1. P10-2: Exclusive Study on the  N Weak Interaction in A=4  -Hypernuclei (update from P10) S. Ajimura (Osaka Univ.) Osaka-U, KEK, OsakaEC-U, RIKEN, Seoul-U, JAEA, Torino Spokespersons: A. Sakaguch, S. Ajimura (Osaka Univ.)

2. 2 Subjects of this proposal Properties of  N weak interaction –study on non-mesonic weak decay (NMWD) in hypernuclei   N weak interaction spin/isospin structure parity information –measurement of np-ratio (  n /  p ) of 4  He  n→nn,  p→np Studies toward test of “  I =1/2 rule” –“  I=1/2 rule” valid or not in NMWD –Study on A=4 hypernuclei ( 4  He and 4  H) –1st step for the study determination of partial decay amplitudes

3. 3 Weak decays in  -Hypernuclei  N  Mesonic weak decayMWD Mesonic weak decay (MWD) similar with free  decay spin/isospin structure well known NNN  N Non-Mesonic weak decayNMWD Non-Mesonic weak decay (NMWD) new decay modes spin/isospin structure: unknown NNN N 3S1/1S03S1/1S0 I =0 or 1

4. 4 Status of NMWD studies Old puzzle solved recently –np-ratio (   n  nn /   p  pn  n /  p ) inconsistent –Experimental and theoretical improvements –(Exp.) Back-to-back coincidence for final two nucleons (E462/508) n p n p n  N→nN n n n p p p p p n n n p  NN→nNN FSI re-scattering n n n p p p p p n n n p in reality

5. 5 A new puzzle arises –Decay asymmetry inconsistent  )Theory(7.0  NM p  )Exp.(0  NM p  HY p n Large contribution ? Asymmetry written by amplitudes assuming initial S state initialfinalamplitudeisospinparity 1S01S0 1S01S0 a1no 3P03P0 b1yes 3S13S1 1S11S1 c0no 3D13D1 d0 1P11P1 e0yes 3P13P1 f1

6. 6 NMWD of 4-, 5-body hypernuclei –allowed initial  N states assuming initial S state initialfinalamplitudeisospinparity 1S01S0 1S01S0 a1no 3P03P0 b1yes 3S13S1 1S11S1 c0no 3D13D1 d0 1P11P1 e0yes 3P13P1 f1 p p n Λ n n p Λ p p n n Λ 0+0+ 0+0+ 0+0+ 0+0+  n→nn: 1 S 0, 3 S 1 1 S 0 1 S 0, 3 S 1  p→np: 1 S 0 1 S 0, 3 S 1 1 S 0, 3 S 1

7. 7 Status of amplitude determination Current status constraint from 5  He data other constraints are loose Our prospects new constraint from 4  He np-ratio better than 15% error

8. 8 Production of 4  He High intensity  beam ~10M  + /spill (not limited by primary beam) High resolution Efficient K+ detection Use 4 He(  +,K + ) 4  He reaction 19,000 4  He/day  500,000 4  He in 4 weeks

9. 9 Energy resolution –K1.8 bemline + SKS  excellent resolution Liquid 4 He 2 g/cm 2   Ex ~ 2 MeV BE( 4  He) = 2.42  0.04 MeV –Separation from QF  production essential   p+     +A  nnX, npX

10. 10 Decay arm system –Large acceptance and high efficiency for NN –Good PID capability (n/p/  /  ) nn pp n/  TOF p/  E/  E/range n/p charge-veto

11. 11 Yield estimation – 19,000 4  He/day  500,000 4  He in 4 weeks – 1,300  p  np and 75  n  nn in 4 weeks high beam intensity large acceptance large acceptance and high efficiency in case of 1% BR

12. 12 Background estimation Background sources – QF  -production (  p+  ,   +A  nnX) cut in Ex spectrum – Mesonic weak decay of hypernuclei 4  He  3 He+p+  ,   +A  nnX    0.3    n  0.01  Reduction of background – veto: no  track in CDC – less material at target LHe target  2 g/cm 2 range(  )  5 g/cm 2 range 2 g/cm 2

