News and Events
A boon for physicists: new MicroBooNE results at Neutrino 2018
The MicroBooNE collaboration has produced their first collection of science results and presented them at the Neutrino 2018 conference in Heidelberg (D), attended by 800 physicists. MicroBooNE started operations in the fall of 2015. The detector, about the size of a school bus, is filled with 170 tons of liquid argon and has recorded interactions of hundreds of thousands of neutrinos, produced by an accelerator complex at Fermilab near Chicago (USA). It features a time projection chamber (TPC) that record the particle tracks created by the neutrino colliding an argon nucleus, similar to a 3D digital camera recording images of fireworks.Scientists at LHEP and AEC of the University of Bern are experts in the technology of liquid argon TPC and are playing a leading role in the experiment, including the physics measurements. Important systems to calibrate the detector with a UV-laser and to record signals from cosmic rays that also hit the detector were designed and built in Bern and are operated now in the experiment in the USA.
MicroBooNE is the first low-energy neutrino experiment to make detailed observations of the subatomic processes that happen when a muon neutrino hits and interacts with an argon nucleus, leading to showers of secondary particles including protons, pions, muons and more. The new results reported at the Neutrino 2018 conference include first measurements of the multiplicity – or number of particles – generated in these collisions, along with absolute yields per incident neutrino of collisions producing a neutral pion or a more inclusive final state.
The measurements are of great importance for the groundbreaking measurements to be performed in the search for a fourth neutrino type and in preparation for the Deep Underground Neutrino Experiment (DUNE) in which the University of Bern will also play a central role, together with other Swiss Universities. DUNE will be operational in the 2020ies and explore the role of neutrinos in the evolution of the universe.
OPERA collaboration presents its final result
On May 22nd 2018, the OPERA collaboration presented its final result on the muon neutrino to tau neutrino oscillation experiment. LHEP has been one of the leading groups in the OPERA experiment. Our journey started at the end of the last century. We took data in the CNGS (CERN to Gran Sasso) neutrino beam in 2008 - 2012, and the first tau neutrino event was reported in 2010. Followed by four more tau neutrino events, we announced the discovery of tau neutrino appearance in 2015. The result of OPERA gave a significant contribution to the 2015 Nobel Physics Prize for the discovery of neutrino oscillations. In this final result published in Physical Review Letters, the OPERA collaboration applied a new analysis strategy proposed by LHEP. A total of 10 tau neutrino events, twice more statistics than the previous result, allowed us to provide better constraints of the oscillation parameters in appearance mode. All data of these 10 events are now publicly available on the CERN open data portal. Beyond the contribution to neutrino physics, the particle detection and microscope technologies of OPERA are being applied to a wider field of research, such as the muon radiography of the Swiss glaciers (Geoscience) and intravital microscopy to study immune cells (Immunology).
PixLAr beam test completed
The collaboration between LHEP/AEC and Fermilab (USA) led to another new result: a novel pixelated charge readout system, an essential part of ARGONCUBE project, was successfully tested within the PixLAr experiment at beam test facility at Fermilab. Several millions of particle tracks were detected and reconstructed in a small-scale liquid argon detector. The PixLAr experiment is an evolution of the LArIAT experiment, improved, apart of pixel charge readout, with the novel photon detector designed at LHEP - ArCLight. Both systems have demonstrated excellent performance. A detailed data analysis is ongoing.
A Study in Tau: DsTau-Project
The DsTau project is a newly launched project, aiming to study tau neutrino production in proton interactions. This study is prerequisite for future tau neutrino experiments, e.g. the SHiP experiment. Recently, the international DsTau collaboration (consisting of members from five countries) has submitted a proposal to the CERN SPS and PS Experiments Committee (SPSC). In January 2018, SPSC recommended a pilot run this year and a subsequent DsTau physics run in 2021 after the long shutdown of the CERN accelerator complex. DsTau is led by Dr. Akitaka Ariga from AEC/LHEP at the University of Bern. For more information, please visit the DsTau project web site: http://dstau.lhep.unibe.ch
ARGONCUBE module becomes reality
In January 2018, the assembly of the very first ARGONCUBE module (so called Mod-0) started at LHEP. The first one of four modules will serve as a test bed for several novelties of the ARGONCUBE concept - modular scalable large-mass Liquid Argon Time Projection Chamber of the future. The module will incorporate a small argon purity monitor and a light readout system. First wet tests at cryogenic temperatures are expected in May 2018.
