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Observation of Excess Events in the XENON1T Dark Matter Experiment

Press release, June 17, 2020. For immediate release. A pre-print of this publication reporting the data analysis and details of the observed excess is available on arxiv.org, and in the meantime also directly here for download. These results were first presented on June 17 in a dedicated webinar by graduate student Evan Shockley from the University of Chicago. The slides of this presentation and a recording are available.

Scientists from the international XENON collaboration announced today that data from their XENON1T, the world’s most sensitive dark matter experiment, show a surprising excess of events. The scientists do not claim to have found dark matter. Instead, they say to have observed an unexpected rate of events, the source of which is not yet fully understood. The signature of the excess is similar to what might result from a tiny residual amount of tritium (a hydrogen atom with one proton and two neutrons), but could also be a sign of something more exciting—such as the existence of a new particle known as the solar axion or the indication of previously unknown properties of neutrinos.

The XENON1T detector. Visible is the bottom array of photomultiplier tubes, and the copper structure that creates the electric drift field.

XENON1T was operated deep underground at the INFN Laboratori Nazionali del Gran Sasso in Italy, from 2016 to 2018. It was primarily designed to detect dark matter, which makes up 85% of the matter in the universe. So far, scientists have only observed indirect evidence of dark matter, and a definitive, direct detection is yet to be made. So-called WIMPs (Weakly Interacting Massive Particles) are among the theoretically preferred candidates, and XENON1T has thus far set the best limit on their interaction probability over a wide range of WIMP masses. In addition to WIMP dark matter, XENON1T was also sensitive to different types of new particles and interactions that could explain other open questions in physics. Last year, using the same detector, these scientists published in Nature the observation of the rarest nuclear decay ever directly measured.

The excess observed in XENON1T in the electronic recoil background at low energies, compared to the level expected from known backgrounds indicated as the red line.

The XENON1T detector was filled with 3.2 tonnes of ultra-pure liquefied xenon, 2.0 t of which served as a target for particle interactions. When a particle crosses the target, it can generate tiny signals of light and free electrons from a xenon atom. Most of these interactions occur from particles that are known to exist. Scientists therefore carefully estimated the number of background events in XENON1T. When data of XENON1T were compared to known backgrounds, a surprising excess of 53 events over the expected 232 events was observed.

This raises the exciting question: where is this excess coming from?

One explanation could be a new, previously unconsidered source of background, caused by the presence of tiny amounts of tritium in the XENON1T detector. Tritium, a radioactive isotope of hydrogen, spontaneously decays by emitting an electron with an energy similar to what was observed. Only a few tritium atoms for every 10 25 (10,000,000,000,000,000,000,000,000!) xenon atoms would be needed to explain the excess. Currently, there are no independent measurements that can confirm or disprove the presence of tritium at that level in the detector, so a definitive answer to this explanation is not yet possible.

More excitingly, another explanation could be the existence of a new particle. In fact, the excess observed has an energy spectrum similar to that expected from axions produced in the Sun. Axions are hypothetical particles that were proposed to preserve a time-reversal symmetry of the nuclear force, and the Sun may be a strong source of them. While these solar axions are not dark matter candidates, their detection would mark the first observation of a well-motivated but never observed class of new particles, with a large impact on our understanding of fundamental physics, but also on astrophysical phenomena. Moreover, axions produced in the early universe could also be the source of dark matter.

Alternatively, the excess could also be due to neutrinos, trillions of which pass through your body, unhindered, every second. One explanation could be that the magnetic moment (a property of all particles) of neutrinos is larger than its value in the Standard Model of elementary particles. This would be a strong hint to some other new physics needed to explain it.

Of the three explanations considered by the XENON collaboration, the observed excess is most consistent with a solar axion signal. In statistical terms, the solar axion hypothesis has a significance of 3.5 sigma, meaning that there is about a 2/10,000 chance that the observed excess is due to a random fluctuation rather than a signal. While this significance is fairly high, it is not large enough to conclude that axions exist. The significance of both the tritium and neutrino magnetic moment hypotheses corresponds to 3.2 sigma, meaning that they are also consistent with the data.

