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Search for Charginos and Sleptons in ATLAS and Identification of Pile-up with the Tile Calorimeter
Stockholm University, Faculty of Science, Department of Physics. (Elementary Particle Physics)ORCID iD: 0000-0003-1661-6873
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The standard model of particle physics (SM) describes the elementary particles and their interactions. Supersymmetry (SUSY), a symmetry beyond those included in the standard model could resolve some of the SM shortcomings. It can provide a candidate for Dark Matter and a solution to the hierarchy problem. The Large Hadron Collider (LHC) has the potential to produce the particles predicted by SUSY. This thesis presents two searches for SUSY particles in proton-proton collision data recorded by the ATLAS experiment.

The first search described in this thesis looks for direct production of chargino and slepton pairs in a final state characterized by the presence of two leptons and missing transverse energy. The second search looks for production of chargino pairs via vector boson fusion (VBF) in a final state containing of two leptons, two jets and missing transverse energy. This is the first attempt in ATLAS to search for supersymmetric particles produced via VBF. A possible observation of such process would prove that the exchanged neutralino is a Majorana particle. These analyses are done using L=20.3 fb-1 proton-proton collisions at √s=8 TeV collected in 2012. No significant excess over background is observed. New exclusion limits at 95% confidence level on chargino, neutralino and slepton masses and cross section for chargino pair production via VBF are set.

The energy measurements of the particles created in LHC collisions are performed by the ATLAS calorimeters. Energy deposits from different collisions in the same read-out window and in the same calorimeter channel (pile-up) can spoil the energy measurements by the calorimeter. It is shown that the quality factor computed offline for each collision and for each channel in the Tile Calorimeter (TileCal) can be used to identify channels that need a special treatment to account for large energy depositions from pile-up. Efficient criteria to detect pile-up in TileCal are proposed.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2015. , 193 p.
Keyword [en]
ATLAS experiment, particle physics, high energy physics, supersymmetry, SUSY, beyond standard model, chargino, neutralino, slepton, vector boson fusion, VBF, Tile calorimeter, TileCal, pile-up, quality factor, Large Hadron Collider, LHC, CERN
National Category
Subatomic Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-120018ISBN: 978-91-7649-240-6 (print)OAI: oai:DiVA.org:su-120018DiVA: diva2:850106
Public defence
2015-10-08, lecture room FB52, AlbaNova University Center, Roslagstullsbacken 21, Stockholm, 14:00 (English)
Opponent
Supervisors
Available from: 2015-09-16 Created: 2015-08-31 Last updated: 2015-09-16Bibliographically approved
List of papers
1. Identification of Pile-up Using the Quality Factor of Pulse Shapes in the ATLAS Tile Calorimeter
Open this publication in new window or tab >>Identification of Pile-up Using the Quality Factor of Pulse Shapes in the ATLAS Tile Calorimeter
2011 (English)In: IEEE Nuclear Science Symposium Conference Record, ISSN 1082-3654, 1188-1193 p.Article in journal (Refereed) Published
Abstract [en]

The ATLAS experiment records data from the proton-proton collisions produced by the Large Hadron Col­ lider (LHC). The Tile Calorimeter is the hadronic sampling calorimeter of ATLAS in the region |η|<1.7. It uses iron absorbers and scintillators as active material. The LHC will provide collisions every 25 ns, putting very strong requirements on the energy measurement in presence of energy deposits from different collisions in the same read out window and physical calorimeter channel (pile-up). In 2011 the LHC is running with filled bunches at 50 ns spacing and at intensities which yield up to about 8 proton-proton collisions per bunch crossing. We present a quality factor that can be computed online for each collision and for each calorimeter channel within the 10 μs latency of the ATLAS first level trigger (L1 trigger), and could allow to identify calorimeter channels presenting pile-up. In presence of a poor quality factor the data from the corresponding channel is read out with additional information to allow for an offline dedicated treatment of the signals to account for pile-up.

Keyword
Q-factor, energy measurement, nuclear electronics, particle beam bunching, particle calorimetry, position sensitive particle detectors, readout electronics, trigger circuits, ATLAS experiment, ATLAS first level trigger, ATLAS tile calorimeter, Large Hadron Collider, active material, bunch crossing, calorimeter channel, calorimeter channels energy deposits, energy measurement, hadronic sampling calorimeter, iron absorbers, physical calorimeter channel, proton+proton collisions, pulse shapes, quality factor, read out window
National Category
Subatomic Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-120008 (URN)10.1109/NSSMIC.2011.6154599 (DOI)
Conference
Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), Valencia, Spain, 23-29 October 2011
Available from: 2015-08-31 Created: 2015-08-31 Last updated: 2017-12-04Bibliographically approved
2. Identification of Pile-up Using the Quality Factor of Pulse Shapes in the ATLAS Tile Calorimeter
Open this publication in new window or tab >>Identification of Pile-up Using the Quality Factor of Pulse Shapes in the ATLAS Tile Calorimeter
2012 (English)Report (Other academic)
Abstract [en]

The ATLAS experiment records data from the proton-proton collisions produced by the Large Hadron Collider (LHC). The Tile Calorimeter is the hadronic sampling calorimeter of ATLAS in the region |η|<1.7. It uses iron absorbers and scintillators as active material. The LHC will provide collisions every 25 ns, putting very strong requirements on the energy measurement in presence of energy deposits from different collisions in the same read out window and physical calorimeter channel (pile-up). In 2011 the LHC was running with filled bunches at 50 ns spacing and at intensities which yield up to about 8 proton-proton collisions per bunch crossing. We present a quality factor that is computed offline for each collision and for each calorimeter channel, and provide criteria to detect pile-up in TileCal channels. The quality factor can be used to select channels that need a special treatment to account for large energy deposition from pile-up.

Publisher
25 p.
National Category
Subatomic Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-120016 (URN)
Note

Technical report from the ATLAS experiment

Available from: 2015-08-31 Created: 2015-08-31 Last updated: 2015-09-02Bibliographically 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 1126-6708, E-ISSN 1029-8479, 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: 2017-12-05Bibliographically approved

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