Master student Projects

Projects for Master students in the Nikhef ATLAS group

date: August 2013

This is an overview with all available Master student projects in the Nikhef ATLAS group.

If you have your own research proposal, need more detailed information on the (availability) of individual proposals or would like to discuss about other available projects in the group you are always welcome to contact either the contact person for the project and/or the Nikhef ATLAS group leaders:

Stan Bentvelsen ___ [ E-mail: stanb_at_nikhef.nl, Tel 020-5925140, Nikhef room H250]

Paul de Jong ______ [ E-mail: h26_at_nikhef.nl, Tel 020-5922087, Nikhef room H253]

For an overview of the theses written in the Nikhef ATLAS group you can look at the Nikhef ATLAS group theses page

Master projects in the Nikhef ATLAS group

1) The polarisation of single top quarks at the LHC

Supervisors: Marcel Vreeswijk (staf) and Rogier van der Geer (PhD student)

Research description:

Single top quark production is a rare process which is only in reach of the Tevatron-collider and the Large Hadron Collider since the last decade.  The cross section for t-channel top quark production at the  LHC allows precision measurements of the spin structure of the Wtb vertex as a probe for new physics.  For this research the data recorded with the ATLAS detector at 8TeV will be used. The observables consist of the decay angles of the top quark from which  (anomalous) couplings can be extracted. Several contributions to the research are possible, which involve: -selection of events and reconstruction of the observables; -unfolding techniques  from measured distribution to the underlying truth distribution; -which quantities can be extracted from the distribution and how do these translate to general couplings?

For this project you need to have affinity with quantum mechanics and computer skills are needed. The ROOT programme and  C++ and/or Python macros are used.   You become part of our research group (~5 persons); we have weekly meetings with colleagues at CERN and depending on the results  -and perhaps some luck-  you may co-author an (internal ATLAS) research note.  The project will be soon at full pace, requiring you to be flexible.

2) Searching for a supersymmetric partner of the top quark

Supervisors: Paul de Jong (staf), Carolina Deluca (post-doc) and Pierfrancesco Butti (PhD student)

Research description:

The goal of this project is to search for a partner of the heaviest quark in the Standard Model, the top quark, as predicted by various theories of new physics beyond the Standard Model. A supersymmetric top quark partner in particular is well motivated: it could be relatively light, i.e. accessible at the LHC, and solve some of the problems of the Standard Model such as the stability of the Higgs mass. Searching for a supersymmetric partner of the top quark at the LHC is difficult: Standard Model top quark production processes form a significant background. In this project we will try to attack this with advanced multi-variate analysis techniques. Eventually, the goal is to examine all ATLAS data collected at 8 TeV for a possible signal.

3) Beyond Standard Model search in multi lepton final state

Supervisors: Olya Igonkina (staf), Pier-Olivier DeViveiros (post-doc) and Joern Mahlstedt (PhD student)

Research description:

The Standard Model of particle physics (SM), though extremely successful, provides no insight on the mass structure of the quark and lepton families. Compositeness models - models in which leptons and quarks are non-fundamental particles made of 'preons' - can be used to explain these features. In such models, the 3 SM lepton families are the lowest energy bound-states of these 'preons'. Interestingly, such models imply the presence of higher energy excited states, which can be searched for at the LHC.

The goal of this project is to devise a search for these at ATLAS, in events where an excited lepton is produced along with a SM lepton. The excited lepton decay can have up to 3 SM lepton, giving a rise to 4 lepton topology. Such a signature could significantly improve the current limits on excited lepton production. The main focus of the project will be to determine which event observables are most sensitive to these signals by studying the various signals and the SM backgrounds. With this project, the student would gain close familiarity with Monte Carlo generators, the standard HEP analysis tools (ROOT, C++, etc.) and with modern experimental techniques (statistical analysis, SM background estimates, etc.).

4) New Physics: search for lepton flavor violating decays tau -> mu gamma

Supervisors: Olya Igonkina (staf), Saminder Dhaliwal (post-doc) and Ivan Angelozzi (PhD student)

Research description:

The lepton flavor violation is a mechanism which is forbidden by Standard Model and is not observed so far in experiment. However, it could explain a large amount of matter (and lack of antimatter) found in the Universe. Such mechanism could manifest itself in decays of tau lepton into a muon and a photon. There are a lot of taus produced in ATLAS, but momenta are rather small, which make the search a challenging and interesting task. The Standard model W->mu nu gamma decays are major background to the search and have to be studied with data, e.g. using the similarity to Z->mu mu gamma data.

All steps of this measurement (analysis of ATLAS data, tuning cuts on MC, understanding the background and trigger performance) are part of the master project. The programing of the code, the data analysis with ROOT and ATLAS software, work with grid tools will be everyday tasks.

Relevant papers:

Review of various lepton flavor violating processes: arXiv:1201.5093

Plans of competing experiments Belle-2 and Super-B : arXiv:1109.2377

Relation between leptogenesis and lepton flavor violation : arXiv:0904.1182

5) Higgs: CP mixing and CP violation in the Higgs sector

Supervisors: Pamela Ferrari, Stan Bentvelsen (staf), Nikos Karasthatis (Koen Oussoren) (PhD student).

Research description:

After having discovered the Higgs, the LHC experiments have determined its spin, providing a proof of the spin 0 nature of the Higgs boson and indicating that a positive parity is strongly preferred. Anyhow, even if it is excluded that the present Higgs boson might be a pseudoscalar, the possibility that is an admixture of a CP-even and CP-odd state is far from being ruled out. From present studies few hundred fb−1 at a centre of mass energy of 14 TeV might be needed to study it or narrow down significantly this possibility. The presence of CP violation in the Higgs sector could provide an answer to one of the most important open questions of particle physics nowadays: the reason for the matter antimatter asymmetry in the universe, which cannot be explained by the amount of CP violation observed in the quark sector of the Standard Model. The project will investigate the precision with which result on CP violation will be obtained using mainly the Higgs to WW decay channel.

