test ofpseudoscalar
QCD symmetries
•
factor
gP
experimentsform
not precise,
controversial,
4 s discrepancy
W to theory
gpNN
n
p
Fp
T. Gorringe, H. Fearing, Induced pseudoscalar coupling of the proton weak interaction, nucl-th/0206039, Jun 2002
V. Bernard et al., Axial Structure of the Nucleon, Nucl. Part. Phys. 28 (2002), R1

Cap
D project
+dn+n+
n
n
d
W
-

• fundamental EW 2-body reaction
• high impact for fundamental astrophysics reactions
• 10x precision improvement feasible by Cap techniques
D
pEFT: Class of axial current reactions
related by single unknown parameter L1A
• basic solar fusion reaction
p + p  d + e+ + 
• key reactions for solar neutrino
detection and supernova neutrino
+dp+p+e
+d p+n+
• short distance, axial two body currents,
ab-inito pEFT(NNLO) vs. SNPA vs. MEEFT
d capture close terrestrial analogue
n
n
d

EFT
L1A
• soft enough ?
d
W
W
-
p
p
MEC
e+
e
• precision measurement
possible ?
D
Precision Measurement of Muon Capture on the Proton
“Cap experiment”
- + p  + n
www.npl.uiuc.edu/exp/mucapture/
Petersburg Nuclear Physics Institute (PNPI), Gatchina,Russia
Paul Scherrer Institut, PSI, Villigen, Switzerland
University of California, Berkeley, UCB and LBNL, USA
University of Illinois, Urbana-Champaign, USA
Universite Catholique de Louvain, Belgium
TU Munich,Garching, Germany
Boston University, USA
University of Kentucky, USA
Cap
@ PSI
experimental challenges
- p
()  + n
p -  e+e+
- = heavy electron
(Rich) physics effects

• Interpretation:
where does capture occur ?
LS
Critical because of strong spin
dependence of V-A interaction
n+
LT
p
p
F=0
• Background:
Wall stops and diffusion
Transfer to impurities p+Z  Z +p
• Rate and statistics (BR =
• SR effect for +
10-3)
F=1
Lortho
pp
J=1
n+
Lpara
pp
J=0
n+
Cap
Cap experimental strategy I
New idea: active target of ultra-pure H2 gas 10 bar
“Lifetime” or “Disappearance” Method
measure t+ and t-  LS = 1/t- - 1/t+ , t to 10-5
Our experiment observes e+ and e– decay products.
Muon capture reduces the μ– lifetime compared to the μ+ lifetime by 0.15% !
log(counts)
ePC2
μ+
μ–
1010
events
ePC1
TPC
time
High precision measurement of the lifetime difference:
Ls = L( )' - L = (t ( )' ) - (t eSC)
e
Cap
-1
-

-

-1
Cap experimental strategy II
Physics
100% LH2
• Unambigous interpretation
At low density (1% LH2) mostly
capture from p(F=0) atomic state.
p
ppO
10% LH2
ppO
1 % LH2
p
p
ppP
ppP
• Clean muon stop definition:
Wall stops and diffusion
eliminated by 3-D muon tracking
ppO
ppP
time (s)
• In situ gas impurity control (cZ<10-8, cd<10-6)
hydrogen chambers bakeable to 150 C, continuous purification
TPC monitors impurities in-situ
10-8 sensitivity with gas chromatograph
• +SR: calibrated with transverse field 70 G
Statistics
• 1010 statistics: Complementary analysis methods
Cap
Cap experimental setup
Key ideas: active target of ultra-pure H2 gas 10 bar for muons,
separate large tracking detector for electrons.
Muon
Detectors
SC
(t = 0)
PC1
PC2
TPC
μ
Cap
Electron
Detectors
ePC1
ePC2
eSC (Hodoscope)
e
Cap experimental setup: TPC
The time projection chamber (TPC), is
our active gas-filled target. It detects in
muons and reaction products in 3D.
 stop
rare impurity capture
+Z  Z’+n+
2003 run
Assembly:
March → August
Data-Taking:
September → mid-October.
commissioning / first physics
Cap
time spectra 2003
Cap status and plans
Planned schedule Cap
• technical proposal spring 2001,
received “high priority status”
Steve’s and
Tom’s thesis
• commissioning 2003
• final detector upgrades 2004
• data run 2003
4% precision
• data run 2004
1% precision
• …. Cap II or d project
Contact me if you
are interested !
Cap
Experimental facility
Location
Paul Scherrer Institute (PSI),
Switzerland
Muon Source
• PSI accelerator (ring cyclotron)
generates 590 MeV proton beam
• protons hit graphite target
and produce pions
• pions decay to muons
Muon Beam Properties
• Particles: μ+ or μ–
• Momentum ~ 30-40 MeV/c
• rate ~ 50 kHz