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Material radioassay and selection for XENON1T

To attain the high sensitivity needed to detect a dark matter particle with a xenon time-projection chamber, all other sources of particle interactions need to be eliminated or minimized. These interactions are classified as background events. Radiogenic backgrounds, in particular, come from radioactive isotopes within the detector materials that decay and lead to alpha, beta, or gamma emissions. Neutrons from spontaneous fission of heavy isotopes or from secondary reactions within the detector materials also contribute to the radiogenic background and can mimic a dark matter signal.

To minimize the radiogenic background, the goal of the XENON1T radioassay program is to measure the radioactivity of all materials that are needed to build the detector and to select only the most radiopure materials for the final construction. To do this, we use mass spectrometry techniques and high-purity germanium spectrometers that are capable of measuring radioactivity at the level of 10-6 decays per second in a kilogram of material (Bq/kg). As comparison, a typical banana has an activity of ~102 Bq/kg!

Because natural radioactivity is present in the soil, the water, and in the air, it is also present in the XENON detector materials. The Figure shows a measurement obtained with a germanium spectrometer of the gamma rays emitted from a sample of photomultiplier tubes. The background (purple spectrum) is subtracted from the sample (pink spectrum) in order to quantify the expected activity from a XENON1T component or material sample.

A high-purity germanium spectrometer measurement of gamma rays emitted from a sample of XENON1T photosensors. Some prominent isotopes from different sources are labeled: primordial uranium and thorium decay chains (green), potassium (red), man-made (orange) and cosmogenic (orange) isotopes.

The most common radioactive isotopes present in the Earth are primordial uranium and thorium, each of which decays into a series of other radioactive isotopes (marked in green in the Figure). Potassium (red) is also a common, primordial isotope that is found in soil, and subsequently in food and in your body. Other isotopes that are found in detector materials come from interactions with cosmic rays (yellow) or from man-made activities (blue), i.e. industrial or medical use, nuclear power plant emissions, nuclear accidents, and military testing.

The measured activities of each material selected for detector construction are used in simulations of XENON1T to determine the expected background. This allows for a prediction of the attainable sensitivity of the detector to dark matter interactions. The radioassay measurement results from over 100 material samples are presented in our new paper “Material radioassay and selection for the XENON1T dark matter experiment”.

Search for magnetic inelastic dark matter with XENON100

Search for magnetic inelastic dark matter with XENON100

There is the long-standing claim of the DAMA/LIBRA collaboration about a detection of dark matter via the highly significant observation of an annually modulating signal in radiopure sodium iodide crystals. This signal, however, is in conflict with exclusion limits from various other dark matter experiments, including XENON100. Several alternatives to the classical WIMP scenario have thus been proposed in order to reconcile these null results with DAMA/LIBRA. One of these models is magnetic inelastic dark matter (MiDM). The MiDM model is motivated by comparing certain properties of the different detector targets and how they possibly influence the expected event rates. Iodine, used in DAMA/LIBRA, is distinguished by its high atomic mass and high nuclear magnetic moment. This enhances the signal of MiDM compared to other targets, such as xenon, and opens up new parameter space for the DAMA/LIBRA signal that is not in conflict with other null results.

The exclusion limit (at 90% confidence level, CL) on MiDM interactions from of XENON100 for a dark matter mass of m=123GeV/c2. Also shown are the 68% and 95% (green) CL regions of the MiDM best fit to the DAMA/LIBRA modulation signal. The limit from COUPP data is shown as well.

In the framework of MiDM the dark matter particle is expected to scatter inelastically off the nucleus, thereby gets excited, and de-excites after a lifetime of the order of order μs with the emission of a photon with an energy of δ~100 keV. Given the mean velocity of the dark matter particle, it travels a distance of O(m) before it de-excites. Furthermore it is assumed that the dark matter particle has a non-zero magnetic moment, μχ. The combination of a low-energy nuclear recoil followed by  an electronic recoil from the de-excitation creates a unique delayed coincidence signature which has been searched for the first time using the XENON100 science run II dataset with a total exposure of 10.8 ton × days. No MiDM candidate event has been found, thus we calculate an upper limit on the interaction strength. The Figure shows the resulting limit for a dark matter mass of 123 GeV/c2 which completely excludes the DAMA/LIBRA modulation signal as being due to MiDM.The sensitivity of this type of analysis will be greatly improved for current ton-scale (e.g., XENON1T) and future multi-ton dual-phase LXe TPCs (e.g., XENONnT, LZ and DARWIN). This is not only due to the increased target mass, but also thanks to the higher probability of detecting the de-excitation inside the larger active volume.

