Weak Decay of Λ Hypernuclei; - Status and Prospects -. H. Bhang (Seoul National University) 2007 APCTP workshop on “Frontiers in Nuclear and Neutrino Physics” APCTP, Postech Feb. 26-28, 2007. I. Current Status of NMWD study II. Experimental Signatures of the 3-body process in NMWD
(Seoul National University)
2007 APCTP workshop on
“Frontiers in Nuclear and Neutrino Physics” APCTP, Postech
Feb. 26-28, 2007
I. Current Status of NMWD study
II. Experimental Signatures of the 3-body process in NMWD
III. Final State Interaction and 3-body process2N-NMWD.
The Decay Modes ofΛHypernuclei
q~ 100 MeV/c
Γπ- ( Λ pπ- )
Γπo ( Λ nπo )
Γp ( Λp np )
Γn ( Λn nn )
q~ 400 MeV/c
Γ2N (ΛNN nNN)
Γn, Γp, nm,Г2N etc.
1. B-B Weak Interaction ;
Λ + N -> N + N (ΔS=1 B-B W.I. )
- The fundamental importance of NMWD is that it is practically the only place to study the strangeness changing baryonic
ΛN->NN weak interaction.
2. Г; Long standing puzzle on : Γn/Γp(≡np ratio)
3. Asymmetry ; Decay asymmetry wrt the polarization axis of Λ.
It is due to the interference of PC and PV amplitudes of weak interaction. Provides the information on the composition of the amplitudes.
6. The 3-body interaction process, 2N NMWD:Current indication is a large Г2N. The enhancement of 3-Body process in ΔS=1 weak interaction is very interesting and it could be global phynomena in nuclear medium.
Гn/Гp puzzle and the previous searches
Gn / Gp
All these derived from p spectra
1. Γn/Γp Puzzle :
~ 1 ~0.1
Nn(> 40 MeV) =0.69
Np(> 40 MeV) =0.40
p n p,n singles spec np,nn pair no.
~ 1.0 ~0.5 ~0.5 ~ 0.5
E307etc. E369 E462/E508
NModels of ΛNNN interaction (I)
I. Meson Exchange Models; ΔI=1/2 rule adopted.
very small Γn/Γp
- Due to large momentum, they involve short distance behaviors.
- pseudo-scalar and vector mesons; π,K,. . .
- No drastic effect.
- 2π exchange is important in nuclear force.
- found its contribution important.
NModels of NMWD and the Γn/Γppuzzle (II)
II. Hybrid quark-hadron Model
- ΔI=1/2, 3/2 both allowed.
- Oka, Sasaki, . .
- considerable improvement on Γn/Γp
III. Contact four fermion interaction model
Resolution of the Γn/Γppuzzle (II)
Upgraded Γn/Γp theoretical values
p+KComparison with recent results
- It is well known that the strangeness changing weak decay strongly
favors ΔI=1/2 transition, though it is not well understood yet.
- The OBE models for NMWD adopt it while DQ and 4 point interaction models do not.
= (Γπ-+ Γπ0) + (Γp + Γn)
Γm Γnm (?)
Decay widths;the strategy to determine the decay width of each channel of NMWD is
1st ; Determine Γnm (= Γ - Γm).
2nd ; Determine r=Γp/Γn, then,
3rd ; Γp = Γnm /(1+r), Γn = Γnm r/(1+r),
This does not work if Γ2nis large.
Γn ( Λn nn )
“A three-body force arises when two nucleons interact to produce a virtual excited state which contains some entity other than nucleons and while this state exists one of its constituent parts interacts with a third nucleon. The effect cannot be attributed to a succession of of two-nucleon interactions.” – M.A. Preston -
Which ones are 3-B interaction?
It is known that the Δis by far the most important in the nucleus.
Ex. • Pot. En. ; V2N ~ 1-2 MeV
• Nuclear matter energy; ~ few % of 2-body contribution.
• Binding En. of 3H, 3He ; only 1%.
Theoretical Prediction of 3-body process (Γ2N) of NMWD.
•Absorption of virtual pion by 2p-2h states.
• The real pion has a large width in nuclear medium due to the coupling to 2p-2h.
• The strength of the real pion becomes a Breit-Wigner distribution and the part of the tail becomes Pauli-unblocked.
• However, this pion is almost on-shell and absorbed via 2p-2h state. It is well established that pions are absorbed dominantly on the pn pair. In the process 3 nucleons are emitted.
practically 2n n enhancement
But it is not yet experimentally confirmed.
To exclude FSI effect and 3-body decay in Гn/Гpand to identify 2N channel,
Exclusive meas. of each decay channel.
Quenching of SinglesYield/ LE n enhancement
FSI & 3-Body process. different yield characteristics.
3. FSI ; n & p are indistingushable (isospin indep.) HE similarity.
4. LE behavior ; Channel Cross-over LE p enhancement.
Instead, What observed LE n enhancement.
5. What would be the source of the LE n enhancement???
1. Sharp peak in Ynp(He)
at Q value(Λpnp).
FSI negligible in He.
2. Broad spec in Ynn(He).
Energy resolution? No,
Seems 3-Body phace space!!
3. bb dominance
4. Nbb(nn)/Nbb(np) Γn/Γp
1. No more sharp peak at Q value(Λpnp).
FSI significant in C.
2. Ynn(C); Even further degraded.
Again points to 3B decay.
3. bb dominance in np pairs,
but not anymore in nn pairs.
4. Nnbb/Nbb(R) is much enhanced in nn pair over that of np.
Rnn/Rnp ~ 2.3±0.93
attribute this 2N NMWD
depending on methods.
- Two mechanisms for quenching ; FSI and 3-body (how)
- FSI characteristics ; n and p are indistinguishable (HE spec..)
- LE behavior; quite different due to the imbalance of cross over.
p enhancement expected in LE.
However, what observed in LE is n enhancement.
3-B enhances n ! ! !
2. Broad Energy sum spec. of nn ; show3-B phase space dist..
3. Enhancement of nn pairin non-back-to-back kinematic region;
Most direct identification;
i. 2-body events seperated kinematically.
ii. FSI events can be removed by the exp. reference (pp events).
. However, the current statistics are very much limited at the moment.
4. INC incooperated with 3-B process reproduced both singles and coincidence yields well, but only with a large Г2N.
Assign it to Г2N.
2. Just Rough Estimation;
1) Nnp(nbb) all FSI eff.
Same FSI on Nnn
Г2N ~ The residual Nnn
after FSI sub.
Г2N / ГNM~0.150.09±
but using INC for FSI
Ratio of N(nbb) to N(bb)
What is this enhancement?
FSI? No, np and nn should have similar ang. distribution
INC(1N+2N) Reproduction of Singles yields
are reasonably well reproduced.
1. Though the limitation of statistics of data and INC uncertainty, all the current aspects indicate the large Γ2N.
2. The first road block toward the decay widths, the Γn/Γppuzzle, has been finally removed.
3. Now the only road block is the determination of the contribution of the 3-body NMWD process, 2N-NMWD. ,
4. We absolutly need to determine the contribution of the 3-body process in NMWD before the main observables, Γn and Γp
Proposal for J-PARC (P18)
1. B-B Weak Interaction ;
Λ + N -> N + N (ΔS=1 B-B W.I. )
2. Long standing puzzle on : Γn/Γp(≡np ratio)
3. Asymmetry ; The relative phase concern of PC and PV part of NMWD interaction.
6. The 3-body interaction process, 2N NMWD: Predicted to be a significant component of NMWD, though not experimentally identified yet.
Γ2N ~ Γ1N
Nonmesonic W.D.Why enhancements?
Why do we expect such enhancements of 3-body process in NMWD?
this might be, at least, one reason of the large enhancement.
I would like to call your attention, especially those of young theorists, to this problem ! ! ! Thanks.
• A nucleus as a Fermi gas.
• ρ(x) V(x)
• FSI is simulated as a cascade free NN scattering along with Fermi blocking imposed.
• Density geometry parameters are adopted from the reactions, (p,p’) and (p,n) data with which Mass and Energy dependence were checked
• These parameters are fixed for the decay INC calc.
M. Kim, JKPS 46 (’05) 805
• We know that FSI(He) not strong.
Then what are those in Ynnnbb(He)?
• R(np) enhancement in C over He.
• R(nn) enhancement over R(np) both in He and C
2N? where R=Nbb/Nnbb
This model tends to produce 2 HE neutron and one LE proton. Then protons are often cut off at the threshold.