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?

• Normal nucleus -- composed of nucleon (proton, neutron)• At the quark level: p=(uud), n=(udd)• There are six quark flavors in nature:

• =(uds), +(uus), =(uss), ... exist Hyperons • Hypernucleus: not only nucleons but hyperons (i.e., quarks other than u and d)• Known hypernuclei: strangeness (s) only. -hypernuclei (~50 species)

-hypernucleus ( only)

-hypernuclei (a few events) H

e

4

u c td s b

Notation

ZAZA • A: Total number of baryons (nucleon & hyperon)

• Z: Total charge (NOT number of protons!)• : hyperon (other examples -- , ...)

• Some examples: 1. 3p + 3n + 1 2. 2p + 2n + 2 3. 1p + 2n + 1+

2p + 1n + 10 3p + 0n + 1

(they are indistinguishable)

Li7

He6

He4He4

How to produce?• Bring strangeness somehow into nuclei• Stopped K method - traditional method - K usmeson has strangeness - 100% reaction, about 10% makes hypernuclei as hyperfragments in A ~ 14 targets. Dirty.• In-flight (K,), (K,) reactions - elementary process NK - small momentum transfer (can be 0) - large cross section• (+,K+) reaction - relatively new method, production ofss pair - large momentum transfer (q > 350 MeV/c) - small cross section, but intense 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 (c < 10 cm) information from hypernculei is important (mostly -hypernuclei N interaction)• In -hypernuclei: No Pauli effect, weak coupling simpler structure extraction of N interation is rather straightforward

Some features of N interaction (1)• One pion exchange is forbidden

(I=0)

(I=1)

- Violates isospin symmetry- weakness of N interaction e.g., no two body bound state weak tensor force- short range interaction heavier mesons (K, , ...), quark-gluon picture

N

N

Some features of N interaction (2)• Two types of spin-orbit force i.e., V(r) ・ LN ‥‥ -spin dependent VN(r) N ・ LN N-spin dependent‥‥ or Vs(r) ( ・ LN symmetric (SLS)‥‥ Va(r) (N) ・ LN anti-symmetric (ALS)‥‥

• In np, ALS breaks charge symmetry (~1/1000 of SLS)• Does not vanish even at flavor SU(3) limit (c.f., N(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, V~ 0)

Overall binding energy of hypernuclei

• from A=3 to 208 • U ~ 28 MeV ~ 2/3 UN

well reproduces data weakness of N 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 : B= 0.13 ±0.05 MeV : B = 2.04 ± 0.04 MeV (ground state, 0+) 1.00 ± 0.06 MeV (excited state, 1+) : B = 2.39 ± 0.03 MeV (0+) 1.24 ± 0.06 MeV (1+) : B = 3.12 ± 0.03 MeV

• If we use N interaction which reproduces A=3,4 binding energies, overbinds by ~1 MeV in calculations

overbinding problem of 5He

He4

H4H3

He5

He5

• 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?

baryon level

p n ⇒

no pauli blocking

quark level

u d s⇒

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)

N 　 N

• Similar to Fujita-Miyazawa 3NF• Maybe stronger MM ~ 80 MeV ~ 1/4(MMN)• (T=0) (T=1) must excite to T=1 state (Ex > 30 MeV) less significant in

• NN three body force?

He5

• 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

• has no charge, no isospin difference of p and n interaction is CSB.• 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 - /mixing in free space exchange force (tensor) - N-N coupling via mass difference of ~8 MeV) three-body force as well as two body force. - K0 and K± mass difference (~1%), also in K*

- / mixing spin-orbit• These are strongly spin dependent spin/state dependence is important

He4H4

Spin-dependence of N 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 N effective potential in hypernuclei

V(r) ・ N spin-spin‥‥ V(r) L ・ LN spin-orbit (‥‥ -spin dependent) VN(r) N ・ LN spin-orbit (N-spin dependent)‥‥ VT(r){3( ・ r)(N ・ r)/r2 ・ N} tensor‥‥

• In p-shell hypernuclei, we usually take

= ∫*N(r)V(r)(r) dr

and regard it as a paramter. (N is almost the same over p-shell)• Similarily, S, SN, and T are defined from V, VN, VT.

How to get?

