ANAIS (Annual modulation with NaI
Scintillator
A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbed ...
s) is a
dark matter direct detection experiment located at the
Canfranc Underground Laboratory
The Canfranc Underground Laboratory (LSC) is a world-class deep underground laboratory designed for research in neutrino physics, dark matter and other unusual phenomena in nature that require very low environmental radioactivity to be observed. ...
(LSC), in Spain, operated by a team of researchers of the CAPA at the University of Zaragoza.
ANAIS' goal is to confirm or refute in a model independent way the
DAMA/LIBRA experiment positive result: an annual modulation in the low-energy detection rate having all the features expected for the signal induced by weakly interacting dark matter particles (
WIMPs) in a standard galactic
halo. This modulation is produced as a result of the Earth rotation around the Sun. A modulation with all the characteristic of a
Dark Matter (DM) signal has been observed for about 20 years by DAMA/LIBRA, but it is in strong tension with the negative results of other DM direct detection experiments. Compatibility among the different experimental results in most conventional WIMP-DM scenarios is actually disfavored, but it is strongly dependent on the DM particle and halo models considered. A comparison using the same target material,
NaI
Nai or NAI may refer to:
Music
* ''Nai'' (album), an album by singer Anna Vissi
* Nai (pan flute), a wind instrument, also known as a pan flute (Romania and Moldova)
* "Nai" (song), a 2007 CD single by Irini Merkouri
Organizations
* National A ...
(Tl), is more direct and almost model-independent.
Experimental set up and performance
Source:
ANAIS-112 experimental setup consists of 112.5 kg of
NaI
Nai or NAI may refer to:
Music
* ''Nai'' (album), an album by singer Anna Vissi
* Nai (pan flute), a wind instrument, also known as a pan flute (Romania and Moldova)
* "Nai" (song), a 2007 CD single by Irini Merkouri
Organizations
* National A ...
(Tl), distributed in 9 cylindrical modules, 12.5 kg each and built by Alpha Spectra Inc., arranged in a 3 × 3 configuration.
Among the most relevant features of ANAIS- 112 modules, it is worth highlighting its remarkable optical quality, which combined to using high
quantum efficiency
The term quantum efficiency (QE) may apply to incident photon to converted electron (IPCE) ratio of a photosensitive device, or it may refer to the TMR effect of a Magnetic Tunnel Junction.
This article deals with the term as a measurement of ...
Hamamatsu
photomultipliers (PMTs) results in a very high light collection, at the level of 15 photo
electrons (phe) per
keV in all the nine modules. The signals from the two PMTs coupled to each module are digitized at 2 GS/s in a 1.2 μs window with high resolution (14
bits). The
trigger requires the coincidence of the two PMT trigger signals in a 200 ns window, while the PMT individual
trigger is set at the single phe level.
Another interesting feature is a
Mylar window in the middle of one of the lateral faces of the detectors, which allows to calibrate simultaneously the nine modules with external
x-ray/
gamma
Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter re ...
sources down to 10
keV in a
radon-free environment. A careful low energy calibration of the region of interest (ROI), from 1 to 6 keV, is carried out by combining information from external calibrations and background. External calibrations with a
109Cd source are performed every two weeks, and every 1.5 months energy depositions at 3.2 and 0.87
keV from
40K and
22Na internal contaminations in one ANAIS module are selected by profiting from the coincidence with a high energy
gamma
Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter re ...
in a second module.
The ANAIS-112 experiment is installed inside a shielding consisting of an inner layer of 10 cm of archaeological
lead and an outer layer of 20 cm of low activity lead. This lead shielding is encased into an anti-
radon box, tightly closed and kept under overpressure with
radon-free nitrogen gas. The external layer of the shielding (the
neutron shielding) consists of 40 cm of a combination of water tanks and
polyethylene bricks. An active veto made up of 16 plastic
scintillators is placed between the anti-
radon box and the
neutron shielding, covering the top and sides of the set-up allowing to effectively tag the residual
muon
A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...
flux onsite along the ANAIS-112 data taking.
ANAIS-112 was commissioned during the spring of 2017 and it started the data-taking phase at the hall B of the LSC on 3 August 2017 under 2450 m.w.e. rock overburden. The "live time" of the experiment, useful for analysis, is more than 95%, allowing for the high duty cycle achieved. Down time is mostly due to the periodical calibration of the modules.

A background understanding has been achieved, except in the
-2keV energy region, where the background model underestimates the measured event rate. Crystal bulk contamination is the dominant background source, being
210Pb,
40K,
22Na,
3H contributions the most relevant ones in the region of interest. Considering altogether the nine ANAIS-112 modules, the average background in the ROI is 3.6 cpd/kg/keV after three years of data taking,
while DAMA/LIBRAphase2 background is below 0.80 cpd/kg/keV in the
–2
The dash is a punctuation mark consisting of a long horizontal line. It is similar in appearance to the hyphen but is longer and sometimes higher from the baseline. The most common versions are the endash , generally longer than the hyphen ...
keV energy interval, below 0.24 cpd/kg/keV in the
–3keV energy interval, and below 0.12 cpd/kg/keV in the
–4keV energy interval.
Annual modulation analysis and results
The development of filtering protocols based on the pulse shape and light sharing among the two PMTs has been crucial to fulfill the ANAIS-112 goal since the trigger rate in the ROI is dominated by non-bulk
scintillation
Scintillation can refer to:
*Scintillation (astronomy), atmospheric effects which influence astronomical observations
*Interplanetary scintillation, fluctuations of radio waves caused by the solar wind
*Scintillation (physics), a flash of light pro ...
events. The determination of the corresponding
efficiency
Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...
is very important, and it is calculated using
109Cd,
40K and
22Na events. It is very close to 100% down to 2 keV, and then decreases steeply to about 15% at 1 keV, where the analysis threshold is set.
A
blind protocol for the annual modulation analysis of ANAIS-112 data has been applied: single-hit events in the ROI are kept blinded during the event selection. Up to now, three unblindings of the data have been carried out: at 1.5 years,
at 2 years,
and 3 years,
which correspond to exposures of 157.55, 220.69, and 313.95 kg×y, respectively. ANAIS-112 annual modulation search is performed in the same regions explored by DAMA/LIBRA collaboration,
–6keV and
–6keV, fixing the period to 1 year and the maximum of the modulation to 2 June.
To evaluate the statistical significance of a possible modulation in ANAIS–112 data, the events rate of the nine detectors is calculated in 10-days bins, and it is
minimized χ
2 = Σ
i (n
i − μ
i)
2/σ
2i, where n
i is the number of events in the time bin t
i (corrected by live time and detector efficiency), σ
i is the corresponding
Poisson uncertainty, accordingly corrected, and μ
i is the expected number of events at that time bin, that depends on the background model and can be written as: μ
i =
0φbkg(ti) + Smcos(ω(ti − t0))">0φbkg(ti) + Smcos(ω(ti − t0))∆E∆t.
Here, R
0 represents the non-modulated rate in the experiment,
is the
probability distribution function (PDF) in time of any non-modulated component, S
m is the modulation amplitude, ω is fixed to 2π/365 d = 0.01721 rad d
−1, t
0 to −62.2 d (time origin has been taken on 3 August and then the cosine maximum is on 2 June), M is the total detector mass, ∆E is the energy interval width, and ∆t the time bin width. R
0 is a free parameter, while S
m is either fixed to 0 (for the null hypothesis) or left unconstrained, positive or negative (for the modulation hypothesis).
The null hypothesis is well supported for the 3-years data in both energy regions, being the results for the two background models (a single exponential or a PDF based on the
Monte Carlo background model) compatible. The standard deviation σ(S
m) is slightly lower when detectors are considered independently, as expected following a priori sensitivity analysis. Therefore, this fit is chosen to quote the ANAIS-112 annual modulation final result and sensitivity for three-year exposure. The best fits are incompatible with the DAMA/LIBRA result at 3.3 and 2.6 σ in
-6and
-6keV energy regions, for a sensitivity of 2.5 (2.7)σ at
–6keV (
–6keV). ANAIS-112 results for 1.5,
2
and 3 years
of data-taking fully confirm the sensitivity projection.
ANAIS-112 results support the prospects of reaching a sensitivity above 3σ in 2022, within the scheduled 5 years of data taking.
Several consistency checks have been carried out (changing the number of detectors entering into the fit, considering only the first two years or the last two years, or changing the time bin size), concluding that there is no hint supporting relevant systematical uncertainties in the result. The performance of a large set of
Monte Carlo pseudo-experiments sampled from the background model guarantees that the fit is not biased. A frequency analysis have also been conducted, and the conclusion is that there is no statistically significant modulation in the frequency range searched in the ANAIS-112 data.
Future prospects
ANAIS-112 sensitivity limitation is mostly due to the high background in the ROI, but in particular in the region from 1 to 2 keV. In this context, the application of
machine learning techniques based on
Boosted Decision Trees (BDTs), under development at present, could improve the rejection of these non-bulk scintillation events. Preliminary results point to a relevant sensitivity improvement. Extending the data taking for a few more years, could allow testing DAMA/LIBRA at the 5σ level. Operation at Canfranc Underground Laboratory has been granted until the end of 2025.
One possible systematics affecting the comparison between DAMA/LIBRA and ANAIS result is a possible different detector response to nuclear recoils, because both experiments are calibrated using x-rays/gammas. It is well known that scintillation is strongly quenched for energy deposited by nuclear recoils with respect to the same energy deposited by electrons. Measurements of Quenching Factors (QF) in NaI scintillators are affected by strong discrepancies. ANAIS-112 detectors QF are being determined after measurements at TUNL.
In addition, a complete calibration program for the experiment using neutron sources onsite is being developed.
ANAIS-112 published results are available in open access at the webpage of the Dark Matter Data Center: https://www.origins-cluster.de/odsl/dark-matter-data-center/available-datasets/anais
Data are available upon request.
Funding Agencies
ANAIS experiment operation is presently financially supported by MICIU/AEI/10.13039/501100011033 (Grants No. PID2022-138357NB-C21 and PID2019-104374GB-I00), and Unión Europea NextGenerationEU/PRTR (AstroHEP) and the Gobierno de Aragón. Funding from Grant FPA2017-83133-P, Consolider-Ingenio 2010 Programme under grants MULTIDARK CSD2009-00064 and CPAN CSD2007-00042, the Gobierno de Aragón and the LSC Consortium made possible the setting-up of the detectors. The technical support from LSC and GIFNA staff as well as from Servicios de Apoyo a la Investigación de la Universidad de Zaragoza (SAIs) is warmly acknowledged.
External links
ANAIS Experiment WebsiteCanfranc Underground Laboratory WebsiteThe DAMA project WebsiteThe Dark Matter Data Center
References
{{Dark matter
Experiments for dark matter search