Coincidence Measurement of the Weak Decay of 12ΛC Hy

pernucleus and the Three-body Weak Decay Process

H. Bhang(Seoul National University)

NP2007 at J-PARC June 01-02, 2007

I. Previous searches and Motivation (J-PARC E18)II. Yield Characteristics and How to extract 2N-NMWD.III. J-PARC E18 and Decay Counter Setup:

- Efficiency and Solid angles, - Yield Estimation and Beam time.

IV. Summary

Nonmesonic q~ 400 MeV/c

I. Decay Modes and Motivation

Γ tot(=1/τ )

Γm

Γ nm

Γπ - ( Λ pπ - )

Γπ o ( Λ nπ o )

Γ p ( Λp np )

Γ n ( Λn nn )

Mesonic

q~ 100 MeV/c

Γ 2N (ΛNN nNN)

(1N)(2N)

3-Body ProcessFocus:

• Baryon-Baryon Weak Interaction• Long standing puzzle on Γ n/Γ p.

• 2N NMWD; Predicted theoretically,

but not exp. idendtified yet.

Previous Searches

Hyp. Nuc.

Γnm Γn/Γp

BNL

5ΛHe 0.41±0.

14.93±0.55

12ΛC 1,14±0.

21.33±1.12/0.

81

KEK’95

12ΛC 0.89±0.

181.87±0.91/1.

59

Г n/Г p puzzle and the previous searches

Γ n/Γ pexp >> Γ n/Γ p

th(OPE)

~ 1 ~0.1

1. Γ n/Γ p Puzzle :

2. Recent Development of Γ n/Γ ptheory : 0.3 ~ 0.7

K.Sasaki (Direct Quark), Nucl. Phys. A669 (2000) 371

D. Jido (Heavy Meson Exc), Nucl. Phys. A694 (2001) 525

10 0.5 1.5

n / p

OPE

All these derived from p spectra

Coincidence Measurement (KEK-PS Coincidence Measurement (KEK-PS E462E462/E508)/E508)

Ep

En

π

θ

To exclude FSI effect and 3-body decay in Гn/Гp and to identify 2N channel,

Exclusive meas. of each decay channel.

π+

SKS

K+

Nn / Np (E>60MeV)~2.00±0.09±0.14

Nn / Np (60<E<110MeV) ~2.17±0.15±0.16

Singles spectra in NMWD

Okada et al., PLB 597 (2004) 249

Nn/Np ~ 2 Г n/ Г p~1/2

Coincidence Yields (NN correlations)

np = 0.45±0.11±0.03 B.Kang et al., PRL 96 (’06)

np = 0.51±0.13±0.0 M. Kim et al., PLB (’06)

Nnn/Nnp Г n/ Г p

12ΛC

5ΛHe

INC(1N)INC(1N)

1. Quenching in both p and n spectra from that of INC.2. What would be the mechanism for the nucleon Quenching?

FSI & 3-Body process.3. FSI ; n & p are indistingushable (isospin indep.) HE similarity. LE behavior ; Cross over effect LE enhancement in p expected.4. Instead, What observed LE enhancement in n spec.

5. What would be the source of the LE n enhancement???

Quenching of Singles Yield

Theoretical Prediction of 3-body process (Γ 2N) of NMWD.

Ramos-Oset Model ;

Λ

N

π

• Absorption of virtual pion by 2p-2h states.

• Λpπ - is dominant at the weak vertex and

• Pions are absorbed dominantly on the pn pair. In the process 3 nucleons are emitted;

1p(LE) + 2n (HE)

• Γ 2N ~ 0.2 Γ nm

15 counts

8 counts

Enhancement of nn pair yields in the nbb region (12ΛC)

1. Enhancement of Nnn in nbb, over that of Nnp, by a factor, Rnn/Rnp~(2.30.93).

Assign it to Г 2N.

2. Just Rough Estimation;

1) Nnp(nbb) all FSI eff.

Same FSI on Nnn

Г 2N ~ The residual Nnn

after FSI sub.

Г 2N / Г NM ~0.150.09±

2) Similarly,

but using INC for FSI

Г 2N / Г NM ~0.270.12

RNN=NNN(nbb)/NNN(bb);

Ratio of N(nbb) to N(bb)

Singles and Coin. Yields Reproduction with INC.

In order to explain the quenching,

Г 2N=0.4Г nm

1. Singles Quenching

2. LE n enhancement

3. Pair Quenching

are reasonably well reproduced.

For 2N-NMWD, we adopted the kinematics of uniform phase space sharing of 3 nucleons (Dalitz kinematics).

Tactics to extract 2N-NMWD

1. 2N and FSI events can not be seperated from each other kinematiclly, but their yield characteristics can be utilized to distinguish them. .

