Dark Matter annual modulation results by DAMA / LIBRA

The DAMA/LIBRA experiment at LNGS (sensitive mass of about 250 kg highly radiopure NaI(Tl)) is mainly devoted to the investigation of Dark Matter (DM) particles in the Galactic halo by exploiting the model independent DM annual modulation signature. DAMA/LIBRA and the former DAMA/NaI (the first generation experiment having an exposed mass of about 100 kg) have released so far results for an exposure of 1.17 ton × year, collected over 13 annual cycles. They provide a model independent evidence of the presence of DM particles in the galactic halo at 8.9 σ C.L. Some obtained results are shortly summarized and future perspectives mentioned.

What accelerators can do: to demostrate the existence of some of the possible DM candidates What accelerators cannot do: to credit that a certain particle is the Dark Matter solution or the "single" Dark Matter particle solution… DM direct detection method using a model independent approach and a low-background widely-sensitive target material + DM candidates and scenarios exist (even for neutralino candidate) on which accelerators cannot give any information e.g. signals from these candidates are completely lost in experiments based on "rejection procedures" of the e.m. component of their rate Some direct detection processes: • Conversion of particle into e.m. radiation → detection of γ, X-rays, e - • v sun ~ 232 km/s (Sun velocity in the halo) • v orb = 30 km/s (Earth velocity around the Sun) • γ = π/3, ω = 2π/T , T = 1 year • t 0 = 2 nd June (when v ⊕ is maximum)

Expected rate in given energy bin changes because the annual motion of the Earth around the Sun moving in the Galaxy
The annual modulation: a model independent signature for the investigation of Dark Matter particles component in the galactic halo 1) Modulated rate according cosine 2) In a definite low energy range 3) With a proper period (1 year) 4) With proper phase (about 2 June) 5) Just for single hit events in a multi-detector set-up 6) With modulation amplitude in the region of maximal sensitivity must be <7% for usually adopted halo distributions, but it can be larger in case of some possible scenarios

Requirements of the annual modulation
To mimic this signature, spurious effects and side reactions must not only -obviously -be able to account for the whole observed modulation amplitude, but also to satisfy contemporaneously all the requirements With the present technology, the annual modulation is the main model independent signature for the DM signal.
Although the modulation effect is expected to be relatively small a suitable large-mass, low-radioactive set-up with an efficient control of the running conditions would point out its presence.
The DM annual modulation signature has a different origin and, thus, different peculiarities (e.g. the phase) with respect to those effects connected with the seasons instead Results on rare processes:

Results on DM particles:
The pioneer DAMA/NaI: ≈100 kg highly radiopure NaI(Tl) model independent evidence of a particle DM component in the galactic halo at 6.3σ C.L. total exposure (7 annual cycles) 0.29 ton×yr As a result of a second generation R&D for more radiopure NaI(Tl) by exploiting new chemical/physical radiopurification techniques (all operations involving crystals and PMTs -including photos -in HP Nitrogen atmosphere)

Installation
• Dismounting/Installing protocol (with "Scuba" system) • All the materials selected for low radioactivity • Multicomponent passive shield (>10 cm of Cu, 15 cm of Pb + Cd foils, 10/40 cm Polyethylene/paraffin, about 1 m concrete, mostly outside the installation) • Three-level system to exclude Radon from the detectors • Calibrations in the same running conditions as production runs • Installation in air conditioning + huge heat capacity of shield • Monitoring/alarm system; many parameters acquired with the production data • Pulse shape recorded by Waweform Analyzer Acqiris DC270 (2chs per detector), 1 Gsample/s, 8 bit, bandwidth 250 MHz • Data collected from low energy up to MeV region, despite the hardware optimization was done for the low energy

Some on residual contaminants in new ULB NaI(Tl) detectors
α/e pulse shape discrimination has practically 100% effectiveness in the MeV range The measured α yield in the new DAMA/LIBRA detectors ranges from 7 to some tens α/kg/day

2-4 keV
The data favor the presence of a modulated behavior with proper features at 8.8σ C.L. The remaining detector has χ 2 /d.o.f. = 1.28 exceeding the value corresponding to that C.L.; this also is statistically consistent, considering that the expected number of detectors exceeding this value over 25 is 1.25.
• The mean value of the twenty-five points is 1.066, slightly larger than 1. Although this can be still ascribed to statistical fluctuations, let us ascribe it to a possible systematics.
• This possible additional error (≤ 4 % or ≤ 0.5%, respectively, of the DAMA/LIBRA modulation amplitude) can be considered as an upper limit of possible systematic effects DAMA/LIBRA (6 years) total exposure: 0.87 ton×yr The line corresponds to an upper tail probability of 5%.
Slight differences from 2 nd June are expected in case of contributions from non thermalized DM components (as e.g. the SagDEG stream) The 40 K double coincidence events are not modulated DAMA/LIBRA 0.87 ton×yr Any modulation contribution around 3 keV in the single-hit events from the hypothetical cases of: i) 40 K "exotic" modulated decay; ii) spill-out effects from double to single events and viceversa, is ruled out at more than 10 σ The experimental S m cannot be due to 40 K for many reasons.
No role for 40 K in the experimental S m also see arXiv:0912.0660 S m (thermal n) < 0.8 × 10 -6 cpd/kg/keV (< 0.01% S m observed ) In all the cases of neutron captures ( 24 Na, 128 I, ...) a possible thermal n modulation induces a variation in all the energy spectrum Already excluded also by R 90 analysis HYPOTHESIS: assuming very cautiously a 10% thermal neutron modulation: Can a possible thermal neutron modulation account for the observed effect?
• Two consistent upper limits on thermal neutron flux have been obtained with DAMA/NaI considering the same capture reactions and using different approaches.
" Capture rate = Φ n σ n N T < 0.022 captures/day/kg Evaluation of the expected effect: 24m Na (T 1/2 =20ms) σ n = 0.43 barn σ n = 0.10 barn NO E (MeV) MC simulation of the process 1.4·10 -3 cpd/kg/keV 7·10 -5 cpd/kg/keV When Φ n = 10 -6 n cm -2 s -1 : • Thermal neutrons flux measured at LNGS : Φ n = 1.08 10 -6 n cm -2 s -1 (N.Cim.A101(1989)959) • Experimental upper limit on the thermal neutrons flux "surviving" the neutron shield in DAMA/LIBRA: Ø studying triple coincidences able to give evidence for the possible presence of 24 Na from neutron activation: Φ n < 1.2 × 10 -7 n cm -2 s -1 (90%C.L.) By MC: differential counting rate above 2 keV ≈ 10 -3 cpd/kg/keV Moreover, a possible fast n modulation would induce: " a variation in all the energy spectrum (steady environmental fast neutrons always accompained by thermalized component) already excluded also by R 90 " a modulation amplitude for multiple-hit events different from zero already excluded by the multiple-hit events Can a possible fast neutron modulation account for the observed effect? NO S m (fast n) < 10 -4 cpd/kg/keV (< 0.5% S m observed ) HYPOTHESIS: assuming -very cautiously -a 10% neutron modulation: In the estimate of the possible effect of the neutron background cautiously not included the 1m concrete moderator, which almost completely surrounds (mostly outside the barrack) the passive shield Measured fast neutron flux @ LNGS: Φ n = 0.9 10 -7 n cm -2 s -1 (Astropart.Phys.4 (1995)23) Thus, a possible 5% neutron modulation (ICARUS TM03-01) cannot quantitatively contribute to the DAMA/NaI observed signal, even if the neutron flux would be assumed 100 times larger than measured by various authors over more than 15 years @ LNGS • Experimental upper limit on the fast neutrons flux "surviving" the neutron shield in DAMA/LIBRA: Ø through the study of the inelastic reaction 23 Na(n,nʹ′) 23 Na*(2076 keV) which produces two γ's in coincidence (1636 keV and 440 keV): Φ n < 2.2 × 10 -7 n cm -2 s -1 (90%C.L.) Ø well compatible with the measured values at LNGS. This further excludes any presence of a fast neutron flux in DAMA/LIBRA significantly larger than the measured ones.