June, 2011 For the CNS2, P. Belli, C. Brofferio, M. G. Giammarchi, A. Ianni, M. Pallavicini

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1 June, 2011 For the CNS2, P. Belli, C. Brofferio, M. G. Giammarchi, A. Ianni, M. Pallavicini Research Area 2 This research area deals with: dark matter direct detection, neutrinoless double beta decay search, neutrinos from a core collapse supernova and rare processes. Direct Detection of Dark Matter At present, the argument of the Dark Matter (DM) in the Universe is one of most stimulating and appealing in physics and astrophysics. Many efforts and results have been already achieved in the field of the direct detection of DM particles giving a strong boost to the field. A large set of candidates is available in the literature for DM particles, for their interaction types and for the overall phenomenology. At present the level of knowledge is such that large uncertainties exist on general aspects and on parameters related to astro, nuclear and particle physics, namely the velocity distribution of DM in the galactic halo, the escape velocity, the interaction cross section, the nuclear form factor etc. Thus, a model-independent approach for the DM detection with a widely sensitive detector in fully controlled conditions is mandatory. The present and the future experiments in this field can be mainly classified as follows: i) experiments based on the identification of signals due to DM particles through the study of a model-independent signature; ii) counting rate experiments exploiting techniques of subtraction of the electromagnetic signal. The case of experiments based on the identification of the signal. Presently, the only feasible signature is the study of the peculiar annual modulation of the flux of DM particles. The DAMA experiment at the Gran Sasso Laboratory, the former DAMA/NaI and the continuously running DAMA/LIBRA, exploits such DM signature to search for the presence of DM particles in the galactic halo in a model-independent way. At present a strong evidence of such a modulation has been observed in DAMA/LIBRA. After the upgrade at the end of 2010, when all the PMTs were replaced by PMTs with higher quantum efficiency, the DAMA/LIBRA set-up will have many other annual cycles to be investigated with a lower energy threshold. This will allow the achievement of new information on the DM signal peculiarities, on the candidate(s), on the model and on the investigation of second-order effects.

2 Very recently, the CoGeNT collaboration [1] has reported a 2.8σ evidence of the annual modulation of very low energy events ( kev ee ) detected with p- type point contact germanium detectors. These low energy events cannot be explained by known backgrounds. The data-set covers a 14-month time span. If this hint is interpreted as the annual modulation of WIMPs, the expected mass range and cross section with nuclei are compatible with the ones observed by DAMA/LIBRA. The necessity to have a signature of DM particles needs to be considered for future experiments. A model-independent search with an improved apparatus is one of the goals for the next generation experiments. Amongst the different possibilities we recall: i) the DAMA/1ton (1 ton of NaI(Tl)), or a very long running of DAMA/LIBRA; ii) a massive bolometer (such as CUORE) and a massive liquid noble gas detector; iii) a CoGeNT log term measurement. The sensitivity to the annual modulation depends on the characteristics of the target, on the properties of the candidate and on the operative conditions. The use of different targets has, therefore, a complementary nature. We recommend to pursue the positive experiences and promising technological developments. Another signature of DM is the directionality due to the Earth s rotation about its axis. However, this probe can be adopted only for candidates inducing nuclear recoils. The asymmetry due to the directionality can be at least a factor 10 greater than the annual modulation amplitude. At present, the method is still in an R&D phase: the main technical difficulties are related to the identification of nuclear tracks at low energy. Amongst the experimental efforts we recall: DRIFT in the UK and CYGNUS, an international collaboration with UK, Japan, USA and France. Presently, there is no Italian involvement (we remind just the contribution of DAMA in the suggestion of the use of anisotropic scintillators). Therefore, some R&D proposal may be put forward in the future, depending on the overall technical, physics and economical situation. The case of counting rate experiments based on the techniques of subtraction of the electromagnetic component. These experiments are mainly based on the use of double read-out bolometer detectors and of two-phase liquid noble gas detectors; thus, they are sensitive just to the DM candidates inducing recoiling nuclei. In this framework the activity of the INFN and of the Gran Sasso Laboratory is reported below. i) The WArP set-up, which makes use of 100 litres of liquid argon as target mass with a veto against neutrons of about 8 tons of liquid argon. The WArP design is