XENON1T is now upgrading to its next phase–XENONnT–with an active xenon mass three times larger and a background that is expected to be lower than that of XENON1T. With better data from XENONnT, the XENON collaboration is confident it will soon find out whether this excess is a mere statistical fluke, a background contaminant, or something far more exciting: a new particle or interaction that goes beyond known physics.

The XENON1T Data Acquisition System

Featuring several kilometers of cables, dozens of analog electronics modules, crates of purpose-built specialty computers, and backed by a small server farm, the XENON1T data acquisition system (DAQ) was designed to put our data onto disks. The XENON Collaboration recently published a technical paper on our DAQ in JINST, of course also available on arXiv.

The XENON1T detector measures light, which creates analog electrical signals in 248 independent photo-sensors. The DAQ is responsible for converting these analog signals to a digital, storage-ready format, deciding what types of aggregate signal indicate the presence of a physical interaction in the detector, and recording all the interesting data onto disk for later storage and analysis.

A photo of the XENON1T DAQ room, deep underground at the Gran Sasso lab. Pictured left to right: the DAQ server rack, (red) digitizers (amplifiers facing backwards), cathode high voltage supply, muon veto DAQ, slow control server rack.

There are a couple novel aspects of this system. The first is that the data is streamed constantly from the readout electronics onto short-term storage, recording all signals above a single photo-electron with high (>93%) efficiency. This is different from a conventional data acquisition system, which usually would require certain hardware conditions to be met to induce acquisition,  also called a trigger. We defer our trigger to the software stage, giving us a very low energy threshold.

The software trigger itself was implemented as a database query, which is another novel aspect of the system. Pre-trigger data was stored in a MongoDB NoSQL database and the trigger logic scanned the database looking for signals consistent with S1’s (light) and S2’s (charge). If the algorithm found a matching signal, it would retrieve all the nearby data from the database and write it to storage. Because of the speed of NoSQL databases, this worked the same in both dark matter search mode, where we record just a few counts per second, and calibration modes, where we could record hundreds of counts per second.

To complete the high-tech upgrade of our system, we also ran the user interface as a web service. This means the system could be controlled from laptops, smartphones, or tablets anywhere with a 4G connection, contributing to the high uptime of the detector.

The DAQ is currently being updated to double its capacity to read out the XENONnT detector, so stay tuned.

Search for light dark matter interactions enhanced by the Migdal effect in XENON1T

When a particle elastically scatters off a xenon nucleus, it has been assumed that electron clouds immediately follow the motion of the nucleus, but in reality it takes some time for the atomic electrons to catch up, resulting in ionization and excitation of the atom. This effect is called the Migdal effect, which was predicted by A. B. Migdal and recently reformulated in the context of Dark Matter searches by Ibe. et alWhile the elastic scattering of WIMPs produces nuclear recoils, the Migdal effect predicts secondary electronic recoils that can accompany a nuclear recoil. Unlike nuclear recoils, electronic recoils lose negligible energy as heat, because electrons have small masses compared with xenon nuclei. This results in a lower energy threshold for electronic recoil signals – in XENON1T, down to about 1 keV. Therefore, searching for the electronic recoil signals induced by the Migdal effect enables a significant boost of XENON1T’s sensitivity to low-mass dark matter, based on this lowered threshold. In this search, we adopted an approach that utilizes the ionization signal only (so-called S2-only analysis), as well as both scintillation and ionization signals (S1-S2 analysis), which enables to lower the detection threshold. We interpreted the results in different cases: spin-(in)dependent (SI/SD) WIMP-nucleon interaction and the scenario where the interaction is mediated by a scalar force mediator (light mediator). The results for the spin-(in)dependent WIMP-nucleon interaction are shown in the following figure:
 
We set the most stringent upper limits on the SI and SD WIMP-nucleon interaction cross-sections for masses below 1.8 GeV and 2 GeV, respectively. Together with the standard nuclear recoil search, XENON1T results have thus reached unprecedented sensitivities to both low-mass (sub-GeV) and high-mass (GeV – TeV) WIMPs. An open access pre-print of the paper can of course be found on the arxiv.

Light Dark Matter Search Results from XENON1T

XENON1T recently released a preprint with new world-leading constraints on light dark matter particles.

The challenge of light dark matter

The XENON1T detector aims find the signals of dark matter bouncing off xenon atoms.
If such a collision happens, it produces two signals: a small light flash (S1), and a cloud of free electrons that can be drifted up and extracted out of the detector (S2).

Figure: How dark matter would make S1 and S2 signals in the XENON1T detector.

However, dark matter lighter than about six times the proton mass (6 GeV/c^2) cannot push the heavy xenon atoms (131 GeV/c^2) enough to make efficiently detectable S1s. XENON1T needs both S1 and S2 to accurately reconstruct where in the detector the event happened. The time between the S1 and S2 signals reveals the depth of the event. Events at the top and bottom edge of the detector are common due to radioactive backgrounds. If we cannot reject these events, dark matter searches will not be efficient. Thus, most strong constraints on light dark matter have, until now, come from different detectors, mostly using ultra-low temperature crystals made of Germanium, Silicone, or Calcium Tungstate.

The S2-only technique

XENON1T’s new preprints use an “S2-only analysis”, where events without S1s are still considered. Advances in detector construction and analysis techniques led to a thousand times lower background level than previously achieved in S2-only searches.

For example, the S2 electron cloud becomes broader as it drifts upward, like a drop of ink spreading out in water. The deeper the event, the broader the cloud, and the longer the S2 signal lasts. Thus XENON1T could reject most of the events at the top and bottom, even without the S1, by rejecting very short and very long S2 signals.

The results

Most theorists predict that dark matter would collide with the heavy xenon nuclei and produce “nuclear recoils”. For these, the S2-only technique is sensitive to 2-3x lower energies than traditional analyses. Thus, we get improved constraints on light dark matter:

Figure: New XENON1T limits (black lines) on light dark matter. The colored lines show previous results, including other results from XENON1T in blue.

In some models, dark matter collides with electrons around the nucleus, and produces “electronic recoils”. These make much larger S2 signals than nuclear recoils of the same S1 size. S2-only searches thus improve the energy threshold for these models by as much as a factor of ten. Combined with the lower background, XENON1T’s S2-only results thus improve the constraints on such models by several orders of magnitude:

Figure: New XENON1T limits on scattering of dark matter on electrons. (The dashed line is the same analysis repeated with more conservative assumptions.)

For more information, please see our arXiv preprint at https://arxiv.org/abs/1907.11485.

 

Signal Reconstruction, Calibration and Event Selection in XENON1T

Since the first release of dark matter search results based on the 1 tonne-year exposure of the XENON1T experiment, the collaboration has published more WIMP signal searches based on the same dataset. Those articles are usually written in a brief way and are focusing on the communication of the scientific results.

In order to give more details on the XENON1T dark matter analysis, we have previously published a paper focusing on the signal and background models and the statistical inference using this data. It has been complemented by a new article that reveals details on the challenges of detector characterization and data preparation before it is ready to be used for model building and statistical inference in order to make statements on dark matter.

The XENON1T experiment performed two science runs between October 2016 and February 2018, reaching a total data livetime of 279 days. During that time the detector had to be operated in a very stable mode in order to ensure undistorted signals. If some conditions change over time they have to be modeled over time in order to account for them in the take them into account during data analysis and include them into the models. One example for those changes are the ones at the photosensors. Each sensor has an individual amplification factor, i.e. gain, that is a function of the applied high voltage. few sensors developed malfunctions during the science runs because of which the amplification factor decreased over time or the voltage had to be reduced resulting in a sudden decreased of the amplification. Those variations are shown in red and black for two sensors as a function of time in the following figure while green, blue and magenta show stable sensors which are representative for the majority of the XENON1T light detectors.

 

Measured photosensor amplification factor as a function of time for three representative stable sensors (green, blue and magenta) and two examples where the amplification decreased due to malfunctions (red and black).

As soon as the detector operation conditions are modeled the data is put through selection criteria that reduce the number of background-like signatures and therefore enhance the signal to background ratio. The criteria are grouped into four general types:

 

Acceptance of dark matter signal events after incrementally applying data selection criteria in order to reduce background-like signatures. The acceptance is shown as function of the signal parameters S1 and S2.

