Hypernuclear Physics in the S=-1 sector

Transcription

1 Hypernuclear Physics in the S=-1 sector O. Hashimoto Department of Physics, Tohoku University, Sendai , Japan (Dated: June 1, 2003) Hypernuclear physics in the S=-1 sector, particularly its spectroscopic study, has made significant progress in recent years. Spectroscopy as a tool to investigate Λ hypernuclear structure and ΛN interaction is discussed referring recent spectroscopic data; hypernuclear mass spectra through the (π +,K + ) reaction using a large acceptance high resolution spectrometer ( SKS ) at KEK 12 GeV PS, hypernuclear γ ray spectroscopy in which precision information on the spin dependent ΛN interaction can be extracted. In addition, a new high-precision reaction spectroscopy experiment by the (e,e K + ) reaction(jlab E89-009), and the experiment under preparation with a high-resolution kaon spectrometer(hks) under construction, which is expected to realize the (e,e K + ) reaction spectroscopy with more than 50 times higher efficiency and twice better resolution(3-400 kev(fwhm)). The future prospect of hypernuclear physics is discussed based on these recent development. PACS numbers: I. INTRODUCTION The Λ hypernuclei provide invaluable opportunities for the investigation of hadronic many-body system with strangeness -1 and the strong and weak interaction in nuclei involving strangeness. There are several aspects that hypernuclear investigation plays key roles in shedding lights on strangeness nuclear physics and current issues of hypernuclear physics can be grouped into the following 4 subjects. Firstly, we intend to reveal nuclear deeply bound states, which are difficult and limited to be investigated by ordinary means of nuclear physics experiments, through the hypernuclear spectroscopic studies. Baryon structure in nuclear medium can be better studied by bringing a strangeness degree of freedom into deep inside a nucleus. It becomes possible partly due that a hyperon is free from Pauli exclusion from nucleons. Deeply bound states of Λ hyperon, even they are excited above the nucleon emission threshold, still have reasonably narrow spreading widths of less than a few hundreds kev and can be studied spectroscopically. It is strongly in contrast to ordinary nuclei, where nucleon deeply bound states become very wide due to the spreading of the states. Secondly, it is expected that new forms of nuclei and nuclear structure can be investigated, having a new quantum number strangeness. With the new quantum number and the interaction between a hyperon and nucleon, nuclear structure which cannot be seen in ordinary nuclei made up from nucleons can be realized in hypernuclei, providing invaluable information on baryonic matter. Thirdly, hyperon-hyperon and hyperon-nucleon interaction on the flavor SU(3) basis can be well studied by the spectroscopic information of hypernuclei. Hyperon scattering experiments, in principle, can give basic data on the interactions but practically it is quite difficult at present to carry out such experiments, because hyperon lifetimes are of the order of 10 9 s and hyperon beams suitable for the scattering experiments are not easily obtained. Spectroscopic investigation yields complimentary information on the hyperon-nucleon and hyperonhyperon interactions. Theoretically, the YN, YY effective interactions are constructed through G-matrix calculation, starting from phenomenological interactions in free space. Analytical functions of the effective potentials are often given in the form of three-range gaussian, V ΛN (r) = (a i + b i K F + c i kf 2)exp( r2 /b 2 i ).[1] Wide variety of hypernuclear properties, such as level structures, splittings due to the spin-dependent interaction, reaction cross sections etc. are then calculated and can be directly compared with experimental data. These calculations are reasonably reliable partly because the ΛN interaction is much weaker than NN interaction and no antisymmetrization against nucleons is required. Therefore, once the information on the hypernuclear structure are obtained experimentally, we can trace back the procedure backward and can investigate the effective and free hyperon-nucleon interactions. It is one of the most important aspect of hypernuclear investigation. Lastly, baryon-baryon weak interaction in nuclear medium is also studied through nonmesonic weak decay of hypernuclei. Strong interaction masks the small components of weak interaction process in the nucleonnucleon interaction, but Λ-nucleon weak interaction possibly manifest itself in the nonmesonic weak decay process thanks to the strangeness degree of freedom. It is emphasized in this paper that the good-quality reaction spectroscopy as well as the γ ray spectroscopy is the basis of the hypernuclear studies. And experimental efforts toward the future hypernuclear spectroscopy will be described. II. THE (π +,K + ) REACTION SPECTROSCOPY AND THE SKS SPECTROMETER The reaction spectroscopy by the (π +,K + ) reaction was first applied to a carbon target at BNL AGS, and later it was considerably improved and also extended

