MuCapHigh-Precision Measurement of

Muon Capture on the Protonm BVR35

Progress reportpresented by

Claude Petitjean, PSI12 Febuary 2004

http://www.npl.uiuc.edu/exp/mucapture/

MuCap Experiment

• experimental goal• principle of measurement• apparatus• time projection chamber (TPC)• data from run in fall 2003• some results• milestones reached in 2003• upgrades for run in fall 2004• Beam request 2004

MuCap collaborationV.A. Andreev, A.A. Fetisov, V.A. Ganzha, V.I. Jatsoura, A.G. Krivshich, E.M. Maev, O.E. Maev,

G.E. Petrov, G.N. Schapkin, G.G. Semenchuk, M. Soroka, A.A. VorobyovPetersburg Nuclear Physics Institute (PNPI), Gatchina 188350, Russia

P.U. Dick, A. Dijksman, J. Egger, D. Fahrni, M. Hildebrandt, A. Hofer, L. Meier, C. Petitjean, R. Schmidt

Paul Scherrer Institute, PSI, CH-5232 Villigen, Switzerland

T.I. Banks, T.A. Case, K.M. Crowe, S.J. Freedman, F.E. Gray, B. LaussUniversity of California Berkeley, UCB and LBNL, Berkeley, CA 94720, USA

K.D. Chitwood, S.M. Clayton, P.T. Debevec, D. W. Hertzog, P. Kammel, B. Kiburg,R. McNabb, F. Mulhauser, C. J. G. Onderwater, C. Ozben, C.C. Polly, A. Sharp, D. Webber

University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

L. Bonnet, J. Deutsch, J. Govaerts, D. Michotte, R. PrieelsUniversité Catholique de Louvain, B-1348 Louvain-La-Neuve, Belgium

R. M. Carey. J. PaleyBoston University, Boston, MA 02215, USA

T. Gorringe, M. Ojha, P. ZolnierzcukUniversity of Kentucky, Lexington, KY 40506, USA

F.J. Hartmann Technische Universität München, D-85747 Garching, Germany

�Cap

Experimental goalmeasure the rate Λs of muon capture on the proton

p n�

� ��

��� �

to 1% precision. Muon capture is a semileptonic weak interaction process, and occurs predominantly from the hyperfine singlet atomic bound state.

a 1% measurement of Λs determines gp – the “weak nucleonic charged current induced pseudoscalar form factor” – to < 7% precision.

The QCD prediction (Heavy Baryon Chiral Pert. Theory) is ~2% precise.

problem of previous experiments

�T

�

pp�

pp�

p�p�

�ortho

�para

F=1

J=1

J=0

�OP

n+�

n+�

interpretation of rate �S

due to formation of ppµ

molecules in lq hydrogen

and badly known rate �OP

of ortho-para transitions

�Sn+�

F=0

Muon Capture and gP

20 40 60 80 100 120

2.5

5

7.5

10

12.5

15

17.5

20

ChPT�

g P

RMC

�Cap proposed

Saclay exp theory

OMC Saclay

update from Gorringe & Fearing

interpretation of gP vs �OP

---------o---------New TRIUMF exp

�OP (ms-1)

Experimental Technique“Lifetime” Method

For µ–, muon capture competes with muon decay:

(99.85%)

( ) (0.15%)ee

p n�

�

� � �

� �

� �

�

��

�

� �

This rate decreases the observed µ– lifetime from the vacuum lifetime, which we measure separately with µ+ :

(100%)ee�

� � �� �

�

Thus we need 10 ppm measurements of the µ+/- lifetimes or a statistics of at least 1010 for both µ-decays

Experimental Technique“Lifetime” or “Disappearance” Method

Since our experiment can only observe e+ and e– decay products, muon capture produces a small downward deflection of the µ– lifetime curve from the µ+ “vacuum” lifetime curve :

log(counts)

time

µ+

µ –

The capture rate is easily calculated from the measured lifetimes:

� � � �1 1

( )' ( )'s � � � �� �

� �

� � � �� � � �� � �

Technical tasks of MuCap experiment

tasks goals to be reached

• ultra-clean 10-bar hydrogen gas target 0.01 ppm Z>1 impurities• deuterium depleted hydrogen („protium“) < 1 ppm deuterium• 100% stop identification no wall stops � TPC• clean electron identification & tracking 2 cylindrical wire chambers• unique µ� e decay assignment µ-stop – e-vertex matching• high data rate 30 kHz µ stops• high statistics > 1010 events for µ+ and µ-

• µSR under control for µ+ 50-100 Gauss magnetic field

Experimental Setup – Apparatus

µSC µPC1 µPC2 TPC

ePC2ePC1

eSC (Hodoscope)

µ beam

MuonDetectors

ElectronDetectors

• e-detectors cover 75% of 4π

assembly/tests: March-Aug 2003 data-taking: Sept-Oct 2003

Ucathodes = 5-6 kV

E = 2 kV/cm - vdrift = 0.5 cm/µssensitive volume (12 x 15 x 30) cm3

TPC design drawing with glass frames and ceramic structure

bakeable to 130 C

Online display of µ beam, µ stops in TPC and e in ePC1

ePC1/eSC(phi,z)

event display of muon stops & electrons

event display of µ stop + impurity capture

development of impurities during the 2 good data weeks

full scale 1 ppm

summed decay time spectra showing pile-up suppression

µ – decay time spectra of 1 week with clean protium filling

MuCap 2003: exponential fits to each eSC counter pair

eSC Segment Number2 4 6 8 10 12 14 16

]-1

Ra

te [

s

4498

4500

4502

4504

4506

4508

45102

x10

Decay-Rate Fits vs. eSC Segment Number

(assuming 50 MHz clock)fitλ

Decay-Rate Fits vs. eSC Segment Number

eSC Segment Number2 4 6 8 10 12 14 16

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

e-Detector Accidentals Fraction

100×Accidentals

e-Detector Accidentals Fraction

MuCap 2003: µSR enhanced fit of µ+ decay time spectra

milestones 2003• protium production with < 0.5 ppm deuterium• ultra-clean protium gas filling (< 0.1 ppm impurities) – after

installation of a new Palladium filter• new TPC@4.8 kV stable operation (perfect µ stop detector!)• full e detector consisting of ePC1 and eSC• new frontend electronics for ePC1• new compressor electronics for deadtime free ePC1 data collection• new DAQ running with 4 MB/s data rate (80% duty cycle)• new µSR magnet with ~50 Gauss field• new slow control system• full electronics for TPC, wire chambers and plastic counters• collimated muon beam injection system• analysis software in mature development

upgrades for 2004

• protium: maintain < 0.5 ppm deuterium & < 0.1 ppm Z>1 impurities• new gas circulation, cleaning & diagnostics (Gatchina)• e-detector: include new ePC2 (� vertex tracking)• tune TPC to 5.5 kV (� online control of µd diffusion)• new 0.5 mm Beryllium window (� increase good µ stops x 2)• µ-detectors: new outside µPC1&2 (� reduce µ-absorption/range-tails)• µ-beam: new tune in πE3 area (� < 3% momentum spread)• new spare TPC from Gatchina (� >6 kV to see e tracks)• upgrade DAQ for larger data flow (� ~100% duty cycle)• upgrade µSR magnet with Alu coils (� reduce scattering of e)• new online data analysis

goal for 2004: collect a significant part of 1010

pileup-protected good µ-decays for µ+ and µ-

protium gas circulation and purifying system (Gatchina)

MCstudy of µ stop distributions for various beam windows

MuCap experiment: time plans of upgrades for 2004 run

MuCap experiment: beam request for 2004

In 2004 we hope to approach closely the goal formulated in the proposal:

a measurement of the µp singlet capture rate to ± 1% accuracy

To accomplish this goal, we need area πE3 for the following activities:

- setup of beamline with el.stat. separator, assemble full apparatusand electronics, test all components in the beam: 2 weeks

- tune new 35 MeV/c muon beam: 1 week- test & optimize complete detector with beam: 1 week- data production towards 1010 good µ- & µ+ decays: 2 x 2.5 weeks- contingency for beam losses or other failures 1 week

� we request 10 weeks in area πE3(preferably in Oct-Dec 2004, at the end of the 2004 production cycle)