The SKS Spectrometer and Spectroscopy of Light  Hypernuclei (E336 and E369) KEK PS Review December 4-5, 2000 Osamu Hashimoto Tohoku University

Outline Motivation Some history The SKS spectrometer E336 experiment –Light  hypernuclear spectroscopy for 7  Li, 9  Be,( 10  B,) 12  C, 13  C, 16  O E369 experiment – 12  C 1.5 MeV resolution spectrum – 89  Y high quality spectrum

Significance of hypernuclear investigation A new degree of freedom –Deeply bound states –Baryon structure in nuclear medium –New forms of matter H dibaryon... New structure of hadronic many-body system with strangeness –Nucleus with a new quantum number –Characteristic structure –Electromagnetic properties Hyperon-nucleon interaction(B-B interaction) –A valuable tool hyperon scattering experiments limited –Potential depth, shell spacing, spin-dependent interaction Weak interaction in nuclear medium –Weak decay processes Nonmesonic decay Decay widths, polarization

Hypernuclear bound states

YN, YY Interactions and Hypernuclear Structure Free YN, YY interaction From limited hyperon scattering data (Meson exchange model: Nijmegen, Julich) YN, YY effective interaction in finite nuclei (YN G potential) Hypernuclear properties Energy levels, splittings Cross sections Polarizations Weak decay widths

Excited states of  hypernuclei n or p  n p  BB BpBp BnBn 208  Pb 207  Tl 207  Pb Weak decay nonmesonic mesonic  Narrow widths < a few 100 keV Likar,Rosina,Povh Bando, Motoba, Yamamoto

 hypernuclear spectroscopy Narrow widths of nucleon-hole  -particle states –less than a few 100 keV  N interaction weaker than NN  N spin-spin interaction weak  isospin = 0 No exchange term A  hyperon free from the Pauli exclusion principle Smaller perturbation to the core nuclear system  hypernuclear structure vs.  N interaction Precision spectroscopy required

Issues of  hypernuclear physics Single particle nature of a  hyperon in nuclear medium New forms of hadronic many-body systems with strangeness –core excited states, genuine(supersymmetric) states, clustering structure,…. YN and YY interactions –central, spin-spin, spin-orbit, tesor Hyperon weak decay in nuclear medium –Lifetimes as a function of hypernuclear mass –Nonmesonic weak decay  n/  p ratios,  I=1/2 rule

S=-1 hyperon production reactions for  hypernuclear spectroscopy  Z = 0  Z = -1 comment neutron to  proton to  (  +,K + ) (  -,K 0 ) stretched, high spin in-flight (K -,  - ) in-flight (K -,  0 ) substitutional at low momentum stopped (K -,  - ) stopped (K -,  0 ) large yield, via atomic states virtual ( ,K) spin flip, unnatural parity (p,p’K 0 ) (p,p’K + ) virtual ( ,K) (p,K + ) (p,K 0 ) very large momentum transfer (e,e’K 0 ) (e,e’K + )

(  +,K ＋ ) Cross section vs. momentum transfer for some hypernuclear production reactions Stopped (K -,  ) ( ,K ＋ ) (p,K ＋ ) Inflight(K -,  ) Hypernuclear Cross section Momentum transfer (MeV/c) mb/sr nb/sr  b/sr

Elementary cross section of the (  +,K + ) reaction

Comparison of the (  +,K + ) and (K -,  - ) reaction

The (  +,K + ) spectroscopy Large momentum transfer –angular momentum stretched states are favorably populated –neutron-hole  -particle states are excited Higher pion beam intensity compensates lower cross sections –10  b/sr for (  +,K + ) vs 1 mb/sr for (K -,  - ) Pion beams are cleaner than kaon beams 1 GeV/c pion beam is required For the spectroscopy a good resolution  beam spectrometer and a good-resolution and large-solid angle spectrometer

Required Resolution Good resolution 1-2 MeV High resolution a few 100 keV (1)  hypernuclei (K -,  - ),( ,K + ),(e.e’K + ),… Major shell spacing( Heavy hypernuclei)~ 1 MeV Spin dependent int.(Light hypernuclei)< MeV (2)  hypernuclei (K -,  - ),( ,K + )  wide  N --->  N a few MeV for 4  He, Coulomb assisted states (3)  hypernuclei (K -,K + )  5-10 MeV or narrower( 1 MeV ?)  N ---> 

The SKS spectrometer Good energy resolution MeV FWHM Large solid angle msr Short flight path m Efficient kaon identification Optimized for the (  +,K + ) spectroscopy Large superconducting dipole at KEK 12 GeV PS The performance of the SKS spectrometer was demonstrated by the 12  C excitation spectrum

Brief history of hypernuclear physics experiments with the SKS spectrometer ,4 Workshop on nuclear physics using GeV/c pions Proposal #140 submitted Workshop on physics with a medium-resolution spectrometer in GeV region E150 approved –Study of  hypernuclei via (  +,K + ) reaction with a conventional magnet ---> PIK SPECTROMETER Construction budget of the SKS approved ( INS ) Proposal #140 conditionally approved as “E140a” –Study of  hypernuclei via (  +,K + ) reaction with a large- acceptance superconducting kaon spectrometer The SKS magnet successfully excited to 3 Tesla in the North Experimental Hall Proposal #269 approved E269 data taking E140a data taking E278 data taking E307 data taking E352 data taking E336 data taking E369 data taking E419 data taking E438 data taking E462 data taking

KEK PS Experiments with the SKS spectrometer E140a (Hashimoto, Tohoku) –Systematic spectroscopy of  hypernuclei E269(Sakaguti, Kyoto) –Pion elastic scattering in 1 GeV/c region E278 (Kishimoto, Osaka) –Nonmesonic weak decay of polarized 5  He E307 (Bhang, Seoul) –Lifetimes and weak decay widths of light and medium-heavy  hypernuclei E336 (Hashimoto,Tohoku) –Spectroscopic investigation of light  hypernuclei E352 (Peterson, Colorado) –Pion-nucleus scattering above the  resonance E369 (Nagae,KEK) –Spectroscopy of 89  Y E419 (Tamura,Tohoku) –Gamma ray spectroscopy of 7  Li E438 (Noumi,KEK) –Study of  N potential in the (pi-,K+) reactions E462 (Outa, KEK) –Weak widths in the decay of 5  He

Pion beam : 3 x 10 6 /10 12 ppp at 1.05 GeV/c Yield rate : events/g/cm 2 /10 9 pions for 12  C gr ( ～ events/day ) E140a 10 B, 12 C, 28 Si, 89 Y, 139 La, 208 Pb 2 MeV resolution, heavy  hypernuclei E336 7 Li, 9 Be, 12 C, 13 C, 16 O high statistics, angular distribution absolute cross section E C, 89 Y best resolution(1.5 MeV), high statistics Absolute energy scale MeV at B  ( 12  C ) = 10.8 MeV examined by 7  Li, 9  Be Momentum scale linearity MeV/c Energy resolution(FWHM)2.0 MeV for 12  C 1.5 MeV Summary of  hypernuclear spectra obtained with the SKS spectrometer

Heavy  hypernuclei Three heavy targets with neutron closed shells Y 50 g 9/2 closed 2.2 MeV La 82 h 11/2 closed 2.3 MeV Pb 126 i 13/2 closed 2.2 MeV Background as low as 0.01  b/sr/MeV KEK PS E140a Hypernuclear mass dependence of  -hyperon binding energies were derived taking into account major and sub-major hole states

Absolute energy scale M HY -M A = -B  + B n - M n +M   M HY ～  p  /   -  p K /  K (1)  M HY adjusted so that B  ( 12  C) = 10.8 MeV (2) Energy loss corrected for  + and K + in the target ±0.1 MeV +  B  ( 12  C) Binding energies of 7  Li, 9  Be ground states are consistent with the emulsion data well within ±0.5 MeV.

La & Pb Spectra

Fitting by assuming ….

Background level in heavy spectra

Heavy  hypernuclear spectra smoother than those of DWIA calculation  binding energies are derived taking into account #1 and #2. (1) Spreading of highest l neutron-hole states of the core nucleus (2) Contribution of deeper neutron hole states of the core nucleus (3) Other reaction processes not taken into account in the shell-model + DWIA calculation. (4) Larger ls splitting ?

 binding energies

Heavy  hypernuclear spectra smoother than those of DWIA calculation 1.Spreading of highest l neutron-hole states of the core nucleus 2. Contribution of deeper neutron hole states of the core nucleus 3. Other reaction processes not taken into account in the shell-model + DWIA calculation. 4. Larger ls splitting ? E369  binding energies are derived taking into account #1 and #2.

