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Laboratory for High Energy Physics


News at LHEP

09.08.2016 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 1.51x1021 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.

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07.07.2016 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.

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23.06.2016 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.

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11.05.2016 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.

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19.04.2016 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.

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02.02.2016 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.

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23.11.2015 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.

The XENON1T Story from XENON1T travel log on Vimeo.

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09.11.2015 Breakthrough Prize in Fundamental Physics to Bern neutrino researchers

Thirteen neutrino physicists from Bern, all members of the T2K experiment, have been awarded the prestigiuos 2016 Breakthrough Prize in Fundamental Physics. The prize is shared among five experiments investigating neutrino oscillations: Daya Bay (China), KamLAND (Japan), K2K/T2K (Japan), Sudbury Neutrino Observatory (Canada) and Super-Kamiokande (Japan).

The Laudatio for the T2K Collaboration reads: "...for the observation of electron neutrino appearance in the muon neutrino beam, which is the first observation of the appearance of a neutrino flavour. This discovery sets the stage for the study of differences in the neutrino oscillation process relative to their antiparticles (antineutrinos), called CP violation, that may elucidate how the universe came to be matter dominated. The total price sum is 3 million US dollars, to be shared among all scientists of all of the honored experiments.

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31.10.2015 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.

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09.10.2015 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.

The OPERA detector at LNGS.

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.

13.09.2015 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.

31.08.2015 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.

29.07.2015 XENON100 challenges long-standing DAMA claim

The event rate in a dark matter detector is expected to show an annually modulating signature due to the movement of the Earth around the Sun. This movement increases or decreases the mean velocity of the dark matter particles hitting the detector, depending on whether the Earth's velocity is added or subtracted to the one of the Sun, leading to a change of the shape of the recoil spectrum. Such as signature has been detected by the DAMA/LIBRA collaboration over the course of several years with a very high significance of >9σ. However, the claim that it is caused by dark matter interactions is in conflict with the non-observation of dark matter signals in many other detectors, which typically feature a considerably lower background and better systematics. The possibility that the DAMA/LIBRA signal could be caused not by nuclear recoil signals, as generally expected from the neutral dark matter particles, but by electronic recoils has now been also challenged by two new articles by the XENON100 collaboration.

The XENON100 data is much lower than the DAMA spectrum in different leptophilic models.

The XENON100 detector is a low-background dual-phase time projection chamber (TPC), filled with cryogenic liquid xenon as target and detector material. It is installed underground in the Italian Gran Sasso laboratory (LNGS). In the first article, published in Science, the collaboration has examined the average electronic recoil background rate of XENON100, which stems from radiactive beta- and gamma-decays in the detector itself. No excess of signals above the expected background is observed which allows placing stringent limits on a variety of leptophilic dark matter models which would lead to electronic recoils. In particular, three models capable to explain DAMA/LIBRA are excluded with significances between 3.6σ and 4.6σ.

For the first time a dual-phase TPC was operated stably for more than one year, which makes the direct search for an annually modulating signal in the electronic recoil data of XENON100 possible. The result of this study is published in the second article, which is accepted by Physical Review Letters: no globally significant modulation signal for periods up to 500 days is found, and the DAMA/LIBRA signal interpreted as being due to dark matter interactions with a period of 1 year is disfavored at 4.8σ.

Members of the astroparticle physics group of the LHEP Bern were involved in the construction of the XENON100 detector, its operation and data analysis. While XENON100 is still running, the collaboration focus has shifted towards XENON1T, a 35x larger and 100x more sensitive instrument which is expected to deliver first data by the end of 2015.

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16.07.2015 From high altitudes to deep underground tunnels

Scientists from the AEC Bern and the University of Zurich have studied the the cosmogenic activation of xenon after placing a bottle filled with pure xenon gas at the Jungfraujoch research station (3470 m). Xenon is successfully used as a target and detector material in several experiments searching for rare events, such as dark matter interaction or neutrinoless double beta decay. The intrinsic radioactive background in these experiments needs to be as low as possible, and activity intrinsic to the xenon, which is usually cooled down to -100°C and used in liquid form, is especially worrisome. In order to study the creation of radioactive isotopes by cosmic rays, which constantly hit the Earth's surface, a xenon sample has been placed at the Jungfraujoch for about one year. The cosmic ray flux up there is more than 11x higher than at sea level, therefore simulating a much longer exposure to cosmic rays at lower altitudes.

