Searches for Heavy Charged Long-Lived Particles with the ATLAS detector

In dieser Arbeit wird eine Suche nach schweren langlebigen geladenen Teilchen mit dem ATLAS Detektor am Large Hadron Collider vorgestellt. Diese Suche analysiert einen Datensatz von 36.1 fb−1 Proton–Proton-Kollisionen. Die erwartete Signatur solcher schwerer langlebiger geladener Teilchen im Detektor ist ähnlich der eines Myons, jedoch haben diese hypothetischen Teilchen eine höhere Masse. Dies hat zur Folge, dass diese Teilchen mit einer deutlich niedrigeren Geschwindigkeit erzeugt werden als hochenergetischen Standard-Modell-Teilchen. Des Weiteren führt die niedrige Geschwindigkeit der Teilchen zu Ionisations-Energieverlusten deutlich über denen von Myonen, welche den Hauptuntergrund für diese Suche darstellen. Schwere langlebige Teilchen können sowohl elektrisch- also auch far- bgeladen sein. Farbgeladene schwere langlebige Teilchen hadronisieren zusam- men mit Quarks zu sogenannten R-Hadronen. Diese können durch den Austausch der Quarks in hadronischen Interaktionen ihre Gesamtladung ändern. Zur Identifikation von schweren geladenen langlebigen Teilchen, werden Mes- sungen des Ionisations-Energieverlust im Pixel Detektor sowie Flugzeitmessun- gen im Tile-Kalorimeter, in den Monitored-Drift-Tubes und in den Resistive-Plate- Chambers, verwendet. Die Suche ist mit dedizierten Signal Regionen, sowohl für farbgeladene langlebige Teilchen als auch für paar-produzierte langlebige Teilchen die nur elektrisch geladen sind, ausgestattet. Da kein signifikanter Überschuss an Daten üeber der erwarteten Anzahl an Ereignissen gefunden wurde, kön- nen die Ergbnisse verwendet werden um Ausschlussgrenzen auf den Wirkungs- querschnitt und die Masse der Teilchen zu bestimmen. Paar-produzierte Sbot- tom, Stop oder Gluino R-hadronen können bis zu einer Masse von 1250 GeV, 1340 GeV beziehungsweise 2000 GeV ausgeschlossen werden. Für farbneutrale paar-produzierte langlebige Teilchen können Massen bis 430 GeV für Staus und 1090 GeV für Charginos ausgeschlossen werden.In this thesis a search for Heavy Charged Long-Lived Particles (HCLLPs) in a dataset of 36.1 fb−1 of proton–proton collisions with the ATLAS detector at the Large Hadron Collider is presented. HCLLPs manifest in the detector by signatures of heavy muon-like particles and are expected to be produced with velocities, significantly lower than the speed of light. Accordingly they are also expected to have an ioni- sation energy loss larger than that of muons, which are the main background for this search. Beside their electrical charge, HCLLPs can also be colour charged and hadronise together with Standard Model quarks to so called R-hadrons. Those R-hadrons can have a very special signature as they are able to change their overall charge through the exchange of the Standard Model quarks in hadronic interac- tions. The observables used to identify HCLLPs in this analysis are a dE/dx estimate with the pixel detector and time-of-flight measurements with the Tile Calorimeter, the Monitored Drift Tubes and the Resistive Plate Chambers. The signal regions are de- signed to cover signatures of colour-charged particles that can undergo a change of charge as well as pair-produced colour singlets that are charged throughout the whole detector. No significant excess over the estimated background was observed and the results are used to set upper limits on the production cross section as well as lower mass limits for several models. The obtained lower mass limits for sbot- tom, stop and gluino R-hadrons are 1250 GeV, 1340 GeV and 2000 GeV, respectively, while for colour singlets such as staus and charginos 430 GeV and 1090 GeV are ob- tained

Searches for Heavy Charged Long-Lived Particles with the ATLAS detector

In dieser Arbeit wird eine Suche nach schweren langlebigen geladenen Teilchen mit dem ATLAS Detektor am Large Hadron Collider vorgestellt. Diese Suche analysiert einen Datensatz von 36.1 fb−1 Proton–Proton-Kollisionen. Die erwartete Signatur solcher schwerer langlebiger geladener Teilchen im Detektor ist ähnlich der eines Myons, jedoch haben diese hypothetischen Teilchen eine höhere Masse. Dies hat zur Folge, dass diese Teilchen mit einer deutlich niedrigeren Geschwindigkeit erzeugt werden als hochenergetischen Standard-Modell-Teilchen. Des Weiteren führt die niedrige Geschwindigkeit der Teilchen zu Ionisations-Energieverlusten deutlich über denen von Myonen, welche den Hauptuntergrund für diese Suche darstellen. Schwere langlebige Teilchen können sowohl elektrisch- also auch far- bgeladen sein. Farbgeladene schwere langlebige Teilchen hadronisieren zusam- men mit Quarks zu sogenannten R-Hadronen. Diese können durch den Austausch der Quarks in hadronischen Interaktionen ihre Gesamtladung ändern. Zur Identifikation von schweren geladenen langlebigen Teilchen, werden Mes- sungen des Ionisations-Energieverlust im Pixel Detektor sowie Flugzeitmessun- gen im Tile-Kalorimeter, in den Monitored-Drift-Tubes und in den Resistive-Plate- Chambers, verwendet. Die Suche ist mit dedizierten Signal Regionen, sowohl für farbgeladene langlebige Teilchen als auch für paar-produzierte langlebige Teilchen die nur elektrisch geladen sind, ausgestattet. Da kein signifikanter Überschuss an Daten üeber der erwarteten Anzahl an Ereignissen gefunden wurde, kön- nen die Ergbnisse verwendet werden um Ausschlussgrenzen auf den Wirkungs- querschnitt und die Masse der Teilchen zu bestimmen. Paar-produzierte Sbot- tom, Stop oder Gluino R-hadronen können bis zu einer Masse von 1250 GeV, 1340 GeV beziehungsweise 2000 GeV ausgeschlossen werden. Für farbneutrale paar-produzierte langlebige Teilchen können Massen bis 430 GeV für Staus und 1090 GeV für Charginos ausgeschlossen werden.In this thesis a search for Heavy Charged Long-Lived Particles (HCLLPs) in a dataset of 36.1 fb−1 of proton–proton collisions with the ATLAS detector at the Large Hadron Collider is presented. HCLLPs manifest in the detector by signatures of heavy muon-like particles and are expected to be produced with velocities, significantly lower than the speed of light. Accordingly they are also expected to have an ioni- sation energy loss larger than that of muons, which are the main background for this search. Beside their electrical charge, HCLLPs can also be colour charged and hadronise together with Standard Model quarks to so called R-hadrons. Those R-hadrons can have a very special signature as they are able to change their overall charge through the exchange of the Standard Model quarks in hadronic interac- tions. The observables used to identify HCLLPs in this analysis are a dE/dx estimate with the pixel detector and time-of-flight measurements with the Tile Calorimeter, the Monitored Drift Tubes and the Resistive Plate Chambers. The signal regions are de- signed to cover signatures of colour-charged particles that can undergo a change of charge as well as pair-produced colour singlets that are charged throughout the whole detector. No significant excess over the estimated background was observed and the results are used to set upper limits on the production cross section as well as lower mass limits for several models. The obtained lower mass limits for sbot- tom, stop and gluino R-hadrons are 1250 GeV, 1340 GeV and 2000 GeV, respectively, while for colour singlets such as staus and charginos 430 GeV and 1090 GeV are ob- tained

Measurement of the W-boson mass in pp collisions at s√=7TeV with the ATLAS detector

A measurement of the mass of the W boson is presented based on proton–proton collision data recorded in 2011 at a centre-of-mass energy of 7 TeV with the ATLAS detector at the LHC, and corresponding to 4.6 fb−1 of integrated luminosity. The selected data sample consists of 7.8×106 candidates in the → channel and 5.9×106 candidates in the → channel. The W-boson mass is obtained from template fits to the reconstructed distributions of the charged lepton transverse momentum and of the W boson transverse mass in the electron and muon decay channels, yielding =80370=80370±7 (stat.)±11(exp. syst.)±14 (mod. syst.) MeV±19MeV, where the first uncertainty is statistical, the second corresponds to the experimental systematic uncertainty, and the third to the physics-modelling systematic uncertainty. A measurement of the mass difference between the + and − bosons yields +−−=−29±28 MeV.publishedVersio

Search for new phenomena with top quark pairs in final states with one lepton, jets, and missing transverse momentum in pp collisions at √s = 13 TeV with the ATLAS detector

A search for new phenomena with top quark pairs in final states with one isolated electron or muon, multiple jets, and large missing transverse momentum is performed. Signal regions are designed to search for two-, three-, and four-body decays of the directly pair-produced supersymmetric partner of the top quark (stop). Additional signal regions are designed specifically to search for spin-0 mediators that are produced in association with a pair of top quarks and decay into a pair of dark-matter particles. The search is performed using the Large Hadron Collider proton-proton collision dataset at a centre-of-mass energy of √s = 13 TeV recorded by the ATLAS detector from 2015 to 2018, corresponding to an integrated luminosity of 139 fb−1. No significant excess above the Standard Model background is observed, and limits at 95% confidence level are set in the stop-neutralino mass plane and as a function of the mediator mass or the dark-matter particle mass. Stops are excluded up to 1200 GeV (710 GeV) in the two-body (three-body) decay scenario. In the four-body scenario stops up to 640 GeV are excluded for a stop-neutralino mass difference of 60 GeV. Scalar and pseudoscalar dark-matter mediators are excluded up to 200 GeV when the coupling strengths of the mediator to Standard Model and dark-matter particles are both equal to one and when the mass of the dark-matter particle is 1 GeV.publishedVersio

Search for the Higgs boson decays H → ee and H → eμ in pp collisions at √s=13 TeV with the ATLAS detector

Searches for the Higgs boson decays H → ee and H → eμ are performed using data corresponding to an integrated luminosity of 139 fb−1 collected with the ATLAS detector in pp collisions at √s = 13 TeV at the LHC. No significant signals are observed, in agreement with the Standard Model expectation. For a Higgs boson mass of 125 GeV, the observed (expected) upper limit at the 95% confidence level on the branching fraction B(H → ee) is 3.6 × 10−4 (3.5 × 10−4) and on B(H → eμ) is 6.2 × 10−5 (5.9 × 10−5). These results represent improvements by factors of about five and six on the previous best limits on B(H → ee) and B(H → eμ) respectively.publishedVersio

Search for invisible Higgs boson decays in vector boson fusion at √s = 13 TeV with the ATLAS detector

We report a search for Higgs bosons that are produced via vector boson fusion and subsequently decay into invisible particles. The experimental signature is an energetic jet pair with invariant mass of O(1) TeV and O(100) GeV missing transverse momentum. The analysis uses 36.1 fb−1 of pp collision data at √s=13 TeV recorded by the ATLAS detector at the LHC. In the signal region the 2252 observed events are consistent with the background estimation. Assuming a 125 GeV scalar particle with Standard Model cross sections, the upper limit on the branching fraction of the Higgs boson decay into invisible particles is 0.37 at 95% confidence level where 0.28 was expected. This limit is interpreted in Higgs portal models to set bounds on the wimp–nucleon scattering cross section. We also consider invisible decays of additional scalar bosons with masses up to 3 TeV for which the upper limits on the cross section times branching fraction are in the range of 0.3–1.7 pb.publishedVersio

Search for large missing transverse momentum in association with one top-quark in proton-proton collisions at s√= 13 TeV with the ATLAS detector

This paper describes a search for events with one top-quark and large missing transverse momentum in the final state. Data collected during 2015 and 2016 by the ATLAS experiment from 13 TeV proton–proton collisions at the LHC corresponding to an integrated luminosity of 36.1 fb−1 are used. Two channels are considered, depending on the leptonic or the hadronic decays of the W boson from the top quark. The obtained results are interpreted in the context of simplified models for dark-matter production and for the single production of a vector-like T quark. In the absence of significant deviations from the Standard Model background expectation, 95% confidence-level upper limits on the corresponding production cross-sections are obtained and these limits are translated into constraints on the parameter space of the models considered.publishedVersio

Measurement of the cross section for isolated-photon plus jet production in pp collisions at √s = 13 TeV using the ATLAS detector

The dynamics of isolated-photon production in association with a jet in proton–proton collisions at a centre-of-mass energy of 13 TeV are studied with the ATLAS detector at the LHC using a dataset with an integrated luminosity of 3.2 fb−1. Photons are required to have transverse energies above 125 GeV. Jets are identified using the anti- algorithm with radius parameter and required to have transverse momenta above 100 GeV. Measurements of isolated-photon plus jet cross sections are presented as functions of the leading-photon transverse energy, the leading-jet transverse momentum, the azimuthal angular separation between the photon and the jet, the photon–jet invariant mass and the scattering angle in the photon–jet centre-of-mass system. Tree-level plus parton-shower predictions from Sherpa and Pythia as well as next-to-leading-order QCD predictions from Jetphox and Sherpa are compared to the measurements.publishedVersio

Measurement of the CP-violating phase ϕs in B0s→J/ψϕ decays in ATLAS at 13 TeV

A measurement of the B0s→J/ψϕ decay parameters using 80.5fb−1 of integrated luminosity collected with the ATLAS detector from 13 TeV proton–proton collisions at the LHC is presented. The measured parameters include the CP-violating phase ϕs, the width difference ΔΓs between the B0s meson mass eigenstates and the average decay width Γs. The values measured for the physical parameters are combined with those from 19.2fb−1 of 7 and 8 TeV data, leading to the following: ϕs=−0.087±0.036 (stat.)±0.021 (syst.) rad ΔΓs=0.0657±0.0043 (stat.)±0.0037 (syst.) ps−1 Γs=0.6703±0.0014 (stat.)±0.0018 (syst.) ps−1 Results for ϕs and ΔΓs are also presented as 68% confidence level contours in the ϕs–ΔΓs plane. Furthermore the transversity amplitudes and corresponding strong phases are measured. ϕs and ΔΓs measurements are in agreement with the Standard Model predictions.publishedVersio

Measurement of ZZ production in the ℓℓνν final state with the ATLAS detector in pp collisions at √s = 13 TeV

This paper presents a measurement of ZZ production with the ATLAS detector at the Large Hadron Collider. The measurement is carried out in the final state with two charged leptons and two neutrinos, using data collected during 2015 and 2016 in pp collisions at s√ = 13 TeV, corresponding to an integrated luminosity of 36.1 fb−1. The integrated cross-sections in the total and fiducial phase spaces are measured with an uncertainty of 7% and compared with Standard Model predictions, and differential measurements in the fiducial phase space are reported. No significant deviations from the Standard Model predictions are observed, and stringent constraints are placed on anomalous couplings corresponding to neutral triple gauge-boson interactions.publishedVersio