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

This talk provides an overview of experimental analysis methods and evidence related to the study of tri-baryon and di-baryon strange systems with FINUDA. The talk also discusses the experimental observations, theoretical developments, and potential future prospects in the field.

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

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  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

  3. Introduction • General understanding of KN (KA) interaction • Small binding energies: 10-30 MeV • Large decay widths: 80-100 MeV • Recent theoretical developments (Akaishi-Yamazaki) • Existence of states with the kaon deeply bound (50-100 MeV) to few nucleons, andnarrow 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: • pΛ,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. Missing mass observations of a possible tri-baryon signal: KEK-PS E471 KEK-PS E471 miss. mass method Suzuki et al, Phys. Lett. B597 (2004) 263 • 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 4He(K-stop, n) S+(3140) 4He(K-stop, p) experimental signature of AY expectations? A few inconsistencies… S0(3115) proton momentum (MeV/c) KEK-PS E471 miss. mass method, Suzuki et al, NPA 754 (2005) 375

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

  7. Study of inclusive proton momentum spectra in reactions induced by K- on 6Li and 12C targets No 4He target in FINUDA but6Li 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 Study of tri-baryion strange aggregates with FINUDA: missing mass analysis 6Li 12C

  8. Semi-inclusive proton spectra: coincidence with a - 6Li 12 C • 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 • Peak of the K-n→Λ -reaction evident • Very strong correlation for back-to-back selected events • Less evident correlations on 12C

  9. Impact parameters distributions for coincidence events EXPERIMENTAL DATA SIMULATIONS K-+d p+S- p p - - • The p and the high momentum- are produced in two different vertices events with p- > 275 MeV/c p - • 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) • Where does the Σ-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? Impact parameter (cm) Impact parameter (cm)

  10. 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 b) K-+”4He” S0+p and S0 Lnn e) K- +”4He” S0+p and S0 S-d • Angular distributions (opening angle between p and -) • Observed a back-to-back correlation • 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+S- c) K-+”4He” S0+p and S0 S-np d) K-+”4He” S0+p and S0 S-np and S-p Ln conversion

  11. K- interaction on “quasi” deuteron in 6Li or 4He? T. Yamazaki, to be published • The momentum distribution of a deuteron bound in a 6Li nucleus has two peaks: • Sharp peak at ~50 MeV/c • Broad peak at ~ 300 MeV/c d in 6Li • The momentum of a deuteron in a 4He 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 d inside the α in 6Li • The momentum of the deuteron in 12C has two peaks again • Lower at ~100 MeV/c • May explain why the absorption reaction is not observed in 12C targets • 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)

  12. 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 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

  13. Newest observations of a tri-baryon strange state with the invariant mass method: Λd system Λd invariant mass • FOPI - GSI • Ni+Ni @ 1.93 AGeV • Use of invariant mass spectroscopy to search for short-lived ΛX resonances • Λ+d (K-ppn) • m = (3159 ± 20) MeV • Γ= (100 ± 50) MeV • FINUDA • Hints for a Λd bound state (low statistics) • Λ+d (K-ppn) • m = (3243 ± 5) MeV • Γ= (36.8 ± 10) MeV • Y = (5.4 ± 10) 10-3/K-stop • Unique feature in FINUDA: low deuterons can be spectroscopized • Details in S. Piano’s talk

  14. 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!

  15. (K-pp) system identification • The apparatus acceptance (- momentum threshold) cuts Λ‘s slowerthan 300 MeV/c • 6 MeV FWHM • 1. Observation of a Λ 2 1 • 2. angular correlation between the Λand the decay proton cosθYN < -0.8 • 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

  16. (Λp) invariant mass: observation of a bound state fit region FINUDA Coll., PRL 94(2005)212303 • An enhancement is observed • Two nucleon absorption • K- + (pp)  Y* N  p peak expected @ m(K-2p) = 2.37 GeV… NOTHING! • K- + (pp)  0p   p 74 MeV lower distribution, and broadened • Kaon-nuclear bound state formation • K- (pp)  X  p  0p   p mK+2mp Acceptance correction M = (2255 ± 9) MeV B = 115+6-5 (stat)+3-4 (sys) MeV = 67+14-11(stat)+2-3 (sys) MeV Yield ≈0.1%/stopped K- ~ 100 events available so far Further details in the poster by H. Fujioka

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

  18. 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 6Li • Signature of the K-”d” →Σ- p reaction occurring on a quasi-deuteron substructure of 6Li nucleus • Absence 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 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

  19. Summary: di-baryon observations • Di-baryon strange system • FINUDA 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 Λand 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

  20. Conclusions • 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

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