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Searches for exotic stable massive particles with the ATLAS experiment
Stockholm University, Faculty of Science, Department of Physics. (Elementarpartikelfysik)
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents two signature-based searches for exotic long-lived particles using experimental data collected by the ATLAS experiment at the Large Hadron Collider (LHC) at CERN. At the LHC, proton-proton collisions take place at the world-record center-of-mass energy of 7 TeV and luminosities of up to  3×1033 cm-2s-1 . New physics has long been anticipated at the LHC and in this thesis two searches for hitherto unobserved particles are presented.

Long-lived heavy hadrons have been predicted by models of supersymmetry, and these were the target of the first physics analysis. Using measurements of timing and energy loss, a search for so-called R-hadrons was performed. No excess was observed in the data, and new lower limits were established on the masses of meta-stable supersymmetric particles with color charge: mgluino > 562-586 GeV (depending on choice of scattering model), mstop > 309 GeV and msbottom > 294 GeV. These constraints are the most stringent limits published to date.

In addition, a ``blue-sky'' search was made for objects possessing high electric charge. Ionization energy loss measurements were combined with variables describing the shape of electromagnetic showers to separate the sought signal from background processes. No event in the experimental data passed the selection and the results are in agreement with a background-only hypothesis. Upper limits between 1-12 pb were computed for the production cross sections of particles with electric charge 6e ≤ |q| ≤ 17e and mass 200 ≤ m ≤ 1000 GeV. This is the first dedicated search for such particles at the LHC.

Finally, a system designed to monitor the LHC beams and timing signals based on beam pick-up detectors is described together with results from early LHC operation.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2011. , 185 p.
Keyword [en]
ATLAS, ATLAS experiment, supersymmetry, squark, gluino, stable massive particles, long-lived particles, physics analysis, search analysis, LHC, Large Hadron Collider
National Category
Subatomic Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-63068ISBN: 978-91-7447-341-4 (print)OAI: oai:DiVA.org:su-63068DiVA: diva2:446620
Public defence
2011-11-04, sal FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 14:00 (English)
Opponent
Supervisors
Available from: 2011-10-13 Created: 2011-10-07 Last updated: 2011-10-10Bibliographically approved
List of papers
1. The software of the ATLAS beam pick-up based LHC monitoring system
Open this publication in new window or tab >>The software of the ATLAS beam pick-up based LHC monitoring system
2010 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 219, no 1Article in journal (Refereed) Published
Abstract [en]

The ATLAS BPTX stations are comprised of electrostatic button pick-up detectors, located 175 m away along the beam pipe on both sides of ATLAS. The pick-ups are installed as a part of the LHC beam instrumentation and used by ATLAS for timing purposes. The signals from the ATLAS BPTX detectors are used both in the trigger system and for a stand-alone monitoring system for the LHC beams and timing signals. The monitoring software measures the phase between collisions and clock with high accuracy in order to guarantee a stable phase relationship for optimal signal sampling in the sub-detector front-end electronics. It also measures the properties of the individual bunches and the structure of the beams. In this paper, the BPTX monitoring software is described, its algorithms explained and a few example monitoring displays shown. In addition, results from the monitoring system during the first period of single beam running in September 2008 are presented.

Place, publisher, year, edition, pages
IOP Publishing: , 2010
National Category
Subatomic Physics
Identifiers
urn:nbn:se:su:diva-63067 (URN)10.1088/1742-6596/219/2/022040 (DOI)
Available from: 2011-10-07 Created: 2011-10-07 Last updated: 2017-12-08Bibliographically approved
2. The ATLAS beam pick-up based timing system
Open this publication in new window or tab >>The ATLAS beam pick-up based timing system
2010 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 623, no 1, 558-560 p.Article in journal (Refereed) Published
Abstract [en]

The ATLAS BPTX stations are composed of electrostatic button pick-up detectors, located 175 m away along the beam pipe on both sides of ATLAS. The pick-ups are installed as a part of the LHC beam instrumentation and used by ATLAS for timing purposes.

