News

17.12.2020 Final Results of GERDA

GERDA has reported its final results on the search for the neutrinoless double-beta (0νββ) decay of 76Ge in the recent issue of Physical Review Letters. No signal has been observed, but all goals of the final phase of the experiment have been achieved.

The reported lower limit for the 0νββ half-life of 1.8x1026 yr coincides with the expected value for the sensitivity of the experiment; a more stringent value for the decay of any 0νββ isotope has been measured never before. Similarly, the reported background rate of 5.2x10-4 counts/(kg∙yr∙keV) in the signal region is second to none in the field, demonstrating not only the feasibility of a background-free experiment at high exposure but also providing the foundation for a next generation experiment with significantly higher sensitivity. More details can be found here.
 

03.12.2020 PhD awarded to Dr. Chiara Capelli

Congratulations to Dr. Chiara Capelli who successfully defended her Ph.D. thesis at the beginning of December! During her thesis work, she performed a first search for the neutrinoless double beta decay of 136-Xe with XENON1T data. In particular, she worked on the signal reconstruction in the MeV energy range and achieved the best energy resolution to date in a two-phase xenon time projection chamber, namely 0.8% at 2.458 keV, the Q-value of the 136-Xe decay. This work was published and featured in EPJ-C 80 (2020) 8, 758. For XENONnT, she was responsible for the design, construction and installation of the light calibration system of the VUV-sensitive photosensors, currently used for calibrating the PMTs in the liquid xenon phase. Chiara Capelli also participated in the installation and initial commissioning of the XENONnT TPC: she remained at LNGS (together with another PhD student in our group, Giovanni Volta) during the start of the new pandemic and completed the installation of the TPC underground. 

11.11.2020 Three short videos about our research

Every year in November - during an event organised by our department - we present with posters and flyers our research topics to prospective bachelor and master students. In 2020, the organisation of such an event was not possible due to the corona pandemic. Our students and postdocs thus created three short videos to present our main research on dark matter, neutrinoless double beta decay and other topics in the framework of the DARWIN, GERDA/LEGEND and XENON projects. You can find these videos under this link. If you are a prospective bachelor and master student, and interested in a concrete project, please get in touch! We also offer small R&D projects in our laboratories in the Physics Department.

30.10.2020 DARWIN in the Nature News and Spektrum

During this month, the DARWIN experiment has received considerable attention from the media. Two articles have appeared, one in the Nature News and the other in the German magazine Spektrum, describing the DARWIN project and its huge effort to give the WIMPs the last chance to be detected. We want to congratulate the whole DARWIN Collaboration and recommend to everybody the interesting reading.

In a search for solar axions, bosonic dark matter, and an enhanced neutrino magnetic moment, the XENON1T collaboration found an excess of events in the low-energy electronic recoil region. The collaboration is not making any claims about its nature, as the source of these events is not yet known. The signature of the excess is similar to what might result from a tiny residual amount of tritium in the detector, but could also be a sign of something more exciting—such as the existence of solar axions or the indication of previously unknown properties of neutrinos. The results are now published in Physical Review D and featured in a Viewpoint article in the journal entitled “Dark Matter Detector Delivers Enigmatic Signal”. The results also yielded a flurry of theoretical explanations of the excess, a synopsis of which is featured in the Physical Review, entitled "Theorists React to Potential Signal in Dark Matter Detector".

02.10.2020 PhD awarded to Dr. Adam Brown

Congratulations to Dr. Adam Brown who successfully defended his Ph.D. thesis on October 1st! During his thesis work, he performed a study of inelastic dark matter interactions in XENON1T, tested photomultiplier tubes for the XENONnT experiment with our MarmotX liquid xenon detector and built the bases, connectors and cables necessary to power and readout the signals from the PMTs.  Adam Brown was also involved in the commissioning of the XENONnT time projection chamber, especially in the assembly of both photosensor arrays at LNGS. 

25.09.2020 DARWIN Demonstrator: Successful Installation of the Heat Exchanger

Our DARWIN team is making progress towards the commissioning of the full-length DARWIN demonstrator, under construction here at the University of Zurich. This week we installed the heat exchanger, a component that will increase the efficiency of our recirculation system by reducing the cooling power requirements. The successful installation follows the detailed study and design by PhD student Kevin Thieme. We are looking forward to the next step: the installation of the cryogenic tower.

11.09.2020 DARWIN Collaboration Meeting (Online)

The DARWIN Collaboration Meeting was held from September 9 to 11, 2020. This time the meeting had to be online since given the covid-19 situation we could not travel to LNGS as previously planned. During three days we discussed the status of the R&D projects towards DARWN as well as the sensitivity and background studies which are ongoing. Our group contributed with 6 presentations. Thanks to the effort of all the participants we had a very successful and fruitful virtual meeting; however we hope to meet in person next time.

02.09.2020 DARWIN's sensitivity to the neutrinoless double beta decay published in EPJ-C

The DARWIN Collaboration has recently published the projected sensitivity of the DARWIN observatory to the neutrinoless double beta decay of 136Xe. The analysis, based on a detailed Monte Carlo simulation of the background, shows that the experiment can achieve a half-life sensitivity of 2.4 x 1027 years for an exposure of 5 t (natural xenon) and 10 years.  Our group has significantly contributed to this study, which confirms that DARWIN, the ultimate  dark matter experiment, will be in addition comparable in its science reach to dedicated double beta decay experiments. The details of the analysis can be found in Eur. Phys. J. C 80. 808 (2020).

20.08.2020 Successful Bachelor thesis defence

Our student Livio Redard-Jacot recently completed their Bachelor thesis as part of the group. During this unusual online semester, he successfully completed the simulated electron recoil background spectrum of the DARWIN experiment at the low energies (below 1.5 MeV). With these simulations he estimated the sensitivity of DARWIN to solar axions and axion-like particles, a topic receiving a lot of attention since the last results of the XENON Collaboration. The details of the analysis can be found in his thesis, titled "Sensitivity of the DARWIN experiment to axions and axion-like particled." (PDF, 3933 KB)

02.07.2020 Neutrino 2020 Conference

The XXIX International Conference on Neutrino Physics and Astrophysics (Neutrino 2020) was organized by Fermilab and held online from June 22 to July 2, 2020. It is an important international conference series held biannually in the field of neutrino physics, where progresses on both theory and experiment are presented.

Our group contributed a total of 6 posters on XENON1T/nT, DARWIN, GERDA, and LEGEND-200 experiments:

  1. Analysis of High Energy Events in XENON1T (Chiara Capelli)
  2. The XENONnT direct Dark Matter detection experiment (Giovanni Volta)
  3. Sensitivity of the DARWIN observatory to the neutrinoless double beta decay of 136-Xe (Yanina Biondi)
  4. Solar Neutrino Detection Sensitivity in DARWIN via Electron Scattering (Shayne Reichard)
  5. Energy Calibration for the GERDA Experiment (Junting Huang, Chloe Ransom)
  6. The calibration system of the LEGEND-200 experiment (Yannick Müller)

25.06.2020 Forschungskredit Postdoc Fellowship

We congratulate our group member Dr. Junting Huang for having been awarded the Forschungskredit Postdoc Fellowship 2020 for his project "Lepton and Baryon Number Violation Searches with High Purity Germanium Detectors". Junting is a member of the GERDA and LEGEND collaborations, which operate ultra-pure germanium crystals in a large liquid argon cryostat to search for neutrinoless double beta decay and other rare interactions.

17.06.2020 Observation of Excess Events in XENON1T

Today we announced that data from our XENON1T detector, the world's most sensitive dark matter experiment, show a surprising excess of events. We do not claim to have found dark matter. Instead, we 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, 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. Read the press release issued by the university and faculty. The manuscript detailing the analysis is here: arXiv:2006.09721.

27.05.2020 Results with Xurich II and SiPM arrays published in EPJ-C

We have built the first dual-phase xenon TPC (Xurich II) with a SiPM top array. It provides a good spatial resolution of ~1.5 mm for the horizontal event position reconstruction. This novel detector concept was characterised down to energies of 2.82 keV with internal 37Ar and 83mKr sources. The figure on the right shows the anti-correlation of the obtained charge and light yields at varying drift fields which we also compared with predictions from NEST. Read the full story in Eur. Phys. J. C 80, 477 (2020).

