We present a search for low-mass ($leq 20 GeV/c^{2}$) weakly interacting massive particles(WIMPs), strong candidates of dark matter particles,using the low-background CsI(Tl) detector array of the Korea Invisible Mass Search (KIMS) experiment. With a
total data exposure of 24,324.3kg$cdot$days,we search for WIMP interaction signals produced by nuclei recoiling from WIMP-nuclear elastic scattering with visible energies between 2 and 4keV. The observed energy distribution of candidate events is consistent with null signals, and upper limits of the WIMP-proton spin-independent interaction are set with a 90% confidence level. The observed limit rejects most of the low mass region of parameter space favored by the DAMA annual modulation signal.
The unbound excited states of the neutron drip-line isotope 24O have been investigated via the 24O(p,p)23O+n reaction in inverse kinematics at a beam energy of 62 MeV/nucleon. The decay energy spectrum of 24O* was reconstructed from the momenta of 23
O and the neutron. The spin-parity of the first excited state, observed at Ex = 4.65 +/- 0.14 MeV, was determined to be Jpi = 2+ from the angular distribution of the cross section. Higher lying states were also observed. The quadrupole transition parameter beta2 of the 2+ state was deduced, for the first time, to be 0.15 +/- 0.04. The relatively high excitation energy and small beta2 value are indicative of the N = 16 shell closure in 24O.
Correlations of back-to-back coincident $Lambda d$ and $Lambda t$ pairs from the stopped $K^-$ reaction on $^4$He had been investigated, thereby $Lambda d$ and $Lambda t$ branches of non-mesonic three- and four-nucleon absorption processes of antikao
n at rest were identified as well-separable processes, respectively. The branching ratio of the three-nucleon process, ($^4$He-$K^-$)$_{atomic} rightarrow Lambda d n$, is estimated to be $(0.9 pm 0.1 (stat) pm 0.2 (syst)) times 10^{-3}$ from the normalized $Lambda d$ spectrum in a $Lambda d n$ final state, while the fraction of the four-nucleon process, ($^4$He-$K^-$)$_{atomic} rightarrow Lambda t$, is obtained to be $(3.1 pm 0.4 (stat) pm 0.5 (syst)) times 10^{-4}$ per stopped $K^-$.
We have observed an intense high-energy component in an inclusive $^4$He(stopped $K^-$, $d$) spectrum. For back-to-back $Lambda d$ pairs a prominent event cluster has been found just below the $m_{^4textrm{He}}+m_{K^-}-m_{n}$ mass threshold in the $L
ambda d$ invariant mass spectrum of $Lambda dn$ events, which is evidence for a three-nucleon absorption process of $K^-$ in $^4$He. In addition, an appreciable strength is revealed below $sim$3220 MeV/$c^2$. Well separated $Sigma^0 dn$ events show a peak similar to the case of $Lambda dn$.
We have investigated correlations of coincident $Lambda N$ pairs from the stopped $K^-$ reaction on ${}^4$He, and clearly observed $Lambda p$ and $Lambda n$ branches of the two-nucleon absorption process in the $Lambda N$ invariant mass spectra. In a
ddition, non-mesonic reaction channels, which indicate possible exotic signals for the formation of strange multibaryon states, have been identified.
A search for tribaryon states was performed at KEK-PS. We adopted the $^4$He($K^-_{stopped}, p)$ reaction to populate the states with strangeness -1, charge 0 and isospin 1. No significant narrow structure was observed in the mass region from 3000 to
3200 MeV/c^2 in an inclusive missing mass spectrum. The upper limit of the formation branching ratio was determined to be ($1sim3)times 10^{-4}$, ($0.7sim2)times 10^{-3}$ and ($2sim8) times 10^{-3}$/($stopped K^{-}$) with 95 % confidence level for narrow states with an assumed width of 0, 20 and 40 MeV/c^2, respectively.
We have measured the Balmer-series x-rays of kaonic $^4$He atoms using novel large-area silicon drift x-ray detectors in order to study the low-energy $bar{K}$-nucleus strong interaction. The energy of the $3d to 2p$ transition was determined to be 6
467 $pm$ 3 (stat) $pm$ 2 (syst) eV. The resulting strong-interaction energy-level shift is in agreement with theoretical calculations, thus eliminating a long-standing discrepancy between theory and experiment.
Very recently, we have performed a couple of experiments, {it{KEK PS-E549/E570}}, for the detailed study of the strange tribaryon $S^0(3115)$ obtained in {it{KEK PS-E471}}. These experiments were performed to accumulate much higher statistics with im
proved experimental apparatusespecially for the better proton spectroscopy of the $^4$He({it{stopped K}}$^-$, {it{N}}) reaction. In contrast to the previous proton spectrum, no narrow ($sim$ 20 MeV) peak structure was found either in the inclusive $^4$He({it{stopped K}}$^-$, {it{p}}) or in the semi-inclusive $^4$He({it{stopped K}}$^-$, {it{p}}$X^pm$) reaction channel, which is equivalent to the previous $E471$ event trigger condition. Detailed analysis of the present data and simulation shows that the peak, corresponding to $S^0(3115)$, has been an experimental artifact. Present analysis does not exclude the possible existence of a much wider structure. To be sensitive to such structure and for better understanding of the non-mesonic $K^-$ absorption reaction channel, detailed analysis of the data is in progress.
