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.
[Press Release May 2017 – for immediate release. Preprint is on the arxiv]
“The best result on dark matter so far! … and we just got started!”.
This is how scientists behind XENON1T, now the most sensitive dark matter experiment world-wide, hosted in the INFN Laboratori Nazionali del Gran Sasso, Italy, commented on their first result from a short 30-day run presented today to the scientific community.
Dark matter is one of the basic constituents of the Universe, five times more abundant than ordinary matter. Several astronomical measurements have corroborated the existence of dark matter, leading to a world-wide effort to observe directly dark matter particle interactions with ordinary matter in extremely sensitive detectors, which would confirm its existence and shed light on its properties. However, these interactions are so feeble that they have escaped direct detection up to this point, forcing scientists to build detectors that are more and more sensitive. The XENON Collaboration, that with the XENON100 detector led the field for years in the past, is now back on the frontline with the XENON1T experiment. The result from a first short 30-day run shows that this detector has a new record low radioactivity level, many orders of magnitude below surrounding materials on Earth. With a total mass of about 3200kg, XENON1T is at the same time the largest detector of this type ever built. The combination of significantly increased size with much lower background implies an excellent dark matter discovery potential in the years to come.
The XENON Collaboration consists of 135 researchers from the US, Germany, Italy, Switzerland, Portugal, France, the Netherlands, Israel, Sweden and the United Arab Emirates. The latest detector of the XENON family has been in science operation at the LNGS underground laboratory since autumn 2016. The only things you see when visiting the underground experimental site now are a gigantic cylindrical metal tank, filled with ultra-pure water to shield the detector at his center, and a three-story-tall, transparent building crowded with equipment to keep the detector running, with physicists from all over the world. The XENON1T central detector, a so-called Liquid Xenon Time Projection Chamber (LXeTPC), is not visible. It sits within a cryostat in the middle of the water tank, fully submersed, in order to shield it as much as possible from natural radioactivity in the cavern. The cryostat allows keeping the xenon at a temperature of -95°C without freezing the surrounding water. The mountain above the laboratory further shields the detector, preventing it to be perturbed by cosmic rays. But shielding from the outer world is not enough since all materials on Earth contain tiny traces of natural radioactivity. Thus extreme care was taken to find, select and process the materials making up the detector to achieve the lowest possible radioactive content. Laura Baudis, professor at the University of Zürich and professor Manfred Lindner from the Max-Planck-Institute for Nuclear Physics in Heidelberg emphasize that this allowed XENON1T to achieve record “silence”, which is necessary to listen with a larger detector much better for the very weak voice of dark matter.
A particle interaction in liquid xenon leads to tiny flashes of light. This is what the XENON scientists are recording and studying to infer the position and the energy of the interacting particle and whether it might be dark matter or not. The spatial information allows to select interactions occurring in the central 1 ton core of the detector. The surrounding xenon further shields the core xenon target from all materials which already have tiny surviving radioactive contaminants. Despite the shortness of the 30-day science run the sensitivity of XENON1T has already overcome that of any other experiment in the field, probing un-explored dark matter territory. “WIMPs did not show up in this first search with XENON1T, but we also did not expect them so soon!” says Elena Aprile, Professor at Columbia University and spokesperson of the project. “The best news is that the experiment continues to accumulate excellent data which will allow us to test quite soon the WIMP hypothesis in a region of mass and cross-section with normal atoms as never before. A new phase in the race to detect dark matter with ultra-low background massive detectors on Earth has just began with XENON1T. We are proud to be at the forefront of the race with this amazing detector, the first of its kind.”
As always, feel free to contact the XENON collaboration at email@example.com.
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.
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.
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:
Analyzing terabytes of background and calibration data in the search for just a couple of dark matter-induced events is a difficult process that we published in a dedicated paper: