Weak decay of hypernuclei - Theoretical status

0.17 MB·English

by  G. Garbarino

EPJ manuscript No.
(will be inserted by the editor)
Weak decay of hypernuclei – Theoretical status
Gianni Garbarino
Dipartimento diFisica Teorica, Universit`a diTorino and INFN,Sezione di Torino, I–10125 Torino, Italy
7
0 Received: date/ Revised version: date
0
2
Abstract. The physics of the weak decay of hypernuclei is briefly reviewed from a theoretical point of

n view. Special regard is devoted to the recent progress concerning the determination of the non–mesonic
a decay widths and theasymmetry parameters. Whileconvincing evidencehasbeen achieved for asolution
J of the long–standing puzzle on the ratio Γn/Γp, the discrepancies between theory and experiment on the
7 decay asymmetries clearly highlight the exigence of dedicating further efforts in exploring new aspects of
1 the dynamicsunderlyingthenon–mesonic weak decay.

1 PACS. 21.80.+a – 13.75.Ev – 25.40.-h
v
9
4 1 Introduction 2 Weak decay modes of Λ hypernuclei –
0
General properties
1
0 Hypernucleicanbe consideredasapowerful“laboratory”
7
for unique investigations of baryon–baryon strangeness–
0 changing weak interactions.The best studied systems are WhenahypernucleuscontainingasingleΛhyperonissta-
/
h nuclei containing a Λ hyperon. In such nuclei the Λ can ble with respect to electromagnetic and strong processes,
t decaymesonically,byemittinganucleonandapion,Λ→ itisinthegroundstate,withthehyperoninthe1slevelof
-
l πN, as it occurs in free space; in addition, the hyperon the Λ–nucleus mean potential. From such a state the hy-
c
u (weak) interaction with the nucleons opens new decay pernucleus then decays via a strangeness–changing weak
n channels, customarily indicated as non–mesonic modes. interaction, through the disappearance of the Λ.
: In particular, one can distinguish between one– and two– The mesonic decaymode is the maindecay channelof
v −
nucleon induced decays, ΛN → nN and ΛNN → nNN, aΛinfreespace,includingthetwochannelsΛ→π pand
i
X accordingwhetherthe hyperoninteractswithasinglenu- Λ→π0n withdecayratesΓπ− andΓπ0,respectively.The
r cleon or with a pair of correlatednucleons. experimental ratio for the free decay, Γπfr−ee/Γπfr0ee ≃ 1.78,
a Thefieldofhypernuclearnon–mesonicweakdecayhas isverycloseto2andthusstronglysuggeststhe∆I =1/2
recently experienced a phase of renewed interest, thanks ruleontheisospinchange.Asitoccursinthedecayofthe
to the ideation and accomplishment of innovative exper- Σ hyperonandinpionickaondecays,thisruleisbasedon
iments as well as to the advent of elaborated theoretical experimental observations. Unfortunately, its dynamical
models. For many years the main open problem in the origin is not yet convincingly understood on theoretical
decay of hypernuclei has been the discrepancy between grounds.
theoretical and experimental values of the ratio Γn/Γp The Q–valueforthe mesonicdecayQM ≃mΛ−mN−
between the neutron– and proton–induced decay widths, m ≃40MeVcorrespondstoamomentumofthefinalnu-
π
Γn ≡Γ(Λn→nn) and Γp ≡Γ(Λp→np). This topic will cleon of about 100 MeV. As a consequence, in nuclei the
be discussed in this contribution together with the most Λ mesonic decay is disfavoredby the Pauliprinciple, par-
recent evidences for a solution of the puzzle. ticularly in medium and heavy systems. It is strictly for-
Another interesting and open question we shall deal biddeninnormalinfinitenuclearmatter(wheretheFermi
with concerns the asymmetric non–mesonic decay of po- momentumisk0 ≃270MeV),whileinfinite nucleiitcan
F
larizedhypernuclei.Alsointhiscase,asfortheΓ /Γ puz- occur because of three important effects:
n p
zle,weexpectimportantprogressfromthepresentandfu- 1. In nuclei the hyperon has a momentum distribution
tureimprovedexperiments,whichwillprovideaguidance that admits larger momenta for the final nucleon;
foradeepertheoreticalunderstandingofthehypernuclear 2. The final pion feels an attraction by the medium such
decay mechanisms. thatforfixedthree–momentumq ithasanenergysmaller
Forcomprehensivetheoreticalreviewsonhypernuclear than the free one [ω(q)< pq2+m2], and consequently,
π
weakdecaywereferthereadertoRefs.[1,2]andreferences due to energy conservation, the final nucleon again has
therein. The experimental viewpoint on the same subject more chance to come out above the Fermi surface;
has been discussed at this conference by H. Outa [3]. 3. At the nuclear surface the local Fermi momentum is
2 Gianni Garbarino: Weak decay of hypernuclei– Theoretical status
considerably smaller than k0, and the Pauli blocking is
F
less effective in forbidding the decay.
