Physics highlights from the FINUDA experiment

Transcription

1 Physics highlights from the FINUDA experiment Alessandro Feliciello I.N.F.N. - Sezione di Torino

2 Outline 2 1) Introduction 2) the experimental setup the DAΦNE machine the FINUDA apparatus 3) the strangeness nuclear physics program high-resolution spectroscopy of Λ-hypernuclei Λ-hypernucleus decay neutron-rich Λ-hypernuclei K + N charge exchange reaction (deeply) bound kaon-nuclear states

3 3

4 The DAΦNE machine

5 The DAΦNE Φ-factory 5

6 The DAΦNE e + e - collider 6 energy 510 MeV luminosity 5 x cm -2 s -1 σ x (rms) 2.11 mm σ y (rms) mm σ z (rms) bunch length crossing angle frequency (max) 35 mm 30 mm 12.5 mrad MHz e mrad Φ e mrad bunch/ring up to 120 p r Φ part./bunch current/ring 5.2 A (max)

7 Hypernuclear physics

8 What is a hypernucleus? 8 bound system of nucleons and one or more iperons M Λ = 1115,68 MeV M n = 939,57 MeV Λ(uds) n(udd) q Λ = 0 q n = 0 Λ is a fat n strangeness makes it distinguishable inner nuclear shell are not Pauli-blocked to Λ p n n p n p p Λ p n n p Λ p n n Λ p

9 Single Λ-hypernucleus production 9 A hypernucleus is the outcome of a genetic engineering manipulation applied to the nuclear physics domain 1) strangeness exchange (both in flight and at rest): K π + A + Z Z + A Z A Λ A Λ Z + π 2) associated strangeness production: + K + 3) electro-production : e A ' + + Z e + K + Λ A ( Z 1)

10 Single Λ-hypernucleus production two body reactions K - stop π + + n Λ + K γ(e) + p Λ( + e ) + K + K + N Λ + π + Λ recoil momentum magic momentum, no Λ recoil LNF Hyperon recoil momentum (MeV/c) Projectile momentum (GeV/c)

11 What one can do with a Φ-factory? 11 source of (nearly) monochromatic, collinear, background free, tagged neutral and charged kaons KLOE CP, CPT violation chiral dynamics and more... K S K L (34%) FINUDA hypernuclear physics K + K - (49%) Double Annular Φ-factory for Nice ρπ (13%) DEAR/SIDDHARTA exotic atoms Experiments

12 The FINUDA way L cm -2 s -1 σ = 3.26 μb 300 Hz 150 Hz 12

13 Open questions 13 (low-energy) YN interaction detailed knowledge of the hypernuclear fine structure evaluation of the spin dependent terms of the ΛN interaction measurement of angular distribution of γ rays determination of spin and parity of each observed level Impurity nuclear physics measurement of transition probability B(E2) information on the size and deformation of hypernuclei measurement of nucleus core shrinking glue-like role of Λ Properties of hyperons in nuclear matter (medium effect) measurement of transition probability B(M1) g-factor value for Λ in nuclear matter

14 One step beyond 14 stop 7 Li(K -,π - ) 12 7 K CLi Λ The FINUDA DAΦNE ΔE ~ 1.9 MeV FWHM ΔE ~ 2 kev FWHM H. Tamura, Nucl, Phis. A 691 (2001) 86c

15 The γ-ray spectroscopy domain The region of high excitation energy in heavy Λ-hypernuclei cannot be explored with γ-spectroscopy 17

16 The FINUDA apparatus nothing by chance

17 The FINUDA apparatus high resolution high acceptance magnetic spectrometer realistic (feasible) reasonable cost 21

18 Interaction-target region 23 beam pipe 500 μm Be tube double Si microstrip array (400 μm) Φ internal scintillator barrel (2.3 mm) target modules ( g/cm 2 ) kaon stop point in the target

19 Target design study π 9 Be target thickness: 1.8 mm 1 2 K - K - K - K boost

20 Target envelope by K - stopping points boost 6 Li 6 Li /2004 Φsetup boost 12 C 12 C Isim 1 Isim 8 Isim 2 Φ Isim 7 51 V 27 A l 7 Li 12 C /2007 setup