3 unique. However, so far it has demonstrated how difficult is to build and to run a set-up using such technique, and mostly with so large mass targets. ii) XENON 10/100 set-ups. These detectors have achieved important results. However, the light response is still subject of debate and speculations [2] and the radiopurity and performance of the 100 litres set-up is still not completely known. Recently, new data have been reported [3] on XENON100 set-up and the INFN has approved the installation of XENON1t in Hall B at the Gran Sasso underground Laboratory. iii) CRESST bolometer with a target-detector of CaWO 4 : the latest results show an excess of nuclear recoils of Oxygen with respect to the considered modelling of expected events surviving the subtraction procedures. Unfortunately, the future of CRESST will not be at the Gran Sasso Laboratory. Around the World other experiments have reached various levels of sensitivity and some of them have already reported a number of events in excess with respect to some modelling of the expected background events. This is the case of CDMS-II, Edelweiss, and CoGeNT. All these possible excesses can be well compatible with the DAMA model-independent result in the framework of several scenarios, taking into account the present experimental and theoretical uncertainties. To conclude on the DM search we underline the following points: i) a new model-independent signature is fundamental; ii) results from DAMA/LIBRA with the lower threshold are fundamental and should be delivered as soon as possible; ii) it is important to cover as much as possible the space of parameters (cross section and mass) in the WIMP hypothesis; iii) in the framework of WIMPs searches within INFN (that is taking into account the depth of the Gran Sasso Laboratory) experiments must show a high level of radiopurity and a high level of rejection of cosmogenic neutron background together with the possibility to search for light WIMPs; iv) it is fundamental to use complementary information from indirect searches. Neutrinoless double beta decay Neutrinoless Double Beta Decay (0νDBD) is a unique probe to investigate lepton number conservation and the Dirac/Majorana nature of neutrinos. Even if not observed it could help in the comprehension of neutrino physics because of its interplay with other observations, sensitive to neutrino characteristics. In case of evidence, the measurement of the decay rate would provide, after a proper evaluation of the nuclear part of the process, the value of the effective neutrino mass:

4 3! 2 m = m U exp( i" ) # j= 1 j and a proof that the neutrino is a Majorana particle. The exchange of a virtual massive Majorana ν L is the simplest and more straightforward explanation for 0νDBD. Other more exotic mechanisms (e.g. a heavy W R mediated process, or a series of supersymmetric possibilities) are however possible. Future signals in different nuclei (two or more) may help, at least in principle, to discriminate among different decay mechanisms, even taking into account current nuclear uncertainties. 0νDBD can be considered therefore also a powerful test for a number of theoretical predictions beyond Standard Model [4]. The relevant parameters to be taken into account when evaluating the sensitivity of an experiment in comparison with the others are: ε - detector efficiency, M - source mass, ΔE - energy resolution, and B - background per unit mass, time and energy. The strategy of DBD experimentalists in the past was just to choose an isotope with a reasonably high Q-value decay, since this increases strongly the phase space and therefore the expected decay rate, and a favourable nuclear matrix element (NME), because in literature you could find differences between isotopes up to a factor 5. Then a proper detector was chosen. Two main philosophies were pursued: either pushing on active mass, high efficiency and good energy resolution, using a detector that naturally contained the DBD source, like Ge diodes or bolometers, or improving tracking capabilities, so as to disentangle actively as much as possible background from real signal, loosing in efficiency, energy resolution and source mass, but gaining a factor 100 at least in B, like in the NEMO3 case. The clear involvement of DBD in neutrino physics after the discovery of neutrino oscillations has urged the experimentalists in the rush towards a sensitivity at the level of y, with which the quasi-degenerate hierarchy hypothesis can be assessed and the 76 Ge claim scrutinized with high accuracy. But this effort would be useless if the NME were still known with big uncertainties. In a common effort to solve this problem there have been recently many discussions and exchanges between theoretical groups, to understand discrepancies and evaluate errors. When available, β decay and 2νDBD experimental data have been used to fix parameters in QRPA, and other approaches, like Shell Model (SM) or Interacting Boson Model (IBM) are now applied to calculate in an independent way the same NMEs. There are big expectations on the possibility to see reasonably soon a convergence to a unique result for the calculation of NMEs of interest for DBD. See the recent "`International Student Workshop on Neutrinoless Double Beta Decay"' held in LNGS in Nov [5] ej i

