KEK Annual Report 2005

KEK-PS E570
– Precision spectroscopy of Kaonic Helium 3d → 2p X-rays –In E570, we have measured the strong-interaction shift of 3d → 2p X-rays of kaonic helium 4 atom with a precision better than ∼2 eV. The measurement will give the answer tothe longstanding “kaonic helium puzzle” and provide a crucial information to understandthe basis of the Akaishi-Yamazaki prediction of deeply-bound kaonic nuclei [1] being oneof the interpretations of the strange multibaryon candidates recently reported at KEK[2], DAΦNE [3] and BNL [4].
The experiment E570 was performed in October 2005 for the beamtime of 80 shifts
and in December 2005 for the extension beamtime of 40 shifts. In E570, a significant improvement over the past experiments [5, 6, 7] was achieved by incorporating the following:• SDDs : Instead of a conventional Si(Li) X-ray detector, we used eight silicon driftdetectors (SDDs). In the SDD, the electrons produced by an X-ray hit are radiallydrifted toward the anode at the center and are collected there, so that the anodesize (and hence its capacitance) can be kept small, independent of the detector area.
This results in a good energy resolution despite a large effective area of some 100mm2 . In addition, the small anode area makes it possible to reduce the active layer
thickness, while still keeping the small capacitance. The thin active layer (260 µmin the case of E570 SDDs, compared with 4 mm for Si(Li) counters used in thepast experiments) helps reduce continuum background caused by the soft-Compton
process. The typical energy resolution is ∼185 eV (FWHM) at 6.4 keV whichcorresponds to the energy of 3d → 2p X-rays of kaonic helium atoms.
• Fiducial volume cut : Continuum background events could be drastically reduced
by applying a “fiducial volume cut”, which requires that the reaction vertex obtainedby tracing an incident kaon and a secondary charged particle is within the liquid
helium volume. With the fiducial cut, a good S/N ratio of ∼4 was achieved asshown in Fig. 2, which is about 5 times better than that of the past experiments.
• In-beam energy calibration : The energy calibration was done by characteristicX-rays induced by the incident beam (mainly contaminating pions in the kaon beam)on pure titanium and nickel foils, and simultaneously measuring the kaonic helium
atom X-rays. Since the energy of the 3d → 2p kaonic helium atom X-ray, ∼6.4 keV,lies between the characteristic X-ray energies, 4.5 keV(Ti) and 7.5 keV(Ni), thiswill provide an accurate in-situ calibration. Figure 1 shows a characteristic X-ray
spectrum for a typical SDD with calibration self trigger. We have observed 4.0 ×103
titanium Kα X-rays per one shift (8 hours) for each SDD.
Figure 2 shows a preliminary kaonic helium atom X-ray energy spectrum for about
50% of the total E570 statistics. The 3d → 2p peak at ∼6.4 keV has been clearly observed,
together with other transitions feeding the 2p state, 4d → 2p and 5d → 2p. In total, E570
accumulated about 1.5 × 103 counts of nearly background-free 3d → 2p X-rays, which
implies, with a 185 eV (FWHM at 6.4 keV) resolution, we will achieve a statistical error√of σ ∼ 2 eV (=185/2.35/ 1.5 × 103 eV). The offline analysis including the systematicerror estimation is now in progress.

Figure 1: A typical characteristic X-rayspectrum with calibration self trigger.

Figure 2: Preliminary kaonic helium atom
X-ray energy spectrum for half of all E570
statistics.

References[1] Y. Akaishi and T. Yamazaki, Phys Rev. C65, 044005 (2002) .
[2] T. Suzuki et al., Phys. Lett. B597, 263-269 (2004).
[3] M. Agnello et al., Phys. Rev. Lett. 94, 212303 (2005).
[4] T. Kishimoto et al., Nucl. Phys. A 754, 383c (2005).
[5] C. E. Wiegand and R. Pehl, Phys. Rev. Lett. 27, 1410 (1971).
[6] C. J. Batty et al., Nucl. Phys. A326, 455 (1979).
[7] S. Baird et al., Nucl. Phys. A392, 297 (1983).