Search for Electronic Recoil Event Rate Modulation with 4 Years of XENON100 Data

We report on a search for electronic recoil event rate modulation signatures in the XENON100 data accumulated over a period of 4 yr, from January 2010 to January 2014. A profile likelihood method, which incorporates the stability of the XENON100 detector and the known electronic recoil background model, is used to quantify the significance of periodicity in the time distribution of events. There is a weak modulation signature at a period of $431_{-14}^{+16}$ day in the low energy region of (2.0–5.8) keV in the single scatter event sample, with a global significance of $1.9\sigma$; however, no other more significant modulation is observed. The significance of an annual modulation signature drops from $2.8\sigma$, from a previous analysis of a subset of this data, to $1.8\sigma$ with all data combined. Single scatter events in the low energy region are thus used to exclude the DAMA/LIBRA annual modulation as being due to dark matter electron interactions via axial vector coupling at $5.7\sigma$.

Figure 1

Temporal evolution of the most relevant quantities across run I (left column), run II (middle column), and run III (right column), spanning a range of 4 yr. The vertical dashed lines indicate detector maintenance periods. The panels (from top to bottom) show the detector or room pressures, various temperature readings, height of the liquid xenon level, signal acceptances, and low-E MS and SS event rates (acceptance corrected). The uncertainty bands on the acceptance models are derived from the weighted mean of the $1\sigma$ error bars. The best-fit unmodulated background in the XENON100 detector (blue dashed line) is overlaid in the bottom panel, along with the expected modulation signal (red dashed line) using the amplitude reported by DAMA/LIBRA (as calculated in the text).

Figure 2

Temporal evolution of (top) ${}^{222}\mathrm{Rn}$ activity via characteristic alpha spectroscopy and (bottom) krypton concentration via RGMS measurement in the XENON100 detector. The ${}^{222}\mathrm{Rn}$ evolution is modeled by Eq. (1) and shown by the red line in the top panel. The ${}^{85}\mathrm{Kr}$ concentration is modeled by linearly increasing functions during air leaks in run II and run III, and constant otherwise due to its long decay lifetime of ${\tau }_{\mathrm{Kr}}=15.6\mathrm{yr}$, as shown by the red line in the bottom panel. The shaded regions show the range of each run.

Figure 3

The expected mean (solid lines) and central 68.3% region (shaded bands) of test statistics as a function of period for simulated data. Uncertainties on all parameters are taken into account. The horizontal global significance lines are derived from the null hypothesis tests and shown here for comparison to Fig. 4.

Figure 4

Test statistics as a function of modulation period for single scatters in the low-$E$ region (top), multiple scatters in the low-$E$ region (middle) and single scatters in the high-$E$ region (bottom). The phase is unconstrained. The previous run II-only result [12] is overlaid for comparison.

Figure 5

The XENON100 best-fit black dot, and 68% (light red shaded region) and 90% (green shaded region) confidence level contours as a function of amplitude and phase relative to January 1, 2011 for one year period. The corresponding run II-only results [12] are overlaid with a black square and dashed lines. The phase is less constrained than in run II due to the smaller amplitude. The expected DAMA/LIBRA signal (cross, statistical uncertainty only) and the phase expected from a standard DM halo (vertical dashed line) are shown for comparison. Top and side panels show $-\mathrm{\Delta }({\mathrm{TS}}_1)$ as a function of phase and amplitude, respectively, along with two-sided significance levels.