6) Higgs: WW to WW boson scattering at the LHC

Supervisors: Stan Bentvelsen, Pamela Ferrari (staf), and Rosemarie Aben (PhD student)

Research description:

Understanding the scattering of two W-bosons (WW->WW) is essential in the Standard Model. Calculations show that, in the absence of a Higgs particle, this process grows with the cm energy of the W-bosons, and ultimately become larger than unity. This is unphysical and is one of the main motivations for including the Higgs particle in the model. Now the Higgs particle has been found, it remains to be seen if the particle is responsible for restoring unitarity.

The measurement of this process at ATLAS may need a few more years of data taking. But its interesting to see if the current amount of data (including the whole of 2012) reaches sensitivity to this process.

This project aims at the observation of the scattering of two W-bosons - in the so-called 'vector boson fusion' process. One of the first goals is to isolate events where the two W-bosons produce one Z-boson (WW->Z), which subsequent decay is measured in the ATLAS detector. In addition the study includes the effect if a Higgs particle in the process. Ultimately we have to see what is needed to isolate the WW->WW process in data.

7) Higgs Physics: Is the observed Higgs-like particle at 125 GeV fundamental or composite?

Supervisors: Peter Kluit and Ivo van Vulpen (staf), and Antonio Castelli (PhD student)

Research description:

Now that a Higgs-like particle has been observed in several final states [1] it is important to test experimentally whether it is composite. In the Standard Model the Higgs is elementary. The test can be done by looking in the final state H (composite) -> H (observed) + gamma.

For the analysis the clean four-lepton final state of the H (observed) will be used: H -> Z Z^* -> 4 l. By combining the four-lepton candidates with a photon a search for a resonant composite particle - or excitation of the H (observed) ground state - can be performed by looking for a peak in the invariant mass spectrum. The full data taken from 2010 to 2012 with about 30 observed signal events [2] will be used for this search. No ATLAS or CMS result exists for this topic.

Literature:

1: ATLAS Collaboration, Observation of a New Particle in the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC, Phys. Lett. B 716 (2012) 1-29,arXiv:1207.7214

2: ATLAS Collaboration, Measurements of the properties of the Higgs-like boson in the four lepton decay channel with the ATLAS detector using 25 fb−1 of proton-proton collision data, http://cds.cern.ch/record/1523699

8) Astroparticle physics at the LHC – from the caverns of CERN to the top of the atmosphere

Supervisors: David Berge (staff)

Research description:

Understanding particle acceleration up to very high energies in the Universe requires Earth-bound experimental techniques that exploit the Earth’s atmosphere as detection medium. Only the shear size of the atmosphere provides a sufficiently large sensitive area to measure the very rare highest energy particles from the cosmos as they impinge on the Earth. The idea of the atmospheric measurement is simple: a cosmic particle hitting the atmosphere is being absorbed by developing into an air shower, a spray of secondary particles that originates in the collision of the primary cosmic particle with air molecules, and successive interactions of those secondary particles in the atmosphere. Such air showers can be traced and therefore measured on Earth, providing information about the energy, type, and direction of the primary cosmic particle, by different means. Important examples of such atmospheric detection techniques include the measurement of muons with particle counters at the Earth’s surface and the measurement of Cherenkov or Fluorescence light emitted during the air shower development. The connection between measured quantities like particle numbers or light intensity and original quantities like particle energy or type is in all cases inferred using simulations of particle collisions and cascades in the atmosphere.

The goal of this master project is to exploit data of proton collisions measured with ATLAS, an experiment at the Large Hadron Collider (LHC), the highest energy human particle collider currently operating at CERN in Geneva (Switzerland), to test and improve simulations of particle collisions in the atmosphere up to the highest known energies (a few times 1020 eV). The student will work on ATLAS data analysis and Monte Carlo simulations of particle collisions, both for simulating proton colliding in ATLAS and cosmic-ray protons colliding with air molecules in the atmosphere. The ultimate goal is to improve Monte Carlo model predictions used for experiments like the upcoming CTA and Auger.

9) Astroparticle physics at the LHC – going after the Dark in ATLAS

Supervisors: David Berge (staff)

Research description:

It is currently believed that most of the matter in the Universe is a new species of so-called dark matter. This new form of matter dominates over all the known forms of matter. If the dark matter of the universe is a new particle that can be produced in proton-proton collisions at CERN's Large Hadron Collider (LHC), this new particle could couple to the recently discovered Higgs boson and could be searched for at the LHC in Higgs decays. Since a dark matter particle must be very weakly interacting it is expected that such a particle would leave the LHC detectors unseen. Looking for such invisible Higgs decays to undetectable particles is an important search for both new physics beyond the Standard Model of particle physics and dark matter. The master student will work on a search for new physics at the ATLAS experiment looking for signatures of invisibly decaying Higgs bosons, in particular in the context of dark matter. She or he will learn how to analyse high-energy particle-physics data and will focus on analysis optimisations and inter-disciplinary comparisons of the ATLAS dark matter search to direct and indirect dark matter searches on Earth and in space.

10) Detector R&D: the ATLAS inner tracker upgrade

Supervisors: Nigel Hessey (staff)

Research description:

CERN plans to upgrade the LHC to provide a significantly higher luminosity. This will affect the ATLAS detector, and ATLAS is designing a new inner tracker in order to cope with the significant challenges associated to the high-luminosity LHC. The Nikhef ATLAS group is working on the detector layout, and designing a new endcap silicon strip detector. We have projects in silicon sensor development, CO2 cooling, Helium gas cooling, and carbon fibre composites.