Weak Decay of Hypernuclei Status and Prospects

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Weak Decay of ? 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 III.
Final State Interaction and 3-body process
2N-NMWD. IV. Summary
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The Decay Modes of ? Hypernuclei
Mesonic q 100 MeV/c
Gp- ( ? ? pp- ) Gpo ( ? ? npo )
Gm Gnm
Gtot(1/t)
Previous Searches ?n/?p puzzle
Gp ( ?p ? np ) Gn ( ?n ? nn )
(1N)
Nonmesonic q 400 MeV/c
(2N)
G2N (?NN? nNN)
3-Body Process
decay observables Gn, Gp, ?nm, ?2N etc.
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Non-Mesonic Weak Decay (NMWD) Issues

• 1. B-B Weak Interaction
• ? N -gt N N (?S1 B-B W.I. )
• - The fundamental importance of NMWD is that
  it is practically the only place to study the
  strangeness changing baryonic
• ?N-gtNN weak interaction.
• 2. ? Long standing puzzle on Gn/Gp (np ratio)
• 3. Asymmetry Decay asymmetry wrt the
  polarization axis of ?.
• It is due to the interference of PC and PV
  amplitudes of weak interaction. Provides the
  information on the composition of the amplitudes.
  
• ?I1/2 Rule Its validity not well established
  yet. Can be tested in light hypernuclei.
• Final State Interaction It is an important
  element in order to understand NMWD.
• 6. The 3-body interaction process, 2N
  NMWDCurrent indication is a large ?2N. The
  enhancement of 3-Body process in ?S1 weak
  interaction is very interesting and it could be
  global phynomena in nuclear medium.

HYP03 Conf.
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?n/?p puzzle and the previous searches
1. Gn/Gp Puzzle
Gn/Gpexp gtgt Gn/Gpth(OPE) 1 0.1
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Recent Experiments at KEK-PS
p n p,n singles spec np,nn
pair no. meas. meas. 1.0 0.5
0.5 0.5
E307etc. E369 E462/E508
12C(p,K)12?C

• Residual FSI effects
• No 2N NMWD assumed!!

?Ambiguity Sources!!
??n/?p(12?C) 0.510.14
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Models of ?N?NN interaction (I)

• I. Meson Exchange Models ?I1/2 rule adopted.
• OPE model(1967) Vp by Adams.
• VpV? McKellarGibson(84), Bando(85)
• ? very small Gn/Gp
• Heavy meson exchange(HME) model Dubach, Oset,
  Ramos, Parreno. .
• - Due to large momentum, they involve short
  distance behaviors.
• - pseudo-scalar and vector mesons p?,K,. . .
• - No drastic effect.
• 2p(s, ? channel) Exchange Model Itonaga,
  Schmatikov
• - 2p exchange is important in nuclear force.
• - found its contribution important.

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Models of NMWD and the Gn/Gp puzzle (II)

• II. Hybrid quark-hadron Model
• 6-q Bag model Vp Cheung et al.,
• Direct Quark(DQ) Mechanism VDQ VME
• - ?I1/2, 3/2 both allowed.
• - Oka, Sasaki, . .
• - considerable improvement on Gn/Gp
• Phase problem in K exchange amplitude Sasaki,

• III. Contact four fermion interaction model
• Block and Dalitz prl 11 ('63) 96
• Jun J., prc 63 ('01) 044012
• Parreno A. prc 70 ('04) 051601.

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Resolution of the Gn/Gp puzzle (II)

• Upgraded Gn/Gp theoretical values
• After the correction of the phase error in K
  exchange term, the n/p ratio was enhanced
  significantly. Since the K exchange term was
  employed in most of the calculations, the
  correction also improved the n/p ratios of HME
  model calculations.
• Now the theoretical n/p ratios of various models
  agree with the recent exp. ratio quite well. The
  n/p ratio puzzle was finally resolved.

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Comparison with recent results
OPE
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?I1/2 rule in Nonmesonic Weak Decay.

• - It is well known that the strangeness changing
  weak decay strongly
• favors ?I1/2 transition, though it is not well
  understood yet.
• - The OBE models for NMWD adopt it while DQ and 4
  point interaction models do not.
• This becomes one of the most urgent issues of
  NMWD.
• Its test can be done in decay of light
  Hypernuclei, 4?H, 4?He.
• This will be one of the main theme of J-PARC
  Hypernuclear decay experiments.
• Proposal 10-2.

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How to measure partial decay widths (Gp, Gn, G2n)

• G 1/t Gm Gnm
• (Gp- Gp0) (Gp Gn)
• Gm Gnm (?)
• ? Decay widths the strategy to determine the
  decay width of each channel of NMWD is
• 1st Determine Gnm ( G - Gm).
• 2nd Determine rGp/Gn, then,
• 3rd Gp Gnm /(1r), Gn Gnm r/(1r),
• ? This does not work if G2n is large.

Gn ( ?n ? nn )
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3-body Interaction
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 ? is 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.
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Theoretical Prediction of 3-body process (G2N) 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.

• Yield Characteristics 1p(LE) 2n (HE)
• ? practically 2n ? n enhancement
• G2N 0.2 G1N
• ?But it is not yet experimentally confirmed.

