A.Vorobyov on behalf of MuCap collaboration

Determination of the nucleon’s pseudoscalar form factorin the MuCap experiment

HSQCD 2014 Gatchina 30.06.2014

Overview

• Recent progress in studies of muon capture rates on proton, deuteron, and He3 made by the MuCap collaboration at PSI.

• First determination of the nucleon’s pseudoscalar form factor.

• Critical probe for Heavy Baryon Chiral Perturbation Theory.

• Determination of Low Energy Constants for Effective Field Theories of two and three nucleon systems

MuCap collaboration

Petersburg Nuclear Physics Institute Paul Scherrer Institute , Switzerland

University of Wasington, USA Regis University, USA

University of Kentucky, USABoston University, USA

University of South Carolina, USAUniversité Catholique de Louvain, Belgium

4

Muon Capture on Proton Muon Capture on Proton

- + p + n- + p + n

Chiral Effective Theories

5

Muon Capture on Proton Muon Capture on Proton

- + p + n- + p + n

p n

µν

W

6

Muon Capture on Proton Muon Capture on Proton

- + p + n- + p + n

p n

µν

W

- + p µ+ n

- + p µ+ n

Muon Capture on Proton

µ νµ

p n

Wqc

2 = - 0.88 mµ2

gv(qc2) = 0.9755(5) values and q2 dependence known from

gM(qc2) = 3.5821(25) EM form factors via CVC

gA (qc2) =1.251(4) gA(0)=1.2701(25) from neutron β-decay,

gP(expt) = ? q2 dependence from neutrino scattering

(

(

µp-capture offers a unique way to determine gP(qc

2)

Theoretical predictions for gP

n

p

-

gNN

F

• Partial conservation of axial current (PCAC)• Heavy Baryon chiral perturbation theory (ChPT)

W

g P(qc2) = (8.74 0.23 ) – (0.48 0.02) = 8.26 0.23

PCAC pole term

ChPT leading order one loop two-loop <1%

Recent reviews:V. Bernard et al., Nucl. Part. Phys. 28 (2002), R1T. Gorringe, H. Fearing, Rev. Mod. Physics 76 (2004) 31

45 years of Efforts to Determine g45 years of Efforts to Determine gPP

9

“ Radiative muon capture in hydrogen was carried out only recently with the result that the derived gP was almost 50% too high. If this result is correct, it would be a sign of new physics... ’’

— Lincoln Wolfenstein (Ann.Rev.Nucl.Part.Sci. 2003)

OMC RMC

- + p n + +

Emilio Zavattini 1927-2007

1969 Bologna-Pisa-CERN

1973 Dubna group

H2 –target 8 atm

Pioneers of muon capture experiments

gP = 12 ± 5

gP = 9 ± 7

H2 –target 41 atm

Status of µp-capture experiments

Year Exptl.place H2 target ΛC, s-1 Method

1962 *)

1962

1962

1963

1969

1974

1981

Chicago

Columbia

CERN

Columbia

CERN

Dubna

Saclay

liquid

liquid

liquid

liquid

gas, 8 atm

gas, 41 atm liquid

428 ± 85

515 ± 85

450 ± 50

464 ± 42

651 ± 57

686 ± 88

460 ± 20

neutron detection

- " –

- " –

- " –

- " –

- " -

Life time measurement

- + p µ+ n Br=0.16%

Two experimental methods to measure Λ C

*) First direct observation of µp-capture

Neutron detection Life time measurement

ΛC = 1/τµ+ - 1/τµ- ΛC Nn/Nµ

µ±p → e± νe νµ

CPT inv

T = 12 s-1

pμ↑↓

singlet (F=0)

S= 710 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

λ pp

From which muonic state the muon capture occurs ?

