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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. 4. He. S.

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introduction to hypernuclear physics
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
what is hypernucleus

4

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

notation

7

Li

L

Notation
  • A: Total number of baryons (nucleon & hyperon)
  • Z: Total charge (NOT number of protons!)
  • L: hyperon (other examples -- S, X, ...)
  • Some examples:
  • 1. 3p + 3n + 1L
  • 2. 2p + 2n + 2L 
  • 3. 1p + 2n + 1S+
  • 2p + 1n + 1S0 
  • 3p + 0n + 1S-
  • (they are indistinguishable)

6

He

LL

how to produce
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
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 l n interaction 1
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 l n interaction 2
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
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)

light hypernuclei 1 overbinding problem

3

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
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)
solution to the overbinding problem 2

5

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
light hypernuclei 2 charge symmetry breaking

4

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 l n interaction
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
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
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
hyperons in nuclei

7

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
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
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
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
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
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
g n g p puzzle
Gn/Gp puzzle
  • Simplest diagram for non-mesonic weak decay
  • -- one pion exchange
  • Virtual mesonic decay
  • + absorbsion
  • This model predicts
  • Gn (nLnn) << Gp(pLpn)
  • - 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
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)
other topics in weak decay

4

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
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
experiments using hyperball

7

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

kek ps e419 1 overview

7

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

kek ps e419 2 results

7

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
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)
bnl ags e930 1

9

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
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)
hybrid emulsion experiment kek ps e373

6

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
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
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
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,....