2 to heavier ssytems both at BNL and KEK.[2, 3] Although the hypernuclear cross sections are often about 2 orders of magnitude smaller than those by (K,π ) reaction, which had been widely used for hypernuclear study, high intensity pion beams can compensate them. The reaction has an advantage to excite deeply bound hypernuclear states due to its large momentum transfer, converting a neutron in a high-l orbital to a Λ hyperon in a low-l orbital. With an intension to take full advantage of the (π +,K + ) reaction for the hypernuclear reaction spectroscopy, a superconducting kaon spectrometer(sks) was constructed by INS, University of Tokyo and installed at KEK 12 GeV PS.[4] The SKS spectrometer has the good resolution of 1.5-2MeV FWHM and simultaneously spans a large solid angle of 100 msr, accepting, for example, 60 % of 12 Λ C ground-state yield in the (π+,k + ) reaction.?? The large acceptance allows us to perform efficient coincidence experiments such as study of hypernuclear weak decay and γ ray spectroscopy. A series of experiments for the investigation of hypernuclei using the SKS spectrometer has been carried out in the past years. In Fig. 1, mass spectra of the 12 C(π +,K + ) 12 Λ C reaction, which is now a standard of the hypernuclear reaction spectroscopy, are shown. ter at BNL-AGS with 3 MeV(FWHM) resolution and the other by the SKS spectrometer at KEK with 2 MeV(FWHM) resolution. Spectra by the SKS spectrometer was further improved to 1.45 MeV(FWHM) for a thinner target.[6] These spectra clearly demonstrate the importance of the energy resolution in the spectroscopy. With the better resolution, satellite peaks between the two prominent ones were for the first time observed and were interpreted as a Λ hyperon coupling to the excited states of the 11 C core in connection with the ΛN interaction. Those high precision hypernuclear mass spectra firmly established the field of hypernuclear spectroscopy. In addition, the E140a experiment of KEK-PS, the first of the series of experiments, intended to investigate the nature of a Λ hyperon deeply bound to heavy nuclei. The binding energies of a Λ hyperon bound by a wide range of hypernuclei such as Λ Y, Λ La and even Λ Pb were obtained from the excitation spectra. The spectra of Λ La and Λ Pb were depicted in Fig 2 It was revealed that the single-particle nature of a Λ hyperon persists to the first order even in nuclei as heavy as A=208.[7] Much improved statistics and resolution were obtained for 89 Λ Y in the most recent E369 experiment.[6] The spectrum is also shown in Fig.3. It is now well demonstrated that the (π +,K + ) reaction spectroscopy is a powerful tool for the investigation of hypernuclear physics. However, it is also clear that the resolution of these spectra are not high enough to resolve the detail related to the nuclear structure and/or ΛN interaction. And it is certainly required to further explore high quality hypernuclear spectroscopy. III. HYPERNUCLEAR γ RAY SPECTROSCOPY FIG. 1: Missing mass spectra of the 12 C(π +,K + ) 12 Λ C reaction with 3 and 2 MeV(FWHM) resolution. One was obtained using the Moby-Dick spectrome- The γ ray spectroscopy offers high precision measurements for those γ decaying hypernuclear states. However, it has been difficult to perform γ ray spectroscopy for Λ hypernuclei, since the hypernuclear yield rates are very much limited due to small cross section and low beam intensities of kaon and pion beams. The spectroscopy with Ge detectors, with which we can expect a few kev energy resolution, has been thought furthermore difficult since the photo-peak efficiency is usually low compared to NaI detectors and also operation of Ge detectors under high counting rate environment involves technical difficulties. The KEK E419 experiment was the first that succeeded in observing hypernuclear γ rays with Ge detectors.[8, 9] It was carried out by installing the HY- PERBALL system, which consists of 14 Ge detectors equipped with a fast transistor-reset preamplifiers, in the target region of the SKS spectrometer system. The overall photopeak detection efficiency of ɛω = 2.5 % for 1 MeV γ rays was achieved. The BGO counters surrounding each Ge detector were used to suppress background coming from π 0 s and Compton γ rays. Two hypernuclear γ rays from 7 ΛLi, which were excited through the 7 Λ Li(π+,K + ) reaction, were observed as shown in Fig. 4.

