1 Studies of K - -nuclear states with FINUDA Alessandra Filippi I.N.F.N. Torino - Italy HYP2006 Conference – Mainz October 11, 2006

2 Outline 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

3Introduction General understanding of KN (KA) interaction Small binding energies: MeV Large decay widths: MeV Recent theoretical developments (Akaishi-Yamazaki) kaon deeply bound ( MeV) to few nucleonsnarrow widths (23-40 MeV) Existence of states with the kaon deeply bound ( MeV) to few nucleons, and narrow widths (23-40 MeV) Preferred production on light nuclear systems A 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 shallower The expected widths are large and the states should elude observation Possible signals only from heavy systems

4 Experimental evidences: search methods Missing mass spectroscopy Measurement of the momentum of the monochromatic recoiling particle in a A(K -,N)X reaction KEK-PS E471, AGS E930 FINUDA Invariant mass spectroscopy Based on the kaonic-nuclear state feature of decaying into hyperons (K - pp) → Λ + p (K - ppn) → Λ + d Typically: p Λ,p ~ 500 MeV/c p  < 200 MeV/c p decay p ~ 500 MeV/c Necessary to fully reconstruct all the particles emitted in the decay: suitable for few-body systems The decay occurs at rest: angular correlation information between the emitted particles can be exploited FOPI (heavy ion collisions) FINUDA (K - stop, zero temperature)

5 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 tri-baryon signal: KEK-PS E471 experimental signature of AY expectations? A few inconsistencies… Dedicated experiment: Stopped K - L 4 He 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 method Suzuki et al, Phys. Lett. B597 (2004) 263 S + (3140) S 0 (3115) 4 He(K - stop, n) 4 He(K - stop, p)

6 Experimental features of the signals observed in K - stop 4 He reactions S + (3140) S + (3140), T=0, S = -1 S 0 (3115) S 0 (3115), T=1, S = -1 M = (sys) ±2.3(stat) MeV/c 2, B = 174±4 MeV Γ < 21.6 MeV/c 2 inclusive n momentum spectrum K - ppn state: signal observed in the inclusive n momentum spectrum, in coincidence with a fast  - Angular correlation between the incoming kaon and the formation neutron: enhancement for backward events Indication of a preferred decay into Σ ± NN K-A bound state with T=0, Q=+1 theoretically predicted at M = 3194 MeV/c 2 M = (sys) ± 0.9(stat) MeV/c 2, B ≈ 193 MeV Γ< 21.6 MeV/c 2 (95% C.L.) semi-inclusive p momentum spectrum K - pnn state: signal observed in the semi-inclusive p momentum spectrum, in coincidence with a fast  - ΣNN preferred decay channel

7 The FINUDA Experiment at DAΦNE 4 m Use of several targets at the same time, in the same experimental conditions Cylindric geometry typical of a collider experiment

8 Study of tri-baryion strange aggregates with FINUDA: missing mass analysis Study of inclusive proton momentum spectra in reactions induced by K - on 6 Li and 12 C targets No 4 He target in FINUDA but 6 Li is long time known to be a (d+ α ) cluster Interesting 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 6 Li 12 C

9 Semi-inclusive proton spectra: 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 6 Li targets 6 Li 12 C K - n→Λ  - Peak of the K - n→Λ  - reaction evident Very strong correlation for back-to-back selected events Less evident correlations on 12 C

10 EXPERIMENTAL DATA Impact parameters distributions for coincidence events The p and the high momentum  - are produced in two different vertices events with p - > 275 MeV/c p  - Where does the Σ - hyperon come from? From the decay of a DBKS produced in the K - absorption on 4 He? Produced together with a prompt p in the K - (np) →Σ - p absorption reaction on two nucleons? p p -- -- SIMULATIONS K - + d  p+  - The - comes from the decay of a Σ - hyperon The  - comes from the decay of a Σ - hyperon 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)

11 Prompt Σ - or from DBKS decay? Expected momentum distributions: Several decay channels may be excluded due to their kinematic features Hard to discriminate the two cases Angular distributions (opening angle between p and  - ) Observed a back-to-back correlation Σ - No correlation expected if both Σ - and p are produced in a DBKS decay The Σ - is promptly produced in the K - absorption on two nucleons a) K - +"d"  p+  - b) K - +” 4 He”  S 0 +p and S 0  nn c) K - +” 4 He”  S 0 +p and S 0   np d) K - +” 4 He”  S 0 +p and S 0   np and   p  n conversion e) K - +” 4 He”  S 0 +p and S 0   d

12 K - interaction on “quasi” deuteron in 6 Li or 4 He? The momentum distribution of a deuteron bound in a 6 Li nucleus has two peaks: Sharp peak at ~50 MeV/c Broad 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 4 He nucleus has, on the contrary, just one peak at ~ 180 MeV/c More bound Less likely to single out a “quasi”-deuteron in a α particle The momentum of the deuteron in 12 C has two peaks again Lower at ~100 MeV/c May explain why the absorption reaction is not observed in 12 C targets T. Yamazaki, to be published d inside the α in 6 Li d in 6 Li