Results on ArCLight sensor published
The ArCLight photon counting device combines advantages of two well-known photon detectors: vacuum photomultiplier tubes (PMT) and silicon photo-multipliers (SiPM). Large sensitive area of the one, merged with robustness and compactness of the other. This results in a very compact, flat, almost fully-dielectric photon readout device, perfectly suited for use in high electric field environment, such as in Liquid Argon TPCs. First results on characterization of the device have been published: http://www.mdpi.com/2410-390X/2/1/3/
Axion Dark Matter Search
The nEDM collaboration has published new results on a laboratory-based search for axion dark matter in the journal Physical Review X. It provides valuable constraints for the properties that these hypothetical particles can have — and thus a guide to where to look next. LHEP is a member of the international collaboration which performs its high-precision experiment at the ultracold neutron source at the Paul Scherrer Insititute.
Particle Physics at "Nacht der Forschung"
As every three years, the night was lit up by science and dance, mixed in an interactive experience. For one night the visitors of the University of Bern could explore the wonders of research of the infinitely small. From seeing particle detectors in action to discussions with theorists as well as playing with interactive simulations, "treating cancer" patients with hadron therapy, collide protons by kicking them, exploring detectors in virtual reality, or enjoying an ice cream made with liquid nitrogen. Anyone visiting and presenting had a lot of fun learning and sharing. We cordially thank everybody that came to this year's "Nacht der Forschung".
Link to the official video of "Nacht der Forschung".
Successful Neutron Beam Time at PSI
The members of the Fundamental Neutron Physics Group of LHEP have just completed their first beam time of four weeks at the neutron spallation source SINQ at the Paul Scherrer Institute. First experiments were performed using a high-precision neutron Ramsey prototype apparatus. The setup is ultimately intended for a novel neutron electric dipole moment search using a neutron beam. This first results will provide valuable input for the future development of the setup. Another beam time is scheduled for January/February 2018 - this time at the European research facility Institute Laue-Langevin in Grenoble (France).
First SBND neutrino data with Cosmic Ray Tagger from Bern
On the morning of June 22, a first subdetector of the Short-Baseline Near Detector (SBND) began taking data at Fermilab (USA, Chicago). The subdetector called Cosmic Ray Tagger (CRT) is designed and constructed at LHEP. The CRT is composed of many finely grained modules capable of measuring a particle interaction instance with the accuracy of a nanosecond in time, and a centimeter in space. With the newly operational Cosmic Ray Tagger in its current configuration (beam telescope), SBND is currently characterizing the flux of particles called muons. These muons are produced by neutrinos from the Booster Neutrino Beamline and they carry information about parameters of the parent neutrino beam at SBND pit. Data taken during this characterization stage will be a great asset on the way to simulation and analysis for the whole SBND detector later on.
Muon radiography of Alpine glaciers
The interdisciplinary project "Eiger-mu GT (Eiger muon glacier tomography)" has published a new paper recently. It was highlighted by Geophysical Research Letters, one of the most prestigious journals in geophysics. The paper reports the results from the first feasibility test of muon radiography at Alpine glaciers. The researchers installed small detectors, made of emulsion films, at several locations inside the Jungfrau railway. After analyzing the films with the high-resolution microscopes in LHEP, they succeeded in resolving the interface between glacial ice and granite rocks in the very uppermost part of the Aletsch glacier, the largest glacier in the Central Swiss Alps. The project is a collaboration between the Laboratory for High-Energy Physics (LHEP) and the Institute of Geological Sciences (GEO), University of Bern, supported by Swiss National Science Foundation.
