The Physicist

-…My teacher said: “There are three types of alchemist: those who are vague because they don’t know what they’re doing; those who are vague because they do know what they’re doing, but who also know that the language of alchemy is addressed to the heart, and not to the mind

-And the third type?

-Those, who will never hear about alchemy, but who will succeed through the lives they lead, in discovering the Philosopher’s Stone…

P. Coelho, O Alquimista

For as long as I can remember I have always been captivated by Physics—it is not just a demanding and invigorating discipline that takes in everything from the subatomic to the cosmic, it is the pursuit of answers to the biggest questions in the universe, the laws and theorems that underpin our very existence.

Being a Physicist means that one is not only equipped with the scientific knowledge of the laws of physics and the various related formalisms, but also with the important skill of being able to ask (the right) questions. It is with this knowledge that a physicist can begin developing possible models for explaining observations and making predictions for the outcome of experiments.

It is no surprise then, that as I moved through my academic career I was drawn to the world of Subatomic Physics. A world I was lucky enough to enter as the discipline was at the centre of one of the most important scientific developments of the modern age – the construction of the Large Hadron Collider at CERN, a project I was fortunate enough to be a part of working as a member of the team developing and constructing the ATLAS detector.

The Higgs particle, which explains the spontaneous Electroweak Symmetry Breaking and determines the masses of the particles via their interactions with it, was still an aspiration. That aspiration was fulfilled with the announcement of its discovery on 4 July 2012, by both the ATLAS and CMS detectors.


My BSc & MSc Projects

  1. Study of the effect of the misalignment of the muons’ detectors at the ATLAS experiment, on the discovery of the Higgs particle H4μ (MSc Thesis and Presentation)
    • The ATLAS (A Toroidal LHC ApparatuS) detector is one of the four detectors which will be installed in the new accelerator, at LHC, CERN. One of the experiments’ research aims is the discovery of the Higgs particle and thus, the confirmation or not of the Standard Model. The Muon Spectrometer of the detector has special importance, because the decay channels of the Higgs particle which have muons at the final state are clear signatures of the existence of the Higgs particle. This project focuses on the Higgs decay through the channel: H→ZZ→4μ. The alignment of the muon detectors has to be very accurate, so that its contribution on the measurement of the muons’ momentum, to be low compared to the intrinsic resolution of the detectors themselves. Although the alignment of the muons detectors at the barrel and the end-caps regions is well controlled, the relative alignment of the end-caps with respect to barrel is not controlled with the same accuracy. In this project, we study the influence of such misalignments (translations and rotations) on the search of the Higgs particle (width and discovery potential), which decays through the channel mentioned above. The first part of the project refers to the Physics of the ATLAS detector, to its specific construction features and finally to the alignment scheme of the muon detectors at the barrel and forward region. In the second part, there is a detailed reference to the data acquisition and analysis, using the muon spectrometer alone. The method, the mathematical formulation and the C++ programming application concerning the fitting of the muons’ tracks under the constraint of the parent’s Z invariant mass (Constraint Fitting) are presented. This method results in diminishing the reconstructed Higgs width, therefore in improvement of the Signal/Background ratio. Finally, the applied misalignments (translations and rotations) of the one end-cap with respect to the barrel are presented and the effects on the determination of the mass and the width of the Higgs and Z particles are studied. At the conclusions, we mention the usage of the well-known Z width in order to isolate the kind and the amount of the probable misalignments.
  2. Detection of cosmic ray muons with 4×4 MDT chamber and track reconstruction using LabView (BSc Thesis)
    • The project refers to the participation of the Department of Physics (at the Aristotle University of Thessaloniki) at the construction of the Muon Spectrometer for the ATLAS Experiment (LHC – CERN). I begin with a brief introduction concerning the drift chambers and their principles, allowing the detection and, therefore the study, of the particles’ tracks. I describe in details the setup for data acquisition (hodoscope and electronics) and I produce the delay curve for the hodoscope, thus providing the triggering. The time spectrum of a Monitored Drift Tube (MDT) is given. Finally, I present the software (developed using the LabView software tool) used for the track reconstruction, from the cosmic-rays hits given by the 4×4 MDT chamber.
  3. Beam Phase Space and Beam Emittance 
    • Initially, there is a review of the already known terms (from the classical, hamiltonian mechanics) of the canonical phase space and the Liouville’s theorem. The definition of the beam emittance of accelerating particles is then given. I also deal with the phase portraits, as a way for the geometrical description of the motion. Finally, I conclude with the establishment of the emittance using this time the statistical method.
  4. The Quantum Mechanics of the Photoelectric Effect
    • In this report, a detailed calculation of the differential cross section for the photoelectric effect is carried out (Fermi’s Golden Rule, matrix element, interaction hamiltonian….). I comment on the result and I plot the angular distribution of that differential cross section. In addition, I compare the total cross sections for several processes, as a function of the energy. Finally, a detailed reference to the mass absorption coefficient is done.
  5. 2-Dimensional track Reconstruction in uniform magnetic field
    • During this project, I studied the reconstruction of 2-dimensional tracks of charged particles moving through a vertical and homogeneous magnetic field. The study is done with Monte Carlo track generation. I studied how several parameters (magnetic field, momentum, etc) could affect the efficiency of the reconstruction, while I calculate the residuals for each detector layer’s. Finally, I approached the parameters of a real-life experiment and I discussed further developments that can be done.
  6. γ-Rays Bursts [Report and Presentation]
    • Initially, there is a detailed reference of the time spectra and of the topology of a gamma-ray burst (GRB). I refer to the optical emission of a GRB and I develop the several models ( thermonuclear model, pulsars etc ) that are used to describe the GRB’s. At the end, the event GRB990123 (time spectrum, light curve etc) is described as a characteristic example of such a burst.
  7. The EUSO experiment  [Report and Presentation]
    • The report refers to the basic ideas of the EUSO (Extreme Universe Space Observatory). I mention the physical processes that the detector is going to study (Čerenkov and fluorescence radiation) and how this is going to be implemented. Furthermore, I refer to the specific parts of the detector (main telescope, optical lenses, filters, focal surface), analysing the working principles of them. Finally, the EUSO observing efficiency (duty cycle, expected EUSO performance, comparison with ground based observations) is discussed in detail.
  8. Calibration of the hodoscope for the X5/GIF beam area
    • The setup and the calibration procedure of the hodoscope, which will provide the cosmic trigger, for the BIS MDT chamber (Beatrice) at X5/GIF are described. The hardware and electronics configuration (logic diagram), used for data taking, is presented. The tests and modifications to the original setup are given, and the improvements of the system are described. The analysis of cosmic data, calculation of the calibration constants of the scintillators, the time resolution of the system, and conclusions are presented.
  9. High Level Trigger Studies for the ATLS Detector
    • An overview of the studies that have been done, during my first PhD year, on the High Level Trigger for the ATLAS Detector is presented. The work consists of the performance studies on the IDScan tracking algorithm for the Level-2, the e/γ vertical slice for triggering physics events in which electrons and photons are involved and finally, the very preliminary effort for optimising the size of the Region of Interest, which plays a crucial role, as explained in the main text. My future plans for the further performance and physics studies are given at the end of this report.
  10. The ATLAS Level-2 Trigger (LVL2)
    • The LVL2 trigger provides the next stage of event selection after the hardware-based LVL1 trigger. It uses the Region of Interest (RoI) guidance received from LVL1 to seed the validation and enhancement of the LVL1 trigger using selected full granularity event data. The LVL2 trigger consists of three stages: (i) The selection of the E/M clusters using the full calorimeter granularity, (ii) The track searching in the Inner Detector (ID) and (iii) The decision making based on the cuts on the reconstructed parameters.


