Precise Measurement of the Nonmesonic Weak Decay of A=4, 5 - PowerPoint PPT Presentation

ハイ パー核の弱崩壊の実験 Precise Measurement of the Nonmesonic Weak Decay of A=4, 5 Λ Hypernuclei (J-PARC LOI) 阪大理 味村 周平 – Weak decay of hypernuclei – n/p ratio, asymmetry parameter – Proposed experiment – Yield estimation – Summary 物理学会 2003 年秋季大会 10pSG-8

Weak decay of Λ hypernuclei - mesonic decay: Λ → N π - nonmesonic decay: Λ N → NN main decay mode medium/heavy hypernuclei large momentum transfer (~400MeV/c) strangeness changing weak interaction YN weak → baryon-baryon weak interaction observables: τ : lifetime Γ n/ Γ p= Γ ( Λ n → nn)/ Γ ( Λ p → np): n/p ratio α : asymmetry parameter

Nonmesonic decay initial final amplitude isospin parity 1 S 0 1 S 0 1 no a 3 P 0 1 yes b 3 S 1 1 S 1 0 no c 3 D 1 0 no d 1 P 1 0 yes e 3 P 1 1 yes f assuming initial S state - n/p ratio: ratio of final isospin 1 to sum of 0 and 1 2 + b n 2 + 3 f n 2 a n 2 + b p 2 + 3( c p 2 + d p 2 + e p 2 + f p 2 ) a p - asymmetry parameter: interference between parity conserving and parity changing amplitudes ( ) ( ) Nabetani et al.,   * * − + − − + * 2 3 Re 2 3 2 ae b c d f c d     PRC 60(1999)017001 ( ) + + + + + 2 2 2 2 2 2 3 a b c d e f

n/p ratio Meson exchange – Ramos, Parreno, … - d amplitude dominance (n/p ratio ~ 0.1) � recently, inclusion of K/K* exchange increases f amplitude � n/p ratio ~ 0.4 (Parreno et al., PRC 65(2002)015204) Quark model – Oka, Inoue, Sakaki, … They combined meson-exchange and direct quark exchange. n/p ratio ~ 0.7 significant effect of quark-exchange in 1 S 0 proton decay (Sasaki et al., NPA669(2000)331, NPA678(2000)455) 2 π / ρ, 2 π / σ – Itonaga, … [12pSE-1]

n/p ratio – exp. 5 Λ He 0.93 ± 0.55 (J. Szymanski et al., PRC 43(1991)849) 12 Λ C 1.33 +1.12 / -0.81 (J. Szymanski et al., PRC 43(1991)849) 1.87 ± 0.59 +0.32 / -1.00 (H. Noumi et al., PRC52(1995)2936) (derived from single proton/neutron spectrum) E462/E508 exp. – exclusive measurement of weak decay of 5 Λ He, 12 Λ C 0.4 ~ 0.6 (preliminary) [12pSE-2] - Now the theories becomes compatible with experimental results. - We have to measure the nonmesonic weak decay with coincidence of final two nucleon

Asymmetry parameter E160 – asymmetric proton emission from polarized 12 Λ C/ 11 Λ B Λ C: A= − 0.01 ± 0.11, P Λ =0.06~0.09 12 Λ B: A= − 0.19 ± 0.10, P Λ =0.16~0.21 11 PL B282(1992)293 asymmetry parameter: − 1.3 ± 0.4 � E278 – 5 Λ He • asymmetry parameter: +0.24 ± 0.22 PRL 84(2000)4052 • polarization was determined experimentally E462/E508 – 5 Λ He, 12 Λ C/ 11 Λ B Λ He: +0.07 ± 0.08(stat.) (preliminary) [12pSE-3] 5 We confirm E278 result, however the asymmetries are derived from single proton spectra. Theory – Meson-ex/DQ-ex � − 0.7 for both s-/p-shell hypernuclei

Asymmetry parameter • Recent experimental results suggest small asymmetry parameter, which contradicts theoretical prediction. • BUT the theory explain branching ratio fairly well. → Initial 1 S 0 contribution has to be important for asymmetry. (decay rates are mainly determined by 3 S 1 amplitudes) ( ) ( )   * * − + − − + * 2 3 Re 2 3 2 ae b c d f c d     ( ) + + + + + 2 2 2 2 2 2 3 a b c d e f We need … • to measure 1 S 0 amplitudes directly, • to measure asymmetry parameter with back-to-back coincidence of final two nucleons.