The Jungfraujoch research station (right building) [Image: www.ifjungo.ch]

The intrinsic radioactivity of the xenon gas before and after the activation was measured by means of a low-background Germanium spectrometer, installed underground in the Italian Gran Sasso Laboratory (LNGS), where about 1500m of rock reduce the cosmic ray flux to almost negligible levels. The creation of five different isotopes could be observed, out of which only one (125Sb) could potentially lead to background in a dark matter search. In order to test the systematics of the procedure, a sample of 10.4 kg of ultra-pure copper has been activated simultaneously, and the new results largely agree with an older study. However, the comparison of the results with widely-used computer codes to calculate the cosmogenic activation revealed that they tend to significantly underpredict the activation.

The Bern astroparticle physics group is involved in the XENON and DARWIN projects, which build and operate detectors to find rare interactions induced by dark matter particles.

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16.06.2015 A fifth tau-neutrino detected at Gran Sasso

The OPERA neutrino experiment located at the Gran Sasso Underground Laboratory (LNGS) in Italy has announced the detection of its 5th tau-neutrino candidate. This allows the collaboration claiming the discovery of the muon- to tau-neutrino oscillation channel, with a statistical sensitivity exceeding 5 standard deviations. This channel is particularly relevant since is the one occurring for atmospheric neutrino and it had not yet been seen in direct appearance, as done now (unambiguously) by OPERA. The Figure shows the trajectories of the elementary particles identified in the OPERA emulsion detectors, which make up the central part of the experiments. Their micrometer resolution to identify tracks is crucial to be able to identify the tau-lepton which decays almost immediately after its creation.

The Bern OPERA group has played a key role over the almost two decades of life of the experiment with more than 30 researchers and students involved and important contributions in detector construction, data analysis and management.

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09.06.2015 Towards measurements of lowest radioactivity levels

A new detector system is currently being assembled for the first time in the base level of the ExWi building in Bern. It is a low-background high purity germanium (HPGe) spectrometer, a small cylindrical crystal of about 6.5cm height and 8.5cm diameter, which is surrounded by several tons of copper and lead in order to achieve an extremely low radioactive background level. This detector is the heart of the GeMSE facility (Germanium Material and Meteorite Screening Experiment), a joint interdisciplinary project of groups from AEC/LHEP and the NMBE/Geology Bern, which will measure the radiactive contamination in meteorites (to learn more about their age and the size of their mother body) and in raw materials (to build low-background detectors from them). For this purpose, a large sample cavity around the detector is avalable inside the shielding.

The current installation in Bern is just a trial run, and the final experiment will be placed underground in the Vue-des-Alpes Laboratory (Kanton Neuchatel, about 50 mins away from Bern), where it is shielded from cosmic rays by rock which thickness is equivalent to more than 600m of water. This reduces the flux of cosmic muons by a factor 1900 while cosmic neutrons and protons are completely gone and is mandatory to achive the design goal in terms of background. In addition, a muon veto system will be installed to reduce the muon-induced background by another factor six. The HPGe detector has already been characterized and tested extensively in the Bern Tieflabor (providing a shielding equivalent to 70 m water) and the underground installation is planned for Summer 2015.

The LHEP research group involved in this project is a member of the XENON experiment which aims at the direct detection of dark matter by means of sensitive low-background detectors. The researchers from NMBE/Geology have assembled one of the largest collections of meteorites world-wide, which stems from many dedicated search campaigns in Oman and Saudi Arabia.

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20.05.2015 T2K confirms muon-antineutrino disappearance

The T2K experiment has recently released its first results on the disappearance of muon antineutrinos. It is based on the analysis of neutrino and antineutrino interactions generated by 2.3x1020 protons on target, delivered between May 2014 and March 2015 by a beam mainly composed of muon antineutrinos. Out of the 59.8 events which were predicted in case of no neutrino oscillations, only 17 muon-like events were observed in the Super-Kamiokande detector, which is 295 km away from the neutrino source. This confirms unambiguously the hypothesis of antineutrino disappearance.

