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Ghent Activities in High Energy Physics

Astro-Particle Physics at the South Pole

The IceCube Neutrino Observatory (short: IceCube) is, encompassing a volume of 1 cubic kilometer, the world largest neutrino detector. The detector is located at the South Pole, and uses the Antarctic ice as a medium to detect high-energy neutrinos. These neutrinos originate from cosmic sources and reach energies which cannot be obtained in any research facility on Earth. Having researchers from 38 institutions in 10 countries, the IceCube collaboration includes nearly 250 people.

The Experiment

Neutrinos have very small cross sections for any type of interactions. If we want to detect neutrinos, we consequently need very large detector volumes. In the IceCube experiment, 5160 digital optical modules (DOMs) are mounted on 86 vertical strings and are frozen at depths between 1450 and 2450 m beneath the South Pole surface. IceTop, the surface component of IceCube, consists of 81 detector stations (each including 4 DOMs) covering an area of 1km2 above the IceCube strings. The combination of IceTop and IceCube provides a powerful tool to measure cosmic ray composition and energy by detecting the electromagnetic component at the surface in coincidence with the muon bundle in the deep underground detector.

If a neutrino interacts, it creates an electron, a muon or a tau. When these charged particles travel through the ice, they emit Cherenkov radiation. Due to their small energy loss during propagation and long decay length, muons give the easiest signature to reconstruct. Hence, when a muon travels through the ice, the emitted Cherenkov radiation will be detected by the DOMs, which consist of a photomultiplier tube (PMT) and associated electronics. Using these signals, the actual direction and energy of the muon can be reconstructed. Since December 2010, the IceCube detector is fully deployed. However, it has been providing data since the completion of the nine string array, in 2006.

Physics

Neutrinos are unaffected by the Earth and galactic magnetic field, and, due to their weakly interacting nature, they are essentially unabsorbed when they travel very large distances. Moreover, they are unaffected by high-density regions which are opaque for photons. Hence, neutrinos can provide us with direct information from the most energetic processes in our universe. IceCube is searching for high-energy neutrinos from the most violent astrophysical sources like gamma-ray bursts, galactic supernova remnants and active galactic nuclei. These phenomena yield information and knowledge about, for example, the origin and evolution of our universe. Furthermore, a neutrino telescope can be used to search for possible dark matter candidates and neutrino oscillations. Cosmic ray particles that penetrate the Earth atmosphere produce a large cascade of many thousands or even millions of particles. Since the experiment consists of both an underground and surface detector, IceCube is ideally suited to study these cosmic rays and to determine the energy and mass of the primary particle.

Working topics of the Ghent group

At this moment, the contribution of the IceCube group in Ghent is manifested in three areas. From the start of our connection to IceCube, the Ghent group has been working on the study of cosmic rays. Only recently we have been involved in two other areas. On the one hand, we are trying to find tau neutrinos using IceCube. Additionally, we are partly responsible for the development of a new detector for a possible low-energy extension for IceCube. In 2011, one of the former Ghent PhD students has been a winterover at South Pole for the IceCube experiment.
The Solid Experiment

The Solid Collaboration aims to resolve the reactor anomaly, where the flux of neutrinos close to nuclear reactors is measured to be lower than expected from calculations. A possible reason for this deficit is that the neutrinos are oscillating into a new fundamental particle, the sterile neutrino, which cannot be detected. By deploying a segmented detector close to the BR2 reactor core, Mol, Belgium, the Solid experiment would be able to confirm or rule out the existence of such a particle.

