Muon Capture as a Probe of the Nucleon’s Axial Structure –

the Cap Experiment

Peter Kammel

University of Illinois at Urbana-Champaign

www.npl.uiuc.edu/exp/mucapture

PANIC05, October 25, 2005

Contents

• Physics context

• Muon capture on the proton theory - experiment

• Axial currents in the 2N system

E-W Current probes Strong Interactions

Charged current

interaction

nucleon level

quark level(1- 5)

u d

p n pQCD

Basic challenge: derive low energy hadron structure and interactions from QCD

• lattice QCD

• EFT based on chiral symmetry for q/ small

Formfactor parametrizemicroscopic QCD structure

W

nucleon current <n| V - Ap>

+ second class currents

nucleon current <n| V - Ap>

+ second class currents

Muon Capture on the Proton

- + p + n rate S BR~10-3

- + p + n + BR~10-8, E>60 MeV

nucleon weak CC formfactors q2= -0.88m2

gV = 0.9755(5) gA = 1.245(3)

gM = 3.5821(25) gP = ?

nucleon weak CC formfactors q2= -0.88m2

gV = 0.9755(5) gA = 1.245(3)

gM = 3.5821(25) gP = ?

gV, gM, gA

determined by SM symmetries and data,contribute <0.3% uncertainty to S

gP determined by chiral symmetry of QCD: n

p

-

gNN

F

gP= (8.74 0.23) – (0.48 0.02) = 8.26 0.23

PCAC pole term Wolfenstein

ChPT leading order one loop two-loop <1% N. Kaiser Phys. Rev. C67 (2003) 027002

Lincoln Wolfenstein, Ann. Rev. Nucl. Part. Sci. 2003

…it became customary to assume the standard V-A coupling and then deduce the pseudoscalar gP coupling from the data. I thought this was misleading because in the absence of new physics gP was determined very accurately from the pion-pole contribution. The radiative muon capture in hydrogen was carried out only recently with the results that the derived gP was almost 50% too high. If this results is correct, it would be a sign of new physics that might contribute effectively to V, A or P.

One of many experimental challenges

T = 12 s-1

pμ↑↓

singlet (F=0)

S= 664 s-1

n+

triplet(F=1)

μ

pμ↑↑

ppμ

para (J=0)ortho (J=1)

λop

ortho=506 s-1 para=200 s-1

ppμ ppμ ppμ

• Interpretation requires knowledge of pp population

• Strong dependence on hydrogen density

ppP

ppO

p

100% LH2

p

ppP

ppO

1 % LH2

time (s)

rate proportional to H2 density !

Precise Theory vs. Controversial Experiments

20 40 60 80 100 120

2.5

5

7.5

10

12.5

15

17.5

20

PT

OP (ms-1)

g P

- + p + n + @ Triumf

Cap precision goal

exp theory

update from Gorringe & Fearing

• no overlap theory & OMC & RMC

• large uncertainty in OP gP 50% ?

• no overlap theory & OMC & RMC

• large uncertainty in OP gP 50% ?

TRIUMF 2004

- + p + n @ Saclay

Goals of Cap*

Unambiguos Interpretation In-situ experimental handle on all systematics Much higher statistics S with 1% precision

gP with 7% precision

* Cap collaborationPetersburg Nuclear Physics Institute (PNPI), Gatchina, Russia

Paul Scherrer Institute (PSI), Villigen, Switzerland University of California, Berkeley (UCB and LBNL), USAUniversity of Illinois at Urbana-Champaign (UIUC), USA

Université Catholique de Louvain, BelgiumTU München, Garching, Germany

University of Kentucky, Lexington, USABoston University, USA

• gP basic and least known weak nucleon form factor

• solid QCD prediction via ChPT (2-3% level)

• basic test of QCD symmetries

• experiments not precise, controversial, discrepancy to theory

• gP basic and least known weak nucleon form factor

• solid QCD prediction via ChPT (2-3% level)

• basic test of QCD symmetries

• experiments not precise, controversial, discrepancy to theory

Recent reviews:T. Gorringe, H. Fearing, Rev. Mod. Physics 76 (2004) 31V. Bernard et al., Nucl. Part. Phys. 28 (2002), R1

How will Cap achieve this ?

