7th. June. 2004 KEK-PS External Review

Non-Mesonic Weak Decays of 5ΛHe and 12ΛChypernuclei in (π +,K+) reactions

RIKEN S.Okada
for KEK-PS E462 / E508 collaborations
5KEK-PS
KEK-PSE462
E462 (2000(2000 ––2001)
2001):: 5ΛΛHeHe
12
KEK-PS
E508(2002):
KEK-PS E508 (2002) : 12ΛΛCC

KEK-PS E462 / E508 collaborators
Osaka Univ.a, KEKb, GSIc, Seoul Univ.d,
Tohoku Univ.e, Univ. Tokyof,
Tokyo Inst. Tech.g, KRISSh, RIKENi
S. Ajimuraa, K.Aokib, A.Banuc, H. C. Bhangd, T. Fukudac,
O. Hashimotoe, J. I. Hwangd, S. Kameokae, B. H. Kangd,
E. H. Kimd, J. H. Kimd, M. J. Kimd, T. Marutaf, Y. Miurae,
Y. Miyakea, T. Nagaeb, M. Nakamuraf, S. N. Nakamurae,
H. Noumib, S. Okadag, Y. Okayasue, H. Outab, H. Parkh,
P. K. Sahab, Y. Satob, M. Sekimotob, T. Takahashie,
H. Tamurae, K. Tanidai, A. Toyodab, K. Tsukadae,
T. Watanabee, H. J. Yimd
* Spokesperson* Present main analysis member (Ph.D student)Weak decay of Λ hypernucleusΛΛweakweakdecay
decayininfreefreespacespace

Λ→ ｐ + π− : 63.9±0.5 %Λ → ｎ + π0 : 35.8±0.5 %τΛ = 263.2±2.0 ps→ Well known.

WeakWeakdecay
decaymode
modeofof ΛΛhypernucleus
hypernucleusΓm1/τHY =Γtot
ΓnmΓπ_ (Λ→ ｐ + π−) MesonicΓπ0 (Λ → ｎ + π０ ) q~100MeV/cΓp (Λ +“ｐ”→ ｎ+ ｐ)
Γn (Λ +“ｎ”→ ｎ+ ｎ)

Non-Mesonic(NMWD)

q~400MeV/c

Study of the mechanism of baryon-baryon weakinteraction.

Non-mesonic weak decayΛ+ p → n + p: Γp = a2+b2+c2+d2+e2+f2

Λ + N→ N + N

Λ+n→n+n: Γn = a2+b2+f2
Γn / Γp ratio: The most important observable to study
the isospin structure of the NMWD.

Simple theoretical model

One Pion Exchange(OPE) model

NΛ

WNSπN

strong tenser coupling(ΔL=2, ΔS=2)→ dominant term
3S →3D (amplitude “d”)
11

OPE : Γn / Γp ~0.1
Exp. : Γn / Γp ~ 1with large error
Γn / Γp ratio puzzle

Theoretical approach
q~400MeV/c(← large!) → short-distance interaction( range ~ 0.5 fm)

One Meson Exchange(OME) mechanismNNNΛ
π,K,η,ρ,ω,K*Direct Quark (DQ)
mechanism

NNΛN• Kaon exchange model (OME) dominant term 3S1→1P1 (amplitude “f”)

→ large Γn / Γp (~0.4-0.7)

Experimental difficulty
Most of the experiments measured only proton energy specra Γn / Γp = (Γnm – Γp) / Γp(because of the difficulty in detecting neutrons)

Proton energy loss inside a target and detectors
Final state interaction (FSI) effect( not well established theoretically) to distinguish between the FSI and 2N-induced process (ΛNNnNN)
Final state interaction(FSI) effectΛNN→NNN(2N-induced process)rescatteringNNπNNp np nnp
np nnp pNΛW

(One of the theoretical model)

The present experiment
KEK-PS E462/E508
NMWD : ΛN→NNCoincidence
NMWD

pnΛ npNMWD

pp nnnSelect ΛN→NN events
w/o FSI effect & ΛNN→NNN.

npn nDirect measurementof the Γn / Γp ratio

N(Λn → nn )× (Ω n × Ω n )av .

pCoincidence1)
1)Angular
Angularcorrelation
correlation((back-to-back,back-to-back,cosθ<-0.8cosθ<-0.8))2)2)Energy
Energycorrelation
correlation((Q~E(N1)+E(N2)Q~E(N1)+E(N2)~152MeV~152MeV))× ε n2 × (1 − R FSI )N(Λp → np )× (Ω n × Ω p )av .
× ε n × ε p × (1 − R FSI )* cosθ<−0.8* E(N1)+E(N2) cut

Γn N(nn − pair coin ) ε p=×Γp N(np − pair coin ) ε nFeature of the present experiment• Λ +“p”→ n
Measure both np- and nn- pair!
+pΛ +“n”→+Direct nmeasurementof the Γn / Γp ratio (Nnn/Nnp ratio)n• Detect all decay particles (p, n, π-, γ (π0))
with very high statistics.
• Select Light hypernuclei
 Light s-shell hypernucleus, 5ΛHe( to minimize the FSI effect ) Typical p-shell hypernucleus, 12ΛC
( to investigate the p-wave effect )Excitation-energy spectra for 6ΛLi and 12ΛC

The ground state of 6ΛLi isabove the threshold of 5ΛHe + p.

4.6 104
events6 Li (g.s.) → 5 He + pΛΛ5ΛHe

6 Li (g.s.) promptly decays into 5 HeΛΛemitting a low-energy proton.

 Good S/N ratio ~ 10
 High statistics(one-order of magnitude
higher than those ofprevious experiments.)

