Muon Capture as a Probe of the Nucleon’s Axial Structure – the  Cap Experiment Peter Kammel University of Illinois at Urbana-Champaign PANIC05, October 25, 2005 Contents Physics context Muon capture on the proton theory - experiment Axial currents in the 2N system

E-W Current probes Strong Interactions Charged current interaction nucleon level quark level   (1-  5 ) u d p n pQCD Basic challenge: derive low energy hadron structure and interactions from QCD lattice QCD EFT based on chiral symmetry for q/  small Formfactor parametrize microscopic QCD structure W nucleon current + second class currents nucleon current + second class currents

Muon Capture on the Proton  - + p   + n rate  S BR~10 -3  - + p   + n +  BR~10 -8, E>60 MeV nucleon weak CC formfactors q 2 = -0.88m  2 g V = (5) g A = 1.245(3) g M = (25) g P = ? nucleon weak CC formfactors q 2 = -0.88m  2 g V = (5) g A = 1.245(3) g M = (25) g P = ? g V, g M, g A determined by SM symmetries and data, contribute <0.3% uncertainty to  S g P determined by chiral symmetry of QCD: n  p --  g  NN FF g P = (8.74  0.23) – (0.48  0.02) = 8.26  0.23 PCAC pole term Wolfenstein ChPT leading order one loop two-loop <1% N. Kaiser Phys. Rev. C67 (2003) Lincoln Wolfenstein, Ann. Rev. Nucl. Part. Sci …it became customary to assume the standard V-A coupling and then deduce the pseudoscalar g P coupling from the data. I thought this was misleading because in the absence of new physics g P was determined very accurately from the pion-pole contribution. The radiative muon capture in hydrogen was carried out only recently with the results that the derived g P was almost 50% too high. If this results is correct, it would be a sign of new physics that might contribute effectively to V, A or P.

One of many experimental challenges  T = 12 s -1 pμ ↑↓ singlet (F=0)  S = 664 s -1 n+ triplet (F=1) μ pμ ↑↑ ppμ para (J=0)ortho (J=1) λ op  ortho=506 s -1  para=200 s -1 ppμ Interpretation requires knowledge of pp  population Strong dependence on hydrogen density pp  P pp  O pp 100% LH 2 pp pp  P pp  O 1 % LH 2 time (  s) rate proportional to H 2 density !

Precise Theory vs. Controversial Experiments  PT OP (ms -1 ) gPgP  - + p   + n + Triumf  Cap precision goal exp theory update from Gorringe & Fearing no overlap theory & OMC & RMC large uncertainty in OP  g P  50% ? no overlap theory & OMC & RMC large uncertainty in OP  g P  50% ? TRIUMF 2004  - + p   + Saclay

Goals of  Cap* n Unambiguos Interpretation n In-situ experimental handle on all systematics n Much higher statistics  S with 1% precision  g P with 7% precision *  Cap collaboration Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia Paul Scherrer Institute (PSI), Villigen, Switzerland University of California, Berkeley (UCB and LBNL), USA University of Illinois at Urbana-Champaign (UIUC), USA Université Catholique de Louvain, Belgium TU München, Garching, Germany University of Kentucky, Lexington, USA Boston University, USA g P basic and least known weak nucleon form factor solid QCD prediction via ChPT (2-3% level) basic test of QCD symmetries experiments not precise, controversial, discrepancy to theory g P basic and least known weak nucleon form factor solid QCD prediction via ChPT (2-3% level) basic test of QCD symmetries experiments not precise, controversial, discrepancy to theory Recent reviews: T. Gorringe, H. Fearing, Rev. Mod. Physics 76 (2004) 31 V. Bernard et al., Nucl. Part. Phys. 28 (2002), R1

How will  Cap achieve this ? Lifetime method   →e decays measure   to 10ppm,    S = 1/   - 1/   to 1%  Unambiguous interpretation capture mostly from F=0  p state at 1% LH 2 density Clean  stop definition in active target (TPC) to avoid  Z capture Ultra-pure gas system and purity monitoring  p + Z   Z + p TPC bakeable, high vacuum materials & continuous purification online/offline purity analysis (0.01 ppm level) Isotopic purity at ~1 ppm level  p + d   d + p, large diffusion In situ/offline analysis (0.5 ppm level) unique  Cap capabilities fulfill all requirements simultaneously   disappears faster by ~0.1%

 Cap detector Design Reality 2004

3D tracking w/o material in fiducial volume Muon stops in active target p -- 10 bar ultra-pure hydrogen, 1% LH kV/cm drift field ~5 kV on 3.5 mm anode half gap bakable glass/ceramic materials Observed muon stopping distribution E e-e-

Time spectra  -e impact parameter cut huge background suppression diffusion (deuterium) monitoring --  +  SR in 80G  + as reference identical detector systematics different physics blind analysis

Impurity detection in TPC rare impurity capture  +Z  (Z-1)+n+ Triggered FADC + C irculating H ydrogen U ltrahigh P urification S ystem (CHUPS)* + Gas chromatography *PNPI+UIUC with CRDF funding

