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Weak Decay of Hypernuclei (Experimental status) Stefania Bufalino

Workshop on Strangeness in Nuclei ECT*, 4-8 October, 2010. Weak Decay of Hypernuclei (Experimental status) Stefania Bufalino INFN-Torino on behalf of the FINUDA Collaboration. Talk Outline Hypernuclear weak decay (why?) Hypernuclear decay studies in FINUDA Mesonic weak decay (MWD)

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Weak Decay of Hypernuclei (Experimental status) Stefania Bufalino

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  1. Workshop on Strangeness in Nuclei ECT*, 4-8 October, 2010 Weak Decay of Hypernuclei(Experimental status)StefaniaBufalino INFN-Torinoon behalf of the FINUDA Collaboration

  2. Talk Outline • Hypernuclear weak decay (why?) • Hypernuclear decay studies in FINUDA • Mesonic weak decay (MWD) • Non-Mesonic weak decay (NMWD) • Perspectives

  3. Physics motivations & open questions • MWD - Jpassignment: new indirect spectroscopic tool -LN interaction potential -p--nucleus optical potential • NMWD - 4-baryon strangeness-changing weak interaction • - DI=1/2 from s-shell (4LH) and heavy hypernuclei - Gn/Gppuzzle (solved ? … systematics) - G2N, FSI contributions

  4. FINUDA @ DAFNE e+ + e- f (1020)  K+ + K- (127 MeV/c) K-stop + AZ  ALZ + p- Detector capabilities: Selective triggerbased on fast scintillation detectors (TOFINO, TOFONE) CleanK-vertexidentification (ISIM P.ID.+x,y,zresolution + K+ tagging) , K, p, d, … P.I.D.(OSIM&LMDsdE/dx, TOF) High momentumresolution (6‰ FWHM for - @270 MeV/c for spectroscopy) (1% FWHM for p- @270 MeV/c for decay study) (6% FWHM for - @110 MeV/c for decay study) (2% FWHM for p @400 MeV/c for decay study) (tracker resolution + He bag + thin targets) Solid angle ~ 2srad Solenoid: B=1T 2003 data taking: 190 pb-1 (2x6Li, 7Li, 3x12C, 27Al, 51V) 2006 data taking: 966 pb-1 (2x6Li, 2x7Li, 2x9Be, 13C, D2O) 10 pb-1/day

  5. p - MWD & NMWD studies in FINUDA Coincidence measurement chargedMesonicchannel chargedNon-Mesonicchannel K-stop + AZ  ALZ + p- ALZA(Z+1) +p- K-stop + AZ  ALZ+p- ALZA-2(Z-1) + p + n S-EX 260-280 MeV/c MWD 80-110 MeV/c NMWD 170-600 MeV/c - -

  6. MWD:p-shell hypernuclei • MWD Pauli forbidden (pN~100 MeV/c) • theoretical calculations with pion distorted wave predict MWD to be less suppressed for p-shell (A~10) • p feels attraction in nuclear medium due to the p-wave part of the optical potential  dispersion relation modified inside the nucleus  pion carries lower • energy for fixed momentum q: w(q) = (q2+mp2)1/2  Energy conservation increases the final nucleon chance to come out above the Fermi surface • Enhancementof MWD: • Bando et al., Progr. Theor. Phys. Suppl. 72 (1984) 109 • Osetet al., NPA 443 (1985) 704 • Extensivecalculations: • Motoba et al., Progr. Theor. Phys. Suppl. 117 (1994) 477 • Gal Nucl. Phys. A 828 (2009) 72.

  7. MWD & NMWD in FINUDA: strategy Inclusive production p-spectra K-np background corrected 11LB 12LC 12LC p NMWD kinetic energy (MeV) NMWD MWD 11LB 11LB decayp-and pspectra (Lqfdecay)/K-np background subtracted & acceptancecorrected p p-

  8. Jpassignment: 7LLi Agnello PLB 681 (2009) 139 • Correspondencewith the calculatedstrenghtfunctions • T. Motobaet al, Progr. Theor. Phys. Suppl. 117 (1994) 477. • A. Gal, Nucl. Phys. A 828 (2009) 72. • Formationofdifferentexcitedstatesof the daughternucleus • Initialhypernucleusspin • Jπ(7LLig.s.) = 1/2+ (Sasao, PLB 579 (2004) 258. 7Be: 3/2-gs & 1/2- (429keV) 3-body decays Gal NPA 828 (2009) 72 T. Motoba (Private Communication))

  9. Jpassignment: 9LBe 9B: 3/2-gs & 1/2-(2.75 MeV) FINUDA DT ~ 4 MeV FWHM @38 MeV • Correspondencewith the calculatedstrenghtfunctions • T. Motobaet al, Progr. Theor. Phys. Suppl. 117 (1994) 477. • A. Gal, Nucl. Phys. A 828 (2009) 72. • Initialhypernucleusspin • Jπ(9LBeg.s.) = 1/2+ • O.Hashimoto NPA 639 (1998) 93c. Agnello PLB 681 (2009) 139 T. Motoba (Private Communication))

  10. Jpassignment: 11LB 11C: 3/2-gs & 7/2- (~6.5 MeV) Agnello PLB 681 (2009) 139 • Correspondencewith the calculatedstrenghtfunctions • H. Bando et al, Pers. Meson Science (1992) p.571 • A. Gal, Nucl. Phys A 828 (2009) 72. • Twocontributionsof the 11C ground state5/2- and its7/2-excited state • Initialhypernucleusspin • Jπ(11ΛBg.s.) = 5/2+: experimentalconfirmation • (Sato et al., PRC 71 (2005) 025203)bydifferentobservable T. Motoba (Private Communication)

  11. Jpassignment: 15LN Agnello PLB 681 (2009) 139 15O: 1/2-gs & sd(~6 MeV) • Correspondencewith the calculatedstrenghtfunctions • T. Motobaet al, Nucl. Phys. A 489 (1988) 683. • A. Gal, Nucl. Phys. A 828 (2009) 72. • 15ΛNg.sspinnotknown. Jπ(15ΛNg.s.) = 3/2+ • D.J.Millener, A.Gal, C.B.DoverPhys. Rev. C 31 (1985) 499. • Spinorderingnotobtainedfromg-raysof16LO M.Ukaiet al. Phys. Rev.C 77 (2008) 054315. • First experimentaldeterminationof • Jπ(15ΛNg.s.) = 3/2+ fromdecay rate value (and spectrumshape) T. MotobaNPA 489 (1988) 683.

  12. Mesonicdecayratio: Gp-/ GL Gp- / GL = Gtot/ GLBRp- Gtot/GL= (0.990±0.094) + (0.018±0.010) A present data T. Motoba PTPS 117 (1994) 477 previous data fit from measured values for A=4-12 hypernuclei A.Gal NPA 828 (2009) 72 p distortion, MWD enhancement proved ! strong nuclear structure effects A

  13. NMWD:p spectra @ LNF • coincidence measurement: method • Spectrum of negative pions for events in which a proton is detected in coincidence with ap- • Asking for the proton coincidence a clear peak emerges at 272 MeV/c (ground state) 12LC Proton energy spectrum from 12LCp-induced NMWD before and after the acceptance correction Not acceptance corrected Not acceptance corrected 339 events 339 events Acceptance corrected

  14. Coincidence measurement: FINUDA method background reaction: K-npS-p S-nπ- coincidence Coincidence spectra: 12LC p- p simulation + reconstruction + selection + normalization Data normalization region NMWD p subtraction M. Agnello et al., NPA 804 (2008), 151

  15. 5LHe 7LLi 12LC • Similar shape for 5LHe, 7LLi and 12LC • Peak at ~ 80 MeV (Q/2 value), broadened by N Fermi motion, • visible even for 12LC  no strong FSI effect in low energy region • FSI & 2N contribution in the low energy region ?

  16. Comparisons with theory and KEK results Garbarino PRC 69 (2004),054603 12LC 15 MeVthreshold ! FINUDA NPA 804 (2008),151 KEK E462/E508 PLB 597 (2004), 249 FINUDA NPA 804 (2008),151 5LHe FINUDA NPA 804 (2008),151 FINUDA NPA 804 (2008),151 KEK E462/E508 PLB 597 (2004), 249 Garbarino PRC 69 (2004),054603

  17. Comparisons with theory and KEK results • Comparison between FINUDA and KEK data: normalization beyond 35 MeV (KEK data threshold) • compatibility for 5LHe, not for 12LC • Comparison between FINUDA and theory: normalization beyond 15 MeV (FINUDA data threshold) • compatibility for 5LHe, not for 12LC Strong disagreement between experiments and with theory • KEK: thick targets  strong correction FINUDA: thin targets & transparent detectors • KEK: p energy from TOF and range + dE/dx poor energy resolution above 100 MeV, distortion FINUDA: p momentum from magnetic analysis, 2% energy resolution FWHM @ 80 MeV, no distortion • Inputs of calculations

  18. NMWD p spectra p-shell hypernuclei LNF FINUDA – M.Agnello et al., PLB 685 (2010) 247 Background subtracted & acceptance corrected

