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1% precision measurement of singlet nuclear muon capture rate Λ s - semi-leptonic weak interaction process - determines the induced pseudoscalar coupling g P to 6% - constitutes a vigorous test of low energy HBChPT thesis works by T.I. Banks (UC Berkeley), S.M. Clayton (UI Urbana Campaign), B.E. Kiburg, S. Knaak (both UIUC, now UW Seattle) MuCap proposal 1997 Petitjean LTP-Seminar
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Unblinding the MuCap experimentthe final results of μp capture rate ΛS
and of electro-weak coupling constant gP
LTP Seminar
April 23, 2012
Claude PetitjeanPSI
theor. & exp. gP bands vs. Ortho-Para transition rate
MuCap homepage http://muon.npl.washington.edu/exp/MuCap/Petitjean LTP-Seminar 23.04.12
outline- goal of MuCap experiment, theory
- experimental challenges MuCap apparatus with TPC- systematics: - impurities - formation of ppμ molecules Argon doped run - interferences with electron charges - μp scattering & diffusion list of systematic corrections- unblinding of main runs 2006/07- new results on ΛS gP
Petitjean LTP-Seminar 23.04.12
1% precision measurementof singlet nuclear muon capture rate Λs
p n
- semi-leptonic weak interaction process- determines the induced pseudoscalar coupling gP to 6%- constitutes a vigorous test of low energy HBChPT
thesis works by T.I. Banks (UC Berkeley), S.M. Clayton (UI Urbana Campaign), B.E. Kiburg, S. Knaak (both UIUC, now UW Seattle)
MuCap proposal 1997
Petitjean LTP-Seminar 23.04.12
scientific case of μ capture on the protonμ capture probes axial structure of nucleon
μ capture neutron β decay
hadronic vertex determined by QCD: q2 dep. form-factors (gV,gM,gA,gP)μp-capture is the only process sensitive to the nucleon form factor gP
pn
νe
e-n
νμ
p
μ-W W
μ- + p νμ+ n
(analogue)
prediction of heavy baryon chiral perturbation theory (V. Bernard et al. 1994): gP
theory = 8.26 0.23- gp least known of the nucleons weak form factors- solid QCD prediction by HBChPT at the 2-3% level (NNLO < 1%)- basic test of chiral symmetries and low energy QCD
- V. Bernard et al., Nucl. Part. Phys. 28 (2002), R1 - T. Gorringe, H. Fearing, Rev. Mod. Physics 76 (2004) 31 - P. Kammel, K. Kubodera, Annu. Rev. Nucl. Part. Sci. 60 (2010) 327
recent reviews:
Petitjean LTP-Seminar 23.04.12
dependence of ΛS from gA & gPgA from neutron β decay ↓ ↓ ↓
← - 1% ← ΛS
th = (711.5 ± 4.6) s-1
(incl. rad. correction)← + 1%
a 1% Λs measurementdetermines gp to ~ 6%
the QCD prediction(HBChPT) is ± 2-3%
Petitjean LTP-Seminar 23.04.12
experimental challenges of μ- in
pure hydrogen- high precision measurement of absolute capture rate
- high statistics (1010 events)
- all μ must stop in hydrogen (no wall stops!)