22.04.2020 Dr. Neil McFadden earns Chair's Award for Best Dissertation from University of New Mexico

Postdoc Neil McFadden was awarded the Physics Department Chair's Award for Best Dissertation from the University of New Mexico, where he defended his thesis. His thesis defense, titled "Studying Properties Of Xenon Doped Argon and Developing Optical Simulation Techniques for the LEGEND collaboration, a Neutrinoless Double Beta Decay Experiment ", granted him the honor of graduating with Distinction. 

17.04.2020 Stefan Hochrein honored with the Semester Award

We are very happy to announce that our former bachelor student (now master) Stefan Hochrein was honoured with the Semester Award Autumn-2019 for his outstanding scholarly work. He conducted his bachelor thesis on Kr-83m calibration of the first dual-phase xenon time projection chamber with silicon photomultiplier readout in our group and was supervised by our phd student Kevin Thieme with the help of our postdoc Patricia Sanchez. We wish him all the best for his promising future.

25.03.2020 The XENONnT TPC is installed at LNGS

The installation of the XENONnT Time Projection Chamber (TPC) was successfully completed at the Gran Sasso Underground Laboratory (LNGS). Several PhD students and postdocs in our group participated in the assembly and installation on site. In particular, Adam Brown, Chiara Capelli and Giovanni Volta remained at LNGS and continued to work underground during the difficult time of the Covid-19 pandemic, with a few members from the collaboration. Together, they ensured that the photosensors and the light calibration system are operational, and that the TPC is in a safe mode in vacuum, inside the cryostat. The XENONnT experiment will start the commissioning phase later in 2020, once access to LNGS is restored.

15.03.2020 New group members

Two new  members recently joined the group working on XENON, DARWIN and LEGEND. Alexander Bismark, from Freiburg University is a new PhD student working on XENONnT and the future DARWIN. Neil McFadden, a postdoc who completed his PhD at the University of New Mexico, Albuquerque, is working on the DARWIN demonstrator and on the LEGEND project. We are happy to welcome them in our group!

30.01.2020 PhD awarded to Dr. Michael Miloradovic

Congratulations to Dr. Michael Miloradovic, who very successfully defended his Ph.D. thesis on January 30. His thesis work deals with the characterisation of new, inverted coaxial detectors for GERDA, with a background model for the natural Ge detectors, with pulse shape simulations for the BEGe and inverted coaxial detectors, as well as with the production and characterisation of new, low-neutron emission calibration sources for the last phase of the GERDA project.

30.01.2020: Summer/Fall 2019 Students

We would like to congratulate three students who successfully worked with us in the second half of 2019: Basak Çigdem Özcan worked on the characterisation of SiPMs as potential replacements for PMTs in xenon detectors. She is now finishing her studies in physics at the Bilkent University in Turkey. Nicole Schermer tested new SiPM pre-amplifier boards with low thermal output for use in liquid xenon. She is now continuing her master studies at the Technical University of Munich, in Germany. As part of her masters thesis project, Rachana Yajur worked on light collection efficiency simulations for SiPM rings in the 2.6 m tall DARWIN demonstrator TPC. She also tested a SiPM in a TPC mock-up in order to validate the simulation results. She is completing her masters studies at the Indian Institute of Technology in Madras. We thank all three students for the hard work and wish them much success for the future.

17.12.2019: Light Dark Matter Results Published

XENON1T has now published the world-leading constraints on light dark matter in Physical Review Letters as an Editor's Suggestion. XENON1T’s new letter details an “S2-only analysis”, where events without S1s are still considered. Advances in detector construction and analytic techniques have led to a background levels that is one thousand times lower than what was achieved in previous S2-only searches.

For example, the S2 electron cloud becomes broader as it drifts upward, like a drop of ink that spreads 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.

Theorists predict that dark matter would collide with the heavy xenon nuclei and produce nuclear recoils. In this case, the S2-only technique is sensitive to energies 2-3 times lower than traditional analyses. We consequently find improved constraints on light dark matter. On the other hand, dark matter might collide with electrons that surround the nucleus, producing electronic recoils. These interactions create 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 enhance the constraints on such models by several orders of magnitude.

This letter was also featured in a synopsis from APS Physics.

