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Introduction to Hypernuclear Physics

K. Tanida (RIKEN)

CNS summer school, Aug. 21, 2002

- Outline:
- What is hypernucleus?
- BB interaction and structure of hypernuclei
- Hyperons in nuclei
- Weak decay of hypernuclei
- Results from recent experiments
- Future prospects

He

S

What is hypernucleus?- Normal nucleus -- composed of nucleon (proton, neutron)
- At the quark level: p=(uud), n=(udd)
- There are six quark flavors in nature:
- L=(uds), S+(uus), X0=(uss), ... exist Hyperons
- Hypernucleus: not only nucleons but hyperons
- (i.e., quarks other than u and d)
- Known hypernuclei: strangeness (s) only.
- L-hypernuclei (~50 species)
- S-hypernucleus ( only)
- LL-hypernuclei (a few events)

u c t

d s b

How to produce?

- Bring strangeness somehow into nuclei
- Stopped K- method
- - traditional method
- - K-(`us)meson has strangeness
- - 100% reaction, about 10% makes hypernuclei as
- hyperfragments in A ~ 14 targets. Dirty.
- In-flight (K-,p-), (K-,p0) reactions
- - elementary process NK Lp
- - small momentum transfer (can be 0)
- - large cross section
- (p+,K+) reaction
- - relatively new method, production of`ss pair
- - large momentum transfer (q > 350 MeV/c)
- - small cross section, but intense p beam available
- Other methods
- - (e,e'K+), heavy ion collision, ...

Baryon-Baryon interaction and structure of hypernuclei

- GOAL: unified understanding of NN, YN and YY interactions
- Flavor SU(3) symmetry (symmetry in u, d, s quarks)
- NN interaction -- experimentally well known from elastic
- scattering data
- phenomenologically well reproduced by meson-exchange
- and quark-cluster models.
- YN, YY interaction -- poor scattering data
- low yield, short lifetime (ct < 10 cm)
- information from hypernculei is important
- (mostly L-hypernuclei LN interaction)
- In L-hypernuclei: No Pauli effect, weak coupling
- simpler structure
- extraction of LN interation is rather straightforward

Some features of LN interaction (1)

- One pion exchange is forbidden

N

L

p(I=1)

L(I=0)

N

- Violates isospin symmetry
- weakness of LN interaction

e.g., no two body bound state

weak tensor force

- short range interaction

heavier mesons (K, h, w, s, ...), quark-gluon picture

Some features of LN interaction (2)

- Two types of spin-orbit force
- i.e.,
- VL(r) sL・LLN ‥‥ L-spin dependent
- VN(r) sN・LLN ‥‥ N-spin dependent
- or
- Vs(r) (sL+sN)・LLN ‥‥ symmetric (SLS)
- Va(r) (sL-sN)・LLN ‥‥ anti-symmetric (ALS)
- In np, ALS breaks charge symmetry (~1/1000 of SLS)
- Does not vanish even at flavor SU(3) limit
- (c.f., SN(I=3/2) channel ALS=0 at SU(3) limit)
- Towards understanding of the source of LS force
- -- vector meson exchange? (ALS < SLS)
- -- quark-gluon picture? (ALS ~ SLS, VL ~ 0)

Overall binding energy of hypernuclei

- from A=3 to 208
- UL ~ 28 MeV ~ 2/3 UN
- well reproduces data
- weakness of LN
- interaction
- Single particle picture
- good (later in detail)

(D. J. Millener et al., PRC38 (1988) 2700)

4

5

5

4

H

He

He

H

He

L

L

L

L

L

overbinding problem of

5

He

L

Light hypernuclei (1) -- overbinding problem- Binding energy of hypernuclei, A=3~5
- : BL = 0.13 ±0.05 MeV
- : BL = 2.04 ± 0.04 MeV (ground state, 0+)
- 1.00 ± 0.06 MeV (excited state, 1+)
- : BL = 2.39 ± 0.03 MeV (0+)
- 1.24 ± 0.06 MeV (1+)
- : BL = 3.12 ± 0.03 MeV
- If we use LN interaction which reproduces A=3,4 binding
- energies, overbinds by ~1 MeV in calculations

- First pointed out by Dalitz et al. in 1972 （NPB47 109）,
- but not solved for nearly 30 years.

Solution to the overbinding problem? (1)

- Quark Pauli effect?

quark level

baryon level

L

p

n

u

s

d

⇒

⇒

no pauli blocking

partial Pauli blocking

- Is this significant? seemingly no
- Large baryon size is required to solve the problem
- (H. Nemura et al., PTP 101 (1999) 981, Y. Suzuki et al., PTP 102 (1999) 203)

He

L

Solution to the overbinding problem? (2)- LNN three body force?

- Similar to Fujita-Miyazawa 3NF
- Maybe stronger
- ML-MS ~ 80 MeV ~ 1/4(MD-MN)
- L(T=0) S(T=1)
- a must excite to T=1 state (Ex > 30 MeV)
- less significant in

p

S

p

NLN

- Sorry, reality is not so simple, but this is promising.
- For details, see recent papers, e.g.,
- Y. Akaishi et al., PRL84 (2000) 3539.
- H. Nemura et al., nucl-th/0203013

4

He

H

L

L

Light hypernuclei (2) -- charge symmetry breaking- L has no charge, no isospin
- difference of Lp and Ln interaction is CSB.
- L in is more strongly bound than by 0.35 ± 0.05 MeV
- Coulomb force correction makes the difference larger!
- After Coulomb force correction, this difference is ~5 times
- larger than in 3H -- 3He case
- The reason is not yet understood, possiblities include
- - L/S0mixing in free space p0 exchange force (tensor)
- - LN-SN coupling via mass difference of S+, S0, S- (~8 MeV)
- three-body force as well as two body force.
- - K0 and K± mass difference (~1%), also in K*
- - r/wmixing spin-orbit
- These are strongly spin dependent
- spin/state dependence is important

Spin-dependence of LN interaction

- No experimental data so far from scattering experiments
- (analysis of KEK-PS E452 is ongoing)
- All information is from hypernuclei
- Data are mostly for light (s- and p-shell) hypernuclei
- Spin dependent terms LN effective potential in hypernuclei
- Vs(r) sL・sN ‥‥ spin-spin
- VL(r) sL・LLN ‥‥ spin-orbit (L-spin dependent)
- VN(r) sN・LLN ‥‥ spin-orbit (N-spin dependent)
- VT(r){3(sL・r)(sN・r)/r2- sL・sN} ‥‥ tensor
- In p-shell hypernuclei, we usually take
- D = ∫f*LN(r)Vs(r)fLN(r) dr
- and regard it as a paramter. (fLN is almost the same over p-shell)
- Similarily, SL, SN, and T are defined from VL, VN, VT.

How to get?

J+1/2

DE

J(=0)

A

Z

Z

J-1/2

A+1

L

Z

Z

- L is in s state state splits into two
- Spatial wavefunctions are the same
- DE is determined only by LN spin-dependent interaction.
- Examples in pure single-particle limit
- p3/2-shell(7Li, 9Be, 11B+L): DE = 2/3D + 4/3SL- 8/5T
- p1/2-shell(13C,15N+L): DE = -1/3D + 4/3SL + 8T
- (more detailed calculation: see D. J. Millener et al. PRC31 (1985) 499)
- DE is usually small -- we need high resolution measurement
- experimental data appear later in this talk.

LL interaction

- Unique channel in SU(3) BB interaction classification
- Repulsive core may vanish in this channel
- possibile existense of H-dibaryon (uuddss, J=I=0)
- Original prediction by Jaffe (PRL38 (1977) 195)
- - H is 80 MeV bound from LL
- No experimental evidence so far
- - at least, deeply bound H is rejected
- LL - XN (- SS) coupling important (DE = 28 MeV)
- LL interaction study performed by
- - LL hypernuclei (example later in this talk)
- - LL final state interaction in (K-,K+) reaction
- (J. K. Ahn et al., PLB444 (1998) 267 )
- Present data suggests LL interaction is weakly attractive

Li

L

Hyperons in nuclei- A hyperon behaves as an impurity in nuclei
- May change some properties of nuclei,
- - size, shape, collective motion, ...
- Theoretical prediction:
- - A L makes a loosely-bound light nuclei, such as 6Li, smaller
- glue-like role (Motoba et al., PTP70 (1983) 189)

a

a

+

d

d

L

L

6Li

- Recent experiment gives evidence for such shrinkage
- later in this talk
- Other properties are also interesting, but no experimental data

Test of single-particle states at the center of nucleus

- Hyperons are free from Pauli blocking
- -can stay at the center of nucleus (especially for L)
- - is a good probe for depth of nucleus
- KEK-PS E369 observed
- clear and narrow peaks for
- sL and pL states of
- (H. Hotchi et al.,
- PRC64 (2001) 044302)
- There are single-
- particle states in center
- of nuclei

89

Y

L

pL

sL

magnetic moment

- Good observable to see hyperon (L) property in nuclear matter.
- - is it changed from free space? If so, how?
- Meson current
- S mixing?
- partial quark deconfinment?
- Everyone wants to measure, but no one ever did!
- - lifetime too short (~ 200 ps)
- spin precession angle ~1degfor 1T magnetic field
- Alternative (indirect) measurement:
- B(M1) \ (gcore - gL)2 (planned in KEK-PS E518)

Weak decay of hypernuclei

- In free space...
- L p + p-(63.9%, Q = 38 MeV)
- n + p0 (35.8%, Q = 41 MeV)
- DI=1/2 rule holds well.
- - initial state: I=0, final state: I=1/2 or 3/2
- if If = 1/2, branch is 2:1
- 3/2, 1:2
- - this rule is global in strangeness decay, but no one knows why
- This decay (called mesonic decay) is suppressed in hypernuclei
- due to Pauli blocking for the final state nucleon.
- Instead, non-mesonic decay occurs in hypernuclei, such as
- p + L p + n,
- n + L n + n, ....

Mesonic decay

- Dominant only in very light hypernuclei (A<6)
- Well described by (phase space)*(Pauli effect)*(p distortion)

p- decay partial width

free L

- Exp. data from
- H.Outa et al., NPA639
- (1998) 251c
- V. J. Zeps et al., NPA639
- (1998) 261c
- Y. Sato, Doctor thesis
- (Tohoku Univ., 1998)

Lifetime

- Almost constant for A > 10 -- non-mesonic decay dominant
- short range nature of nonmesonic decay

- exp. data from
- H. Park et al.,PRC61
- (2000) 054004
- H. Outa et al., NPA639
- (1998) 251c
- V. J. Zeps et al., NPA639
- (1998) 261c
- J. J. Szymanski et al.,
- PRC43 (1991) 849
- R. Grace et al., PRL55
- (1985) 1055

Gn/Gp puzzle

- Simplest diagram for non-mesonic weak decay
- -- one pion exchange

- Virtual mesonic decay
- + absorbsion
- This model predicts
- Gn (nLnn) << Gp(pLpn)
- - 3S1 3D1 tensor coupling
- has the largest amplitude,
- but this is forbidden for
- (nn) final state.

N

N

p

Weak

Strong

L

N

- However, experimental data indicate
- Gn/Gp ~ 1 (e.g., H. Hashimoto et al., PRL88 (2002) 042503)
- Gn/Gppuzzle

Solution?

- Additional meson exchange?
- K (+ h, r, w, K*,....) meson

- Improve the situation, but
- still below exp. data.
- (e.g., E. Oset et al.,
- NPA691 (2001) 146c)
- Some models also incorporate
- 2p exchange processes
- (e.g., K. Itonaga et al.,
- NPA639 (1998) 329c)

N

N

K

Strong

Weak

L

N

- Direct quark mechanism?
- - s-quark decays directly without meson propagation
- (e.g., M. Oka, NPA691 (2001) 364c)
- Two nucleon induced processes? (LNN NNN)

He

L

Other topics in weak-decay- Does DI = 1/2 rule holds in non-mesonic decay?
- - some models require DI=3/2 component to solve Gn/Gp puzzle
- - nature of DI = 1/2 rule. Is it really global?
- p+ decay -- observed only in
- - decay via S+ component in hypernuclei?
- - two step processes (L np0, p0p p+n)?
- Parity conserving/non-conserving amplitudes
- - parity conserving part cannot be studied in NN system
- - interferance decay asymmetry in polarized hypernuclei
- Weak production of hyperon
- - pn pL reaction using polarized protons
- - parity-violation and T-violation
- - experiments planned at RCNP (Osaka, Japan) and
- COSY (Juelich, Germany)

Results from recent experiments

- Hyperball project
- - High-resolution g-ray spectroscopy using Ge detectors

- Motivation
- - study of LN spin-dependent interaction via hypernuclear
- structure
- high-resolution is required
- g-ray spectroscopy using Ge detectors
- Hyperball
- - 14 Ge detecotors of 60% relative efficiency
- - BGO ACS
- - solid angle: 15% of 4p
- - photo-peak efficiency ~3% at 1 MeV

9

Li

Be

L

L

Experiments using hyperball- KEK-PS E419 (1998)
- - spin-spin force in
- - glue-like role
- BNL-AGS E930 (1998)
- - spin-orbit force in
- BNL-AGS E930 (2001)
- - tensor force in
- - in analysis
- KEK-PS E509 (2002)
- - stopped K
- - in analysis
- KEK-PS E518 (2002)
- -
- - coming this September

16

O

L

11

B

L

Li

L

7

Li

Li

Λ

Λ

KEK-PS E419(1) -- overview- The first experiment at KEK (Tsukuba, Japan)
- studied hypernucleus using 7Li(p+,K+g) reaction

7/2+

3+

2.19

5/2+

K+

E2

E2

3/2+

M1

1+

1/2+

p+

0

(MeV)

6Li

Li

L

KEK-PS E419(2) -- Results- Two peaks observed
- These attributed to
- M1(3/2+ 1/2+) and
- E2(5/2+ 1/2+)
- transitions in
- Eg = 691.7±0.6±1.0 keV
- 2050.1±0.4±0.7 keV
- Peak shape analysis
- (Doppler shift attenuation
- method)
- B(E2)=3.6±0.7 e2fm4
- For details, see
- H. Tamura et al., PRL84(2000)5963
- K. Tanida et al., PRL86(2001)1982

KEK-PS E419(3) -- discussion

- Eg(M1) = 692 keV gives strength of LN spin-spin force
- -6Li(1+) state has pure 3S1 (a+d) structure
- D = 0.48 ~ 0.50 MeV
- (D. J. Millener, NPA691(2001)93c,
- H. Tamura et al., PRL84(2000)5963)
- B(E2) is related to hypernuclear size or cluster distance
- between a and d as B(E2) \ <r2>2
- (T. Motoba et al., PTP70(1983)189)
- Without shrinkage effect, B(E2) is expected to be
- 8.6±0.7 e2fm4 from B(E2) data of 6Li.
- Present result (3.6±0.7 e2fm4) is significantly smaller
- strong evidence forglue-like role
- (3.6/8.6)1/4 = 0.81±0.04 shrinkage of 19±4%
- (K. Tanida et al., PRL86(2001)1982)

Be

L

9

Be

Λ

Λ

BNL-AGS E930(1)- Experiment performed at BNL (New York, USA)
- Measured g ray from created by 9Be(K-,p-) reaction

3/2+

L=2

2+

3.04

5/2+

- DE(5/2+,3/2+)
- LN spin-orbit force, SL
- (core structure: 2a rotating
- with L=2)

E2

0+

1/2+

0

(MeV)

8Be

BNL-AGS E930(2)

5/2+,3/2+ 1/2+

2000 2500 3000 3500

Eg(keV)

- Two peaks separated!
- |DE| = 31±3 keV - very small indeed
- surprisingly small spin-orbit force (~ 1/100 of NN case)
- (H. Akikawa et al., PRL88(2002)082501)

He

LL

5

He

L

Hybrid emulsion experiment -- KEK-PS E373- Hybrid emulsion -- C(K-,K+) reaction to produce X-
- then stop it in emulsion
- NAGARA event found (H. Takahashi et al., PRL87(2001)212502)

- Track #1 is the
- Binding energy of
- is obtained to be
- BLL = 7.3±0.3 MeV
- (from a+2L)
- In order to extract LL
- interaction, we take
- DBLL = BLL - 2BL( )
- = 1.0±0.3 MeV
- weakly attractive

6

He

LL

Future prospect

- Near future (a few years)
- - experimental studies continue at KEK, BNL, JLAB,...
- KEK-PS
- - E521: study of neutron rich hypernuclei by (p-,K+) reaction
- - E518: g-ray spectroscopy of
- - E522: study of LL final state interaction
- BNL-AGS
- - E964: study of LL hypernuclei with hybrid-emulsion method
- and X-ray spectroscopy of X- atoms
- CEBAF(JLAB, Virginia, USA)
- - E01-011: spectroscopy of hypernuclei with (e,e'K+) reaction
- - E02-017: weak decay study
- - E94-107: high-resolution study with (e,e'K+) reaction
- More activities expected at Frascati (Italy), Dubna (Russia),
- Juelich, GSI(Germany), RCNP (Osaka, Japan).

11

B

L

Future prospect(cont'd)

- Within 5 years...
- - KEK-PS and BNL-AGS will be shut down
- - JHF 50 GeV PS will come instead!
- Much more intense kaon (and other) beam available at JHF.
- - Systematic g-ray spectroscopy of single L hypernuclei
- not only LN force, but LNN force
- - Hyperon-Nucleon scattering (LN, SN, XN)
- - Spectroscopy of X hypernuclei with (K-,K+) reaction
- - Production of relativistic hypernuclei using primary beams
- measurement of magnetic moment
- - Study of LL hypernuclei and their weak decay
- - Charmed hypernuclei (charm quark instead of strange)
- Hypernucleus will be a main subject at JHF
- - Rich field for both theoretical and experimental studies.

At the end... (summary)

- Hypernucleus is interesting!
- There are more that I couldn't talk today.
- I tried to include references as much as possible
- - please look at them if you are interested in
- Feel free to contact me at
- tanida@rarfaxp.riken.go.jp
- if you have questions, comments,....

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