Strangeness in nuclear physics A. Gal ∗ Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel E. V. Hungerford † University of Houston, Houston, TX 77204, USA D. J. Millener ‡ Brookhaven National Laboratory, Upton, NY 11973, USA (Dated: February 2016) Extensions of nuclear physics to the strange sector are reviewed, covering data and models of Λ and other hypernuclei, multi-strange matter, and anti-kaon bound states and condensation. Past achievements are highlighted, present unresolved problems discussed, and future directions outlined. PACS numbers: 13.75.Ev, 13.75.Jz, 21.80.+a, 25.80.-e, 26.60.+c, 97.60.Jd Contents 1. Electroproduction at Mainz (MAMI) 33 2. Electroproduction at Jlab 34 C. Experiments at PANDA 34 I. Introduction 1 D. Weak decay of hypernuclei 35 A. Brief historical overview 1 1. mesonic decays 35 B. General features of Λ hypernuclear formation 2 2. nonmesonic decays 35 1. The ( K − stop , π − ) reaction 3 3. Λ hypernuclear lifetimes 36 2. The in-flight ( K − , π − ) reaction 3 E. Multi-strange systems 36 3. The ( π + , K + ) reaction 4 F. Experiments at heavy-ion facilities 37 4. The ( e, e ′ K + ) reaction 5 G. K -nucleus bound-state searches 38 5. Addendum: hypernuclear lifetime measurements 6 IX. Summary 38 II. Λ Hypernuclear Structure Calculations 8 A. The effective Y N interaction and s -shell hypernuclei 8 Acknowledgments 39 B. p -shell hypernuclei, γ -ray measurements, and spin dependence of the Λ N interaction 8 References 39 III. Weak Decays of Λ Hypernuclei 14 A. Mesonic decays 14 I. INTRODUCTION B. Nonmesonic decays 16 A. Brief historical overview IV. Ξ Hypernuclei 20 V. Λ − Λ Hypernuclei 21 In the early 1950s a quantum number, conserved un- der the strong interaction, was introduced (Gell-Mann, 24 VI. Strange Dense Matter 1953) in order to explain the behavior of the “strange” A. Strange hadronic matter 24 particles which had been observed in emulsions exposed B. Neutron stars 25 to cosmic rays. Almost simultaneously, the first hyper- ¯ VII. K -Nuclear Interactions and Bound States 27 nucleus, formed by a Λ hyperon bound to a nuclear frag- ment, was observed in an emulsion exposed to cosmic rays 31 VIII. Future Experiments and Directions (Danysz and Pniewski, 1953). For the next 20 years or A. Spectroscopy using meson beams 31 so, hypernuclei were explored using emulsion detectors, 1. Hyperon production and hyperon-nucleon first with cosmic rays, and then with beams from existing interactions 31 2. Reaction spectroscopy with mesons 32 accelerators. Within the last 40 years, modern particle 3. Experiments using emulsion detectors 32 accelerators and electronic instrumentation has increased 4. Spectroscopy using electromagnetic transitions 32 the rate and breadth of the experimental investigation of B. Spectroscopy with electron accelerators 33 strangeness in nuclei. As always, theoretical interest has closely followed the experimental development. The behavior of a Λ in a nuclear system is a nu- clear many-body problem, since the forces between the ∗ Electronic address: avragal@vms.huji.ac.il baryons are predominantly hadronic and the time scale of † Electronic address: hunger@uh.edu the strong interaction is about 10 − 23 s compared to the ‡ Electronic address: millener@bnl.gov
2 weak-interaction lifetime of a Λ lifetime in the nuclear TABLE I Experimental Λ separation energies, B Λ , of light medium (Bhang et al. , 1998; Park et al. , 2000) of ap- hypernuclei from emulsion studies. These are taken from proximately 10 − 10 s. Therefore, the combined hypernu- a compilation (Davis and Pniewski, 1986) of results from clear system can be treated using well developed nuclear- c et al. , 1973), omitting 15 (Cantwell et al. , 1974; Juriˇ Λ N (Davis, theory models such as the shell or mean-field models with A reanalysis for 12 1991). Λ C (D� luzewski et al. , 1988) gives an effective Λ-nucleus interaction. New dynamical sym- 10.80(18) MeV. metries may also arise in hypernuclei, e.g. by treating Hypernucleus Number of events B Λ ± ∆ B Λ (MeV) the Λ hyperon shell-model orbitals on par with those of 3 Λ H 204 0 . 13 ± 0 . 05 nucleons within the Sakata version of SU(3) symmetry 4 Λ H 155 2 . 04 ± 0 . 04 (Sakata, 1956). This approach was found useful in hy- 4 Λ He 279 2 . 39 ± 0 . 03 pernuclear spectroscopic studies (Auerbach et al. , 1981, 5 Λ He 1784 3 . 