Bern cyclotron laboratory is promoting science in particle accelerator applications
Saverio Braccini and Paola Scampoli from AEC-LHEP edited a special issue of Modern Physics Letters A (MPLA) dedicated to cyclotrons and their applications (http://www.worldscientific.com/toc/mpla/32/17). The idea originated from the 12th workshop of the European Cyclotron Network (CYCLEUR 2016) held in Bern on 23-24 June 2016 together with the 2nd Bern Cyclotron Symposium. Experts form the main cyclotron facilities in Europe came together to discuss about current cutting-edge results and future prospects on a broad spectrum of scientific topics. In the last years, an increasing number of medical cyclotrons allowed for routine production of isotopes for diagnosis and therapy as well as for beams for cancer hadron-therapy. At the same time, research was conducted to foster advances in several directions by means of dedicated infrastructures. The book collects specific contributions giving a picture of the rich scientific research programs based on cyclotrons.
Ultracold Neutron Workshop in Bern
For three days Bern became the capital of ultracold neutron physics. More than 40 international scientists from Belgium, France, Germany, Poland, the United Kingdom, the United States, and Switzerland participated in a three days workshop at the University of Bern. The workshop, which was hosted by LHEP and the Albert Einstein Center, focused on the low-energy high-precision Neutron Electric Dipole Moment (nEDM) experiment carried out at the Paul Scherrer Institute (Switzerland).
The search for a nEDM is currently considered to be one of the flagship experiments in fundamental physics at low energy and presents a route for finding new physics beyond the standard model of particle physics. A permanent nEDM violates discretes symmetries, e.g. parity and time-reversal symmetry. Such new sources of symmetry violation can be directly related to the observed matter-antimatter asymmetry of our universe.
ERC Starting Grant
Florian Piegsa from LHEP has been awarded with a prestigious ERC Starting Grant (ERC). Over the next five years, the grant will allow him to further establish his reseach projects at the University of Bern and to push forward a new experimental appraoch to search for a neutron electric dipole moment. The corresponding experiments will be carried out in Bern, at the Paul Scherrer Institute (PSI) and international neutron research facilities. These activities will foster the already existing strong links and collaborations between PSI and the University of Bern.
The new LHEP-wepage is online since April 2017.
New research group in LHEP
Florian Piegsa, previously working in the Institute for Particle Physics at ETH Zurich, has joined LHEP on a SNSF-professorship position in October 2016. His new research group is investigating the fundamental properties of the neutron in precision low-energy particle physics experiments. The research activities of the group are summarized here.
T2K presents first results on CP violation
The question why the Universe is matter dominated, instead of being made of equal parts matter and antimatter, is still unsolved as of today. One of the conditions required to develop the observed dominance of matter over antimatter is the violation of the Charge-Parity (CP) symmetry. This says that the laws of physics should be the same if viewed upside-down in a mirror (P), with all matter exchanged by antimatter (C). If CP violation occurs in neutrino physics, it will manifest itself as a difference in the oscillation probabilities of neutrinos and antineutrinos. The international T2K Collaboration recently observed that the electron antineutrino appearance rate is lower than expected from the electron neutrino appearance rate, assuming that CP symmetry is conserved. The image shows an anti-electron neutrino.
The new result was announced at the 38th International Conference on High Energy Physics in Chicago. With nearly twice as much antineutrino data than before, T2K continues to see the trends observed in 2015: a preference for maximal disappearance of muon neutrinos and a discrepancy between the electron neutrino and electron antineutrino appearance rates. When analyzed in a three-neutrino framework, and combined with measurements of electron antineutrino disappearance from reactor experiments, the T2K data favor maximal CP violation (δCP=–0.5π). The CP conserving values (δCP=0 and δCP=π) are outside of the 90% confidence level interval. This result is based on a total data set of 1.51x10e21 protons on target, which is 19% of the planned exposure.
In the T2K experiment, a muon neutrino beam is produced at Japan's east coast and sento to the gigantic Super-Kamiokande underground detector, 295 kilometers away. The T2K Bern group is deeply involved in the near detecor (ND280) data analysis with the aims of constraining and improving the neutrino cross section parameters used as inputs to the T2K oscillation analyses. It also measures neutrino cross sections, which are essential for current and future neutrino oscillation experiments.
