Search for the lightest MSSM Higgs boson in cascades of supersymmetric particles in ATLAS

Standard Modellen (SM) beskriver det meste av det vi har observert i eksperimenter fram til i dag. Allikevel er SM ikke sett på som en komplett teori, men heller en effektiv teori som fungere bra opp til et visst energinivå. Både forekomsten av en del uforklarte parameter samt at gravitasjon ikke er innlemmet i SM er sterke hint om at det må finnes en mer komplett teori. Dette har motivert teoretikere de siste 30 årene til å konstruere modeller som kan forklare disse manglene ved dagens modell. I tillegg er det i SM pr. i dag fortsatt en liten del som ikke er bekreftet av eksperimenter; den såkalte Higgs mekanismen som gir en forklaring på hvorfor partiklene har masse. Denne mekanismen forutsier eksistensen av en ny partikkel: Higgs partikkelen, som altså enda ikke er funnet. En av de mest betrodde teoriene som er konstruert som en videreutvikling av SM er supersymmetri (SUSY). SUSY er en symmetri mellom materie og krefter (eller fermioner og bososner). SUSY forutsier at hver SM partikkel har en superpartner med et annet spinn og masse. I tillegg vil Higgs sektoren bli utvidet til å inneholde flere Higgs bosoner. I den enkleste supersymmetriske forlengelse av SM, MSSM (Minimal Supersymmetric Standard Model), er det hele fem Higgs bosoner. Det letteste av disse Higgs bosonene vil være relativt lett (mindre enn 135GeV) og veldig likt Higgs bosonet forutsagt av SM. I denne oppgaven er det fokusert på hvordan man kan oppdage Higgs bosonet i partikkeleksperimentet ATLAS som er en del av LHC (Large Hadron Collider) på CERN. Oppgaven konsentrerer seg om Higgs produksjon fra en kaskade av supersymmetriske partikler, som vil oppstå når man får to supersymmetriske partikler produsert direkte fra proton-proton kollisjonen. Det klart mest dominerende henfallet til en lett Higgs vil vare til et b-kvark par. Dette er en vanskelig kanal å bruke, da andre prosesser, både SM og SUSY, som produserer b-kvarker har stort tverrsnitt på LHC. Dette medfører at signalet vil være vanskelig å skille fra all bakgrunnen. Spesielt er SM QCD og ttbar bakgrunnen problematisk. Allikevel, ved å bruke spesielle kjennetegn på supersymmetriske kaskader er det mulig å skille vekk mye av bakgrunnen. For å få et bedre signal er det i tillegg viktig at man har god forståelse for hvordan detektoren virker, da man er veldig avhengig av gode og effektive måter for å skille b-kvarker fra andre kvarker. I oppgaven har jeg tatt for meg flere forskjellige supersymmetriske scenarioer innen MSSM og vurdert mulighetene og forskjellige metoder for å oppdage det letteste supersymmetriske Higgs bosonet i ATLAS. Da LHC ikke begynner å operere før sent i 2008 er hele denne studien gjort med simulert data

Increased thromboinflammatory load in hereditary angioedema

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Cognitive function and health-related quality of life four years after cardiac arrest

Aim: Neuropsychological testing has uncovered cognitive impairment in cardiac arrest survivors with good neurologic outcome according to the cerebral performance categories. We investigated cognitive function and health-related quality of life four years after cardiac arrest. Methods: Thirty cardiac arrest survivors over the age of 18 in cerebral performance category 1 or 2 on hospital discharge completed the EQ-5D-5L and HADS questionnaires prior to cognitive testing using the Cambridge Neuropsychological Test Automated Battery. The results were compared with population norms. Results: Twenty-nine per cent of patients were cognitively impaired. The pattern of cognitive impairment reflects dysfunction in the medial temporal lobe, with impaired short-time memory and executive function slightly but distinctly affected. There was a significant reduction in quality of life on the EQ-VAS, but not on the EQ index. Conclusion: Cognitive impairment four years after cardiac arrest affected more than one quarter of the patients. Short-term memory was predominantly affected

Search for heavy lepton resonances decaying to a ZZ boson and a lepton in pppp collisions at s=8\sqrt{s}=8 TeV with the ATLAS detector

A search for heavy leptons decaying to a $Z$ boson and an electron or a muon is presented. The search is based on $pp$ collision data taken at $\sqrt{s}=8$ TeV by the ATLAS experiment at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, Three high-transverse-momentum electrons or muons are selected, with two of them required to be consistent with originating from a $Z$ boson decay. No significant excess above Standard Model background predictions is observed, and 95% confidence level limits on the production cross section of high-mass trilepton resonances are derived. The results are interpreted in the context of vector-like lepton and type-III seesaw models. For the vector-like lepton model, most heavy lepton mass values in the range 114-176 GeV are excluded. For the type-III seesaw model, most mass values in the range 100-468 GeV are excluded

Measurement of the production cross section of prompt J/ψ mesons in association with a W ± boson in pp collisions at √s = 7 TeV with the ATLAS detector