6.2 104
eventsDecay counter system

Neutron
Neutral particle：
・TOF(target→NT)
・T3 VETO

Solid angle: 26%= 9(T)+9(B)+8(S) %Charged particle：
・TOF (T2→T3)
・tracking（PDC）

ProtonNeutral decay particle ID
Neutral particles from 12ΛC
Neutron energy resolution(estimated from γ width)→ 7MeV(FWHM) at 75MeV Good γ n separation
 Good S/N ratio (~30)( previous exp.
S/N ratio (5ΛHe) ~1 )
~30 times (for 5ΛHe)higher than previousexp.

Constant
backgroundvery small

5MeV< En < 150MeV

 High statistics(~ 5000 neutrons)
~200 times (for 5ΛHe)~30 times (for 12ΛC)
higher than previousexp.

1 / β spectra(TOF spectra)Charged decay particle ID
Gaussian fit(±2σ cut)Charged particles
from 5ΛHe

Deuterons wereseparated fromthe protons.
(for the first time!)

PID function

derived from• dE/dx (at T2)• Total energy deposit (sequentially fired counters (T2,T3,T4).)• TOF (between T2 and T3)

Excitation spectra w/ coincident decay particles for 5ΛHe
previous experiment at BNL

inclusive

w/ protonw/ pionw/ neutron

w/ gamma

The g.s. peak is clearlyseen in all spectra withcoincident decay particles.

Excitation spectra w/ coincident decay particles for 12ΛC
inclusive

w/ protonw/ pionw/ neutron

w/ gamma

Neutron and Proton energy spectra of 5ΛHe and 12ΛC
To avoid suffering from
FSI effect &ΛNN→NNN,High energy threshold

135MeVNn / Np (60<E<110MeV)~ 2.17±0.15±0.16
5 He → n + α : Q~135MeV
Λ(rate : 0.049±0.01Γπ-)apply upper energy limit of 110MeV !

Nn / Np (E>60MeV)~ 2.00±0.09±0.14

Corrected
proton energy
loss inside thetarget !!

apply a simple relation
Γn / Γp = (Nn / Np - 1) / 2

Γn / Γp ~ 0.5

Coincidence analysis for 5ΛHen+pcos θ＜−0.8n+nAngular correlationcos θ＜−0.8

estimated contamination from π− absorptionenergy sum

Q-Value~152MeVMeV

Coincidence analysis for 12ΛCn+pcos θ＜−0.8n+n

Angular correlationcos θ＜−0.8

estimated contamination from π− absorptionenergy sum

Q-Value~152MeV

MeV

Γn / Γp ratio
Theo.

NΛ

W

Direct Quark
mechanism

Meson Exchangemechanism

One Pion Exchange(OPE)

NN

N

N

NN

Λ

NS

πN

Γn / ΓpΛ

0.5

0π,K,η,ρ,ω…

11.5Exp.

Previous exp. (at BNL)

0.93±0.55 (Szymanski et al.) for 5ΛHe

5 He (E462)
ΛNnn / Nnp (5ΛHe)= 0.45±0.11±0.03
12 C (E508)
ΛNnn / Nnp (12ΛC)= 0.40±0.09 (preliminaly)Summary ΛN→NN was directly observed for the first time !
5 He : Γ / Γ ratio ~ NΛnpnn / Nnp = 0.45±0.11±0.03
12 C : Γ / Γ ratio ~ NΛnp
nn / Nnp = 0.40±0.09 (preliminaly)
 Nn / Np ratio with a high threshold (60MeV) was approximately
equal to 2 ( Γn / Γp ~ 0.5 )  consistent with coincidence results.

 excludes the earlier experimental claim Γn / Γp ~ 1.
 rules out theoretical calculations based on the OPE (Γn / Γp~0).
The longstanding “Γn / Γp ratio puzzle” is solved.
It supports recent calculations based on short-range interactions
such as heavy-meson exchange and the direct quark exchange.

Other results
Lifetime

π- branching ratio

π0 branching ratio

π - decay width for 5ΛHe

π0 decay width for 5ΛHe and 12ΛC

Γπ- /ΓΛ of 5ΛHe

E462

Statistical errors were much improved !!

E462 / E508 publication
International conference PANIC02 : oral (invited) SENDAI03 : oral 6
 HYP2003 : oral (invited)Doctor thesis
Japan 1Master thesis
Japan 4, Korea 2
International conference INPC2004 : oral 1Doctor thesis
Japan 2, Korea 3 (or 2)
Refereed journals
Letter paper 5, Full paper

1 , poster
2, oral1

23, poster3

To J-PARC
Non-mesonic weak decay of 4ΛHe and 4ΛHSpin / isospin dependence

see S.Ajimura : J-PARC LOI 21

Γnm(4ΛH) = ( 3Rn1+ Rn0+ 2Rp0 )Γnm(4ΛHe) = (2Rn0 + 3Rp1 + Rp0 )Γnm(5ΛHe) = ( 3Rn1+ Rn0 + 3Rp1 + Rp0 )RNS … N : Λnnn, Λpnp
S : spin = 0 or 14Ë

Hn n pΛ

4ËHep p nΛ

5Ë Hep p n nΛ

ρ4 / 6
ρ4 / 6
ρ5 / 8

4He (K-,π-) 4 He
Λ
4He (π+,K+) 4 HeΛor

 n+n back-to-back

4He (K-,π0) 4 H
Λ p+n back-to-back(π0 spectrometer ) Need one-order higher statistics.  J-PARC