2. Our tactics to extract 2N NMWD is based on

1) Singles and pair quenching,

2) LE n enhancement,

3) Nnn(nbb) enhancement, ; Most direct evidence.

4) Energy sum correlation and

5) PP events for the experimental FSI information.

3. Direct evidence would be the Nnn enhancement in non-back-to-back (nbb) kinematic region, but the statistics are limited yet.

Status of NMWD of Λ hypernuclei I. Main Issue ; B-B Weak Interaction ;

Λ + N -> N + N (ΔS=1 B-B Weak Interaction )

- Γ n/Γ p (≡np ratio) and Ay have been mainly studied so far.

II. urgent Issues to be solved ;

- ΔI=1/2 rule

- 3-body process of Weak Decay;

Is there really such processes?

How much contribution?

Why 3-body effect is so strong that it is comparabe to 2-body effect?

- Branching ratios of NMWD; It has been so long, but accurate branching ratios are not available yet.

Since the contribution of 3-body process seems significant, we have to measure its contribution.

J-PARC E22

J-PARC E18

II. Decay Counter SetupBasic concepts are based on the setup of E462/E508 expeiments. CDC+T1(Timing for charged one)+T2(neutron) Side veto to reject passing through ptls Share most of the detection system with E22

bb: back-to-backnbb: non-back-to-back

E508 E18

Nπ 2x1012 5x1012

dNπ/dt 4x106/spill

107/spill

T(arget) 4.3g/cm2 4.3g/cm2

NHY(g.s.) ~62K 2.5*62K

Ybb (np) 116 ~1160

Ybb (nn) 43 ~430

Ybb (pp) 8 ~90

Ynbb (np) 12 ~375

Ynbb (nn) 23 ~300

Ynnn 3 (~45)

Ynnp 2 (~80)

σ stat(Γ n/Γ p) 28% ~10%

σ stat(Γ 2N) ~10%

Detection Efficiency of nucleon pairs and Expected Yields

pp

npnn

Summary

1. Now the Г n/ Г p puzzle problem solved finally, the measurement of 2N-NMWD, 3-body decay process, is most urgent before we determine the decay observables of NMWD.

2. The signatures of (2N)-NMWD processes were found both in the singles and coincidence data. All of them indicates fairly large Γ 2N comparabel to Γ 1N.

3. We still can not say much on each decay width of each channel of NMWD. It is because ot the large Γ 2N and its large error bar.

4. We proposed the proposal P18 whose purposes are to determine accurately; 1) Γ 2N : The strength of 3-body decay proces 2) Γ n, Γ p ; have to determine their magnitudes.

5. P18 naw got the phase-1 (physics) approval and became E18. We have to prepare for the phase-2 approval.

KEK, RIKEN, Seoul Univ.,Tohoku Univ., Osaka Univ., Univ. Tokyo, Osaka Elec. Comm. Univ.G , Tokyo Inst.

Tech., KRISS, Pusan N. Univ., Kyungbuk N. Univ.

J.K. Ahn, K. Aoki, H. Bhang, S. Choi, T. Fukuda, O. Hashimoto, S. Kameoka, B. H. Kang,

J. H. Kim, M. J. Kim, W. Kim, Y. Lee, T. Maruta, Y. Miura, Y. Miyake, T. Nagae, M. Nakamura, S. N.

Nakamura, H. Noumi, S. Okada, Y. Okatasu, H. Outa, H. Park, P. K. Saha, Y. Sato, M. Sekimoto, T. Takahashi, H. Tamura, K. Tanida, A. Toyoda, K. Ts

hoo, K.Tsukada, T. Watanabe, H. J. Yim.

H. Bhang; bhang@snu.ac.krH. Outa ; outa@riken.jpH. Park ; hyeonseo@kriss.re.kr

J-PARC 50-GeVJ-PARC 50-GeV E E118 collaboration8 collaboration

Extra

(3-Body)

PID for neutral and charged particles

Essentially same as E508

Charged particle PID will be improved than that of E508

LE particles; CDC + E

HE particles; ΔE(0.6cm) + E(2cm)

Proposal

Main Objects : To measure the 3-body decay process, namely Γ 2N,

the 2-nucleon induced NMWD in 10% error level To measure all decay widths of NMWD in 10% error level.

Reaction : 12C(π +,K+) at Pπ =1.05 GeV/c with 107π /spill. Spectrometer & Detector :

SKS Kaon spectrometer; 100 mSr. Coincidence Detectors; 2π Sr

Yield Estimation and Expected Results: For 80 shifts, Nnbb(nn) ~ 300 (23) Nnbb(np) ~ 375 (12) Nbb(pp) ~ 90 (8) N(NNN) ~ (125) (5)