The mesonic channel can provide valuable informa-
tion onthe pion–nucleus optical potentialsince the decay
widths Γπ− and Γπ0 turn out to be very sensitive to the
π− and π0 self–energies in the nuclear medium [4].
In hypernuclei the Λ decay also occurs through pro-
cesses which involve a weak interaction of the hyperon
with one or more nucleons. When the pion emitted by
the weak vertex is virtual, it gets absorbed by the nu-
clear medium, resulting in non–mesonic processes of the
following type:
Λn→nn (Γn), Λp→np (Γp), ΛNN →nNN (Γ2).
Moremassivemesonsthanthepioncanalsomediatethese
decays[5,6].Hybridmodelsadoptingdirectquarkmecha-
nisms in additionto meson–exchangepotentials havealso
beenproposed[7].Intheseapproaches,thebaryon–baryon
shortrange repulsionoriginatesfrom quark–exchangebe-
tween baryons.
The non–mesonic mode is only possible in nuclei and,
nowadays, the systematic study of hypernuclear decay is
Fig. 1. Decay widths of the Λ in finitenuclei as a function of
the only practical way to get phenomenological informa-
thenuclear mass number A (taken from Refs. [1,8]).
tion on the hyperon–nucleon weak interactions. This is
especially facilitatedby the factthat the finalnucleons in
thenon–mesonicprocesseshavelargemomenta(p ≃420
N
(340) MeV for the one–nucleon (two–nucleon) induced
two–body induced decay mechanisms. These limitations
channel).Therefore,thenon–mesonicmodeisnotblocked
lead to very indirect experimental determinations of the
by the Pauli principle; on the contrary, it dominates over
decay rates Γ and Γ from single–nucleon spectra mea-
the mesonic mode for all but the s–shell hypernuclei. Be- n p
surements. Particularly in the last few years, this persis-
ingcharacterizedbyalargemomentumtransfer,thenon–
tent,puzzlingstatushasencouragedarenewedinterestfor
mesonicdecaymodeisonlyslightlyaffectedbythedetails
hypernuclear non–mesonic decay. Thanks to recent theo-
of hypernuclear structure, thus providing useful informa-
retical [7,12,13,14,15,16,17,18,19] and experimental [3,
tion directly on the hadronic weak interaction.
20,21,22,23,24] progress, we can now safely affirm that
ThetotalweakdecayrateofaΛhypernucleusisΓT =
the Γ /Γ puzzle has been solved. A decisive role in this
ΓM+ΓNM, where ΓM = Γπ− +Γπ0, ΓNM = Γ1+Γ2 and achievnemepnthasbeenplayedbythefirstmeasurementsof
Γ1 = Γn+Γp, while the lifetime is τ = h¯/ΓT. It is inter- nucleon–coincidence spectra together with a non–trivial
esting to observe that there is an anticorrelationbetween
interpretation of data, which required theoretical analy-
mesonicandnon–mesonicdecaymodessuchthatthetotal
sesoftwo–nucleoninduceddecaysandaccuratestudiesof
decayrate is quite stable fromlightto heavyhypernuclei.
nuclear medium effects on the weak decay nucleons. We
This behavior is evident from Figure 1 and is due to the
summarizetheseimportantdevelopmentsinthefollowing.
rapid decrease of the mesonic width and to the satura-
tion property of the ΛN →nN interaction for increasing One–pion–exchange (OPE) models predict small ra-
nuclear mass number. tios,typicallyaround0.1-0.2forthemoststudiedsystems,
5He and 12C. This is mainly due to the particular form
Λ Λ
of the OPE potential, which has a strong tensor compo-
3 The ratio Γ /Γ nent, ΛN(3S1) → nN(3D1), requiring isospin 0 np pairs
n p
intheantisymmetricfinalstate.Onthecontrary,theOPE
model has been able to reproduce the total non–mesonic
Up to very recent times, the main challenge of hypernu-
ratesmeasuredfortheabovementionedhypernuclei[1,2].
