Λハイパー核の弱崩壊実験 S. Ajimura (RCNP) Nonmesonic weak decay of hypernuclei E22 experiment (4LHe) Future experiment (4LH)

Weak decays in L-Hypernuclei Mesonic weak decay (MWD) similar with free L decay spin/isospin structure well known p L N N N Non-Mesonic weak decay (NMWD) new decay modes spin/isospin structure: unknown I=0 or 1 N N L LN weak interaction N N N 3S1/1S0

Status of NMWD studies Old puzzle solved recently np-ratio (GLnnn/GLp  pnGn/Gp) inconsistent Experimental and theoretical improvements (Exp.) Back-to-back coincidence for final two nucleons (E462/508) n p LN→nN LNN→nNN FSI re-scattering in reality

Asymmetry written by amplitudes A new puzzle arises Decay asymmetry inconsistent HY p n ) Exp. ( » NM p a Û ) Theory ( 7 . - » NM p a Asymmetry written by amplitudes Large contribution ? initial final amplitude isospin parity 1S0 a 1 no 3P0 b yes 3S1 1S1 c 3D1 d 1P1 e 3P1 f assuming initial S state

NMWD of 4-, 5-body hypernuclei allowed initial LN states n n p Λ p p n Λ p p n n Λ 0+ 0+ 0+ 0+ Ln→nn: 1S0, 3S1 1S0 1S0, 3S1 Lp→np: 1S0 1S0, 3S1 1S0, 3S1 initial final amplitude isospin parity 1S0 a 1 no 3P0 b yes 3S1 1S1 c 3D1 d 1P1 e 3P1 f assuming initial S state

Status of amplitude determination Our prospects new constraint from 4LHe np-ratio better than 15% error Current status constraint from 5LHe data other constraints are loose Sakaguch-plot: assuming DI=1/2

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

Energy resolution K1.8 bemline + SKS  excellent resolution Liquid 4He 2 g/cm2  DEx ~ 2 MeV BE(4LHe) = 2.42  0.04 MeV Separation from QF L production essential L  p+p- p-+A  nnX, npX

Decay arm system Large acceptance and high efficiency for NN Good PID capability (n/p/p/g) n n p p n/g TOF p/p E/DE/range n/p charge-veto

Yield estimation 19,000 4LHe/day  500,000 4LHe in 4 weeks 1,300 Lpnp and 75 Lnnn in 4 weeks high beam intensity large acceptance large acceptance and high efficiency in case of 1% BR

Background estimation Background sources QF L-production (Lp+p-, p-+AnnX) cut in Ex spectrum Mesonic weak decay of hypernuclei 4LHe3He+p+p-, p-+AnnX Gp-  0.3 G  Gn  0.01 G Reduction of background veto: no p track in CDC less material at target LHe target  2 g/cm2 range(p-)  5 g/cm2 range 2 g/cm2

Background MC simulation Simulation of worst case 1/5 of p- stop in material around target 1/5 Gp- ~ 0.06 ⇔ Gn ~ 0.01 GEANT4 base simulation Li C R~1/110 R~1/30 R: Reduction factor (En1+En2>50MeV)

Nonmesonic decay of A=4 hypernuclei Allowed initial states for A=4, 5 hypernuclei hypernucleus Ln→nn Lp → np 4LH 1S0, 3S1 1S0 4LHe 5LHe Gp(4LH), Gn(4LHe) ⇒ we can measure 1S0 amplitudes directly. If DI=1/2 rule holds, Gn(4LHe)/Gp(4LH)=2. ⇒ we can check the validity of the DI=1/2 rule in B-B weak interaction. Existing experimental results Gn(4LHe) /GL=0.01+0.04/-0.01 (KEK), 0.04±0.02(BNL) NP A639(1998)261c Gp(4LHe) /GL= 0.16±0.02(KEK), 0.16±0.02(BNL) NP A639(1998)251c

(p-,K0) reaction The final state of K0 is two “charged” pions. It is easy to measure the energy of pions and their opening angle, compared with gamma-ray in p0 case. background Multi-pion production in the target or other material around target should be most serious background. Such background can be rejected by measuring K0 decay point.

Beam Momentum bgct = 9.7 cm @ PK=1.53 GeV/c 1/5 At standard momentum for (p,K) reaction, 1050 MeV/c, bgct for kaon is 3.9cm. bgct = 9.7 cm @ PK=1.53 GeV/c

High Intensity and High Resolution beamline (proposed by H High Intensity and High Resolution beamline (proposed by H. Noumi: 2nd NPFC L08) handle 109 p/spill x 100 of K1.8 Dp/p ~ 10-4

Resolution OK ? – angle/momentum of pion Ex resolution Dp/p= 1x10-5: DEx= 0.013 – 0.014 1x10-4: 0.133 – 0.142 1x10-3: 1.332 – 1.417

Summary We propose to measure the nonmesonic weak decay of 4-body L hypernuclei (4LHe/H). select initial spin state (1S0/3S1) check the validity of DI=1/2 rule in baryon weak interaction We are now preparing E22 exp. (4LHe) at K1.8 beamline. High intensity and high resolution beamline by H. Noumi, is suitable for measurement of 4LH.