# ANAIS-112 sensitivity in the search for dark matter annual modulation

## Abstract

The annual modulation measured by the DAMA/LIBRA experiment can be explained by the interaction of dark matter WIMPs in NaI(Tl) scintillator detectors. Other experiments, with different targets or techniques, exclude the region of parameters singled out by DAMA/LIBRA, but the comparison of their results relies on several hypotheses regarding the dark matter model. ANAIS-112 is a dark matter search with 112.5 kg of NaI(Tl) scintillators at the Canfranc Underground Laboratory (LSC) to test the DAMA/LIBRA result in a model independent way. We analyze its prospects in terms of the *a priori* critical and detection limits of the experiment. A simple figure of merit has been obtained to compare the different experiments looking for the annual modulation observed by DAMA/LIBRA. We conclude that after 5 years of measurement, ANAIS-112 can detect the annual modulation in the \(3\sigma \) region compatible with the DAMA/LIBRA result.

## 1 Introduction

The ANAIS experiment [1, 2] is intended to search for dark matter annual modulation with ultrapure NaI(Tl) scintillators at the Canfranc Underground Laboratory (LSC) in Spain, in order to provide a model independent confirmation of the signal reported by the DAMA/LIBRA collaboration [3, 4, 5] using the same target and technique. Projects like DM–Ice [6], COSINE-100 [7, 8], SABRE [9] and PICO–LON [10] also envisage the use of large masses of NaI(Tl) for dark matter searches. Results obtained by other experiments with other target materials and techniques (like those from CDMS [11], CRESST [12], EDELWEISS [13], KIMS [14], LUX [15], PICO [16], XENON [17] or DarkSide [18, 19] collaborations) have been ruling out for years the most plausible compatibility scenarios. Nevertheless, DAMA/LIBRA has accumulated up to now twenty annual cycles in the [2,6] \(\hbox {keV}_{\text {ee}}\) energy region (\(\hbox {keV}_{\text {ee}}\) for keV electron-equivalent) with \(12.8\sigma \) statistical significance (phase and period fixed) [3, 4, 5]. Moreover, DAMA/LIBRA-phase2 has been able to accumulate six annual cycles in the [1,6] \(\hbox {keV}_{\text {ee}}\) energy region with \(9.5\sigma \) statistical significance because all the photomultipliers (PMTs) were replaced by a second generation PMTs Hamamatsu R6233MOD, with higher quantum efficiency and with lower background with respect to those used in phase1 [5].

*R*is the interaction rate, \(E_R\) is the recoil energy, \(t_0\) is the expected time of the maximum (or minimum, depending on the sign of \(S_m\)), 152.5 days after 1st January, and

*T*is the expected period of one year. The time-averaged differential rate is denoted by \(S_0\), whereas the modulation amplitude is given by \(S_m\) [21]. The value of \(S_m\) measured by DAMA/LIBRA is \(0.0102\pm 0.0008\) and \(0.0105\pm 0.0011\) cpd/kg/keV\({_{\text {ee}}}\) within [2,6] and [1,6] \(\hbox {keV}_{\text {ee}}\) intervals, respectively (cpd stands for

*counts per day*) [5].

In this paper, we analyze the ANAIS-112 prospects in terms of the *a priori* critical and detection limits of the experiment and a simple figure of merit has been obtained to compare the different experiments looking for the annual modulation observed by DAMA/LIBRA. The structure of the paper is as follows: Sect. 2 describes the ANAIS-112 experimental layout; Sect. 3 focuses on the procedure to search for a modulation signal in the [2,6] \(\hbox {keV}_{\text {ee}}\) energy region, considering a single energy bin and afterwards the energy binning and segmented detector in nine modules; Sect. 4 focuses on the [1,6] \(\hbox {keV}_{\text {ee}}\) energy region in view of the last DAMA/LIBRA-phase2 results. Finally, conclusions are presented in Sect. 5.