clear weak decay studies has been to provide a theoret-
ical explanation of the large experimental values for the Other interactionmechanisms beyond the OPE might
ratio Γ /Γ [1,2]. During this period, large uncertainties then be responsible for the overestimation of Γ and the
n p p
involved in the extraction of this ratio from data did not underestimation of Γ . Many attempts have been made
n
allowtoreachanydefinitive conclusion.These “old”data in order to solve the Γ /Γ puzzle. We recall here the in-
n p
[9,10,11] were quite scarceand not precise due to the dif- clusion in the ΛN → nN transition potential of mesons
ficulty of detecting the products of the non–mesonic de- heavier than the pion [5,6,12,13,14], the implementation
cays, especially the neutrons. Moreover,up to now it has of interaction terms that explicitly violate the ∆I = 1/2
not been possible to distinguish between nucleons pro- rule[25,26]andthedescriptionoftheshortrangebaryon–
duced by the one–body induced and the (non–negligible) baryon interaction in terms of quark degrees of freedom
Gianni Garbarino: Weak decay of hypernuclei– Theoretical status 3
[7], which automatically introduces ∆I = 3/2 contribu- Table 3. Predictions of Refs. [15,16] for the ratio Nnn/Nnp
tions. corresponding to an energy thresholds Tth of 30 MeV and to
N
the back–to–back kinematics (cos θNN ≤−0.8). The data are
A few investigations with transition potentials includ- from KEK–E462 and KEK–E508 [3,22,23,24].
ingheavy–meson–exchangeand/ordirectquark(DQ)con- 5He 12C
tributions have recently improved the situation, without Λ Λ
Nnn/Nnp Γn/Γp Nnn/Nnp Γn/Γp
providing an explanation of the origin of the puzzle. As
OPE 0.25 0.09 0.24 0.08
discussed in the next paragraphs, the proper determina-
OMEa 0.51 0.34 0.39 0.29
tion of Γ /Γ from data indeed required an analysis of
n p OMEf 0.61 0.46 0.43 0.34
the two–nucleon induced decay mechanism and an accu- EXP 0.45±0.11 0.40±0.10
rate evaluation of the final state interactions suffered by
thedetectednucleons.InTables1and2wesummarizethe
resultsofthosecalculationswhichpredictedratiosconsid-
erably enhanced with respect to the OPE values together A weak–decay–model independent analysis of the co-
with experimental data. The variety of models adopted incidence dataofTable3 hasbeen performedinRef.[15].
in the quoted works has been extensively discussed in The results are given in Table 1 either by neglecting the
Ref. [1]. Here we only mention that, with respect to the two–nucleon stimulated decay channel (1N) or by adopt-
OPE results, the addition of kaon–exchange is found to ing Γ2/Γ1 = 0.20 for 5ΛHe and Γ2/Γ1 = 0.25 for 1Λ2C
considerablyreduceΓNM whileincreasingΓn/Γp byafac- (1N + 2N), as predicted in Ref. [8]. The Γn/Γp values
torofabout4.Thisresultismainlyduetoi)theenhance- determined in this way are in agreement with the pure
ment of the parity–violating ΛN(3S1) → nN(3P1) tran- theoretical predictions of Refs. [12,13,14,17,18] (see Ta-
sition contributing especially to neutron–induced decays ble 1) but are substantially smaller than those obtained
andii)thetensorcomponentofkaon–exchange,whichhas experimentallyfromsingle–nucleonspectraanalyses[9,10,
oppositesignwithrespecttotheOPEone,thusincreasing 11,27]. In our opinion, this result represents a strong ev-
Γn and reducing Γp. Also the DQ mechanism revealed to idence for a solution of the long–standing puzzle on the
be important to obtain larger Γn/Γp. All the approaches Γn/Γp ratio.
of Tables 1 and 2 but the one of Ref. [18] reproduce the Thisconclusionhasbeencorroboratedbyarecentstudy
observed non–mesonic widths. We note that the use of a [19],analogoustotheoneofRef.[16]butperformedwithin
more realistic Λ wave function would lead to a reduction anuclearmatterformalismadaptedtofinitenucleiviathe
of about 25% [19] of the non–mesonic rate evaluated in local density approximation (see the results in Table 1).