5 Meanwhile experimentalists are trying to improve a factor 100 in sensitivity. To do that, one has to concentrate the efforts on a consistent gain in mass and background. Among the INFN funded experiments we find GERDA and CUORE, now under construction in LNGS, that are the natural evolution of the HdM and CUORICINO detectors. They should both reach a sensitivity in the range of y, as requested for present experiments to be on the stage. Recently the interest on DBD has attracted also neutrino oscillation communities, such as SNO and KamLAND collaborations. They are ready to implement their 1 kton detector for DBD physics. SNO will dilute 1 ton of natural Nd (enrichment is not possible at present) in the LAB (linear alkylbenzene) liquid scintillator with which the SNO sphere is now going to be filled (SNO+ Project). This corresponds to only 56 kg of 150 Nd, but thanks to the very high phase space, it will be none the less competitive with the other detectors, provided they will be able to control background. Their natural Nd has shown in fact a contamination in U and Th at the level of 10-8 g/g: a factor 10 6 higher than what is foreseen to have a competitive experiment. But they are confident they will clean it, also thanks to their experience on background reduction in SNO. KamLAND will dilute enriched Xe, which on the contrary can be enriched easily and purified very well, in a part of its liquid scintillator. This mixture (400 kg of Xe in 16 tons of LS) will be contained in a small balloon suspended in the center of the KamLAND sphere. This approach was studied in detail more than ten years ago as a possible application in BOREXINO, but never developed. What has to be considered in these detectors is that solar 8 B neutrinos now become an unavoidable background and that the poor energy resolution of KamLAND, well above 5% FWHM, will spoil any possible improvement in sensitivity when the tail of the 2νDBD spectrum will cover the 0νDBD peak region. As a general remark on present generation experiments, most of the challenging work in the construction of the detectors is related to the tireless struggle against background. All materials used must be checked and selected. All possible sources of radioactivity must be identified and removed. When this is not possible, active or passive shielding against them must be realized. To avoid cosmogenic activation, materials are often stocked, even for long periods, underground. Sometimes they are even produced underground, or at least machined or prepared underground. There is an enormous effort against Rn contamination and most often very strict procedures are applied for material handling and stocking. But the level of contaminations that must be avoided are so tiny, even below 1 µbq/kg, that sometimes it is impossible to check them with conventional techniques and new approaches, like Neutron Activation Analysis (NAA) or Inductive Coupled Plasma Mass Spectrometry (ICP-MS), have been extensively used. In some cases, especially conceived instruments have been realized. Depending on the characteristics of the experiment, active or passive

