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:

XENON at EPS-HEP2019

XENON was on the agenda at the European Physical Society Conference on High Energy Physics 2019 (EPS-HEP2019), which was held in Ghent, Belgium in the middle of July. The talk, presented by Adam Brown from the University of Zurich group, concentrated on results from XENON1T and also provided an overview of the work which is well underway to build the next generation detector, XENONnT.

Among results shown were our searches for elastic WIMP scattering and the recently published observation of double electron capture in 124Xe. The slides can be downloaded here. While the XENONnT upgrade currently in progress at Gran Sasso features many improvements of the XENON1T detector, Adam summarized four major improvements in one colorful slide.

XENON talk at Patras Workshop

A talk on the XENON project was given at the 15th Patras Workshop on Axions, WIMPs and WISPs, which was held in Freiburg (Germany) in the first week of June. Andrea Molinario from the Gran Sasso Science Institute and Laboratori Nazionali del Gran Sasso presented the most recent results from the data analysis of XENON1T, in particular the search for WIMP-nucleon spin-dependent and spin-independent interactions. The sensitivity of this search will be much-improved upon by the upcoming XENONnT phase of the experiment.

The first observation of 124Xe double electron capture and the measurement of the half-life of the process were also shown (this topic had a dedicated talk by Sebastian Lindemann). In the second part of his talk, Andrea gave an update on the status of XENONnT. The presentation is available here.

XENON1T talk at Low Radioactivity Technique 2019

Our latest XENON1T paper on details of our analysis was presented at the Low Radioactivity Techniques, a conference focused on low background experiments. In the talk (that you can find here), the response model of the detector, the challenges of background modeling, as well as the used techniques were described. In a low background experiment is often hard to asses the expected distribution of events due to lack of statistics and to many subtle effects. In the talk a novel technique was described to introduce a well-motivated systematic uncertainty to the background model based on a calibration sample, which can be relevant to other low background experiments.

The XENONnT dual-phase xenon TPC requires two regions with different electric fields to drift, extract, and accelerate the small number of ionization electrons that are created by a possible dark matter interaction with xenon nuclei. These fields will be created with a total of five electrodes that are biased at constant electric potentials from top to the bottom of the TPC. The challenges to build these large electrodes with almost 1.5 meters in diameter with very thin wires include stringent requirements on their optical transparency, wire sagging, field uniformity and high voltage stability.

Such a challenging project is carried out by a collaborative effort of many expertises within the XENON collaboration. The design and production of the electrodes are led by Dr. Carla Macolino and realized by researchers from the Laboratoire de l’Accélérateur Linéaire, Rice University, University of California San Diego, and University of Coimbra, accompanied by further technical design and electric field simulation support from University of Chicago and Freiburg University. A special instrument was designed and built by the University of Münster to measure the tension of every individual wire. Finally, strict cleaning requirement is satisfied from the expertise at MPI for Nuclear Physics and technical support from Nikhef.

Shown are the actual XENONnT electrodes during construction and quality control in above-ground clean room laboratories.

 

XENON on skis

XENON was present at the ALPS conference in Austria. Chiara Capelli from University of Zurich gave a talk on behalf of the XENON collaboration. The talk focused on the latest XENON1T results on spin-independent and spin-dependent WIMPs, and on the newest results on two-neutrinos double electron capture, with a final status on the XENONnT upgrade. The talk is available here.

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.

 

Data for reproducing the limits of: “Constraining the spin-dependent WIMP-nucleon interaction with XENON1T”

To add our limits to your own figures you can use the official data points from the XENON1T collaboration, as we believe that data should be shared. They can be both found here and on the ArXiv page (‘Download Other formats’). The format is csv and the following files are included:
– x1t_n_only_limit.csv (Fig 2.)  x1t_n_only_limit
– x1t_p_only_limit.csv (Fig 3.)  x1t_p_only_limit
– x1t_isoscalar_mediator_mass_95CL_limit.csv (Fig 4.)  x1t_isoscalar_mediator_mass_95CL_limit
The files for Fig 2 and 3 contain the mass points between 6 and 1.000 GeV and their corresponding 90% CL limits, with cross sections in cm^2. The values for the plus and minus 1 and 2 sigma bands at those mass points are also included.
The file for Fig 4 contain the mass points between 6 and 10.000 GeV and their corresponding cross sections at 95% CL in cm^2. Please contact us if you have any questions regarding this data.

On March 8, 2019, Shigetaka Moriyama presented the status of the XENONnT experiment at the international symposium on “Revealing the history of the Universe with underground particle and nuclear research” in Sendai, Japan. The symposium is held by a Japanese research community working on underground experiments and developing low background techniques. Its members are interested in the physics goals of XENONnT as well as its radon reduction technique and will enhance the experiment with Super-Kamiokande’s water Cherenkov technology developed in Kamioka, Japan, for the SK-Gd project. Super-Kamiokande developed this technology to measure the diffuse relic neutrino flux from past supernovae.

At the Sendai meeting, this community is summarizing its achievements over last five years and aims to secure new funding for the next five years by expanding its activity through internationalization and the inclusion of new physics topics such as history of stars, galaxies, and the origin of the heavy elements in the Universe.

Its HP is here and the slides are available here.