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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.

CMS

Shown here is a record breaking event from the 2010 LHC run at the Compact Muon Solenoid,

a collision event with both an electron and very high missing transverse energy. The electron is represented by the red trapezoid (the length is proportional to the electron's energy), while the transverse energy is represented by the red arrow. Missing transverse energy is a quantity used to identify particles that did not leave a detectable signature. The IIHE is actively involved in the study of this kind of collisions, in collaboration with other groups of the CMS experiment. If the rate of these kind of collisions would be unexpectedly high, it would be a hint of the existence of, for example, extra dimensions.

Phenomenology

The pheno group — A hint for supersymmetry?

Particle physics phenomenology studies the implications of a theoretical model on experiments in high-energy particle physics and the other way round. From the experimental side, the CMS Collaboration observed in a certain search region 12 events more than expected based on the Standard Model of Particle Physics. Can this be explained by theories that go beyond the Standard Model like supersymmetry? Scientists from the pheno group at the IIHE as well as from the theory group at the ULB collaborated to answer this question. The figure shows how the number of events predicted by a simple supersymmetric model depends on the parameters of the model. The two free parameters, the mass of the stau and the selectron, are shown on the x- and y-axis while the number of events is indicated by the colours. Since we are looking for 12 events coming from new physics, we see from the figure that the model with selectron mass 145 GeV and stau mass 90 GeV can account for the observation of CMS.

CMS

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.

IceCube

Here you see an event recorded by IceCube in January 2008, when the detector was still in construction!

At that time, 22 strings were already taking data and 18 other strings were freshly deployed. Every colored bubble indicates the detection of one or more Cerenkov photons created by the cross of a charged particle by one of the sensors deployed in the ice. The size of the circles reflects the intensity of the signal. The color indicates the arrival time from red (early) to blue (late). These informations combined with the geometry of the detector allow first guess reconstructions of the initial track.

CMS

Looking in usually ignored collisions for physics beyond the Standard Model

It is commonly agreed that the standard model is not the ultimate theory and breaks down at higher energies. One of its most famous extensions is called supersymmetry or SUSY. Even though the CERN LHC data is already extensively examined for signatures predicted by this theory, no evidence has been found. However, supersymmetric models in which particles would have large lifetime (so would seem not to come from the collision point), have been mostly overlooked until now. IIHE physicists have performed a search that focuses on checking the LHC data for evidence of such a model. The picture depicts the transverse view of the CMS interaction point, showing a typical event from one of the possible signal with long life time. The definition of the leptons' impact parameter, d0, which is largely correlated with to the particle lifetime, is shown by the arrows.

IceCube

IceCube results challenge current understanding of Gamma Ray Bursts

Favoured candidates for the emission of Ultra High-Energy Cosmic Rays are Active Galactic Nuclei (AGN) and Gamma Ray Bursts (GRB), both spectacular emitters of high-energy gamma rays arising from particle acceleration in relativistic jets. However, the composition of the particles involved in these processes as well as the acceleration mechanism are very uncertain. The IceCube Neutrino Observatory at the South Pole is honing in on how the most energetic cosmic rays might be produced. IceCube is performing a search for cosmic high-energy neutrinos, which are believed to accompany cosmic ray production, and as such explores the possible sources for cosmic ray production. In a paper published in the 2012 April 19 issue of the journal Nature (Volume 484, Number 7394), the IceCube collaboration describes a search for neutrino emission related to 300 gamma ray bursts observed between May 2008 and April 2010 by the SWIFT and Fermi satellites. Surprisingly, no related neutrino events were found - a result that contradicts 15 years of predictions and challenges most of the leading models for the origin of the highest energy cosmic rays, as shown in the figure.

IceCube

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."

CMS

Shown here is a result of the 2012 LHC run at the Compact Muon Solenoid,

studying the invariant mass of electron pairs produced at the Large Hadron Collider. Shown is the data, as black dots, and the simulation predicting what we should expect according to the particle physics Standard Model (coloured bands). The IIHE is actively involved in the study of this kind of collisions, in collaboration with other groups of the CMS experiment. The data points agree very well with the predictions from the Standard Model, which means that up to now no new physics beyond the Standard Model could be observed that produces electron pairs. This could change when the LHC runs at a higher collision energy in 2015 and the high mass region to the right of the spectrum can be explored. New physics could show up as a peak in the high mass region of the spectrum, and could look like a small version of the peak of the Z boson that can be seen at a mass of about 90 GeV.

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