The SKS Spectrometer and Spectroscopy of Light Hypernuclei

(E336 and E369)

KEK PS Review

December 4-5, 2000

Osamu HashimotoTohoku University

Outline

• Motivation

• Some history

• The SKS spectrometer

• E336 experiment – Light hypernuclear spectroscopy for 7

Li, 9Be,( 10

B,) 12C, 13

C, 16O

• E369 experiment– 12

C 1.5 MeV resolution spectrum

– 89Y high quality spectrum

Significance ofhypernuclear 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 interactionFrom limited hyperon scattering data

(Meson exchange model: Nijmegen, Julich)

YN, YY effective interaction in finite nuclei(YN G potential)

Hypernuclear properties

Energy levels, splittingsCross sectionsPolarizations

Weak decay widths

iiFiFii

N

rkckba

rv

)/exp()(

)(222

Excited states of hypernuclei

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

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

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) stat

es, 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 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

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 spectrometerand

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 MeVSpin dependent int.(Light hypernuclei) < 0.1-1 MeV

(2) hypernuclei (K-,-),(,K+) wide N ---> N

a few MeV for 4He, Coulomb assisted states

(3) hypernuclei (K-,K+) 5-10 MeV or narrower( 1 MeV ?)

N --->

The SKS spectrometer

• Good energy resolution --- 2 MeV FWHM• Large solid angle --- 100 msr• Short flight path --- 5 m• 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

Brief history of hypernuclear physics experiments

with the SKS spectrometer

• 1985 2,4 Workshop on nuclear physics using GeV/c pions• 1985. 6 Proposal #140 submitted• 1985.10 Workshop on physics with a medium-resolution

spectrometer in GeV region• 1985.10 E150 approved

– Study of hypernuclei via ( +,K+) reaction with a conventional magnet ---> PIK SPECTROMETER

• 1987. 4 Construction budget of the SKS approved ( INS )• 1989. 3 Proposal #140 conditionally approved as “E140a”

– Study of hypernuclei via ( +,K+) reaction with a large-acceptance superconducting kaon spectrometer

• 1991. 9 The SKS magnet successfully excited to 3 Tesla in the North Experimental Hall

• 1992. 3 Proposal #269 approved• 1992.11 E269 data taking• 1993. 2 - E140a data taking• 1993 10 E278 data taking• 1995. 1-11 E307 data taking• 1995.11-2 E352 data taking• 1996. 4-10 E336 data taking• 1997.11-2 E369 data taking• 1998.5-7 E419 data taking• 1999. 10-12 E438 data taking• 2000. 11-12 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 89Y

• 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 5He

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

Heavy hypernuclei

• Three heavy targets with neutron closed shells

8939Y50 g9/2 closed 2.2 MeV

13957La82 h11/2 closed 2.3 MeV

20882Pb126 i13/2 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

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.

La & Pb Spectra

Fitting by assuming ….

Background level in heavy spectra

Heavy hypernuclear spectrasmoother 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 spectrasmoother 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 16O

states observed by 3 different reactions

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

22+(p3/2

-1 x p1/2,3/2)

02+(p3/2

-1 x p1/2,3/2)

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

HN(Core) 　　 : Core nucleus

t : kinetic energy

vN : effective N interaction

( Nijmegen, Julich ... )

H = HN(Core) + t + vN

E336 Summary

Pion beam : 3 x 106/1012ppp at 1.05 GeV/c

Spectrometer : SKS improved from E140aBetter 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

Momentum scale linearity +- 0.06 MeV/c

Energy resolution(FWHM) 2.0 MeV for 12C

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 positive parity states

• Intershell mixing• 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 1.46 ± 0.05#2(12

-) 2.58 ± 0.17 2.70 ± 0.13 0.25 ± 0.03#3(13

-) 6.22 ± 0.18 0.24 ± 0.03#3’ 8.31 ± 0.38 0.16 ± 0.03#4(2+) 10.68 ± 0.12 10.97 ± 0.05 1.80 ± 0.07

Angular distributions and absolute cross sections

6.89 ± 0.42

Statistical errors only

E369 spectrum best resolution 1.45 MeV

12C spectra by SKS

E336

2 MeV(FWHM)

1.45 MeV(FWHM)

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

Angular distributionof the 12C(+,K+)12

C reaction

E336

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 (E369)

• > 2 times greater ?

“Puzzle”

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

High quality spectra required

Recent hypernuclear ray spectroscopy

Small ls splitting in 13C, 9

Be observed

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

Angular distributionof the 13C(+,K+)13

C reaction

E336

Angular distributionof the 16O(+,K+)16

reaction

E336

13C

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

#2 [12C(2+,0) x s1/2]3/2+ 4.81 ± 0.09

#3 [12C(0+,0) x p3/2]3/2- 9.59 ± 0.24 ± 0.5*

#4 [12C(1+,0) x s1/2]1/22+ 11.52 ± 0.20 ± 0.5*

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

#5 [12C(2+,0) x p1/2]5/22- 15.24 ± 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 E929 at BNL (Kishimoto)

★ 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) = 1.36 ± 0.26 ± 0.7 MeV Ex(1/2-) = 10.98 ± 0.03 MeVEx(3/2-) = 10.83 ± 0.03 MeVE = 0.152 ± 0.054 ± 0.036 MeV

E929 at BNLKishimoto et. al.

Excitation spectrumof the 16O(+,K+)16

reaction

E336

9Be

★ microscopic three-cluster modelYamada et.al.

9Be = + x +

x = ** = 3N + N

★ supersymmetric states Gal et.al.(’76) genuine hypernuclear states Bando 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 spectrumof the 13C(+,K+)13

C reaction

E336

Cluster states of 9Be

SupersymmetricGenuine hypernuclear states

T.Motoba, Il Nuovo Cim. 102A (1989) 345.

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 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 : 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

• 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.