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? u c t d s b 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 S He 4

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) Li 7 L 6 He LL

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 L N 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)

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 L H 3 L H 4 L He 4 L He 5 L He 5  overbinding problem of 5 L He 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 ⇒ u d s ⇒ 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) 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 L He 5 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 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 L He 4 L H 4

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.

Z Z Z Z How to get? DE J+1/2 J(=0) A J-1/2 A+1 L 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 d +  a L d L 6Li 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 +  L d L 6Li L Li 7 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 ~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 N N p L N 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/Gp puzzle

Solution? N N K L N 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 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) L He 4

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 - 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 KEK-PS E518 (2002) - - coming this September Li 7 L Be 9 L 16 O L 11 B L

KEK-PS E419(1) -- overview 6Li Li The first experiment at KEK (Tsukuba, Japan) studied hypernucleus using 7Li(p+,K+g) reaction Li 7 L 7/2+ 3+ 2.19 5/2+ K+ E2 E2 3/2+ 1+ M1 1/2+ p+ (MeV) 6Li 7 Λ Li

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 Li 7 L

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) 8Be Be Experiment performed at BNL (New York, USA) Measured g ray from created by 9Be(K-,p-) reaction Be 9 L 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+ (MeV) 8Be 9 Λ Be

BNL-AGS E930(2) Two peaks separated! 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 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 He 6 LL 6 He LL L He 5

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,....