15.10.2019: New group members on GERDA and LEGEND

Three new members recently joined the group working on GERDA/LEGEND: Yannick Mueller from Karlsruhe Institute of Technology, Gabriela Rodrigues Araujo from the Technical University of Munich, and Junting Huang from the University of Texas at Austin. Yannick and Gabriela are Ph.D. students, and Junting is a postdoc. We are happy to welcome them in our group!

05.09.2019 Probing Majorana neutrinos with double-β decay

The latest results from the GERDA experiment were published in Science this week. GERDA reached the 1.1x1026 y sensitivity to the half-life of the neutrinoless double beta decay of 76-Ge. Combined with other experimental results ussing 76-Ge detectors, the sensitivity to the effective Majorana neutrino mass is 0.07 - 0.16 eV. The leading performance of GERDA in terms of background suppression, energy resolution and sensitivity opens the way to LEGEND, a next-generation Ge experiment with sensitivity to half-lives of 1027 y. A first phase, with 200 kg 76Ge is under preparation at LNGS.:DOI: 10.1126/science.aav8613 and arXiv:1909.02726

01.09.2019 Scientifica 2019: Science Fiction – Science Facts

Under this motto, the University of Zürich and the ETH organised the Zürich science days of this year. Scientifica 2019 took place from Friday, August 30 to Sunday, September 1 and it was a great success. During these three days, more than 30,000 visitors took the opportunity to discuss directly with the researchers about science facts that sound like science fiction today, but which might be commonplace tomorrow. Our group participated actively in the booth called “Dark Matter-Antimatter-Does it matter?” and in the opening ceremony with a talk called "Die dunkle Seite des Universums". Thanks to all the participants (Yanina Biondi, Chiara Capelli, Ricardo Peres, Giovanni Volta, Patricia Sanchez and Laura Baudis) for sharing their enthusiasm for our research with the numerous visitors.

29.08.2019 PhD awarded to Dr. Rizalina Mingazheva

Congratulations to Dr. Rizalina Mingazheva, who very successfully defended her Ph.D. thesis on August 29. Her thesis work deals with the energy calibration of the GERDA experiment, as well as with background studies and new physics searches. In particular, she performed the first search for bosonic super-WIMPs with masses up to 1 MeV with GERDA data. Rizalina will stay with our group for another four months, as a postdoc on GERDA and LEGEND.

22.08.2019 Three successful Bachelor thesis defences

Three students recently completed their Bachelor thesis as part of the group. Stefanie Javet analysed data from a local experiment to search for an annual modulation in the decay rate of a variety of beta-decaying isotopes, placing an upper limit on the size of any such modulation, in her thesis (PDF, 5475 KB) titled "Data processing and analysis of the modulation experiment”. Simon Buse contributed to the ongoing effort searching for neutrinoless double-beta decay (0νββ) in XENON1T, performing the calibration of O(MeV) energy deposits and selecting the volume of XENON1T with the best sensitivity to 0νββ, in his thesis (PDF, 10535 KB) titled “Energy Calibration and Fiducial Volume Selection for the XENON1T Experiment in the Neutrinoless Double Beta Decay Regime”. Stefan Hochrein demonstrated the potential advantages of silicon photomultipliers (SiPMs) for future large dual-phase xenon time projection chambers (TPCs) by performing a 83mKr calibration of the local Xurich detector, a dual-phase TPC recently upgraded with SiPMs, in his thesis (PDF, 11861 KB) titled "Calibration of the first dual-phase xenon time projection chamber with silicon photomultiplier readout”.

22.08.2019 Xenoscope - DARWIN demonstrator frame assembly

In the context of the ERC-funded Xenoscope project, our group is designing and building a full height DARWIN demonstrator. Last week, technicians of FMS-Technik AG together with our PhD students Frédéric and Kevin installed the 4 m × 5 m × 4 m support structure of the demonstrator in the assembly hall of the Physik Institute of the University of Zürich. This first milestone  enables us to start the assembly of the various systems of the detector. 

Watch a time-lapse video of the assembly by clicking here!

