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Overview of Strangeness Nuclear Physics

Benjamin F. Gibson, T-5

International Workshop on JHF Science (JHF98) Tsukuka, Japan March 4-7, 1998

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Overview of Strangeness Nuclear Physics

B. F. Gibson Theoretical Division, Los Alamos National Laboratory

Los Alamos, New Mexico 87545, USA

ABSTRACT

Novel as well as puzzling aspects of strangeness (S = -1 and S = -2) nuclear physics are highlighted. Opportunities to gain new insights into hypernuclear spectroscopy, structure, and weak decays and to contribute to the continuing effort to understand the fundamental baryon-baryon force are outlined. Connections to strangeness in heavy-ion reactions and astrophysics are noted.

Strangeness nuclear physics entails the study of 1) strong, weak, and electromag- netic properties of the hyperons (Y), 2) the fundamental YN and YY interactions, and 3) the structure and the weak decays of nuclear systems containing one or more hyperons. Interjecting the strangeness (flavor) degree of freedom has revealed physics which is novel and, at times, puzzling; it has stretched our intuition and expanded our insight beyond that acquired from 3/4 of a century of intense research in nuclear and particle physics. Strangeness introduces new dynamical symmetries, triggers non mesonic weak decays, and forms the basis for the SU(3) structure in the baryon- baryon force. Moreover, hypernuclear physics tests whether the sophisticated models of conventional (non strange) nuclei and reactions extrapolate beyond the S = 0 realm in which they were constructed or are merely exquisite interpolation tools.

Several basic issues in the YN and YY strong force sector remain to be addressed. The additional flavor degree of freedom in hadron interactions provides a testing ground to explore:

0 SU(3) breaking

0 the realm of validity of chiral perturbation theory

0 quark exchange versus boson exchange pictures

It is agreed that SU(3) symmetry must be broken kinematically ( e . g . , M y # MN) as well as dynamically (e.g., m, # m K , g, # gK), but just how and by how much is

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undetermined. The YN data are sparse; more are sorely needed. The NN and Nl1 interactions differ significantly; the latter has no long range one-pion-exchange tail to hide shorter range properties. Moreover, how NN-NA coupling differs from Nil- NE and AA-NZ coupling is crucial. All of us are familiar with the quest to discover the H dibaryon, the six quark (uuddss) singlet state, as proof that a quark-based picture is required. However, antisymmetrization in quark models of the YN and YY interaction may prove to differ sufficiently from boson exchange models to probe that question, if such data are in hand.

Strangeness physics is not just a simple extension of conventional S = 0 phenom- ena. The physics is new, is different, and can be puzzling. Limited manpower and resources have revealed such novel aspects of A hypernuclei as:

0 anomalous binding energies

0 vanishingly small spin-orbit force

0 important NA-NC coupling

0 significant three-body forces

0 striking charge symmetry breaking

0 new dynamical symmetries

0 puzzling non mesonic weak decays

However, our purpose is not to dwell upon the past but to focus upon the future. In the case of hypernuclear structure (spectroscopy) there are a number of significant issues to be addressed. For S = -1 physics we list a few:

0 quantitative measurement of the small NA spin-spin force

0 elucidation of dynamical symmetries due to injecting a new distinguishable baryon

0 separation of Coulomb compression from charge symmetry breaking effects

0 quantitative measurement of the NA spin-orbit force

0 existence of C hypernuclei

0 magnetic moments of A hypernuclei

Throughout these forefront investigations one should keep in mind the complemen- tary nature of (K-,7ro) in converting a proton into a A compared with the neutron to A conversion in (K-, 7r-) and (7r+,K+) reactions. Also, one should take advantage

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of the complementary aspects of the (K-, T O ) reaction in preferentially exciting nat- ural parity transitions in contrast to the spin-flip excitations of the (y,K’) reaction. Moreover, y-ray spectroscopy holds promise of providing unprecedented precision for exploring a number of the important spectroscopy issues. Experiments at the AGS as proposed by the likes of Hashimoto and Tamura and Hungerford should prepare the way for an exciting future at the JHF.