Pµ - PPµ(ortho) - PPµ(para) population

ppP

ppO

p

100% LH2

p

ppP

ppO

1 % LH2

time (s)

Precise Theory vs. of Exp. Efforts

20 40 60 80 100 120

2.5

5

7.5

10

12.5

15

17.5

20

ChPT

OP (ms-1)

gP

- + p + n + @ TRIUMF 1996

theory

- + p + n @ Saclay 1981

gP = 12 ± 5

H2 –target 8 atm1969

exp1• exp2

Main requirements

• The H2 gas pressure should not exceed 10 bar to provide dominant (µ-p)1S state.

• The muon capture rate Λs in the reaction

- + p (µ-p)1S → µ+ n BR=0.16%

should be measured to 1% precision

• To reach such precision, one should measure the µ- life time in hydrogen with 10-5 precision.

ΛS

Theory predicts gP with ~ 3% precision

Uncertainties in other form factors set the limit : =0.45% or =3%

To approach this limit, one should measure ΛS with ~0.5% precision

Sensitivity of ΛS to form factors

Contributes 0.45% uncertainty to measured S

Examples:

10% 56%

1.0% 6.1%

0.5% 3.8%

High precision muon capture experiments based on

New experimental methoddeveloped at PNPI

Unique muon channel at PSI

MuCap setup PSI meson factory

Strategy of MuCap experiment

• H2 gas target at 10 atm

(µ-p)1S

Strategy of MuCap experiment

• H2 gas target at 10 atm

(µ-p)1S

• Lifetime method ΛS = 1/τµ+ - 1/τµ-

10 -5 precision both in µ- and µ+ life times → δΛS/ΛS = 1%

Strategy of MuCap experiment

• H2 gas target at 10 atm

(µ-p)1S

• Lifetime method ΛS = 1/τµ+ - 1/τµ-

10 -5 precision both in µ- and µ+ life times → δΛS/ΛS = 1%

• >1010 muon decay events High data taking rate

Strategy of MuCap experiment

• H2 gas target at 10 atm

(µ-p)1S

• Lifetime method ΛS = 1/τµ+ - 1/τµ-

10 -5 precision both in µ- and µ+ life times → δΛS/ΛS = 1%

• >1010 muon decay events High data taking rate

• Clean muon stop selection No wall effects

Strategy of MuCap experiment

• H2 gas target at 10 atm

(µ-p)1S

• Lifetime method ΛS = 1/τµ+ - 1/τµ-

10 -5 precision both in µ- and µ+ life times → δΛS/ΛS = 1%

• >1010 muon decay events High data taking rate

• Clean muon stop selection No wall effects

• Low background < 10-4

Strategy of MuCap experiment

• H2 gas target at 10 atm

(µ-p)1S

• Lifetime method ΛS = 1/τµ+ - 1/τµ-

10-5 precision both in µ- and µ+ life times → δΛS/ΛS = 1%

• >1010 muon decay events High data taking rate

• Ultra clean protium impurities with Z>1 less than 10 ppb (1 ppb = 10-9) deuterium concentration less than 100 ppb

• Clean muon stop selection No wall effects

• Low background < 10-4

…………………. . . . . . . . . . . .

E

- 40kV

µ

G +5kVG

Time projection chamber, TPC

z

Y

x

Cathode stripsAnode wires

3D detection of muon trackX- cathode stripsY- drift timeZ- anode wires

y

x

z

Sensitive volume 15 x 12 x 28 cm3

Muon stop selection inside the sensitive volume far enough from all chamber materials

H2 gas 10 atm room temperature

…………………. . . . . . . . . . . .