04.07.2019 Best poster and Bjorn Wiik diploma for Yanina Biondi

Our DARWIN PhD student Yanina Biondi was selected to participate in the 57th International Erice School of Subnuclear Physics "In search for the unexpected". She presented a poster about "Xenoscope: Towards DARWIN – the Ultimate Dark Matter Detector" and was awarded the diploma for the best poster. She was a very active participant at the school and was also awarded the Bjorn Wiik diploma for new talents in physics. Congratulations Yanina!

27.06.2019 GERDA collaboration meeting in Zurich

From June 24 to June 26 we hosted the GERDA collaboration meeting in the Main Building of the University of Zurich. The first two days were spent to discuss the status of the experiment and analysis work (summary of data taking, the calibration and energy scale, the background model and the pulse shape analysis) as well as exotic searches (e.g., searches for SuperWIMPs, or for a bosonic component of neutrinos). We also had a guest talk from Patricia Sanchez on neutrinoless double beta decay searches with DARWIN. The last day was dedicated to an in-depth analysis discussion and plans for the unblinding of the last GERDA data set in spring 2020. GERDA will end its last run in December 2019, when the works for LEGEND-200 at LNGS will start.  All the participants enjoyed a lovely dinner and boat cruise on Lake Zurich with MS Bachtel in the evening of June 24!

 

25.04.2019 First observation of two-neutrino double electron capture in Xe-124 with XENON1T


Two-neutrino double electron capture (2νECEC) is a second-order weak-interaction process with a predicted half-life that surpasses the age of the Universe by many orders of magnitude. Until now, indications of 2νECEC decays have only been seen for two isotopes, 78Kr and 130Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance. The 2νECEC half-life is an important observable for nuclear structure models and its measurement represents a meaningful step in the search for neutrinoless double electron capture—the detection of which would establish the Majorana nature of the neutrino and would give access to the absolute neutrino mass.

The XENON collaboration has detected the first direct observation of 2νECEC in 124Xe with the XENON1T dark-matter detector. The significance of the signal is 4.4 standard deviations and the corresponding half-life of 1.8 × 1022 years (statistical uncertainty, 0.5 × 1022 years; systematic uncertainty, 0.1 × 1022 years) is the longest measured directly so far. This study demonstrates that the low background and large target mass of xenon-based dark-matter detectors make them well suited for measuring rare processes and highlights the broad physics reach of larger next-generation experiments.

The results are now published as a Nature Letter and can be found here: Nature Volume 568 Issue 7753, 25 April 2019 (arxiv link coming soon).

 

 

18.03.2019 XENON1T Detector Disassembly


The XENON1T team has recently disassembled the Time Projection Chamber (TPC) at the Laboratori Nazionali del Gran Sasso to prepare for the installation of the new XENONnT detector. Shown at the right is a picture taken during the photomultiplier tube (PMT) array disassembly. After removal of the bottom array of PMTs, one can see the electrodes, reflectors, and top array of PMTs inside the detector. The PMTs will be safely stored until installation in XENONnT later this year.

09.03.2019 2019 Science Info Day

The 2019 Science Info Day at the University of Zurich took place on 9 March at Irchel Campus. The Science Info Day is the Faculty of Science's orientation event for prospective students. During this day, high school students, their parents, and teachers could learn about the degree programs offered by the Faculty of Science. The program included guided tours, short presentations and experiment demonstrations. This year, two members of our group, Yanina Biondi and Patricia Sanchez, showed our labs and explained the R&D activities which we are working on.

19.12.2018: Chris Marentini honored with Dectris-Award

Congratulations to our former master student Chris Marentini, who was honored with the Dectris-Award for the best Master Thesis in experimental physics. His Master Thesis was conducted within our group on "Characterization of Novel VUV-Silicon Photomultipliers and their Application in Xenon-Based Dual-Phase TPCs" (PDF, 10730 KB), which was supervised by Dr. Julien Wulf. We wish him all the best in his future endeavors.

12.12.2018: Darwin Collaboration Meeting in Zurich

We are pleased to announce that the meeting of the DARWIN Collaboration will take place in Zürich from Monday, 17th December 2018, to Tuesday, 18th December 2018, and it is organised by the University of Zurich. The meeting programme consists of  plenary talks and discussions about R&D towards DARWIN, design considerations, MC simulations, sensitivity studies and science channels. All information related to the meeting and the final programme can be found in the indico website.