The resulting measurement of the antineutrino oscillation mixing angle sin2(θ23) is the most precise measured so far, even though is was only obtained with limited statistics. The comparison of this measurement with previous muon neutrino disappearance results provides a test of the CPT symmetry, which predicts the same rates for both, neutrinos and antineutrinos. Both results are fully compatible with each other, in agreement with the CPT expectation.

T2K will continue data taking in both beam configurations (neutrino, anti-neutrino mode) with the final goal of comparing electron neutrino and antineutrino appearance probabilities, to address the open question of a possible CP violation in the leptonic sector of the Standard Model.

Physicists from the Bern T2K group are very active in the analysis of ND280 near detector data, whose main role is to constrain flux and cross section uncertainties for the neutrino oscillation studies.

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05.05.2015 The LHC is back

After 14 months of upgrades, the Large Hadron Collider (LHC) is now back online at a record energy, ready to explore new fundamental physics which might be now accessible and to unveil the existence of new particles. We are ready for data taking and are eagerly awaiting the first collisions expected around first of June.

The LHEP group has been part of the LHC project since its beginning and is playing a central role in the ATLAS experiment. In the last 14 months, works at CERN concentrated on boosting the accelerator capabilities and on improving the detectors. A team from Bern contrinuted to the construction and installing of a new layer of high precision position detectors (pixel detector, IBL) in the very center of the ATLAS detector. It has already seen first particle tracks and is now ready to precisely record the wealth of tracks from the high energy collisions. Bern scientists will continue to maintain and calibrate the new detector during operation, ensuring that it acquires high-quality data.

One of the first proton collisions recorded in ATLAS after the restart.

The LHC is expected to run for the next 3 years, first at a center-of-mass energy 13 TeV (6.5 TeV per beam) colliding protons every 50 nanoseconds, then going up to 14 TeV with collisions every 25 nanoseconds. The energy is doubled compared to before, allowing ATLAS and CMS to study the Higgs boson discovered in 2012 even more precisely, and making the LHC a prime environment to search for new particles, for examples the ones predicted in supersymmetric models, addressing some of the most fundamental questions in physics today. The Bern group leads data analysis efforts searching for such particles, developing the analysis tools to separate interesting events from large backgrounds.

Nobody knows what the LHC will uncover in the next years and there is not even a guarantee that something new will show up. However, it is fascinating to participate in this adventure exploring the building blocks of our Universe, getting insights from the smallest accessible scales to cosmology.

15.04.2015 Forefront position in interdisciplinary research

last year, we have initiated four new interdisciplinary projects, which are all fundeded by the Swiss National Science Foundation (SNSF) with a total of about 3 million CHF. The common aspect of all projects is that they bring well-established techniques and methods from experimental particle physics into other fields, covering very different research topics: from meteorite research and geology to medical and biological applications.

More information can be found in the full press release:
Universität Bern: Spitzenposition in interdisziplinärer Forschung (German)

09.03.2015 New Neutrino Project with AEC Participation

A new Short-Baseline Neutrino (SBN) physics program based on the use of three LAr-TPC detectors located along the Booster Neutrino Beam (BNB) at Fermilab is planned in the United States. The proposal of the project has been published recently: arXiv:1503.01520, and outline of the facilities at Fermilab is shown in the Figure.
The SBN project will deliver a rich physics opportunity, including the ability to resolve a class of experimental anomalies in neutrino physics, and to perform the most sensitive search for sterile neutrinos at the eV mass-scale through both appearance and disappearance oscillation channels.

Additional physics of the SBN Program includes the study of neutrino-argon cross sections with millions of interactions using the well characterized neutrino fluxes of the BNB. The SBN detectors will also record events from the off-axis flux of the NuMI neutrino beam with its higher electron neutrino content and different energy spectrum. Finally, the SBN Program is an excellent opportunity to further develop this important technology for the future long-baseline neutrino program (LBNF). The AEC-LHEP Bern group played a seminal role in the design of the SBN. Our researchers are involved in both the MicroBooNE and the LAr1-ND detectors of the SBN. MicrobooNE is presently in its commissioning phase.

