Search for Event Rate Modulation in XENON100 Electronic Recoil Data

We have searched for periodic variations of the electronic recoil event rate in the (2–6) keV energy range recorded between February 2011 and March 2012 with the XENON100 detector, adding up to 224.6 live days in total. Following a detailed study to establish the stability of the detector and its background contributions during this run, we performed an unbinned profile likelihood analysis to identify any periodicity up to 500 days. We find a global significance of less than $1\sigma$ for all periods, suggesting no statistically significant modulation in the data. While the local significance for an annual modulation is $2.8\sigma$, the analysis of a multiple-scatter control sample and the phase of the modulation disfavor a dark matter interpretation. The DAMA/LIBRA annual modulation interpreted as a dark matter signature with axial-vector coupling of weakly interacting massive particles to electrons is excluded at $4.8\sigma$.

Figure 1

Temporal evolution of the relevant XENON100 detector parameters studied for this analysis. The dashed blue lines indicate a detector maintenance period. (a)–(c) Xe pressure, LXe temperature, and LXe level. (d) Radon level in the 34 kg LXe fiducial mass, as measured via in situ $\alpha$ spectroscopy. (e) Average cut acceptance in the low-$E$ range of (2.0–5.8) keV, as derived from weekly ER calibrations. (f) ER event rate in the 34 kg fiducial mass for single scatters in the low-$E$ range.

Figure 2

The expected mean (solid lines) and central 68.3% region (shaded bands) of $-2\log ({\mathcal{L}}_0/{\mathcal{L}}_1)$ as a function of period for simulated data with a fixed average rate $C=6.0\text{events}/(\mathrm{keV}·\text{tonne}·\text{day})$, a linear increase in rate $K=(2.54\pm 0.53)\times {10}^{-3}\text{events}/(\mathrm{keV}·\text{tonne}·\text{day})/\text{day}$, an amplitude $A=2.7\text{events}/(\mathrm{keV}·\text{tonne}·\text{day})$, and three periods $P$ (days). Uncertainties on all parameters are taken into account. The horizontal local significance lines are derived from the null hypothesis tests described in the text and are shown here for comparison to Fig. 3.

Figure 3

$-2\log ({\mathcal{L}}_0/{\mathcal{L}}_1)$ as a function of modulation period for single scatters (SS) in the low-$E$ region (top panel), multiple scatters (MS) in the low-$E$ region (middle panel) and single scatters (SS) in the higher energy region (bottom panel). The phase is unconstrained.

Figure 4

The XENON100 best-fit and 95% and 99.73% confidence level contours as a function of amplitude and phase relative to January 1, 2011, for period $P=1\mathrm{year}$. The expected DAMA/LIBRA signal with statistical uncertainties only and the phase expected from a standard dark matter (DM) halo are overlaid for comparison. Top and side panels show $-2\log ({\mathcal{L}}_1/{\mathcal{L}}_{\max })$ as a function of phase and amplitude, respectively, along with two-sided significance levels.