Table of contents

For the sixth time the International Symposium on large TPCs for Low-Energy Rare-Event Detection has been organized in Paris on 17–19 December 2012. As for the previous conference, we were welcomed in the Astroparticle and Cosmology Laboratory (APC).

Around one hundred physicists from all over the world gathered to discuss progress in the dark matter and low-energy neutrino search. The new results from the LHC were also widely discussed. The Higgs discovery at 125 GeV, without any sign of other new heavy particles, does not provide us with any information on the nature of dark mater.

Alternatives to the favored SUSY model, in which the role of the WIMP is played by a stable neutralino, predict low mass candidates below a few GeV. Developing low threshold detectors at sub-keV energies becomes mandatory, and interest for Axion or Axion-like particles as dark matter is revived.

We have seen increasing activity in the field and new infrastructures for these searches have been developed. We heard news of activities in the Canfranc laboratory in Spain, Jinping in China, SURF in the USA and about the extension project of Fréjus (LSM) laboratory. We would like to thank the organizing and advisory committees as well as the session chairpersons: J Zinn-Justin, G Wormser, D Nygren, G Chardin, F Vannucci, D Attié, T Patzak and S Jullian.

I Giomataris, P Colas and I G Irastorza

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

In this contribution, recent results on the search for dark matter candidates at the LHC in monojet and monophoton final states are presented based on the data collected by the ATLAS and CMS experiments at = 7 TeV.

The International Axion Observatory (IAXO) is a new generation axion helioscope aiming at a sensitivity to the axion-photon coupling of g_aγ ≳ few × 10⁻¹² GeV⁻¹, i.e. 1–1.5 orders of magnitude beyond the one achieved by CAST, currently the most sensitive axion helioscope. The main elements of IAXO are an increased magnetic field volume together with extensive use of x-ray focusing optics and low background detectors, innovations already successfully tested in CAST. Additional physics cases of IAXO could include the detection of electron-coupled axions invoked to explain the white dwarf cooling, relic axions, and a large variety of more generic axion-like particles (ALPs) and other novel excitations at the low-energy frontier of elementary particle physics.

We have studied low-background techniques in order to improve the background level for Micromegas detectors. These detectors are good candidates for rare event searches thanks to the low mass and the radiopurity of the materials used in the construction; moreover they have good discrimination capabilities. The motivation of these studies is the reduction of the background level in the CAST experiment where three of the four detectors operating currently are Micromegas of the microbulk type. The last result of the experience acquired in low-background techniques has been the reduction of the background level by a factor 4.5 for two detectors in the CAST experiment, corresponding to an improvement of a factor 2 in signal strength.

A central part of the CAST upgrade program is the further reduction of the background rates and the extension of the experiment's sensitivity to lower energies. Both aims could be reached with a highly pixelized readout of the detector as, for example, with an InGrid, the combination of a Timepix ASIC and a Micromegas grid. The high granularity and efficiency of such a device make it possible to detect and to distinguish single primary electrons. Therefore, we have studied the energy resolution by electron counting and the separation of photon events from tracks with an event shape analysis.

We report on the design, construction and commissioning of the Large Underground Xenon (LUX) dark matter detector at the Sanford Laboratory in Lead, SD, USA. From its inception in 2007, to its construction at a surface laboratory in lead in 2009–2010, its operation in 2011, and its re-installation 1 mile underground in 2012, we review the relevant achievements already obtained and give an outlook on how LUX will become the most sensitive detector in the field in 2013.

A robust signal for sidereal anisotropy in nuclear recoils would support, perhaps more decisively than any other evidence, a discovery claim for a WIMP component of Dark Matter. I present a concept based on columnar recombination in dense xenon gas, sensing nuclear recoil direction relative to a TPC drift field. The central advance is that nuclear recoil directionality information is obtained through a comparison, event-by-event, of the ionization signal and recombination signal that are produced prior to drifting the track ionization. The optimum xenon density for this concept may be near ten bars, unlike conventional techniques that employ track visualization – with severe restrictions on gas density to about 1/10 bar. No restriction is imposed by diffusion during drift, facilitating the realization of a large monolithic room temperature xenon gas Time Projection Chamber at the ton-scale, with unprecedented sensitivity for both directionality and cross-section. Remarkably, the desired operating conditions for 0-νββ ¹³⁶Xe experiment may be identical.

