1,720,967 research outputs found
A quantum algorithm for the n-gluon MHV scattering amplitude
No full textWe propose a quantum algorithm for computing the n-gluon maximally helicity violating (MHV) tree-level scattering amplitude. We revisit a newly proposed method for unitarisation of non-unitary operations and present how this implementation can be used to create quantum gates responsible for the color and kinematic factors of the gluon scattering amplitude. As a proof-of-concept, we detail the full conceptual algorithm that yields the squared amplitude and implement the corresponding building blocks on simulated noiseless quantum circuits for n = 4 to analyze its performance. The algorithm is found to perform well with parameter optimizations, suggesting it to be a good candidate for implementing on quantum computers also for higher multiplicities
Electromagnetic form factors of the Sigma*-Lambda transition
No full textWe introduce and examine the analytic properties of the three electromagnetic transition form factors of the Sigma*-Lambda hyperon transition. In the first part of the thesis, we discuss the interaction Lagrangian for the hyperons at hand. We calculate the decay rate of the Dalitz decay Sigma* Lambda -> e+e- in the one-photon approximation in terms of the form factors, as well as the differential cross section of the scattering e+e- -> Sigma*bar Lambda in the one-photon approximation. In the second part of the thesis, we build up the machinery for calculation of the form factors using dispersion relations, performing an analytic continuation from the timelike, q2 > 0, to the spacelike, q2 < 0, region of the virtual photon invariant mass q2. Due to an anomalous cut in the triangle diagram arising from a two-pion saturation of the photon-hyperon vertex, there is an additional term in the dispersive integral. We use the scalar three-point function as a model for the examination of the dispersive approach with the anomalous cut. The one-loop diagram is calculated both directly and using dispersion relations. After comparison of the two methods, they are found to coincide when the anomalous contribution is added to the dispersive integral in the case of the octet Sigma exchange. By examination of the branch points of the logarithm in the discontinuity, we deduce the structure of the Riemann surface of the unitarity cut and present trajectories of the branch points. The result of our analysis of the analytic structure yields a correct dispersive relation for the electromagnetic transition form factors. This opens the way for the calculation of these form factors in the low-energy region for both space- and timelike q2. As an outlook, we present preliminary calculations for the hyperon-pion scattering amplitude using the unitarity and the anomalous contribution in a once-subtracted dispersion relation. Finally we present the corresponding preliminary unsubtracted dispersive calculations for the form factors
Precision Standard Model Phenomenology for High Energy Processes
Get PDFThe present status of particle physics is that the Standard Model has been completed with the discovery of theHiggs boson in 2012, but there is a multitude of phenomena in nature which is not accounted for by this model.Researchers are investigating possibilities for detecting new physics at the current particle physics facilities, withthe Large Hadron Collider (LHC) at the frontier. As no significant sign of new physics has been observed as oftoday, precision phenomenology becomes increasingly important. This thesis and the four papers included in itcontribute to this field of precision predictions for various important processes at the LHC.In paper I and paper IV, the Drell-Yan process is investigated, and specifically, the decay coefficients whichparameterize the spherical distribution of the outgoing leptons in the process. In the first work, we investigatethe next-to-leading-order (NLO) electroweak corrections to the coefficients of the neutral-current process. In thesecond work, a similar study, but including also next-to-next-to-leading-order quantum chromodynamic (QCD)corrections, is performed for the decay coefficients of the charged-current Drell-Yan process. The latter processand the corresponding coefficients are of great importance for measuring the W boson mass at the LHC.In paper II, the top quark pair production and the spin correlations for the process are investigated. The spincorrelation information of the top quarks may reveal underlying new physics when probed at high precision.Therefore, this work computes approximate complete-NLO corrections, including electroweak corrections to thespin correlation coefficients and related leptonic distributions, contributing to the state-of-the-art high precisionStandard Model predictions for these observables.Finally, paper III is the theoretical base of a crucial improvement to matrix-element generators. We proposein this paper to utilize a next-to-leading-colour truncation of the colour matrix in the large-Nc limit, in order toreduce the complexity of the cross section computation when a large number of QCD patrons are involved in theprocess. The results suggest that such a truncation of the colour expansion will facilitate for efficient computationof multi-jet events, which are a dominant background for many important processes and new physics searches athadron colliders, such as the LHC