21 Tracking system + neutron detector 26 2 low-mass drift chamber array 8 m 3 He bag trajectories are measured in 4 points longitudinal and stereo straw tube arrays outer scintillator barrel (10.0 cm) neutron counter

22 Concept becomes reality may

23 FINUDA key features 29 very thin nuclear targets ( g/cm 2 ) high resolution spectroscopy coincidence measurement with large acceptance decay mode study irradiation of different targets in the same run systematic error reduction

24 Physics output (S = -1) 33 nuclear models spectroscopy neutron rich Λ-hypernuclei medium effect n p p n n p p n Λ p p n (weak) decay 4B weak interaction quark substructures low-energy N -Y interaction deeply bound K states

25 Spectroscopy of Λ-hypernuclei

26 The fight against background 35

27 The fight against background 36

28 37 comparison with: M. Agnello et al., HYP2006 KEK E336 esperiment: lower energy emulsion experiment: g.s. energy value within 1σ

29 38 (1.04 ± 0.12 ± 0.14) x 10-3 comparison with: H. Tamura et al., Nucl. Phys. A 754 (2005) 58c (γ-ray): excited states spin-flip amplitude 0 1/2 + 5/2 +

30 comparison with: KEK E336 esperiment: 2 signals vs. 8; first 2 in agreement emulsion experiment: higher energy 40

31 (0.37 ± 0.04 ± 0.05) x 10-3 comparison with: H. Tamura et al., Nucl. Phys. A 754 (2005) 58c (γ-ray): excited state compatible 41

32 comparison with: KEK E336 esperiment: similar situation emulsion experiment: g.s. within 2 σ 42

33 43 (0.62 ± 0.08 ± 0.08) x 10-3 comparison with: H. Kohri et al., Phys. Rev. C 65 (2002) (γ-ray): excited states compatible with peaks #2 and # 4

34 44 comparison with: KEK E336 esperiment: g.s. not compatible H. Tamura et al. Prog. Theor. Phys. Suppl. 117 (1994) 1 (stopped K - ): g.s. in agreement F. Cusanno et al., Phys. Rev. Lett. 103 (2009) (electroproduction): g.s. in agreement

35 45 16 O Λ (0.36 ± 0.06 ± 0.05) x N Λ comparison with: H. Tamura et al., Prog. Theor. Phys. Suppl. 117 (1994) 1 (stopped K - ): excited 6.3 MeV in agreement

36 Un unsatisfactory summary 46 particle (p) stable levels only formation probability: decreasing trend vs. A however: different measurements for the same g.s. formation probability give different value need for a coherent picture systematics on A is a helpful tool

37 Λ-hypernucleus decay

38 Λ-hypernucleus decay 48 free Λ decay hypernucleus mesonic decay hypernucleus non-mesonic decay p N ~ 100 MeV/c N W Λ π N Λ W π p F ~ 270 MeV/c p N ~ 400 MeV/c N Λ W S π N N Λ n + π MeV (36%) Λ p + π MeV (64%) τ Λ = 263 ps I = ½ rule (not understood) suppressed by Pauli blocking Λ + n n + n MeV Λ + p n + p MeV dominant in all but the lightest hypernuclei

39 Some notations 49 Γ tot = Γ MWD + Γ NMWD Γ MWD = Γ π0 + Γ π- (Λ n + π 0 )+(Λ p + π - ) Γ NMWD = Γ 1N + Γ 2N Γ 1N = Γ n + Γ p (Λn nn) + (Λp np) Γ 2N = Γ np + Γ pp + Γ nn (Λnp nnp)+(λpp npp)+(λnn nnn) Γ tot = Γ π0 + Γ π- + Γ n + Γ p + Γ np + Γ pp + Γ nn

40 Physics motivations & open questions Experimental observables τ, Γ π, π0 / Γ Λ, (single) particle decay spectra, Γ n, p, 2N / Γ Λ, Γ n /Γ p MWD - J π assignment: new indirect spectroscopic tool - ΛN interaction potential - π -nucleus optical potential NMWD - 4-baryon strangeness-changing weak interaction - ΔI=1/2 from s-shell ( 4 ΛH) and heavy hypernuclei - Γ n /Γ p puzzle (solved? systematics) - Γ 2N, FSI contributions KEK & LNF results

41 4 baryon weak interaction 51 The hypernucleus non-mesonic decay provides primary means of studying the baryon-baryon weak interaction N N W N-N scattering π ±, ρ, ω N S S = 0 only information on the parity violating part of weak interaction is accessible parity conserving part is masked by strong interaction N Λ + N N + N N Λ W π, ρ, η, ω, Κ, Κ in nuclear medium only both information on the parity violating and parity conserving parts of weak interaction can be extracted q ~ 400 MeV/c probes short distance N S S = 1 N = ½ rule applies also to non-mesonic weak decay? The role of explicit quark/gluon substructures can be put in evidence?

42 MWD & NMWD studies in FINUDA Coincidence measurement charged Mesonic channel charged Non-Mesonic channel K - stop + A Z A ΛZ + π - S-EX MeV/c π - A Λ Z A (Z+1) + π - MWD MeV/c K - stop + A Z A ΛZ+ π - A Λ Z A-2 (Z-1) + p + n NMWD MeV/c p π - π - 52

43 non-mesonic weak decay

44 NMWD: p LNF coincidence measurement: method 339 events 339 events 12 Λ C Proton energy spectrum from 12 ΛC p-induced NMWD before and after the acceptance correction Not acceptance Not corrected acceptance corrected Spectrum of negative pions for events in which a proton is detected in coincidence with a π - Asking for the proton coincidence a clear peak emerges at 272 MeV/c (ground state) Acceptance corrected 54

45 Coincidence measurement: FINUDA method background reaction: K - np Σ - p Coincidence spectra: 12 Σ - n π ΛC - coincidence normalization region π - Data NMWD p p simulation + reconstruction + selection + normalization subtraction M. Agnello et al., NPA 804 (2008)

46 5 Λ He 7 Λ Li 12 Λ C Similar shape for 5 ΛHe, 7 ΛLi and 12 ΛC Peak at ~ 80 MeV (Q/2 value), broadened by N Fermi motion, visible even for 12 ΛC no strong FSI effect in low energy region FSI & 2N contribution in the low energy region? 56

47 Comparisons with theory and KEK results 15 MeV threshold! 12 Λ C FINUDA NPA 804 (2008),151 Garbarino PRC 69 (2004), KEK E462/E508 PLB 597 (2004), 249 FINUDA NPA 804 (2008),151 5 Λ He FINUDA NPA 804 (2008),151 FINUDA NPA 804 (2008),151 KEK E462/E508 Garbarino PLB 597 (2004), 249 PRC 69 (2004),054603

48 Comparisons with theory and KEK results Comparison between FINUDA and KEK data: normalization beyond 35 MeV (KEK data threshold) Kolmogorov-Smirnov test: 75% compatibility for 5 ΛHe, 20% for 12 ΛC Comparison between FINUDA and theory: normalization beyond 15 MeV (FINUDA data threshold) Kolmogorov-Smirnov test: 80% compatibility for 5 ΛHe, 5% for 12 ΛC 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 80 MeV, no distortion Inputs of calculations

49 NMWD p spectra p-shell hypernuclei LNF FINUDA M.Agnello et al., PLB 685 (2010) 247 Background subtracted & acceptance corrected 59

50 NMWD: Γ 2N LNF FINUDA M.Agnello et al., PLB 685 (2010) 247 Γ 2N /Γ NMWD & Γ n /Γ p independent on A assumption W. Alberico and G. Garbarino, Phys. Rev. 369 (2002) 1. NMWD p gaussian fit free μ A low : spectrum area below μ 1N + 2N + FSI A low μ from fit A high 12 Λ C A high : spectrum area above μ 1N + FSI 2N (>70 MeV) ~ 5% 2N tot assumption G. Garbarino, A. Parreno and A. Ramos, Phys.Rev.Lett. 91 (2003) Phys.Rev. C 69 (2004)