European cyclotron experts meet in Bern
The 12th workshop of the European Cyclotron Network (CYCLEUR 2016) was held in Bern on 23-24 June 2016, organized by the Albert Einstein Center for Fundamental Physics (AEC) and supported by the swissHADRON foundation. It reassembled cyclotron experts from about 40 cyclotron laboratories in Europe, Canada, Korea and Tunisia. It was followed by the 2nd Bern Cyclotron Symposium, where specific topics were presented by invited speakers. With about 25 talks, this event gave an updated overview on scientific activities at cyclotron laboratories as well as in industry.
The main highlights are reported in the following. Accelerator physics developments for radioisotope production and proton therapy are focused on compact and effective solutions for medical applications. Novel beam monitoring detectors are instrumental for optimal production of non-standard radioisotopes for medicine. In this domain, increasing interest is shown on theranostics, which means the use of isotopes of the same element for diagnostics and therapy. In particular, scandium and gallium are proposed for PET and scandium and astatine for metabolic therapy. A recent field of application is the use of radioactive nanoparticles. Radiation protection plays a crucial role and traces of specific radioisotopes can be used to detect the artificial production of radioactivity as in the case of nuclear explosions.
The participants had the opportunity to visit the Bern cyclotron laboratory, where industrial GMP PET radioisotope production is performed together with multi-disciplinary research activities.
The science opportunities of DARWIN
The DARWIN collaboration has recently published a detailed article on the multi-ton dark matter observatory DARWIN, its science channels, its background and on the R&D towards its realization. DARWIN's main goal is to explore all experimentally accessible parameter space in the search for weakly interacting massive particles (WIMPs), a prime dark matter candidate. The study, signed by 119 authors and with key contributions from the Bern DARWIN group can be found here: arXiv:1606.07001.
With a design target mass of 40 tons of liquid xenon, DARWIN will be able to search for
- WIMP dark matter in the spin-independent, spin-dependent and inelastic channels,
- axions and axion-like particles via the axio-electric effect,
- low energy solar neutrinos (pp-neutrinos, 7Be neutrinos),
- coherent neutrino-nucleus scattering,
- neutrinos from supernova explosions,
- neutrinoless double-beta decay of 136Xe,
- and other rare nuclear processes.
The Figure from the publication shows the sensitivity of DARWIN to the effective Majorana neutrino mass via a search for the neutrinoless double-beta decay of 136Xe. Two different exposures (30 t x y and 150 t x y) at two different background levels are shown. The 'ultimate' case assumes that background from the detector materials can be removed completely, thus the remaining backgrounds are from 222Rn in the Xe target, 8B solar neutrinos and the two-neutrino double beta decay.
LHC and ATLAS back into operation
On March 25, 2016, the most powerful collider in the world, the Large Hadron Collider (LHC) at CERN, has resumed operation after its annual winter break, with a center of mass energy of 13 TeV. The Laboratory of High Energy Physics and the Albert Einstein Center at Bern (LHEP/AEC) play a key role in ATLAS, one of the four large experiments. The accelerator complex and the experiments have been turned on and tested over the last weeks and detectors have now started the data taking. The Figure below shows one of the the first collision events with stable beams, recorded on April 23, 2016. The LHC operators will increase the intensity of the beams gradually until the maximal rate of collisions is reached.
During the winter break the detectors were further improved. The ATLAS experiment went through an optimization of the track recording detectors (silicon pixel detector), which are located closest to the collision points right in the center of ATLAS. The Bern ATLAS group has important responsibilities for this detector system. In fact, it leads the largest upgrade performed during the shutdown, which concerned the installation of new detector readout components to double the readout speed in order to overcome bandwidth saturation. New optical readout components were specifically developed and built in Bern, together with new software which was integrated in the final readout system. The picture below shows test of the optical plugin in the laboratory in Bern.
