Osamu Hashimoto

Department of PhysicsTohoku 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

12C, 16

O,13C, 9

Be, 7Li, 10

B(5) Future prospect and summary

Seoul National University

Osamu Hashimoto

Department of PhysicsTohoku 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

12C, 16

O,13C, 9

Be, 7Li, 10

B(4) Future prospect and summary

Seoul National University

n or p

n

p

B

Bp

Bn

208Pb

207Tl

207Pb

Weak decay nonmesonic mesonic

Narrow widths< a few 100 keVLikar,Rosina,PovhBando, 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 structurevs.

N interaction

Precision spectroscopy required

S=-1 hyperon production reactionsfor hypernuclear spectroscopy

Z = 0 Z = -1 commentneutron to proton to

(+,K+) (-,K0) 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’K0) (p,p’K+) virtual (,K)

(p,K+) (p,K0) very large momentum transfer

(e,e’K0) (e,e’K+)

(+,K)

Cross section vs. momentum transferfor some hypernuclear production reactions

Stopped (K-,)

(,K)

(p,K)

　Inflight(K-,)

Hy p

ernu

c le a

r C

r oss

sec

tio n

Momentum transfer (MeV/c)

mb/sr

nb/sr

b/sr

0 500 1000

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 --- 2 MeV FWHM• Large solid angle --- 100 msr

– about 60 % of 12C ground state yield

• Short flight path --- 5 m– 40 % kaon survival rate

• Efficient kaon identification

Optimized for the (+,K+) spectroscopy

Large superconducting dipoleat 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 5He

• 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 89Y

• E419 (Tamura,Tohoku)

– Gamma ray spectroscopy of 7Li

Weak decay of 209Bi Outa

hypernuclei by the (+,K+) reaction Noumi

Absolute energy scale

MHY-MA = -B + Bn - Mn+M

MHY ～ p/ -pK/K

(1) MHY adjusted so that B(12

C) = 10.8 MeV

(2) Energy loss corrected for + and K+ in the target

±0.1 MeV + B(12C)

Binding energies of 7Li, 9

Be ground states are

consistent with the emulsion data well within ±0.5 MeV.

Heavy hypernuclei

• Three heavy targets with neutron closed shells

8939Y50 g9/2 closed 2.2 MeV

1.7 MeV139

57La82 h11/2 closed 2.3 MeV

20882Pb126 i13/2 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 E140aKEK 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 spectrasmoother 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 106/1012ppp at 1.05 GeV/c

Spectrometer : SKS improved from E140a Better tracking capability with new drift chambers

Targets :7Li 1.5 g/cm2(99%,Metal) 440 G+

9Be 1.85 g/cm2(metal) 434 G+

13C 1.5 g/cm2(99% enriched,powder) 362 G+

16O 1.5 g/cm2(water) 593 G+

12C 1.8 g/cm2(graphite) 313 G+

Absolute energy scale +- 0.1 MeV at B(12

C ) = 10.8 MeV examined by 7

Li, 9Be

Momentum scale linearity +- 0.06 MeV/c

Energy resolution(FWHM) 2.0 MeV for 12C

1.5 MeV

High quality spectra 2 MeV resolution and good statistics

Absolute cross section and angular distribution

Pion beam : 3 x 106/1012ppp at 1.05 GeV/cYield rate : 5 - 8 events/g/cm2/109 pions for 12

Cgr

( ～ 5 - 800 events/day )

E140a 　　　　　 10B, 12C, 28Si, 89Y, 139La, 208Pb

2 MeV resolution, heavy hypernucleiE336 7Li, 9Be, 12C, 13C, 16O

high statistics, angular distributionabsolute cross section

E369 12C, 89Ybest resolution(1.5 MeV), high statistics

Absolute energy scale +- 0.1 MeV at B(12

C ) = 10.8 MeV examined by 7

Li, 9Be

Momentum scale linearity +- 0.06 MeV/c

Energy resolution(FWHM) 2.0 MeV for 12C

1.5 MeV

Summary of hypernuclear spectra obtained with the SKS spectrometer

12C

• The (13-) state at 6.9 MeV is located higher than the

corresponding 12C 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 --- 5-10 times better statistics consistent with E140a spectrum