Nonmesonic decay of A=4, 5 hypernuclei Allowed initial states for A=4, 5 hypernuclei Λ n → nn Λ p → np hypernucleus 4 1 S 0 , 3 S 1 1 S 0 Λ H 4 1 S 0 1 S 0 , 3 S 1 Λ He 5 1 S 0 , 3 S 1 1 S 0 , 3 S 1 Λ He � Γ p( 4 Λ H), Γ n( 4 Λ He) ⇒ we can measure 1 S 0 amplitudes directly. � If ∆ I=1/2 rule holds, Γ n( 4 Λ He)/ Γ p( 4 Λ H)=2. ⇒ we can check the validity of the ∆ I=1/2 rule in B-B weak interaction. Existing experimental results Γ n( 4 Λ He) / Γ Λ =0.01 +0.04 / − 0.01 (KEK), 0.04 ± 0.02(BNL) NP A639(1998)261c Γ p( 4 Λ He) / Γ Λ = 0.16 ± 0.02(KEK), 0.16 ± 0.02(BNL) NP A639(1998)251c

Proposed experiment Subject Reaction Spectrometer Λ H: Λ p → np 4 He(K − , π 0 ) π 0 -spectrometer 4 ∆ M hyp ~ 2MeV Λ He: Λ n → nn 4 He(K − , π − ) 4 mag. spectrometer ∆ M hyp ~ 2MeV 6 Li( π + ,K + p) 5 Λ He: asymmetry mag. spectrometer ∆ M hyp ~ 2MeV large acceptrance We need to develop: liq. He target, π 0 -spectrometer, decay counter system For 5 Λ He experiment, magnetic spectrometer has to have coverage of scattering angle from –15 to 15 deg to produce polarized hypernuclei. SKS is the best choice among existing spectrometers.

π 0 spectrometer E1 π 0 η 2 = E M π π − η − 0 0 2 ( 1 cos )( 1 ) X E2 − E E = 1 2 X + E E L 1 2 Energy resolution 2   2 ∂  ∂  E E   ∆ = ∆ +  ∆ η  π π 0 0 E E     γ π 0 ∂ ∂ η   E   γ at X ~ 0 ( ) 3 CsI: C ~ 0.15 ⇒ ∆ E π 0 ~ 0.0022 MeV ∆ ≅ ∆ = 2 , E C E C E γ γ π 0 ∆ E 2 γ β ( ) 2 E 700MeV/c π 0 : <0.5 mrad for ∆ E π 0 (rms) < 1MeV ∆ ≅ ∆ η E π 0 ∆ η 2 M ⇒ ∆ L/L < 0.16 %

π 0 spectrometer – acceptance 6 0 400x400 cm 2 5 0 4 0 300x300 cm 2 3 0 2 0 Acceptance (%) 200x200 cm 2 1 0 |X|<0.5 0 0 5 1 0 1 5 2 0 2 5 3 0 scattering angle, θ π 0 200x200 cm 2 の結晶を 200cm 離し て、 conversion efficiency (~0.5) を含め 200cm acceptance は約 100msr π 0

Conceptual design of decay counter system � π/ p separation ∆ E – E ∆ E – TOF - thin plastic counter Outer plastic stacks surrounding target( ∆ E) - outer plastic stack(E) Drift Chambers charged veto - tracking by DC Beam � neutron detection - outer plastic stacks and charge veto - γ /n separation and energy measurement can be done by TOF between beam hodoscope and outer plastic stacks

Yield estimation 4 Λ H 4 Λ He 5 Λ He 5 × 10 6 K − /3.4 sec 5 × 10 6 K − 1 × 10 7 π + beam intensity 0.125 g/cm 2 target thickness 1.25 4 cross section 0.2 mb/sr 0.5 0.005 spectrometer acceptance 0.10 sr 0.05 0.03 0.5 × 0.5 spectrometer efficiency 0.8 0.5 decay counter acceptance 0.5 0.5 0.5 efficiency for decay p 0.8 0.8 0.8 efficiency for decay n 0.2 0.2 0.2 branching ratio ( Λ n → nn) 0.1 0.01 - branching ratio ( Λ p → np) 0.01 0.1 0.2 nn events/200 shifts 10000 5500 - np events/200 shifts 4000 220000 4000 expected error level 1.6% 1.5% 4%

Summary • We propose to measure the nonmesonic weak decay of A=4,5 hypernuclei with back-to-back coincidence of final nucleons. • Key observables are: Decay rate of Λ n → nn of 4 Λ He Decay rate of Λ p → np of 4 Λ H Asymmetry parameter for Λ p → np of 5 Λ He • Intense and pure secondary beam available at 50GeV-PS can give us a chance to derive conclusive experimental results for the nonmesonic weak decay of hypernuclei.