First axion results from the XENON100 experiment

We present the first results of searches for axions and axionlike particles with the XENON100 experiment. The axion-electron coupling constant, $g_{\mathrm{Ae}}$, has been probed by exploiting the axioelectric effect in liquid xenon. A profile likelihood analysis of 224.6 live days $\times$ 34-kg exposure has shown no evidence for a signal. By rejecting $g_{\mathrm{Ae}}$ larger than $7.7\times {10}^{-12}$ (90% C.L.) in the solar axion search, we set the best limit to date on this coupling. In the frame of the DFSZ and KSVZ models, we exclude QCD axions heavier than 0.3 and $80\mathrm{eV}/{\mathrm{c}}^2$, respectively. For axionlike particles, under the assumption that they constitute the whole abundance of dark matter in our galaxy, we constrain $g_{\mathrm{Ae}}$ to be lower than $1\times {10}^{-12}$ (90% C.L.) for masses between 5 and $10\mathrm{keV}/{\mathrm{c}}^2$.

Figure 1

(Top) Event distribution in the flattened ${\log }_{10}(S{2}_b/S1)$ vs $S1$ space for science data (black points) and calibration (grey points). Straight dashed lines show the selection cut on the flattened ${\log }_{10}(S{2}_b/S1)$ (horizontal red lines) and the three PE threshold cut (red vertical line). (Bottom) Global acceptance for ER events, evaluated on calibration data.

Figure 2

Conversion function between energy recoil in keV and $S1$ in PE. The $n^{\exp }$ central value and the $\pm 1\sigma$ uncertainty are indicated with solid blue and black dashed line, respectively.

Figure 3

Background model $N_b\times f_b$ (grey line), scaled to the correct exposure, as explained in the text. The analytic function $f_b$ is based on the ${}^{60}\mathrm{Co}$ and ${}^{232}\mathrm{Th}$ calibration data (empty blue dots) and is used in Eq. (4). The three-PE threshold is indicated by the vertical red dashed line.

Figure 4

Event distribution of the data (black dots) and background model (grey) of the solar axion search. The expected signal for solar axions with $m_A<1\mathrm{keV}/{\mathrm{c}}^2$ is shown by the dashed blue line, assuming $g_{\mathrm{Ae}}=2\times {10}^{-11}$, the current best limit, from EDELWEISS-II [31]. The vertical dashed red line indicates the low $S1$ threshold, set at three PE. The top axis indicates the expected mean energy for ERs as derived from the observed S1 signal.

Figure 5

The XENON100 limits (90% C.L.) on solar axions are indicated by the blue line. The expected sensitivity, based on the background hypothesis, is shown by the green/yellow bands $(1\sigma /2\sigma )$ around the XENON100 limits. Results by EDELWEISS-II [31] and XMASS [32] are shown, together with the ones from a Si(Li) detector by Derbin et al. [33]. Indirect astrophysical bounds from solar neutrinos [34] and red giants [35] are represented by light grey horizontal lines. The benchmark DFSZ and KSVZ models are represented by dark grey lines [4, 5, 6, 7].

Figure 6

Event distribution in the galactic ALPs search region between 3 and 100 PE (black dots with error bars). The grey line shows the background model used for the profile likelihood function. The vertical dashed red line indicates the $S1$ threshold. The expected signal in XENON100 for various ALP masses, assuming $g_{\mathrm{Ae}}=4\times {10}^{-12}$, is shown as blue dashed peaks. The top axis indicates the expected mean energy for ERs as derived from the observed $S1$ signal.

Figure 7

The XENON100 limits (90% C.L.) on ALP coupling to electrons as a function of the mass, under the assumption that ALPs constitute all of the dark matter in our galaxy (blue line). The expected sensitivity is shown by the green/yellow bands $1\sigma /2\sigma$. The other curves are constraints set by CoGeNT [39] (light brown dashed line), CDMS [40] (blue dashed line, more dotted), and EDELWEISS-II [31] (ochre dashed line, extending up to $40\mathrm{keV}/{\mathrm{c}}^2$). Indirect astrophysical bound from solar neutrinos [34] is represented as a continuous light grey line. The benchmark KSVZ model is represented by a dark grey line [6, 7].