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J-PARC E03
is an experiment to measure X rays from exotic atoms including Ξ-,
which has two strange quarks, for the first time in the world.
The physics motivation for the study of Ξ atoms is the strong
interaction of baryons in the S= -2 sector. It has attracted
a lot of attention for various reasons, and has been the biggest
motivation for the construction of the J-PARC 50 GeV proton synchrotron.
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Firstly, based on the SU(3)
classification, new interactions appear up to S= -2. Especially,
the isospin 0 channel (ΛΛ-Ξ N-ΣΣ) is
the unique SU(3) singlet, so that investigation of the S= -2
systems is essential.
The possibility of strong
mixing of Ξ N components into ΛΛ hypernuclei is
another interesting subject, because the mass difference of Ξ
N and ΛΛ is as small as 28 MeV. This is much smaller
than in the case of S= -1 (Λ N-Σ N, ΔM ~
80 MeV), and S=0 (Δ N-NN, ΔM ~ 300 MeV), and the coupling
effect is inversely proportional to the mass difference. If the
conversion from Ξ N to ΛΛ is found to be strong,
then a significant amount of Ξ should mix in ΛΛ
hypernuclei. Therefore, a measurement of the conversion strength
is very interesting.
In addition, the knowledge
of the depth of the Ξ-nucleus potential is also important for
estimating the existence of strange hadronic matter with Ξ's.
For a long time, it was believed that Σ- hyperons
would appear in neutron stars earlier (i.e., at lower densities)
than even lighter Λ hyperons due to their negative charge.
However, recent data strongly suggest that the interaction of Σ-
with neutron-rich nuclear systems is strongly repulsive, which means
Σ- hyperons may no longer appear in neutron stars.
Disappearance of Σ- hyperons would not necessarily
lead to crucial changes of neutron star features if they were substituted
effectively by Ξ- hyperons. In this point of view,
it becomes more important to investigate the Ξ dynamics in the
nuclei than it was considered previously.
However, despite the importance
of S= -2 systems as described above, very little is known experimentally.
Reflecting this situation, there is no established interaction model
in S= -2 channels. Various models are proposed, but they give
remarkably different Ξ N and hence Ξ A interactions. This
fact demonstrates that the experimental information on the ΞA
optical potential ($U_{Ξ}$) including its mass dependence, is
crucially important in order to discriminate reasonable interaction
models.
Here we are planning to
measure X rays from Ξ- atoms to obtain information
on the Ξ A interaction for the first time in the world. This
method has been successfully applied for the study of the interaction
of negatively-charged hadrons, such as π-, K-,
$\bar{p}$, and Σ-, and is thus promising.
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The planned experiment will be performed at the K1.8 beamline of
J-PARC together with the KURAMA spectrometer and a germanium (Ge)
detector array, Hyperball-J. Ξ-'s are produced by
the quasi-free p(K-,K+) Ξ-
reaction at 1.8 GeV/c where the cross section of the elementary
process is at maximum. The produced Ξ- is then brought
to stop in the same target (iron plate of 6 cm width, 1.5 cm height,
and 3 cm thickness).
The X-ray detector system,
Hyperball-J, is an upgraded version of Hyperball (constructed in
1998), and Hyperball2 (constructed in 2005), which have been used
for hypernuclear γ spectroscopy experiments. It consists of
about thirty Ge detectors, each surrounded by fast PWO counters
for background suppression instead of the previous BGO counters.
The total photo-peak efficiency of Hyperball-J is about 16% for
the Ξ--Fe X ray of interest [(6,5) to (5,4)] at around
286 keV. The Ge detectors are constantly monitored and calibrated
by a system using lutenium oxyorthosilicate (LSO) scintillators,
which include 176Lu as a natural radioactive source.
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We expect 2500 counts of
X rays and the statistical accuracy of the level shift will be 0.04
keV, even if the width of the (6,5) to (5,4) X ray would be as large
as 4 keV. Then, the actual accuracy is determined by systematic
effects, such as energy calibration and background subtraction,
and is expected to be about 0.05 keV (or better). For the expected
energy shift of an order of 1 keV, this accuracy is good enough
to determine the strength of the real part of the optical potential.
The sensitivity for the
level width is not so high, we will have sensitivities down to Γ
~ 1 keV. In addition to the direct measurement of level width, there
is another method to obtain information on the imaginary part of
the Ξ- A optical potential.
The comparison of the yields
for $(n,l)=(6,5)\to(5,4)$ and $(n,l)=(7,6)\to(6,5)$ gives an estimation
of the branching ratio of the nuclear absorption at the $(n,l)=(6,5)$
state, after correcting for the other small contributions feeding
the $(n,l)=(6,5)$ state, such as from $(n,l)=(8,6)$. Though such
a correction is slightly model-dependent, we can estimate the strength
of the imaginary potential using the X-ray transition rate which
is precisely calculable. This is especially important when the absorption
is so strong that the X-ray peak for $(n,l)=(6,5)\to(5,4)$ is not
observed. Even in such an extreme case, we will be able to give
quite useful information on the strength of the Ξ N to ΛΛ
coupling.
The
proposal for the first experiment was submitted in April 2006
as J-PARC P03. The proposal was discussed in the meetings of
J-PARC Program Advisory Committee (PAC), and stage-1 (scientific)
and stage-2 (full) approval was granted in August 2006 and March
2008, respectively. No essential difficulty is anticipated in the
experimental setup itself, and the first run is expected in 2011.
We would like to establish the experimental method in the first
experiment.
After the first experiment,
we will design the next experiment as soon as the result is obtained.
If we find the energy shift and width are small, we will use heavier
targets, such as 27Co and 30$Cu. In the opposite
case, we would choose even lighter targets, such as 25Mn.
We also will measure more X rays using targets in other mass regions.
Eventually, our goal is to measure X rays from ~ 10 targets, namely,
from 1 or 2 optimal targets for each (4 <= n <= 9) and to reconstruct
the Ξ A optical potential. Also, measurements of γ rays
from double-Λ hypernuclei may be possible as a byproduct.
Presently, this is the only practical way to perform double-Λ
hypernuclear γ-ray spectroscopy, and we will plan a dedicated
experiment if such a measurement is found to be really possible
in J-PARC E03."
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