<p>Resonant states are multiply excited states in atoms and ions that have enough energy to decay by emitting an electron. The ability to emit an electron and the strong electron correlation (which is extra strong in negative ions) makes these states both interesting and challenging from a theoretical point of view. The main contribution in this thesis is a method, which combines the use of <i>B </i>splines and complex rotation, to solve the three-electron Schrödinger equation treating all three electrons equally. It is used to calculate doubly excited and triply excited states of ⁴<i>S</i> symmetry with even parity in He^-. For the doubly excited states there are experimental and theoretical data to compare with. For the triply excited states there is only theoretical data available and only for one of the resonances. The agreement is in general good. For the triply excited state there is a significant and interesting difference in the width between our calculation and another method. A cause for this deviation is suggested. The method is also used to find a resonant state of ⁴<i>S</i> symmetry with odd parity in H^2-. This state, in this extremely negative system, has been predicted by two earlier calculations but is highly controversial.</p><p>Several other studies presented here focus on two-electron systems. In one, the effect of the splitting of the degenerate H(<i>n=</i>2) thresholds in H^-, on the resonant states converging to this threshold, is studied. If a completely degenerate threshold is assumed an infinite series of states is expected to converge to the threshold. Here states of ¹<i>P</i> symmetry and odd parity are examined, and it is found that the relativistic and radiative splitting of the threshold causes the series to end after only three resonant states. Since the independent particle model completely fails for doubly excited states, several schemes of alternative quantum numbers have been suggested. We investigate the so called DESB (Doubly Excited Symmetry Basis) quantum numbers in several calculations. For the doubly excited states of He^-mentioned above we investigate one resonance and find that it cannot be assigned DESB quantum numbers unambiguously. We also investigate these quantum numbers for states of ¹<i>S </i>even parity in He. We find two types of mixing of DESB states in the doubly excited states calculated. We also show that the amount of mixing of DESB quantum numbers can be inferred from the value of the cosine of the inter-electronic angle. In a study on Li^-the calculated cosine values are used to identify doubly excited states measured in a photodetachment experiment. In particular a resonant state that violates a propensity rule is found.</p>

Resonant states are multiply excited states in atoms and ions that have enough energy to decay by emitting an electron. The ability to emit an electron and the strong electron correlation (which is extra strong in negative ions) makes these states both interesting and challenging from a theoretical point of view. The main contribution in this thesis is a method, which combines the use of B splines and complex rotation, to solve the three-electron Schrödinger equation treating all three electrons equally. It is used to calculate doubly excited and triply excited states of 4S symmetry with even parity in He-. For the doubly excited states there are experimental and theoretical data to compare with. For the triply excited states there is only theoretical data available and only for one of the resonances. The agreement is in general good. For the triply excited state there is a significant and interesting difference in the width between our calculation and another method. A cause for this deviation is suggested. The method is also used to find a resonant state of 4S symmetry with odd parity in H2-. This state, in this extremely negative system, has been predicted by two earlier calculations but is highly controversial. Several other studies presented here focus on two-electron systems. In one, the effect of the splitting of the degenerate H(n=2) thresholds in H-, on the resonant states converging to this threshold, is studied. If a completely degenerate threshold is assumed an infinite series of states is expected to converge to the threshold. Here states of 1P symmetry and odd parity are examined, and it is found that the relativistic and radiative splitting of the threshold causes the series to end after only three resonant states. Since the independent particle model completely fails for doubly excited states, several schemes of alternative quantum numbers have been suggested. We investigate the so called DESB (Doubly Excited Symmetry Basis) quantum numbers in several calculations. For the doubly excited states of He- mentioned above we investigate one resonance and find that it cannot be assigned DESB quantum numbers unambiguously. We also investigate these quantum numbers for states of 1S even parity in He. We find two types of mixing of DESB states in the doubly excited states calculated. We also show that the amount of mixing of DESB quantum numbers can be inferred from the value of the cosine of the inter-electronic angle. In a study on Li- the calculated cosine values are used to identify doubly excited states measured in a photodetachment experiment. In particular a resonant state that violates a propensity rule is found.

