Determination of non-mesonic weak decay widths of 5ΛHe and 11ΛB Hypernuclei

Physics Letters B 748 (2015) 86–88

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Determination

of

non-mesonic

weak

decay

widths

of

_!

5

He

and

11

_!

B

Hypernuclei

E. Botta

a

,

b

_,

_{T. Bressani}

a

,

b

_,

_{S. Bufalino}

a

,

b

_,

_{A. Feliciello}

b

,

∗

a_{DipartimentodiFisica,UniversitàdiTorino,viaP.Giuria1,Torino,Italy} b_{INFNSezionediTorino,viaP.Giuria1,Torino,Italy}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received21April2015 Accepted29June2015 Availableonline2July2015 Editor:D.F.Geesaman

Keywords:

!-Hypernuclei

Non-mesonicweakdecaywidths

Therecent determinationofthepartialdecaywidthsfortheone-protonandthetwo-nucleoninduced Non-Mesonic Weak Decay of!-Hypernuclei inthe A ₌5–16 rangepermitted toreconstruct thefull patternofdecaywidthsfor 5

!Heand 11!B.Aconsistencycheckon12!Cdecaywidthsconfirmsthevalidity

oftheadoptedmethod.

©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

WeakInteraction mediates the

!-Hypernuclei

(Hypernuclei in thefollowing) decayto non-strangenuclear systemsthrough dif-ferentchannels(WeakDecays,WD).

The simplest process is the so-called Mesonic Weak Decay (MWD),which iscloselyrelatedto thetwo maindecay channels into N

π

ofthe constituent

!

hyperoninfree space. The associ-ated decay widths are labeled

#

π− and

#

_π0 and their sum

#

M. The MWDis themostimportantdecay modeforthe s-shell Hy-pernuclei,itremains quitesignificant,atthe levelof

∼

15–20% of thetotaldecaywidth, inthecaseof p-shellHypernuclei,whileit becomesnegligibleformedium- tohigh-A Hypernuclei.

Theso-calledNon-Mesonic WeakDecay (NMWD)channelsare insteadlinked to theoccurrenceof WeakInteractionsamongthe constituent

!

hyperonandone or morenucleons ofthe nuclear core of a Hypernucleus. The process

!

N

→

N N is the simplest one:usuallyitisreferredtoasproton-stimulateddecay,inthecase ofthe

!

p

→

np elementaryreaction, witha decaywidth

#

p, or neutron-stimulateddecayinthecaseofthe

!

n

→

nninteraction, withacorresponding

#

n.Theimportanceoftheone-nucleon(1N) inducedNMWD wasimmediatelyrecognizedsince theearlydays ofHypernuclearPhysics[1]asauniquetooltoachievequantitative informationonthe

!

N

→

N N WeakInteraction,veryhardly acces-sibleinfreespace.Suchastatementstillholdstoday,despitemore thansixdecadesofremarkableprogressinthetechnologyof

accel-*

Correspondingauthor.

E-mailaddress:[email protected](A. Feliciello).

erators,ofparticledetectorsandofcomputingdevices.Quitelater, afurtherNMWDchannelwaspredicted,associatedtothe elemen-tary

!(

N N

)

→

n

(

N N

)

interactiononapairofcorrelatednucleons inthe nuclearcoreoftheHypernucleus(2N induced)[2],witha correspondingdecaywidth

#

2N.

#

2N maybefurthersplitintothe

threebranchescorrespondingtothethreeprojectionsofthetotal IsospinoftheN Npair.Howeverthedominantconfigurationisthe one correspondingto the I3

₌

0 component(np)predictedto ac-count for

∼

83%ofthetotal

#

2N width[3].

#

N M is thenthesum of

#

p,

#

nand

#

2N.TheNMWDchannelsarethedominantonesin

allbutthes-shellHypernuclei.

We do not report here the specific decayschemes for all the abovefive(orseven)channels,whicharestraightforward.Actually, a lot of different configurations for the residual nuclear system produced in the different decay processes are possible (a single nucleusin the groundorlow-lying excited state or two ormore fragments,. . . ).Thetotaldecaywidth

#

T foragivenHypernucleus isfinallygivenby:

#

T

= #

M

+ #

N M

= #

π−

+ #

_π0

+ #

p

+ #

n

+ #

2N

.