12 ± 0 . 02 1983). Furthermore, by coupling SU(3)-Sakata with 6 Λ He 31 4 . 18 ± 0 . 10 SU(2)-spin, the resulting SU(6) symmetry group presents 7 Λ He 16 not averaged a natural extension of Wigner’s SU(4) spin-isospin sym- 7 Λ Li 226 5 . 58 ± 0 . 03 metry group in light nuclei (Dalitz and Gal, 1981). 7 Λ Be 35 5 . 16 ± 0 . 08 Λ hypernuclei also offer a test-ground for microscopic 8 Λ He 6 7 . 16 ± 0 . 70 approaches to the baryon-baryon interaction. Thus, since 8 Λ Li 787 6 . 80 ± 0 . 03 8 one-pion exchange (OPE) between a Λ hyperon and a Λ Be 68 6 . 84 ± 0 . 05 9 nucleon is forbidden by isospin conservation, the Λ N in- Λ Li 8 8 . 50 ± 0 . 12 9 Λ Be 222 6 . 71 ± 0 . 04 teraction has shorter range, and is dominated by higher 9 Λ B 4 8 . 29 ± 0 . 18 mass (and multiple) meson exchanges when compared to 10 Λ Be 3 9 . 11 ± 0 . 22 the NN interaction. For example, two-pion exchange 10 Λ B 10 8 . 89 ± 0 . 12 between a Λ hyperon and a nucleon proceeds through in- 11 Λ B 73 10 . 24 ± 0 . 05 termediate Σ N states (Λ N → Σ N → Λ N ), potentially 12 Λ B 87 11 . 37 ± 0 . 06 leading to non-negligible three-body Λ NN forces (Gibson 12 Λ C 6 10 . 76 ± 0 . 19 and Lehman, 1988). The analogous mechanism of inter- 13 Λ C 6 11 . 69 ± 0 . 12 mediate ∆ N states ( NN → ∆ N → NN ) in generating 14 Λ C 3 12 . 17 ± 0 . 33 three-body NNN forces in two-pion exchange seems to be less important in nuclear physics, not only because the NN interaction is dominated by OPE, but also be- cause of the considerably higher mass of the ∆ resonance with respect to that of the Σ hyperon. Such theoretical hypernucleus will normally de-excite by a nuclear Auger expectations may be explored in hypernuclear few-body process, or by γ emission. The resulting ground state and spectroscopic calculations. then decays by the weak interaction, emitting π mesons Finally, the Λ can be used as a selective probe of the as in the free Λ decay, and also nucleons in a four-fermion nuclear medium, providing insight into nuclear proper- in-medium interaction Λ N → NN . Therefore, obser- ties that cannot be easily addressed by other techniques. vation of the energetics of hypernuclear formation and Thus, since from a hadronic as opposed to a quark per- decay can provide information on binding energies and spective, the Λ remains a distinguishable baryon within spins of hypernuclear ground states. To conserve baryon the nucleus, and samples the nuclear interior where there number, a reaction producing a hypernucleus commonly is little direct information on the single-particle structure replaces a nucleon with a Λ. of nuclei. Because of this, various aspects of hypernuclear studies such as Λ decay, or the spectra of heavy hyper- The acquisition of hypernuclear binding energies, well- nuclear systems, can illuminate nuclear features which depths, and positions of the hypernuclear levels began in would be more obscured in conventional nuclei. the 1960s. Early work included K − absorption in emul- Useful material on the subject of this review can sions and bubble chambers, where hyperfragments were be found in the proceedings of the recent trien- identified by their mesonic decays. These efforts success- nial conferences on Hypernuclear and Strange Particle fully established the binding energies of a number of light Physics ( ???? ), recent special volumes ( ??? ), schools ( ? ), hypernuclei in their ground states (g.s.) where the Λ is and several review articles (Botta et al. , 2012; Hashimoto in the lowest s 1 / 2 orbit, as summarized in Table I. In and Tamura, 2006; ? ). 1972, the existence of a 12 Λ C particle-unstable state with a Λ in the p orbit was confirmed (Juriˇ c et al. , 1972), and the reaction K − + 12 C → π − + p + 11 Λ B in emulsion was used to study excited states of 12 B. General features of Λ hypernuclear formation Λ C decaying by proton emission to 11 Λ B. In this case, the emitted proton energy A hypernucleus is characterized by its spin, isospin, was measured in the emulsion, and the level structure and in the case of Λ hypernuclei, a strangeness of − 1. If interpreted in terms of three p -shell Λ states located at the Λ is injected into the nuclear system, the resulting about 11 MeV excitation energy (Dalitz et al. , 1986).
Recommend
More recommend