Ramos-Oset Model
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Exclusive Measurements (KEK E462/E508) for
5?He/12?C
To exclude FSI effect and 3-body decay in ?n/?p
and to identify 2N channel, ? Exclusive meas.
of each decay channel.
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Quenching of Singles Yield/ LE n enhancement

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

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Broad Esum spectrum in NN correlations (5?He)
5?He
1. Sharp peak in Ynp(He) at Q
value(?p?np). ? FSI negligible in He. 2. ?Broad
spec in Ynn(He). FSI? No. Energy resolution?
No, ? Seems 3-Body phace space!! 3. bb
dominance 4. Nbb(nn)/Nbb(np) ? Gn/Gp
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Broad Esum spectrum in NN correlations (12?C)

• 1. No more sharp peak at Q value(?p?np).
• ? 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 2.30.93
• attribute this ? 2N NMWD
• ?2N/?NM 0.15 0.27,
• depending on methods.

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Strategy to measure 3-body NMWD

• Quenching (Singles and pair nucleons) observed.
• - 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 ? show 3-B
  phase space dist..  
• 3. Enhancement of nn pair in 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 ?2N.

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Enhancement of nn in nbb region ? ?2N

• Enhancement of Nnn in nbb, over that of Nnp, by
  a factor, Rnn/Rnp(2.3?0.93).
• ? Assign it to ?2N.
• 2. Just Rough Estimation
• 1) Nnp(nbb) ? all FSI eff.
• ? Same FSI on Nnn
• ? ?2N The residual Nnn
• after FSI sub.
• ? ?2N / ?NM 0.15?0.09
• 2) Similarly,
• but using INC for FSI
• ? ?2N / ?NM 0.27?0.12

RNNNNN(nbb)/NNN(bb) Ratio of N(nbb) to N(bb)
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Quenching of Pair Yields.

• Quenching of pair yields ? Quenching of
  singles.
• Enhancement of Nnn in non-back-to-back region.
• What is this enhancement?
• FSI? No, np and nn should have similar ang.
  distribution

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INC(1N2N) Reproduction of Singles yields
INC(1N2N)

• INC calculation included 2N-NMWD with
• 3-body kinematics of equal phase space sharing.
• In order to explain the quenchings, we need
  ?2N0.4?nm.

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Singles and Coin. Yields Reproduction with
INC(1N2N).
?2N0.4?nm

• Singles Quenching
• LE n enhancement
• Pair Quenching
• are reasonably well reproduced.

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Motivation of the proposal (P18)

• 1. Though the limitation of statistics of data
  and INC uncertainty, all the current aspects
  indicate the large G2N.
• 2. The first road block toward the decay widths,
  the Gn/Gp puzzle, 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, Gn and Gp
• ? Proposal for J-PARC (P18)

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Non-Mesonic Weak Decay (NMWD) Issues

• 1. B-B Weak Interaction
• ? N -gt N N (?S1 B-B W.I. )
• 2. Long standing puzzle on Gn/Gp (np ratio)
• 3. Asymmetry The relative phase concern of PC
  and PV part of NMWD interaction.
• ?I1/2 Rule
• Final State Interaction It seems one of the
  most important element to understand NMWD.
• 6. The 3-body interaction process, 2N NMWD
  Predicted to be a significant component of NMWD,
  though not experimentally identified yet.

J-PARC P10(2)
J-PARC P18
HYP03 Conf.
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G2N G1N
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Why enhancements?

• Why do we expect such enhancements of 3-body
  process in NMWD?
•   In the nucleus p highly off shell.
•   In hypernuclear decay almost on shell
•      ? this might be, at least, one reason of the
  large enhancement.

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The Implication of the Enhancement of the 3-B
process

• The mechanism of the enhancement is very
  interesting.
• The enhancement of the 3-B interaction process in
  the weak interaction in ?S1 sector could be
  global in the nucleus.
• Its implication could be profound.

I would like to call your attention, especially
those of young theorists, to this problem ! ! !
Thanks.
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IV. Summary

• Discussed the NMWD, the only window to study ?S1
  Baryonic weak interaction. The long stood ?n/?p
  puzzle has finally been resolved. However, there
  remains important issues remained to be solved.
• The ?n and ?p, remain to be measured. However, it
  seems that ?2N comparabel to ?1N and has to be
  determined beforehand. Its enhancement may not
  be an isolated one in NMWD, but could be global
  one of ?S1 weak interaction in nuclear medium.
• Asymmetry parameter Large discrepancy remained
  between the exp.(small) and theoretical
  values(large neg.). It remains to be cleared yet,
  but it leaves more homework to theorists.
• ?I1/2 rule This is an empirical rule. Though
  it holds very well in the strangeness changing
  weak decay, it is not well understood yet. Its
  validity in the baryonic weak interaction has not
  been established. Its experimental test is
  considered one of the most important one in the
  baryonic weak interaction study.
• Two proposals for J-PARC experiments (P10-2 and
  P18) are proposed focused on these issues. We
  expect these can be answered with the high
  intensity J-PARC beam.

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EXTRA
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INC (IntraNuclear Cascade) calculation
(p,p)
Mass Dependence

• A nucleus as a Fermi gas.
• ?(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
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Enhancement of nn in nbb region
We know that FSI(He) not strong. Then what
are those in Ynnnbb(He)? R(np) enhancement in C
over He. ? FSI R(nn) enhancement over R(np)
both in He and C ? 2N? where RNbb/Nnbb
This model tends to produce 2 HE neutron and one
LE proton. Then protons are often cut off at the
threshold.