ar X iv : 0 70 4 . 04 23 v 2 [ as tr o - ph ] 1 4 A ug 2 00 7 Limits on the WIMP - nucleon interactions with CsI ( Tl ) crystal detectors

The Korea Invisible Mass Search (KIMS) experiment presents new limits on the WIMP-nucleon cross section using data from an exposure of 3409 kg·d taken with low-background CsI(Tl) crystals at Yangyang Underground Laboratory. The most stringent limit on the spin-dependent interaction for a pure proton case is obtained. The DAMA signal region for both spin-independent and spin-dependent interactions for the WIMP masses greater than 20 GeV/c 2 is excluded by the single experiment with crystal scintillators. The existence of dark matter has been widely supported by many astronomical observations on various scales [1][2][3]. Weakly interacting massive particles (WIMPs) are a good candidate for dark matter well motivated by cosmology and supersymmetric models [4]. The Korea Invisible Mass Search (KIMS) experiment has developed low-background CsI(Tl) crystals to detect the signals from the elastic scattering of WIMP off the nucleus [5][6][7]. Both 133 Cs and 127 I are sensitive to the spin-independent (SI) and spin-dependent (SD) interactions of WIMPs. Recently, the role of CsI in the direct search for SD WIMP for pure proton coupling has been pointed out [8]. It is worth noting that 127 I is the dominant target for the SI interactions in the DAMA experiment. The pulse shape discrimination (PSD) technique allows us to statistically separate nuclear recoil (NR) signals of WIMP interactions from the electron recoil (ER) signals due to the gamma ray background [9][10]. The KIMS experiment is located at the Yangyang Un-deground Laboratory (Y2L) at a depth of 700 m under an earth overburden. Details of the KIMS experiment and the first limit with 237 kg·d exposure data can be found in the previous publication [11]. Four low-background CsI(Tl) crystals are installed in the Y2L and operated at a temperature of T = 0 • C. Throughout the exposure period, the temperature of the detector was kept stable to within ±0.1 • C. Green-enhanced photomultiplier tubes (PMTs) are mounted at both ends of each crystal. The signals from the PMTs are amplified and recorded by a 500 MHz FADC. Each event is recorded for a period of 32 µs. Both PMTs on each crystal must have at least two photoelectrons within a 2 µs window to form an event trigger. We obtained 3409 kg·d WIMP search data TABLE I: Crystals used in this analysis and amount of data for each crystal Crystal mass (kg) data (kg·days) with four crystals, as shown in Table I. The energy …