Comparison of excitation energies of 16  O states observed by 3 different reactions (p 1/2 -1 x  s 1/2 ) (p 3/2 -1 x  s 1/ (p 1/2 -1 x  p 3/ (p 1/2 -1 x  p 1/ (p 3/2 -1 x  p 1/2,3/2) (p 3/2 -1 x  p 1/2,3/2)

Light  hypernuclei Playground for investigating  hypernuclear structure and LN interaction Recent progress in shell-model calculations and cluster-model calculations prompt us to relate the structure information and interaction, particularly spin-dependent part.

Hypernuclear Hamiltonian H N (Core) : Core nucleus t  :   kinetic energy v  N : effective  N interaction ( Nijmegen, Julich... ) H = H N (Core) + t  +  v  N

E336 Summary Pion beam : 3 x 10 6 /10 12 ppp at 1.05 GeV/c Spectrometer : SKS improved from E140a Better tracking capability with new drift chambers Targets : 7 Li1.5 g/cm 2 (99%,Metal) 440 G  + 9 Be1.85 g/cm 2 (metal) 434 G  + 13 C1.5 g/cm 2 (99% enriched,powder) 362 G  + 16 O1.5 g/cm 2 (water) 593 G  + 12 C1.8 g/cm 2 (graphite) 313 G  + Absolute energy scale MeV at B  ( 12  C ) = 10.8 MeV Momentum scale linearity MeV/c Energy resolution(FWHM)2.0 MeV for 12  C

12  C The (1 3 - ) state at 6.9 MeV is located higher than the corresponding 12 C excited state. The nature of the state is under discussion –  N spin-spin interaction – Mixing of other positive parity states Intershell mixing The width of the p-orbital is peak broader –consistent with ls splitting E140a spectrum E336 spectrum times better statistics consistent with E140a spectrum Example of a good resolution spectroscopy Core-excited states clearly observed Phys. Rev. Lett. 53(‘94)1245 Peak # E140a E336(Preliminary) Ex(MeV) Ex(MeV) Cross section( )(  b) #1(1 1 - ) 0 0 MeV 1.46 ± 0.05 #2(1 2 - ) 2.58 ± ± ± 0.03 #3(1 3 - ) 6.22 ± ± 0.03 #3’ 8.31 ± ± 0.03 #4(2 + ) ± ± ± 0.07 Angular distributions and absolute cross sections 6.89 ± 0.42 Statistical errors only E369 spectrum best resolution 1.45 MeV

12  C spectra by SKS E336 2 MeV(FWHM) 1.45 MeV(FWHM)

11 C vs 12  C /2 - 3/ /2 - 1/2 - 3/ / / C 12  C (1 - 2 ) (1 - 3 ) (2 + )? C x s  11 C x p  MeV

Angular distribution of the 12 C(  +, K + ) 12  C reaction E336

 Hypernuclear spin-orbit splitting Very small widely believed V  SO = 2±1MeV –CERN data Comparison of 12  C, 16  O spectra  E(p3/2-p1/2) < 0.3 MeV –BNL data Angular distribution of 13 C (K-,  -) 13  C  E (p3/2-p1/2) = MeV Larger splitting ? recent analysis – 16  O emulsion data analysis ( Dalitz, Davis, Motoba)  E(p3/2-p1/2) ～ E(2+) - E(0+) = 1.56 ± 0.09 MeV –SKS(  +,K + ) data new 89  Y spectrum (E369) > 2 times greater ? “Puzzle” Comparison of (K -,   ) and (  +,K + ) spectra provides information the splitting High quality spectra required Recent hypernuclear  ray spectroscopy Small ls splitting in 13  C, 9  Be observed

16  O :p 1/2 -1 x  s 1/ :p 3/2 -1 x  s 1/ :p 1/2 -1 x  p 3/ :p 1/2 -1 x  p 1/2 In-flight (K -,  - ) CERN populated Stopped (K -,  - ) and populated ★ SKY at KEK-PS ★ Emulsion new analysis Dalitz et.al. K O →  - + p + 15  N E(2 1 + ) - E(0 1 + ) = ± 0.09 MeV ? (  +,K + ) SKS 4 distinct peaks populated ls partner

