Muon Capture on the ProtonFinal results from the MuCap experiment

gP

Peter WinterUniversity of Washington for the MuCap collaboration

Overview

Brief motivation for MuCap

Experimental overview

Final MuCap result

-

- + p n +

q2 = -0.88m2

Nucleon form factors

uud

dud

-

p n

- + p n +

M ~ GFVud · (1-5) · n(V-A)p

q2 = -0.88m2

Nucleon form factors

Observable: Singlet capture rate LS

Nucleon form factors

V = gV(q2)

+ i gM(q2) q/2MN

M ~ GFVud · (1-5) · n(V-A)p

A = gA(q2) 5

+ gP(q2) q/m5

Contributes 0.45% uncertainty to LStheory

dgP

gP

dLS

LS

1.0% 6.1%

• ChPT based on the spontaneous symmetry breaking

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

• basic test of chiral symmetries and low energy QCD

Pseudoscalar form factor gP

gP(q2) = - - gA(0)mNmrA2

2mNm¹gA(0)

q2-m¼2

1

3gP(q2) = -

2mNmgA(0)

q2-m2

PCAC pole term(Adler, Dothan, Wolfenstein)

NLO (ChPT)Bernard, Kaiser, Meissner

PR D50, 6899 (1994)

gP = 8.26 ± 0.23

p n

gNN

f

Recent review: Kammel, P. and Kubodera, K., Annu. Rev. Nucl. Part. Sci. 60 (2010), 327

How to access gP?

In principle any process directly involving axial current:

- b decay: Not sensitive since gP term

proportional to q

- n scattering difficult to measure

Muon capture most direct source for gP

Muon capture

- Ordinary muon capture (OMC): m- p n n

- Radiative muon capture (RMC): m- p n n gBR = ~10-8 for Eg>60 MeV

- m- 3He n 3H or other nuclei

Muon capture

- Ordinary muon capture (OMC): m- p n n

- Radiative muon capture (RMC): m- p n n gBR = ~10-8 for Eg>60 MeV

- m- 3He n 3H or other nuclei

Methods to measure OMC rate

Direct method:- Measure outgoing neutrons- Typical experiments ~10% precision in LS

Lifetime method:

LS l- - l+

LS = 0.15% l-!

l+ known to 1 ppm!

D.B. Chitwood et al., Phys. Rev. Lett. 99, 03201 (2007)

D. Webber et al., Phys. Rev. Lett. 106, 041803 (2011)

MuLan 2007 and 2011

GF = 1.1663818(7) x 10-5 GeV-2 (0.6 ppm)

MuCap key elements

• Lifetime method

• Low gas density

•Active gas target (TPC)

•Ultra pure gas system with in-situ monitoring

• Isotopically pure hydrogen gas

:f Hydrogen density, (LH2: f=1)

Muon kinetics

μ-

pμ↑↑

pμ↑↓

p

LT ~ 12s-1

S ~ 700s-1

f>0.01<100ns

Muon kinetics

• ppm formation depends on density f• Interpretation requires knowledge of lOF and lOP

pμ↑↓

LS ~ 700s-1

lOP

ortho (J=1)

ppμflOF

para (J=0)

ppμ

flPF

LOM ~ ¾ LS

LPM ~ ¼ LS

pμ↑↓

LS ~ 700s-1

lOP

ortho (J=1)

ppμflOF

para (J=0)

ppμflPF

LOM ~ ¾ LS

LPM ~ ¼ LS

pμ↑↓

LS ~ 700s-1

lOP

ortho (J=1)

ppμflOF

para (J=0)

ppμ

flPF

LOM ~ ¾ LS

LPM ~ ¼ LS

Muon kinetics

Lower density dramatically decreases sensitivity to molecular complications

Previous results

20 40 60 80 100 120

2.5

5

7.5

10

12.5

15

17.5

20

lOP (ms-1)

mCap precision goal

exp theory TRIUMF 2005

m p n n @ SACLAY

m p n n g @ TRIUMFgP

• no overlap theory, OMC & RMC• large uncertainty in lOP gP ± 50%

ChPT

Requirement of clean target

pμ↑↓

LS ~ 700s-1

lOP

ortho (J=1)

ppμflOF

para (J=0)