24.02.2015 XENON1T Cryostat closed for the first time

The majority of the matter in the Universe consists of the so-called dark matter. The particle(s) which makes up this mysterious matter must be very different from all particles we know as of today, and remains still unknown. A prime candidate is the weakly interacting massive particle (WIMP), which is prediced by several extensions of the standard model of particle physics. The XENON1T experiment, which is currently being installed in the Laboratori Nazionali del Gran Sasso (LNGS) in the Italian Abbruzzo mountains, will search for the dark matter particle with an unprecedented sensitivity. It consists of about 3 tons of cryogenic liquid xenon, which is hold by a double-wall cryostat. On February 24, the outer cryostat has been closed for the first time, allowing for the evacuation of the insulation vacuum.

Both, inner and outer cryostat, are made of selected stainless steel with a very low intrinsic contamination with radioactive isotopes and are installed inside a big water shield (currently empty), which will act as a Cerenkov muon veto. In order to reduce the radiative heat loss -- the liquid xenon is -95°C cold -- the inner cryostat is wrapped with superinsulation (the shiny foil visible on the photo). The outer cryostat, its flange is visible at the bottom, is then installed around the inner one. The XENON group at LHEP Bern is responsible for the design and installation of the XENON detector, a time projection chamber which will be eventually installed inside the inner cryostat, as well as for the data acquisition system to read the signals. It is expected that XENON1T will see first signals by fall 2015.

09.02.2015 Using particle physics expertise for medical applications

Proton therapy is a high precision technique in cancer radiation therapy, which allows irradiating specific tumours with minimal damage to the surrounding healthy tissues. Clinical beams can be administered by means of several techniques among which pencil beam scanning is the most advanced. It is based on a few-mm wide beam with variable energy, which is magnetically driven to cover the target volume. Even tiny doses delivered outside of the tumour are relevant for the recurrence of secondary cancers and for the induction of secondary effects. The pencil beam is surrounded by a halo of protons, whose effects have to be carefully assessed. To study this issue, nuclear emulsion films interleaved with tissue equivalent material were used for the first time by the AEC-LHEP medical applications group. The results have been published by the Journal of Instrumentation.

The high-precision tracking performance of emulsion film detectors allowed identifying protons in the halo, in order to carefully measure their position and angular distribution. On this basis, the corresponding dose was evaluated by Monte Carlo simulations. Measurements with this technique were performed with the clinical beam of the Center for Proton Therapy (CPT) at the Paul Scherrer Institute (PSI). The dose profile in the halo region as a function of the distance from the centre of the beam is shown in the figure. It decreases with the distance and increases with the depth in the tissue equivalent material of the detector. This study shows that the extra dose due to the beam halo is negligible if compared to the doses given to the organs at risk during the treatment.

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26.01.2015 Large public interest in dark matter

A large audience of almost 200 interested people attended a presentation on "The dark side of the Universe", given by LHEP member M. Schumann in the framework of the Physik am Freitag event. This series of outreach talks, mainly targeted at high school students as well as everybody who is interested in ongoing physics research, is organized by the department of physics and astronomy at the University of Bern and takes place annually in January/February.

Even though it is not yet known what dark matter actually is made of, we know from a plethora of cosmological and astronomical observations that it exists. This was highlighted in the presentation, which was accompanied by several experiments, for example on the Doppler effect, used to measure the rotation curve of galaxies (which are dominated by the presence of dark matter). The second part of the talk focused on how dark matter can be detected, for example with the low-background detectors of the XENON project, in which the Astroparticle Physics group of LHEP/AEC Bern plays an active role. The current instrument, XENON100, will soon be surpassed by the next-generation experiment XENON1T. It will be 100x more sensitive than XENON100 and will is expected to take first data by the end of 2015.

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Article on the event in the uniaktuell magazine (in German only)

12.01.2015 LHEP participates in Japanese-Swiss Meeting

On December 14, 2014, members of AEC/LHEP joined the 3rd Japan-Switzerland Joint Committee Meeting on Science & Technology Cooperation. The meeting was opened by speeches by the Ambassadors, followed by reports on topical cooperations in the domains of particle physics and disaster management add by discussions between the Japanese and Swiss funding agencies. As LHEP employs a considerable fraction of Japanese researchers, who have brought very important expertise to Switzerland, the participation of LHEP members was a prime example of scientific exchange, especially in terms of researcher mobility.