E

40kV

µ

G 5kVG

Cathode stripsAnode wires

e ePC1

ePC2

eSC hodoscope

µSC

e-µ space correlation reduces background

tµ = t eSC – t µSC

µPC

Cryogenic circulation-purification hydrogen gas system

TT1 LT1H1 H2 H3 H4TT2 TT3 TT4

T1

MF

C-1

MF

C-2

MF

C-3

Co

lum

n1

Co

lum

n2

Co

lum

n3

L-

nit

rog

en t

ank

Vacuumline

Vacuumline

Membranepump

HV pumping system

Compressed air

Oil-free vacuum line

Relief

Refilling ports

Gas/Liquidseparator

Ref

illin

g p

ort

FA1 FA2

Lower L-nitrogen

tank

Upper L-nitrogen

tank

Radiation shielding

PT2

PT1

PT4 PT3

TPC

F8

F11

F12

F11

F9

F10

SV1

V12V11

V21

V35 V19

V10

MFC-4MFC-5

Sa

mp

le

T2

T3

LT1

V14

Sampling valve

V13CV12

F12

F21

F22

F31

F32

CV31 CV32

F5

F4

CV22CV21CV11

F11

F6F7

FromDewar

Oil freevacuumline

Releasevolume

Compressor

Co

mpr

esso

r un

it

Pur

ifica

tion

unit

Control Gas Panel

Reserve volume

Vacuum system

Purifier

CHUPS

V6

RV2

RV1V1V2

V26

V27

V30

HS

V29

V24

V28

V31

V3

4

V37 V38

V9

V25V5 V4

G - getter

PT - pressure sensorTT - temperature sensor

LT - level meter- hydrogen line

- nitrogen line

- compressed air line

- electronic signal

- pumping lineSV - operating valve

V manual valve - F - filter

H - heaterM - manometer

T - thrermalizerEF - electrostatic filterHS - humidity sensor

LT - level detector

MFC - mass-flow controller

F - filter adsorber-

SV3

SV2

G

V32

V33

V3

V36

T5

T4

V15

V23

V16

V17

Oil freevacuumline

TT10H10 TT6DAC_24 TT5 N2 less than 10 ppbO2 less than 10 ppb

Gas purification system

-5 0 5 10 15 2010

100

1000

Nitr

ogen

con

cent

ratio

n, p

pb

Time from CHUPS start, hours

Run 2006H

2 flow: 3 slpm

TPC provides control for impurities on a level of 10 ppb

Isotopic purity of protium

Deuterium concentration in hydrogen gas

Natural gas 140 ppmBest on market 2 ppmProduced at PNPIfor MuCap ≤ 6 ppb

Cryogenic isotopic exchange column

Accelerator mass spectrometryat ETH in Zurich

Reached sensitivity 6 ppb

PSI muon channel can provide ~ 70 kHz muons

MuCap could use only ~ 7kHz (to prevent pile up)

New beam system (Muon-on-demand) constructed in 2005 allowed to increase

the usable beam intensity up to 20 kHz

-

+12.5 kV -12.5 kV

Kicker Plates

50 ns switching time

detector

TPC ~3 times higher rate

Data taking rate

Muon-on demand system

run2006

10 000 hrs of beam time

µ e

Reduction of background by µ-e vertex cut

The impact cut can reduce the background to a level of 10-4-10-5

Determination of Determination of SS

32

molecular formation

bound state effect

MuCap: precision measurement

MuLan

Final result

• λµ– = 455854.6 ± 5.4stat ± 5.1syst s-1 (MuCap)• λµ+ = 455170.05 ± 0.46 s-1 (μLAN experiment),

ΛSMuCap = 714.9 ± 5.4stat ± 5.1syst s-1 .

MuCap collaboration, Phys. Rev. Lett. 110, 022504 (2013).

*) Based on updated calculations of Λs fromA. Czarnecki, W.J. Marciano, A. Sirlin, Phys. Rev. Lett., 99, 032003 (2007)

gPMuCap(qc

2) = 8.06 ±0.48±0.28 *)

gP HBCPT = 8.26 0.23 V. Bernard, L. Elouadrhiri, and U.-G. Meissner, J. Phys.G28, R1 (2002).

34gP(theory) = 8.26 ± 0.23

gP(MuCap) = 8.06 ± 0.55

The MuCap result does not depend on molecular OP-transitions

Axial Vector gAxial Vector gAA

35

•PDG 2008gA(0)= 1.2695±0.0029

•PDG 2012gA(0)= 1.2701 ±0.0025

•Future ? gA(0)= 1.273 ?