Science with a medical PET cyclotron
Beyond routine radioisotope production for medical purposes, compact medical cyclotrons can be at the heart of multidisciplinary research facilities. The cyclotron laboratory in Bern is a prime example, as described by the AEC-LHEP scientists Saverio Braccini and Paola Scampoli in a recent article published in the April issue of the CERN Courier.
Medical PET cyclotrons are usually employed by hospitals and radiopharmaceutical industries for the routine production of radioisotopes. To match the patient's examination schedule, they run during the night or early in the morning, while their beams are not used during daytime and could in principle be used for other projects. This represents an opportunity to exploit the science potential of these accelerators well beyond Positron Emission Tomography (PET) applications. To perform multidisciplinary research, beams of variable shape and intensity must be available together with the possibility of accessing the beam area. For this purpose, the Bern facility is equipped with a transport line leading the beam to an experimental area, which is always accessible for scientific activities.
Thanks to this solution, the AEC-LHEP medical application group is conducting scientific activities in several research fields, such as as nuclear and detector physics, material science, radiation hardness, and radiation protection. The Bern facility daily serves the local University Hospital (Inselspital) and other Swiss healthcare centers with FDG, the most common PET radiotracer, and actively searches for alternative medical radioisotopes. In particular, scandium-43 has been proposed as novel radioisotope, having nearly ideal nuclear decay properties for PET.
Eiger-µ GT launched
The new interdisciplinary project "Eiger-µ GT (Eiger muon glacier tomography)" has been launched recently. It is a collaboration between the Laboratory for High-Energy Physics (LHEP) and the Institute of Geological Sciences (GEO), University of Bern, aiming to "see" inside glaciers of the Swiss Alps using cosmic-ray muons. Thesse are are most abundant charged particles in cosmic rays and can penetrate several kilometers of rock. The project will rely on this high penetration power to investigate the thickness of the glacier in way similar to medical X-ray radiographies in hospitals.
The first target is the Eiger glacier, which straddles at the western flank of the famous Eiger mountain. Several small detectors, made of higly sensitive emulsion films with a micrometer resolution, were installed at several locations inside the Jungfrau railway tunnel in December 2015. The detectors will sit in the tunnel until the end of March 2016, when they will be recovered and read out using the scanning microscopes at LHEP Bern. The reconstruction of the arrival direction of the muons, using their tracks in the emulsion films, will allow the reconstruction of the material between the detector and the mountain surface, which is important to answer several geological questions.
XENON1T gears up to search for dark matter
After several years of design, R&D and construction work, the new XENON1T experiment is now close to completion. The instrument, which uses about 3.5 tons of cryogenic liquid xenon as detector material to search for galactic dark matter, was recently inaugurated at the Italian Gran Sasso laboratory, where it is protected from cosmic rays by 1400 m of rock. The astroparticle physics group of AEC/LHEP Bern is a key member in this project. It is responsible for core components such as the design, construction and assembly of the central time projection chamber and its electronic readout. The video summarizes more than 2 years of construction effort by more than 120 scientists from 21 insitutions in about 5 minutes.
Beam currents in the pA-range obtained at the Bern medical cyclotron
Medical cyclotrons for the production of radioisotopes are designed to operate with beam currents of the order of 100 microampere (µA). These particle accelerators have a large potential for multi-disciplinary research provided that access to the beam area is possible and currents orders of magnitude lower are achievable. To obtain stable proton beams down to the pA range, the AEC-LHEP medical applications group developed a method based on ion source, radio-frequency and magnetic field tuning. The results were published recently in Measurement Science and Technology.
The 18 MeV cyclotron at the Bern University Hospital (Inselspital) is used every night for the production of radioisotopes for Positron Emission Tomography (PET) while, during the day, the proton beam is available for scientific activities. Researchers can access the irradiation area by means of a second bunker, where a transport line provides beams of variable shape and intensity, a peculiar feature for a hospital-based facility. While currents above 10 µA are standard for this kind of accelerator, its operation at lower intensities is challenging, especially if high stability is required for specific experimental activities. By operating the ion source at the minimum of 1 mA and by tuning the radio-frequency peak voltage together with the magnetic field produced by the main coil, stable beams down to 1.5 pA were obtained. A further decrease of intensity can be obtained by means of collimators. The importance of this method relies on the fact that it opens the way to the exploitation of radioisotope production medical cyclotrons in fields such as novel detector physics, material science, dosimetry and radiation biophysics.