Both, Swiss and Japanese, delegations concluded with a press release which promises to reinforce further the cooperation in science and technology. More information and the press releases can be found here: www.stofficetokyo.ch/3rd-joint-committee-meeting-on-st-cooperation

20.11.2014 OPERA achieves its science goal

The OPERA experiment aimed at the first observation of oscillations from muon neutrinos into tau neutrinos in appearance mode. It recently reported the "observation" of the appearance of tau neutrinos in a pure muon neutrino beam from CERN, after travelling over 730 km through the Earth's crust. We have analyzed about 20'000 neutrino events recorded in 5 years of data taking and confirmed the observation of 4 tau neutrino events, leading to a 4.2 σ statistical significance.

It was a long long way to reach this goal. First discussions about the experiment started back in 1995. The proposal was approved in 2001 and detector construction was completed in 2008. The physics data taking using the CNGS neutrino beam, from CERN to the Gran Sasso underground laboratory (LNGS), took place from 2008 to 2012. The OPERA Collaboration is composed of about 200 physicists from 11 countries.

The AEC-LHEP opera group of the University of Bern has contributed to the construction of the detector. OPERA's heart is a target consisting of 1300 ton of bricks composed of layers of lead and nuclear emulsion films, whose position resolution is required in order to identify the signature of the rare tau neutrino events. Our group participates in the analysis of the nuclear emulsion films and is currently the largest laboratory in analyzing neutrino interactions in Europe.

The results has been published in Progress of Theoretical and Experimental Physics (PTEP): Observation of tau neutrino appearance in the CNGS beam with the OPERA experiment

27.10.2014 LHC ATLAS simulations on Cray Supercomputers

Using a framework developed by the AEC-LHEP ATLAS group (see picture: Michael Hostettler (left) under the supervision of Sigve Haug), computing tasks for the ATLAS detector, one of the two large multipurpose instruments at the Large Hadron Collider (LHC), have been run on a Cray High-Performance Computers (HPC) system. Development and testing was done on a Cray XK7 machine located at the Swiss National Supercomputing Centre (CSCS). This innovative milestone shows that LHC can use supercomputers to address its future big data and extreme processing needs. There are current efforts to implement the same setup on HPC systems in Germany, Czechia and China.

The Large Hadron Collider at CERN in Geneva is one of the most important scientific instruments serving the human quest for the fundamental building blocks in nature. During its first the run from 2010 to 2013 LHC revealed a particle consistent with the long-sought Higgs boson, a manifestation of the mechanism explaining mass. Next year LHC will resume operation at higher collision energies, i.e. new heavier and unknown particles may be accessible for creation and studies (energy is proportional to mass).

Today the computational infrastructure needed to exploit the scientific potential of LHC embraces hundreds of computing centres all over the world. In future the computational needs will increase significantly. Thus, innovation is needed both on the technical and also on the operational side. One possibility is to take advantage of the world’s flagship High-Performance Computers, so-called supercomputers. A handful of these would in principle provide the required computational power. The CSCS in Lugano has several supercomputers, in particular Europe’s fastest machine, the number six in the world according to the TOP500 list. CSCS is thus a strong asset for Swiss science.

The AEC-LHEP ATLAS group is involved in various aspects of the experiment, including computing, hardware development, construction and installation, as well as data analysis.

29.09.2014 Radioactivity induced by protons in air studied at the Bern medical cyclotron

Radioactivity induced by a 15-MeV proton beam extracted in air was studied for the first time at a medical cyclotron. The results have been published by Radiation Protection Dosimetry. The experiment was carried out at the beam transport line of the 18-MeV cyclotron at the Bern University Hospital (Inselspital) by the AEC-LHEP medical applications group. The Bern cyclotron (Figure) is daily used for the production of radioisotopes for Positron Emission Tomography (PET) and for multi-disciplinary research activities using the beam line, a special feature for a hospital-based facility.