51 NMWD: Γ 2N FSI & ΛNN contribution evaluation: systematics 61

52 NMWD: Γ 2N FSI & ΛNN contribution evaluation A low = 0.5 N p (Λp np) + N p (Λnp nnp) + N p FSI-low A high = 0.5 N p (Λp np) + N p FSI-high N p (Λnp nnp) N p (Λp np) = Γ np Γ p Γ 2 Γp assumption Γ np : Γ pp : Γ nn = 0.83 : 0.12 : 0.04 E. Bauer and G. Garbarino, Nucl. Phys. A 828 (2009) 29. R = A low = A low + A high 0.5 N p (Λp np) +N p (Λnp nnp) + N p FSI-low N p (Λp np) + N p (Λnp nnp) + N p FSI-low + N p FSI-high 62

53 FSI linear on A up to A=16 Γ 2 = Γ p [R(A) ba] [R(A) ba] R(A) = a + b A = = weighted mean Γ 2 /Γ p 1 + Γ 2 /Γ p + b A Assumption: Γ 2 /Γ 1 and Γ n /Γ p independent from A supported by exp and theory Γ 2 = Γ NM Γ 2 /Γ p = Γ n /Γ p Γ 2 /Γ p Bhang et al., EPJ A33 (2007) 259. Bauer et al., NPA 828 (2009) 29 Bhang et al., EPJ A33 (2007) 259: ~ ΛC M. Kim et al., PRL 103 (2009) : ΛC J.D.Parker et al., PRC 76 (2007), : 0.24 (95% CL) 4 ΛHe 63

54 NMWD: Γ 2N NMWD: n+p FINUDA γ prompt n detection efficiency ~10% 1/β>1.47 n + π - bound 1/β n energy resolution ~9% at 80 MeV TOF allows n/γ discrimination background prevails if no correlations or selections are imposed 7 MeV threshold 5 Λ He single n spectra shape cannot be directly analyzed due to background 64

55 Triple coincidence analysis Analysis of (π-,n,p) coincidence N n (cosθ - 0.8, E p < μ 20 MeV): 2N + FSI and small contribution of 1N N n = number of n in coincidence with (π-,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 μ value of the gaussian fits of the proton spectra from FINUDA Coll. and G. Garbarino, PLB 685 (2010)

56 NMWD: Γ 2N N n /N p R(A) = R(A) = N n (cos θ - 0.8, E p < μ-20 MeV) = N n (Λnp nnp) + N FSI N p (E p >μ p single spectra fit) 0.5 N p (Λp np) + N FSI a + b A = Γ 2 + b A Γ 0.5 Γ 2 /Γ p not dependent on A p weighted mean Γ 2 /Γ p = Γ 2 /Γ NM = % due to systematics A direct measurement error lowered by a factor 3 66

57 NMWD: Γ 2N Triple coincidence (n+n+p) FINUDA exclusive Λnp nnp 7 ΛLi 4 He+p+n+n decay event π p π = MeV/c E tot = MeV Q-value = 167 MeV p miss = MeV/c E(n1) = MeV E(n2) = 16.9 MeV E(p) = 51.0 MeV θ (n1 n2) = 95 θ(n1 p) = 102 θ(n2 p) = 154 no n-n or p/n scattering First direct experimental evidence of 2N-induced NMWD!!! 67

58 mesonic weak decay

59 MWD: p-shell hypernuclei MWD Pauli forbidden (p N ~100 MeV/c) theoretical calculations with pion distorted wave predict MWD to be less suppressed for p-shell (A~10) π 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: ω(q) = (q 2 +m π2 ) 1/2 energy conservation increases the final nucleon chance to come out above the Fermi surface Enhancement of MWD: Bando et al., Progr. Theor. Phys. Suppl. 72 (1984) 109 Oset et al., NPA 443 (1985) 704 Extensive calculations: Motoba et al., Progr. Theor. Phys. Suppl. 117 (1994) 477 Gal Nucl. Phys. A 828 (2009)

60 MWD & NMWD in FINUDA: strategy inclusive production π - spectra K - np background subctracted 11 Λ B MWD 12 Λ C 11 Λ B NMWD NMWD 11 Λ B 12 Λ C p kinetic energy (MeV) π - p decay π - and p spectra (Λ qf decay)/k - np background subtracted & acceptance corrected 70

