Introduction to Hypernuclear Physics

1
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

2
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
3
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
4
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 ofss
  pair
• - large momentum transfer (q gt 350 MeV/c)
• - small cross section, but intense p beam
  available
• Other methods
• - (e,e'K), heavy ion collision, ...

5
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 lt
  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

6
Some features of LN interaction (1)

• One pion exchange is forbidden

N
L
p(I1)
L(I0)
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

7
Some features of LN interaction (2)

• Two types of spin-orbit force
• i.e.,
• VL(r) sLLLN ?? L-spin
  dependent
• VN(r) sNLLN ?? N-spin
  dependent
• or
• Vs(r) (sLsN)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(I3/2) channel ? ALS0 at SU(3)
  limit)
• Towards understanding of the source of LS force
• -- vector meson exchange? (ALS lt SLS)
• -- quark-gluon picture? (ALS SLS, VL 0)

8
Overall binding energy of hypernuclei

• from A3 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)
9
Light hypernuclei (1) -- overbinding problem

• Binding energy of hypernuclei, A35
• 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 A3,4
  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.

10
Solution to the overbinding problem? (1)

• Quark Pauli effect?

quark level
baryon level
p
n
L
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)

11
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(T0) ? S(T1)
• ? a must excite to T1 state (Ex gt 30 MeV)
• ? less significant in

p
S
p
N L N

• 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

12
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/S0 mixing 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/w mixing ? spin-orbit
• These are strongly spin dependent
• ? spin/state dependence is important

13
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) sLsN ?? spin-spin
• VL(r) sLLLN ?? spin-orbit (L-spin
  dependent)
• VN(r) sNLLN ?? spin-orbit (N-spin
  dependent)
• VT(r)3(sLr)(sNr)/r2 - sLsN ?? tensor
• In p-shell hypernuclei, we usually take
• D ?fLN(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.

14
How to get?
J1/2
DE
J(0)
A
Z
Z
J-1/2
A1 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, 11BL) DE 2/3D
  4/3SL - 8/5T
• p1/2-shell(13C,15NL) 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.

15
LL interaction

• Unique channel in SU(3) BB interaction
  classification
• Repulsive core may vanish in this channel
• ? possibile existense of H-dibaryon (uuddss,
  JI0)
• 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

16
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

17
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
18
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 1deg for 1T
  magnetic field
• Alternative (indirect) measurement
• B(M1) \ (gcore - gL)2 (planned in KEK-PS
  E518)

19
Weak decay of hypernuclei

• In free space...
• L ? p p- (63.9, Q 38 MeV)
• n p0 (35.8, Q 41 MeV)
• DI1/2 rule holds well.
• - initial state I0, final state I1/2 or 3/2
• if If 1/2, branch is 21
• 3/2, 12
• - 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, ....

20
Mesonic decay

• Dominant only in very light hypernuclei (Alt6)
• 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)

21
Lifetime

• Almost constant for A gt 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

22
Gn/Gp puzzle

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

• Virtual mesonic decay
• absorbsion
• This model predicts
• Gn (nL?nn) ltlt 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/Gp puzzle

23
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)

24
Other topics in weak-decay

• Does DI 1/2 rule holds in non-mesonic decay?
• - some models require DI3/2 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 ? pn)?
• 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)

25
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

26
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
27
KEK-PS E419(1) -- overview

• The first experiment at KEK (Tsukuba, Japan)
• studied hypernucleus using 7Li(p,Kg)
  reaction

7/2
3
2.19
5/2
K
E2
E2
3/2
M1
1
1/2
p
0 (MeV)
6Li
28
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 691.70.61.0 keV
• 2050.10.40.7 keV
• Peak shape analysis
• (Doppler shift attenuation
• method)
• ? B(E2)3.60.7 e2fm4
• For details, see
• H. Tamura et al., PRL84(2000)5963
• K. Tanida et al., PRL86(2001)1982

29
KEK-PS E419(3) -- discussion

• Eg(M1) 692 keV gives strength of LN spin-spin
  force
• - 6Li(1) state has pure 3S1 (ad) 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) \ ltr2gt2
• (T. Motoba et al., PTP70(1983)189)
• Without shrinkage effect, B(E2) is expected to
  be
• 8.60.7 e2fm4 from B(E2) data of 6Li.
• Present result (3.60.7 e2fm4) is significantly
  smaller
• ? strong evidence for glue-like role
• (3.6/8.6)1/4 0.810.04 ? shrinkage of 194
• (K. Tanida et al., PRL86(2001)1982)

30
BNL-AGS E930(1)

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

3/2
L2
2
3.04
5/2

• DE(5/2,3/2)
• ? LN spin-orbit force, SL
• (core structure 2a rotating
• with L2)

E2
0
1/2
0 (MeV)
8Be
31
BNL-AGS E930(2)
5/2,3/2 ? 1/2
2000 2500 3000
3500
Eg(keV)

• Two peaks separated!
• DE 313 keV - very small indeed
• ? surprisingly small spin-orbit force ( 1/100
  of NN case)
• (H. Akikawa et al., PRL88(2002)082501)

32
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 7.30.3 MeV
• (from a2L)
• In order to extract LL
• interaction, we take
• DBLL BLL - 2BL( )
• 1.00.3 MeV
• ? weakly attractive

6
He
LL
33
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
34
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.

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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_at_rarfaxp.riken.go.jp
• if you have questions, comments,....