Tag Archives: dark matter

XENON1T, the most sensitive detector on Earth searching for WIMP dark matter, releases its first result

[Press Release – 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.

XENON1T at LNGS

XENON1T installation in the underground hall of Laboratori Nazionali del Gran Sasso. The three story building on the right houses various auxiliary systems. The cryostat containing the LXeTPC is located inside the large water tank on th left, next to the building. (Photo by Roberto Corrieri and Patrick De Perio)

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 XENON1T TPC

Scientists assembling the XENON1T time projection chamber. (Photo by Enrico Sacchetti)

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.

XENON1T first results limit

The spin-independent WIMP-nucleon cross section
limits as a function of WIMP mass at 90% confidence
level (black) for this run of XENON1T. In green and yellow
are the 1- and 2σ sensitivity bands. Results from LUX
(red), PandaX-II (brown), and XENON100 (gray)
are shown for reference.

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 contact@xenon1t.org.

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.

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.

gAe_Galactic_noS2width_sensitivity-exclusion_withCLS

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.

gAe_Solar_noS2width_sensitivity-exclusion

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.

 

Dark Matter is Out There

Dark matter has been discovered. We know from measurements of the relic abundance of light elements that were generated just minutes after the Big Bang that the known, baryonic, matter is not sufficient to explain the energy-matter density of the Universe today. A cold dark matter component has been measured from the incredibly accurate observations of the Cosmic Microwave Background, which was emitted just 300,000 years after the Big Bang. And dark matter must exist in order to turn the tiny fluctuations in the Cosmic Microwave Background into the huge density fluctuations that are observed in the Universe today.

Our Milky Way

Our Milky Way contains much more mass in the form of the mysterious dark matter than meets the eye. Picture by Thomas Tuchan.

Gravitational lensing and dispersion measurements of galaxy clusters, the largest bound systems that have been observed, show that dark matter is the dominating mass component. Detailed studies of half a dozen or so merging galaxy clusters have clearly ruled out possible alternative explanations involving modifications of the gravitational law, and are now starting to probe the properties of dark matter itself. We also know that dark matter exists in our own galaxy, the Milky Way, which shows rotational velocities that are independent of radius at high radii, just as in any other spiral galaxy we observe. This flat rotation curve is clearly inconsistent with that expected from Kepler’s laws but is naturally explained by the fact that galaxies are immersed in a halo of dark matter that dominates their mass.

Taken together, we have discovered dark matter with independent measurements spanning vast time scales from a few minutes after the Big Bang all the way to today, and at length scales from the Cosmos as a whole to individual galaxies. Yet, what dark matter is made out of remains entirely unknown. Thus, research into the nature of dark matter is of utmost importance to our view of the Cosmos. It is pursued with a variety of diverse approaches that test dark matter interactions with other known particles, with itself, and at a range of different energies.

Dark matter can be expected to have couplings, albeit weak, to standard matter, so that it can be searched for with laboratory experiments. This direct search for dark matter is pursued with a variety of complementary technologies and experiments. The XENON project in particular is one of the most sensitive direct searches for dark matter.