The g -ray spectroscopy of light hypernuclei at J-PARC (E13)

1. eh mq= 2mqc mq : Constituent quark mass Level scheme of 7LLi K. Shirotori for the Hyperball-J collaboration Department of Physics, Tohoku Univ., Japan Contacts E-mail : sirotori@lambda.phys.tohoku.ac.jp Web : http://lambda.phys.tohoku.ac.jp/~sirotori/ Introduction Hypernuclei of interest g-ray measured hypernuclei The study of hypernuclei is one of the ways to understand the baryon-baryon (BB) interactions, through the investigation of hyperon-nucleon interactions, the properties of baryons in the nuclear matter, and impurity effects of L on the core nucleus. The LNinteraction is studied through the L hypernuclear level structure and its precise structure can only be observed from the g-ray spectroscopy by using germanium (Ge) detectors. The method of g-ray spectroscopy with Ge detectors has been successfully used to study structure of light p-shell L hypernuclei. • 7LLi One of the purposes of the experiment is to measure the reduced transition probability (B(M1) of the L spin-flip M1 transition. The magnetic moment of a L inside of a nucleus will be extracted from the 7LLi (M1; 3/2+→1/2+) transition probability. • 4LHe The level structure and the mass spectrum of 4LHe compared with that of 4LH measured in out dated experiments give the information on charge symmetry breaking of the LN interaction. From g-ray yield, the cross sections of the spin-flip 4LHe(1+) and non-spin-flip 4LHe(0+) states for several K- beam momenta will also be measured to study the spin-flip/non-spin-flip property of hypernuclear production in the (K-, p-) reaction. • 10LB, 11LB,19LF Another is to investigate the LN interaction further our previous studies in p-shell hypernuclei (10LB and 11LB ). In addition, we also study 19LF level which gives the strength of the effective LN interaction in the sd-shell hypernuclei for the first time. The g-ray spectroscopy of light hypernuclei at J-PARC (E13) For the first hypernuclear g-ray spectroscopy experiment at the J-PARC K1.8 beam line (J-PARC E13), several light hypernuclei (4LHe, 7LLi, 10LB, 11LB and 19LF) are planned to be studied. Hypernuclei of interest have been chosen from the past experimental results. Exited states of L hypernuclei are produced via the (K-, p-) reaction at the incident Kaon beam momentum of 1.5 GeV/c. Kaon beams and scattered pions are identified and momentum-analyzed by using the K1.8 beam line spectrometer and the modified SKS (Superconducting Kaon Spectrometer), SksMinus, respectively. g rays from the hypernuclei are measured by the Ge detector array, Hyperball-J, placed around the target. Through the coincidence measurement between these spectrometer systems and Hyperball-J, g rays from produced hypernuclei are identified. 7LLi : Change of the baryon property in nuclear medium For the accrete measurement, we produce 3/2+ state from the feeding of the upper level, 1/2+(T=1) state to only select the forward scattering angle. For the proper stopping time, the recoil velocity can be as small as possible, and the feeding of 7/2+ state should be minimized because its branching ratio haven't been experimentally determined. From the simulation and the yield estimate, we expect the accuracy of B(M1) of less than 5% included systematic errors. The reduced transition probability B(M1) is related to the lifetime (t) of the excited state via 1/t∝ B(M1). The lifetime is obtained by analyzing the partly Doppler-broadened peak shape of the gray from recoiling hypernucleus which is slowing down in the target. The lifetime of excited states has to be of the same order as the stopping time. This method is called Doppler shift attenuation method (DSAM). The expected lifetime of the spin-flip M1 transition (3/2+→1/2+) is ~0.5 ps [2]. The target is chosen to be Li2O (2.01 g/cm3) in which the stopping time of the recoil 7LLi is 2–3 ps at the (K-, p-) reaction at 1.5 GeV/c beam. It is close to the ideal condition. The magnetic moment of baryons is described well by the constituent quark model picture. Each constituent quark has a magnetic moment of Dirac particle having the constituent quark mass. If the baryon mass, in turn the constituent quark mass, is changed in the nuclear medium by possible partial restoration of chiral symmetry, the change of the magnetic moment of baryon should be observed. Thus we approach to understand the origin of mass. Nuclear medium Free space Change of const. quark mass smaller mq ⇒ larger mq ? Simulation result Cross section of 7LLi [3] The direct measurement of the magnetic moment of L hypernuclei is extremely difficult because of their short lifetime. The magnetic moment of L in the nucleus can be measured by the spin-flip B(M1) transition between the upper and lower level of the hypernuclear spin-doublet states (right figure). g-ray spectrum of 7LLi [1] Doppler shift attenuation method (DSAM) Lifetime ~ Stopping time Shape of g-ray spectrum = Eg_shifted (recoiling) + Eg_not-shifted (stopped) ⇒Compared with response function by simulation ⇒Extracted lifetime simulated spectrum Forward scattering 4LHe : Charge symmetry breaking of LN interaction and spin-flip property of hypernuclear production For the future hypernuclear g-ray spectroscopy at J-PARC, the measurement of the cross section of the spin-flip state is important for the study of the (K-, p-) reaction in the nuclear medium. 