+ avoid formation of ppμ molecules (Ortho-Para problem)
- ultra-high gas purity avoid μp → μZ transfers to impurities - ultra-high isotopic purity avoid μp → μd transfers (diffusion problem)
solutions of MuCap experiment
lifetime method(likeSaclay)
ΛS = λ(μ-p) – λ(μ+)muon beam with kicker
hydrogen TPC operatedin low density H2 gas
Φ ~ 0.01 (lq=1)
use of UHV materialscontinuous gas circulationsystem (CHUPS)
cZ>1 ≤ 20 ppb
HD separation columnD-depletion cd < 7 ppb
log(
coun
ts)
te-t
μ+
μ –
λμ+
λμ-
ΛCAP reduces lifetime
by ~1.6x10-3
→ e
Petitjean LTP-Seminar 23.04.12
e
cut out view of MuCap detector used in 2003-07
Petitjean LTP-Seminar 23.04.12
the MuCap detector with rolled back TPC
Petitjean LTP-Seminar 23.04.12
MWPC with Ucath = -(5-6)
kVEdrift = 2 kV/cm - vdrift = 0.55 cm/μssensitive volume (12 x 15 x 30) cm3
all wires soldered on special glass frames pure metal & ceramic
structurebakeable to 130 C
the Hydrogen TPC as active muon stop targetdeveloped 2001-03 at PSI
filling 10 bar ultra-pure protium gas
UHV = -30 kV
Petitjean LTP-Seminar 23.04.12
details of the TPC 75 anode wires
35 x 4 cathode wires2-D hor. readout + vert. drift time → 3-D reconstruction of μ tracksreliable operation achieved in 2004-07
first MWPC modul
etested
in 2001
mounting the final TPC in 2003 details of wiringPetitjean LTP-Seminar 23.04.12
first TPC assembly by PSI engineers in 2002
Petitjean LTP-Seminar 23.04.12
TPC performance shown in the event displayan
odes
strip
s
TPC signals showing a clean muon stop event with nuclear recoil from Z>1 capture allows monitoring of
impuritiesPetitjean LTP-Seminar 23.04.12
precision lifetime measurementis understanding the systematics!
the major issues in MuCap:
I impurities calibration with doped gas mixtures *II formation of ppμ molecules (rate λppμ) ** ortho-para transitions (rate λop)III electron interference with muon tracks *IV μ + p scatters *V diffusion of μdVI diffusion of μp *
* thesis Brendan Kyburg** thesis Sara Knaack
Petitjean LTP-Seminar 23.04.12
I. impurities: removal with CHUPS (Cont. H2 Ultra-Purification System, developed in Gatchina)- cryogenic adsorption/desorption cycles in active Carbon- Zeolite in liquid nitrogen absorbs all Z>2 impurities: cN2 <5ppb, cH2O
~17ppb
CHUPS duringmain runs
our main impurity source was water vapor
from walls & materials
H2O
N2
gas chromatography
humidity sensorPetitjean LTP-Seminar 23.04.12
cleaning effect of CHUPS gas circulation
thesis B. Kiburg
calibration
calibration run with 11 ppm N2
Petitjean LTP-Seminar 23.04.12
effect on μ lifetime determined with extrapolation method
ppm impurity admixtures correction to ΛS: -7.8 ± 1.87 (2006); -4.54 ± 0.93 (2007) [s-1] (-1.1%) (-0.6%) thesis B. Kiburg
syst. error due to badly known ratio
α = YN/Yall
we include: 0<α<1
Petitjean LTP-Seminar 23.04.12
ΛPM ~213s-
1
ΛT ~12s-1
pμ↑↓
singlet(F=0)
ΛS ~710s-1
n+
triplet(F=1)
μ-
pμ↑
↑
n+
ppμ ppμ
para (J=0)ortho (J=1)
ΛOM ~540s-1
λop
n+
φλppμ
n+
- ppμ formation rate λppμ was known to ± 30% only- ortho para transition λop known to ± 50% only
- capture rates ΛS - ΛOM - ΛPM very different
τ~10ns
- our observed capture rate is not pure ΛS- at our gas density (10 bar) φ = 1.13% of lq H2 gives to ΛS a 3% correction with large error bar
- determine φλppμ by a special Argon doped run– remeasure λop from n-time spectra (to be done!)
II. ppμ molecule formation – the ortho-para problem
problem:
solution:Petitjean LTP-Seminar 23.04.12
ppP
ppO
p
100% lq. H2
p
ppP
ppO
1% lq. H2
→ time (μs)
development of atomic & molecular states of μ in H2
(a) liquid hydrogen, φ = 1 (b) hydrogen gas at φ = 0.01 pμ depopulated in 1-2 μs pμ remains dominant
~ 81% ppμ formation ~ 5% ppμ formation badly known ortho-para ratio! small effects from op transitions!