13 K - “d” →Σ - p capture rate Few data on multinucleon absorption rates in definite channels available for comparison Only old measurements from bubble chambers Multinucleonic capture rates with nucleon emission: In 2 H: ~1% In 4 He: ~16% In 12 C: ~20% In 4 He, single channels with hyperons: K - 4 He →Σ - pd: (1.6 ±0.6)%/K - stop K - 4 He → Σ - ppn: (2.0±0.7)%/K - stop FINUDA measurement: K - 6 Li → Σ - p X: (X = any non-strange system with B=4) (1.62±0.23 (stat) (sys) )%/K - stop Good agreement with existing observations FINUDA Coll., NPA 775 (2006), 35

14 Conclusions on tri-baryon strange system study with the missing mass method A signal is observed in the inclusive momentum spectrum of a proton emitted in a K - reaction on 6 Li at ~ 500 MeV/c Very similar to the signal observed by KEK-PS E471 in coincidence with fast π -, coming from the decay of aΣ - hyperon Back-to-back (π - p) correlation: prompt production of the hyperon in a K - absorption reaction: K - “d” →Σ - p Experimental width compatible with the smearing due to the “quasi”-deuteron momentum in 6 Li Evaluated capture rate for the K - d →Σ - p reaction compatible with existing measurements (~2%/stopped K - ) No need to invoke the formation of a DBKS to explain the signal Simple kinematic effect of a “quasi”-two body reaction induced by the K - on a substructure of 6 Li nucleus Agreement with Oset-Toki expectations

15 Λ+d (K - ppn)Λ+d (K - ppn) m = (3159 ± 20) MeVm = (3159 ± 20) MeV Γ= (100 ± 50) MeVΓ= (100 ± 50) MeV Newest observations of a tri-baryon strange state with the invariant mass method: Λd system FOPI - GSI 1.93 AGeV Use of invariant mass spectroscopy to search for short-lived ΛX resonances FINUDA Λ Hints for a Λd bound state (low statistics) Λ Λd invariant mass Λ+d (K - ppn)Λ+d (K - ppn) m = (3243 ± 5) MeVm = (3243 ± 5) MeV Γ = (36.8 ± 10) MeVΓ = (36.8 ± 10) MeV Y = (5.4 ± 10) /K - stopY = (5.4 ± 10) /K - stop Unique feature in FINUDA: low deuterons can be spectroscopized Details in S. Piano’s talk

16 Invariant mass studies of di-baryon strange states with FINUDA X(K - pp)→Λp  - p p(Λ) ≈ 500 MeV/c p(p) ≈ 500 MeV/c p(  - ) ≈ 200 MeV/c short track!

17 (K - pp) system identification 1. Observation of a Λ angular correlation between the Λ and the decay proton cos θ YN < -0.8 The apparatus acceptance (  - momentum threshold) cuts Λ‘s slower than 300 MeV/c 6 MeV FWHM Events with a Λ-p coincidence: ~ 5% Light targets only (3x 12 C, 2x 6 Li, 1x 7 Li) The hyperon and the p should be oppositely emitted, apart from FSI inside the nucleus

18 fit region (Λp) invariant mass: observation of a bound state m K +2m p FINUDA Coll., PRL 94(2005) An enhancement is observed Two nucleon absorption K - + (pp)  Y* N   p m(K - 2p) = 2.37 GeV K - + (pp)  Y* N   p peak m(K - 2p) = 2.37 GeV… NOTHING! K - + (pp)   0 p   p K - + (pp)   0 p   p 74 MeV lower distribution, and broadened Kaon-nuclear bound state formation K - (pp)  X   p   0 p   p   0 p   p B = (stat) (sys) MeV  = (stat) (sys) MeV Yield 0.1%/stopped K - Yield ≈ 0.1%/stopped K - M = (2255 ± 9) MeV Acceptance correction ~ 100 events available so far Further details in the poster by H. Fujioka

19 Search for the K - pn system with FINUDA: first observations at low statistics K - pn   n  tagging and back-to-back  n selection  (1115)  (1197) K - pn Two kinds of K - pn states expected: Isobaric analog state of K - pp (pn: I=1) K - d (pn: I=0) (much less bound) K - pn   - p  - tagging and back-to- back  - p selection ~100 events

20 Summary: tri-baryon observations Tri-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 6 Li Signature of the K - ”d” → Σ - p reaction occurring on a quasi- deuteron substructure of 6 Li nucleus Absence of a similar signal in reactions on 12 C No need to invoke the presence of a DBK system to explain the observed signal Simple kinematic effect 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 analysis First observations by FINUDA for a bound state Details in the talk by S. Piano More data awaited in the next data taking

21 Summary: di-baryon observations Di-baryon strange system FINUDA can fully exploit the invariant mass analysis procedure A peak observed in the ( Λ p) 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 proton Could 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

22 Prospects for the new data taking New targets setup: mainly light elements chosen 2x 6 Li, 2x 7 Li, 2x 9 Be, 1x D 2 O, 1x 13 C All of them good to search for DBKS signatures 6 Li (  d) – 9 Be (  n) useful for tri-baryon system searches D 2 O: free d to compare with quasi-free d in 6 Li behavior 1 fb -1 to be collected in ~6-7 months data taking Possible to study DBKS existence and features as well as FSI contributions (from angular distributions) in single targets K - pp → Λ+ p4000 K - pn → Λ+ n1600 K - pn → Σ - + p1600 K - ppn → Λ+ d1200 Minimum number of expected events for a ≥ 1 fb -1 run (high resolution tracks) 8x if 3 point tracks are included ~ 20 times the statistics collected in the first data taking

23Conclusions 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 taking Significative improvement in the study of low- statistics reactions