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IIHE - Interuniversity Institute for High Energies (ULB-VUB)The IIHE was created in 1972 at the initiative of the academic authorities of both the Université Libre de Bruxelles and Vrije Universiteit Brussel.
Its main topic of research is the physics of elementary particles.
The present research programme is based on the extensive use of the high energy particle accelerators and experimental facilities at CERN (Switzerland) and DESY (Germany) as well as on non-accelerator experiments at the South Pole.
The main goal of this experiments is the study of the strong, electromagnetic and weak interactions of the most elementary building blocks of matter. All these experiments are performed in the framework of large international collaborations and have led to important R&D activities and/or applications concerning particle detectors and computing and networking systems.
Research at the IIHE is mainly funded by Belgian national and regional agencies, in particular the Fonds National de la Recherche Scientifique (FNRS) en het Fonds voor Wetenschappelijk Onderzoek (FWO) and by both universities through their Research Councils.
The IIHE includes 19 members of the permanent scientific staff, 20 postdocs and guests, 31 doctoral students, 8 masters students, and 15 engineering, computing and administrative professionals.
The needle in the haystack
Physicists working in the CMS experiment regularly have to spend their time searching for a needle in a haystack. In other words we look for the rarest of rare collisions that represent very unlikely physics processes. An example of work done at the IIHE is the search for the production of four top quarks (the needle) in the huge dataset recorded by CMS in 2012 (the haystack). Our results put an extremely tight limit on the production of four top quarks, indeed the tightest limit at the LHC so far. As four top quarks are also produced in many new theories of physics such as supersymmetry, this limit can tell us a lot about the validity of these theories.
Dark matter searches with IceCube
According to the most recent observations and based on the standard model of cosmology, dark matter makes up 26.8% of the energy density in our Universe The argument that yet to be detected Weakly Interacting Massive Particles (WIMPs) make up the dark matter is compelling. Over time, WIMPs may accumulate in the center of the Sun and Earth, and annihilate with each other. The decay products may vary, and most of them will interact and decay in the massive body. If neutrinos are created from those secondaries, they will escape and provide a neutrino ﬂux. This neutrino flux could be measured by the IceCube Neutrino Detector. Data taken by AMANDA and IceCube have been analysed at the IIHE to search for WIMPs in the centre of the Sun and Earth; no significant excess above background was observed so far.
Astroparticle Physics revolves around phenomena that involve (astro)physics under the most extreme conditions.
Cosmic explosions, involving black holes with masses a billion times greater than the mass of the Sun, accelerate particles to velocities close to the speed of light and display a variety of relativistic effects. The produced high-energy particles may be detected on Earth and as such can provide us insight in the physical processes underlying these cataclysmic events. Having no electrical charge and interacting only weakly with matter, neutrinos are special astronomical messengers. Only they can carry information from violent cosmological events at the edge of the observable universe directly towards the Earth. At the Inter-university Institute for High Energies (IIHE) in Brussels we are involved in a world wide effort to search for high-energy neutrinos originating from cosmic phenomena. For this we use the IceCube neutrino observatory at the South Pole, the world's largest neutrino telescope which is now completed and taking data.
The IIHE is a diverse institute
At the last count, the IIHE includes 19 members of the permanent scientific staff, 20 postdocs and guests, 31 doctoral students, 8 masters students, and 15 engineering, computing and administrative professionals. The numbers vary over time with people graduating etc, but the IIHE is a diverse institute: Approximately half of the people here are not from Belgian background and at any time we have tens of different nationalities. Many languages are spoken and the working language is mostly English. Typically, between 25 and 30% of the people working at our lab identifies as female.
First results from a realistic modeling of radio emission by particle cascades in ice
In the previous decade several new experiments (ANITA, NuMoon, ARA, ARIANNA) were proposed to detect high energy (>EeV) neutrino induced particle cascades in dense media such as ice, salt, and moon rock. At the highest energies, these neutrino's are extremely rare and a large detector volume is needed to detect them. Due to the long attenuation length, the detection of the produced radio signals is the most promising tool to search for these rare events. In light of these new experimental efforts, the EVA-code, originally constructed to model radio emission from cosmic-ray-induced air showers, is under development to model radio emission from particle cascades in the South-Pole ice. The ice geometry is included into the code, as well as a parameterized model for the particle cascade. Furthermore, the original EVA-code already incorporated Cherenkov effects in the emission for radio signals moving on curved paths due to a density gradient in the medium. The figure below shows a preliminary result for the electric field as seen by an observer positioned at the ice-air interface. The particle cascade starts at 330 meters depth traveling approximately 10 meters straight upward in the ice until it dies out. The pulses as seen by observers at different lateral distances ranging from 10 m to 300 m are shown. It is seen that the pulse becomes sharper moving outward toward the Cherenkov cone at a lateral distance of approximately 330 meters."
Observation of a New Particle with a Mass of 125 GeV
In a joint seminarar at CERN and the “ICHEP 2012” conference in Melbourne, researchers of the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) presented their preliminary results on the search for the standard model (SM) Brout-Englert-Higgs boson in their data recorded up to June 2012. CMS observes an excess of events at a mass of approximately 125 GeV with a statistical significance of five standard deviations (5 sigma) above background expectations. The probability of the background alone fluctuating up by this amount or more is about one in three million. The evidence is strongest in the two final states with the best mass resolution: first the two-photon final state and second the final state with two pairs of charged leptons (electrons or muons). We interpret this to be due to the production of a previously unobserved particle with a mass of around 125 GeV.
Monojets as a possible signature for dark matter production at the Large Hadron Collider
Dark Matter is, almost a century after it was conceived, still only known to us through gravitational effects. Depending on its properties, there exists the exciting possibility of producing dark matter particles at colliders like the LHC. With the CMS detector, IIHE scientists search for direct production of dark matter particles in collisions like the one shown here: a jet (a spray of particles from a quark or gluon) recoiling against particles that escapes detection. This particular collision was the highest energy event of this type recorded by the CMS detector so far. Although it is most probably a background collision, dark matter could manifest itself in our detector exactly in such a "monojet" signature.
IceCube observes first hint of astrophysical high-energy neutrinos
Two neutrino candidate events detected at the IceCube Neutrino Observatory, dubbed "Bert and Ernie", are the two highest energy neutrinos ever observed so far, with an estimated deposited energy of about 1 PeV. The IceCube event displays of these two events are shown in the figures below, where for comparison one should realize that a single event covers an area comparable with the Maracana football stadium in Rio de Janeiro! The probability that these two events are not background, i.e. anything else in the detector besides astrophysical neutrinos, is at the 2.8 sigma level and does not allow claiming a first observation of astrophysical neutrinos. Further details may be found in Physical Review Letters 111 (2013) 081801. To improve the detection sensitivity, a follow-up search on the same data period has been conducted. The new analysis selects high-energy neutrino events with vertices well contained in the detector volume and exploits veto algorithms by using the outer layers of IceCube sensors. By means of this new analysis method 26 new events have been detected. The entire sample of 28 events has properties consistent in flavour, arrival direction and energy with generic expectations for neutrinos of extraterrestrial origin.
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