Neutrinos are not so standard

Neutrinos are by far the most abundant massive particle in the Universe. For long considered being massless, and nicely fitting into the Standard Model, an important glitch eventually appeared with the discovery of neutrino oscillations. The observation of neutrino oscillation implies neutrinos to be massive and exhibit a non-degenerate mass hierarchy. In a three neutrinos mixing framework, the three standard neutrino flavours (e,μ,τ) are unitary linear combinations of three massive neutrinos. However a few striking anomalies have been uncovered by several distinct experiments that do not fit with this current three neutrinos picture. First hints came from experiments searching for solar neutrinos with large liquid Gallium detectors such as GALLEX and SAGE. Their calibration measurements presented a large deficit of neutrino events at a level of 2.8 σ from what was theoretically expected. Secondly, both the LSND experiment at Los Alamos, and the MiniBooNE experiment at Fermilab measured an excess of electron anti neutrino events from a muon anti neutrino beam. Finally, improved calculations of the expected reactor anti neutrino fluxes recently increase the predicted flux by 3% in average, causing a deficit in the measured rate. This is known as the reactor neutrino anomaly.

The sterile neutrino hypothesis

Due to the wide variety of experiments pointing to the same direction, it is very unlikely to explain these results by a purely experimental effect, and therefore the explanation should involve some yet to-be-identified physical feature. A current explanation could be the existence of one (or more) non-standard neutrino(s). Such a neutrino must be light and oscillate with standard neutrinos at short distance. However we know from LEP results, that only 3 neutrinos can couple to the Z boson, so an extra neutrino cannot interact weakly and therefore must be sterile.

The Solid Experiment

The international Solid Collaboration proposes to perform new direct measurements of the electron anti neutrino flux at an unprecedented distance (5-12m) from the compact research reactor core BR2 at Mol. The BR2 research reactor is an ideal site to conduct such an experiment. It has one of the smallest reactor cores, providing a near point-like source, which enables high resolution on oscillation patterns, and it is powerful enough to accumulate sufficient neutrino interactions per day. On the other hand, the detector uses a novel composite scintillator technology combining plastic scintillator Poly-Vinyl Toluene (PVT) cubes of 5cm edge with a thin sheet of 6LiF:ZnS(Ag), which is highly sensitive to thermal neutrons. The finely segmented 3D-array detector, composed of 20 frames of 256 cubes, has a total front surface of 1.2 x 1.5m and a mass of 2.88t. This design provides a new way of detecting Inverse Beta Decay (IBD) products close to the interaction point and offers the possibility to combine both an excellent energy and spatial resolution. The reactor-emitted anti neutrino interacts with protons in the PVT cubes, giving rise to a neutron and a positron:

νe + p → n + e+ (Eν > 1.805 MeV)

In addition to the fast positron signal, the outgoing neutron thermalizes and gets absorbed a few centimeters away on the Li6 layer, a few hundreds of ns to a few hundreds of µs later:

n + 6Li + p → 3H + α + 4.78 MeV.

A neutrino event is then defined by time-coincidence detection of a prompt scintillation signal from the positron and a delayed signal from the neutron. As the average kinetic energy of the neutron is about 10 keV, it can get absorbed in another cube than the positron and therefore offers the possibility of directional measurement. The scintillation light is captured by wavelength shifting fibers and read out with MPPC avalanche photodiodes, both in the horizontal and vertical directions of each plane. The signature of a neutron event is very different from a signal of electromagnetic origin, making an IBD selection easier. With an expected 41% efficiency and 2 years running from summer 2016 (150 days/year) a total of 250k events can be collected - sufficient to cover the current reactor anomaly region below 5 eV2 at better than 99% C.L- by end of 2017.

Activities in Gent

The construction of the full Solid detector is currently planned to start during summer 2015. The main parts of the construction effort will be held in Gent: from the construction of the modules and testing the different components of the detector, including the characterization of the properties of several thousands of individual cubes, hundreds of wavelength shifting fibers and MPPC photodetectors, to assembling and testing the different detector layers as they become available and eventually ensure all individual components to be ready for deployment. Our department already played a big role in the construction of the first sub module during summer 2014. Our group is also coordinating the efforts to develop a dedicated software framework to facilitate and expedite the sharing of the analysis developments in the different institutions of the Solid collaboration, as well as the design and management of databases in order to have access to a thorough description of the detector characteristics and calibration, run configuration, or environment conditions. In collaboration with the VUB different analyses have and are being performed. These include an extensive attenuation study of the individual planes of the first Solid sub module and a study of the noise in the raw data and the appropriate corrections to make.
Physics with accelerators at CERN

What is LHC ?

