Cross section of elementally process [5]

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

K. Shirotori for the Hyperball-J collaborationDepartment of Physics, Tohoku Univ., Japan

Hypernuclei of interestHypernuclei of interest 7

Li One of the purposes of the experiment is to measure the reduced transition probability (B(M1) of the spin-flip M1 transition. The magnetic moment of a inside of a nucleus will be extracted from the 7

Li (M1; 3/2+→1/2+) transition probability.

4He

The level structure and the mass spectrum of 4He compared with that of 4

H measured in out dated experiments give the information on charge symmetry breaking of the N interaction. From -ray yield, the cross sections of the spin-flip 4He(1+) and non-spin-flip 4

He(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-, -) reaction.

10B, 11

B, 19F

Another is to investigate the N interaction further our previous studies in p-shell hypernuclei (10

B and 11B ). In addition, we also study 19

F level which gives the strength of the effective N interaction in the sd-shell hypernuclei for the first time.

For the first hypernuclear -ray spectroscopy experiment at the J-PARC K1.8 beam line (J-PARC E13), several light hypernuclei (4

He, 7Li, 10

B, 11B and 19

F) are planned to be studied. Hypernuclei of interest have been chosen from the past experimental results. Exited states of hypernuclei are produced via the (K-, -) 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. 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, rays from produced hypernuclei are identified.

IntroductionIntroduction

77Li : Change of the baryon property in nuclear mediumLi : Change of the baryon property in nuclear medium

44He : Charge symmetry breaking of He : Charge symmetry breaking of N interaction and spin-flip property of hypernuclear productionN interaction and spin-flip property of hypernuclear production

ContactsE-mail : sirotori@lambda.phys.tohoku.ac.jpWeb : http://lambda.phys.tohoku.ac.jp/~sirotori/

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.

The direct measurement of the magnetic moment of hypernuclei is extremely difficult because of their short lifetime. The magnetic moment of 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).

The reduced transition probability B(M1) is related to the lifetime () of the excited state via 1/ ∝ B(M1). The lifetime is obtained by analyzing the partly Doppler-broadened peak shape of the ray 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 7

Li is 2–3 ps at the (K-, -) reaction at 1.5 GeV/c beam. It is close to the ideal condition.Level scheme of 7

LiCross section of 7

Li [3]

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.

Change of const. quark masssmaller mq ⇒ larger q ?

Free space Nuclear medium

mq : Constituent quark mass

eh2mqcq=

Simulation result

-ray spectrum of 7Li [1]

Lifetime ~ Stopping time

Shape of -ray spectrum

= E_shifted (recoiling)

+ E_not-shifted (stopped)

⇒Compared with response function by simulation

⇒Extracted lifetime

Doppler shift attenuation method (DSAM)

simulated spectrum

If the charge symmetry holds in the baryon-baryon interaction, the p interaction and n interaction should be the same because the has no isospin and charge. In the case of 4

H and 4He 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 p interaction seems to be more attractive than that of the n interaction.

Old data of 4H and 4

He by NaI [4]

Level energies of 4H and 4

He For the future hypernuclear -ray spectroscopy at J-PARC, the measurement of the cross section of the spin-flip state is important for the study of the (K-, -) reaction in the nuclear medium. 4

He will be studied because 4He has only one

excited state, 4He(1+), and this state is a pure spin-flip state. In the experiment,

the 1+→0+ -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).

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 (4

He(1+)) and the spin-non-flip state (4

He(0+)) is too small (~1 MeV) for the resolution of the spectrometer. The -ray spectroscopy is suitable to measure the cross section of the spin-flip state in 4

He.

1010B, B, 1111

B, B, 1919F : Study of F : Study of N interaction N interaction

ReferencesReferencesH .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”

Experimental apparatusExperimental apparatus The first complete set of parameters of the N interaction were determined from hypernuclear -ray spectroscopy of 7

Li, 9Be and 16

O. Then the consistency was checked by the other hypernuclei. The data from other spin-doublet state of 7

Li and -spin-orbit state of 13C give

the consistent parameters. However, the 10B and

11B data are inconsistent. Those inconsistencies

suggest the necessity of a correct treatment of the core nuclear wave function and an inclusion of the N-N coupling effect [6]. Thus more data are necessary.

If the energy spacing of the ground state doublet of 19F 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 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 of p-shell hypernuclei can be extracted.

Spin-dependent interaction + N-N coupling effect

How effect ?

Expected level scheme of 19F [7] In addition, 19

F is one of the candidate of the B(M1) measurement of the ground state doublet, because the core nucleus of 18F has similar structure to the 6Li of the 7

Li core (both closed shell nuclei with p-n pair, 7Li+ pn +

⇔ 19F : 16O + pn + ). B(M1)?

Forward scattering

Setup of J-PARC E13 experiment

Missing mass analysis : magnetic spectrometers(identification of hypernuclear bound states )

Beam K-, + Scattered -, K+

-ray measurement by Ge detector array

rays from hypernuclei : Reaction- coincidence

The origin of CSB is not understood yet. CSB is related to the N-N coupling effect. For the explanation, it is suggested that the mass difference of intermediate +, 0 and - causes the CSB. The difference is some 8 MeV which is 10% of the mass difference of and . Therefore, the contribution of the NN force is suggested. To understand the mechanism of CSB, systematic study of mirror hypernuclei is necessary.

In E13 experiment, the -ray spectroscopy experiment of 4He will be performed

with high statistics and much better energy resolution by germanium detectors.

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 on the core nucleus. The N interaction is studied through the hypernuclear level structure and its precise structure can only be observed from the -ray spectroscopy by using germanium (Ge) detectors. The method of -ray spectroscopy with Ge detectors has been successfully used to study structure of light p-shell hypernuclei.

-ray measured hypernuclei

Hyperball

Hyperball2