Petitjean LTP-Seminar 23.04.12
20 40 60 80 100 120
2.5
5
7.5
10
12.5
15
17.5
20
op (ms-1)
- + p + n + @ TRIUMF
MuCap precision goal
Saclay 1981 theory TRIUMF 2006
- + p + n @ Saclay
experimental situation on gP before MuCap
experiments & op rates are inconsistent Saclay experiment (in lq. H2) cannot be interpreted!
gP
HBChPT (8.26)
Petitjean LTP-Seminar 23.04.12
new ppμ measurement using 18.5 ppm Argon admixture
μp
ppμ
Λppμ = ppμ2.30x10-2s-1
ΛpAr = cAr pAr 4.46 x10-2 s-1
μAr
μ+ArCl+n+
ΛAr 1.3 s-1
kinetics scheme with rates Λppμ, ΛpAr, ΛAr
new result measured at MuCap conditions:
ppμ = 1.99 ± 0.058stat μs-1
(prelim.)(thesis Sara Knaack, to be published)correction to observed ΛS: ΔΛppμ = (18.2±2.5) s-1
(2.5%)
5x108 eventsΧ2/NDF = 0.98
Petitjean LTP-Seminar 23.04.12
(c) decay electron deposits energy (d) can generate or
augment pixels
(a) μ enters TPC & ionizes gas(b) charge drifts towards
MWPC
III. electron interference with muon tracks:charge deposition from decay electrons can generate or modify
pixelsof a muon track acceptance of events may become time
dependent!
thesis B. KiburgPetitjean LTP-Seminar 23.04.12
• EL Pixel• separated from track • added near the track
• EH Pixel• EL pixel “upgraded”• this modifies NCEH in
a complicated way
example of an interference between a muon trackand the decay electron producing blue pixels
crucial parameter is NCEH (# of EH pixels)
thesis B. KiburgPetitjean LTP-Seminar 23.04.12
μ + p scattersgenerates EH pixel on one anode cut NCEH = 1
events
IV. μ + p scatters: can fake a μ stop, but actually stops in
surrounding high Z material & distorts the lifetime
thesis B. KiburgPetitjean LTP-Seminar 23.04.12
fast neutron time component in NCEH=1 eventsdue to μ + p scatters leaving the TPC
thesis B. KiburgPetitjean LTP-Seminar 23.04.12
thesis B. Kiburg
lifetime fits vs. NCEH
upper e-detector: no μ-e interference,but effect from μ-e scatters
lower e-detector: μ-e interference withμ track is time and space dependent
Petitjean LTP-Seminar 23.04.12
• match NCEH =2+ • determine
NCEH =1 distortion
• allows μ + p scatter estimate
correction of μ + p scatter effects: the μ+ data has NCEH interference, but no μ + p scatter distortion in NCEH = 1
thesis B. Kiburg
correction to ΛS: -12.4 ± 3.2 (2006); -7.20 ± 1.25 (2007) [s-1] (-1.7%) (-1.0%)
Petitjean LTP-Seminar 23.04.12
V. diffusion of μd: remove all deuterium by a H-D isotope separation
column (developed by Gatchina & PSI 2006/07)principle: - H2 gas circulates from bottom to cold head at top & gets liquefied
- liquid droplets fall down & evaporize gas phase gets depleted from D - the D-enriched liquid H2 at the bottom is slowly removed
AMS protium analysis at ETHZ:in 2004: cd = (1.45±0.15)10-6
in 2006-07: cd < 6*10-9
World Record: cd < 6 ppbPetitjean LTP-Seminar 23.04.12
VI. μp diffusion: it distorts the lifetime slope!
impact parameter
our choice of impact parameter is 120 mm resulting in a correction due to μp
diffusion of
ΔΛS = -3.0 ± 0.1 s-1 (0.4% of ΛS)
thesis B. KiburgPetitjean LTP-Seminar 23.04.12
source old run8 (2004)
Phys. Rev. Lett. 99,032002 (2007)
new run10 (2006)
P. Winter et al.
new run11 (2007)
B. Kiburg et al.
remarks
unit s-1 s-1 s-1
μ+p scatter 0 ± 3.0 -12.40 ± 3.22* -7.20 ± 1.25* err. prelim.