ppμ

flPF

LOM ~ ¾ LS

LPM ~ ¼ LS

μd

cdlpd

Ld

diffusion

Deuterium removal unit

cd < 6 ppb

Requirement of clean target

pμ↑↓

LS ~ 700s-1

lOP

ortho (J=1)

ppμflOF

para (J=0)

ppμ

flPF

LOM ~ ¾ LS

LPM ~ ¼ LS

μd

cdlpd

μZ

cZLZ

Ld

LZ ~ LS Z4

diffusion

High-Z in MuCap

cN, cH2O < 10 ppb

Circulating H2 Ultra-Purification System

NIM A578 (2007), 485

• Active TPC • No materials in

fiducial volume

Requirement of clean target

pμ↑↓

LS ~ 700s-1

lOP

ortho (J=1)

ppμflOF

para (J=0)

ppμ

flPF

LOM ~ ¾ LS

LPM ~ ¼ LS

μd

cdlpd

μZ

cZLZ

Ld

LZ ~ LS Z4

diffusion

The facility: pE3 beamline at PSI

http://www.psi.ch

m

e

MuCap

t

TPC - the active target• 10 bar ultra-pure H2

• bakeable materials

• No materials in fiducial volume

TPC - the active target• 10 bar ultra-pure H2

• bakeable materials

• No materials in fiducial volume

m-p

E

e-

m-

26

A sample event

TPC active volume TPC active volume

Fiducial volume Fiducial volume

Front face view

muon beam direction

vert

ica

l dir

ect

ion

TPC side view

transverse directionve

rtic

al d

ire

ctio

n

10 times increased statistics

Year Statistics[1010 muon decays]

Comment

m- m+2004 0.16 0.05 published *2006 0.55 0.16 This talk2007 0.50 0.40 This talkTotal ~1.21 ~0.61

Remember: l+ known to 1 ppm from MuLan!

*V.A. Andreev et al., Phys. Rev. Lett. 99, 03202 (2007)

Lifetime spectra

Normalizedresiduals

Consistency checks

Consistency: Rate versus run

Data run number (~3 minutes per run)

Rate versus azimuth

Blinded measurement

500 MHz precise master clock

Analyzers add secret offsetDouble blinded analysis!

Detune clock

Hide from analyzers

Double blinded

~700 s-1

Relative unblinded

~700 s-1

rates with secret offset, stat. errors only

Unblinded

~700 s-1

Systematic corrections and errors

Systematic errors Run 2006 Run 2007 Comment

l (s-1) dl (s-1) l (s-1) dl (s-1)

High-Z impurities -7.8 1.87 -4.54 0.93

mp scatter -12.4 3.22* -7.20 1.25* * = prelim.

mp diffusion -3.1 0.1 -3.0 0.1

Fiducial volume cut 3 3

Entrance counter inefficiencies

0.5 0.5

Choice of electron detector def.

1.8* 1.8* * =prelim.

Total -23.3 5.14§ -14.74 3.88§ § = correlated

Impurity monitoring

2004 run: cN < 7 ppb, cH2O~30 ppb

2006 / 2007 runs: cN < 7 ppb, cH2O~10 ppb

Imp. Capture: m- Z (Z-1) n n

Final high-Z impurity correction

l

0

Production DataCalibration Data(oxygen added to production gas)

ExtrapolatedResult

Observed capture yield YZ

Lifetime deviation is linear with the Z>1 capture yield.

External corrections to l-

molecular formation

bound state effect

LS (MuCap prelim.*) = 714.5 ± 5.4stat ± 5.4syst s-1

LS (theory) = 711.5 ± 3.5 ± 3 s-1

* Small revision of molecular correction might affect LS < 0.5s-1 and syst. error

Precise and unambiguous MuCap resultsolves longstanding puzzle

gP(theory) = 8.26 ± 0.23

gP(MuCap prelim.) = 8.07 ± 0.5

Subset of the MuCap collaboration• Boston University• Regis University, Colorado• Université Catholique de

Louvain, Belgium• James Madison University

• Petersburg Nuclear Physics Institute, Gatchina, Russia

• Paul Scherrer Institute, CH • University of California • University of Illinois at

Urbana-Champaign• University of Washington• University of Kentucky

Supported by NSF, DOE, Teragrid, PSI and Russian Acad. Science and CRDF

Precise and unambiguous MuCap resultsolves longstanding puzzle

gP(theory) = 8.26 ± 0.23

gP(MuCap prelim.) = 8.07 ± 0.5