## 2 The ANAIS-112 experiment

ANAIS-112 consists of nine modules made by Alpha Spectra (AS), Inc. Colorado and then shipped to Spain along several years, arriving at LSC the first of them at the end of 2012 and the last by March, 2017. Each crystal is cylindrical (4.75\(''\) diameter and 11.75\(''\) length), with a mass of 12.5 kg. NaI(Tl) crystals were grown from selected ultrapure NaI powder and housed in OFE (Oxygen Free Electronic) copper; the encapsulation has a mylar window allowing low energy calibration. Two Hamamatsu R12669SEL2 PMTs were coupled through quartz windows to each crystal at LSC clean room. All PMTs have been screened for radiopurity using germanium detectors in Canfranc. The shielding for the experiment consists of 10 cm of archaeological lead, 20 cm of low activity lead, 40 cm of neutron moderator, an anti-radon box (continuously flushed with radon-free nitrogen) and an active muon veto system made up of plastic scintillators designed to cover top and sides of the whole ANAIS set-up. The hut housing the experiment is at the hall B of LSC under 2450 m.w.e.

The light output measured for all AS modules is at the level of \(\sim \) 15 phe/\(\hbox {keV}_{\text {ee}}\) [2], which is 1.5 times larger than that determined for the best DAMA/LIBRA detectors [5]. This high light collection, possible thanks to the excellent crystal quality and the use of high quantum efficiency PMTs, has a direct impact in energy threshold. Triggering below 1 \(\hbox {keV}_{\text {ee}}\) is confirmed by the identification of bulk \(^{22}\)Na and \(^{40}\)K events at 0.9 and 3.2 \(\hbox {keV}_{\text {ee}}\), respectively, thanks to coincidences with the corresponding high energy photons following the electron capture decays to excited levels [2].

At the region of interest, crystal bulk contamination is the dominant background source. Contributions from \(^{40}\hbox {K}\), \(^{210}\hbox {Pb}\) (powder/crystal growing contaminants), \(^{22}\hbox {Na}\) and \(^{3}\hbox {H}\) (cosmogenics) are the most relevant [22, 23]. In almost all detectors the \(^{40}\hbox {K}\) peak at 3.2 \(\hbox {keV}_{\text {ee}}\) is clearly visible. The background level at 2 \(\hbox {keV}_{\text {ee}}\) ranges from 2 to 5 cpd/kg/\(\hbox {keV}_{\text {ee}}\), depending on the detector, and then increases up to 5–8 cpd/kg/\(\hbox {keV}_{\text {ee}}\) at 1 \(\hbox {keV}_{\text {ee}}\). A detailed analysis of the background contributions can be found in [23].

## 3 Searching for a modulation signal in the [2,6] \(\hbox {keV}_{\text {ee}}\) energy interval

*b*of the counting rate

*B*

*a*is the mean annual rate and \(\tau =2\pi (t-t_0)/T\), see Eq. (1). We will take the simplest approximation of only considering one bin (4 \(\hbox {keV}_{\text {ee}}\) wide) and the whole detection mass; afterwards we will take into account the energy binning and the segmentation of the 112.5 kg in 9 modules.

### 3.1 Model independent modulation

The test statistic [25] to evaluate the null (\(b=0\)) and the alternative (\(b\ne 0\)) hypotheses is the least squares estimator of the amplitude, \(\hat{b}\), of expected value \(E(\hat{b})=b\) and variance \(var(\hat{b})\). Asymptotically, \(\hat{b}\) follows a normal distribution.

#### 3.1.1 A single energy bin

*n*time bins, where the dependent variable \(B_i\) is the measured rate in the \(i^{th}\) time bin \(\tau _i\), \(w_i=1/var(B_i)\) and the independent variable is \(\text {cos }\tau _i\), gives Eq. (6.12) on page 105 of Ref. [26]:

*M*is the total detection mass, \(\varDelta E\) and \(\varDelta t\) are the width of the energy and live time bins, respectively. Note that the number of observed events is divided twice by the efficiency in \(var(B_i)\).

*B*and \(\varepsilon \) of the nine modules for the \(\sim \)10% unblinded data. The background of all modules in [2,6] \(\hbox {keV}_{\text {ee}}\) is listed in the 2nd column of Table 1 and the cut efficiencies, \(\varepsilon \), are shown in Fig. 1. These are comparable in the [2,6] \(\hbox {keV}_{\text {ee}}\) interval, with average \(\varepsilon =0.97\).