The assessment of the radioactivity present in air is an important issue in radiation protection. A reliable and efficient monitoring is crucial for radioisotope production and research facilities. Due to the growth of PET imaging, the number of medical cyclotrons is continuously increasing. Their safe and reliable operation is of paramount importance. Several research applications require the extraction of the beam into air as, for example, PIXE and PIGE ion beam analysis and the bombardment of cell samples for radiation biology. In this case, radioisotopes are produced in air by nuclear reactions induced by the beam and an accurate assessment of the radioactivity is therefore needed.

In this study, the produced radioactivity was calculated and measured by means of high sensitivity proportional counters located at the main exhaust of the Bern cyclotron laboratory. Good agreement was found between calculations and measurements. On this basis, radioactivity induced in air by proton beams in the energy range of medical cyclotrons can be assessed for specific applications.

Read More:
S. Braccini et al., Study of the Radioactivity induced in Air by a 15-MeV Proton Beam, Radiation Protection Dosimetry (2014) doi: 10.1093/rpd/ncu199

10.09.2014 Science Night 2014

LHEP scientists organized and participated in the "Interactive AEC Village" at the second Nacht der Forschung (Science Night) of the University of Bern, which took place until midnight of September 6, 2014. The project, subtitled "Understanding how the world works", was a joint endeavor with our AEC colleagues from the institute of theoretical physics (ITP) featuring a collection of experiments, shows, and display items.

The event was a big success, with several thousand interested visitors from Bern and the greater Bern area stopping by at our booth in front of the University's main building to learn about the physics pursued at LHEP and the AEC. Among many topics, the visitors could build the ATLAS detector from Lego, learn how proton beams can be used to treat tumors, digest extremely cold ice cream (made using liquid nitrogen and liquid argon), ask theorists all kind of crazy questions, investigate which background radiation can impact physics measurements, try to work against the pressure of the vacuum, and explore what happens when liquid nitrogen droplets fall on a hot surface.

04.08.2014 A Moiré Deflectometer for Antimatter

The AEgIS experiment reported on a first successful employment of a Moiré deflectometer to observe the shift of an antiproton beam in a weak magnetic field (~10 Gauss). The results have been published by Nature Communications and represent an important step towards a similar measurement with electrically neutral anti-hydrogen. This is the final goal of the collaboration, aiming at the first measurement of the gravitational acceleration of anti-matter.

A Moiré deflectometer consists of three equally spaced elements: two gratings and a very position sensitive emulsion detector. The horizontal slits of the two gratings (see Figure a) select classical particle trajectories which form a fringe pattern one the emulsion detector. Anti-protons hitting the gratings annihilate when they touch "ordinary" matter and only those which go through the slits survive and reach the detector. The Moiré pattern is then measured by the emulsion detector, featuring a position resolution of 1 µm (Figure b). Once the transit time of the particles through the device is know, the measumement of the shift of the Moiré pattern leads to the measurement of the absolute force acting on the particle beam. For the new results, a mean force of ~500 aN (atto = 10-18) was measured.

The emulsion detector was developed and analyzed by the AEC/LHEP AEgIS group of the University of Bern, which has long-term expertise with this special technology. The thick tracks in the Figure are fragments of nuclei of the emulsion and the lighter ones stem from pions, which are secondary particles produced in the annihilation of the anti-protons.

Read More:
AEgIS Collaboration: A moiré deflectometer for antimatter, Nature Communications 5, 4538 (2014).

17.07.2014 XENON100 sets tight limits on axions

One of the best motivated candidates for dark matter is the weakly interacting massive particle (WIMP) which is expected to be about 10-1000x heavier than a proton. However, there are also other candidates, one of them being the ultra-light axion or similar-behaving axion-like-particles (ALPs). In some sense, these are expected to behave like a photon with a tiny mass. XENON100, a ultra-sensitive dark matter experiment developed to search for WIMPs, has recently placed very tight constrains on ultra-light dark matter, in the form of axions and ALPs (publication).

