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

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.

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Backup Slides

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

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

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

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

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

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

28 700,000

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

30 High Intensity and High Resolution beamline (new configuration)

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Others

33 Super Kaon Spectrometer (SKS)

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

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

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