Ref.[18].AlthoughnocalculationofTable1isabletoex- At variance with Ref. [16], a microscopic model more in
plainthedataofRefs.[9,10,11,27],extractedfromsingle– line with the functional approach of Ref. [30] has been
nucleonmeasurements,somepredictionsareinagreement followed for the two–nucleon induced decays, also includ-
withtherecentdeterminationsofRefs.[16,19]obtainedby ing the channels Λnn→nnn and Λpp→npp besides the
fittingthenucleon–nucleoncoincidencedataofRefs.[3,22, standard mode Λnp → nnp of the previous phenomeno-
23,24]. Inthe remainderofthe presentSectionwe discuss logical approach.
theserecentachievements,whichmadepossibleasolution ForthcomingcoincidencedatafromKEK,BNL[31],J–
of the Γn/Γp puzzle. PARC [32] and FINUDA [33] could be directly compared
with the results of Refs. [15,16,19]. This will permit to
TheauthorsofRefs.[15,16]evaluatedneutron–neutron achieve better determinations of Γ /Γ and to establish
n p
and neutron–proton energy and angular correlations for the first constraints on Γ2/Γ1.
5He and 12C and analyzed the corresponding data ob-
Λ Λ
tained by the experiments KEK–E462 and KEK–E508.
A one–meson–exchange (OME) model was used for the
4 The asymmetry puzzle
ΛN → nN transition in a finite nucleus framework. The
two–nucleoninduceddecaychannelΛnp→nnpwastaken
into account via the polarization propagator method in Despite the recent progress just discussed, the reaction
the local density approximation [8], a model applied for mechanism for the non–mesonic decay does not seem to
the first time to hypernuclear decay in Ref. [28]. The in- be fully understood. Indeed, a new intriguing problem,
tranuclearcascadecodeofRef.[29]wasemployedtosimu- of more recent origin, is open: it concerns a strong dis-
latethenucleonpropagationinsidetheresidualnucleus.In agreement between theory and experiment on the asym-
Table 3 the ratios N /N predicted by the OPE model metry of the angular emission of non–mesonic decay pro-
nn np
and two OME models (OMEa and OMEf, using NSC97a tons from polarized hypernuclei. This asymmetry is due
and NSC97f potentials, respectively) of Refs. [15,16] are to the interference between parity–violating and parity–
given for the back–to–back kinematics (cosθ ≤ −0.8) conserving Λp→np transition amplitudes [34], while the
NN
and nucleon kinetic energies T ,T ≥ 30 MeV. The pre- widely considered rates Γ and Γ are dominated by the
n p n p
dictions for Γ /Γ are also quoted.The OME results well parity–conservingpartoftheinteraction.Thestudyofthe
n p
reproduce the data, thus indicating a ratio Γ /Γ ≃ 0.3 asymmetric emission of protons from polarized hypernu-
n p
for both hypernuclei, in agreement with some of the pure clei is thus supposed to provide new constraints on the
theoretical determinations of Table 1. dynamics of hypernuclear decay.
4 Gianni Garbarino: Weak decay of hypernuclei– Theoretical status
Table 1. Theoretical and experimental determinations of the Γn/Γp ratio.
Ref. and Model 5He 12C
Λ Λ
Sasaki et al. [7] 0.70
π+K+ DQ
Jido et al. [12] 0.53
π+K+2π/σ+2π+ω
Parren˜o and Ramos [13] 0.34÷0.46 0.29÷0.34
π+ρ+K+K∗+ω+η
Itonaga et al. [14] 0.39 0.37
π+2π/σ+2π/ρ+ω
Barbero et al. [17] 0.24 0.21
∗
π+ρ+K+K +ω+η
Bauer and Krmpoti´c [18] 0.29
π+ρ+K+K∗+ω+η
BNL [9] 0.93±0.55 1.33+1.12
−0.81
KEK [10] 1.87+0.67
−1.16
KEK [11] 1.97±0.67
KEK–E307 [27] 0.87±0.23
KEK–E462 [23] 0.45±0.11±0.03
KEK–E508 [24] 0.51±0.13±0.05
KEK–E462/E508 (analysis of Ref. [16]) 0.40±0.11 (1N) 0.38±0.14 (1N)
0.27±0.11 (1N +2N) 0.29±0.14 (1N +2N)
KEK–E462/E508 (analysis of Ref. [19]) 0.37±0.14 (1N)
0.34±0.15 (1N +2N)
Table2.Theoreticalandexperimentaldeterminationsofthenon–mesonicwidthΓNM (inunitsofΓΛfree).Onlytheone–nucleon
induced decay channel has been taken into account in thetheoretical evaluations.