13. 13 Background MC simulation Simulation of worst case –1/5 of  stop in material around target 1/5  - ~ 0.06 ⇔  n ~ 0.01 –GEANT4 base simulation LiC R: Reduction factor (En1+En2>50MeV) R~1/110 R~1/30

14. 14 Time schedule –Ready in 2009 –Collaboration with E05 and E15 K1.8 and SKS Ready Decay arm Ready

15. 15 Summary of proposal We propose to measure the nonmesonic weak decay of 4  He. –select initial spin state ( 1 S 0 / 3 S 1 ) –first step to check the validity of  I=1/2 rule 1300 np-decay and 75 nn-decay are expected in 4 weeks if B.R.(nn)=1%. Main background,   absorption, will not affect the measurement Experiment will be ready in FY 2009.

16. 16

17. Backup Slides

18. 18  S=1 weak interaction Quark description Case of  free decay  S = 1 I  0I  0I  0I  0  N   I =1/2 or  I =3/2 S= -1 I =0 S=0 I =1/2 S=0 I =1   /   =2   /   =0.5 Exp.   /   =1.78 “  I =1/2 rule” s ud W u p/s = 0.38 isospin and parity structure well known P

19. 19 –(Exp.) Overestimation of  n  nn decay Assumption of clean  N  nN process Old puzzle n p n p n  N→nN n n n p p p p p n n n p  NN→nNN FSI re-scattering n n n p p p p p n n n p in reality

20. 20  N  nN coincidence necessary –Back-to-back emission of n+N –Sum of n+N kinetic energies Back-to-backBack-to-back

21. 21 New puzzle in asymmetry  NM =0.08  0.08 +0.08 p -0.00 Large contribution ? Asymmetry written by amplitudes

22. 22 Partial decay amplitudes Block and Dalitz treatment –Initial S-wave (s-shell hypernuclei) –isospin=0 or 1 in final states –Represent spin/isospin and parity structure spinisospinparity

23. 23 n p n p n  N→nN n n n p p p p p n n n p  NN→nNN FSI re-scattering n n n p p p p p n n n p counts Q/2 Energy spectra (image) Energy distribute low energy region up to Q/2 broad peak around Q/2 continuous distribution Expected Spectrum

24. 24 Theory: - 0.6 ～ - 0.7 Asymmetry parameter of 5 Λ He  NM =0.08±0.08 +0.08 p statistical  contami -0.00 Nucl.Phys.A754 (2005) 168c nucl-ex/050916

25. 25 Asymmetry parameter of 12 C, 11 B   NM = － 0.14±0.28 +0.18 p statistical  contami -0.00 E160 : － 0.9±0.3

26. 26 Yield of 4  He with (K-,  -) reaction –33,000 4  He/day: about  2 of (  +,K+) full beam (30GeV) SPES2

27. 27 Charged particle ： ・ TOF (T2→T3) ・ tracking （ PDC ） Neutral particle ： ・ TOF (target→NT) ・ T3 VETO p n π K Decay counter Setup (KEK-PS K6 & SKS) Decay arm N: 20cm×100cm×5cm T3: 10cm×100cm×2cm T2: 4cm×16cm×0.6cm Solid angle: 26% 9(T)+9(B)+8(S)% n p polarization axis

28. 28 700,000

29. 29 High Intensity and High Resolution beamline – handle 10 9  /spill –  p/p ~ 10 -4

30. 30 High Intensity and High Resolution beamline (new configuration)

31. 31

32. Others

33. 33 Super Kaon Spectrometer (SKS)

34. 34 Production cross section – 4  He(g.s., 0+) production –estimation with DWIA by T. Harada

35. 35 Neutron energy resoltion →  7MeV(FWHM) at 75MeV Neutral PID Constant background very small 1 /  spectra Neutral particles from 12  C Good  n separation Charged particles from 5  He PID function Charged PID Good  p d separation Decay particle identification

36. 36 Nn-pair detection efficiency –About  1.5 in back-to-back region averageaverage back-to-backregionback-to-backregion

37. 37