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Search for Weakly Produced Supersymmetric Particles in the ATLAS Experiment
Stockholm University, Faculty of Science, Department of Physics.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Large Hadron Collider located at CERN is currently the most powerful particle accelerator and ATLAS is an experiment designed to exploit the high energy proton-proton collisions provided by the LHC. It opens a unique window to search for new physics at very high energy, such as supersymmetry, a postulated symmetry between fermions and bosons.

Supersymmetry can provide a solution to the hierarchy problem and a candidate for Dark Matter. It also predicts the existence of new particles with masses around 1 TeV, thus reachable with the LHC. This thesis presents a new search for supersymmetry in a previously unexplored search channel, namely the production of charginos and neutralinos directly decaying to electroweak on-shell gauge bosons, with two leptons, jets, and missing transverse momentum in the final state. The search is performed with proton-proton collision data at a center of mass energy of √s = 8 TeV recorded with the ATLAS experiment in 2012. The design of a signal region sensitive to the new signal is presented and a data driven technique to estimate the Z+jets background is developed.

Precise measurements of hadronic jet energies are crucial to search for new physics with ATLAS. A precise energy measurement of hadronic jets requires detailed knowledge of the pulse-shapes from the hadron calorimeter signals. Performance of the ATLAS Tile Calorimeter in this respect is presented using both pion test-beams and proton–proton collision data.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2014. , 124 p.
Keyword [en]
ATLAS, ATLAS experiment, supersymmetry, chargino, neutralino, physics analysis, LHC, Large Hadron Collider
National Category
Subatomic Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-108060ISBN: 978-91-7447-992-8OAI: oai:DiVA.org:su-108060DiVA: diva2:753514
Public defence
2014-11-14, sal FA32, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2 and Paper 4: Technical report from the  ATLAS experiment.

Available from: 2014-10-23 Created: 2014-10-08 Last updated: 2015-09-23Bibliographically approved
List of papers
1. Pulse shapes for signal reconstruction in the ATLAS Tile Calorimeter
Open this publication in new window or tab >>Pulse shapes for signal reconstruction in the ATLAS Tile Calorimeter
2010 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, Vol. 617, no 03-jan, 96-98 p.Article in journal (Refereed) Published
Abstract [en]

The ATLAS detector will record proton-proton collisions produced in the Large Hadron Collider at center of mass energies of up to 14 TeV. Its design allows for precision measurements as well as searches for new physics. Energy measurement using the calorimeters is crucial to these efforts. Studies of the pulse shapes used for signal reconstruction in the Tile Calorimeter are presented. The shapes were measured using test beam data and found to vary slightly with signal size. The impact of this variation on the reconstructed energy has been determined using a toy Monte Carlo.

Keyword
ATLAS, Calorimeter, Test beam, Pulse shape
National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-68183 (URN)10.1016/j.nima.2009.09.114 (DOI)000279258700026 ()
Note
authorCount :2Available from: 2012-01-03 Created: 2012-01-03 Last updated: 2014-10-08Bibliographically approved
2. Measurement of the ATLAS Tile Calorimeter pulse-shapes with √s = 7 TeV collision data
Open this publication in new window or tab >>Measurement of the ATLAS Tile Calorimeter pulse-shapes with √s = 7 TeV collision data
2010 (English)Report (Other academic)
Abstract [en]

The energy reconstruction of the ATLAS Tile Calorimeter requires a predefined pulse-shape function. This pulse-shape function must match the pulse-shapes in data to avoid bias in the reconstructed energy. Using 60 nb-1 of √s = 7 TeV collision data, this note investigates deviations between measured pulse-shapes and the pulse-shape used for reconstruction and the possible bias on the energy reconstruction are studied. Measured pulse-shape widths exhibit slight channel-by-channel variations of the order ±1% with respect to the width of the reference pulse-shape. This implies less than 0.5% bias on the reconstructed energy. No energy dependence of the pulse-shapes is found.

22 p.
National Category
Subatomic Physics
Identifiers
urn:nbn:se:su:diva-108041 (URN)
Note

Technical report from the  ATLAS experiment

Available from: 2014-10-07 Created: 2014-10-07 Last updated: 2014-10-09Bibliographically approved
3. Search for direct production of charginos, neutralinos and sleptons in final states with two leptons and missing transverse momentum in pp collisions at root s=8TeV with the ATLAS detector
Open this publication in new window or tab >>Search for direct production of charginos, neutralinos and sleptons in final states with two leptons and missing transverse momentum in pp collisions at root s=8TeV with the ATLAS detector
Show others...
2014 (English)In: Journal of High Energy Physics (JHEP), ISSN 1029-8479, E-ISSN 1126-6708, no 5, 1-52 p.Article in journal (Refereed) Published
Abstract [en]

Searches for the electroweak production of charginos, neutralinos and sleptons in final states characterized by the presence of two leptons (electrons and muons) and missing transverse momentum are performed using 20.3 fb(-1) of proton-proton collision data at root s = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. Limits are set on the masses of the lightest chargino, next-to-lightest neutralino and sleptons for different lightest-neutralino mass hypotheses in simplified models. Results are also interpreted in various scenarios of the phenomenological Minimal Supersymmetric Standard Model.

Keyword
Supersymmetry, Hadron-Hadron Scattering
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-105872 (URN)10.1007/JHEP05(2014)071 (DOI)000336654400001 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

AuthorCount:2886;

Available from: 2014-07-08 Created: 2014-07-07 Last updated: 2015-09-02Bibliographically approved
4. Fake Isolated Muon Background in Searches for Supersymmetry with two Muons in the Final State
Open this publication in new window or tab >>Fake Isolated Muon Background in Searches for Supersymmetry with two Muons in the Final State
Show others...
2010 (English)Report (Other academic)
Abstract [en]

We study a data-driven technique to derive the contamination from fake isolated muons in a search for supersymmetric (SUSY) particles, in final states with two high pT muons. The signal sample is characterized by jets, missing transverse energy and two high pT muons, isolated from calorimeter energy. In real data, a fraction of the selected events will arise from processes where at least one of the isolated muons is not truly isolated, but rather arises in conjunction with a jet. We illustrate how to determine the contribution from fake isolated muons, to the signal sample. The method is demonstrated with a simple SUSY analysis of the di-muon sample. We study a range of potential systematic effects arising from the extrapolation from the control sample to the signal region.

34 p.
National Category
Subatomic Physics
Identifiers
urn:nbn:se:su:diva-108052 (URN)
Note

Technical report from the  ATLAS experiment

Available from: 2014-10-07 Created: 2014-10-07 Last updated: 2014-10-09Bibliographically approved

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