Angular distribution of the 13 C(  +, K + ) 13  C reaction E336

Angular distribution of the 16 O(  +, K + ) 16   reaction E336

13  C #1[ 12 C(0 +,0) x  s 1/2 ]1/ #2 [ 12 C(2 +,0) x  s 1/2 ]3/ ± 0.09 #3 [ 12 C(0 +,0) x  p 3/2 ]3/ ± 0.24 ± 0.5* #4 [ 12 C(1 +,0) x  s 1/2 ]1/ ± 0.20 ± 0.5* [ 12 C(1 +,1) x  s 1/2 ]1/2 4 + #5 [ 12 C(2 +,0) x  p 1/2 ]5/ ± 0.08 [ 12 C(2 +,1) x  s 1/2 ]3/2 4 + ★ p 1/2 → s 1/2  observed by the (K -,  - ) reaction E(  p 1/2 ) = ±0.1±0.2 MeV M. May et.al. Phys. Rev. Lett. 78(1997) ★ p 3/2,1/2 → s 1/2  ray measurement E929 at BNL ( Kishimoto) ★ The (  +,K + ) reaction excites the p 3/2 state [ 12 C(1 + ) x  s 1/2 ]1/2 + near the 3/2 - peak [ 12 C(0 + ) x  p 3/2 ]3/2 - [ 12 C(0 + ) x  p 1/2 ]1/2 - ls partner *A systematical error considering possible contamination from the #4(1/2 2 +) peak is quoted. Peak # configuration E x (MeV) [ 12 C(J c ,T c ) x  lj]J  n  E = E(  p 1/2 ) - E(  p 1/2 ) = 1.36 ± 0.26 ± 0.7 MeV E x (1/2 - ) = ± 0.03 MeV E x (3/2 - ) = ± 0.03 MeV  E = ± ± MeV E929 at BNL Kishimoto et. al.

Excitation spectrum of the 16 O(  +, K + ) 16   reaction E336

9  Be ★ microscopic three-cluster model Yamada et.al. 9  Be =  + x +  x =  *  * = 3N + N ★ supersymmetric statesGal et.al.(’76) genuine hypernuclear statesBando et.al.(’86) (  +  ) x p 1 -,3 -,... Cluster excitation taken into account ★ microscopic variational method with all the rearrangement channels Kamimura, Hiyama A typical cluster  hypernucleus The present spectrum compared with Yamada’s calculation BNL spectrum (1) The genuinely hypernuclear states,1 -, 3 - identified (2) Higher excitation region shows structure not consistent with the calculated spectrum

Excitation spectrum of the 13 C(  +, K + ) 13  C reaction E336

Cluster states of 9  Be Supersymmetric Genuine hypernuclear states T.Motoba, Il Nuovo Cim. 102A (1989) 345.

7  Li  + d +  3 He + t +  5  He + p + n Cluster model approach Shell model approach Richter et.al. Bando et.al. Kamimura,Hiyama T=1 states around B  = 0 MeV strength observed Ground : [ 6 Li(1 + ) x s 1/2 ] 1/2 + First excited : [ 6 Li(3 + ) x s 1/2 ] 5/2 + E2  transition 5/2 + →1/2 + : 2.03 MeV

What did we learn from MeV hypernuclear reaction spectroscopy ? Improvement of the resolution, even if it is small, has a great value –3 MeV → 2 MeV → 1.5 MeV Hypernuclear yield rate also plays a crucial role –feasibility of experiments –expandability to coincidence experiments hypernuclear weak decay gamma ray spectroscopy

 spin-orbit splitting from the width of 12  C 2 + peak p  peak assumed to be “equal strength doublet” & 2 MeV resolution –splitting : MeV consistent with the emulsion result(Dalitz) – MeV |2 1 + > ～ 11 C(3/2 - ) x |  p 3/2> (97.8%) |2 2 + > ～ 11 C(3/2 - ) x |  p 1/2> (99.0%)

Summary The value of good-resolution (  +,K + ) spectroscopy has been demonstrated with the use of a large acceptance superconducting kaon spectrometer.(SKS) Taking the advantage of the (  +,K + ) reaction that selectively excites bound  hypernuclear states,  single-particle binding energies are derived up to 208  Pb.(E140a) Light  hypernuclear spectroscopy has been extensively performed for p-shell  hypernuclei and compared with theoretical calculations based on shell and cluster models..(E336) High quality hypernuclear structure information plays an important role in the investigation of the  N interaction, particularly spin dependent part. High quality hypernuclear spectroscopy was carry out for 89  Y. Splittings of  major shell orbitals were observed and is under discussion in terms of spin-orbit splitting and/or structural effect.(E369) SKS serves also as an efficient tagger of  hypernuclear production and has been intensively used for coincidence measurements of weak and gamma decay processes.