New Limits on Interactions between Weakly Interacting Massive Particles and Nucleons Obtained with CsI(Tl) Crystal Detectors

New limits are presented on the cross section for weakly interacting massive particle (WIMP) nucleon scattering in the KIMS $\mathrm{CsI}(\mathrm{T}\ell )$ detector array at the Yangyang Underground Laboratory. The exposure used for these results is $24524.3\mathrm{kg}·\mathrm{days}$. Nuclei recoiling from WIMP interactions are identified by a pulse shape discrimination method. A low energy background due to alpha emitters on the crystal surfaces is identified and taken into account in the analysis. The detected numbers of nuclear recoils are consistent with zero and 90% confidence level upper limits on the WIMP interaction rates are set for electron equivalent energies from 3 to 11 keV. The 90% upper limit of the nuclear recoil event rate for 3.6–5.8 keV corresponding to 2–4 keV in $\mathrm{NaI}(\mathrm{T}\ell )$ is 0.0098 counts/kg/keV/day, which is below the annual modulation amplitude reported by DAMA. This is incompatible with interpretations that enhance the modulation amplitude such as inelastic dark matter models. We establish the most stringent cross section limits on spin-dependent WIMP-proton elastic scattering for the WIMP masses greater than $20\mathrm{GeV}/{\mathrm{c}}^2$.

Figure 1

LMT10 distributions at 3 keV for (a) SA with test crystal 1, (b) gammas with test crystal 1, (c) neutrons with test crystal 2, (d) gammas with test crystal 2, (e) Compton scattering events in detector 0 used in the WIMP search. This data sample is obtained at $(25.2\pm 0.2)°\mathrm{C}$ for comparisons with other reference data.

Figure 2

(a) Two-dimensional plot of $f_{N,i}$ (horizontal) versus $f_{S,i}$ (vertical). (b) Projections of $f_{S,i}$ and (c) $f_{N,i}$ for the 6 keV bin of detector 9. (d) The fitted LMT10 distribution for the 6 keV bin of detector 9.

Figure 3

Nuclear recoil event rates for all detectors. The black horizontal bar indicate 90% C.L. upper limits, the red vertical lines denote the 68% C.L. interval, and the red horizontal bars the most probable values.

Figure 4

Total nuclear recoil event rates from the combined results from nine detectors (without detector 0, 8, and 11).

Figure 5

The 90% exclusion limits on (Left) SI WIMP-nucleon and (Right) SD WIMP-proton cross sections. In both plots DAMA results interpreted by Savage et al. [16] are used ($3\sigma$ contours are drawn). The SI plot includes NAIAD [6], CRESST-II [7], EDELWEISS-II [8], ZEPLIN-III [9], XENON100 [10], and CDMS [11] limits. The SD plot includes PICASSO [12] and COUPP [13] limits.

Figure 6

The allowed parameter space for DAMA/LIBRA [17] and the limits reported here for a 70 GeV WIMP mass in iDM model. $\delta$ is the mass split between the ground and excited states of the WIMP. The astronomical parameters from Ref. [17] are used.