Search for Two-Neutrino Double Electron Capture of Xenon-124 with XENON100

Besides the hunt for dark matter particles, the XENON detectors can be used to search for many other rare processes. One interesting case arises from one of the xenon isotopes itself, namely 124Xe, which is slightly abundant in natural xenon (0.1%). While it is considered stable since its direct decay into 124I is energetically forbidden, there is a rare process in nature, so far only indirectly observed, which would lead to a decay of 124Xe into the isotope 124Te. This requires, in the most probable case, the simultaneous capture of two electrons from the closest atomic shell turning two protons into two neutrons. Since this happens rarely, the corresponding half-life is predicted to be as large as 1022 years, which overshoots the lifetime of the universe by some 12 orders of magnitude. Nevertheless, as the XENON detectors are built for the rare event detection of dark matter particles, they are also very well suited for a search of such a rare process. What would one expect to be the trace of such a decay within the detector? Although the nuclear reaction

124Xe + 2e124Te* + 2νe

would suggest that neutrinos are the signal to search for, as they are a direct product of the decay with a total energy of 2.8MeV, their weak interaction cross-section makes them not detectable. But there are two electrons now missing from the atom’s shell, which is usually from the closest one (K-shell). So there are two “holes” left at an energetically favored position. In a cascade-like process, electrons from upper shells are now dropping down, filling these holes. This releases their former higher binding energy of a characteristic value in the form of secondary particles such as X-rays or Auger electrons. These particles cascade is releasing a summed energy of 64 keV, which is the signature we expect to see with our detector.

Looking for this signal in our well-known XENON100 data from 2011 to 2012 with 225 live days of exposure, we found no signal excess above our background. This way, a lower limit on the half-life of the decay with a value of 6.5×1020 years could be determined using a Bayesian analysis approach. This is close to the optimistic theoretical predictions, but a bit less sensitive than the XMASS detector, which estimated the half-life to be larger than 4.7 x 1021 years.
However, the results from XENON100 can be seen as the preparation for the next step, XENON1T. As XENON1T has about 2kg of 124Xe in its two-ton active xenon target (a factor of 70 more compared to the 29g used in XENON100) it will be more sensitive to this rare decay. Moreover, the background in XENON1T is a factor of 30 smaller in the region of interest. After only five live days of measurement it is thus expected to explore regions no experiment has explored before, and after 2 live years of measurement, we can probe half lives up to 6 x 1022 years (see Fig.1). It has to be emphasized that this data comes for free while searching for dark matter particles, since both searches require the same settings.


Figure 1: Expected sensitivity of XENON1T as a function of live time in days. The aimed duration for the dark matter search is marked at 2 ton years, which would translate into
two years of measurement using 1 ton of the detector mass as a fiducial volume. After 5
days new parameter space is explored.

The XENON1T detector is also prepared to search for competing decay modes of the double electron capture, as it has an improved response to high energy signals. The so far unobserved emission of two positrons and two neutrinos as well as a mixture with one positron emitted and one electron captured simultaneously. While any detection of these decay modes would certainly lead to a deeper understanding of standard nuclear physics another possible decay branch could open the door to physics beyond the Standard Model: The neutrinoless double beta decay. If this hypothetical mode, where no neutrinos would be emitted, would be detected, it would reveal that they are their own anti-particles and annihilate in this process of double beta decay. This would prove the violation of lepton number conservation and, additionally, it could tell something about the mass of neutrinos, which is known to be very small (<eV) but is not determined today. Unfortunately, the expected life time of these decays given by theoretical calculations is even larger than for the process with the emission of two neutrinos.

Lowering the radioactivity of the XENON1T photosensors

E. Aprile et al (XENON Collaboration), Lowering the radioactivity of the XENON1T photosensors, arXiv:1503.07698, Eur. Phys. J. C75 (2015) 11, 546.

The XENON1T experiment employs 242 photomultiplier tubes (PMTs) in the time projection chamber, arranged into two circular arrays. Because the overall background goal of the detector is incredibly low, with less than 1 expected event in a tonne of liquid xenon and one full year of data, the PMTs must be made out of ultra-pure materials. These materials were selected for their content in traces of 238-U, 232-Th, 40-K, 60-Co, 137-Cs and other long-lived radionuclides.