Measured \(\sim \)10% unblinded background (Fig. 3) in the [2,6] \(\hbox {keV}_{\text {ee}}\) energy interval for all modules after filtering and efficiency correction (Fig. 1) have been applied (2nd column). Considering the energy binning, the relevant quantity is \(\left\langle B/\varepsilon \right\rangle \) (3rd column), where the background is divided twice by the efficiency, see Sect. 3.1.2. The average values for ANAIS-112 are listed in the last row

Module |
| \(\left\langle B/\varepsilon \right\rangle \) (cpd/kg/\(\hbox {keV}_{\text {ee}}\)) |
---|---|---|

D0 | 4.58 ± 0.05 | 4.74 ± 0.05 |

D1 | 4.66 ± 0.05 | 4.82 ± 0.05 |

D2 | 2.44 ± 0.04 | 2.54 ± 0.04 |

D3 | 3.16 ± 0.04 | 3.24 ± 0.04 |

D4 | 3.12 ± 0.04 | 3.22 ± 0.04 |

D5 | 2.96 ± 0.04 | 3.11 ± 0.04 |

D6 | 2.90 ± 0.04 | 3.02 ± 0.04 |

D7 | 2.61 ± 0.04 | 2.72 ± 0.04 |

D8 | 2.29 ± 0.04 | 2.37 ± 0.04 |

ANAIS-112 | 3.19 ± 0.01 | 3.31 ± 0.01 |

The usual results of the experiments looking for dark matter are exclusion plots (upper limits) at 90% C.L. in the plane cross section WIMP-nucleon versus WIMP mass [21]. By definition of \(L_D\), \(L_D\simeq 2L_C\) if \(var(\hat{b})\simeq var(\hat{b}\mid b=0)\) and both are set to the same C.L. (see Fig. 2 of Ref. [27]). ANAIS-112 fulfills the former condition because \(b\ll a\), see Eq. (9). Under the same conditions as above, any upper limit, \(L_U\), satisfies \(L_U\le L_D\) [27]. Furthermore, \(L_C\le L_U\) (both to the same C.L.) if the outcome of \(\hat{b}\) is \(\ge 0\). If \(\hat{b}<0\), it should be \(\mid \hat{b}\mid \sim \sigma (\hat{b})\) because if \(\hat{b}\ll -\sigma (\hat{b})\), it would imply a negative modulation, opposite to the observed by DAMA/LIBRA. Briefly, any \(L_U\) given by ANAIS-112 will be less than \(L_D\), likely greater than \(L_C\) or, at least, not much smaller than \(L_C\).

It is worth noting that, assuming a background linearly decreasing with time as an approximation of the decay of the long-lived \(^{210}\hbox {Pb}\) and \(^{3}\hbox {H}\) [22] during data taking, the obtained \(L_D\) is very similar to the one obtained assuming a constant background [28]. The contribution of \(^{210}\hbox {Pb}\) (\(^{3}\hbox {H}\)) in the [2,6] \(\hbox {keV}_{\text {ee}}\) has been estimated for the first year of data taking as 1.246 (0.826) cpd/kg/\(\hbox {keV}_{\text {ee}}\) [23]. Therefore, adding a linear term to Eq. (2), a three parameter linear least-squares fit [26] can be carried out and \(L_D=7.20\cdot {}10^{-3}\) cpd/kg/\(\hbox {keV}_{\text {ee}}\) is obtained.

#### 3.1.2 Energy binning and segmented detector

A more accurate \(L_C\) and \(L_D\) value can be obtained taking into account the energy binning and the background and efficiency differences among the modules of ANAIS-112 (segmented detector). In addition, the energy binning and the segmented detector in nine modules should be considered to obtain the possible energy dependence of the modulation amplitude *b*(*E*).

*(a) Energy binning*

*N*bins the

*jth*modulation amplitude in \(\left[ E_j,E_{j+1}\right] \) (\(j=1,2,\ldots ,N\)) is

*b*is the arithmetic mean of \(b_j\). For \(N=40\) (\(\delta E=0.1\) \(\hbox {keV}_{\text {ee}}\)) and \(M=112.5\) kg, \(\hat{b}_j\)’s are also virtually normal variables for one-day time bins. When the \(\hat{b}_j\)’s are statistically independent

*(b) Segmented detector*

*jth*energy bin of the module

*k*(\(k=1,2,\ldots ,9\)) is:

*jth*energy bin of the module

*k*. Now, \(\hat{b}_j^k\)’s are virtually normal variables for one-week time bins. Thus, the variance of the estimator of

*b*in the module

*k*is:

### 3.2 Dark matter hypothesis

The one-tailed \(L_D\) of Eq. (11), deduced from the figure of merit Eq. (10), can be translated to the \((\sigma _{SI},M_{WIMP})\) plane, see the solid black line of the Fig. 4. For ANAIS-112, it is numerically equivalent to the maximum likelihood ratio test under the dark matter hypothesis. For \(M_{WIMP}>180\) GeV the modulation amplitude is negative in the [2,6] \(\hbox {keV}_{\text {ee}}\) energy interval, a result non considered in the one-tailed test because it is opposite to the DAMA/LIBRA signal.

## 4 ANAIS-112 in the [1,6] \(\hbox {keV}_{\text {ee}}\) energy interval

The case of a single energy bin is not a good approximation because \(B(E)/\varepsilon (E)\) changes steeply below 2 \(\hbox {keV}_{\text {ee}}\) (Fig. 3). In order to estimate \(L_C\), a one-tailed test is carried out again, \(L_C=1.28\cdot {}\sigma (\hat{b})\) and \(L_D=2L_C\).

\(\left\langle B/\varepsilon \right\rangle \) calculated from measured \(\sim \)10% unblinded background in the [1,6] \(\hbox {keV}_{\text {ee}}\) energy interval for all modules after filtering and efficiency correction have been applied. The average values for ANAIS-112 are listed in the last row

Module | \(\left\langle B/\varepsilon \right\rangle \) (cpd/kg/\(\hbox {keV}_{\text {ee}}\)) |
---|---|

D0 | 6.42 ± 0.06 |

D1 | 7.04 ± 0.06 |

D2 | 3.59 ± 0.04 |

D3 | 4.91 ± 0.05 |

D4 | 4.60 ± 0.05 |

D5 | 4.58 ± 0.05 |

D6 | 4.48 ± 0.05 |

D7 | 3.67 ± 0.04 |

D8 | 3.29 ± 0.04 |

ANAIS-112 | 4.73 ± 0.02 |

Taking each module separately, according to the Table 2 and the Eq. (19), \(L_D=(7.55\pm 0.02)\cdot {}10^{-3}\) cpd/kg/\(\hbox {keV}_{\text {ee}}\), very close to Eq. (25) because the nine values \({\left\langle B/\varepsilon \right\rangle }^k\) are close to \({\left\langle B/\varepsilon \right\rangle }\).

### 4.1 Dark matter hypothesis

## 5 Conclusions

We have estimated the detection limit at 90% C.L., when the critical limit is at 90% C.L., of ANAIS-112 for the annual modulation observed by DAMA/LIBRA. It is based on the measured background following the unblinding of \(\sim \)10% of the first year of data of the nine modules D0–D8. In the two considered scenarios (the [2,6] \(\hbox {keV}_{\text {ee}}\) and the [1,6] \(\hbox {keV}_{\text {ee}}\)), we conclude that after 5 years of measurement, ANAIS-112 can detect the annual modulation in the \(3\sigma \) region compatible with the DAMA/LIBRA result. The sensitivity in [2,6] \(\hbox {keV}_{\text {ee}}\) is very similar to that obtained in previous paper [33], where the background estimation was based on the measured activity of the six modules D0–D5. On the other hand, the sensitivity in [1,6] \(\hbox {keV}_{\text {ee}}\) is now much better due to the improvements introduced in the efficiency estimate below 2 \(\hbox {keV}_{\text {ee}}\).

We give a simple figure of merit that gives good estimates of \(L_C\) and \(L_D\) if the ratio \(B(E)/\varepsilon (E)\) is nearly constant (energy and detector independent), as it is our case within [2,6] \(\hbox {keV}_{\text {ee}}\). Furthermore, in order to compare the sensitivity of different experiments looking for the annual modulation, several approaches depending on the available information are also provided.

## Notes

### Acknowledgements

Professor J.A. Villar passed away in August, 2017. Deeply in sorrow, we all thank his dedicated work and kindness. This work has been supported by the Spanish Ministerio de Economía y Competitividad and the European Regional Development Fund (MINECO-FEDER) (FPA2014-55986-P and FPA2017-83133-P), the Consolider-Ingenio 2010 Programme under grants MULTIDARK CSD2009-00064 and CPAN CSD2007-00042, and the Gobierno de Aragón and the European Social Fund (Group in Nuclear and Astroparticle Physics, ARAID Foundation and I. Coarasa predoctoral grant). We also acknowledge LSC and GIFNA staff for their support.

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