Overview of the Research Activities of our group

Intro

The HERMES experiment [1] is an experiment located at DESY Hamburg [2]. 27,5 GeV electrons accelerated by the HERA accelerator are scattered off atomic nuclei. It is a so-called fixed-target experiment. The research is a collaboration between 25 universities and research centra spread over the world: USA, Italy, Germany, Russia, Belgium, UK, Netherlands, China, Canada, Japan, Poland and Armenia. Our group has a rich history as a member of the HERMES collaboration.

Physics

The HERMES experiment tries to measure the spin structure of protons and neutrons[3]. To do so, polarized electrons are scattered off polarized targets. From double-spin asymmetries, the spin structure function g₁ can be determined. As a consequence of the construction and functionality of a RICH detector (see further), the HERMES collaboration managed to determine the individual quark flavor contribution to the nucleon spin (the so-called Delta-q functions). The up quarks appear to be polarized parallel to the proton spin, whereas the down quarks are polarized antiparallel.

The results of HERMES confirmed previous results from the EMC collaboration showing that the nucleon spin is only for about one-third generated by the individual quark spins. Apart from the intrinsic quark spin, the quarks' orbital angular momenta are believed to significantly contribute to the nucleon spin, since the spin operator J is the sum of the intrinsic spin operator S and the angular momentum operator L: J=L+S. Apart from the quarks' contributions, also the gluons contribute to the nucleon spin [4]. Quark orbital angular momenta can be determined by analyzing exclusive reactions like deeply virtual Compton scattering and exclusive meson production. In exclusive reactions, the initial and final state are fully determined. To improve the detection efficiency of exclusive reactions, a recoil detector surrounding the HERMES target cell was installed in 2006.

Between 2002 and 2005 a transverse magnet was installed around the target cell in order to achieve a transversely polarized proton target. From analyzing azimuthal dependencies in the scattering of electrons off transversely polarized protons, the so-called transversity distribution [5] can be determined. The transverse quark polarization is different from the longitudinal, since rotations are not mutually commutable. The measurement of the transverse spin is very important in solving the nucleon spin mystery.

Apart from specific spin physics topics, other interesting physics topics are studied by analyzing HERMES data. One of these topics, for example, is the creation of hadrons in deep inelastic scattering, the so-called hadronization [6]. This describes how a hadron of type h is created from a quark of flavor q. Another example is the effect of the nuclear medium on the creation of hadrons in the deep inelastic scattering process [7] . HERMES has the advantage of being able to inject various target nuclei in the target cell, allowing an analysis of the A (the mass number) dependence of particular variables of the hadron formation process. The various nuclei used are: hydrogen, deuterium, helium, carbon, neon, krypton and xenon.

Specific contributions by our research group

The experimental particle physics group of Gent University is contributing to nearly all analysis topics within the HERMES experiment. Hardware wise we are responsible for the maintenance and calibration of the trigger scintillator arrays (the so-called hodoscopes). Moreover, our group was actively involved in the construction and development of a dual Ring-Imaging Cherenkov Detector (RICH) [8]. That detector is used to separate pions, kaons and protons based on the Cherenkov radiation these hadrons emit in the two radiator materials. The Cherenkov light is measured by PMTs. Our group contributed half of the about 4000 PMTs required for the HERMES RICH detector and designed the matrix housing the PMTs.

Apart from the RICH and the hodoscopes, our group designed the photon subdetector of the Recoil Detector [9]. This photon counter is constructed in three layer each layer consisting of a convertor layer and a scintillating layer. The scintillator layer is read out by PMTs. Our group is responsible for the simulation and the tracking based on the photon detector.

Intro

The IceCube experiment [1] is the successor of the AMANDA project [2]. The collaboration has about 200 members from institutes in Belgium, Germany, Japan, Netherlands, New-Zealand, UK, USA and Sweden. The experiment is located at the South Pole and exists out of up to 80 strings to which so-called digital-optical modules are attached. The total detector volume will amount to a cubic kilometer once all strings will be installed in the Antartic ice. In 2011 all 80 strings of the IceCube experiment should be installed and deployed. The DOMs are located at a depth between 1,400 and 2,400m below the surface. At the polar surface itself, the IceTop detector is being constructed. This detector consists of frozen water tanks with two DOMs.

The IceCube experiment mainly investigates the abundance and origin of high-energetic cosmic radiation. The energies seen in cosmic radiation cannot be created in any research facility on Earth. To investigate the highest energies ever seen we have to analyze cosmic radiation.

Physics

The goal of the IceCube experiment is the observation of cosmic neutrinos [3]. Neutrinos in general have very small cross sections for any type of reactions. If we want to detect neutrinos, we consequently need large detector volumes. If a neutrino interacts in the Antarctic ice, a muon is created. IceCube detects both upgoing and downgoing muons. The muons create Cherenkov radiation in the ice, which is detected by the DOMs.

When a high energetic cosmic particle, like a high energetic proton, or any other nucleus, hits the Earth's atmosphere, a particle shower is created. The particles in the shower are detected by the IceTop detector and depending on the amount of energy deposition, particle identification can be achieved. The muons in the particle shower additionally are observed as downgoing muons in the IceCube detector. The energy range of the initial particles generating the showers which are detected simultaneously by IceTop and IceCube is situated between the knee and the ankle of the energy flux of cosmic energies: the showering particles have energies between 10¹⁵ and 10¹⁸ eV/nucleon. Using the IceTop and the IceCube detector the flux drop at the knee as well as the flux enhancement at the ankle are investigated [4].

