First Results on Dark Matter Annual Modulation from the ANAIS-112 Experiment

ANAIS is a direct detection dark matter experiment aiming at the testing of the DAMA/LIBRA annual modulation result, which, for about two decades, has neither been confirmed nor ruled out by any other experiment in a model independent way. $\mathrm{ANAIS}-112$, consisting of 112.5 kg of sodium iodide crystals, has been taking data at the Canfranc Underground Laboratory, Spain, since August 2017. This Letter presents the annual modulation analysis of 1.5 years of data, amounting to 157.55 kg yr. We focus on the model independent analysis searching for modulation and the validation of our sensitivity prospects. $\mathrm{ANAIS}-112$ data are consistent with the null hypothesis ($p$ values of 0.67 and 0.18 for [2–6] and [1–6] keV energy regions, respectively). The best fits for the modulation hypothesis are consistent with the absence of modulation ($S_m=-0.0044\pm 0.0058\mathrm{cpd}/\mathrm{kg}/\mathrm{keV}$ and $-0.0015\pm 0.0063\mathrm{cpd}/\mathrm{kg}/\mathrm{keV}$, respectively). They are in agreement with our estimated sensitivity for the accumulated exposure, which supports our projected goal of reaching a $3\sigma$ sensitivity to the DAMA/LIBRA result in five years of data taking.

Figure 1

Rate of events corresponding to different populations along the first year and a half of $\mathrm{ANAIS}-112$ operation, calculated in ten-day binning. Data from all the modules have been added together and corrected, when necessary, by the corresponding efficiencies. Events surviving all the filters are shown in panels (a)–(f): in energy region [1–6] keV, (a); [3–5] keV, (b); [6–20] keV, (c); $M2$ events in energy region [1–6] keV, (d); $M2$ events in energy region [2–5] keV in coincidence with a high energy gamma in a second detector in the ${}^{40}\mathrm{K}$ window, (e); and $M2$ events in energy region [0–2] keV in coincidence with a high energy gamma in a second detector in the ${}^{22}\mathrm{Na}$ window, (f). Events arriving less than one second after a muon veto trigger before the cuts are shown in panel (g). Fits are shown as red solid lines, and corresponding fit parameters and chi-squared and $p$ values are also given in the plot. Green dashed lines correspond to the background model, normalized according to a factor, $f$, which is also given in the plots.

Figure 2

$\mathrm{ANAIS}-112$ data in the energy windows [1–6] keV (bottom panel) and [2–6] keV (top panel) surviving all the cuts and efficiency corrected [27]. Data are displayed after subtracting the constant and exponential functions fitted to Eq. (2). Fits are also shown in the same way, both in the modulation (three free parameters) and the null hypothesis (two free parameters). ${\chi }^2$ and $p$ values displayed allow the comparison of both hypothesis, and DAMA/LIBRA results on modulation amplitude in both energy windows are shown in green [8].

Figure 3

$\mathrm{ANAIS}-112$ sensitivity to the DAMA/LIBRA signal in $\sigma$ C.L. units (see text) as a function of real time in the [2–6] keV (upper panel) and [1–6] keV (lower panel) energy regions. The black dots are the sensitivities derived in this work, $\sigma (S_m)$. The blue bands represent the 68% C.L. DAMA/LIBRA uncertainty [8].

Figure 4

Modulation amplitude per 1 keV energy bins combining data from all the modules. We show for reference the corresponding DAMA/LIBRA result [8], and the $1\sigma$ and $2\sigma$ $\mathrm{ANAIS}-112$ bands following the analysis done in Ref. [29]. Top panel compares the $p$ values of the fits to Eq. (2) with those corresponding to the null hypothesis for every energy bin.