My University Teaching

  1. University of Athens – Physics Department


My Presentations in Conferences & Seminars

  1. Hellenic High Energy Physics Workshop. [Heraklion, April, 07th 2001]
  2. Seminar at Ludwig-Maximilians University (LMU). [Munich, March, 07th 2003]
  3. Seminar at University College London (UCL). [London, June, 11th 2004]
  4. 2005 Workshop on Recent Advances in Particle Physics and Cosmology organised by the Hellenic Society for the Study of High Energy Physics [Thessaloniki, April, 21st – 24th 2005]
  5. Particle Physics 2006 organised by the High Energy Particle Physics group of the Institute of Physics. [Warwick, 10 – 12th April 2006]
  6. Physics at LHC 2006 [Crakow, 3 – 8th July 2006]
  7. Seminar at Petris Technologies Ltd  [London, 1st May 2007]
  8. Public Seminar in Physics [Kilkis, 11th April 2009]
  9. The Telegraph – Festival of Education  [Wellington College, 23rd June 2016]


My Presentations in Collaboration Meetings

  1. Physics and Event Selection Algorithms (PESA) – E/G Algorithms.
  2. UCL – ATLAS group meetings


My Publications

  1. Study of WW scattering in the absence of a light Higgs boson using the ATLAS Detector at the LHC. Stefanidis S, PhD Thesis, University College London.[CERN-THESIS-2003-063]
  2. Study of the effect of the misalignment of the muons’ detectors at the ATLAS experiment, on the discovery potential of the Higgs particle H→4μ. Stefanidis S., MSc Thesis, High Energy Physics Library (University of Athens). [Ref. No: 2906/4-12-03].
  3. Detection of cosmic ray muons with 4×4 MDT chamber and track reconstruction using LabView. S. Stefanidis, BSc Thesis, Physics Dept. Library (Aristotle University of Thessaloniki) [Ref. No: D463].
  4. Electroweak Symmetry Breaking without the Higgs Boson. Stefanidis S. Proceedings of the Conference ‘Physics at LHC’. [Acta Physica Polonica B].
  5. Optimisation of the size of the EmTau Region of Interest for the ATLAS Level-2 Trigger using the Electromagnetic Calorimeter. Konstantinidis N.P, Stefanidis S, Sutton M.R. [CERN-ATL-COM-DAQ-2005-038].
  6. A new compact MDT wire tension meter. D. Fassouliotis, P. Ioannou, C. Kourkoumelis, S. Stefanidis, A. Kulemzine. [ATL-MUON-2001-003].
  7. Improvements/Experience derived from wiring 10% of BIS MDTs. D. Fassouliotis, P. Ioannou, C. Kourkoumelis, V. Pancheluga, T. Papas, S. Stefanidis, V. Birioukov. [ATL-MUON-2001-005].
  8. Influence of the alignment of the muon chambers to SM Higgs reconstruction H→4μ. D. Fassouliotis, C. Kourkoumelis, S. Stefanidis, D. Levin. [ATL-COM-PHYS-2003-041].