Modelling how an electronic or nuclear recoil will look like in the detector is crucial both to know the shape of a WIMP signal, and to model the backgrounds well. XENON1T uses a comprehensive fit to multiple calibration sources to constrain the distributions of  backgrounds and signals in the analysis space; S1, S2 and the radius from the center axis of the detector. Some background components are harder to model directly, and are estimated by using sidebands or other data samples. In the XENON1T analysis, coincidences between unrelated, lone S1 and S2 events were modeled this way, in addition to the surface background– events occurring close to or at the detector wall.

The models of each background and the signal, for two separate science runs, are put together in a likelihood, which is a mathematical function of the WIMP signal strength as well as nuisance parameters. These are unknowns that could change the analysis, such as the true expectation value for each background component. The likelihood also contains multiple terms representing measurements of nuisance parameter, which constrain them when the likelihood is fitted to the data collected by XENON1T. The value of the likelihood evaluated at a specific signal strength has a random distribution which is estimated using simulated realizations of the experimental outcome. The final statistical limits are computed by comparing the likelihood computed on the actual data with the distributions found from the simulations:

Observing the Rarest Decay Process Ever Measured

[Press Release April 2019 – for immediate release. Paper published in Nature and preprint on the arxiv.]

The universe is almost 14 billion years old. An inconceivable length of time by human standards – yet compared to some physical processes, it is but a moment. There are radioactive nuclei that wdecay on much longer time scales. Using our XENON1T detector at the INFN Gran Sasso National Laboratory, we were able to observe the decay of Xenon-124 atomic nuclei for the first time.

The half-life of a process is the time after which half of the radioactive nuclei present in a sample have decayed away. The half-life measured for Xenon-124 is about one trillion times longer than the age of the universe. This makes the observed radioactive decay, the so-called double electron capture of Xenon-124, the rarest process ever seen happening in a detector. “The fact that we managed to observe this process directly demonstrates how powerful our detection method actually is – also for signals which are not from dark matter,” says Prof. Christian Weinheimer from the University of Münster (Germany) whose group lead the study. In addition, the new result provides information for further investigations on neutrinos, the lightest of all elementary particles whose nature is still not fully understood. XENON1T is a joint experimental project of about 160 scientists from Europe, the US and the Middle East. The results were published in the science journal “Nature”.

A sensitive dark matter detector

The Gran Sasso Laboratory of the National Institute for Nuclear Physics (INFN) in Italy, where scientists are currently searching for dark matter particles is located about 1,400 meters beneath the Gran Sasso massif, well protected from cosmic rays which can produce false signals. Theoretical considerations predict that dark matter should very rarely “collide” with the atoms of the detector. This assumption is fundamental to the working principle of the XENON1T detector: its central part consists of a cylindrical tank of about one meter in length filled with 3,200 kilograms of liquid xenon at a temperature of –95° C. When a dark matter particle interacts with a xenon atom, it transfers energy to the atomic nucleus which subsequently excites other xenon atoms. This leads to the emission of faint signals of ultraviolet light which are detected by means of sensitive light sensors located in the upper and lower parts of the cylinder. The same sensors also detect a minute amount of electrical charge which is released by the collision process.

In double electron capture, two electrons and two protons simultaneously convert into two neutrons and two neutrinos. X-rays are emitted when the electron vacancies are subsequently filled.

The new study shows that the XENON1T detector is also able to measure other rare physical phenomena, such as double electron capture. To understand this process, one should know that an atomic nucleus normally consists of positively charged protons and neutral neutrons, which are surrounded by several atomic shells occupied by negatively charged electrons. Xenon-124, for example, has 54 protons and 70 neutrons. In double electron capture, two protons in the nucleus simultaneously “catch” two electrons from the innermost atomic shell, transform into two neutrons, and emit two neutrinos. The other atomic electrons reorganize themselves to fill in the two holes in the innermost shell. The energy released in this process is carried away by X-rays and so-called Auger electrons. However, these signals are very hard to detect, as double electron capture is a very rare process which is hidden by signals from the omnipresent natural radioactivity.

The measurement

The peak at 64keV from double electron capture of Xenon-124 is clearly visible in this plot of the background spectrum from XENON1T.