6 muon or neutron veto are realized. But the possible improvements are approaching the asymptotic line. If DBD searches will have to unravel the neutrino mass hierarchy question, the half-life sensitivity will have to reach y. Isotope masses will have therefore to reach the 1 ton scale, and enrichment at the highest level will have to be pursued. At present, this puts at a disadvantage 48 Ca and 150 Nd, that can't be enriched by centrifugation, but also the world production rate could become an issue if 1 ton enriched material has to be collected in a reasonable time. At the same time, any technique with an energy resolution worse than 2-3% will encounter serious problems in disentangling the 2νDBD events from the 0νDBD at the Q-value. Since neutrinos cannot be tagged, the 2 events are totally identical, and even if the shape of the continuum spectrum from the 2νDBD will initially help squeezing a little more juice from data, it will always remain as the sword of Damocles on low energy resolution experiments. Aiming at a zero-background experiment, a ``must" to fully exploit the effort to produce a 1 ton isotope mass, the winning strategy should be to have a tracker Active Detector (where the DBD isotope is part of the detector itself), as the Xe gas TPC proposed by NEXT or EXO. In the future a major upgrade of the Borexino detector could also yield to an experiment of sensitivity down to 5-10 mev: up to 15 t of Xenon might be contained in the Borexino Stainless Steel Sphere if pressurized to 5 bars and the energy resolution in the DBD region could reach 2% or less by increasing the number of PMTs, and a background at the level of 10-5 c/kev/kg/y (scintillator mass) could be obtained. The idea was already mentioned in the Borexino program and might be considered for a future upgrade. The COBRA experiment, under development in LNGS, foresees a complete tracking capability of the CdZnTe room temperature semiconductor devices with which the array detector is composed. The results that can be achieved with pixellization are well known and already applied for instance on Si diodes, with 55 µm pitch pixels: with such a spatial resolution it is easy to demonstrate that electrons (therefore DBD events) can be disentangled from alphas, gammas and muons. But this idea still needs a lot of R&D and it would require a huge number of channels (10 8 or so, to be compared with the 10 3 of CUORE) for 418 kg of detector, corresponding to 183 kg of isotope mass, if 116 Cd will be enriched at the 90% level. Without pixellization the background is still very high (5 c/kev/kg/y) and the technique is less competitive. In cases when the background comes mostly from a specific channel, for instance alpha decays, then one can develop selective discrimination ability for the detector. This is the case for scintillating bolometers (LUCIFER) that are under development as fundamental upgrade for future CUORE-like detectors. The technique, initially approached 20 years ago in Milano and that reached a mature

7 stage with the Dark Matter Search experiments CRESST (running in LNGS) and ROSEBUD, is now applied to many different bolometers of interest for 0νDBD searches, like ZnSe, CdWO 4, CaMoO 4 or ZnMoO 4. The idea is to read both the scintillation channel, with a Light Detector Bolometer (undoped Ge or Si used as bolometers), and the normal heat channel of the absorber scintillating bolometer. A clear discrimination between β, γ and α has been demonstrated, showing the possibility to reduce background easily below 0.1 c/kev/kg/y, provided the Q- value of the DBD is above the natural gamma spectrum end-point, at 2.6 MeV. In some cases, pulse shape discrimination with high efficiency in the heat channel alone seems feasible. This of course would simplify very much the construction and the readout of these detectors, making them even more attractive. On the contrary, as far as Compton scattering events coming from gamma background are concerned, one should try to discriminate Multi Site Events (MSE) from Single Site Events (SSE). In the Ge-diode past experiments (HdM and IGEX) this was done by pulse shape analysis, but with a reduced efficiency. MAJORANA is a proposed experiment with Ge-diodes that can be considered the natural evolution of IGEX experience. Their efforts have been concentrated on the reduction of cosmogenic activation of Cu and Ge by underground electroforming of all the required Cu and by Ge crystal zone refinement. They are confident that the last step towards an almost zero-background experiment is related to the MSE discrimination. They have therefore developed and then shared with the GERDA collaboration the electrode segmentation technique on standard n-type HPGe and are now studying p-type point-like electrode Ge (PPC- BEGe) that should result in a very high efficiency pulse shape discriminator for Compton events, together with an impressive reduced electronic noise and an almost eliminated risk of degraded alpha background signals. They have been funded for a MAJORANA Demonstrator of 30 kg natural BEGe detectors and, in a second phase, 30 kg of enriched (86%) Ge diodes. GERDA and MAJORANA will eventually merge in a single collaboration for a 1 ton isotope mass experiment using the best features attained by the experience of the two groups. A completely revolutionary approach is that of tagging the DBD events in an efficient way, instead of recognizing and eliminating all the different background species. What has been proposed by the EXO collaboration is to tag the daughter ion. The 136 Ba ++, formed by the 136 Xe DBD, will very quickly capture an electron, leading to the 136 Ba + ion, which is stable in Xe. These ions can be identified via their atomic spectroscopy by optical pumping with blue and red lasers. You can try to shoot the ion with the laser while still in the liquid (or gaseous) detector, which was the first idea of the EXO collaboration, but it requires a very complicated system to zero in on the ion, or you extract with the highest efficiency the ions produced using an ion trap and then you expose them to the laser to tag them as 136 Ba + ions and count them, which at present seems more

