Quest for Connecting Quarks to the Cosmos

J-PARC 50-GeV PS E03




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.

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.


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.

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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