In this case

gPMuCap = 8.24±0.55

gPTheory = 8.26±0.23 A. Garcia

PDG12

Synopsis: Sizing Up Quark Interactions Measurement of Muon Capture on the Proton to 1% Precision and Determination of the Pseudoscalar Coupling gPV. A. Andreev et al. (MuCap Collaboration)Phys. Rev. Lett. 110, 012504 (2013)Published January 3, 2013

Even though the radioactive decay of nuclei is mainly driven by the weak force, interactions between the quarks that make up the protons and neutrons in the nucleus can also affect the process. Calculating these effects with quantum chromodynamics (QCD), the theory describing the strong force interactions between quarks, is, however, mathematically cumbersome at the low energies associated with the nucleus. Instead, calculations are more tractable using an effective QCD theory, in which interactions are between bound quarks (mesons, protons and neutrons). Now, researchers running the muon capture (MuCap) experiment at the Paul Scherrer Institute in Switzerland have confirmed a long-standing prediction of the theory, known as chiral perturbation theory, boosting confidence that it can be used to accurately describe quark interactions in simple nuclei. Muon capture is like a beta-decay process run in reverse: a muon (a particle with the same charge as an electron, but 200 times the mass) interacts with a proton to produce a neutron and a neutrino. Among other factors, a dimensionless quantity called the “pseudoscalar coupling,” determines the rate of the reaction.Chiral perturbation theory says the coupling factor has a value of Gp , without a lot of wiggle room. But experimental data going back to the 1960s have shown the coupling could be anywhere between   and   . The MuCap collaboration, which measures the rate of the muon capture process by stopping a beam of muons in a low-density gas of pure hydrogen, has analyzed 30 terabytes of data to extract the pseudoscalar coupling with unprecedented precision. The value of their result, reported in Physical Review Letters, is 8.06+/-0.55 —in excellent agreement with the theoretical prediction. – Jessica Thomas

PHYSICS spotlighting exceptional research American Physical Society

Muon capture rates

on deuteron and He3

38

Quest for “unknown” Axial LECQuest for “unknown” Axial LEC LEC - low energy constants in Effective Field TheoriesLEC - low energy constants in Effective Field Theories

2-body system2-body system– 1 LEC to be determined from µd capture1 LEC to be determined from µd capture – experimental information scarce: ~100% uncertaintyexperimental information scarce: ~100% uncertainty– Measurement of µd capture rate to 1% precision will Measurement of µd capture rate to 1% precision will reduce

uncertainty to ~15%

3-body system3-body system– 2 LECs and additional complexity enter2 LECs and additional complexity enter– tritium beta decaytritium beta decay– Muon capture on He3Muon capture on He3

potential current

+ d + d n + n + n + n +

model-independent connection via EFT & Lmodel-independent connection via EFT & L1A1A

“MuSun”

Next experiment

Mesurement of muon capture rate with 1% precision

basic solar fusion reaction

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

+ d p + p + e- (CC)

+ d p + n + (NC)

MEC

EFT

L1A

40

Precise Experiment NeededPrecise Experiment Needed

Muon Capture on He-3

µ- + 3He → 3H + νµ Et = 1.9 MeV

This capture rate was measured in MuCap experimentwith 0.3 % precision

Λstat = 1496 ± 4 s-1 Physics Letters B417,224 (1998)

The world precision was improved by a factor of 50

Since then, this result was a subject of various analyseswith the goal to obtain the value of gp

+ 3He → 3H +

•MuCap: 1496±4 /s (0.3%)

•Pisa-JLab theory: 1494±21 /s

gP(qc2)=8.2±0.7

He captureHe capture

42

Conclusions

• The MuCap experiment is able to perform measurements of muon capture rates on proton and light nuclei on unprecedented level of precision.

• This allowed for the first time to measure with high precision the nucleon’s pseudoscalar form factor, thus providing a critical probe of the Heavy Baryon Chyral Perturbation Theory.

• The precision measurements of µd and µHe3 capture rates

will allow to fix the Low Energy Constants in the Effective Field Theories of light nuclei.