(1)

In order to compare dataabout differentHypernuclei, partial

#s

are usually given in units of

#

!, the total decay width of the

free

!;

equation(1)isthenrewrittenas:

#

T

#

!

=

#

M

+ #

N M

#

!

=

#

π−

#

!

+

#

_π0

#

!

+

#

p

#

!

+

#

n

#

!

+

#

2N

#

!

.

(2)

It appearsthat inprincipleit isnecessarytomeasure six quanti-tiestofullydescribethedecayfeaturesofaHypernucleus(thefive

http://dx.doi.org/10.1016/j.physletb.2015.06.072

0370-2693/©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

E. Botta et al. / Physics Letters B 748 (2015) 86–88 87

#s

anditslifetime).Severalexperimentsreportedpartial measure-ments ofthe differentWD widths.A recentreview on Hypernu-clearWeakDecaysmaybefoundinRef.[4].

Moreover,inadditiontotheexperimental hardnessintrinsicto theNMWDobservation,onehastofacetheeffectsofintranuclear cascadeprocesses (Final State Interaction, FSI), already quite sig-nificantforlighthypernuclearsystemslikethe 5

!HeHypernucleus.

Asystematicsummary ofthe measurementsofallthe

#s

for 12

!C

performedattheKEKPSisreportedinRef.[5]:itistheonly com-plete determination carried out so far. The main breakthroughs withrespect to previous experiments were the determination of

#

2N

/ #

!, which was found to be equalto (0.27

±

0.13), and an

updatedestimationoftheFSIeffects.

#

2N was determinedbythe

FINUDACollaboration forthe Hypernuclei inthe A

=

5–16 range thankstoastudyoftheprotonspectraandoftheneutron–proton coincidence spectrain suitable kinematical configurations,as de-scribedinRefs.[6–8].Aconstantvalueof

#

2N

/ #

p

=

0

.

36

±

0

.

14stat+₋0₀._.05₀₄sys_sys (3)

wasobtained,whichwillbeusedinthefollowing.From(3)avalue of

#

2N

/ #

N M

=

!

0.20

±

0.08stat+₋00..0403syssys

"

was calculated.Moreover, inthemostrecentpaper[8],adeterminationof

#

p

/ #

! foreight

Hypernucleiwasgivenaswell.Then,theavailabilityof

#

p

/ #

!and

#

2N

/ #

! inthe A

=

5–16 rangeoffersthepossibilityof

construct-ingfullWDpatternsforsomeHypernucleiintheabovementioned

A range, by merging the information on

#

T

/ #

!,

#

π−

/ #

! and

#

_π0

/ #

! provided byother experiments.Theprocesseswith

neu-tral particles in the final state are obviously the hardest to be measured.However, goodquality data samples exist on

#

_π0

/ #

!

for 5

!He and

11

!B. Then, by exploiting equation (2) it was

possi-bletoderive by difference

#

n

/ #

!,even though withlargeerror,

andfinally to obtain the full pattern ofthe WD widths for 5

!He

and11

!B.

2. TheNMWDwidthsof 5

!He

Thefirst experimentwhich produced a full setof WDwidths for 5

!Heand12!Candwhichpioneeredallthefollowingeffortswas

performedattheBNLAGS[9].Avaluefor

#

n

/ #

! of(0.20

±

0.11)

was deduced from (7.7

±

3.3) neutron events, background sub-tracted.Apossible contributionofthe 2N induceddecaywas not takenintoaccountsince atthattime itwas not evenanticipated theoretically. The value

#

n

/ #

p

= (

0.93

±

0.55) whichwas finally published is then unavoidably biasedby this omission. Both the

#

n

/ #

! and

#

p

/ #

!

= (

0.21

±

0.07)valueswere correctedforFSI.

Further measurements of

#

π−

/ #

!, of

#

π0

/ #

! and of

#

T

/ #

!

were reported in Refs. [10,11], in Ref. [12] and in Ref. [10], re-spectively.A very recentdetermination of

#

p

/ #

! was published

inRef.[8];onthebasisofsucharesultandbyexploitingequation

(3)anevaluationof

#

2N

/ #

!wasobtainedaswell.

