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Studies of KStudies of K---nuclear states -nuclear states with FINUDAwith FINUDA

Alessandra Filippi

I.N.F.N. Torino - ItalyHYP2006 Conference – Mainz

October 11, 2006

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Outline of the talkOutline of the talk

Introduction

Overview of experimental analysis methods and evidences

Study of tri-baryon strange systems with FINUDA

Study of di-baryon strange systems with FINUDA

Summary, Prospects for the coming data taking, Conclusions

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IntroductionIntroductionGeneral understanding of KN (KA) interactionGeneral understanding of KN (KA) interaction

Small binding energies: 10-30 MeVLarge decay widths: 80-100 MeV

Recent theoretical developments (Akaishi-Yamazaki)Recent theoretical developments (Akaishi-Yamazaki)Existence of states with the kaon deeply bound (50-100 kaon deeply bound (50-100 MeV) to few nucleonsMeV) to few nucleons, and narrow widths (23-40 MeV)narrow widths (23-40 MeV)

Preferred production on light nuclear systemsA few experimental observation by KEK-E471 experiment corroborating this hypothesis

Several criticism on the theoretical side:Simple FSI interaction effect? Simple underlying dynamics? (Oset)Possible existence of aggregates (Gal, Weise, Schaffner-Bielich, Wycech) but:

The potential is shallowerThe expected widths are large and the states should elude observation

Possible signals only from heavy systems

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Experimental evidences: search methodsExperimental evidences: search methods

Missing mass spectroscopyMissing mass spectroscopyMeasurement of the momentum of the monochromatic recoiling particle in a A(K-,N)X reaction

KEK-PS E471, AGS E930FINUDA

Invariant mass spectroscopyInvariant mass spectroscopyBased on the kaonic-nuclear state feature of decaying into Based on the kaonic-nuclear state feature of decaying into hyperonshyperons

(K(K--pp) → pp) → ΛΛ + p + p

(K(K--ppn) → ppn) → ΛΛ + d + d Typically: Typically:

ppΛΛ,p,p ~ 500 MeV/c ~ 500 MeV/cpp < 200 MeV/c< 200 MeV/cp p decay pdecay p ~ 500 MeV/c ~ 500 MeV/c

Necessary to fully reconstruct all the particles emitted in the decay: suitable for few-body systemsThe decay occurs at rest: angular correlation information between the emitted particles can be exploited

FOPI (heavy ion collisions)

FINUDA (K-stop, zero temperature)

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proton momentum (MeV/c)proton momentum (MeV/c)KEK-PS E471 miss. mass method, Suzuki et al, NPA 754 (2005) 375

Missing mass observations of a possible Missing mass observations of a possible tri-baryon signal: KEK-PS E471tri-baryon signal: KEK-PS E471

experimental signature of AY expectations?

A few inconsistencies…

Dedicated experiment:

Stopped K-

L4He target

Good neutron detector

- momentum measurement by light release in scintillators (scarce)

p momentum peak at ~500 MeV/c, measured by TOF

Corresponding to a missing mass of 3115 MeV

KEK-PS E471 miss. mass methodSuzuki et al, Phys. Lett. B597 (2004) 263

S+(3140)

S0(3115)

4He(K-stop, n)

4He(K-stop, p)

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The FINUDA Experiment at DAThe FINUDA Experiment at DAΦΦNENE

4 m

Use of several targets at the same time, in the same experimental

conditions

Use of several targets at the same time, in the same experimental

conditions

Cylindric geometry typical of a collider

experiment

Cylindric geometry typical of a collider

experiment

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Study of tri-baryion strange aggregates Study of tri-baryion strange aggregates with FINUDA: missing mass analysiswith FINUDA: missing mass analysisStudy of inclusive proton momentum spectra in reactions induced by K- on 6Li and 12C targets

No 4He target in FINUDA but 6Li is long time known to be a (d+α) clusterInteresting information available from this nuclear system

Resolution on p momentum: ~ 1%High quality tracks (four points each)Proton identification by means of dE/dx on Si-microstrips detector (p.id. efficiency > 94%)

Main contribution to the raw spectra shape: multinucleon absorption

6Li 12C

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Semi-inclusive proton spectra: Semi-inclusive proton spectra: coincidence with a coincidence with a --

Coincidence measurements are possible in FINUDA thanks to the possibility of fully reconstructing the event topologies, measuring the particle momenta with high resolution

Strong correlation observed between fast protons and fast - in K- induced events on 6Li targets

6Li 12 C

Peak of the KK--

nn→→Λ Λ -- reaction evidentVery strong correlation for back-to-back selected events

Less evident correlations on 12C

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EXPERIMENTAL DATA

Impact parameters distributions for Impact parameters distributions for coincidence eventscoincidence events

The p and the high momentum - are produced in two different vertices

eventswith p- > 275 MeV/c

p

-

Where does the Where does the ΣΣ-- hyperon come from?hyperon come from?