8 feasible and should be implemented in EXO detector, a 1 ton Xe liquid hybrid TPC or, if it doesn't work in liquid, gaseous TPC. A challenging idea, but with a very high potential payoff. Neutrinos from core collapse supernovae The search for neutrinos from core collapse supernovae can be performed in a dedicated experiment or in a set-up developed for other main searches such as solar and atmospheric neutrinos, double beta decay, dark matter. At present, within the INFN, there are three detectors in operation for supernova neutrinos: LVD (mainly dedicated to this goal), Borexino and ICARUS-T600. It was also proposed to make use of the liquid argon in the veto of WArP to detect supernova neutrinos by means of neutrino-nucleus interactions. This latter process can also be exploited in CUORE, where a standard supernova will produce some 50 events, provided the radiopurity will allow to work with a low threshold. At present, in the INFN supernova neutrinos can be detected with liquid scintillators in LVD and Borexino. In liquid scintillators of 1kton scale target mass the main interaction channels are the inverse-beta decay and the neutrinoproton interaction. This latter process is very important to measure the temperature of muon and tau neutrinos. However, it is necessary to operate the detector with a threshold of kev to collect a few 100 events for a standard supernova. Around the World only Borexino can perform such a search. Electron neutrinos from a supernova bring important information on early instants in the collapse process. A massive liquid argon detector, such as ICARUS-T600, can exploit a charged-current interaction channel on 40 Ar for electron neutrinos. The neutrino-nucleus interaction channel can be exploited in the future at the Gran Sasso Laboratory by a new massive noble liquid detector (built for DM searches) and by CUORE (built for double beta decay). LVD and Borexino are part of the SNEWS network (early supernova alert) together with SuperKamiokande and IceCube. By-product results of low-background detectors The low-background detectors developed for experiments on double beta decay, solar neutrinos and on the detection of Dark Matter particles can allow the study and the investigation of other nuclear or astrophysics rare processes. For example: i) the electron decay and the electric charge conservation;

9 ii) the Pauli Exclusion Principle violation; iii) the nucleon (N), di-nucleon (NN), tri-nucleon (NNN) decay in various isotopes into invisible particles; iv) rare α and β nuclear decays in various isotopes; v) the search for solar axions with Bragg diffraction techniques on crystals and/or through M1 transition capture on suitable targets; vi) electric charge non conserving processes (CNC), analogous to the usual electron capture but without changing the charge of the nucleus: (A,Z) + e - (A,Z) * + ν e ; vii) electric charge non conserving processes, analogous to the usual β decay, but some massless uncharged particle (e.g., neutrino or γ or Majoron ) would be emitted instead of the electron; viii) heavy cluster decays in various isotopes; ix) the search for exotic matter. Such topics were already studied and results were obtained in many experiments in underground laboratories. More sensitive measurements can be planned in future; they can be achieved either as by-product results or with a dedicated data taking with specific low-background set-ups. It is recommended to consider also this rich field in present and future activities. Dark Energy There is an overwhelming experimental evidence (study of Ia Supernovae, anisotropies of the cosmic microwave background and large-scale structures formation) indicating that a form of unknown energy (dark energy) pervades our Universe and makes up for about 70% of its total energy budget. The main characteristic of dark energy is that it counters the effect of gravity, thereby making the overall acceleration of the Universe growing faster with time. In spite of the conclusive evidence of dark energy existence, its properties are unknown: it can be due to a cosmological constant or a dynamical fluid (called quintessence, with its own dynamical evolution). A large-scale failure of general relativity also cannot be excluded, since gravity has never been tested in the regime below 10-7 cm/s 2. Moreover, we have no information on time evolution of dark energy. However, a great deal of experimental information on the large-scale dynamical effects of dark energy is available. This has played a key role in the birth of the current "basic paradigm" of a Universe with a spatially constant dark energy, cold dark matter and general relativity as the theory of gravitation.