4LHe will be studied because 4LHe has only one excited state, 4LHe(1+), and this state is a pure spin-flip state. In the experiment, the 1+→0+g-ray transition is measured and the cross section of the spin-flip state is determined at several momenta (e.g. 1.1, 1.3, 1.5, 1.8 GeV/c). If the charge symmetry holds in the baryon-baryon interaction, the Lp interaction and Ln interaction should be the same because the L has no isospin and charge. In the case of 4LH and 4LHe which are the lightest mirror pair of hypernuclei, their energy difference is quite large and the charge symmetry breaking is suggested [4]. From the binding energies, the Lp interaction seems to be more attractive than that of the Ln interaction. Level energies of 4LH and 4LHe Cross section of elementally process [5] With the reaction spectroscopy using only the magnetic spectrometer, it is difficult to resolve the spin-flip state because the energy spacing between the spin-flip state (4LHe(1+)) and the spin-non-flip state (4LHe(0+)) is too small (~1 MeV) for the resolution of the spectrometer. The g-ray spectroscopy is suitable to measure the cross section of the spin-flip state in 4LHe. Old data of 4LH and 4LHe by NaI [4] The origin of CSB is not understood yet. CSB is related to the LN-SN coupling effect. For the explanation, it is suggested that the mass difference of intermediate S+, S0 and S- causes the CSB. The difference is some 8 MeV which is 10% of the mass difference of L and S. Therefore, the contribution of the LNN force is suggested. To understand the mechanism of CSB, systematic study of mirror hypernuclei is necessary. In E13 experiment, the g-ray spectroscopy experiment of 4LHe will be performed with high statistics and much better energy resolution by germanium detectors. 10LB, 11LB, 19LF : Study of LN interaction Experimental apparatus Setup of J-PARC E13 experiment Missing mass analysis : magnetic spectrometers (identification of hypernuclear bound states ) The first complete set of parameters of the LN interaction were determined from hypernuclear g-ray spectroscopy of 7LLi, 9LBe and 16LO. Then the consistency was checked by the other hypernuclei. The data from other spin-doublet state of 7LLi and L-spin-orbit state of 13LC give the consistent parameters. However, the 10LB and 11LB data are inconsistent. Those inconsistencies suggest the necessity of a correct treatment of the core nuclear wave function and an inclusion of the LN-SN coupling effect [6]. Thus more data are necessary. If the energy spacing of the ground state doublet of 19LF is measured, this energy gives information on the spin-spin interaction of sd-shell hypernuclei. The ground state spin of the core 18F is determined only from the spin of nucleons in the sd-orbit (s and d could mix because of the same parity). The interaction between the L in 0s-orbit and the nucleons in the sd-orbit determines the energy spacing of the ground state doublet. From the energy, the interaction parameter of spin-spin interaction of sd-shell hypernuclei which corresponds to D of p-shell hypernuclei can be extracted. Scattered p-, K+ Beam K-, p+ g g-ray measurement by Ge detector array g rays from hypernuclei : Reaction-g coincidence References In addition, 19LF is one of the candidate of the B(M1) measurement ofthe ground state doublet, because the core nucleus of 18F has similar structure to the 6Li of the 7LLi core (both closed shell nuclei with p-n pair, 7LLi : a + pn + L ⇔ 19LF :16O + pn + L). Expected level scheme of 19LF [7] H .Tamura et al., J-PARC proposal “Gamma-ray spectroscopy of light hypernuclei” (2006) [1] K. Tanida et al., PRL 86 (2001) 1982 • [2] E. Hiyama et al., PRC 59 (1999) 2351 • [3] T. Motoba, private communication (2006) • [4] M. Bedjidian et al. PLB 83 (1979) 252 • [5] T. Harada, private communication (2006) • [6] Y. Akaishi et al., PRL 84 (2000) 3539 • [7] D. J. Mollener, private communication (2006) • See also T. Yamamoto poster, “Detail in Hyperball-J” Spin-dependent interaction + LN-SN coupling effect Hyperball How effect ? B(M1)? + LS Hyperball2