Neutrinos and Thermal Leptogenesis

One of the unanswered questions of modern cosmology is the issue of baryogenesis. Why does the universe contain a huge amount of baryons but no antibaryons? What kind of a mechanism can produce this kind of an asymmetry? One theory to explain this problem is leptogenesis. In the theory right-handed neutrinos with heavy Majorana masses are added to the standard model. This addition introduces explicit lepton number violation to the theory. Instead of producing the baryon asymmetry directly, these heavy neutrinos decay in the early universe. If these decays are CP-violating, then they produce lepton number. This lepton number is then partially converted to baryon number by the electroweak sphaleron process. In this work we start by reviewing the current observational data on the amount of baryons in the universe. We also introduce Sakharov's conditions, which are the necessary criteria for any theory of baryogenesis. We review the current data on neutrino oscillation, and explain why this requires the existence of neutrino mass. We introduce the different kinds of mass terms which can be added for neutrinos, and explain how the see-saw mechanism naturally explains the observed mass scales for neutrinos motivating the addition of the Majorana mass term. After introducing leptogenesis qualitatively, we derive the Boltzmann equations governing leptogenesis, and give analytical approximations for them. Finally we review the numerical solutions for these equations, demonstrating the capability of leptogenesis to explain the observed baryon asymmetry. In the appendix simple Feynman rules are given for theories with interactions between both Dirac- and Majorana-fermions and these are applied at the tree level to calculate the parameters relevant for the theory.Eräs modernin kosmologian ongelmista on baryogenesis. Miksi universumissa on valtava määrä baryoneja mutta ei lainkaan antibaryoneja? Minkälainen mekanismi voi tuottaa tällaisen asymmetrian? Eräs baryogenesiksen teoria on leptogenesis. Tässä mallissa standardimalliin lisätään raskaat oikeakätiset neutriinot, joilla on Majorana-massatermi. Jos näitten raskaitten neutriinoiden hajoaminen rikkoo CP-symmetrian, tuottaa hajoaminen eri määrän leptoneja ja antileptoneja. Sähköheikko sphaleroni muuttaa näin tuotetun leptoniluvun baryoniluvuksi. Opinnäytetyön aluksi esitellään nykyiset havainnot universumin baryonitiheydestä. Tämän jälkeen käydään läpi baryogenesiksen teoriaa yleisesti. Erityisesti esitellään nk. Sakharovin ehdot, jotka ovat välttämättömät ehdot jokaiselle baryogenesiksen teorialle. Käymme läpi neutriino-oskillatioista saadut rajat neutriinojen massoille, jonka jälkeen tarkastellaan erilaisia massatermejä joita voidaan lisätä oikeakätisille neutriinoille. Kiikkumekanismi selittää luonnollisesti neutriinojen havaitun massaskaalan ja motivoi Majorana-massatermin lisäämisen. Leptogenesiksen perusidea esitellään kvalitatiivisesti, jonka jälkeen teorian Boltzmannin yhtälöt johdetaan. Niitä approksimoidaan analyyttisesti sekä esitellään niitten numeerisia ratkaisuja. Nämä ratkaisut osoittavat että leptogenesis kykenee selittämään havaitun baryoniasymmetrian. Liitteessä johdetaan Feynmanin säännöt teorialle jossa on sekä Dirac- että Majorana-fermioneja sekä niiden välisiä vuorovaikutuksia. Näitä sääntöjä soveltaen teorialle relevantteja parametrejä johdetaan puutasolla

The TeV-mass curvaton

We consider the constraints for a curvaton with mass m ~ 1 TeV and show that they are not consistent with a purely quadratic potential. Even if the curvaton self-interactions were very weak, they must be accounted for as they affect the dynamical evolution of the curvature perturbation. We show that the only TeV-mass curvaton interaction potential that yields the correct perturbation amplitude, decays before the dark matter freeze-out, and does not give rise to non-Gaussian perturbations that are in conflict with the present limits, is given by V_int= sigma^8/M^4. The decay width of the curvaton should be in the range Gamma= 10^-15...10^-17 GeV. The model typically predicts large non-linearity parameters f_NL and g_NL that should be observable by the Planck satellite. We also discuss various physical possibilities to obtain the required small curvaton decay rate