This thesis examines how the kinetic properties of pair-produced Higgs bosons depend on whether the process is mediated by particles in the Standard Model or a simplified supersymmetric model. The analysis is based on simulated data made using MadGraph. The examination within this thesis is split into two parts, one performed on the output of MadGraph (parton level) and the second performed on the output of a simplified simulation of the ATLAS detector response (reconstruction level). The first part also contains a section showing that the supersymmetric model results are consistent with the Standard Model results in cases where no supersymmetric particles are present in the di-Higgs production and decay, and a section detailing the minor differences in kinematics of the Higgs bosons depending on the mass of the supersymmetric partner of the top quark. The second part of the thesis covers the decay of the Higgs boson into bottom and anti-bottom quarks, and includes a section showing that there is no difference if the decay is done using the MadSpin module or the Pythia module for MadGraph, before exploring the kinematics of the jets produced. Both when studying Higgs bosons at the parton level and when studying b-jets at the reconstruction level, it is found that there are clear differences in the kinematics between the Standard Model and the supersymmetric model. At both levels, a neural network has been designed, trained and tested. For the parton level neural network 59 % of all events are classified correctly, while at the reconstruction level 51 % of all events are classified correctly. These neural network results show that it is possible to train a neural network to learn on data like this, and that with enough di-Higgs events detected, their kinematic properties could be used to indirectly infer the presence of physics beyond the Standard Model.

Tau neutrinos are one of the least understood elementary particles, as they are difficult to detect. One of the few detectors that can detect them is the IceCube neutrino observatory, located on the south pole. When tau neutrinos interact with a nucleus in the ice at IceCube, a tau lepton can form, which decays almost instantaneously. The decay products of the tau lepton will then emit light, which can be detected at IceCube. To gain a better understanding of tau neutrinos, it is therefore crucial to accurately simulate tau lepton decays. In this thesis, the tau lepton decays within the IceCube simulation framework are evaluated by comparing the simulations to TAUOLA, a high-precision tau decay simulation library, Through this work, several issues with the current simulations were discovered. Specifically, the polarization of the tau lepton is neglected, hadronic resonances are not implemented for tau decays, and several decay modes are missing. These issues result in an overestimate of the energy of the visible decay products, which means that the light emitted from the tau decay is dimmer in reality than what is predicted by the current IceCube simulations. To address this, TAUOLA was integrated into the IceCube simulations, thereby enabling high-precision tau decay simulations for future studies. / Tauneutriner är en av de allra minst studerade partiklarna på grund av att de är svåra att detektera. En av få detektorer som han observera tau neutriner är neutrinoteleskopet IceCube vid sydpolen. När tauneutriner interagerar med isen kan en taulepton skapas, som snabbt sönderfaller. De partiklar som bildas vid sönderfallet avger ljus som kan mätas i IceCube. För att kunna studera tauneutriner är det därför fundamentalt att simulera korrekta och noggranna tausönderfall. Under detta examensarbete jämfördes IceCubes simuleringar av tausönderfall mot mjukvarubiblioteket TAUOLA, som är ämnat för högprecisionssimuleringar av tausönderfall. Genom detta upptäcktes ett flertal problem med IceCubes simuleringar: ingen hänsyn tas till polariseringen av tauleptonen, hadroniska resonanser är inte implementerade, och det finns ett flertal sätt som tauleptonen kan sönderfalla på som inte är implementerade. Dessa problem leder till att energin hos de observerbara partiklarna som produceras vid sönderfallet överskattas, vilket innebär att ljuset som avges i verkligheten är mindre än vad simuleringarna förutspår. För att lösa dessa problem integrerades TAUOLA i IceCubes simuleringsramverk, vilket möjliggör precisa tausönderfallssimuleringar för framtida analyser inom IceCube.