Ref. and Model 5He 12C
Λ Λ
Sasaki et al. [7] 0.52
π+K+ DQ
Jido et al. [12] 0.77
π+K+2π/σ+2π+ω
Parren˜o and Ramos [13] 0.32÷0.43 0.55÷0.73
∗
π+ρ+K+K +ω+η
Itonaga et al. [14] 0.42 1.06
π+2π/σ+2π/ρ+ω
Barbero et al. [17] 0.69 1.17
∗
π+ρ+K+K +ω+η
Bauer and Krmpoti´c [18] 1.64
∗
π+ρ+K+K +ω+η
BNL [9] 0.41±0.14 1.14±0.20
KEK [10] 0.89±0.18
KEK [11] 0.50±0.07
KEK–E307 [27] 0.828±0.056±0.066
KEK–E462 [22] 0.424±0.024
KEK–E508 [22] 0.940±0.035
KEK–E462 [3] 0.411±0.023±0.006
KEK–E508 [3] 0.929±0.027±0.016
The intensity of protons emitted in Λp → np decays proton asymmetry parameter takes the following form:
alongadirectionforminganangleΘwiththepolarization
A(Θ,J)=p (J)a cosΘ, (2)
axis is given by (see Ref. [35] for more details): Λ Λ
p being the polarization of the Λ spin and a the in-
Λ Λ
I(Θ,J)=I0(J)[1+A(Θ,J)], (1) trinsic Λ asymmetry parameter, which is expected to be
a characteristic of the elementary process Λp→np.
I0 being the (isotropic) intensity for an unpolarized hy- Nucleon final state interactions (FSI) acting after the
pernucleus.In the shell modelweak–couplingscheme,the non–mesonicprocessmodifytheweakdecayintensity(1).
Gianni Garbarino: Weak decay of hypernuclei– Theoretical status 5
Experimentally, one has access to a proton intensity IM Table 5. Results of Ref. [42] for theasymmetries aΛ and aMΛ.
which is assumed to have the same Θ–dependence as I:
FSI,Tth(MeV) 5He 12C
p Λ Λ
IM(Θ,J)=I0M(J)(cid:2)1+pΛ(J)aMΛ(J)cosΘ(cid:3). (3) nwoithFSFIS,I0, 0 aΛ−=0−.300.68 aΛ−=0−.106.73
The observable asymmetry, aM(J), which is expected to with FSI, 30 −0.46 −0.37
Λ with FSI, 50 −0.52 −0.51
depend on the hypernucleus, is then determined as:
with FSI, 70 −0.55 −0.65
1 IM(0◦,J)−IM(180◦,J) KEK–E462 [38] 0.11±0.08±0.04
aM(J)= . (4) KEK–E462 [39] 0.07±0.08+0.08
Λ p (J)IM(0◦,J)+IM(180◦,J) −0.00
Λ KEK–E508 [38] −0.20±0.26±0.04
KEK–E508 [39] −0.16±0.28+0.18
While inexplicable inconsistencies appeared between −0.00
the first asymmetry experiments of Refs. [36,37], as dis-
cussed in Ref. [1], very recent and more accurate data [3,
22,38,39]favorsmallaM values,compatiblewithavanish-
Λ comparisonwiththeobserveddecayasymmetriesinacal-
ingvalue,forboths–andp–shellhypernuclei.Onthecon-
culation scheme based on a meson–exchange model, this
trary,theoreticalmodelsbasedonOMEpotentialsand/or
result can be interpreted dynamically as the need for the
DQ mechanisms predicted rather large and negative a
Λ introduction of a scalar–isoscalarmeson–exchange.
values (see Table 4). It must be noted that, on the con-
Prompted by the work of Ref. [43], models based on
trary, the mentioned models have been able to account
OME and/or DQ mechanisms [44,45] have been supple-
fairlywellfortheotherweakdecayobservables(Γ and
NM mentedwiththeexchangeofthescalar–isoscalarσ–meson.
Γ /Γ ) measured for s– and p–shell hypernuclei.
n p Despite the rather phenomenological character of these
Concerningthe abovecomparisonbetweentheoryand
works(theunknownσweakcouplingsarefixedtofitnon–
experiment, it is important to stress that, while one pre- mesonic decay data for 5He), they have clearly demon-
dicts a (5He)≃a (12C),there isno knownreasonto ex- Λ
Λ Λ Λ Λ stratedtheimportanceofσ–exchangeinthenon–mesonic
pectthis approximateequalitytobe validforaM.Indeed,
Λ decay.Moredetailedinvestigationsareneededtoestablish
the relationship between I(Θ,J) of Eq. (1) and IM(Θ,J)
the precise contribution of the scalar–isoscalarchannel.
