On Tuesday 20th of June, we presented our latest results on Electronic Recoil Modulations with 4 years of Xenon100 data at the PASCOS 2017 conference held in Madrid. After a short introduction, by M.L. Benabderrahmane, to the dark matter modulation as a signal, the main results have been presented, namely the test statistics of unbinned profile likelihood to search for the modulation period using three different sets of data. The first set contains the single scatter events in the energy range 2-5.8keV, the second set contains Multiple scatter events in the same energy range and the last one contains single scatters in the energy range 6-20keV. The last two samples are used as a sideband. The results of the likelihood gives a period of 431 days which is different from the one observed by the DAMA/LIBRA collaboration. Our single scatter modulation at 431 days has a global significance below 2sigma. The local test statistics for one year period gives a 1.8sigma. Similarity of the spectra between the two control samples and the signal sample disfavors the possibility for a modulation due to Dark Matter interaction.
The traditional approach for WIMP nucleus interaction studies in direct detection experiment is to consider just two types of interactions, the spin independent (SI) and the spin dependent (SD) ones. However, these are not the only types of interactions possible. In recent years, a non-relativistic effective field theory approach has been studied. In this framework, 14 new interaction operators arise. These operators include the SI and SD ones among others. Some of these new operators are momentum dependent and predict a non-exponential event rate as function of energy, in contrast to the traditional expected signals. Moreover, some of these operators predict energy recoils above the upper threshold of the standard analyses done in direct detection experiments. For XENON100, this threshold is 43keV (nuclear recoil).
In this analysis, we extend the upper energy threshold up to ~240 keV. This value is dictated by low statistics in calibration data above it. We divide our signal region into two regimes, low recoil energy, on which we perform the same “standard” analysis done for the SI and SD cases, and high recoil energy, which is the main focus of this work.
We find that our data is compatible with background expectations. Using a binned profile likelihood, we thus produce 90% CL exclusion limits for both elastic scattering and inelastic WIMP scattering for each operator. Find the preprint of this study on the arxiv.
With 2 tonnes of target material, XENON1T is currently the largest liquid xenon detector in the search for dark matter. The detector’s immense volume greatly increases our chances of successfully observing that rare scatter of a dark matter particle off a xenon atom that we have sought for more than a decade, like casting a larger net to catch more fish. However, the size also makes it considerably more difficult for us to know how our detector would respond to the scatters of dark matter or to the scatters of electrons or gamma rays that would obscure our view of dark matter. These electrons and gammas come from ambient radon, impurities in the xenon, and even the detector components themselves. While their effects can be reduced through an appropriate selection of location and the materials, they cannot be completely eliminated. Hence, it is necessary to understand how our detector responds to electrons and gammas in contrast to dark matter. A thorough understanding of these backgrounds is arguably the single most crucial part of our endeavor.
Historically, in XENON10 and XENON100, we used external radioactive sources of cobalt and thorium, which emit gammas. Unfortunately, a drawback of XENON1T’s size is that it hobbles the rate of interactions in the central region of the detector, which is the most important for us in understanding our detector. For this reason, we needed to identify an internal, or dissolved, radioactive source that can be injected directly into the detector’s liquid xenon target volume.
One possibility is a source of 220Rn, which we characterize using the XENON100 detector. The source is well suited to calibrate our detector because it can imitate the effects of ambient radon (222Rn). The radioactive decay of 212Pb (a daughter of 220Rn) generates an electron just like 214Pb (a daughter of 222Rn), and thus their responses are very similar. The only thing we have to verify is that we can spread the 212Pb atoms throughout the detector volume. Due to the particular design of XENON100, many gamma events from other 220Rn daughters prevent us from identifying which events arise from 212Pb. So, we use the distribution of 212Bi, the daughter of 212Pb, to clearly show that 212Pb reaches even the centermost part of the detector. This distribution is shown in the figure below. Once that calibration is done, we do not have to proactively clean our detector, as would be the case with alternatives. Since the half-life of 212Pb is about half a day, we just wait a few days for the detector to return to normal.
The source also provides alpha particles that prove useful in understanding our background. The alpha particles of 216Po and 220Rn enable us to easily pinpoint their locations at the time of decay. And because they happen in quick succession (about 0.1 seconds), the daughter 216Po can be paired with its parent 220Rn. These pairs show us the trajectory of 216Po in the liquid xenon and how long it takes to decay. With both the time and the displacement, we can calculate the average velocity of the 216Po atom. If we then determine the velocities of all identified RnPo pairs, we can create a map of atomic motion in the full xenon volume of a given detector, as shown here:
In the case of XENON100, we find that atoms move at speeds up to 8 mm/s. They move in a single convection cell with a small contribution that results from an electric field applied to the xenon volume. This aspect tells us that an appreciable fraction of 216Po atoms is left ionized after the decay of 220Rn.
In general, a known map of atomic motion motivates two additional techniques to help us understand the backgrounds. Firstly, such a map enables us to identify the presence of “dead regions” through which liquid xenon does not flow very well. This feature is important for keeping the xenon free of impurities. Secondly, with sufficiently slow atomic motion or sufficiently large detector volumes, we could match 214Pb to its parent 222Rn or 218Po following a method similar to the one described previously for 220Rn. This tagging process would clearly distinguish 214Pb background events from the coveted dark matter event.
All of the finer details about this dissolved radon source can be found in the dedicated publication in Phys. Rev. D: E. Aprile et al. (XENON), Results from a calibration of XENON100 using a source of dissolved radon-220, Phys. Rev. D 95, 072008 (2017). This publication is of course also available as a pre-print as arXiv:1611.03585.
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.
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.
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 + 2e– → 124Te* + 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.
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.
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.
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.
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 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.
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.
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.
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: