July 2, 23 Master Thesis Topics 23/4 in Experimental Particle Physics at LS Schaile
At LS Schaile we offer a broad spectrum of master thesis topics in experimental elementary particle physics. Contacts: If you have questions of organisational nature, or if you would like to have some advice which master thesis topic would be most suitable for you please contact Prof. Dr. Otmar Biebel (Otmar.Biebel@physik.uni-muenchen.de) Prof. Dr. Dorothee Schaile (Dorothee.Schaile@physik.uni-muenchen.de) If you have already spotted your favorite topic or you have questions concerning a specific topic you can also address directly the contact person indicated for each topic. Search for the Higgs Boson Figure : The local probability p for a background-only experiment to be more signal-like than the observation as a function of m H for the combination of all channels searched in. The dashed curve shows the expected p under the hypothesis of a Standard Model Higgs boson production at that mass. The horizontal dashed lines indicate the p-values corresponding to significances of σ to σ. The Standard Model of particle physics is very successful in describing interactions between the elementary building blocks of matter. One final essential ingredient of the model has been recently observed: the Higgs boson. The Higgs field and the associated Higgs boson cause the gauge-symmetry of the standard model to be spontaneously broken, giving rise to massive gauge bosons and fermions. Before the discovery the mass of the Higgs boson was constrained by precision measurements of the parameters of the Standard Model, and also by direct searches in collider experiments. From these constraints, the Higgs boson was expected to be light: lighter than the top quark but more massive than the Z boson. Recent data from the ATLAS and CMS experiments at CERN have shown the existence of the Higgs boson with a mass around 25 GeV.
At this mass the dominant Higgs boson decay is into pairs of bottom quarks. Also decays into W boson pairs are a very important channel. The experimental particle physics group at LMU is contributing to measurements of and searches for the Higgs boson in these and other channels. Possible topics for a master thesis are: Search for the Higgs boson in the ZH llbb channel. No significant signal of the H bb decay has been observed yet. Since the decay is directly dependent on the coupling of the Higgs boson to fermions, its observation will confirm the Standard Model predictions for the Higgs boson. The search involves determining selection criteria based on simulated Higgs boson events, and evaluating and reducing backgrounds to Higgs boson production. Available ATLAS data can be used, as well as simulation for the upcoming high energy run of the LHC ( s = 3/4 TeV). Contact: Dr. Michiel Sanders (Michiel.Sanders@physik.uni-muenchen.de) Search for the Higgs boson in the W H lνbb channel. This search is similar to the search in the ZH channel, but the non-higgs-boson backgrounds are rather different, and more advanced methods are needed to reduce and understand these backgrounds. Contact: Dr. Michiel Sanders (Michiel.Sanders@physik.uni-muenchen.de) At LHC the dominant production mechanisms over the full mass range of the Standard Model Higgs boson are gluon-gluon and vector-boson fusion. A very important decay channel are the decays into the heavy gauge bosons of the electroweak interaction, i.e. W - or Z-boson pairs. The master thesis will search for methods to discriminate this Higgs signature against the dominant background arising from different backgrounds and study the production mechanisms and Spin/CP properties of the new boson with LHC data. Contact: PD Dr. Johannes Elmsheuser (Johannes.Elmsheuser@physik.uni-muenchen.de) In the Minimal Supersymmetric Model (MSSM) the decay rate of the neutral Higgs bosons A, H or h into two muons can be significantly higher than for Standard Model Higgs decays for certain regions of parameter space. As the ATLAS detector has excellent muon identification and reconstruction capabilities, this decay channel has a promising discovery potential. Signal selection and background reduction are very similar as in the H W W ( ) decay. The master thesis will investigate how this analysis can be used to set limits for MSSM Higgs bosons and apply the procedure developed to LHC data and simulation data for the planned s = 3/4 TeV run. Contact: PD Dr. Johannes Elmsheuser (Johannes.Elmsheuser@physik.uni-muenchen.de) 2 Physics of the Top Quark and Quantum-Chromo-Dynamics (QCD) The top quark and QuantumChromoDynamics (QCD) are two parts of the standard model of particle physics which deserve detailed studies using the ATLAS experiment at LHC. Since LHC collides protons, QCD is the main interaction taking place between the constituents of the colliding particles. Understanding these processes is mandatory before any noticed deviation can be claimed a signal of a new physics phenomena. The main object of research are jets of many particles originating from a highly energetic quark or gluon produced from a hard scattering process. 2
Figure 2: Semileptonic decay of a tt-pair. The top quark is the heaviest known particle, whose properties are not fully investigated yet. Proton proton collisions at LHC are abundant with top quarks which allows to measure its properties with high precision. Also top quarks and additional jets are produced in the decay of a heavier quark from a hypothetical fourth generation. Our research work is currently directed to: Top quark properties: production cross section, mass, decay modes and branching ratios, topantitop quark spin correlation QCD multijet production, strong coupling constant, soft underlying event, multiparton interactions Topics for MSc theses in one of these areas involve the analysis of measured data using existing software tools, the selection of relevant signal events out of many competing processes, the comparison of the measured results to expectations obtained from simulation studies, the interpretation of the final results in terms of the observed quantity and the systematic uncertainties of the measurement. The actual work will require some software development in the framework of the existing analysis software tools using C++ Contact: Prof. Dr. Otmar Biebel (Otmar.Biebel@physik.uni-muenchen.de) 3 Search for Supersymmetry Supersymmetry (SUSY) is a well motivated extension of the Standard Model. Each fermionic degree of freedom is complemented by a bosonic degree of freedom and vice versa. As the supersymmetric partners of the known Standard Model particles have not been observed yet, it is assumed that supersymmetry is a broken symmetry and therefore these partners have different masses. There are strong arguments, however, that the masses of the supersymmetric partners should not be too heavy and in reach of LHC. SUSY also offers a plausible explanation for Dark Matter in the universe. 3
[GeV] m χ 4 35 3 25 2 5 ATLAS Preliminary χ ± χ τ (τν) τ νν) τνχ ττ(νν) χ L ν L τ( 2 m τ, ν m + m =.5 ± χ, χ χ 2 m χ ± < m χ SR combined - L dt = 2.7 fb, s=8 TeV SUSY Observed limit (± σ ) theory Expected limit (± σ exp ) All limits at 95% CL [GeV] m /2 MSUGRA/CMSSM: tan( β) = 3, A = -2m, µ > Lepton & Photon 23 SUSY τ 95% CL limits. σtheory not included. LSP ATLAS Preliminary Expected -lepton, 2-6 jets 9 - Observed ATLAS-CONF-23-47 L dt = 2. - 2.7 fb, s = 8 TeV Expected -lepton, 7- jets Observed ATLAS-CONF-23-54 Expected - lepton, 3 b-jets 8 Observed ATLAS-CONF-23-6 Expected -lepton + jets + MET Observed ATLAS-CONF-23-62 Expected -2 taus + jets + MET 7 Observed ATLAS-CONF-23-26 Expected 2-SS-leptons, - 3 b-jets Observed ATLAS-CONF-23-7 6 5 g (4 GeV) 4 g ( GeV) 5 5 2 25 3 35 4 45 5 m [GeV] ± χ, χ 2 3 q (6 GeV) q (2 GeV) 2 3 4 5 6 m [GeV] Figure 3: Expected and observed 95% CL exclusion limits in a simplified model with chargino-neutralino production as a function of the chargino and neutralino masses (left), and in the msugra parameter space defined by the scalar and fermion mass parameters (right). Finding the new particles predicted by supersymmetry is one of the major goals of the ATLAS experiment. The easiest way to produce SUSY particles at the LHC is through strong production. However, if coloured super-particles are heavy, as recent limits from ATLAS and CMS, and the Higgs boson search results seem to indicate, weak production may play a major role in the discovery of SUSY. Topics for master theses could involve the analysis of data collected during LHC Run (5 fb in 2 and 2 fb in 22) or focus on upgrade studies for LHC Run 2 (starting in 25). The LMU group is involved in the following areas, where topics for master theses can be offered: Strong production. We focus on the analysis of events with several jets, missing transverse energy and lepton (electron or muon) in the final state. Such analyses are sensitive to gravity-mediated SUSYbreaking models (msugra). A possible topic for a master thesis is to extend the sensitivity of the current analysis to the more general phenomenological Minimal Supersymmetric Standard Model (pmssm). Another domain of interest is the development of an advanced trigger strategy due to the increased luminosity at the LHC. The trigger is a crucial part of the experiment since at this stage the collision events are either recorded for further analysis or rejected. Possible topics include the feasibility of new trigger algorithms, their optimization on SUSY models of interest, and measurement of trigger rates and efficiencies with LHC data. Weak production. Direct production of charginos, neutralinos and sleptons: the detector signature is missing transverse energy and more than one lepton (electron, muon or tau) in the final state. Such searches are sensitive to the Minimal Supersymmetric Standard Model (MSSM). Potential topics for a master thesis include the optimization of the discovery potential and extension of the current analysis strategy to include final states where one or more leptons are tau leptons, the study of the Standard Model backgrounds producing fake leptons coming from misidentified jets or leptons from heavy-flavour decays, and trigger studies that allow us to find ways to cope with the higher instantaneous luminosity at LHC Run 2. Further topics include searches for the direct production of stops, the supersymmetric partner of the top quark, and large-scale scans of pmssm models to spot particular signatures that are not covered by the current searches. 4
Contact: Dr. Federica Legger (Federica.Legger@physik.uni-muenchen.de), Dr. Alexander Mann (Alexander.Mann@phy muenchen.de), Jeanette Lorenz (Jeanette.Lorenz@physik.uni-muenchen.de) 4 Detector-R&D and Particle Identification Figure 4: Irradiation measurements at the MLL Tandem Accelerator. The extreme radiation background conditions at LHC require the development of new and radiation hard particle detectors which shall replace existing and less radiation tolerant detector technologies in the ATLAS detector. Our research is focused on: Micro pattern gaseous particle detectors like MICROmesh GAseous Structures (Micromegas) or Gaseous Electron Multiplier (GEM); such detectors shall have a spatial resolution of 2 to micron and cover an active area of square meters. Szintillating detectors with Silicon Photomultiplier (SiPM) readout; the goal is to achieve spatial resolution in two dimensions by reading out two optically separated trapezoidal szintillators using SiPMs and wavelength shifting fibers. Topics for MSc theses are directly connected to these areas of detector research & development. A thesis involves both hardware and software related work. This includes typically the conceptual planning of an experiment, setting-up the experimental apparatus, performing the measurements and, finally, analyzing and interpreting the results from the measurement. Contact: Prof. Dr. Otmar Biebel (Otmar.Biebel@physik.uni-muenchen.de), Dr. Ralf Hertenberger (Ralf.Hertenberger@physik.uni-muenchen.de) 5