ZAZJ(=0)

J+1/2

J-1/2E

ZA+1Z

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

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 • No experimental evidence so far - at least, deeply bound H is rejected• coupling important (E = 28 MeV)• interaction study performed by - hypernuclei (example later in this talk) - final state interaction in (K,K+) reaction (J. K. Ahn et al., PLB444 (1998) 267 )• Present data suggests 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 makes a loosely-bound light nuclei, such as 6Li, smaller glue-like role (Motoba et al., PTP70 (1983) 189)

d

6Li

+ d

Li7

• 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 ) - is a good probe for depth of nucleus• KEK-PS E369 observed clear and narrow peaks for s and p states of (H. Hotchi et al., PRC64 (2001) 044302) There are single- particle states in center of nuclei s

p

Y89

magnetic moment• Good observable to see hyperon () property in nuclear matter. - is it changed from free space? If so, how?• Meson current 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 g)2 (planned in KEK-PS E518)

Weak decay of hypernuclei• In free space... p + (63.9%, Q = 38 MeV)n + (35.8%, Q = 41 MeV)• I=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 + p + n, n + n + n, ....

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

decay partial width

free • 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

n/p puzzle• Simplest diagram for non-mesonic weak decay -- one pion exchange

N

N• Virtual mesonic decay + absorbsion• This model predicts n (nnn) << p(ppn)

- 3S1 3D1 tensor coupling has the largest amplitude, but this is forbidden for (nn) final state.

Weak Strong

• However, experimental data indicate n/p ~ 1 (e.g., H. Hashimoto et al., PRL88 (2002) 042503)

n/p puzzle

Solution?• Additional meson exchange? K (+ , , K*,....) meson

K

N

N

Strong Weak

• Improve the situation, but still below exp. data. (e.g., E. Oset et al., NPA691 (2001) 146c)• Some models also incorporate 2 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? (NN NNN)

Other topics in weak-decay• Does I = 1/2 rule holds in non-mesonic decay? - some models require I=3/2 component to solve n/p puzzle - nature of I = 1/2 rule. Is it really global?• decay -- observed only in - decay via component in hypernuclei? - two step processes ( n0, 0p +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 p reaction using polarized protons - parity-violation and T-violation - experiments planned at RCNP (Osaka, Japan) and COSY (Juelich, Germany)

He4

Results from recent experiments◎ Hyperball project - High-resolution -ray spectroscopy using Ge detectors

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

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

Li7

Be9

O16

B11

KEK-PS E419(1) -- overview• The first experiment at KEK (Tsukuba, Japan)• studied hypernucleus using 7Li(+,K+) reactionLi7

7/2+

5/2+

3/2+

1/2+

7ΛLiΛLi

M1

E2

1+

3+

6Li 0(MeV)

2.19

+

K+

E2

KEK-PS E419(2) -- Results

• Two peaks observed• These attributed to M1(3/2+ 1/2+) and E2(5/2+ 1/2+) transitions in • E = 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

Li7

KEK-PS E419(3) -- discussion• E(M1) = 692 keV gives strength of N spin-spin force - 6Li(1+) state has pure 3S1 (+d) structure = 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 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 ray from created by 9Be(K,) reactionBe9

3/2+

5/2+

1/2+

9ΛBeΛ

E2

0+

2+

8Be 0(MeV)

3.04L=2

• E(5/2+,3/2+) N spin-orbit force, S

(core structure: 2rotating with L=2)

BNL-AGS E930(2)

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

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

E(keV)2000 2500 3000 3500

Hybrid emulsion experiment -- KEK-PS E373

• Hybrid emulsion -- C(K,K+) reaction to produce

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 B = 7.3±0.3 MeV (from )• In order to extract interaction, we take B= B = 1.0±0.3 MeV weakly attractive

He6

He6

He5

Future prospect• Near future (a few years) - experimental studies continue at KEK, BNL, JLAB,...• KEK-PS - E521: study of neutron rich hypernuclei by (,K+) reaction - E518: -ray spectroscopy of - E522: study of final state interaction• BNL-AGS - E964: study of hypernuclei with hybrid-emulsion method and X-ray spectroscopy of 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).

B11

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 -ray spectroscopy of single hypernuclei not only N force, but NN force - Hyperon-Nucleon scattering (, , ) - Spectroscopy of hypernuclei with (K,K+) reaction - Production of relativistic hypernuclei using primary beams measurement of magnetic moment - Study of 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,....