This is how the XENON collaboration succeeded with this measurement: The X-rays from the double electron capture in the liquid xenon produced an initial light signal as well as free electrons. The electrons were moved towards the gas-filled upper part of the detector where they generated a second light signal. The time difference between the two signals corresponds to the time it takes the electrons to reach the top of the detector. Scientists used this interval and the information provided by the sensors measuring the signals to reconstruct the position of the double electron capture. The energy released in the decay was derived from the strength of the two signals. All signals from the detector were recorded over a period of more than one year, however, without looking at them at all as the experiment was conducted in a “blind” fashion. This means that the scientists could not access the data in the energy region of interest until the analysis was finalized to ensure that personal expectations did not skew the outcome of the study. Thanks to the detailed understanding of all relevant sources of background signals it became clear that 126 observed events in the data were indeed caused by the double electron capture of Xenon-124.

Using this first-ever measurement, the physicists calculated the enormously long half-life of 1.8×1022 years for the process. This is the slowest process ever measured directly. It is known that the atom Tellurium-128 decays with an even longer half-life, however, its decay was never observed directly and the half-life was inferred indirectly from another process. The new results show how well the XENON1T detector can detect rare processes and reject background signals. While two neutrinos are emitted in the double electron capture process, scientists can now also search for the so-called neutrino-less double electron capture which could shed light on important questions regarding the nature of neutrinos.

Status and outlook

XENON1T acquired data from 2016 until December 2018 when it was switched off. The scientists are currently upgrading the experiment for the new “XENONnT” phase which will feature a three times larger active detector mass. Together with a reduced background level this will boost the detector’s sensitivity by an order of magnitude.

 

Modeling and statistical analysis of the XENON1T data

On May 31st 2018, XENON1T released the result of a search for dark matter interacting with xenon atoms using an exposure of 1 tonne-year. Papers presenting the scientific results are written to be brief, and communicate the most important information to the scientific community. Therefore, many details of the instrument, reconstruction of events and analysis work by the entire collaboration must be left out of the science papers. XENON1T has previously published a paper focusing on the operation of the detector itself. A new paper by XENON1T now goes into the details of the analysis of the XENON1T data, and another one, on the event reconstruction and calibration, is being prepared.

XENON1T detects the scintillation light and ionization electrons that energy depositions in the two tonne liquid xenon target produce. In addition to WIMPs, different background sources can produce an S1+S2 signal. The expected S1,S2 distribution may change depending on whether the energy deposition happens by a recoil on an electron of the xenon atom or the nucleus. This is one of the main methods XENON uses to discriminate against backgrounds, since WIMPs, which scatter on the xenon nucleus, have a mean S2 lower than 99.7% of the dominant background component, which is made up of scatters on electrons.

Modelling how an electronic or nuclear recoil will look like in the detector is crucial both to know the shape of a WIMP signal, and to model the backgrounds well. XENON1T uses a comprehensive fit to multiple calibration sources to constrain the distributions of backgrounds and signals in the analysis space; S1, S2 and the radius from the center axis of the detector.
Some background components are harder to model directly, and are estimated by using sidebands or other data samples. In the XENON1T analysis, coincidences between unrelated, lone S1 and S2 events were modeled this way, in addition to the surface background– events occurring close to or at the detector wall.

Models of various backgrounds and the expected WIMP signal in two of the parameters extracted from each even, scintillation S1 and ionization S2 signals.


The models of each background and the signal, for two separate science runs, are put together in a likelihood, which is a mathematical function of the WIMP signal strength as well as nuisance parameters. These are unknowns that could change the analysis, such as the true expectation value for each background component. The likelihood also contains multiple terms representing measurements of nuisance parameter, which constrain them when the likelihood is fitted to the data collected by XENON1T.

The value of the likelihood evaluated at a specific signal strength has a random distribution which is estimated using simulated realizations of the experimental outcome. The final statistical limits are computed by comparing the likelihood computed on the actual data with the distributions found from the simulations: 

Likelihood as function of the signal strength (measured by the WIMP-nucleon cross-section)
The gray area shows likelihoods that corresponds to a 90% exclusion. The confidence interval– the region of signal strength compatible with the observed data– is the region where the likelihood lies below the gray band.


The models and tools used in the XENON1T spin-independent analysis are also used to explore alternative models of dark matter, such as spin-independent interactions and scatterings between WIMPs and pions, with more to come!