A compilation of the above WD widths values is reported in thefirst fourrowsof thesecond columnof Table 1.When more thanonemeasurementisavailableintheliterature,wequotedthe weighted average(w.a.). The

#

n

/ #

! value was calculated by

dif-ferenceaccordingtoequation(2).Consequently,itisaffectedbya quitelargeerror,whichcharacterizesalsothe

#

n

/ #

p ratio,shown intheseventhrowofTable 1.Itturnsouttobeconsistentwiththe value

#

n

/ #

p

= (

0.45

±

0.11stat.

±

0.03syst.

)

obtainedinthedirect

measurementinwhichtheneutronandtheprotonwere detected intightback-to-backtopologyandby applyingfurtherconstraints onthenucleonenergywhichpermittedtheselectionofeventsfor whichtheFSIeffectwerenotsoimportant[13].

Table 1

WDwidthsfor 5

!He,11!Band12!CHypernuclei.Theexistingexperimentaldatafor

#T/ #!,#π−/ #!,#π0/ #!and#p/ #!havebeenintegratedwiththe determina-tionof#2N/ #!,#n/ #!and#n/ #p.Thislatterresultiscomparedwiththeoretical predictionsfromRef.[20],shownin thelastrow.Thevaluesobtainedfor12

!Care comparedwithmeasurementsdescribedinRef.[5],listedinthelastcolumn.

5 !He 11!B 12!C 12!C [5] #T/ #! 0.962±0.034 [9,10] 1.274±0.072 [18,19] 1.241±0.041 [5] 1.241±0.041 #π−/ #! 0.342±0.015 [9–11] 0.228[11,14–16]±0.027 0.120[5,9,15]±0.014 0.123±0.015 #_π0/ #! 0.201±0.011 [12] 0.192±0.056 [17] 0.1656±0.0080 [12,17] 0.165±0.008 #p/ #! 0.217±0.041 [8,9] 0.47±0.11 [8] 0.493±0.088 [5,8] 0.45±0.10 #2N/ #! 0.078±0.034

[thiswork] 0.169[thiswork]±0.077 0.178[thiswork]±0.076 0.27±0.13

#n/ #! 0.125±0.066 [thiswork] 0.21±0.16 [thiswork] 0.28±0.12 [thiswork] 0.23±0.08 #n/ #p 0.58±0.32 [thiswork] 0.46±0.37 [thiswork] 0.58±0.27 [thiswork] 0.51±0.14 #n/ #p [20] 0.508 0.502 0.418 – 3. TheNMWDwidthsof11 !B

ThereisaconsiderableamountofmeasurementsofpartialWD forthisHypernucleus,duetothecircumstancesthatitisproduced together with 12

!C that is the moststudiedHypernuclear system.

Asamatteroffact,inallHypernuclearmassspectraobtainedwith (K−

,

π

−) and(

π

+

,

K+) production reaction on a 12_{C target two}

prominent peaks appear, one corresponding to the formation of thegroundstateof12

!C,describedastheparticle–holeshell-model

configuration

(

s

(!),

p−1

(

n

)),

theother,separatedby

∼

11 MeV in excitation energy, corresponding tothe

(

p

(!),

p−1

(

n

))

configura-tionanddecayingviaStrongInteractionto 11

!Bplus alow-energy

proton (undetected).Measurements of

#

π−

/ #

!, of

#

π0

/ #

! and

of

#

T

/ #

! can be found in Refs. [11,14–16], in Ref. [17] and in

Refs. [18,19], respectively. Values for

#

p

/ #

! were reported in

Ref. [15] and, more recently, in Ref. [8]. As far as the result de-scribedinRef.[15]isconcerned,itisnotclearwhetherthevalue for

#

p

/ #

! was correctedforFSI effect,whichcanamount up to

100%asshowninRef.[8].Then,wedidnotconsidersuchavalue homogeneous withthe one reported in Ref. [8] and we did not use it to evaluate a w.a. for

#

p

/ #

!. By entering the value from

Ref. [8] in the formula(3),

#

2N

/ #

! was determined along with

#

n

/ #

! and

#

n

/ #

p.Allthesevaluesarereportedfromthefifthto theseventhrowofthethirdcolumnofTable 1.