The XENON collaboration joined efforts with Hamamatsu to produce a photosensor that meets the strict requirements of our experiment. The sensor is a 3-inch diameter tube that operates stably at -100 C and at a pressure of 2 atmospheres. It has a high quantum efficiency, with a mean around 35%, for the xenon scintillation light at 178 nm and 90% photon collection efficiency.

PMT_schematicsThe sensor, shown schematically in the left picture, features a VUV-transparent quartz window, with a low-temperature bi-alkali photocathode deposited on it. A 12-dynode electron multiplication system ensures a signal amplification of ~3 millions, which is a crucial feature to detect the tiny signals induced by the rare collisions of dark matter particles with xenon nuclei.

Before the tubes were ready to be manufactured, the construction materials were inspected with gamma-ray spectroscopy and glow-discharge mass spectroscopy (GDMS). For the former, we employed the world’s most sensitive high-purity germanium detectors, GeMPI and Gator, operated deep underground at the Gran Sasso Laboratory. GDMS can detect trace impurities in solid samples and the results were compatible with those from germanium screening. We measured many samples to select the final materials for the PMT production. As an example, specific 226-Ra activities around or below 0.3 mBq/PMT were seen in most of the inspected materials. Such an activity corresponds to 3 x 10-4 226-Ra decays per second and tube, or about 26 decays per day.

BarChart_blogThe relative contribution of the selected materials to the trace contaminations in U, Th, K, Co and Cs of the final product, seen in the left picture, also tells us how to improve further sensor versions for the XENONnT upgrade. Most of the nuclides in the 238-U and 232-Th chains, especially dangerous for their emission of alpha particles, that can the produce fast neutrons in (alpha,n) reactions, are located in the ceramic stem of the tube. In consequence, finding a new material to replace the ceramic might drastically improve the background expectations.

pmts_gatorOnce the final production started, and the tubes were delivered in several batches to our collaboration, they were measured in the Gator detector. Its inner chamber can accommodate 15 PMTs at a time, as seen in the left picture. Each batch was screened for about 15 days, and theobserved activities were mostly consistent from batch to batch. For all measured PMTs, we obtain contaminations in uranium and thorium below 1 mBq/PMT. While 60-Co was at the level of 0.8 mBq/PMT, 40-K dominates the gamma activity with about 13 mBq/PMT. The information from screening was considered in the final arrangement of the PMTs in the XENON1T arrays. PMTs with somewhat higher activities are placed in the outer rings, where they are more distant from the central, fiducial xenon region of the detector.

The average activities per PMT of all trace isotopes served as input contaminations to a full Monte Carlo simulation of the expected backgrounds in XENON1T. The results show that the PMTs will provide about 1% and 6% of the total electronic and recoil background of the experiment, respectively. We can therefore safely conclude that the overall radioactivity of the sensors is sufficiently low, and they will certainly not limit the dark matter sensitivity of the XENON1T experiment.

Search for Event Rate Modulation in XENON100 Electronic Recoil Data

E. Aprile et al. (XENON Collaboration), Exclusion of Leptophilic Dark Matter Models using XENON100 Electronic Recoil Data, Science 2015 vol. 349 no. 6250 pp. 851, and Search for Event Rate Modulation in XENON100 Electronic Recoil Data, Physical Review Letters 115, 091302 (2015) and arxiv.1507.07748

The annual modulation signature

Although we believe that Dark Matter is Out There, we are completely oblivious to the impact of Dark Matter on our daily lives. On the human scale Dark Matter is nearly impossible to detect, the faintest whisper of the galaxy. The vast majority of the time Dark Matter particles pass right through us as if we don’t exist.

It is hypothesized, however, that we may be able to tune our ears to hear the unique song of Dark Matter here on Earth. Doing so successfully would constitute direct proof that Dark Matter exists.

Rather than the swelling symphony that you might expect from the most abundant matter in the Universe, this song will be a random melody, plucked out in individual notes. The tempo of these notes, that is the rate of events in a Dark Matter detector, should vary over the course of one year.

Evidence suggests that both the Sun and the Earth are enveloped by the Dark Matter halo of the Milky Way. As the Earth’s velocity relative to the Sun varies over its one-year orbit, so does it’s velocity relative to the Dark Matter. This should result in the so-called “WIMP wind” that blows harder in June, and softer in December.

This variation itself becomes the song of Dark Matter, repeating every year like clockwork – the annual modulation signature.


Illustration of the expected “WIMP wind” due to the motion of the Sun relative to the DM halo of the Milky Way. Figure from arXiv:1209.3339

Illustration of the expected “WIMP wind” due to the motion of the Sun relative to the DM halo of the Milky Way. Figure from arXiv:1209.3339

XENON100 was the first instrument using liquified xenon that was able to search for such a signature. The liquid xenon that fills the detector emits light when particles interact with it. We take pictures of the light with extremely sensitive devices, and use them to identify the energy and type of interaction. We took data with this detector from February 2011 to March 2012, long enough to observe more than one full cycle of the Dark Matter annual modulation.

What will Dark Matter events look like?

In XENON100, more than one type of event is identifiable. The type depends on whether Dark Matter interacts with the nuclei of the atoms in the detector, or with the electrons surrounding these nuclei. Typically, we assume the interactions of Dark Matter are with the nuclei.

For our newest study, we considered the possibility that Dark Matter instead interacts with the electrons in XENON100, and looked for an annual modulation signature.

One challenge of such a study is that many things can potentially make the rate of events in the detector vary in time, for example random noise in the instrument itself or the decay of radioactive particles. We examined all these possibilities carefully, and determined to what extent they might affect the rate of events in the detector.

The results of our study show some evidence for a rate of events varying periodically over the course of roughly one year, or perhaps longer. This slight change in rate – about half of the average rate in the detector, which is itself very small – can not yet be explained. There’s a one in a thousand chance that it is just a statistical fluke.

Before you go extolling the news from the rooftops, however, take note that our observation is not what we would naively expect from Dark Matter.

Our data shows that the rate of multiple-scatter events (interactions with more than one atom) varies almost as much as that of single-scatter events. Since Dark Matter interacts extremely rarely, we would never expect it to cause multiple-scatter events. In addition, the date of the peak rate in our detector does not match up with what we expect due to the motion of the Earth through the Dark Matter halo.

New perspective on an old claim of Dark Matter discovery

The DAMA/LIBRA collaboration has observed an annual modulation signal in their NaI detectors for more than a decade. They claim that it can be interpreted as a direct detection of Dark Matter. Meanwhile, many experiments that are more sensitive than DAMA/LIBRA (including XENON100) have found no comparable evidence of Dark Matter interacting with atomic nuclei.

However, given the fact that the NaI detectors are unable to differentiate between different types of events, one way to resolve this tension between the different experiments is if the interactions in DAMA/LIBRA are with the electrons.

Although our study shows that XENON100 sees some hint of a signal varying over long periods, the size of that signal is still much smaller than what we would expect to see if we were, in fact, detecting the same signal as DAMA/LIBRA. Thus, we find that it is extremely unlikely to be the case that DAMA/LIBRA observes an annual modulation due to interactions with electrons. The data from XENON100 exclude this possibility with a statistical significance of 4.8σ, corresponding to a probability of about one in a million.

Best-fit amplitude and phase of annual modulation signal in XENON100 from a profile likelihood study. Expected signal from DAMA/LIBRA and expected phase from the standard Dark Matter halo overlaid for comparison.

Best-fit amplitude and phase of annual modulation signal in XENON100 from a profile likelihood study. Expected signal from DAMA/LIBRA and expected phase from the standard Dark Matter halo overlaid for comparison.

Our study answers an important question about how to interpret the DAMA/LIBRA annual modulation signal, but raises many more. Why haven’t we discovered the annual modulation of Dark Matter? What causes the annual modulation in DAMA/LIBRA? What causes the slight variation of rate in XENON100?

More data has since been taken by XENON100 that will hopefully allow the last question to be answered. As to the nature of Dark Matter, well, we will have to keep listening.

First axion results from the XENON100 experiment

E. Aprile et al. (XENON100), First Axion Results from the XENON100 Experiment, Physical Review D 90, 062009 (2014) and arXiv:1404.1455.

Is it better a dark matter WIMP or the Imp from GoT? I don’t know, but I would rather advice you to not forget the axions from GUT – Grand Unification Theories. Axions, if they exist, could solve several yet unsolved problems in understanding our Universe and in the description of the forces that govern the subatomic world. The axions have been postulated by Roberto Peccei and Helen Quinn in 1977 to explain the discrepancy between theory and observation in Quantum Chromodynamics for what concern the Charge-Parity Violation. They could be an excellent dark matter candidate and solve at the same time the CPV problem. What does this mean?

In the Standard Model of particle physics, the fundamental force that regulates the interaction among the quarks is called the Strong Force. Let me remind you that the quarks are thought to be the fundamental constituent of the hadrons, among which we have the nucleons, i.e. the protons and neutrons which made the atoms. We know that the quarks come with a colour. To be clear, this colour is just a conventional name without implying that quarks are literally red, green or blue. It’s just a way to distinguish different kinds of quarks. Because of these colours, the quantum theory formalism that describes the quarks gets the name of chromo: Quantum Chromo Dynamics or QCD.

Now, in the Standard Model we have another force, called the Weak Force. This Weak Force is responsible of the decay of the nuclei; and whenever a neutrino is involved. Why do we care about Weak Interaction if the axons deal with Strong one? This is because of the CP symmetry violation.

Already in 1964 it was found that the Weak Interaction violates the CP symmetry. The fundamental particles may come with a charge (C), like the electron, and with a parity (P), which can be seen as a spatial symmetry. Like the human face which is symmetric (although not perfectly symmetric) between left and right. Before 1964 it was expected that by changing the charge of a particle (performing a so called charge conjugation) you get something different from what you had at the beginning: a positron is not an electron, but it is its charged-conjugated partner. The same thing was expected to happen with the parity conjugation: imagine to put a particle in front of a mirror, the mirrored particle won’t be the same as the original one.

However, it was believed that if you combine these two transformations (if you make a CP conjugation) you obtain the same situation as the one present at the beginning of the process. Well, in 1964, it was proven that this is not the case for the Weak Interactions, that is to say: Weak Interactions violate the CP symmetry. Nowadays we understand this process better and we can precisely describe this violation within the Standard Model of particle physics.

This CP symmetry violation, although perfectly fine with the Standard Model, has not been observed in the Strong Interaction. Imagine that you see a leaf that is about to fall from a branch, but never falls. The fall is predicted by the gravity, but it doesn’t happen. There must be something wrong! Or maybe we must be missing something. Like, the leaf being stuck to the branch. So, what is it happening to the Strong Interactions? Why haven’t we yet observed the CP violation in the Strong sector of the Standard Model?

We don’t know… yet. To solve this problem, Peccei and Quinn have introduced this new particle, the axion, that takes away the CP violation in the Strong Interaction processes, restoring the symmetry. It is like preventing the leaf to fall, and making the violation invisible. Why is this important for us?

Simple: now that the Higgs boson has been discovered and we have a clearer idea on how the particles acquire the mass they have, we are still unable to explain why we are living in a matter-dominated universe rather than an antimatter-dominated one. The definition of what is matter and what is antimatter is a purely human artifact: the two options, matter or antimatter universes, would be completely indistinguishable in terms of the laws of nature. The only difference you might experience is that instead of switching on the light letting the electrons flowing, you would do the same using positrons instead. So why the Nature has chosen the matter (electron) instead of the antimatter (positron)?

We think that the solution lies in understanding the CP violation. And the axion is one of the keystones in the building of this cathedral. There are several experimental groups searching for these particles, and many theoretical physicists are working on various axion models (oscillating between predictions and readjustment, once experimental results get published).

Concerning the experimental searches, it was recently realized that the dark matter detectors (like CDMS, EDELWEISS or xenon-based instruments) can be particularly suitable for such a challenge. About one year ago, we understood that XENON100 could play in the world championship of this competition, maybe winning the AC (not the America’s Cup, but the Axion’s Cup). So we have involved ourselves in this venture.

Supported by several theoretical models (also arising from Grand Unification Theories) we expect the axions to interact with the normal matter by coupling  either to photons, nucleon or electrons. By normal baryonic matter we mean the building blocks that constitute the Universe to which we naturally interacts. Everything you see, everything you touch is normal baryonic matter. Also XENON100 is made only of baryonic matter.

With it we could test the axion-electron coupling. This means that to explore the existence of this very elusive particles, we tried to observe the probability of an axion to kick out an electron from the xenon atoms (see the figure below). This process is called the axio-electric effect.

The axio-electric effect

The axio-electric effect converts an axion A into an electron e-, in the presence of either a nucleus Z+ or another electron e-.

The axio-electric effect is very similar to the photo-electric effect (whose discovery won Albert Einstein the Nobel Prize of Physics in 1921), with a crucial difference though: in our case instead of a photon we consider an axion hitting the electron and ionizing the xenon target. The axio-electric effect was first introduced and formalized by A. Derevianko and others in the late 1990s. What happen when an axion hits our xenon target?