The dark matter directional detection opens a new field in cosmology bringing the possibility to build a map of nuclear recoils that would be able to explore the galactic dark matter halo giving access to a particle characterization of such matter and the shape of the halo. The MIMAC (MIcro-tpc MAtrix of Chambers) collaboration has developed in the last years an original prototype detector based on the direct coupling of large pixelized micromegas with a devoted fast self-triggered electronics showing the feasibility of a new generation of directional detectors. The discovery potential of this search strategy is discussed and illustrated. In June 2012, the first bi-chamber prototype has been installed at Modane Underground Laboratory (LSM) and the first underground background events, the gain stability and calibration are shown.

GBAR is a recently approved experiment at CERN/AD. Its aim is to perform the first test of the Weak Equivalence Principle with Antimatter. The objective is to measure the gravitational acceleration of antihydrogen atoms on Earth with 1% precision in a first phase, and better than a per mil in a second phase. The method is to detect the free fall of ultra-cold atoms. By sympathetic cooling of antihydrogen positive ions ⁺ with beryllium ions, and after photodetachment of the excess positron, antiatoms are formed at m/s velocities and their free fall can be directly measured. Antihydrogen ions are produced by the interaction of keV antiprotons with a dense positronium cloud, the latter being formed by dumping positrons onto a porous material. We describe here briefly the experimental techniques and report on the most recent results. In particular, a flux of around 4 × 10⁶ e⁺s⁻¹ slow positrons is now produced by the source installed at Saclay, cross sections for the production of the ⁺ ions have been computed, and a dedicated beam line for the study of positronium formation and for applications in Materials Science has been realized. The tentative schedule of the GBAR developments is given.

The Enriched Xenon Observatory collaboration searches for neutrino-less double beta decay in ¹³⁶Xe. The first phase of the experiment, EXO-200 has started taking data in 2011 and has already produced important physics results like the first observation of the allowed two-neutrino double beta decay in ¹³⁶Xe. A massive effort for material screening has accompanied the design of EXO-200 and has produced a detector with very low background due to the residual radioactivity. The most recent data analysis campaign has provided a strong limit for the neutrino-less double beta decay in ¹³⁶Xe. For the next phase ton-scale detector, the collaboration is investigating the options for employing a Ba ion tagging technique to provide complete suppression of the radioactive background.

The NEXT-100 time projection chamber, currently under construction, will search for neutrinoless double beta decay (ββ0ν) using 100–150 kg of high-pressure xenon gas enriched in the ¹³⁶Xe isotope to ~ 91%. The detector possesses two important features for ββ0ν searches: very good energy resolution (better than 1% FWHM at the Q value of ¹³⁶Xe) and event topological information for the distinction between signal and background. Furthermore, the technique can be extrapolated to the ton-scale, thus allowing the full exploration of the inverted hierarchy of neutrino masses.

NEXT-100 is an electroluminescent high pressure Time Projection Chamber currently under construction. It will search for the neutrino-less double beta decay in ¹³⁶Xe at the Canfranc Underground Laboratory. NEXT-100 aims to achieve nearly intrinsic energy resolution and to highly suppress background events by taking advantage of the unique properties of xenon in the gaseous phase as the detection medium. In order to prove the principle of operation and to study which are the best operational conditions, two prototypes were constructed: NEXT-DEMO and NEXT-DBDM. In this paper we present the latest results from both prototypes. We report the improvement in terms of light collection (~ 3×) achieved by coating the walls of NEXT-DEMO with tetraphenyl butadiene (TPB), the outstanding energy resolution of 1 % (Full Width Half Maximum) from NEXT-DBDM as well as the tracking capabilities of this prototype (2.1 mm RMS error for point-like depositions) achieved by using a square array of 8 × 8 SiPMs.