The usage of the BPTX signals in ATLAS is twofold: they are used both in the trigger system and for LHC beam monitoring. The BPTX signals are discriminated with a constant-fraction discriminator to provide a Level-1 trigger when a bunch passes through ATLAS. Furthermore, the BPTX detectors are used by a stand-alone monitoring system for the LHC bunches and timing signals. The BPTX monitoring system measures the phase between collisions and clock with a precision better than 100 ps in order to guarantee a stable phase relationship for optimal signal sampling in the sub-detector front-end electronics. In addition to monitoring this phase, the properties of the individual bunches are measured and the structure of the beams is determined.

On September 10, 2008, the first LHC beams reached the ATLAS experiment. During this period with beam, the ATLAS BPTX system was used extensively to time in the read-out of the sub-detectors. In this paper, we present the performance of the BPTX system and its measurements of the first LHC beams.

Place, publisher, year, edition, pages
Elsevier, 2010
Keyword
ATLAS, Beam monitoring, Level-1 trigger, BPTX, LHC, LHC timing signals
National Category
Subatomic Physics
Identifiers
urn:nbn:se:su:diva-63066 (URN)10.1016/j.nima.2010.03.069 (DOI)
Available from: 2011-10-07 Created: 2011-10-07 Last updated: 2017-12-08Bibliographically approved
3. Search for Stable Hadronising Squarks and Gluinos at the ATLAS Experiment at the LHC
Open this publication in new window or tab >>Search for Stable Hadronising Squarks and Gluinos at the ATLAS Experiment at the LHC
Show others...
2011 (English)In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 701, no 1, 1-19 p.Article in journal (Refereed) Published
Abstract [en]

Hitherto unobserved long-lived massive particles with electric and/or colour charge are predicted by a range of theories which extend the Standard Model. In this Letter a search is performed at the ATLAS experiment for slow-moving charged particles produced in proton–proton collisions at 7 TeV centre-of-mass energy at the LHC, using a data-set corresponding to an integrated luminosity of 34 pb−1. No deviations from Standard Model expectations are found. This result is interpreted in a framework of supersymmetry models in which coloured sparticles can hadronise into long-lived bound hadronic states, termed R-hadrons, and 95% CL limits are set on the production cross-sections of squarks and gluinos. The influence of R-hadron interactions in matter was studied using a number of different models, and lower mass limits for stable sbottoms and stops are found to be 294 and 309 GeV respectively. The lower mass limit for a stable gluino lies in the range from 562 to 586 GeV depending on the model assumed. Each of these constraints is the most stringent to date.

Keyword
Supersymmetry, Long-lived particle, R-hadron, Limit
National Category
Subatomic Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-63000 (URN)10.1016/j.physletb.2011.05.010 (DOI)000292719700001 ()
Projects
Atlas
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
Publikationen har totalt 3042 författare, G. Aad et al.Available from: 2011-10-06 Created: 2011-10-06 Last updated: 2017-12-08Bibliographically approved
4. Search for Massive Long-lived Highly Ionising Particles with the ATLAS detector at the LHC
Open this publication in new window or tab >>Search for Massive Long-lived Highly Ionising Particles with the ATLAS detector at the LHC
Show others...
2011 (English)In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 698, no 5, 353-370 p.Article in journal (Refereed) Published
Abstract [en]

A search is made for massive highly ionising particles with lifetimes in excess of 100 ns, with the ATLAS experiment at the Large Hadron Collider, using 3.1 pb−1 of pp collision data taken at . The signature of energy loss in the ATLAS inner detector and electromagnetic calorimeter is used. No such particles are found and limits on the production cross section for electric charges 6e⩽|q|⩽17e and masses 200 GeV⩽m⩽1000 GeV are set in the range 1–12 pb for different hypotheses on the production mechanism.

Keyword
High-energy collider experiment, Long-lived particle, Highly ionising, New physics
National Category
Subatomic Physics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-62973 (URN)10.1016/j.physletb.2011.03.033 (DOI)000290185500003 ()
Projects
Atlas
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
Publikationen har totalt 3039 författare, G. Aad et al.Available from: 2011-10-06 Created: 2011-10-06 Last updated: 2017-12-08Bibliographically approved

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