The similarity to photons makes the coupling of axions to photons (usually in a strong magnetic field) the prime search channel, however, the XENON100 collaboration used another way: Similar to the well-known photoeffect, where a photon can liberate an electron from an atom, an axion can lead to free electrons via the so-called axio-electric effect. These electrons have a distinct energy distribution, depending on whether they were liberated by an axion or an ALP (where the mass mA is important), and can be detected by the XENON100 dark matter detector. Since no excess of events over the expected background was observed after searching for dark matter for 225 days with a 34 kg liquid xenon target, the collaboration could derive strong limits on the coupling strength gAe of axions/ALPs to electrons. For axions emitted from the Sun, this is the best limit achieved so far, excluding axions in a very interesting mass range (see Figure). Another strong result was obtained for ALPs, in particular below mA=10 keV/c2.

The Astroparticle Physics group of AEC/LHEP is an active member of the XENON collaboration, which is operating XENON100, a dual-phase time projection chamber (TPC) filled with 161 kg of liquid xenon. At the same time, the collaboration is currently constructing the next-generation dark matter experiment XENON1T, which will contain more than 3000 kg of liquid xenon. To liquefy the xenon gas, it has to be cooled down to about -90°C at a pressure of 2 bar.

20.06.2014 T2K observes its first anti-neutrino event in anti-neutrino beam mode

After a one-year long shutdown, the T2K experiment has resumed its data taking on May 26th, 2014. During the beamline commissioning, the first beam of anti-neutrinos ever at J-PARC (Japan) was delivered. At this occasion the first anti-neutrino interaction while running in anti-neutrino mode was observed in the T2K near detector ND280. The display of the event is shown in the picture.

In future, T2K will alternate between runs where J-PARC delivers mostly neutrinos with runs of mostly anti-neutrinos by changing the direction of the magnetic horn current. By comparing oscillations of anti-neutrinos with oscillations of neutrinos it might be possible to probe the CP violation in the lepton sector, an important step towards the explanation of the observed baryon asymmetry of the Universe ("Why do we see so much more matter than anti-matter in the Universe?").

The T2K group at LHEP Bern is actively contributing to the analysis of the interactions in the near detector complex ND280.

05.06.2014 Successful upgrade of a part of the ATLAS detector

On May 7th 2014, the IBL (Insertable B-Layer) has been successfully inserted in the ATLAS Pixel Detector. The IBL is a fourth layer of pixels, placed about 3 cm away from the beam line, around a new beam pipe. Because of the high level of radiation in the vicinity of the beam interaction area, advanced silicon technologies (3D and planar) had to be developed, together with a new electronic data readout.

The pixel size was reduced to 50 x 250 micrometers, and a CO2-based cooling system was introduced. New carbon foam structures were invented to support the modules that make up the IBL. These "staves" had to be just firm enough to serve as mechanical support but flexible enough to be inserted.

The IBL journey began Monday, May 5th with the move of the packed IBL from the clean room in laboratory SR1 at CERN, where it has been assembled, to the bottom of the ATLAS cavern. The operation lasted two days. After that, one day has been spent spent on final alignments and preparation of the necessary tools for the final insertion of the IBL into the ATLAS detector, that took only half a day.

The LHEP/AEC ATLAS group was involved in the construction and installation efforts of the detector and its readout, and will now continue to play a leading role in the commissioning of IBL and the data-taking.

05.05.2014 Revealing unknown physics with gaming technology

Recent developments in gaming technology allows us to accelerate the analysis of physics data. A Graphic Processing Unit (GPU) is usually used to render the graphics of computer games, however, lately it is more and more used for general purpose computing as well. The LHEP in Bern is pioneering the application GPUs for experimental high energy physics.

The use of GPUs is attractive for physicists because of their large computing power with thousands of processing cores which can work in parallel. For example a recent GPU (NVIDIA GeForce GTX TITAN) has 2880 cores and the computing capability is 4.5 TFLOPS (tera floating operation per second). This is very large compared to latest multi-core CPUs featuring 0.1-0.5 TFLOPS. In other words: one single GPU corresponds to a full computing cluster. This kind of computing solution is suitable for image-like data processing and the reconstruction of particle trajectories measured in particle detectors.

An advanced application of GPUs reconstructing data from photographic emulsion detectors has recently been published in JINST. The data rate from the photographic emulsion detectors is very large, at the order of 1 GByte/second. One needs a powerful computing solution to process this amount of data in real-time. By using GPUs, we could a data processing speed which is 60 times faster compared to that of a single core of CPU.