3 FIG. 2: Missing mass spectrum of 139 Λ La and 208 Λ Pb(KEK E140a) Λ hyperon.[10] Hypernuclear γ-ray spectroscopy was also carried out recently for the 9 Be(K,π ) 9 ΛBe reaction at BNL AGS, installing the same hyperball system in the target region of the D line spectrometer.[11] It has observed the splitting of the ls partner levels, 3/2 + and 5/2 +, which were not resolved in the previous BNL experiment using NaI detectors. Small ls splitting of 31.4 kev was measured and thus a S Λ value much smaller than the one predicted by the OBE model was derived. The result was interpreted that both symmetric and anti-symmetric ls forces are comparable in the magnitude and cancel out each other resulting in the small ls force. Effort to further increase the detection efficiency with clover-type Ge detectors are under way which will eventually realize twice larger photo-peak efficiency. FIG. 3: Missing mass spectrum of 89 Λ Y. (KEK E369) IV. HYPERNUCLEAR STUDY WITH ELECTRON BEAMS One is the M1 transition between 3/2 + 1/2 + and the energy is 689 ± 4 kev. The γ energy were directly compared with the recent calculation based on a cluster model and the magnitude of spin-spin interaction was derived to be consistent with widely adopted values of =0.5 MeV.[8] The 2050 kev peak which corresponds to the 5/2 + 1/2 + transition has a broader peak shape due to Doppler effect. After line shape fitting, the lifetime of the 5/2 + state was derived to be τ = 5.2 ± 0.8 ± 0.6 ps. The B(E2) value of the transition is strongly dependent on the charge distribution of the hypernucleus. It was found that the spatial distance between α and (p+n) shrinks due to the presence of a Λ hyperon, which was theoretically predicted as the glue like role of In Table I, hypernuclear production reactions for the hypernuclear spectroscopic investigation are listed. Each reaction in the table has its own advantages in the spectroscopic study of hypernuclei. Hypernuclear production with electron beams, which requires high-quality electron beams in the GeV region, has been thought to play a unique role in the hypernuclear study. In contrast to meson beams which have been mostly used for the hypernuclear experiments, the (e,e K + ) reaction involves large spin-flip amplitude even at 0 degrees. Thus, it excites both spin flip and spin non-flip hypernuclear states efficiently. The reaction converts a proton to a Λ hyperon, contrary to the (π +,K + ) and (K,π ) reactions. Therefore, it produces Λ hypernuclei which are not accessible by such reactions. The (e,e K + ) reaction populates neutron rich Λ hypernuclei, for example 7 ΛHe from the 7 Li target, which would be the neutron hallo Λ hypernuclei. Being most important from the experimental point of view, the (e,e K + ) reaction has a potential power that will realize

4 FIG. 4: Energy spectrum of γ rays emitted from the decay of 7 ΛLi(KEK E419). TABLE I: Comparison of Λ hyperon production reactions Z =0 Z = 1 comment neutron to Λ proton to Λ (π +,K + ) (π,k 0 ) stretched, high spin in-flight (K,π ) in-flight (K,π 0 ) substitutional at low momentum stopped (K,π ) stopped (K,π 0 ) large yield, via atomic states (e, e K 0 ) (e, e K + ) spin flip, unnatural parity, virtual (γ,k) (p, p K 0 ) (p, p K + ) virtual (π,k) (p, K 0 ) (p, K + ) very large momentum transfer hypernuclear reaction spectroscopy with sub-mev energy resolution. Even a few 100 kev resolution, which is comparable to that of NaI γ detectors, can be achieved once an appropriate spectrometer system is on hand. Such high resolution is achievable because the electron beam is primary and has excellent beam emittance. A small beam spot size of the order of 0.1 mm at the target and high beam current are easily available. Thus, we can employ small and thin targets, even for enriched isotopes. Elementary strangeness production by the photoelectron beams has been studied for various reaction channels until now. However, there are few investigations on the strangeness production with the nuclear target. Only at Tanashi-ES, quasifree Λ production by the tagged photon beam was observed by measuring charged kaons.[12] Up until recently, the (e,e K + ) spectroscopy has been impractical, since high-duty high-quality beams have not been available. It also requires a scattered electron spectrometer that has good energy resolution and can handle high rate electrons which are associated with bremstrahlung from the target and a kaon spectrometer which has reasonable momentum resolution to realize the spectroscopic measurements. E experiment at Jlab Hall C Very recently, the first hypernuclear spectroscopy experiment(e89-009) by the (e,e K + ) reaction was successfully carried out at Hall C of Jefferson National Laboratory(Jlab). [13] Continuous electron beams in the GeV region, which is vital to carry out the (e,e K + ) reaction experiments, are now available only at Jlab. Kinematical conditions of detecting kaons and scattered electrons in the experiment are depicted in Fig. 5. The electron beam of 1.8 GeV bombarded the target and the scattered electrons at 0 degrees which were associated with virtual photons were bent by a splitter magnet and were analyzed by the ENGE spectrometer. The splitter magnet also deflected kaons at forward angles to the opposite direction so that kaon momentum were analyzed by the short-orbit spectrometer(sos) standard at Hall C. Since angular distribution of the virtual photons which are associated with the strangeness production is very much for-

