Osamu Hashimoto Department of Physics Tohoku University APCTP Workshop on Strangeness Nuclear Physics (SNP'99) February 19-22, 1999 Reaction spectroscopy of  hypernuclei (1) Introduction (2) The  hypernuclear spectroscopy (3) Mass dependence of  binding energy (4) Light  hypernuclear spectra 12  C, 16  O, 13  C, 9  Be, 7  Li, 10  B (5) Future prospect and summary Seoul National University

Osamu Hashimoto Department of Physics Tohoku University APCTP Workshop on Strangeness Nuclear Physics (SNP'99) February 19-22, 1999 Reaction spectroscopy of  hypernuclei (1) Introduction (2) The  hypernuclear spectroscopy (3) Light  hypernuclear spectra 12  C, 16  O, 13  C, 9  Be, 7  Li, 10  B (4) Future prospect and summary Seoul National University

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 Excited states of  hypernuclei

 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

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

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

The SKS spectrometer Good energy resolution MeV FWHM Large solid angle msr –about 60 % of 12  C ground state yield Short flight path m –40 % kaon survival rate 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 Large momentum transfer Higher pion beam intensity compensates lower cross sections Pion beams are cleaner than kaon beams 1 GeV/c pion beam is required Characteristics

The (  +,K + ) experiments with the SKS spectrometer E140a (Hashimoto, Tohoku) –Systematic spectroscopy of  hypernuclei 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 E369 (Nagae,KEK) –Spectroscopy of 89  Y E419 (Tamura,Tohoku) –Gamma ray spectroscopy of 7  Li Weak decay of 209  Bi Outa  hypernuclei by the (  +,K + ) reaction Noumi

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.

Heavy  hypernuclei Three heavy targets with neutron closed shells Y 50 g 9/2 closed 2.2 MeV 1.7 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 The binding energies are not strongly dependent on the assumption KEK PS E140a KEK PS E369 Hypernuclear mass dependence of  -hyperon binding energies was derived with different assumptions

La & Pb Spectra

Background level in heavy spectra

Fitting by assuming ….

 binding energies

Heavy  hypernuclear spectra smoother than those of DWIA calculation Spreading of highest l neutron-hole states of the core nucleus Contribution of deeper neutron hole states of the core nucleus Other reaction processes not taken into account in the shell-model + DWIA calculation. Larger ls splitting ? E369 Nagae

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.

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 examined by 7  Li, 9  Be Momentum scale linearity MeV/c Energy resolution(FWHM)2.0 MeV for 12  C 1.5 MeV High quality spectra 2 MeV resolution and good statistics Absolute cross section and angular distribution

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

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 negative parity states 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 1.47 ± 0.05 #2(1 2 - ) 2.58 ± ± ± 0.03 #3(1 3 - ) 6.05 ± ± 0.03 #3’ 8.10 ± ± 0.03 #4(2 + ) ± ± ± 0.07 Angular distributions and absolute cross sections Intershell mixing --- positive parity state Motoba, Millener, Gal 6.89 ± 0.42 Statistical errors only

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

 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 (Nagae) > 2 times greater ? “Puzzle” Comparison of (K -,   ) and (  +,K + ) spectra provides information the splitting High quality spectra required

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

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 Kishimoto 98 at BNL ★ 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 ) = 1.32 ± 0.26 ± 0.7 MeV

9  Be ★ microscopic three-cluster model Yamada et.al. 9  Be =  + x +  x =  *  * = 3N + N ★ supersymmetric statesGal et.al. genuine hypernuclear statesBando et.al. (  +  ) 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

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 plays a crucial role –feasibility of experiments –expandability to coincidence experiments hypernuclear weak decay gamma ray spectroscopy

Future prospect From MeV to sub-MeV with high efficiency Wide variety of reactions –angular momentum transfer –spin-flip amplitude electromagnetic hyperon production (K,  ) at 1.1 GeV/c –proton or neutron to  hyperon photoproduction neutral meson detection New opportunities –(K -,  0 ) at BNL around 1 MeV Youn –(e,e’K + ) at Jlab 600 keV Hungerford –New (  +,K + ) a few 100 keV Noumi –Gamma-ray spectroscopy a few keV Tamura, Tanida 300 keV

Physics outline 12C spectrum reproduced, the core excited state at Ex=6.6 MeV was puzzling. 10B spectrum similarly favor strong spin singlet strength for the LN interaction 7Li and Be are typical L hypernuclei treated by cluster model. 7Li spectrum is consistent with the gamma ray data. It also show the strength for T=1 states. 9Be spectrum show the band of genuine L hypernuclear states. 8Be* core excited states are also observed with a distinct structure, whose position is not reproduced by the available cluster model. 13C spectrum shows clear shoulder structure at around Ex=10 MeV, which supposedly consists of 12C(0+)xp3/2 and 12C(1+)xs1/2, from which we may deduce the peak position for the p3/2 state. By combining the recent gamma ray data for p1/2, spin orbit splitting may be derived. Pik 16O spectrum can be compared with the CERN Kpi spectrum, from which we may conclude that the spin- orbit splitting is quite small.

 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 MeV hypernuclear reaction spectroscopy has matured to a level that allows quantitative investigation of their structure and  N interaction through the structure information. The (  ,K + ) reaction has established its value for hypernuclear spectroscopy since it favorably excites  hypernuclear bound states. Much better resolution and high detection efficiency are required for the  hypernuclear spectroscopy in the future. Sub-MeV reaction spectroscopy together with gamma-ray spectroscopy will further explore frontiers of strangeness nuclear physics.