 

Constraining the spin-dependent WIMP-nucleon interaction with XENON1T

Since we don’t know how dark matter interacts with more familiar particles, we have to break up our search for weakly interacting massive particles (WIMPs) in terms of their possible interactions with xenon nuclei. While many complex interactions are possible, we generally start with two simple cases: WIMP-nucleus interactions that don’t depend on the nuclear spin, and those that do. XENON1T set a world-leading constraint on the former, “spin-independent” interaction in 2018. Today, we released our first results constraining the latter, “spin-dependent” interaction. The results are shown in the following figure:

The spin-dependent WIMP-nucleon interaction contains a range of possible cases, so experiments typically consider two extreme ones: the case where WIMPs only scatter off protons, and the case where they only scatter off neutrons. Most of the spin in xenon is carried by neutrons, so xenon experiments are better at constraining the neutron-only case. These results set the most stringent limit on this case, using the same data and procedure as the spin-independent result. We also tried out a new method of combining our constraints with complementary searches at particle accelerators, following the example of PICO-60. An open-access pre-print version of the paper is available on the arXiv.

The first limits on spin-independent WIMP-pion interactions

XENON1T was built to observe the recoil of xenon-atoms, which may be caused by the interaction of a Weakly Interacting Massive Particle (WIMP) as it passes through the detector. A recoiling xenon atom produces scintillation light and ionization that XENON1T detects as an S1 and S2 signal, which carry information of the recoil type, energy and position in the detector. The first results of the XENON1T were published on the spin-independent WIMP-nucleon interaction, which is expected to dominate the WIMP scattering rate. However, models of WIMPs exist where this contribution would be suppressed or vanish. XENON has therefore performed searches for alternative WIMP-recoil spectra, such as the one expected if the scattering depends on the nucleon spins.

A careful accounting of all the possible WIMP-nucleon interactions showed that WIMPs can also interact with pions— subatomic particles that contribute to the strong force that binds atoms together. The figure illustrates a WIMP (χ) scattering via a mediator line on a pion (π) exchanged between a proton and a neutron in the xenon nucleus. The xenon atom recoils from the interaction, which can be observed with our detector. Similarly to the spin-independent recoil, the wimp-pion interaction happens in a way where the WIMP scatters coherently, off the entire xenon atom together. 

A WIMP scattering on a pion exchanged within the xenon nucleus

The analysis was performed with the same tools as the main XENON1T spin-independent WIMP search, and 1 tonne-years of data. No significant evidence for a signal was observed, so we set the first limits on the spin-independent WIMP-pion interaction strength. An open access pre-print of the paper can be found on the arxiv.

Latest XENON1T results at ICHEP2018 in Seoul

The XXXIX International Conference on High Energy Physics (ICHEP2018) was taking place from July 4 – 11, 2018 in Seoul, Korea. After a warm welcome in this modern and traditional metropolis with over 10 million citizens, I was invited to present the recent results from XENON1T in a Dark Matter parallel session.

Here is one slide of my talk visualizing the spatial distribution of the unblinded and de-salted events.

Spatial distribution of unblinded and de-salted data.

The left plot shows the X- and Y- distribution, while the right plot indicates the radius R versus depth Z for the same set of data. The enlarged fiducial volume of 1.3 tons with respect to the first result, is highlighted by the pink line. For the analysis, a core volume (green line) was defined to distinguish WIMP-like events over neutron-like background events. The different events are visualized by pie charts, where the color code resembles the relative probability from each background component assigned by the best-fit. The larger a pie is, the more “WIMPy” it is. As you can see, only a few “WIMPy” events were found that are comparable to the background model expectations. From this, we derived the most stringent limits on spin-independent WIMP-nucleon cross sections.

At the end of my talk,  I also reported on the status of XENONnT, which will feature a 10x higher sensitivity than XENON1T. One main task is radon mitigation, one of the dominant backgrounds, which is visualized in this slide.

Radon mitigation for XENONnT

In a first step, a careful material selection needs to be made to avoid radon emanation from the start. Then, a new high throughput radon distillation column is under development to further reduce the radon contribution. Additionally, a new custom-made radon-free magnetically-coupled piston pump was built and installed at XENON1T in June 2018. With that, the radon budget in XENON1T was reduced by almost half (45%), which is an important step for the future XENONnT experiment.

The full talk is publicly available here.