Transcript Document

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?
• Normal nucleus -- composed of nucleon (proton, neutron)
• At the quark level: p=(uud), n=(udd)
• There are six quark flavors in nature:
u
d
c
s
t
b
• 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 ( S4 He only)
LL-hypernuclei (a few events)
Notation
A
L
Z
• 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  L7 Li
6
2. 2p + 2n + 2L  LL
He
3. 1p + 2n + 1S+
2p + 1n + 1S0
 4S He
3p + 0n + 1S(they are indistinguishable)
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.,
or
VL(r) sL・LLN
VN(r) sN・LLN
‥‥ L-spin dependent
‥‥ N-spin dependent
Vs(r) (sL+sN)・LLN
Va(r) (sL-sN)・LLN
‥‥ symmetric (SLS)
‥‥ 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)
Light hypernuclei (1) -overbinding problem
• Binding energy of hypernuclei, A=3~5
3
LH : BL = 0.13 ± 0.05 MeV
4
+
LH : BL = 2.04 ± 0.04 MeV (ground state, 0 )
1.00 ± 0.06 MeV (excited state, 1+)
4
+
LHe: BL = 2.39 ± 0.03 MeV (0 )
1.24 ± 0.06 MeV (1+)
5
LHe: BL = 3.12 ± 0.03 MeV
• If we use LN interaction which reproduces A=3,4 binding
energies, L5He overbinds by ~1 MeV in calculations
 overbinding problem of 5LHe
• 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
p
n
L
d
s
⇒
⇒
no pauli blocking
u
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)
• 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 L5He
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
Light hypernuclei (2) -- charge
symmetry breaking
• L has no charge, no isospin
 difference of Lp and Ln interaction is CSB.
• L in L4He is more strongly bound than L4H 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
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(=0)
A
DE
Z
J+1/2
J-1/2
A+1
L
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
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
d
6Li
+ L

a
7
L
L
d
Li
• 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 89LY
(H. Hotchi et al.,
PRC64 (2001) 044302)
 There are singleparticle states in center
pL
of nuclei
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 ~1deg for 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
N
N
• This model predicts
p
Weak
L
Strong
N
Gn (nLnn) << Gp(pLpn)
- 3S1  3D1 tensor coupling
has the largest amplitude,
but this is forbidden for
(nn) final state.
• However, experimental data indicate
Gn/Gp ~ 1 (e.g., H. Hashimoto et al., PRL88 (2002) 042503)
 Gn/Gp puzzle
Solution?
• Additional meson exchange?
 K (+ h, r, w, K*,....) meson
N
Strong
L
N
K
Weak
N
• 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)
• 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
• 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 L4He
- 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
Experiments using hyperball
• KEK-PS E419 (1998)
- spin-spin force in L7 Li
- glue-like role
• BNL-AGS E930 (1998)
- spin-orbit force in L9 Be
• BNL-AGS E930 (2001)
- tensor force in 16LO
- in analysis
• KEK-PS E509 (2002)
- stopped K
- in analysis
• KEK-PS E518 (2002)
- 11LB
- coming this September
KEK-PS E419(1) -- overview
• The first experiment at KEK (Tsukuba, Japan)
• studied L7 Li hypernucleus using 7Li(p+,K+g) reaction
2.19
7/2+
3+
5/2+
K+
E2
E2
3/2+
1+
0
6Li
(MeV)
M1
1/2+
7
Λ Li
p+
KEK-PS E419(2) -- Results
• Two peaks observed
• These attributed to
M1(3/2+  1/2+) and
E2(5/2+  1/2+)
transitions in L7 Li
• 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 for glue-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)
• Experiment performed at BNL (New York, USA)
• Measured g ray from L9 Be created by 9Be(K-,p-) reaction
3.04
2+
L=2
3/2+
5/2+
E2
0+
0
(MeV) 8Be
1/2+
9
Λ Be
• DE(5/2+,3/2+)
 LN spin-orbit force, SL
(core structure: 2a rotating
with L=2)
BNL-AGS E930(2)
2000
2500
3000
3500
5/2+,3/2+  1/2+
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
• Hybrid emulsion -- C(K-,K+) reaction to produce Xthen stop it in emulsion
• NAGARA event found (H. Takahashi et al., PRL87(2001)212502)
6
He
• Track #1 is the LL
6
• Binding energy of LLHe
is obtained to be
BLL = 7.3±0.3 MeV
(from a+2L)
• In order to extract LL
interaction, we take
DBLL = BLL - 2BL( L5He)
= 1.0±0.3 MeV
 weakly attractive
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 11LB
- 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).
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
[email protected]
if you have questions, comments,....