We present in this work measurements performed with a small Micromegas-TPC using a xenon-trimethylamine (Xe-TMA) Penning-mixture as filling gas. Measurements of gas gain and energy resolutions for 22.1 keV X-rays are presented, spanning several TMA concentrations and pressures between 1 and 10 bar. Across this pressure range, the best energy resolution and largest increase in gain at constant field (a standard figure for characterizing Penning-like energy transfers) is observed within the 0.9%–1.7% TMA range. A gain increase (at constant field) up to a factor 100 and best values of the energy resolution improved by up to a factor 3 with respect to the one previously reported in pure Xe -operated Micromegas, can be obtained. In virtue of the VUV-quenching properties of the mixture, the overall maximum gain achievable is also notably increased (up to 400 at 10bar), a factor ×3 higher than in pure Xe. In addition, preliminary measurements of the electron drift velocity in a modified setup have been performed and show good agreement with the one obtained from Magboltz.

These results are of great interest for calorimetric applications in gas Xe TPCs, in particular for the search of the neutrino-less double beta decay (0νββ) of ¹³⁹Xe.

It is important to note that in this work some figures from [1] have been updated. Precisely, the TMA concentration has been re-estimated after a detailed re-calibration of the mass spectrometer yielding lower TMA concentration values, corrected by a factor in the range 0.5 – 0.7. An erratum to [1] is being submitted at the moment of writing.

The Tokai-To-Kamioka (T2K) experiment was designed to measure the oscillation of muon neutrinos produced at the J-PARC accelerator in Japan. The Super-Kamiokande detector acts as the far detector and a near detector is installed in the path of the neutrino beam to reduce uncertainties on the flux and cross sections. This near detector, called ND280, is located 280 m from the production target of the T2K neutrino beamline in order to measure the characteristics of the beam prior to any oscillation. The first ND280 analysis of the momentum-angle distribution of the muon produced by muon neutrino charged current interactions has been performed. These results have also been used to determine a flux-averaged cross section.

A new experimental search for sterile neutrinos beyond the Standard Model at a new CERN-SPS neutrino beam aiming at measuring the electron and muon neutrino events with a Near and Far detectors (1600 and 330 m from the proton target) is presented. The project will exploit the ICARUS T600 LAr-TPC moved from LNGS to the CERN Far position and a new additional LAr-TPC detector, 1/4 of the T600, located in the Near position. Two magnetic spectrometers will be placed downstream of the two LAr-TPC detectors to greatly complement the physics capabilities. Comparing the two detectors, in absence of oscillations, all cross sections and experimental biases cancel out. Any difference of the event distributions at the two locations should be attributed to the possible existence of oscillations, presumably due to additional neutrinos with a mixing angle sin²(2θ_new) and a mass squared difference Δm²_new larger than the measured for the standard neutrinos. The superior quality of the LAr imaging TPC, in particular its unique electron-π⁰ discrimination allows for full rejection of backgrounds and offers a lossless ν_e detection capability. The determination of the muon charge with the spectrometers allows for the full separation of ν_μ from _μ and therefore controlling systematics from muon mis-identification mainly at high momenta.

The high-energy Universe is potentially a great laboratory for searching new light bosons such as axion-like particles (ALPs). Cosmic sources are indeed the scene of violent phenomena that involve strong magnetic field and/or very long baselines, where the effects of the mixing of photons with ALPs could lead to observable effects. Two examples are archetypal of this fact, that are the Universe opacity to gamma-rays and the imprints of astrophysical magnetic turbulence in the energy spectra of high-energy sources. In the first case, hints for the existence of ALPs can be proposed whereas the second one is used to put constraints on the ALP mass and coupling to photons.

The recently developed Spherical Proportional Counter [1] allows to instrument large target masses with good energy resolution and sub-keV energy threshold. The moderate cost of this detector, its simplicity and robustness, makes this technology a promising approach for many domains of physics and applications, like dark matter detection and low energy neutrino searches. Detailed Monte Carlo simulations are essential to evaluate the background level expected at the sub-keV energy regime. The simulated background here, it refers to the contribution of the construction material of the detector and the effect of the environmental gamma radiation. This detector due to its spherical shape could be also served as an optical photon detector provided it is equipped with PMTs, for Double Beta decay and Dark Matter searches. All calculations shown here are obtained using the FLUKA Monte Carlo code.