22.11.2018: Contributions to the II South American Dark Matter Workshop

The II South American Dark Matter Workshop was held in São Paulo last week, organised by the South American Institute for Fundamental Research (ICTP-SAIFR). Scientists from South America and all over the world met to discuss the latest and most important results in all Dark Matter search branches: theoretical, collider, direct and indirect, and astrophysical. Our group gave two talks: "DARWIN: the ultimate dark matter detector" (Yanina Biondi) and "Status and Results from the XENON1T Dark Matter Experiment" (Adam Brown).

24.10.2018: Characterisation of Silicon Photomultipliers for Liquid Xenon Detectors

Silicon Photomultipliers (SiPMs) are a promising solid-state alternative to the widely used Photomultiplier Tubes (PMTs) in rare-event searches with liquid xenon as target material. We have characterised a vacuum ultraviolet (VUV) sensitive SiPM down to temperatures of 110K and in liquid xenon at 185K. Furthermore, we studied the radioactivity of the raw SiPM materials. The detailed description of our cryogenic setup and the conducted SiPM measurements are published in JINST 13 (2018) no.10, P10022.

20.09.2018: Ph.D. awarded to Dr. Julien Wulf

Congratulations to Dr. Julien Wulf, who very successfully defended his Ph.D. thesis on September 20th. Julien will stay with our group for another six months (starting October 1, 2018) as a postdoc on XENON/DARWIN.

13.09.2018: Contributions to SPS meeting and TeVPA conference

During the last week of August, the annual meeting of the Swiss Physical Society (SPS 2018) and the TeV Particle Astrophysics conference (TeVPA 2018) took place in Lausanne and Berlin respectively. Our group was represented at both events. The current status of the projects XENON1T, GERDA and DARWIN was presented in 5 talks at the SPS meeting. Likewise, two contributions to TeVPA 2018 showed the latest results from GERDA and XENON1T.

01.08.2018: Roman Hiller is awarded a postdoctoral Forschungskredit fellowship

Dr. Roman Hiller, currently searching for the neutrinoless double beta decay with the GERDA experiment in our group, was awarded a Forschungskredit fellowship in 2018. The Forschungskredit award supports young scientists up to postdoctoral level to carry out a research project at the University of Zurich. Roman Hiller received the grant to continue his neutrino physics research in our group for one year starting in August 2018.

18.06.2018: Contribution to Neutrino 2018

The XXVIII International Conference on Neutrino Physics and Astrophysics, Neutrino 2018was held in Heidelberg last week, organised by MPIK and KIT. Scientists from all over the world met to discuss the last and most important results in the field. Our group showed three posters: Neutrino physics with DARWIN (Patricia Sanchez), Double beta decay with XENON1T (Chiara Capelli) and Calibration with GERDA (Rizalina Mingazheva).

11.06.2018: GERDA sets a new sensitivity record for the neutrinoless double beta decay

One of the highlights of this year's Neutrino conference were new results from the GERDA experiment: with a large data release, GERDA has doubled its exposure, for a total of 82.4 kg years. Improvements in background suppression techniques have brought down the background expectation to 0.1 events per exposure in the energy region of interest of the decay. GERDA is now the most sensitive experiment in the field, with a  half-life sensitivity of 1026 yr for the neutrinoless double beta decay of 76Ge.

No signal was detected yet, and an upper limit on the half-life could be set: a 76Ge atom takes on average longer than ten quadrillion times the age of the universe to decay via this rare process which is not allowed in the Standard Model of particle physics and would violate lepton number conservation by ΔL = 2.

 

30.05.2018: XENON1T results from 1 tonne × year search for dark matter

XENON1T reported results on a search for Weakly Interacting Massive Particles (WIMPs) on Monday 28th May, based on 278.8 days of data. Given the 1.3 t fiducial volume this makes it the first experiment to reach the tonne × year exposure regime, while the electronic recoil background rate of ~ 82 events / (t × yr × keV) is the lowest ever achieved by a dark matter search experiment. These achievements allowed us to reach a sensitivity to WIMPs seven times better than previous experiments (PandaX-II, LUX).

The data are consistent with the background expectation and allow us to place the strongest limit on the cross-section for spin-independent WIMP-nucleon scattering, with a minimum of 4.1 × 10−47 cm2 at 30 GeV / c2. These results have been submitted to Physical Review Letters.

