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Nelinearni sigma modeli
ID Ogrinec, Urban (Author), ID Prosen, Tomaž (Mentor) More about this mentor... This link opens in a new window, ID Škoda, Zoran (Comentor)

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Abstract
Nelinearni sigma model (NLSM) je teorija polja, katerega polja zavzemajo vrednosti v Riemannovi mnogoterosti M. NLSM poljske vrednosti lahko torej razumemo kot koordinate na mnogoterosti M, katere metrika je odvisna od polj. NLSM je torej zelo splošen teoretičen koncept, čigar geometrijski pomen je glavni razlog za njegove uspešne aplikacije v teoriji polja, strun in statistični mehaniki. Koristnost NLSM v kvantni teoriji polja (QFT) izvira iz pomembnosti simetrij v fundamentalni fziki, ki je opisljiva s Standardnim modelom (SM), tj. lokalno QFT z umeritveno simetrijo SU(3) × SU(2) × U(1). Ta združuje močno (QCD) z elektrošibko interakcijo in je načeloma konsistenten z eksperimentalnimi odkritji. Kljub uspehu perturbativnega pristopa k asimtotično prosti QCD, je teoretično razumevanje ujetosti (confinement) kvarkov in gluonov v hadronih še vedno odsotno. Razlog je seveda neperturbativna narava ujetosti, katerega formalni dokaz v neabelskih umeritvenih teorijah zahteva neperturbativna teoretična orodja za izpeljavo kvantne nizkoenergijske efektivne akcije. Nekaj časa je bil mrežni pristop (lattice QCD) edini način študija dolgovalovnih pojavov, saj je bil izračun QCD efektivne akcije pretežak. Nedavno sta Seiberg in Witten s pomočjo supersimetrije (SUSY) in dualnosti predlagala rešiti ta problem in razložiti ujetost v QCD. V mnogo teoretičnih študijah je predloagano, da so sestavni deli ultimativne QFT 4D supersimetrične umeritvene teorije. Izračuni v SUSY teorijah so enostavnejši zaradi krajšanj med Feynmanovimi diagrami bozonov in fermionov. NLSM ponuja tudi poljsko teoretičen laboratorij za študij 2D eksaktno rešljivih sistemov na mreži, kot so Isingov model ali Heisenbergov antiferomagnet. O(N) 2D NLSM so pogosti v fziki kondenzirane snovi, v povezavi z antifermagnetnimi spinskimi verigami in kvantnim Hallovim efektom. Efektivni Lagrangijan supertekočega He3 lahko prav tako opišemo z NLSM. Močan argument za študij NLSM je spontani zlom simetrije, ključen za fenomenološke aplikacije QFT. Spontano zlomljene simetrije niso simetrije transformacij fizikalnih stanj, torej ni res, da pustijo vakuum invarianten. Spontano zlomljene simetrije so vedno povezane z degeneracijo vakuumskih stanj. Če je zlomljena zvezna globalna simetrija dobimo po Goldstonovem izreku brezmasni (Nambu-Goldstonov) delec za vsak generator zlomljene simetrije. Spontano zlomljene simetrije določajo tudi nelinearno efektivno akcijo NG polj. Če so ta polja skalarji, nizkoenergijska efektivna akcija postane NLSM. Splošne nelinearne Goldstonove akcije so torej posplošitve NLSM akcij, četudi sploh nimajo skalarjev. V delu si bomo ogledali matematična orodja kot so supergeometrija in BV formalizem za definicijo določene posplošitve NLSM, t.i. AKSZ modela. To vljučuje Q-mnogoterosti, P-strukturo, klasični in kvantni BRST formalizem, liho-simplektične mnogoterosti in BV formalizem. Ogledali si bomo tudi različne modele NLSM iz fizike delcev, kondenzirane snovi in topoloških kvantih teorij polja (TFT), npr. SUSY NLSM in Wess-Zumino-Wittenov model.

Language:Slovenian
Keywords:supergeometrija, BRST formalizem, liho-simplektična mnogoterost, BV algebra, master enačba, AKSZ sigma model, SNLSM, WZW model, fazni prehodi
Work type:Master's thesis/paper
Typology:2.09 - Master's Thesis
Organization:FMF - Faculty of Mathematics and Physics
Year:2024
PID:20.500.12556/RUL-159333 This link opens in a new window
COBISS.SI-ID:200897283 This link opens in a new window
Publication date in RUL:06.07.2024
Views:391
Downloads:35
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Secondary language

Language:English
Title:Nonlinear sigma models
Abstract:
Nonlinear sigma model (NLSM) is a field theory, whose fields take values in Riemannian manifold M. NLSM field values can be understood as coordinates on M, for which metric is field-depenedent. NLSM is a general theoretical concept, whose geometric meaning is the reason for its successful applications in field theory, string theory and statistical mechanics. Usefullness of NLSM in QFT stems from the importance of symmetries in fundamental physics, which could be described via Standard model (SM), i.e. local QFT with gauge symmetry SU(3) × SU(2) × U(1). SM encompasses strong (QCD) and electroweak interaction and is consistent with numerous experimental data. Although perturbative approach successfully describes asymptotically free QCD, theoretical understanding of confinement of quarks and gluons in hadrons is still absent. The reason is the nonpertubative nature of confinement. In nonabelian gauge theories nonperturbative tools were used for the derivation of low energy effective action. For quite some time lattice QCD was the only way to study long distance phenomena. Recently Seiberg and Witten proposed supersymmetry (SUSY) and duality to solve the problem and understand the confinement in QCD. In many theoretical studies it is suggested that the building blocks of the ultimate QFT are 4D SUSY gauge theories, where the calculations are easier due to Feynman diagram cancellations. NLSM offers field theoretic laboratory for the study of 2D exactly solvable systems on a lattice, such as Ising model and Heisenberg antiferromagnet. O(N) 2D NLSM are frequent in condensed matter physics where they appear as antiferromagnetic spin chains or with regards to quantum Hall effect. Effective Lagrangian of superfuid He3 is NLSM as well. Strong argument for the study of NLSM is spontaneous symmetry breaking, which is crucial part of phenomenological QFT. Spontaneously broken symmetries (SBS) are not symmetries of physical state transformations, i.e. they do not leave vacuum invariant, however they are always connected with degenerate vacuum state. If continuous global symmetry is broken, we get Goldstone massless boson for every generator of broken symmetry. SBS determine nonlinear effective action of Goldstone fields. If fields are scalars, low energy effective action becomes NLSM. In the work we will present mathematical tools for the development of NLSM: supergeometry and BV formalism for the definition of AKSZ model (some generalization of NLSM). Specifically this includes Q-manifolds, P-structure, classical and quantum BRST formalism, odd-symplectic manifolds and BV formalism. Different NLSM models from particle and condensed matter physics are described. In the end topological field theories such as SUSY NLSM and Wess-Zumino-Witten model are shortly introduced.

Keywords:supergeometry, BRST formalism, odd-symplectic manifold, BV algebra, master equation, AKSZ sigma model, SNLSM, WZW model, phase transitions

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