Muon Capture as a Probe of the Nucleon’s Axial Structure – the m Cap Experiment

1. Muon Capture as a Probe of the Nucleon’s Axial Structure –the mCap Experiment Peter KammelUniversity of Illinois at Urbana-Champaignwww.npl.uiuc.edu/exp/mucapture PANIC05, October 25, 2005 • Contents • Physics context • Muon capture on the proton theory - experiment • Axial currents in the 2N system

2. nucleon current <n|Va- Aa |p> +second class currents E-W Current probes Strong Interactions • Charged current • interaction quark level gm(1-g5) u d W • Basic challenge:derive low energy hadron structure and interactions from QCD • lattice QCD • EFT based on chiral symmetry for q/Lsmall nucleon level pQCD n p Formfactor parametrizemicroscopic QCD structure

3. gpNN n p p Fp m- nm Muon Capture on the Proton • - + p  m+ n rateLSBR~10-3 • - + p  m+ n + g BR~10-8, E>60 MeV nucleon weak CC formfactorsq2= -0.88mm2 gV = 0.9755(5) gA = 1.245(3) gM = 3.5821(25) gP = ? gV, gM, gA determined by SM symmetries and data,contribute <0.3% uncertainty to LS Lincoln Wolfenstein, Ann. Rev. Nucl. Part. Sci. 2003 …it became customary to assume the standard V-A coupling and then deduce the pseudoscalar gP coupling from the data. I thought this was misleading because in the absence of new physics gP 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 gP 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. gP determined by chiral symmetry of QCD: gP= (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) 027002

4. 1 % LH2 100% LH2 pm ppμ ppmO • Interpretation requires knowledge of ppm population • Strong dependence on hydrogen density ppμ pm rate proportional to H2 density ! ppmO ppmP ppmP time (ms) One of many experimental challenges LT = 12 s-1 triplet (F=1) pμ↑↑ Lortho=506 s-1 Lpara=200 s-1 λop μ ppμ ppμ ortho (J=1) para (J=0) pμ↑↓ singlet (F=0) LS= 664 s-1 n+n

5. Precise Theory vs. Controversial Experiments gP update from Gorringe & Fearing - + p  m+ n + g@Triumf PT mCapprecisiongoal - + p  m+ n @ Saclay TRIUMF 2004 exp theory lOP (ms-1) • no overlap theory & OMC & RMC • large uncertainty in lOP gP  50% ?

6. Goals of mCap* • Unambiguos Interpretation • In-situ experimental handle on all systematics • Much higher statistics • LS with 1% precision • gP with 7% precision • gP 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 * mCap collaboration 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), USAUniversité Catholique de Louvain, BelgiumTU München, Garching, GermanyUniversity of Kentucky, Lexington, USABoston University, USA Recent reviews:T. Gorringe, H. Fearing, Rev. Mod. Physics 76 (2004) 31V. Bernard et al., Nucl. Part. Phys. 28 (2002), R1

7. How will mCap achieve this ? • Lifetime method 1010m→enn decays measure - to 10ppm, • S = 1/- - 1/+ to 1% • Unambiguous interpretationcapture mostly from F=0 mp state at 1% LH2 density • Clean m stop definition in active target (TPC)to avoid mZ capture • Ultra-pure gas system and purity monitoring mp + Z mZ+ pTPC bakeable, high vacuum materials & continuous purification online/offline purity analysis (0.01 ppm level) • Isotopic purity at ~1 ppm level mp + d md+ p, large diffusionIn situ/offline analysis (0.5 ppm level) m-disappears faster by ~0.1% unique mCap capabilitiesfulfill all requirements simultaneously

8. mCap detector Design 2001-2 Reality 2004

9. m- Muon stops in active target 10 bar ultra-pure hydrogen, 1% LH2 2.0 kV/cm drift field ~5 kV on 3.5 mm anode half gap bakable glass/ceramic materials Observed muon stopping distribution E p e- 3D tracking w/o material in fiducial volume

10. Time spectra m- m-e impact parameter cut huge background suppression diffusion (deuterium) monitoring m+ as reference identical detector systematics different physics m+ mSRin 80G blind analysis

11. Impurity detection in TPC rare impurity capture m+Z  (Z-1)+n+n Triggered FADC +CirculatingHydrogenUltrahighPurificationSystem (CHUPS)*+ Gas chromatography *PNPI+UIUC with CRDF funding

12. run 2004 runs 2005-06 mCap Status & Outlook • Final upgrades • Performance • Expected Results • Statistics • Muon-On-Request (MuLan), 2-3x increase in data rate ! • Systematics • Z>1 Impurities • Improveddiagnostics (FADCs, sensors) • faster circulation (CRDF) • Isotopic purity • increase TPC gain for monitoring • CRDF project: new detection method and purification • Kinetics • constrain lop correction by measuring capture neutrons

13. Reactions basic solar fusion reaction p + p  d + e+ +  key reactions for SNO  + d  p + p + e- (CC)  + d  p + n +  (NC) … Theory 1B NN description in good shape 2B not well constrained by theory EFT* SNPA EFTpEFT Quest to determine L1A Experiments on 2N axial current 10% uncertainty at best 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 L1A Axial currents in 2N system • Precise experiment in 2N system needed • determine L1A, astrophysics reactions • test SNPA vs. EFT • verify claimed precision of overall framework pEFT: Class of axial current reactionsrelated by single unknown parameter L1A

14. Muon Capture on the Deuteron Kammel, Chen md capture close terrestrial analogue • soft enough for L1A physics? • 1% precision measurement possible ?  p n d d p n W W ne e- nm m- gP has to be known! - + d  m+ n + n pEFT (error N3LO) EFT* (tritium b-decay) Theory Experiment

15. mD project Collaborators welcome • measurement of absolute rate to <1% (mD I) mCap technique, new cryo TPC Kinetics requires optimized target conditions T<80K, 5% density • measurement of Dalitz Plot to 5 % (mD II) Neutron detector array Kinematics determined by angle and dt • determine rate for relevant low energy rate L’ • study motivation for full DP measurementMECs, gP(q2) New benchmark in EWreactions in 2N system time (ms) 20 En (MeV) En (MeV) L’~90% of intensity mCap N=3,4 with TPC ? (electronic bubble chamber)