Lifetime method

1010 →e decays measure to 10ppm,

S = 1/ - 1/to 1%

Unambiguous interpretation

capture mostly from F=0 p state at 1% LH2 density

Clean stop definition in active target (TPC) to avoid Z capture

Ultra-pure gas system and purity monitoring p + Z Z + p TPC bakeable, high vacuum materials & continuous purification online/offline purity analysis (0.01 ppm level) Isotopic purity at ~1 ppm level p + d d + p, large diffusion In situ/offline analysis (0.5 ppm level) unique Cap capabilities

fulfill all requirements simultaneously

disappears faster by ~0.1%

Cap detector Design 2001-2Reality 2004

3D tracking w/o material in fiducial volume

Muon stops in active target

p-

10 bar ultra-pure hydrogen, 1% LH2

2.0 kV/cm drift field ~5 kV on 3.5 mm anode half gapbakable glass/ceramic materials

Observed muon stopping distribution

E

e-

Time spectra

-e impact parameter cut

huge background suppression

diffusion (deuterium) monitoring

-

+

SR

in 80G

+ as reference

identical detector systematics

different physics

blind analysis

Impurity detection in TPC

rare impurity capture +Z (Z-1)+n+

Triggered FADC

+ Circulating Hydrogen Ultrahigh Purification System (CHUPS)* + Gas chromatography

*PNPI+UIUC with CRDF funding

Cap Status & Outlook

Final upgrades

Performance

Expected Results

StatisticsMuon-On-Request (MuLan), 2-3x increase in data rate !

Systematics Z>1 Impurities

Improved diagnostics (FADCs, sensors) faster circulation (CRDF)

Isotopic purityincrease TPC gain for monitoring CRDF project: new detection method and purification

Kineticsconstrain op correction by measuring capture neutrons

Subsystem Parameter 2003 2004 2005-06

TPC stop fractionhigh voltage (KeV)

0.334.8

0.655.0

0.655.4

eDet 2nd MWPC Electronics eSC FADC

TPC FADC

DAQ Livetime fraction 0.8 0.9 0.9

Purity Z>1 (ppm)deuterium (ppm)

0.53

0.073

0.02, better diag.0.3?, better diag.

Statistics - (109)

+ (109)0.6

2.5

0.5

10

10

Calibration runs C, N, O, D,ppm

run 2

004

runs

200

5-06

Axial currents in 2N system

Reactionsbasic solar fusion reaction

p + p d + e+ + key reactions for SNO

+ d p + p + e- (CC)

+ d p + n + (NC)

… Theory1B NN description in good shape

2B not well constrained by theory

EFT* SNPA EFT EFT

Quest to determine L1A

Experiments on 2N axial current 10% uncertainty at best

Estimated Theory precision from some % to some 0.1% ! during last few 10 years.

Based on 3N info (tritium beta decay),

as no 2N info available of required precision.

MEC

EFTL1A

EFT: Class of axial current reactionsrelated by single unknown parameter L1A

Precise experiment in 2N system needed

• determine L1A, astrophysics reactions• test SNPA vs. EFT• verify claimed precision of overall framework

Muon Capture on the Deuteron

d capture close terrestrial analogue

d p

e e-

p• soft enough for L1A physics?

• 1% precision measurement possible ?

nd

-W

n

W

Kammel, Chen

EFT (error N3LO)

Theory

Experiment

- + d + n + n

gP has to be known !

EFT* (tritium -decay)

20

En (

MeV

)En (MeV)

’~90% of intensity

measurement of absolute rate to <1% (D I)

Cap technique, new cryo TPC

Kinetics requires optimized target conditions T<80K, 5% density

measurement of Dalitz Plot to 5 % (D II)

Neutron detector array

Kinematics determined by angle and dt

• determine rate for relevant low energy rate ’

• study motivation for full DP measurement MECs, gP(q2)

D project Collaborators welcome

Cap N=3,4 with TPC ? (electronic bubble chamber)

time (s)New benchmark in EWreactions in 2N system