  19. NMWD: G2N LNF – FINUDA M.Agnello et al., PLB 685 (2010) 247 NMWD p gaussian fit free m 12LC mfrom fit Alow Ahigh Alow: spectrum area belowm 1N + 2N + FSI W.Alberico and G.Garbarino, Phys. Rev. 369 (2002) 1. assumption Ahigh: spectrum area abovem 1N + FSI 2N(>70 MeV) ~ 5% 2Ntot G,Garbarino, A.Parreno and A.Ramos, Phys.Rev.Lett. 91 (2003) 112501. Phys.Rev. C 69 (2004) 054603. assumption G2N/GNMWD & Gn/Gp independent on A

  20. NMWD: G2N FSI & LNN contribution evaluation: systematics

  21. NMWD: G2N FSI & LNN contribution evaluation Alow = 0.5 N(Lpnp) + N(Lnpnnp) + NpFSI-low Ahigh = 0.5 N(Lpnp) + NpFSI-high assumption Alow N(Lnpnnp) Gnp G2 0.5 = ≈ Alow + Ahigh Gp N(Lpnp) Gnp : Gpp:Gnn= 0.83 : 0.12 : 0.04 E. Bauer and G.Garbarino, Nucl.Phys. A 828 (2009), 29. Gp N(Lnpnnp) + NpFSI-low N(Lpnp) + R = = N(Lnpnnp) + NpFSI-low + NpFSI-high N(Lpnp) +

  22. FSI linear on A up to A=16 systematics: all p-shell G2 [R(A) – bA] - 0.5 = 0.43 ± 0.25 weighted mean = Gp 1 – [R(A) – bA] Assumption: G2/G1andGn/Gpindipendent from A supported by exp and theory 0.5 +G2/Gp + b A R(A) = a + b A = 1 +G2/Gp G2 G2/Gp = 0.24 ± 0.10 = Bauer et al., NPA 828 (2009) 29 Bhang et al., EPJ A33 (2007) 259: ~ 0.4 12LC M. Kim et al., PRL 103 (2009) 182502: 0.29 ± 0.13 12LC J.D.Parker et al., PRC 76 (2007), 035501: ≤ 0.24 (95% CL) 4LHe GNM Gn/Gp+ 1 + G2/Gp Bhang et al., EPJ A33 (2007) 259.

  23. NMWD: G2N NMWD: n+p coincidence @ FINUDA n detection efficiency ~10% nenergyresolution ~9% at 80 MeV TOF allows n/gdiscrimination background prevailsif no correlations or selections are imposed gprompt 1/b>1.47 1/b 7MeVthreshold n+ p-bound 5LHe single nspectrashapecannotbe directlyanalyzed due to background

  24. Triple coincidenceanalysis • Analysis of (p-,n,p) coincidence • Nn(cosθ≥- 0.8, Ep< m-20 MeV): 2N + FSI and small contribution of 1N • Nn = number of n in coincidence with (p-,p) • Number of neutrons for all targets (from A=5 to A=16) • No spectra shape analysis (20 events for each target) • Background study (events from K-np absorption) • Acceptance correction • Normalization to the number of protons with energy greater than the m value of the gaussian fits • of the proton spectra from FINUDA Coll. and G. Garbarino, PLB 685 (2010) 247

  25. NMWD: G2N systematics: all p-shell Nn/Np R(A) = weighted mean G2 Nn(cos ≥- 0.8, Ep<m-20 MeV) G2/Gp = 0.36±0.07 + b A G2/Gp not dependent on A R(A) = a + b A = = 0.5 Gp A Np(Ep>mp single spectra fit) G2/GNM = 0.20±0.03 8% due to systematics N(Lnpnnp) + NFSI • directmeasurement • error lowered by a factor 3 0.5 N(Lpnp) + NFSI

  26. NMWD: G2N Triple coincidence (n+n+p) events @ FINUDA exclusive Lnpnnp7LLi4He+p+n+n decay event pp- = 276.93 MeV/c Etot = 178.3 MeV Q-value = 167 MeV p miss = 216.6 MeV/c E(n1) = 110.2 MeV E(n2) = 16.9 MeV E(p) = 51.0 MeV q (n1 n2) = 95° (n1 p) = 102° (n2 p) = 154° no n-n or p/nscattering p- First direct experimental evidence of 2N-induced NMWD !!

  27. Perspectives • MWD spectra, Jp • NMWD: spectra Gn/Gppuzzle systematics, G2N and FSI contribution J-PARC

  28. THANK YOU!

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