W-boson angular coefficients at LHC at high precision
No full textWe present state-of-the-art high-precision theory predictions for the dominant angular coefficients parametrizing the spin-correlations in the production and decay of a W-boson produced at transverse momentum larger than 30 GeV. The computation at NNLO QCD and NLO EW accuracy are combined to obtain differential distributions in the W-boson transverse momentum and rapidity. The found results show up to 10 % corrections in certain regions of phase space, while the scale bands are significantly reduced as compared to NLO QCD
Closed Timelike Curves in Exact Solutions
No full textThis project aims to study general relativity to the extent to understand the occurrence and behaviors of closed timelike curves (CTCs) in several exact solutions of Einstein’s field equations. The rotating black hole solution, the Gödel universe and the cosmic string solutions are studied in detail to show how CTCs arise in these spacetimes. The chronology-violationing paradoxes and other unphysical aspects of CTCs are discussed. The spacetimes where CTCs arise possess properties which are argumented to be unphysical, such as lack of asymptotic flatness and being innite models. With quantum computational networks it is possible to resolve the paradoxes which CTCs evoke. With all these attempts of resolving CTCs, our conclusion is that CTCs exist quantum mechanically, but there is a mechanism which inhibits them to be detected classically.Detta projekt åsyftar att studera allmän relativitet i den grad att kunna förstå uppkomsten och företeelsen av tidsliknande slutna kurvor (CTC) i några exakta lösningar till Einsteins ekvationer. Dessa lösningar inkluderar Gödel universen, kosmiska strängar och det roterande svarta hålet, där CTC studeras i mer detalj. CTC är kronologi-kränkande företeelser och paradoxen som uppstår presenteras, samt de argument som ligger till grund till att CTC inte är fysikaliskt verkliga objekt. De tidrum där CTC uppkommer delar gemensamma egenskaper som anses ofysikaliska, som att vara icke asymptotiskt platta tidrum, samt att vara oändliga modeller. Med kvantinformatiska nätverk kan CTC illustreras och de klassiska kronologi-paradoxen kan rättas ut. Slutsatsen är att CTC existerar kvantmekaniskt, men det fnns en mekanism i verkligheten som förhindrar dessa att bli detekterade klassiskt
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The colour matrix at next-to-leading-colour accuracy for tree-level multi-parton processes
No full textWe investigate the next-to-leading-colour (NLC) contributions to the colour matrix in the fundamental and the colour-flow decompositions for tree-level processes with all gluons, one quark pair and two quark pairs. By analytical examination of the colour factors, we find the non-zero elements in the colour matrix at NLC. At this colour order, together with the symmetry of the phase-space, it is reduced from factorial to polynomial the scaling of the contributing dual amplitudes as the number of partons participating in the scattering process is increased. This opens a path to an accurate tree-level matrix element generator of which all factorial complexity is removed, without resulting to Monte Carlo sampling over colour
Leading-colour-based unweighted event generation for multi-parton tree-level processes
No full textIn this work, we revisit unweighted event generation for multi-parton tree-level processes in massless QCD. We introduce a two-step approach, in which initially unweighted events are generated at leading-colour (LC) accuracy, followed by a reweighting of these events to full-colour (FC) accuracy and applying an additional unweighting cycle. This method leverages the simple structure of LC integrands, enabling optimized phase-space parameterisations and resulting in high primary unweighting efficiencies, ranging from the percent level for 2 → 4 processes to the per-mille level for 2 → 7 processes. Given that the LC-accurate matrix elements closely approximate the FC-accurate ones, the secondary unweighting efficiencies exceed 50%. Our results suggest that this two-step approach offers an efficient alternative to direct event generation at FC accuracy
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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