This report introduces a signal discrimination framework for particle physics processes, including a novel ensemble learning method using multiple machine learning models. The framework is tested with a signal region of the Higgs boson decay channel H → WW∗ → lνlν with two or more jets. The final state consists of leptons with the same flavor but opposite electrical charges, and the Higgs bosons are produced by Vector-boson fusion (VBF). The background region consists of the three largest processes with the same final state without originating from the signal process. Multiple models are trained and evaluated on Monte Carlo samples corresponding to a subset of the full Run 2 dataset of proton-proton collisions recorded by the ATLAS experiment at CERN’s Large Hadron Collider (LHC). The analysis in this report shows that ensemble methods improve background rejection leading to increased discrimination between the signal and background region compared with individual machine learning models. / Denna rapport introducerar ett ramverk för signalseparering för processer inom partikelfysik, inklusive en ny ensemblemetod. Ensemblemetoden använder sig av flera maskininlärningsmodeller. Ramverket testas med en signalregion bestående av en sönderfallskanal för Higgsbosonen H → WW∗ → lvlv med två eller flera jets. Leptonerna i sluttillståndet är av samma typ men motsatt laddning. Higgsbosonerna produceras genom så kallad "Vector-boson fusion (VBF)". Bakgrundsregionen innehåller de tre största processerna som har samma sluttillstånd utan att komma ifrån signalprocessen. Flertal modeller tränas och utvärderas på Monte Carlo-simuleringar som motsvarar en delmängd av hela Run 2-datasetet bestående av proton-proton kollisioner som samlats in av ATLAS-experimentet vid CERNs Large Hadron Collider (LHC). Rapporten visar att ensemblemetoden förbättrar bakgrundsidentifieringen vilket leder till förbättrad separation mellan signal- och bakgrundsregionen jämfört med individuella maskininlärningsmodeller.

Only a small part of particle physics is mentioned in the physics courses for Swedish high school students, despite the fact that particle physics is a field where a lot of research and progress are being done today. The first two physics courses in Swedish high schools include an overview of the standard model and the particles and interactions within the model. By designing an experiment where students get to use data from the ATLAS experiment in CERN they not only get to learn more about particle physics, they also get the opportunity to learn how to work similar to how researchers work in this field. This project was inspired by the Hands-on-CERN project which is focusing on increasing high school students interest and understanding of particle physics. By imitating Hands-on-CERN’s way of working, an experiment for high school students has been designed. In the experiment students get to discover the existence of quarks, even though they can not be observed as free particles but only in bounded states; so called hadrons. By analyzing data from the ATLAS experiment of the decay of the Z boson, you get to the conclusion that the Z boson more often decay to hadrons than to leptons with a proportion of about 14:71. This proportion depends on the various possibilities of hadrons that the Z boson can decay to, depending on the inner structure of hadrons consisting of quarks, dissimilar to the leptons which have no inner structure. / I fysikkurserna på gymnasiet ingår endast en liten del av partikelfysiken, trots att det är ett område som är väldigt aktuell inom forskning. I Fysik 1 och 2 ingår en översiktlig orientering om standardmodellen och de partiklar och interaktioner som ingår i modellen. Genom att utforma en laboration där eleverna får använda sig av data från ATLAS-experimentet på CERN får de både möjligheten att lära sig mer om partikelfysik och prova att arbeta på ett sätt som liknar forskarnas arbetssätt. Inspiration till arbetet har hämtats från projektet Hands-on-CERN som inriktar sig på att öka gymnasieelevers intresse och kunskaper inom partikelfysik. Genom att efterlikna Hands-on-CERNs sätt att arbeta har en laboration lämplig för gymnasieelever utformats. I laborationen får eleverna möjlighet att upptäcka att kvarkar finns trots att de inte går att observera i fria tillstånd, utan endast i bundna tillstånd; så kallade hadroner. Data från Z-bosonens sönderfall hämtas från ATLAS-experimentet i CERN analyseras, från vilken man kan dra slutsatsen att Z-bosonen sönderfaller oftare till hadroner än till leptoner, ett förhållande på ungefär 14:71. Förhållandet beror på att det finns fler möjliga hadroner som Z- bosonen kan sönderfalla till, vilket i sin tur beror på hadronernas inre struktur bestående av kvarkar, till skillnad från leptonerna som saknar inre struktur.