The Nobel Prize in Physics for Neutrino Research
The Nobel Prize in Physics for the year 2015 has been jointly awarded to Takaaki Kajita and Arthur B. McDonald for the discovery of neutrino oscillations. In 1998, Kajita and collaborators discovered with the Super Kamiokande detector that the flux of atmospheric muon neutrinos observed on Earth depends on the energy and travel distance of neutrinos, as expected if neutrinos do oscillate. Later on, McDonald together with his colleagues of the SNO collaboration, reported the evidence for flavour conversion of solar neutrinos.
The two experimental results were sensational. However, this was not end of the story, since independent measurements with "artificially created neutrinos" (e.g. from accelerators) were needed to firmly assess these ground-breaking results. In this spirit, the OPERA experiment, originally proposed in 1997 by Ereditato - now director of LHEP Bern -, Niwa and Strolin, was designed and built to measure for the first time the same oscillation channel of atmospheric neutrinos in Super Kamiokande, but in appearance mode, namely detecting the event-by-event appearance of tau neutrinos emerging via oscillations from an initially pure muon neutrino beam.
OPERA successfully reported the first tau neutrino event in 2010 and finally reached a five-sigma statistical significance (required to claim the discovery of tau appearance) in spring 2015. The article was recently published in Physics Review Letters. In addition, the T2K collaboration, in which LHEP researchers are involved as well, started data taking in 2009 and reported the appearance of electron neutrinos in a muon neutrino beam in 2013.
Both OPERA and T2K provided the the "final" strong support to the oscillation hypothesis, as recognized by the Nobel Committee:
"Super-Kamiokande’s oscillation results were later confirmed by the detectors MACRO and Soudan, the long-baseline accelerator experiments K2K, MINOS and T2K and more recently also by the large neutrino telescopes ANTARES and IceCube. Appearance of tau-neutrinos in a muon-neutrino beam has been demonstrated on an event-by-event basis by the OPERA experiment in Gran Sasso, with a neutrino beam from CERN."
All LHEP researchers congratulate Takaaki Kajita and Arthur B. McDonald for receiving this years Nobel Prize in Physics.
MicroBooNE sees first events
The MicroBooNE experiment at Fermilab consists of a 170 ton liquid argon time projection chamber (TPC), installed along the short baseline Booster neutrino meanline. The experiment will measure low energy neutrino cross sections and investigate the low energy excess events observed by other experiment, which might be explained by sterile neutrinos. The TPC technology allows for the precise measurement of the tracks of charged particles, a crucial feature for particle identification and energy measurements. After months of commissioning work in the initial phase of the experiment, MicroBoone recently observed its first cosmic ray and UV-laser generated events. This is reported in the press release issue by Fermilab.
The Bern MicroBoone group is responsible for the UV-laser calibration of the detector, for which one event is shown above. The laser track is the one ending with the "red blob" at the TPC cathode. This calibration is crucial as space-charges modify the local electric fields in the TPC, and the straight laser tracks are used to correct for this effect. The MicroBoone experiment will start "hunting" for sterile neutrinos in the near future.
ARGONCUBE is gaining momentum
The ARGONCUBE project is the follow-up of the successful ARGONTUBE R&D program at LHEP Bern, which demonstrated for the first time that charges can be drifted over the world-record length of 5 m in liqid argon. The new concept of the ARGONCUBE liquid argon time projection chamber (TPC) for future neutrino experiments, based on many identical "cubic" modules immersed into a big liquid argon volume, was suggested by the researchers from LHEP Bern. It has attracted the interest of various groups from Portugal, Switzerland, Turkey, UK and USA, who came to Bern on August 27th, 2015 for the first meeting of the Collaboration. A Letter of Intent has been sumbitted to the CERN SPSC, which encouraged the Collaboration to conduct the first phase of the research at LHEP Bern.