μ+p diffusion -2.7 ± 0.5 -3.1 ± 0.1 -3.0 ± 0.1μ+d diffusion -10.2 ± 1.6 0 0 cd < 7
ppbHigh-Z impurities -19.2 ± 5.0 -7.80 ± 1.87 -4.54 ± 0.93entrance counter
inefficiencies 0 ± 3.0 0 ± 0.5 0 ± 0.5
e det. - definition 0 ± 5.0 0 ± 1.8* 0 ± 1.8* err. prelim.
fiducial volume cut
- 0 ± 3.0 0 ± 3.0
total correction -32.1 ± 8.5 -23.3 ± 5.1* -14.7 ± 3.9* *error correlated
systematic corrections to μ and errors [s-1]
Petitjean LTP-Seminar 23.04.12
lifetime fit of full run11 statistics
thesis B. KiburgPetitjean LTP-Seminar 23.04.12
MuCap‘s new physics resultsfrom unblinding the 2006 & 2007 data
at UW Seattle, Dec 16, 2011
(preliminary)
Petitjean LTP-Seminar 23.04.12
lifetime fit results in blinded mode(with unknown offset of master clock)
Petitjean LTP-Seminar 23.04.12
final lifetime results after unblinding Dec 16, 2011
Petitjean LTP-Seminar 23.04.12
old run8 (2004)Phys. Rev. Lett. 99,
032002 (2007)
new run10 (2006)
evaluated byP. Winter et al.
new run11 (2007)
evaluated byB. Kiburg et al.
remarks
statistics 1.6 x 109 5.5 x 109 5.0 x 109 events after cuts
μ- fit 455‘883.5 ± 12.5
455‘880.4 ± 7.0 455‘867.0 ± 7.6 2/NDF ~1.15±0.1
syst. corr. -32.1 ± 8.5 -23.3 ± 5.1 -14.7 ± 3.9 systematical errorμ- corr. 455‘851.4 ±
12.5455‘857.1 ± 7.7 455‘852.3 ± 8.3 stat. error inflated
μ- average all
runs
455‘854.2 ± 7.2
stat.+sys. errors combined
μ+ 455‘170.2 ± 0.5 Phys. Rev. Lett. 106,041803 (2011)
final MuLan result
Δμp +12.3 Phys. Rev. 119(1),365 (1960)
bound state effect
ΔΛpμp +18.2 ± 2.5 pμp formation
ΛS= μ- - μ+
+Δμp +ΔΛpμp
714.5 ± 7.6 (±5.4stat±5.4sys
) preliminary
(within < 0.5s-1)final MuCap
result
ΛS 711.5 ± 4.6 theory
μp lifetimes & evaluation of capture rate ΛS [s-1]
Petitjean LTP-Seminar 23.04.12
the induced pseudoscalar coupling constant gP
δΛS/ΛS = -0.197 δgP/gP δgPMuCap = -0.021 gP
ChPT
gPChPT = 8.26 ± 0.23 gP
MuCap = 8.1 ± 0.5 (preliminairy!)
excellentagreementwith chiralperturbationtheory!
Petitjean LTP-Seminar 23.04.12
gP (MuCap prelim.) = 8.1 ± 0.5gP (theory) = 8.26 ± 0.23
precise & unambiguous MuCap result solves longstanding puzzle
Petitjean LTP-Seminar 23.04.12
V.A. Andreev, T.I. Banks, T.A. Case, D. Chitwood, S.M. Clayton, K.M. Crowe, J. Deutsch, J. Egger,S.J. Freedman, V.A. Ganzha, T. Gorringe, F.E. Gray, D.W. Hertzog, M. Hildebrandt, P. Kammel, B.E.
Kiburg,S. Knaak, P. Kravtsov, A.G. Krivshich, B. Lauss, K.L. Lynch, E.M. Maev, O.E. Maev, F. Mulhauser,C.S. Özben, C. Petitjean, G.E. Petrov, R. Prieels, G.N. Schapkin, G.G. Semenchuk, M. Soroka, V.
Tichenko,A. Vasilyev, A.A. Vorobyov, M. Vznuzdaev, P. Winter
MuCap collaboration & authors of P.R. Lett. 99, 032002 (2007)
Petersburg Nuclear Physics Institute (PNPI), Gatchina, RussiaPaul Scherrer Institute (PSI), Villigen, Switzerland
University of California, Berkeley (UCB and LBNL), USAUniversity of Illinois at Urbana-Champaign (UIUC), USA*
*now University of Washington (UW), Seattle, USAUniversité Catholique de Louvain, Belgium
University of Kentucky, Lexington, USABoston University, USA
parts of the collaboration during parts of the collaboration during the main run in 2006 at PSIthe main run in 2006 at PSI
(graduate students in red)
Petitjean LTP-Seminar 23.04.12