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

1
The g-ray spectroscopy of light hypernuclei at
J-PARC (E13)
K. Shirotori for the Hyperball-J
collaboration Department of Physics, Tohoku
Univ., Japan
Contacts E-mail sirotori_at_lambda.phys.tohoku.ac.j
p 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 LN interaction 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.

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(T1) 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 g 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 7LLi is 23 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 of the 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