61 J π assignment: 7 ΛLi Agnello PLB 681 (2009) Be: 3/2 - gs & 1/2 - (429keV) Correspondence with the calculated strenght functions T. Motoba et al, Progr. Theor. Phys. Suppl. 117 (1994) 477. A. Gal, Nucl. Phys. A 828 (2009) body decays Gal NPA 828 (2009) 72 Formation of different excited states of the daughter nucleus Initial hypernucleus spin J π ( 7 ΛLi g.s. ) = 1/2 + (Sasao, PLB 579 (2004) 258). T. Motoba (Private Communication)) 71

62 J π assignment: 9 ΛBe 9 B: 3/2 - gs & 1/2 - (2.75 MeV) FINUDA ΔT ~ 4 MeV MeV Agnello PLB 681 (2009) 139 kinetic energy (MeV) Correspondence with the calculated strenght functions T. Motoba et al, Progr. Theor. Phys. Suppl. 117 (1994) 477. A. Gal, Nucl. Phys. A 828 (2009) 72. Initial hypernucleus spin J π ( 9 ΛBe g.s. ) = 1/2 + O. Hashimoto NPA 639 (1998) 93c. T. Motoba (Private Communication)) 72

63 J π assignment: 11 ΛB 11 C: 3/2 - gs & 7/2 - (~6.5 MeV) Agnello PLB 681 (2009) 139 Correspondence with the calculated strenght functions H. Bando et al, Pers. Meson Science (1992) p.571 A. Gal, Nucl. Phys A 828 (2009) 72. Two contributions of the 11 C ground state 5/2 - and its 7/2 - excited state Initial hypernucleus spin J π ( 11 ΛB g.s. ) = 5/2 + : experimental confirmation (Sato et al., PRC 71 (2005) ) by different observable T. Motoba (Private Communication) 73

64 J π assignment: 15 ΛN Agnello PLB 681 (2009) O: 1/2 - gs & sd (~6 MeV) Correspondence with the calculated strenght functions T. Motoba et al, Nucl. Phys. A 489 (1988) 683. A. Gal, Nucl. Phys. A 828 (2009) Λ N g.s spin not known. J π ( 15 ΛN g.s. ) = 3/2 + D.J.Millener, A.Gal, C.B.Dover Phys. Rev. C 31 (1985) 499. Spin ordering not obtained from γ-rays of 16 ΛO M.Ukai et al. Phys. Rev.C 77 (2008) First experimental determination of J π ( 15 ΛN g.s. ) = 3/2 + from decay rate value (and spectrum shape) T. Motoba NPA 489 (1988)

65 Mesonic decay ratio: Γ π- / Γ Λ Γ π- / Γ Λ = Γ tot / Γ Λ BR π Γ tot / Γ Λ = ( ) + ( ) A fit from measured values for A=4-12 hypernuclei present data T. Motoba PTPS 117 (1994) 477 previous data A.Gal NPA 828 (2009) 72 strong nuclear structure effects A 75

66 Search for Κ nuclear bound states

67 Κ nuclear bound states 77 few nucleon clusters gathered together by a K deeply bound? nuclear density + K 1.39 / fm 3 medium effect hadron s mass and properties change (partial) chiral symmetry restoration astrophysical interest condensed strange nuclear matter constituent of neutron star cores A.Doté et al., Phys. Rev. C 70 (2004)

68 K nuclear bound states: theoretical debate 78 deep or shallow do they K-nucleus exist? potential? deep: strong B MeV small Γ MeV KN scattering lengths energy level shift and width of kaonic hydrogen (x-ray measurements) Λ(1405) binding energy and width shallow: weak B MeV large Γ MeV density dependent: strong B MeV small Γ 50 MeV

69 K nucleus bound states: theoretical expectations The KN (I=0) strong interaction stabilizes the nuclear matter attracting the surrounding nucleons Simpler system (strange dibaryon): K - pp ( 2 KH) the presence of the K attracts the two unbound protons to form a bound state with B=48 MeV and Γ= 61 MeV The binding energy increases with the increase of the number of I=0 pairs, and the decrease of I=1 ones