5 FIG. 5: Kinematical condition of the (e,e K + ) reaction for Jlab E ward peaked( nearly at zero degrees) and kaon angular distributions are also forward peaked, this configuration has an advantage that very high intensity beam is not required. The mass spectrum of the 12 C(e,e K + ) 12 Λ B reaction is shown in Fig. 6. nb/sr/400kev #1 (#5) #2 (#4) (#6) #3 M HY -M A (MeV) B Λ (MeV) FIG. 6: Hypernuclear missing mass spectrum in the reaction of 12 C(e,e K + ) 12 Λ B The ground state peak, since it is thought the ground state doublet due to the spin-spin interaction is quite small, demonstrates that the experimental resolution of sub-mev is achieved(0.9 MeV(FWHM)). However, the spectrum was obtained spending net 1 month data taking runs and still the statistics are poor and accidental background level is high. The beam intensity was limited because of the huge background due to bremsstrahlung associated electrons, rate of which amounted beyond a few hundred MHz at the focal plane of the electrons(enge) spectrometer. Because of that, the yield rate of 12 Λ B spectrum was low and the signal-toaccidental ratio was also poor. E experiment with the HKS spectrometer Aiming to drastically improve the (e,e K + ) hypernuclear spectroscopy both in the p-shell region and beyond the p-shell region, a new experiment is under preparation at Jlab(E01-011) In the E experiment, we have introduced Tile method in order to overcome the limitation of the 0 degree tagging method.[15] A new high resolution kaons spectrometer(hks) is also new designed and being constructed, because the resolution of hypernuclear mass spectra was determined by the resolution of the previous kaon spectrometer (SOS) and also the hypernuclear yield rates can be improved by the improved factor of its acceptance. In the proposed E experimental configuration[15], which is schematically shown in Fig. 7, scattered electrons and kaons are bent to the opposite directions each other by a splitter magnet as in the case of the E experiment. However, the electron spectrometer is to be tilted vertically by a small angle in order to avoid dominant backgrounds of electrons due to the bremsstrahlung process and Moeller scattering. Since the scattered electrons are momentum dispersed horizontally by the splitter magnet, the scattered electron spectrometer needs to be tilted vertically. The tilt method works because the angular distribution of the bremsstrahlung associated electrons is much more sharply forward-peaked compared to that of the virtual photon associated. An optimum tilt angle is determined taking into account the angular distribution and multiple scattering of virtual photon and bremsstrahlung associated electrons and Moeller electrons. In the present optimized condition, we expect to run a 100 mg/cm 2 12 C target with an electron beam intensity of 30 µa. Even then, singles rate at the focal plane of the Enge spectrometer is much smaller than the previous E experiment, while hadronic rates of the HKS spectrometer are much higher. In addition, the momentum acceptances of the two spectrometers are matched with each other to maximize hypernuclear yields. As a result, it is expected hypernuclear yield rates increases by about a factor of 50 and becomes comparable to the present (π +,K + ) reaction spectrosocpy. The mass resolution will be also improved to kev(fwhm) which is better that that of E by more than a factor of two. Electrophoto strangeness production It is also mentioned here that the process of electrophoto strangeness production has not been fully studied so far, particularly in the threshold region where the (e,e K + ) reaction is relevant. Recent progress of photo-strangeness production experiments at BONN ELSA stimulated elaborated theoretical investigation on phenomenological models. [14]. However, the model predictions on the cross section and angular distribution varied from model to model, if we look into the γ + n Λ+K 0 channel. The channel is unique because no charge are involved in the reaction and it is thought this channel plays an important role for the investigation of electrophoto-production of strangeness, information of which is also required to fully study the (e,e K + ) reaction for the hypernuclear spectroscopy.. Experiments to measure cross sections and angular distributions of neutral kaons in quasifree and elementary processes are under way at the 1.2 GeV booster synchrotron of LNS, Tohoku University. In Fig. 8 is shown a general view of the facility that provides internally tagged photons up to 1.1 GeV with a duty factor of %. We have installed Neutral Kaon Spectrometer(NKS), which is based on the TAGX spectrometer of INS-ES. Neutral kaons(k s) decay to positive and negative pions(b.r %). These pions are detected in coincidence by the central drift chambers installed in the gap of the dipole magnet. Since the