This thesis presents an analysis of misidentified leptons in the Higgs boson decaychannel H → W W ∗ → lνlν. Misidentified leptons, resulting from jets misidentifiedas leptons, mimic the signal of a Higgs boson decay, resulting in a backgroundcontribution to the signal. The analysis is performed on proton-proton collisions ata center-of-mass energy of 13.6 TeV recorded by the ATLAS experiment at CERN’sLarge Hadron Collider. The estimation of misidentified leptons is done using theso-called Fake Factor method, which is used to assess the contributed backgroundwith misidentified leptons in the signal region. The Fake Factor values increasewith increasing momenta for misidentified electrons while it remains constant formisidentified muons. An analysis of the impact parameters d0 and z0 show thatthe accuracy of the Monte Carlo simulation to correctly predict the contribution ofmisidentified leptons from heavy quarks is high. / Denna avhandling presenterar en analys av felaktigt identifierade leptoner i Higgsbosonens sönderfallskanal H → W W ∗ → lνlν. Felaktigt identifierade leptoner frånhadroner imiterar signalen av ett sönderfall från en Higgs boson, vilket resulterar ibakgrund till signalen. Analysen utförs på proton-proton kollisioner med energier på13.6 TeV som samlats in av ATLAS-experimentet vid CERNs Large Hadron Collider.Denna avhandling fokuserar främst på uppskattningen av felaktigt identifieradeleptoner genom användningen av Fake Factor-metoden, vilket används för att bedömaden bidragande bakgrunden med felidentifierade leptoner i signalregionen. Värdenaför Fake Factors ökar med ökande rörelsemängd för felidentifierade elektroner medanför det förblir konstant för felidentifierade muoner. En analys av ”impact parameters”d0 och z0 visar att Monte Carlo-simuleringens noggrannhet att uppskatta antaletfelidentifierade leptoner från tunga kvarkar är hög.

We consider a simple model for invisible neutrino decay as a sub-leading effect in the standard three-flavor neutrino oscillation framework, and use the Cayley–Hamilton formalism to obtain a full set of neutrino oscillation probabilities in matter. These are given as analytical series expansions in the small parameters α ∼ O(λ^2) and s_13 ∼ O(λ), where λ ≡ 0.2 is a “book-keeping parameter” denoting the order of the expansion. We produce explicit formulas for P_eµ, P_eτ , P_µµ, P_µτ , and P_ττ to order O(λ^3), and for P_ee to order O(λ^2), all having first corrections of order O(λ^4). Moreover, we also present vacuum limits of our expressions, as well as discuss the effect of decay on unitarity. We show that all rows in the unitarity table have corrections of order O(λ^2), with the second and third rows having additional corrections of order O(1). In the limit of no decay, unitarity is restored, and we furthermore recover known results for all probabilities. / Vi betraktar en enkel modell för osynligt neutrinosönderfall som en icke-ledande effekt inom det vedertagna ramverket för oscillationer med tre neutrinosmaker och använder Cayley–Hamilton-formalismen för att erhålla en fullständig uppsättning av sannolikheter för neutrinooscillationer i materia. Dessa ges som analytiska serieutvecklingar i de små parametrarna α ∼ O(λ^2) och s_13 ∼ O(λ), där λ ≡ 0.2 är en gemensam “bokföringsparameter” som anger serieutvecklingens ordning. Vi tar fram explicita uttryck för P_eµ, P_eτ , P_µµ, P_µτ och P_ττ till ordning O(λ^3) och för P_ee till ordning O(λ^2), med första korrigeringar för alla sannolikheterna till ordning O(λ^4). Därutöver presenterar vi gränsvärden för våra uttryck i vakuum samt diskuterar sönderfallets inverkan på unitaritet. Vi finner att alla rader i unitaritetstabellen har korrigeringar till ordning O(λ^2) och därtill korrigeringar till ordning O(1) för den andra och tredje raden. I avsaknad av sönderfall återfås unitaritet och vi reproducerar dessutom kända resultatför alla sannolikheter.