The upgoing muons detected in the IceCube detector, on the other hand, are almost certainly originating from the interactions of neutrino's in the Antarctic ice. The Earth itself is used as a filter for the cosmic radiation incident on the northern hemisphere. More than 12,000 km of earth is responsible for filtering all muons created in cosmic air showers on the northern hemisphere. By analyzing the upgoing muons we thus obtain information about neutrino fluxes in the northern hemisphere. These neutrinos are messengers from pointlike sources much as Gamma-Ray Bursts (GRB), Active-Galactic Nuclei (AGN) and supernovae(SN) remnants. These phenomena yield information and knowledge about, for example, the origin of our universe. Neutrinos have the advantage that they are not influenced by external magnetic fields (in contrast to charged particles) nor are they attenuated by opaque matter in the universe (in contrast to photons). As a consequence, we are able to obtain directional information by analyzing neutrinos. The IceCube detector is sensitive to neutrino energies between 10⁷ up to about 10¹⁸ eV.

The highest energies measured in cosmic radiation are about 10²⁰ eV. At least five different experiments have measured one or more of these `Oh my God' events. Up to now, it is hard to explain the origin or the existence of particles with these energies, since the energies measured are above the so-called GZK cutoff [5], corresponding to 10¹⁹ eV. This cutoff is the energy limit predicted by the generally accepted theories. The particle flux at these energies obviously is low. To achieve better statistics we have to consider larger detector volumes, which evidently leads to more expensive detectors.

An interesting way to measure these ultra-high energy neutrinos (UHE) is by means of acoustic detection: if an UHE neutrino interacts in the Antarctic ice, the ice is heated and thus expands. By heath diffusion, the ice is cooled down and as a consequence it will shrink. In other words, an UHE neutrino will create a bipolar pressure wave in the ice, i.e. an acoustic signal. A pressure can be converted into an electric pulse by using piezo-ceramic materials. An advantage of the acoustic detection is the large attenuation length of about 10 km. The attenuation of optical light, by comparison, is about 100 meter. As such, a larger effective detector volume can be obtained with less sensors compared to the optical detection [6].

Specific contributions by our research group

The contribution of our group to IceCube is manifested in two areas: On the one hand we contribute to the optimization of the IceTop detector based on MonteCarlo simulations. On the other hand we are involved in the development of the acoustic neutrino detection. During the winter 2006-2007 the first acoustic sensors were installed in the Antarctic ice. We are helping to deploy these sensors. Moreover, we are involved in the simulation of the acoustic parameters of ice and the thermo-acoustic model. Additionally, we help to optimize the piezo-ceramic sensors used for acoustic neutrino detection, and we try to simulate the neutrino signal in ice by using clear ice and a laser, a particle accelerator, or a piezo-ceramic transducer [7].

Intro

Since 2007 our group is a member of the Compact Muon Solenoid (CMS) Collaboration [1]. CMS is one of the two major collaborations that will study proton-proton collisions at the Large Hadron Collider (LHC) at CERN in Geneva (Switzerland). The LHC is a proton-proton collider designed to operate with a center-of-mass energy of 14 TeV and a luminosity of 10^34 cm-2s-1, which makes this machine the most powerful accelerator ever built [2]. The construction of the LHC was completed in 2008 and the commissioning with beam started in September 2008. The first LHC physics run and the corresponding start of the CMS physics program is foreseen to commence after the summer of 2009. At present, the CMS collaboration has over 2500 members coming from about 170 different institutes from around the world [3].

Physics

The CMS detector is a general purpose, cylindrical collider detector, segmented in 11 modules (cylinder slices): 5 central barrel wheels and 3 endcap disks on each side closing up the detector. The entire detector is about 21m long with a diameter of 15m and weighs approximately 12500 tonnes. It includes a silicon pixel vertex detector, a silicon strip tracking system, a lead tungstate crystal electromagnetic calorimeter and a scintillator tile with brass absorber hadron calorimeter, all surrounded by a superconducting solenoid providing a 4T field for charged particle tracking. Furthermore, a muon system consisting of drift tubes, resistive plate chambers and cathode strip chambers is embedded in the iron return yoke outside the coil of the magnet. The construction and installation of the detector in the CMS underground experimental hall at the LHC was completed around the summer of 2008.

Amongst the main physics goals of the CMS experiment is the search for the elusive Higgs boson and for evidence of possible physics beyond the Standard Model in particle physics, like Supersymmetry or extra dimensions. In the Standard Model, particles acquire their mass through a mechanism involving the coupling to a postulated Higgs field via spontaneous symmetry breaking. The existence of the boson associated with this field, the Higgs boson, still has to be proven experimentally. Furthermore, the Standard Model is incomplete and may just be a low energy limit of a more fundamental theory. It is expected that new physics going beyond the Standard Model will reveal itself at the Terascale which is accessible by the LHC.

Specific contributions by our research group

The Ghent group is involved in the Resistive Plate Chamber (RPC) muon system of the CMS detector [4]. This subsystem is particularly important for muon triggering. At present we are actively participating in the commissioning of the RPC system and we are one of the leading groups which are involved in the extension of the endcap RPC system for the high luminosity phase of the LHC. The group is also involved in studies of the CMS high-level trigger system (HLT). The main physics research interests with CMS are the measurement of top-quark properties, the search for the Higgs-boson and the search for supersymmetry [5].