| 1. |
- Alimena, Juliette, et al.
(författare)
-
Searching for long-lived particles beyond the Standard Model at the Large Hadron Collider
- 2020
-
Ingår i: Journal of Physics G. - : IOP Publishing. - 0954-3899 .- 1361-6471. ; 47:9
-
Tidskriftsartikel (refereegranskat)abstract
- Particles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton-proton collision. Such LLP signatures are distinct from those of promptly decaying particles that are targeted by the majority of searches for new physics at the LHC, often requiring customized techniques to identify, for example, significantly displaced decay vertices, tracks with atypical properties, and short track segments. Given their non-standard nature, a comprehensive overview of LLP signatures at the LHC is beneficial to ensure that possible avenues of the discovery of new physics are not overlooked. Here we report on the joint work of a community of theorists and experimentalists with the ATLAS, CMS, and LHCb experiments-as well as those working on dedicated experiments such as MoEDAL, milliQan, MATHUSLA, CODEX-b, and FASER-to survey the current state of LLP searches at the LHC, and to chart a path for the development of LLP searches into the future, both in the upcoming Run 3 and at the high-luminosity LHC. The work is organized around the current and future potential capabilities of LHC experiments to generally discover new LLPs, and takes a signature-based approach to surveying classes of models that give rise to LLPs rather than emphasizing any particular theory motivation. We develop a set of simplified models; assess the coverage of current searches; document known, often unexpected backgrounds; explore the capabilities of proposed detector upgrades; provide recommendations for the presentation of search results; and look towards the newest frontiers, namely high-multiplicity 'dark showers', highlighting opportunities for expanding the LHC reach for these signals.
|
|
| 2. |
- Klimek, Pawel, 1985-
(författare)
-
Search for Charginos and Sleptons in ATLAS and Identification of Pile-up with the Tile Calorimeter
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
|
|
| 3. |
- Mathisen, Thomas
(författare)
-
Exotic Decays of Vector-Like Quarks and Development of a Test Procedure for the ITk Strip Module at the ATLAS Detector for the HL-LHC
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The naturalness problem of the Standard Model (SM) challenges our understanding of the origin of mass. Several theories have been proposed to tackle the problem, one of them is the Composite Higgs Model (CHM) where the Higgs boson of the SM is not an elementary scalar but instead a composite object bound at a confinement scale of a TeV. The fine-tuning of the Higgs boson mass, arising from the naturalenss problem, is thus reduced by the lower cut-off scale of the SM and an overall symmetry protection in the theory. Consequences of the CHM are new states, both fermions, so called Vector-Like Quarks (VLQ), and scalars with masses low enough to be produced at the LHC.In ATLAS, existing searches for VLQs assume decays into known SM particles. In this thesis, a phenomenological study is presented where the VLQ decays to a Beyond the Standard Model (BSM) scalar or pseduoscalar, quantifying the sensitivity of the current experiments at the LHC to the such decays. The reach is well above 1 TeV in VLQ mass thus directly competing with VLQ decay to known SM states.The phenomenological study is continued with the ATLAS experiment where the VLQ decays into a BSM scalar with a diphoton final state. The data-driven background modeling is covered and expected limits are presented showing good sensitivity to exotic decays of VLQ. The LHC is reaching its limit in energy and luminosity. The next step is an upgrade to the High-Luminosity LHC (HL-LHC), significantly raising the data rates. For the ATLAS experiment to cope with the data rates delivered by the HL-LHC, the detector needs to the upgraded. In this thesis, a detailed description of a test procedure for a trigger and readout scheme for the new inner tracker strip detector modules is presented.
|
|