 Cap Status & Outlook  Final upgrades  Performance  Expected Results Statistics Muon-On-Request (MuLan), 2-3x increase in data rate ! Systematics n Z>1 Impurities Improved diagnostics (FADCs, sensors) faster circulation (CRDF) n Isotopic purity increase TPC gain for monitoring CRDF project: new detection method and purification n Kinetics constrain op correction by measuring capture neutrons SubsystemParameter TPC  stop fraction high voltage (KeV) eDet 2nd MWPC   Electronics eSC FADC TPC FADC   DAQ Livetime fraction Purity Z>1 (ppm) deuterium (ppm) , better diag. 0.3?, better diag. Statistics  - (10 9 )  + (10 9 ) Calibration runs C, N, O, D,  ppm   run 2004 runs

Axial currents in 2N system n Reactions basic solar fusion reaction p + p  d + e + + key reactions for SNO + d  p + p + e - (CC) + d  p + n + (NC) … n Theory 1B NN description in good shape 2B not well constrained by theory EFT* SNPA EFT  EFT n Quest to determine L 1A n Experiments on 2N axial current 10% uncertainty at best n Estimated Theory precision from some % to some 0.1% ! during last few 10 years. Based on 3N info (tritium beta decay), as no 2N info available of required precision. MEC EFT L 1A  EFT: Class of axial current reactions related by single unknown parameter L 1A Precise experiment in 2N system needed determine L 1A, astrophysics reactions test SNPA vs. EFT verify claimed precision of overall framework

Muon Capture on the Deuteron  d capture close terrestrial analogue d p e e-e- p soft enough for L 1A physics? 1% precision measurement possible ? n d -- W n W Kammel, Chen   EFT (error N 3 LO) Theory Experiment  - + d   + n + n  g P has to be known ! EFT* (tritium  -decay)

20 E n (MeV)  ’~90% of intensity  measurement of absolute rate to <1% (  D I)  Cap technique, new cryo TPC Kinetics requires optimized target conditions T<80K, 5% density  measurement of Dalitz Plot to 5 % (  D II ) Neutron detector array Kinematics determined by angle and dt determine rate for relevant low energy rate  ’ study motivation for full DP measurement MECs, g P ( q 2 )  D project Collaborators welcome  Cap N=3,4 with TPC ? (electronic bubble chamber) time (  s) New benchmark in EW reactions in 2N system

 + 3 He → 3 H + g P ( m  2 ) = 8.53±1.54 ±0.5 ?

process value (fm 3) method theory Dim.arguments ±5 2 nucleon e +d  e - +p+p ±2 Orland ?? +d  e + +n+n 3.6 ±5.5reactor, optimistic   +n +p ES,CC,NC 4.0 ±6.3 SNO self calibration  +d  +n+n ±1.5 ?1%  measurement theory uncertainty? 3 nucleon 3 H  3 He+e + +n6.5 ± 2.4  3 He  3 H+ ?g P from other source astro Helioseismology7.0 ± 5.9pp fusion, but no other SoMo uncertainties L 1A estimates Butler, Chen, Vogel Ando et al (2002) EFT with T decay constraint uncertainty ~1% 2 body currents6.5% High E nn contribution negligible  1% experiment on  d measures L 1A to ~20% Chen (private comm):  EFT, q < 10MeV/c, higher order? Ando et al (2002) EFT with T decay constraint uncertainty ~1% 2 body currents6.5% High E nn contribution negligible  1% experiment on  d measures L 1A to ~20% Chen (private comm):  EFT, q < 10MeV/c, higher order?

Parameters

Z>1 impurities Yield Y for  Z  (Z-1)+n+ Y  100 c Z  ~ 1.6 Y Gas chromatography

PCAC: q 2 =0 GT relation: g  NN (0) F  =M g A (0) q 2 <0g p (q 2 )= 2 m M/(m  2 -q 2 ) g A (0) g P =8.7 Sensitivity of capture rate: error from  V ud = 0.16 % assuming optimistic 20% g P error assuming g T <0.1

Nucleon charged current at q 2 = m  2 J  = V  - A  V   g V (q 2 )   + ig M (q 2 )/2M   q  + g S (q 2 )/m q  A   g A (q 2 )     + g P (q 2 ) q  /m   + ig T (q 2 )/2M   q    Vector current in SM determined via CVC g V (0)=1, g(q 2 )=1+q 2 r 2 /6, r V 2 =0.59 fm 2 g M (0)=  p -  n -1= , r M 2 =0.80 fm 2 q 2 dependence from e scatt. Axial vector FF from experiment g A (0)=1.2670(35), r A 2 =0.42±0.04 fm 2 q 2 dependence from quasi-elastic scattering,  e-production 2 nd class FF g S, g T forbidden by G symmetry, e.g. g T /g A =-0.15 ±0.15 (exp), ±0.0053(QCD sum rule, up-down mass difference) error from  V ud = 0.16 % nucleon weak formfactors g V,g M,g A determined by SM symmetries and data contribute <0.4% uncertainty to  S g V = (5) g M = (25) g A = 1.245(3) remains g P = ?  Cap

Continuous purification system commissioned, CRDF CRDF project critical improvement: impurities reduced from 0.5 to 0.07 ppm ! residual impurity signal not completely understood

CHUPS