Primordial Perturbations from a Self-interacting Curvaton

Inflation is a period of accelerated expansion in the very early universe, which has the appealing aspect that it can create primordial perturbations via quantum fluctuations. These primordial perturbations have been observed in the cosmic microwave background, and these perturbations also function as the seeds of all large-scale structure in the universe. Curvaton models are simple modifications of the standard inflationary paradigm, where inflation is driven by the energy density of the inflaton, but another field, the curvaton, is responsible for producing the primordial perturbations. The curvaton decays after inflation as ended, where the isocurvature perturbations of the curvaton are converted into adiabatic perturbations. Since the curvaton must decay, it must have some interactions. Additionally realistic curvaton models typically have some self-interactions. In this work we consider self-interacting curvaton models, where the self-interaction is a monomial in the potential, suppressed by the Planck scale, and thus the self-interaction is very weak. Nevertheless, since the self-interaction makes the equations of motion non-linear, it can modify the behaviour of the model very drastically. The most intriguing aspect of this behaviour is that the final properties of the perturbations become highly dependent on the initial values. Departures of Gaussian distribution are important observables of the primordial perturbations. Due to the non-linearity of the self-interacting curvaton model and its sensitivity to initial conditions, it can produce significant non-Gaussianity of the primordial perturbations. In this work we investigate the non-Gaussianity produced by the self-interacting curvaton, and demonstrate that the non-Gaussianity parameters do not obey the analytically derived approximate relations often cited in the literature. Furthermore we also consider a self-interacting curvaton with a mass in the TeV-scale. Motivated by realistic particle physics models such as the Minimally Supersymmetric Standard Model, we demonstrate that a curvaton model within the mass range can be responsible for the observed perturbations if it can decay late enough.Varhainen maailmankaikkeus laajeni alussa eksponentiaalisen nopeasti. Tätä varhaista laajenemisvaihetta kutsutaan kosmologiseksi inflaatioksi. Kosmologisen inflaation tärkeimpiä ominaisuuksia on sen kyky tuottaa pieniä tiheysvaihteluja varhaisessa maailmankaikkeudessa. Näistä prosentin tuhannesosan tiheysvaihteluista on kehittynyt kaikki näkyvä rakenne maailmankaikkeudessa. Nämä tiheysvaihtelut havaitaan myös lämpötilafluktuaatioina kosmisessa mikroaaltosäteilyssä. Eräs kategoria inflaatiomalleja ovat kurvatonimallit. Näissä malleissa universumin laajeneminen johtuu inflatonikentästä, joka dominoi maailmankaikkeuden energiatiheyttä, mutta joissa toinen kenttä, kurvatoni, fluktuoi kvanttimekaanisten ilmiöden vuoksi inflaation aikana. Myöhemmin inflaation päätyttyä nämä pienet kvanttifluktuaatiot muuttuvat kosmologisiksi tiheysperturbaatioiksi, jotka myöhemmin romahtavat painovoiman vaikutuksesta maailmankaikkeuden rakenteeksi kuten galakseiksi ja galaksijoukoiksi. Tässä työssä tutkitaan sellaisia kurvatonimalleja, jotka vuorovaikuttavat itsensä kanssa. Realistisissa kurvatonimalleissa on usein itseiskytkentöjä, ja vaikka itseiskytkentä voi olla hyvin heikko, se muokkaa kurvatonin kehityshistoriaa dramaattisesti muuttamalla mallin epälineaariseksi. Tämän tuloksena tiheysfluktuaatioiden ominaisuudet ovat hyvin herkkiä alkuehdoille ja kurvatonin dynamiikka tuottaa vaikeasti ennustettavia tuloksia. Eräs kosmologisten perturbaatioiden tärkeimpiä mitattavia ominaisuuksia on niiden poikkeama gaussisesta jakaumasta. Tässä työssä on analysoitu itseiskytkettyjen kurvatonimallien tuottamaa ei-gaussisuutta ja osoitettu, että itseiskytketyt kurvatonimallit kykenevät epälineaarisen luonteensa vuoksi tuottamaan suuria poikkeamia gaussisuudesta. Työssä tutkitaan myös kurvatonimalleja joiden massa on uusien hiukkaskokeiden, kuten LHC:n, tutkimalla alueella ja osoitetaan että tämänkaltaiset mallit voivat tuottaa havaitsemamme kosmologiset perturbaatiot