of Eq. (3) can be strongly affected by FSI of the emitted
protons, thus preventing a direct comparison between a
Λ
and aM. To overcome this obstacle, an evaluation of the
Λ 4.1 Conclusions
FSI effects on the non–mesonic decay of polarized hyper-
nucleihasbeenrecentlyperformed[42]adoptingthesame
framework of Refs. [15,16]. Experimental and theoretical studies of nucleon coinci-
We summarize here some results of this investigation, dencespectrahaverecentlyleadtoasolutionofthelong–
M
which is the first one evaluating a . The simulated pro- standing puzzle on the ratio Γ /Γ . Nowadays, values of
Λ n p
ton intensities turned out to be well fitted by Eq. (3); Γ /Γ around0.3-0.4arecommontoboththeoreticaland
n p
one can thus evaluate aM through Eq. (4). Table 5 shows experimental analyses of s– and p–shell hypernuclei. An
Λ
OME predictions for the intrinsic and observable asym- important role in this achievement has been played by a
metries. As a result of the nucleon rescattering in the non–trivial interpretation of data, which required analy-
nucleus, |aΛ| ∼> |aMΛ| for any value of the proton kinetic sesoftwo–nucleoninduceddecaysandaccuratestudiesof
energy threshold: when Tth = 0, a /aM ≃ 2 for 5He nuclear medium effects on the weak decay nucleons.
p Λ Λ Λ
M 12 M th Despite this improvement,the reactionmechanismfor
and a /a ≃ 4 for C; |a | increases with T and
Λ Λ Λ Λ p
a /aM ≃ 1 for Tth = 70 MeV in both cases. Values of the hypernuclear non–mesonic weak decay does not seem
Λ Λ p to be understood in detail. Indeed, an intriguing problem
aM rather independent of the hypernucleus are obtained
Λ remains open. It regards an asymmetry of the angular
for Tpth = 30, 50 and 70 MeV. The data quoted in the emission of non–mesonic weak decay protons from polar-
table refer to a Tth of about 30 MeV; the corresponding ized hypernuclei. Although nucleon FSI turned out to be
p
predictions of Ref. [42] barely agree with the 12C datum animportantingredientalsowhendealingwiththisissue,
Λ
but are inconsistent with the observation for 5He. further investigations are required to solve the problem.
Λ
Recently, an effective field theory approach based on On the theoreticalside, recent indications on the rele-
tree–levelpion–andkaon–exchangeandleading–ordercon- vanceofthescalar–isoscalarchannelseemtosuggestnovel
tact interactions has been applied to hypernuclear decay reactionmechanismsto improveour knowledgeofthe dy-
[43]. The coefficients of the consideredfour–fermionpoint namics underlying the non–mesonic decay. New and im-
interactionhavebeenfittedtoreproduceavailabledatafor provedexperimentsmoreclearlyestablishingthesignand
thenon–mesonicdecaywidthsof5He,11Band12C.Inthis magnitude of aM for s– and p–shell hypernuclei are also
Λ Λ Λ Λ
way, a dominating central, spin–and isospin–independent necessary. Future experimental studies of the inverse re-
contacttermhasbeenpredicted.Suchtermturnedoutto actionpn→pΛ usingpolarizedprotonbeamsshouldalso
be particularly important to reproduce a small and posi- be encouraged: this process could give a rich and clean
tivevalueoftheintrinsicasymmetryfor5He,asindicated piece of information on the Λ–nucleon weak interaction
Λ
by the recent KEK experiments. In order to improve the and especially on the Λ polarization observables [46].
6 Gianni Garbarino: Weak decay of hypernuclei– Theoretical status
Table 4. Theoretical and experimental determinations of theasymmetry parameters (aΛ and aMΛ, respectively).
Ref. and Model 5He 12C
Λ Λ
Sasaki et al. [7]
π+K+DQ −0.68
Parren˜o and Ramos [13]
π+ρ+K+K∗+ω+η −0.68 −0.73
Itonaga et al. [40]
π+K+2π/σ+2π/ρ+ω −0.33
Barbero et al. [41]
π+ρ+K+K∗+ω+η −0.54 −0.53
KEK–E160 [36] −0.9±0.3
KEK–E278 [37] 0.24±0.22
KEK–E462 [38] 0.11±0.08±0.04
KEK–E462 [39] 0.07±0.08+0.08
−0.00
KEK–E508 [38] −0.20±0.26±0.04
KEK–E508 [39] −0.16±0.28+0.18
−0.00
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