Example of a good resolution spectroscopyCore-excited states clearly observed

Phys. Rev. Lett. 53(‘94)1245

Peak # E140a E336(Preliminary) Ex(MeV) Ex(MeV) Cross section(20-140)(b)#1(11

-) 0 0 MeV 1.47 ± 0.05#2(12

-) 2.58 ± 0.17 2.71 ± 0.13 0.23 ± 0.03#3(13

-) 6.05 ± 0.18 0.22 ± 0.03#3’ 8.10 ± 0.38 0.17 ± 0.03#4(2+) 10.68 ± 0.12 10.97 ± 0.05 1.76 ± 0.07

Angular distributions and absolute cross sections

Intershell mixing --- positive parity stateMotoba, Millener, Gal

6.89 ± 0.42

Statistical errors only

11C vs 12C

6.48

4.80

4.32

2.00

0.00

7/2-

3/2-2

5/2-

1/2-

3/2-1

6.905/2+

6.341/2+

0.00

2.71

6.05

8.10

10.97

11C 12C

1-1

(1-2)

(1-3)

(2+)?

2+11C x s11C x p

MeV

MeV

Hypernuclear spin-orbit splitting

• Very small ----- widely believed VSO = 2±1MeV

– CERN data Comparison of 12C, 16

O spectra

• E(p3/2-p1/2) < 0.3 MeV

– BNL data Angular distribution of 13C (K-,-) 13C

• E (p3/2-p1/2) = 0.36 +- 0.3MeV

• 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 89Y spectrum (Nagae)

• > 2 times greater ?

“Puzzle”

Comparison of (K-,) and (+,K+) spectraprovides information the splitting

High quality spectra required

16O

11- : p1/2

-1 x s1/2

12- : p3/2

-1 x s1/2

21+ : p1/2

-1 x p3/2

01+ : p1/2

-1 x p1/2

In-flight (K-,-) CERN01

+ populated

Stopped (K-,-) 21

+ and 01+ populated

★ 　 SKY at KEK-PS★ Emulsion new analysis Dalitz et.al. 　　　 K- + 16O → - + p + 15

N 　　　 E(21

+) - E(01+) = 1.56 ± 0.09 MeV ?

(+,K+) SKS4 distinct peaks21

+ populated

ls partner

13C

#1 [12C(0+,0) x s1/2]1/21+ 0

#2 [12C(2+,0) x s1/2]3/2+ 4.87 ± 0.09

#3 [12C(0+,0) x p3/2]3/2- 9.63 ± 0.24 ± 0.5*

#4 [12C(1+,0) x s1/2]1/22+ 11.58 ± 0.20 ± 0.5*

[12C(1+,1) x s1/2]1/24+

#5 [12C(2+,0) x p1/2]5/22- 15.43 ± 0.08

[12C(2+,1) x s1/2]3/24+

★ p1/2 → s1/2 observed by the (K-,-) reaction

E(p1/2) = 10.95 ±0.1±0.2 MeV

M. May et.al. Phys. Rev. Lett. 78(1997)★ p3/2,1/2 → s1/2 ray measurement Kishimoto 98 at BNL

★ The (+,K+) reaction excites the p3/2 state

[12C(1+) x s1/2]1/2+ near the 3/2- peak

[12C(0+) x p3/2]3/2-

[12C(0+) x p1/2]1/2-ls partner

*A systematical error considering possible contamination from the #4(1/22

+) peak is quoted.

Peak # configuration Ex(MeV)[12C(Jc

,Tc) x lj]Jn

E = E(p1/2) - E(p1/2) = 1.32 ± 0.26 ± 0.7 MeV

9Be

★ microscopic three-cluster modelYamada et.al.

9Be = + x +

x = ** = 3N + N

★ supersymmetric states Gal et.al. genuine hypernuclear states Bando 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

7Li

+ d + 3He + 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 MeVstrength observed

Ground : [6Li(1+) x s1/2] 1/2+

First excited : [6Li(3+) x s1/2] 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 1--3- 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 : 1.2 +- 0.5 MeV

• consistent with the emulsion result(Dalitz)– 0.75 +- 0.1 MeV

|21+> ～ 11C(3/2-) x |p 3/2> (97.8%)

|22+> ～ 11C(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.