15.03.2018: New lower limit on the half-life of the neutrinoless double beta decay

With the GERDA experiment we search for the lepton number violating neutrinoless double beta decay of 76-Ge with the GERDA experiment. By operating bare Ge diodes in liquid argon, we have achieved an unprecedented low background level for germanium detectors  of 1 x 103   events/(keV kg yr) in the search region. We have increased the exposure for BEGe-type detectors threefold with respect to our previous data release [Nature 544, 2017], and  in the absence of a signal, we set a new lower limit on the half-life of 8.0 · 1025 yr. The manuscript detailing our new results is published in Phys. Rev. Lett. 120, 132503 (2018).

22.01.2018: Search for bosonic super-WIMP interactions with the XENON100 experiment

We searched for vector and pseudo-scalar bosonic super-WIMPs with the XENON100 experiment. The super-WIMPs can be absorbed in liquid xenon and the expected signature is a monoenergetic peak at the super-WIMP’s rest mass. A profile likelihood analysis of data with an exposure of 224.6 live days × 34kg showed no evidence for a signal above the expected background. We thus obtained new upper limits in the (8 − 125) keV/c2 mass range, excluding couplings to electrons with coupling constants of g ae > 3 × 10−13 for pseudo-scalar super-WIMPs and α′/α > 2 × 10−28 for vector super-WIMPs, respectively. We expect to improve upon these results with the XENON1T detector, which operates a larger mass of liquid xenon with reduced backgrounds. Our results were published in Physical Review D 96, 122002 (2017).

18.12.2017: The XENON1T Instrument Explained

The XENON1T experiment at the Laboratori Nazionali del Gran Sasso is the first WIMP dark matter detector operating with a liquid xenon target mass above the ton-scale. Out of its 3.2 t liquid xenon inventory, 2.0 t constitute the active target of the dual-phase time projection chamber. The scintillation and ionization signals from particle interactions are detected with low-background photomultipliers. The detailed description of the XENON1T instrument and its subsystems was published in EPJ-C 77 (2017).

30.10.2017: First XENON1T Results

We have analysed the data from a 34.2 live days run of the XENON1T experiment at LNGS, acquired between November 2016 and January 2017. A profile likelihood analysis showed that the data inside a 1042 kg fiducial mass and an energy region 5-40 keV for nuclear recoils is consistent with the background-only hypothesis. It allowed us to set he most stringent exclusion limits on the spin-independent WIMP-nucleon interaction cross section with a minimum of 7.7 x 10-47 cm2 for 35 GeV/c2 WIMPs at 90% C.L. The results, published in Phys. Rev. Lett. 119, 181301 (2017), also yielded the lowest electronic recoil background ever achieved in a dark matter experiment, of about 2 x 10-4 events/(kg day keV).

25.07.2017: Results on inelastic WIMP scatters with xenon nuclei

We have published the first constraints on the spin-dependent, inelastic cross section of WIMPs with 129Xe nuclei using an exposure of 7.64 x 103 kg days of XENON100 data. The experimental signature is a nuclear recoil observed together with the prompt deexcitation photon around 40 keV. As we saw no evidence for this interaction, a profile likelihood analysis allowed us to set a 90% CL upper limit of 3.3 x 10-38 cm2 for a WIMP mass of 100 GeV/c2. This is the most constraining result to date, and sets the pathway for an analysis of this interaction channel in upcoming, larger dual-phase xenon detectors, such as XENON1T. The results can be found in Phys. Rev. D 98, 022008 (2017)

05.04.2017: First GERDA results published in Nature

Since December 2015, the GERDA experiment restarted its search for lepton number violation via the neutrinoless double beta decay of germanium-76. The first 6 months of data were released in June 2016. With the extended detector mass of 36 kg and an unprecedented low background of 0.001 cts/keV kg yr, the highest sensitivity ever reached with germanium detectors was already equaled (from Phase I of the experiment, 2011-2013). This was achieved with a new background suppression technique, by using the liquid argon cryostat of the detector as a scintillator to veto background events. The new limit on the life time of the decay, as well as the success of the new technique and the record background were reported in April 2017's issue of Nature 544, 3839.