The process pp → W ± J/ψ provides a powerful probe of the production mechanism of charmonium in hadronic collisions, and is also sensitive to multiple parton interactions in the colliding protons. Using the 2011 ATLAS dataset of 4.5 fb−1 of s√ = 7 TeV pp collisions at the LHC, the first observation is made of the production of W ± + prompt J/ψ events in hadronic collisions, using W ± → μν μ and J/ψ → μ + μ −. A yield of 27.4+7.5−6.5 W ± + prompt J/ψ events is observed, with a statistical significance of 5.1σ. The production rate as a ratio to the inclusive W ± boson production rate is measured, and the double parton scattering contribution to the cross section is estimated.publishedVersio

Measurement of the muon reconstruction performance of the ATLAS detector using 2011 and 2012 LHC proton–proton collision data

This paper presents the performance of the ATLAS muon reconstruction during the LHC run with pp collisions at √s = 7–8 TeV in 2011–2012, focusing mainly on data collected in 2012. Measurements of the reconstruction efficiency and of the momentum scale and resolution, based on large reference samples of J/ψ → μμ, Z → μμ and ϒ → μμ decays, are presented and compared to Monte Carlo simulations. Corrections to the simulation, to be used in physics analysis, are provided. Over most of the covered phase space (muon |η| < 2.7 and 5 ≲pT ≲pT 100 GeV) the efficiency is above 99% and is measured with per-mille precision. The momentum resolution ranges from 1.7% at central rapidity and for transverse momentum pT ≃ 10 GeV, to 4% at large rapidity and pT ≃ 100 GeV. The momentum scale is known with an uncertainty of 0.05% to 0.2% depending on rapidity. A method for the recovery of final state radiation from the muons is also presented

Search for resonances decaying into photon pairs in 139 fb−1 of pp collisions at √s = 13 TeV with the ATLAS detector

Searches for new resonances in the diphoton final state, with spin 0 as predicted by theories with an extended Higgs sector and with spin 2 using a warped extra-dimension benchmark model, are presented using 139 fb−1 of √ s =13 TeV pp collision data collected by the ATLAS experiment at the LHC. No significant deviation from the Standard Model is observed and upper limits are placed on the production cross-section times branching ratio to two photons as a function of the resonance mass.publishedVersio

Measurement of distributions sensitive to the underlying event in inclusive Z-boson production in pp collisions at √s = 7 TeV with the ATLAS detector

A measurement of charged-particle distributions sensitive to the properties of the underlying event is presented for an inclusive sample of events containing a TeX -boson, decaying to an electron or muon pair. The measurement is based on data collected using the ATLAS detector at the LHC in proton–proton collisions at a centre-of-mass energy of TeX TeV with an integrated luminosity of TeX fb TeX . Distributions of the charged particle multiplicity and of the charged particle transverse momentum are measured in regions of azimuthal angle defined with respect to the TeX -boson direction. The measured distributions are compared to similar distributions measured in jet events, and to the predictions of various Monte Carlo generators implementing different underlying event models

Search for dark matter in events with heavy quarks and missing transverse momentum in pppp collisions with the ATLAS detector

This article reports on a search for dark matter pair production in association with bottom or top quarks in $20.3 fb^{-1}$ of $pp$ collisions collected at $\sqrt{s} = 8$ TeV by the ATLAS detector at the LHC. Events with large missing transverse momentum are selected when produced in association with high-momentum jets of which one or more are identified as jets containing $b$-quarks. Final states with top quarks are selected by requiring a high jet multiplicity and in some cases a single lepton. The data are found to be consistent with the Standard Model expectations and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between dark matter and Standard Model particles. Limits on the dark-matter--nucleon cross-section for spin-independent and spin-dependent interactions are also provided. These limits are particularly strong for low-mass dark matter. Using a simplified model, constraints are set on the mass of dark matter and of a coloured mediator suitable to explain a possible signal of annihilating dark matter

Search for new phenomena in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in s=\sqrt{s}= 13 pppp collisions with the ATLAS detector

Two searches for new phenomena in final states containing a same-flavour opposite-lepton (electron or muon) pair, jets, and large missing transverse momentum are presented. These searches make use of proton--proton collision data, collected during 2015 and 2016 at a centre-of-mass energy $\sqrt{s}=13$ TeV by the ATLAS detector at the Large Hadron Collider, which correspond to an integrated luminosity of 14.7 fb$^{-1}$, Both searches target the pair production of supersymmetric particles, squarks or gluinos, which decay to final states containing a same-flavour opposite-sign lepton pair via one of two mechanisms: a leptonically decaying Z boson in the final state, leading to a peak in the dilepton invariant-mass distribution around the Z boson mass; and decays of neutralinos (e.g. $\tilde{\chi}_2^0 \rightarrow \ell^+\ell^- \tilde{\chi}_1^0$), yielding a kinematic endpoint in the dilepton invariant-mass spectrum. The data are found to be consistent with the Standard Model expectation. Results are interpreted in simplified models of gluino-pair (squark-pair) production, and provide sensitivity to gluinos (squarks) with masses as large as 1.70 TeV (980 GeV).publishedVersio