It generates a small spark, which is immediately detected by the photomultiplier tubes, which continuously monitor the situation inside XENON100. XENON100 particularly good in discovering the axions through this effect. The secret lies in the cleanliness of the detector. XENON100 is definitively one of the cleanest places of the Universe. In which sense? Everything that is surrounding us is radioactive, emits radiation which continuously hits us: when you wash your hands you receive quite some amount of radiation, particularly if the washbasin is made of ceramic, because of the cobalt contained in the ceramic. This radiation is completely harmless for your body so we never worry about it. But in contrast, if you put the same amount of ceramic inside XENON100, the whole experiment would be spoiled! Hence, every single component has been carefully selected and the detector is operated in such a way that everything that generates a spark in its interior can be considered as good signal, and not some spurious radiation.


To give you an idea of the cleanliness of the XENON100 detector: imagine that you could sit inside the inner part of the XENON detector (wear the proper clothes, since the temperature is about -100 degrees). That place is so radiation-clean that you will have to wait for about a day between one low-energy event and another. All this means that if we see some light we have quite a good chance that this light is coming from something interesting — such as axions.

We have carefully run our experiment for more than a year, taking care of it like a sacred cow. We then skimmed the data that we collected during that time. At the end of the skimming procedure we have found no evidences of axions, as shown below.


What you see in the plot is the following: on the y-axis we show the coupling of the axion with the electron, i.e. a way to describe the probability they interact with the electrons; on the x-axis we shod the hypothetical mass of the axion. Since we don’t know either the coupling nor the mass, we have to plot them in such a graph, in order to check where they like to live (for a given mass the corresponding coupling and vice-versa). In these so-called exclusion plots, we show different experiments (whose names you can find on the plot) which have excluded certain phase space: each point [coupling, mass] above the line for a particular experiment has been rejected, and if the axion exist, it can be only be in the region below these lines. For example, it is highly impossible that an axion in the galaxy can have a mass of 2 keV and a coupling to the electrons 1E-11 (i.e. one in eight hundredth of millionth), since these characteristic have been excluded by CoGeNT, CDMS, EDELWEISS and more recently by XENON100. An axion with a mass of 2 keV and a coupling of 1E-13 is still possible: we haven’t been able to search for that yet. You can think of it like fishing: we try to go deeper and deeper with our fishing rods in different places of the lake. You can immediately see that the XENON100 has reached the deepest level in this search with respect to the other fishermen.

It has taken 40 years before finding the Higgs boson. The hunt for the axion has just started. We are out in front for tracking down these fundamental, elusive particles.


XENON100 is most-cited dark matter experiment in 2013

The latest result from XENON100 on spin-independent WIMP-nucleon interactions, derived from 225 live days of data taking, is among the 20 most-cited particle physics papers of the year 2013. According to the new summary of INSPIRE, the high energy physics information system, our result from 2012, published in Physical Review Letters, is the only dark matter-related paper in the top 40, and is surrounded by high-impact results from ATLAS, CMS, Planck, WMAP, Daya Bay, etc.

Observation and applications of single-electron charge signals in the XENON100 experiment

In XENON100, we observe individual electrons and describe this signal together with its applications in a dedicated publications:

E. Aprile et al. (XENON100), Observation and applications of single-electron charge signals in the XENON100 experiment, J. Phys. G: Nucl. Part. Phys. 41 (2014) 035201, available via arXiv:1311.1088.

The neutron background of the XENON100 dark matter search experiment

In order to search for dark matter, it is imperative that background signals in particular from neutrons are well under control. We describe the successful techniques and leading results from our efforts in a dedicated publications:

E. Aprile et al. (XENON100), The neutron background of the XENON100 dark matter search experiment, arXiv:1306.2303. The paper is also published in Journal of Physics G 40 (2013), 115201.

Response of the XENON100 Dark Matter Detector to Nuclear Recoils

Dark matter is expected to induce nuclear recoils in our detector. We have demonstrated that we have an excellent matching of our expectation and the measured response of the XENON100 detector to such nuclear recoils, with an agreement at the percent level:

E. Aprile et al. (XENON100), Response of the XENON100 Dark Matter Detector to Nuclear Recoils, arXiv:1304.1427. The paper is also published in Physical Review D88 (2013), 012006.