The (K-,K+) production of AA and S hypernuclei will permit one to address a number of open questions. Some of the more important issues appear to be:

Exploring the physics of S = -2 hypernuclei has just begun.

0 confirming the existence of AA hypernuclei

0 determinating the spectroscopy of AA systems

0 confirming the existence of E quasi-particle states

0 determining the spin-orbit forces

This new frontier in flavor physics ( S = -2) attracts much attention because it is unexplored. The strong AA-NS coupling portends a rich spectroscopy, because Pauli effects will be important. E- capture-at-rest to produce two-body final states with a mono energetic neutron [e.g., ‘Li(z-,n),;He] or with two charged clusters [e.g., 12C(E-,7Li),iHe] are competing avenues. In addition to AA physics, that of the Z is important. Knowledge of the Z potential depth provides insight into the stability of Es in such systems as in heavy-ion reactions. S = -2 is a first step toward exploring multi-strange systems with possible implications for strangeness in nuclear astrophysics. First experiments at the AGS, as proposed by people such as Fukuda and May, will begin to prepare us for the JFH era.

The weak decays of A hypernuclei have no analog in the S = 0 sector, even though pionic decay of the A is first cousin to neutron beta decay. A number of issues remain to be resolved:

0 medium modification of A + T + N 0 implications of NA + NN (and AA + EN) for the role of the AI = 1/2 rule,

which governs kaon and hyperon decay

0 anomalous 7rs decay of :He

0 properties of AA + ( N r ) ( N r ) decay

A first direct measurement of the TO decay channel is underway at the AGS. The ratio of neutron stimulated non mesonic decay to proton stimulated non mesonic decay, first brought to light by emulsion studies, is an unsolved puzzle. Whether the AI =

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1 /2 rule should hold for non mesonic decays is still being argued by theorists while we await definitive experimental data.

In summary, S # 0 physics is novel and has crucial implications for our un- derstanding of strongly interacting many-body systems as well as non perturbative aspects of QCD. Hadron beam investigations at the JHF will be complementary to electromagnetic studies (e.g., at TJNAF) and heavy-ion studies (e.g., at RHIC). Compelling issues to be addressed involve the interconnected areas of baryon-baryon forces, S = -1 hypernuclear physics, and S = -2 hypernuclear physics. Without the haron beam that the JHF will provide, reliable interpretation of data from the complementary facilities will be lacking.

Acknowledgement

The work BFG is performed under the auspices of the U. S. Department of Energy. He thanks E. V. Hungerford and A. Gal for discussions and acknowledges numerous conversations with the late C. B. Dover regarding several points.

References

[l] A. Gal, Adv. Nucl. Physics 8, 1 (1975).

[2] LAMPF Workshop on (7r,K) Physics, A. I. P. Conf. Proc. 224, ed. by B. F. Gibson, W. R. Gibbs, and M. B. Johnson, (American Institute of Physics, New York, 1991).

[3] International Symposium on Hypernuclear and Strange Particle Physics, ed. by T. Fukuda, 0. Morimatsu, T. Yamazaki, and K. Yazaki, Nucl. Phys. A585, lc-422c (1992).

[4] Properties and Inteructiuons of Hyperons, Proceedings of the U. S. - Japan Sem- inar, ed. by B. F. Gibson, P. D. Barnes, and K. Nakai, (World Scientific, Singa- pore, 1994).

[5 ] Progress of Theoretical Physics Supplement 117 (1994).

[6] International Symposium on Hypernuclear and Strange Particle Physics, ed. by C. A. Davis, H. W. Fearing, and B. K. Jennings, Nucl. Phys. A547, lc-399c (1995).

[7] B. F. Gibson and E. V. Hungerford 111, Phys. Reports 257 pp. 349-389 (1995).

[8] Proceedings of the Workshop on Hypernuclear Physics, (INS, Univ. of Tokyo), ed. by T. Motoba, Y. Akaishi, and T. Nagae, Genshikaku Kenkyu 41 (1997).

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