4. Theconsistencycheckfrom12

!C

Tohavea consistencycheck oftheabove described procedure adoptedfor_!5Heand11_!B,wecalculatedtheNMWDwidthsof12_!C byusingthesamemethodandwecomparedthemwiththeseries ofresultsfromRef. [5],listedinthelast columnofTable 1. Mea-surementsof

#

_π−

/ #

! andof

#

π0

/ #

! were reportedinRefs. [5, 9,15] andin Refs. [12,17], respectively. For

#

T

/ #

! we took into

accountthelast,precisevaluefromRef.[5].For

#

p

/ #

!we

consid-eredthevaluesfromRefs.[5,8]onlyandweexcludedtheonefrom Ref.[15]forthesamereasonsexplainedabove(seeSection3).This way,we obtainedthe valuesfor

#

2N

/ #

!,

#

n

/ #

p and

#

n

/ #

p, re-ported from the fifth to the seventh row of the fourth column of Table 1. An overall agreement within the errors between the present determination and the previous measurement is evident

88 E. Botta et al. / Physics Letters B 748 (2015) 86–88 andit provides argumentssupporting the validityof themethod

adoptedfor 5

!Heand11!B. 5. Conclusions

Inconclusion,wehavepresentedforthe firsttimethefullset ofWDwidthsfor 5

!Heand11!BHypernuclei.Thevaluesfor

#

n

/ #

!,

determinedbysubtractingthepartialwidths

#

M

/ #

!,

#

p

/ #

!and

#

2N

/ #

! from

#

T

/ #

!, are affected by large errors and

conse-quently also the ones for the

#

n

/ #

p ratio. They are compatible withtheoutcomeofaccuratetheoreticalcalculations[20]reported in thelast rowof Table 1, which predictednuclear structure ef-fectsaslargeas10%in1NinducedNMWDofp-shellHypernuclei. Strongnuclearstructureeffectswereexpected(andtheywere ac-tually observed [11]) for MWD and at a much lower extent for

1N induced NMWD, due to the damping caused by the larger

momentum of the final state particles (

∼

400 MeV/c instead of

∼

100 MeV/c). It could be interesting to verify these predictions byapplyingthemethoddescribedinthisLettertoseveralp-shell Hypernuclei to be observedin a dedicatedexperimentaiming to measure all the values of the WD widths with errors ofthe or-der of5%.Such a setofresultscould be well exploitedtoderive

the physical quantities describing the four-baryon, weak interac-tion

!

N

→

N N,notaccessibleinfreespace.

References

[1]W.Cheston,H.Primakoff,Phys.Rev.92(1953)1537.

[2]W.M.Alberico,A.DePace,M.Ericson,A.Molinari,Phys.Lett.B256(1991)134.

[3]E.Bauer,G.Garbarino,Nucl.Phys.A828(2009)29.

[4]E.Botta,T.Bressani,G.Garbarino,Eur.Phys.J.A48(2012)41.

[5]H.Bhang,etal.,J.KoreanPhys.Soc.59(2011)1461.

[6]FINUDACollaboration,M.Agnello,etal.,G.Garbarino,Phys.Lett.B685(2010) 247.

[7]FINUDACollaboration,M.Agnello,etal.,G.Garbarino,Phys.Lett.B701(2011) 556.

[8]FINUDACollaboration,M.Agnello,etal.,Phys.Lett.B738(2014)499.

[9]J.J.Szymanski,etal.,Phys.Rev.C43(1991)849.

[10]S.Kameoka,etal.,Nucl.Phys.A754(2005)173c.

[11]FINUDACollaboration,M.Agnello,etal.,A.Gal,Phys.Lett.B681(2009)139.

[12]S.Okada,etal.,Nucl.Phys.A754(2005)178c.

[13]B.H.Kang,etal.,Phys.Rev.Lett.96(2006)062301.

[14]A.Montwill,etal.,Nucl.Phys.A234(1974)413.

[15]H.Noumi,etal.,Phys.Rev.C52(1995)2936.

[16]Y.Sato,etal.,Phys.Rev.C71(2005)025203.

[17]A.Sakaguchi,etal.,Phys.Rev.C43(1991)73.

[18]R.Grace,etal.,Phys.Rev.Lett.55(1985)1055.

[19]H.Park,etal.,Phys.Rev.C61(2000)054004.