The Large Hadron Collider (LHC) is an accelerator that smashes groups of protons together at close to the speed of light: 40 million times per second and with seven times the energy of the most powerful accelerators built up to now. Many of these will just be glancing blows but some will be head on collisions and very energetic. When this happens some of the energy of the collision is turned into mass and previously unobserved, short-lived particles could give clues about how Nature behaves at a fundamental level - fly out and into the detector. Learn how the LHC machine works...

What is CMS ?

CMS is a particle detector that is designed to see a wide range of particles and phenomena produced in high-energy collisions in the LHC. Like a cylindrical onion, different layers of detectors measure the different particles, and use this key data to build up a picture of events at the heart of the collision. Scientists then use this data to search for new phenomena that will help to answer questions such as: What is the Universe really made of and what forces act within it? And what gives everything substance? CMS will also measure the properties of previously discovered particles with unprecedented precision, and be on the lookout for completely new, unpredicted phenomena. Learn more about CMS detector...
The CMS community has today about 4300 active people (physicists, engineers, technical, administrative, students, etc.), including 1740 PhD physicists and 845 physics doctoral students. A typical CMS physics paper can have about 2100 signatories! Join the public pages of the CMS community...

What are we doing in Ghent ?

Since 2007 our group is a member of the CMS Collaboration. Our efforts are directed both in refining the analysis of LHC data taken in 2011 and 2012 and in commissioning the CMS detector for a smooth restart in 2015. We are active in the physics studies from data analysis (measurements of the top quark properties, search for SUperSYmmetry) and the monitoring, commissioning and upgrade of muon detectors (Resistive Plate Chambers and Gas Electron Multipliers)

The top quark

The top quark is the heaviest particle in the Standard Model, which is a collection of theories that embodies all of our current understanding about the behaviour of fundamental particles. It was first observed in proton-antiproton collisions at the Fermilab Tevatron collider. Since then its properties have been studied by the Tevatron experiments and found to be in agreement with the expectations of the standard model. At the LHC, top quark production can be studied in pp collisions at much higher energy, allowing extended measurements of the top quark properties. A precise measurement of top quark properties is important as top-quark production may be a background for new physics, and can itself manifest signs of new physics. At the LHC top quarks can be produced singly or in pairs. We focus on the study of pair-produced top quark and perform the following studies:
  • With collision data at 7 and 8 TeV we address the single-lepton channel, where the W boson from one top decays in two quarks and the other in one lepton and a neutrino. We measured the production rate of top quark pairs (cross-section), its mass and some subtle properties that relate the angular momentum of one top quark to the other (spin-correlation)
  • With the first data at 13 TeV we will address the di-lepton channel where the W from both top quark decays in electron or muon and neutrino. We aim to measure the top-antitop cross-section with very early data (0.5-1 fb-1) and the top-antitop plus a W or Z boson cross-section (in the same-sign and multi-lepton channels), with at least 10 fb-1
The group is also contributing to the TOP PAG management through convener-ship and contact positions A collection of public CMS paper on the field can be surfed here

The search for Supersymmetry

Supersymmetry (SUSY) is one of the most appealing extensions of the Standard Model of particle physics. It proposes a symmetry which duplicates the number of existing particles by predicting a force carrying particle (boson) for every matter particle (fermion) - and vice-versa. SUSY leads to several elegant solutions to theoretical problems including: the Hierarchy problem; the gauge-coupling unification; the prediction of promising candidates for dark matter particles. SUSY has been searched for at several experiments, so-far resulting only in non-observations. This lead only to restrictions being placed on the parameters which are defined in the theory. Searches at LHC Run I showed no-evidence for SUSY. We participates in several searches for SUSY:
  • Third generation squarks, particularly lightest supersymmetric partner of the top quark (stop quarks) in decay channels involving only one lepton in the final state
  • Search for SUSY production via the strong interaction, in same-sign di-lepton pairs. This is a prime channel for new physics searches as Standard Model predicts very small production rate for such events
  • Search for Electroweak production of SUSY, with chargino-neutralino pairs, in three lepton + missing energy channel
  • Search for gluino pair production in three-lepton +b-jets channel
  • The all-hadronic regime with b-jet and an energetic W boson, using the so called razor variables. Here we mainly target gluinos decaying to a top +s-top quark
  • The search for long-lived, neutral (neutralino) particle decaying into photons . If the neutralino has a nonzero lifetime, the detected photon can originate from a vertex displaced with respect to the beam axis.

