muon capture on the proton final results from the mucap experiment muon capture on the proton final...

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