The result was also reported at GTC 2014 (GPU Technology Conference, San Jose, 24-27 Mar 2014): "Does Antimatter Fall On The Earth? Measurement Of Antimatter Annihilation with GPUs"

The researchers at LHEP continue to develop faster GPU algorithms for a wide range of applications in physics.

17.04.2014 New results on muon neutrino disappearance studies from the T2K experiment

After having firmly established the appearance of electron neutrinos in a muon neutrino beam at the 7.5 sigma level in summer 2013, the T2K neutrino oscillation experiment has recently reported an update on the muon neutrino disappearance measurement, which is particularly sensitive to the sin223) and Δm232 oscillation parameters. In the T2K experiment, an intense beam of muon neutrinos is sent from the J-PARC nuclear physics site on the East coast of Japan to the Super-Kamiokande neutrino detector in the mountains of western Japan.

In the new analysis, 120 muon events have been observed while 446.0±22.5 were expected in the absence of neutrino oscillations. The reported values sin223) = 0.514+0.055-0.056 for normal hierarchy and sin223) = 0.511±0.055 for the inverted hierarchy are the worlds best measurements of sin223). The result is still consistent with maximal mixing. The corresponding mass-squared splitting is Δm232 = (2.51±0.10) x 10-3 eV2/c4. For the inverted hierarchy, the result is Δm2=(2.48±0.10) x 10-3 eV2/c4.

Physicists from LHEP Bern contributed to the analysis of muon neutrino interactions in the near detector complex ND280, with the goal of predicting the neutrino flux and spectrum at the far detector and to constrain interaction model parameters.

27.03.2014 OPERA observed extremely rare neutrino oscillation

The OPERA experiment at the underground Gran Sasso Laboratory (LNGS, Italy) has detected a fourth τ-neutrino coming from the oscillation of one of the billion μ-neutrinos sent to Gran Sasso from the CERN laboratory in Geneva, 730 km away.

With this fourth event, announced on 25.03.2014 at LNGS, this transition has been now seen unambiguously, namely with a statistical significance of more than 4 standard deviations: this means that the probability that a combination of background events could "fake" such an observation is smaller than 0.001%. The oscillation of μ- into τ-neutrinos, firmly established today, is responsible in particular for the well known deficit of μ-neutrinos coming from the earth atmosphere.

The Bern group of the Albert Einstein Center for Fundamental Physics - Laboratory for High Energy Physics has played and plays a key role in the international OPERA collaboration. Prof. A. Ereditato proposed the experiment in 1997 together with K. Niwa and P. Strolin and has been leader of the collaboration for many years. His group is heavily involved in the analysis of the data presented today. The Bern image analysis laboratory is the second largest of the OPERA collaboration and allows to detect the microscopic signature of neutrino interactions in the emulsion films that constitute the "heart" of the OPERA detector: in some sense the largest photographic camera ever built, with its 10 million individual photographic films!

Article in uniaktuell (in German):

14.02.2014 LHEP well represented in Top 30 of particle physics citations 2013

Several publications from LHEP researchers appear in the list of the 30 top cited particle physics articles during 2013, which is compiled by INSPIRE, the high energy information system. The top ranked article is from ATLAS and reports on the first observation of the Higgs boson. LHEP was a founding member of ATLAS. On place 20, there is the latest result from the XENON100 dark matter experiment, excluding WIMP dark matter over a wide mass range. The last experiment with LHEP participation is T2K, reporting on the indication for electron neutrino appearance from a muon neutrino beam. Other experiments among the top 20 are CMS, Planck, WMAP, DAYA-BAY, RENO and the high-z supernova searches.

As the Review of Particle Physics is cited many more times than a typical research article, it is not considered in the ranking. It is compiled by the Particle Data Group (PDG), with participation of AEC members, and was cited about 10 times per day during 2013.

Further Information:

08.10.20132013 Physics Nobel Prize for the Higgs

The Nobel Prize in Physics 2013 was awarded jointly to François Englert and Peter W. Higgs "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider".

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