The Resistive Plate Chambers

The Ghent group is involved in the Resistive Plate Chamber (RPC) muon system of the CMS detector. Because RPCs are fast gaseous detectors, they are used in the muon triggering subsystem. They consists of two oppositely-charged parallel plates that are made of a very high resistivity material and is separated by a gas volume. When a muon passes through such a volume, an avalanche of electrons is created and the signal is picked up by external metallic strips. RPCs combine a good spatial resolution with a time resolution of just one nanosecond. Several important roles have been covered by the Ghent group:
  • We undertook an intensive RPC production, having built one quarter of all RPCs for the endcap extension (RE4)
  • We are responsible for the monitoring of the quality of data coming from the detector and we manage the off-line shifts at CERN
  • We take a leading role in coordinating the study of detector performance for muon reconstruction
  • We are also active 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)
We are involved in the high-level management of the RPC group.

The Gas Electron Multiplier

A Gas Electron Multipliers (GEM ) is a type of gaseous ionization detector consisting of thin insulator foil, coated with copper layers and perforated with holes with a diameter of few tens of μm. A high voltage is placed across the layers, making large electric fields in the holes. In the presence of appropriate gases, a single electron entering any hole will create an avalanche of other electrons that can be read-out by simple conductive strips laid across a flat plane. With a double- or triple-GEM stacks, gains of one million or more can be achieved. The CMS GEM collaboration is considering GEMs for upgrading the CMS forward muon system in the endcap region. GEM detectors can provide precision tracking and fast trigger information. They would improve the CMS muon trigger and muon momentum resolution and provide missing redundancy in the strong forward and backward direction. The Ghent group was along with CERN one of the initiators of the CMS GEM project. We constructed and tested many GEM prototypes and we are now preparing for the construction of full-scale Triple-GEM muon detectors for the inner ring of the first muon endcap station.
Beyond the LHC

The CALICE Collaboration

The CALICE (CAlorimeter for the LInear Collider with Electrons) Collaboration is a Research and Development group af about 340 physicists and engineers from around the world, working on development of calorimeter detectors for future high energy e+e- experiments. The Linear Collider requires a calorimeter with very fine segmentation and a design optimized for the reconstruction of the energy and direction of all individual particles in multi-jet events using particle-flow techniques. The CALICE Collaboration aims to demonstrate the technical feasibility of such detectors by constructing and putting into test beam several different ECAL and HCAL prototypes, including corresponding electronics and redaout systems, and dedicated detector simulations and data reconstruction software.
Particle and Astro-particle physics theory

Activity in Particle Physics

The theoretical research interests in our group span from hadronic diffraction in high-energy QCD processes to mathematical properties and phenomenology of non-minimal Higgs sectors and flavor symmetries . Currently in the group we are working on various models beyond the Standard Model which make use of extended Higgs sector, such as multi-Higgs-doublet models, with an aim to establish rigorous results which can be applied to broad classes of such models and possibly guide beyond Standard Model-building activity.

Activity in Astro-Particle Physics

We are interested in the physics of astrophysical plasmas, radiation and cosmic-ray acceleration in the shocks of galactic sources such as Super Novae, microquasars and x-binary jets ; and extragalactic sources like Gamma-Ray-Bursts and Active Galactic Nuclei jets. Group members are currently working on relativistic jets, galactic and extragalactic propagation of cosmic rays, and study the consequent radiation in synergy with observational data (e.g. Icecube/Icetop, AUGER, Fermi, etc)