Starting in 2027, the high-luminosity Large Hadron Collider (HL-LHC) will begin operation and allow higher-precision measurements and searches for new physics processes between elementary particles. One central problem that arises in the ATLAS detector when reconstructing event information is to separate the rare and interesting hard scatter (HS) interactions from uninteresting pileup (PU) interactions in a spatially compact environment. This problem becomes even harder to solve at higher luminosities. This project relies on leveraging the time dimension and determining a time of the HS interactions to separate them from PU interactions by using information measured by the upcoming High-Granularity Timing Detector (HGTD). The current method relies on using a boosted decision tree (BDT) together with the timing information from the HGTD to determine a time. We suggest a novel approach of utilizing a graph attentional network (GAT) where each bunch-crossing is represented as a graph of tracks and the properties of the GAT are applied on a track level to inspect if such a model can outperform the current BDT. Our results show that we are able to replicate the results of the BDT and even improve some metrics at the expense of increasing the uncertainty of the time determination. We conclude that although there is potential for GATs to outperform the BDT, a more complex model should be applied. Finally, we provide some suggestions for improvement and hope to inspire further study and advancements in this direction which shows promising potential. / Från och med 2027 kommer \textit{high-luminosity Large Hadron Collider} (HL-LHC) att tas i drift och möjliggöra mätningar med högre precision och utforskningar av nya fysikprocesser mellan elementarpartiklar. Ett centralt problem som uppstår i ATLAS-detektorn vid rekonstruktionen av partikelkollisioner är att separera sällsynta och intressanta interaktioner, så kallade \textit{hard-scatters} (HS) från ointressanta \textit{pileup}-interaktioner (PU) i den kompakta rumsliga dimensionen. Svårighetsgraden för detta problem ökar vid högre luminositeter. Med hjälp av den kommande \textit{High-Granularity Timing-detektorns} (HGTD) mätningar kommer även tidsinformation relaterat till interaktionerna att erhållas. I detta projekt används denna information för att beräkna tiden för enskillda interaktioner vilket därmed kan användas för att separera HS-interaktioner från PU-interaktioner. Den nuvarande metoden använder en trädregressionsmetod, s.k. boosted decision tree (BDT) tillsammans med tidsinformationen från HGTD för att bestämma en tid. Vi föreslår ett nytt tillvägagångssätt baserat på ett s.k. uppvaktande grafnätverk (GAT), där varje protonkollision representeras som en graf över partikelspåren och där GAT-egenskaperna tillämpas på spårnivå. Våra resultat visar att vi kan replikera de BDT-baserade resultaten och till och med förbättra resultaten på bekostnad av att öka osäkerheten i tidsbestämningarna. Vi drar slutsatsen att även om det finns potential för GAT-modeller att överträffa BDT-modeller, bör mer komplexa versioner av de förra tillämpas. Vi ger slutligen några förbättringsförslag som vi hoppas ska kunna inspirera till ytterligare studier och framsteg inom detta område, vilket visar lovande potential.

In 2016, the ATLAS experiment at CERN released data from 100 trillion proton-proton collisions to the general public. In connection to this release the ATLAS Outreach group has developed several tools for visualizing and analyzing the data, one of which is a Histogram analyzer. The focus of this project is to bridge the gap between the general public's knowledge in physics and what is needed to use this Histogram analyzer. The project consists of both the development and an evaluation of a learning resource that explains experimental particle physics for a general public audience. The learning resource is a website making use of analogies and two perspectives on learning: Variation Theory and Cognitive Load Theory. The evaluation of the website was done using a survey with 10 respondents and it focused on whether analogies and the perspectives on learning helped their understanding. In general the respondents found the analogies to be helpful for their learning, and to some degree they found the explanations based on Variation Theory to be helpful. The implementations of Cognitive Load Theory were considered to be helpful by the respondents who noticed them, but the majority did not, implying that improvements of the design are needed. The results indicate that analogies and the two perspectives on learning can be helpful for explaining experimental particle physics, but there might be other learning theories more suitable for this purpose. / ATLAS-experimentet på CERN släppte år 2016 data från 100 biljoner proton-kollisioner fritt till allmänheten. I anslutning till detta har ATLAS Outreach-grupp utvecklat ett flertal verktyg för att visualisera och analysera datan, varav en är en analys med hjälp av histogram. Fokus för detta projekt är att överbrygga klyftan mellan allmänhetens kunskaper i fysik och vad som behövs for att kunna använda Histogram-analysverktyget. Projektet består både av utvecklandet och utvärderingen av ett läromedel som förklarar experimentell partikelfysik med målgruppen allmänheten. Läromedlet är en webbsida som använder sig av analogier och två perspektiv på lärande, Variationsteori och Kognitiv Belastningsteori. Utvärderingen av webbsidan gjordes med en enkät med tio respondenter, med fokus på huruvida analogier och perspektiven på lärande hjälpte deras förståelse. I allmänhet fann respondenterna analogierna hjälpsamma för deras lärande, och de fann Variationsteori hjälpsamt i viss utsträckning. Implementeringarna av Kognitiv Belastningsteori ansågs vara hjälpsamma av de respondenter som lade märke till dem, men majoriteten gjorde inte det, vilket tyder på att förbättringar av implementeringen krävs. Resultaten indikerar att analogier och de två perspektiven på lärande kan vara hjälpsamma för att förklara experimentell partikelfysik, men det kan finnas andra lärandeteorier som uppfyller syftet bättre.