At Kamioka Observatory many activities for low energy rare event search are ongoing. Super-Kamiokande(SK), the largest water Cherenkov neutrino detector, currently continues data taking as the fourth phase of the experiment (SK-IV). In SK-IV, we have upgraded the water purification system and tuned water flow in the SK tank. Consequently the background level was lowered significantly. This allowed SK-IV to derive solar neutrino results down to 3.5 MeV energy region. With these data, neutrino oscillation parameters are updated from global fit; Δm²₁₂ = 7.44^+0.2_−0.19 × 10⁻⁵eV², sin²θ₁₂ = 0.304 ± 0.013, sin²θ₁₃ = 0.0301^+0.017_−0.015. NEWAGE, the directional sensitive dark matter search experiment, is currently operated as "NEWAGE-0.3a" which is a 0.20 × 0.25 × 0.31 m³ micro-TPC filled with CF4 gas at 152 Torr. Recently we have developed "NEWAGE-0.3b". It was succeeded to lower the operation pressure down to 76 Torr and the threshold down to 50 keV (F recoils). XMASS experiment is looking for scintillation signals from dark matter interaction in 1 ton of liquid xenon. It was designed utilizing its self-shielding capability with fiducial volume confinement. However, we could lower the analysis threshold down to 0.3 keVee using whole volume of the detector. In February 2012, low threshold and very large exposure data (5591 kg·days) were collected. With these data, we have excluded some part of the parameter spaces claimed by DAMA/LIBRA and CoGeNT experiments.

The Sanford Underground Research Facility (SURF) at Homestake is presented. The Davis campus is described in detail including the two laboratory modules at the 4850-foot level (>4200 mwe). These modules house the LUX dark matter and MAJORANA DEMONSTRATOR neutrinoless double-beta decay experiments. The Long Baseline Neutrino Experiment plans to place their far detector at SURF. The facility is managed for the US Department of Energy (DOE) by Lawrence Berkeley National Laboratory. The South Dakota Science and Technology Authority (SDSTA) owns and operates the facility. SURF is a dedicated science facility with significant expansion capability.

GridPix is a gas-filled detector with an aluminium mesh stretched 50 μm above the Timepix CMOS pixel chip. This defines a high electric field where gas amplification occurs. A feasibility study is ongoing at Nikhef for the application of the GridPix technology as a charge sensitive device in a dual phase noble gas Time Projection Chamber (TPC), within the framework of the DARWIN design study for next generation dark matter experiments. The smallness of the device and well defined materials allow for high radio-purity and low outgassing. The high granularity of a pixel readout and the high detection efficiency of single electrons of GridPix bring benefits especially in terms of energy resolution for small energy deposits. This feature is interesting also for the measurement of the scintillation yield and the ionisation yield of noble liquids. The accurate measurements of such quantities have a direct impact on the data interpretation of dark matter experiments. The application in dual phase argon or xenon TPCs implies several technological challenges, such as the survival of the device at cryogenic temperature as well as the operation in a pure noble gas atmosphere without discharges. We describe here the recent developments of the project.

A novel concept is proposed for large-volume single-phase noble-liquid TPC detectors for rare events. Both radiation-induced scintillation-light and ionization-charge are detected by Liquid Hole-Multipliers (LHM), immersed in the noble liquid. The latter may consist of cascaded Gas Electron Multipliers (GEM), Thick Gas Electron Multiplier (THGEM) electrodes or others, coated with CsI UV-photocathodes. Electrons, photo-induced on CsI by primary scintillation in the noble liquid, and event-correlated drifting ionization electrons are amplified in the cascaded elements primarily through electroluminescence, and possibly through additional moderate avalanche, occurring within the holes. The resulting charge-signals or light-pulses are recorded on anode pads or with photosensors – e.g. gaseous photomultipliers (GPM), respectively. Potential affordable solutions are proposed for multi-ton dark-matter detectors; open questions are formulated for validating this dream.