10 The main tasks of research on dark energy focuses on determining its nature, its value and time evolution and comparing its action on the overall expansion of the Universe with its role in the formation of galaxy structures. The observational areas that can be exploited are mainly: a) Baryon Acoustic Oscillations (BAO) b) Galaxy Clusters (GC) c) Supernovae (SN) d) Weak Lensing (WL) with the possible addition of gamma ray bursts (if and when the standardization studies will be successful) and the cosmic microwave background (which, in spite of their limited direct resolving power, can yield important constraints). A parameterization of dark energy properties in terms of its current value and an evolution parameter can be made at first order. This approach, suggested by the Dark Energy Task Force [6] (and justified by the minor role played by dark energy in the early Universe and by its dominance today), allows to study on a comparative basis the four major future projects: LST: large survey telescope exploiting one or more amongst a), b), c) and d) SKA: a radio square-km array focused mainly on points a) and d) JDEM: Satellite-based optical/nir survey based on one or more amongst a), b), c) and d) (see above) JDEMx: X satellite survey survey, mainly focused on GC (observation in point b) LST and SKA are Earth-based project that could be of interest to INFN. In particular, the SKA, Square Kilometer Array, focuses on the study of dark energy using BAO and WL techniques. The method consists in a 21-cm and continue emission survey over a whole hemisphere. The R&D that is critical to build the SKA is the estimate of the galactic population between z=0.5 and z=2 to see to what extent one can improve on the HI luminosity function. Several SKA Pathfinders (LOFAR, MWA, ASKAP, SKA1, MerKAAT...) are currently being developed or planned, to assess the feasibility and develop the methodology to be used for the full detector. Techniques under consideration for the SKA project are also being considered within the INFN as a part of the microwave detection development for cosmic rays induced showers in the frame of the Auger project. Molecular bremsstrahlung in the atmosphere can offer the possibility of detecting cosmicshowers induced microwave signals with a duty cycle much higher the currently used fluorescence technique. Dedicated R&D s on the subject are already underway which also involve INFN groups of the collaboration.

11 INFN could also significantly contribute to the development of infrastructures for atmospheric balloons launches, which are important in the research and development necessary for studies on the cosmic microwave background. Recently, the possibility of investigations on the Dark Energy by laboratory tests has been suggested in the literature. In particular, if the dark energy is produced by vacuum fluctuations then there is a chance to probe some of its properties by laboratory tests based on Josephson junctions (see also EOS proposal in INFN- CSN2). These electronic devices can be used to measure a noise spectrum induced by vacuum fluctuations. If the new physics underlying Dark Energy couples to electric charge, a cut-off should be present in the measured power spectrum. Other ideas have also been proposed. Moreover, we stress that these measurements can be performed by "low-cost" multi-disciplinary experiments. Therefore, we encourage developing this strategy both from the theoretical and experimental points of view. Bibliography [1] C.E. Aalseth et al., CoGeNT collaboration, arxiv: v2. [2] J.I. Collar, arxiv: v1. [3] E. Aprile et al., XENON100 collaboration, arxiv: [4] G.L. Fogli, E. Lisi and A.M. Rotunno, PRD 80, (2009); A. Faessler et al., arxiv: v1; A. Faessler et al., arxiv: v1 [5] International Student Workshop on Neutrinoless Double Beta Decay, November 11-13, 2010 at LNGS, [6] Report of the Dark Energy Task Force, arxiv: