Search for WIMP inelastic scattering off xenon nuclei with XENON100

We present the first constraints on the spin-dependent, inelastic scattering cross section of weakly interacting massive particles (WIMPs) on nucleons from XENON100 data with an exposure of $7.64\times {10}^3\mathrm{kg}·\text{days}$. XENON100 is a dual-phase xenon time projection chamber with 62 kg of active mass, operated at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy and designed to search for nuclear recoils from WIMP-nucleus interactions. Here we explore inelastic scattering, where a transition to a low-lying excited nuclear state of ${}^{129}\mathrm{Xe}$ is induced. The experimental signature is a nuclear recoil observed together with the prompt deexcitation photon. We see no evidence for such inelastic WIMP-${}^{129}\mathrm{Xe}$ interactions. A profile likelihood analysis allows us to set a 90% C.L. upper limit on the inelastic, spin-dependent WIMP-nucleon cross section of $3.3\times {10}^{-38}{\mathrm{cm}}^2$ at $100\mathrm{GeV}/c^2$. This is the most constraining result to date, and sets the pathway for an analysis of this interaction channel in upcoming, larger dual-phase xenon detectors.

Figure 1

Signal (1–9) and control (CR1 and CR2) regions for the inelastic WIMP-${}^{129}\mathrm{Xe}$ search in the (cS2,cS1)-plane. (a) shows the signal distribution for a simulated WIMP of mass $100\mathrm{GeV}/c^2$ normalized to 50 events, while (b) is obtained using normalized ${}^{60}\mathrm{Co}$ calibration data and represents the background expectation distribution.

Figure 2

A full simulation (NR and ER response) of the 39.6 keV xenon line activated from ${}^{241}\mathrm{AmBe}$ neutrons is compared with data. (a) compares contours of equal density in the (cS1,cS2)-plane, while (b) shows the same distribution projected in cS1 for several ranges of cS2. The histograms are normalized to unit area. Light and dark blue represent simulation and data, respectively.

Figure 3

Predicted signal yield in each subregion (blue curve), along with systematic uncertainties (shaded area) simulated for a WIMP mass of $100\mathrm{GeV}/c^2$. The signal has been scaled for a total number of 50 events. The subregions are defined in Fig. 1.

Figure 4

Distribution of observed events in the region of interest (data points), along with the normalized distribution from calibration data (filled histogram). The expected signal for a WIMP mass of $100\mathrm{GeV}/c^2$ (blue dashed), normalized to a total of 50 events, is also shown. The bottom panel displays the ratio between data and expected background, where the light and dark blue shaded areas represent the statistical and systematic uncertainty on the background expectation, respectively.

Figure 5

Upper limit (blue curve) on the spin-dependent, inelastic WIMP-nucleon cross section as a function of WIMP mass. The expected one (light shaded area) and two (dark shaded area) standard deviation uncertainties are also shown. This result is compared to the upper limit (at 90% C.L.) obtained by the XMASS experiment (dashed line) [30].