From the decay of a DBKS produced in the K- absorption on 4He?Produced together with a prompt p in the K-(np) →Σ- p absorption reaction on two nucleons?

pp

--

SIMULATIONS

K- + d p+-

The The -- comes from the decay of a comes from the decay of a ΣΣ-- hyperonhyperon

MONTECARLO: The p and the high momentum π- impact parameter distributions are not compatible with the hypothesis of emission from the same source Impact parameter (cm)

Impact parameter (cm) Impact parameter (cm)

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Prompt Prompt ΣΣ-- or from DBKS decay?or from DBKS decay?Expected momentum distributions:

Several decay channels may be excluded due to their kinematic featuresHard to discriminate the two cases

Angular distributions (opening angle between p and -)

Observed a back-to-back correlation

No correlation expected if both ΣΣ-- and p are produced in a DBKS decay

The The ΣΣ-- is promptly produced in the is promptly produced in the

KK-- absorption on two nucleons absorption on two nucleons

a) K- +"d" p+-

b) K- +”4He” S0+p and S0 nn

c) K- +”4He” S0+p and S0 np

d) K- +”4He” S0+p and S0

np and p n conversion

e) K- +”4He” S0+p and S0 d

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KK-- interaction on “quasi” deuteron in interaction on “quasi” deuteron in 66Li or Li or 44He?He?The momentum distribution The momentum distribution of a deuteron bound in a of a deuteron bound in a 66Li Li nucleus has two peaks:nucleus has two peaks:

Sharp peak at ~50 MeV/cSharp peak at ~50 MeV/cBroad peak at ~ 300 MeV/cBroad peak at ~ 300 MeV/c

The width of the observed momentum peak (~40 MeV/c) is compatible with the spread obtained inserting for the “quasi”-deuteron momentum the above distribution (Yamazaki, Akaishi)

The momentum of a deuteron in a 4He nucleus has, on the contrary, just one peak at ~ 180 MeV/c

More boundLess likely to single out a “quasi”-deuteron in a α particle

The momentum of the deuteron in 12C has two peaks again

Lower at ~100 MeV/cMay explain why the absorption reaction is not observed in 12C targets

T. Yamazaki, to be published

d inside the α in 6Li

d in 6Li

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KK-- “d” “d” →→ΣΣ--p capture ratep capture rateFew data on multinucleon absorption rates in definite channels available for comparisonOnly old measurements from bubble chambers

Multinucleonic capture rates with nucleon emission:In 2H: ~1%In 4He: ~16% In 12C: ~20%

In 4He, single channels with hyperons:

K- 4He →Σ- pd: (1.6 ±0.6)%/K-stop

K- 4He → Σ- ppn: (2.0±0.7)%/K-stop

FINUDA measurement:K- 6Li → Σ-p X: (X = any non-strange system with B=4)

(1.62±0.23 (stat)+0.71-0.44 (sys) )%/K-

stop

Good agreement with existing observations

FINUDA Coll., NPA 775 (2006), 35

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• ΛΛ+d+d (K (K--ppn)ppn)• m = (3159 m = (3159 ±± 20) MeV 20) MeV • ΓΓ= (100 = (100 ±± 50) MeV 50) MeV

Newest observations of a tri-baryon strange Newest observations of a tri-baryon strange state with the invariant mass method: state with the invariant mass method: ΛΛd systemd system

FOPI - GSINi+Ni @ 1.93 AGeVUse of invariant mass spectroscopy to search for short-lived ΛX resonances

FINUDAHints for a ΛΛd bound state (low statistics)

ΛΛd invariant mass

• ΛΛ+d+d (K (K--ppn)ppn)• m = (3243 m = (3243 ±± 5) MeV 5) MeV • ΓΓ = (36.8 = (36.8 ±± 10) MeV 10) MeV• Y = (5.4 Y = (5.4 ± 10) 10± 10) 10-3-3/K/K--

stopstop

Unique feature in FINUDA: low deuterons can be spectroscopizedDetails in S. Piano’s talk

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Invariant mass studies of di-baryon Invariant mass studies of di-baryon strange states with FINUDAstrange states with FINUDA

X(K-pp)→Λp

-p p(Λ) ≈ 500 MeV/cp(p) ≈ 500 MeV/cp(-) ≈ 200 MeV/c short track!

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(K(K--pp) system identificationpp) system identification1. Observation of a Λ

1

2

2. angular correlation between the Λ and the decay proton

cosθYN < -0.8

The apparatus acceptance (- momentum threshold) cuts Λ‘s slower than 300 MeV/c6 MeV FWHM

Events with a Λ-p coincidence: ~ 5%

Light targets only (3x 12C, 2x 6Li, 1x 7Li)

The hyperon and the p should be oppositely emitted, apart from FSI inside the nucleus

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fit region

((ΛΛpp) invariant mass: observation ) invariant mass: observation of a bound stateof a bound state

mK+2mp

FINUDA Coll., PRL 94(2005)212303

An enhancement is observedTwo nucleon absorptionTwo nucleon absorption

KK-- + (pp) + (pp) Y* N Y* N pp peak expected @ m(K m(K--2p) = 2.37 2p) = 2.37 GeVGeV… NOTHING!

KK-- + (pp) + (pp) 00p p p p 74 MeV lower distribution, and broadened

Kaon-nuclear bound state Kaon-nuclear bound state formationformation

KK- - (pp) (pp) X X pp 00p p p p

B = 115+6-5 (stat)+3

-4 (sys) MeV= 67+14

-11(stat)+2-3 (sys) MeV

Yield Yield ≈ 0.1%/stopped K0.1%/stopped K--

M = (2255 ± 9) MeV

Acc

epta

nce

corr

ectio

n

~ 100 events available so far

Further details in the poster by H. Fujioka

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Search for the KSearch for the K--pn system with FINUDA: pn system with FINUDA: first observations at low statisticsfirst observations at low statistics

KK--pn pn nn tagging and tagging and back-to-back back-to-back n selectionn selection

(1115)(1115)

(1197)(1197)

Two kinds of KK--pnpn states expected:Isobaric analog state of K-pp (pn: I=1)

K-d (pn: I=0) (much less bound)

KK--pn pn --pp-- tagging and tagging and back-to-back back-to-back - - p selectionp selection

~~100 events100 events

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Summary: tri-baryon observationsSummary: tri-baryon observationsTri-baryon strange system – missing mass analysis

FINUDA observed a peak in the inclusive proton momentum spectrum at about 500 MeV/c, in coincidence with a fast pion, in kaon induced reaction on 6Li

Signature of the K- ”d” → Σ- p reaction occurring on a quasi-deuteron substructure of 6Li nucleusAbsence of a similar signal in reactions on 12C

No need to invoke the presence of a DBK system to explain the observed signal

Simple kinematic effectBEWARE of missing mass analysis method to identify bound states!BEWARE of missing mass analysis method to identify bound states!

FINUDA can take advantage of more information on secondary particles thanks to its p.id. and full event reconstruction capabilities

Tri-baryon strange system – invariant mass analysisFirst observations by FINUDA for a bound state

Details in the talk by S. PianoMore data awaited in the next data taking

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Summary: di-baryon observationsSummary: di-baryon observations

Di-baryon strange systemFINUDA can fully exploit the invariant mass analysis procedure

A peak observed in the (Λp) invariant mass system at about 2255 MeV, 67 MeV wide, that cannot be explained as due to K- absorption on two nucleons: bound by ~115 MeV

Back-to-back Λ and protonCould come from the decay at rest of a composite object

This topology is not a bias of the apparatus acceptance

Further details on the analysis in the poster by H. Fujioka

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ConclusionsConclusions

FINUDA is a powerful facility to study the features of possible K--nuclear bound states

Good acceptance and angular coverage

Good p.i.d. and momentum resolution

First (exploratory) data taking:Observation of complementary features of signals seen by other experiments

Observation of new interesting signatures

Up-coming data takingSignificative improvement in the study of low-statistics reactions