Fast Electroweak Symmetry Breaking and Cold Electroweak Baryogenesis

We construct a model for delayed electroweak symmetry breaking that takes place in a cold Universe with T<<100 GeV and which proceeds by a fast quench rather than by a conventional, slow, phase transition. This is achieved by coupling the Standard Model Higgs to an additional scalar field. We show that the quench transition can be made fast enough for successful Cold Electroweak Baryogenesis, while leaving known electroweak physics unchanged.Comment: 13 pages, 5 figures. Section and extra checks added, conclusions unchanged. Published versio

Gravitational collapse of thin shells: time evolution of the holographic entanglement entropy

Keränen V, Nishimura H, Stricker S, Taanila O, Vuorinen A. Gravitational collapse of thin shells: time evolution of the holographic entanglement entropy. Journal of High Energy Physics. 2015;2015(6): 126.We study the dynamics of gravitationally collapsing massive shells in AdS spacetime, and show in detail how one can determine extremal surfaces traversing them. The results are used to solve the time evolution of the holographic entanglement entropy in a strongly coupled dual conformal gauge theory, which is is seen to exhibit a regime of linear growth independent of the shape of the boundary entangling region and the equation of state of the shell. Our exact results are finally compared to those of two commonly used approximation schemes, the Vaidya metric and the quasistatic limit, whose respective regions of validity we quantitatively determine

Temperature Dependence of Standard Model CP Violation

We analyze the temperature dependence of CP violation effects in the Standard Model by determining the effective action of its bosonic fields, obtained after integrating out the fermions from the theory and performing a covariant gradient expansion. We find non-vanishing CP violating terms starting at the sixth order of the expansion, albeit only in the C odd/P even sector, with coefficients that depend on quark masses, CKM matrix elements, temperature and the magnitude of the Higgs field. The CP violating effects are observed to decrease rapidly with temperature, which has important implications for the generation of a matter-antimatter asymmetry in the early Universe. Our results suggest that the cold electroweak baryogenesis scenario may be viable within the Standard Model, provided the electroweak transition temperature is at most of order 1 GeV.Comment: 4 pages, 2 figures. Additional non-trivial cancellations lead to vanishing of P-odd sector also at finite temperature. Minor additional modifications and clarifications, conclusions otherwise unchanged. Published versio

The Subdominant Curvaton

We present a systematic study of the amplitude of the primordial perturbation in curvaton models with self-interactions, treating both renormalizable and non-renormalizable interactions. In particular, we consider the possibility that the curvaton energy density is subdominant at the time of the curvaton decay. We find that large regions in the parameter space give rise to the observed amplitude of primordial perturbation even for non-renormalizable curvaton potentials, for which the curvaton energy density dilutes fast. At the time of its decay, the curvaton energy density may typically be subdominant by a relative factor of 10^-3 and still produce the observed perturbation. Field dynamics turns out to be highly non-trivial, and for non-renormalizable potentials and certain regions of the parameter space we observe a non-monotonous relation between the final curvature perturbation and the initial curvaton value. In those cases, the time evolution of the primordial perturbation also displays an oscillatory behaviour before the curvaton decay.Comment: Acknowledgments of financial support added, no further change

Holographic dilepton production in a thermalizing plasma

We determine the out-of-equilibrium production rate of dileptons at rest in strongly coupled N=4 Super Yang-Mills plasma using the AdS/CFT correspondence. Thermalization is achieved via the gravitational collapse of a thin shell of matter in AdS_5 space and the subsequent formation of a black hole, which we describe in a quasistatic approximation. Prior to thermalization, the dilepton spectral function is observed to oscillate as a function of frequency, but the amplitude of the oscillations decreases when thermal equilibrium is approached. At the same time, we follow the flow of the quasinormal spectrum of the corresponding U(1) vector field towards its equilibrium limit.Comment: 21 pages, 7 figures. v2: Version accepted for publication in JHEP; minor modifications, added reference