6 Enge Spectrometer Wire Chamber Hodoscope Beam line Photon line Beam Q1 Slit box 1.5T Q2 Splitter magnet Target Vacuum chamber extension HKS High resolution kaon spectrometer Schematic Top View of New Hypernuclear Spectrometer at Jlab 0 1 2m DC ^ AC WC TOF ^ FIG. 7: Schematic drawing of the high-resolution kaons spectrometer dedicated to the (e,e K + ) hypernuclear spectroscopy. FIG. 9: Preliminary missing mass spectrum of neutral kaons produced GeV γ + 12 C. FIG. 8: Schematic view of the 1.2 GeV stretcher/booster ring and internal tagger at LNS-Tohoku university lifetime of neutral kaons is cτ =2.68cm, many of the neutral kaons decay outside the target. Commissioning runs with the natural carbon target was carried out recently. A preliminary π + π missing mass spectrum is shown in Fig. 9, where the peak at the K s mass is seen with the yield of about 120. Based on the success of the commissioning runs, data taking with the C target as well as with the liquid deuterium target will be carried out in V. SUMMARY AND FUTURE PROSPECTS Through the recent progress, we have learned that the hypernuclear spectroscopy is invaluable to the investigation of the Λ hypernuclear structure and ΛN interaction. The recent (π +,K + ) reaction spectroscopy with the SKS spectrometer established the value of the hypernuclear spectroscopy. Thanks to the good resolution of MeV and the high detection efficiency, good quality spectra were measured for wide variety of Λ hypernuclei, providing information both on light and heavy Λ excitation spectra. Hypernuclear γ ray spectroscopy is being rapidly developed and obtained invaluable information on the magnitude of spin-dependent ΛN interaction. Upgrading the Ge detector system to the higher efficiency will be done in the near future and the spectroscopy should become much more efficient. In the near future, we expect the reaction spectroscopy by the (e,e K + ) reaction and the γ ray spectroscopic studies with meson beams will be fully developed. The (e,e K + ) reactions that convert a proton to a Λ hyperon will allow us to investigate neutron-rich Λ hypernuclei not investigated so far and well reveal new aspects of Λ hyper-

7 nuclear structure. A new spectrometer system dedicated to the (e,e K + ) reaction is under construction and the Jlab E experiment is expected to run in 2004, aiming the mass resolution of kev(fwhm). The γ-ray spectroscopy with a few kev resolution, has a significant impact to the Λ hypernuclear physics. It will be further extended with much higher efficiency of a Ge detector array. The reaction spectroscopy, which can study hypernuclear states above the nucleon emission threshold, and the γ-ray spectroscopy which has almost 3 orders of magnitude better energy resolution for the states below the nucleon emission threshold are complimentary to each other in the spectroscopic investigation of Λ hypernuclei. Although the present paper deals mostly with spectroscopy of Λ hypernuclei, spectroscopy of in the S=-2 regime will also open a new field when high intensity kaon beams are available in the future facility such as the 50 GeV PS of J-PARC. The experiments with the SKS spectrometer were carried out under collaboration of Tohoku, KEK, Seoul, Osaka, Tokyo, Hampton, Houston, and NCA&T and those by the electromagnetic strangeness production under collaboration of Tohoku, Jlab, FIU, Houston etc. The author deeply thanks to all the collaborators of these collaborating institutions. [1] Y. Yamamoto et. al., Prog. Theor. Phys. Supplement No. 118, 361 (1994). [2] C. Milner et. al., Phys. Rev. Lett. 54, 1237 (1985). P. H. Pile et al., Phys. Rev. Lett. 66, 2585 (1991). [3] M. Akei et al., Nucl. Phys. A534, 478 (1991). [4] T. Fukuda, et. al., Nucl. Instr. Meth. A361, 485 (1995). [5] T. Hasegawa, et. al., Phys. Rev. Lett. 74, 224 (1995). [6] H. Hotchi et. al., Phys. Rev. C (2001). [7] T. Hasegawa, et. al., Phys. Rev. C53 (1996) [8] H. Tamura et al., Phys. Rev. Lett. 84, 5963 (2000). [9] K. Tanida et al., Phys. Rev. Lett. 86, 1982 (2001). [10] T. Motoba, H. Bando and K. Ikeda, Prog. Theor. Phys. 70, 189 (1983). [11] H. Akikawa et al., Phys. Rev. Lett. 88, (2002). [12] H. Yamazaki et al., Phys. Rev. C 52, 1157 (1995). [13] T. Miyoshi et al., Phys. Rev. Lett. to be published [14] R.A. Adelseck, et. al., Phys. Rev. C42, 108 (1990); R.A. Williams, et. al., Phys. Rev. C46, 1617 (1992); T. Mizutani, et. al., Phys. Rev. C58, 75 (1998). [15] Jlab proposal E01-011, Spokespersons O. Hashimoto, L. Tang, J. Reinhold and S. Nakamura.