70 Search methods for deeply bound K-states Invariant mass spectroscopy Based on the kaonic nuclear states feature of decaying into hyperons (K - pp) Λ + p (K - ppn) Λ + d Typically: p Λ,p ~ 500 MeV/c p π < 200 MeV/c p decay p ~ 500 MeV/c Necessary to fully reconstruct all the particles emitted in the decay The decay occurs at rest: angular correlation between the emitted particles DAΦNE Missing mass spectroscopy Measurement of the momentum of the monochromatic recoiling particle in a A(K,N)X reaction With stopped K - : _ KEK-PS E471, E549 DAΦNE With in flight K - : BNL-AGS E930 KEK-PS E548

71 82 KEK-PS-E471 evidence for strange tribaryons I = 1 B = 193 MeV Γ 21 MeV I = 1 B = 169 MeV Γ 21 MeV

72 83 KEK-PS-E471 evidence for strange tribaryons

73 FINUDA search for B=2 kaon-nuclear states - 1st step:direct observation of a Λ Invariant mass spectrum for a proton and a negative pion system LONG TRACKS, higher resolution the acceptance of the apparatus cuts the Λ s with momentum less than 300 MeV/c, due to the momentum threshold for π - SHORT TRACKS, larger acceptance Less resolution but higher statistics

74 K - pp Λp ppπ - candidate event π p p π p μ + Κ Φ Κ + μ + 86

75 FINUDA search for B=2 kaon-nuclear states - 2nd step: back-to-back Λp events When a kaon interacts with two nucleons and a hyperon-nucleon pair (Λp, Σ 0 p, Σ + n) is produced, they are expected to be emitted in opposite directions, ignoring a F.S.I. inside the nucleus About the 5% of events in FINUDA have a (Λp) coincidence Seen indeed, on every FINUDA target! (be it light or heavy: two nucleon absorption on the nucleus surface?) Event selection: cosθ YN < -0.8

76 FINUDA search for B=2 kaon-nuclear states 89 6 Li 7 Li 12 C acceptance corrected (2m p + m k ) threshold M. Agnello et al., Phys. Rev. Lett. 94 (2005)

77 FINUDA search for B=3 kaon-nuclear states reconstructed by T.O.F. PID by OSIM 90

78 FINUDA search for B=3 kaon-nuclear states - 1st step:direct observation of a Λ Invariant mass spectrum for a proton and a negative pion system in coincidence with a deuteron

79 FINUDA search for B=3 kaon-nuclear states 6 Li ± 14.1 M. Agnello et al., Phys. Lett. B 654 (2007)

80 FINUDA search for B=3 kaon-nuclear states - 2nd step: angular correlation n K - p n n p n p p π - Λ p K - n p n p 93

81 FINUDA search for B=3 kaon-nuclear states 12 C 95

82 FINUDA search for B=4 kaon-nuclear states 96 FINUDA thresholds: Λ: 140 MeV/c t: 430 MeV/c - particle identification -1st step:direct observation of a Λ (in coincidence with t) Direct measurement of K - absorption on 4 He Only one measurement exists, from bubble chamber: 3 events by kin fit 40 events observed in FINUDA

83 K -6 Li Λt + X candidate event 97

84 FINUDA search for B=4 kaon-nuclear states - 2nd step: angular correlation 98 real data MC data Λ, t pairs emitted back-to-back M. Agnello et al., Phys. Lett. B 669 (2008) 229

85 Study of A(K -,Λt)X (A= 6 Li, 7 Li, 9 Be) a) b) K - A Σ 0 t A Σ 0 Λγ d) direct reactions e) 2-steps processes Many body K - absorption role Simulations of different phase space reactions with cos(θ Λt )<-0.9 (filtered through apparatus acceptance) Λ and t momentum distribution compatible with: a) 4-nucleon absorption with (Λt) or (Σt) emission b) 4-nucleon absorption with (Λt)N emission c) NOT with (Λt)π: too small Λ momentum d) 2-step pickup reaction (suppressed?)

86 The FINUDA Collaboration 101 I.N.F.N. Bari and Bari University Brescia University KEK L.N.F. / I.N.F.N. Frascati I.N.F.N. Pavia and Pavia University RIKEN Seoul National University Teheran Shahid Beheshty University I.N.F.N. Torino and Torino University Torino Polytechnic Trieste University and I.N.F.N. Trieste TRIUMF

87 A paradigmatic example of collaboration 103

88 Thank you! 有り難う