Initial state of Heavy-Ion Collisions: Isotropization and thermalization

I discuss how local thermal equilibrium and hydrodynamical flow are reached in heavy-ion collisions in the weak coupling limit.Comment: 8 pages, 5 figs, proceedings of the Quark Matter 201

Neutron star structure from QCD

In this review article, we argue that our current understanding of the thermodynamic properties of cold QCD matter, originating from first principles calculations at high and low densities, can be used to efficiently constrain the macroscopic properties of neutron stars. In particular, we demonstrate that combining state-of-the-art results from Chiral Effective Theory and perturbative QCD with the current bounds on neutron star masses, the Equation of State of neutron star matter can be obtained to an accuracy better than 30% at all densities.Comment: Invited contribution to the EPJA Topical Issue "Exotic Matter in Neutron Stars"; 10 pages, 13 figure

Isotropization and hydrodynamization in weakly coupled heavy-ion collisions

We numerically solve 2+1D effective kinetic theory of weak coupling QCD under longitudinal expansion relevant for early stages of heavy-ion collisions. We find agreement with viscous hydrodynamics and classical Yang-Mills simulations in the regimes where they are applicable. By choosing initial conditions that are motivated by color-glass-condensate framework we find that for Q=2GeV and $\alpha_s$=0.3 the system is approximately described by viscous hydrodynamics well before $\tau \lesssim 1.0$ fm/c.Comment: 6 pages, 4 figures. Shortened for PRL, figs. 1 and 2 modifie

Analytic structure of nonhydrodynamic modes in kinetic theory

How physical systems approach hydrodynamic behavior is governed by the decay of nonhydrodynamic modes. Here, we start from a relativistic kinetic theory that encodes relaxation mechanisms governed by different timescales thus sharing essential features of generic weakly coupled nonequilib- rium systems. By analytically solving for the retarded correlation functions, we clarify how branch cuts arise generically from noncollective particle excitations, how they interface with poles arising from collective hydrodynamic excitations, and to what extent the appearance of poles remains at best an ambiguous signature for the onset of fluid dynamic behavior. We observe that processes that are slower than the hydrodynamic relaxation timescale can make a system that has already reached fluid dynamic behavior to fall out of hydrodynamics at late times. In addition, the analytical control over this model allows us to explicitly demonstrate how the hydrodynamic gradient expansion of the correlation functions can be Borel resummed such that the full nonperturbative information is recovered using perturbative input only.Comment: 21 pages, 15 fig

Dimensional Reduction Near the Deconfinement Transition

When ordinary nuclear matter is heated to a high temperature of ~ 10^12 K, it undergoes a deconfinement transition to a new phase, strongly interacting quark-gluon plasma. While the color charged fundamental constituents of the nuclei, the quarks and gluons, are at low temperatures permanently confined inside color neutral hadrons, in the plasma the color degrees of freedom become dominant over nuclear, rather than merely nucleonic, volumes. Quantum Chromodynamics (QCD) is the accepted theory of the strong interactions, and confines quarks and gluons inside hadrons. The theory was formulated in early seventies, but deriving first principles predictions from it still remains a challenge, and novel methods of studying it are needed. One such method is dimensional reduction, in which the high temperature dynamics of static observables of the full four-dimensional theory are described using a simpler three-dimensional effective theory, having only the static modes of the various fields as its degrees of freedom. A perturbatively constructed effective theory is known to provide a good description of the plasma at high temperatures, where asymptotic freedom makes the gauge coupling small. In addition to this, numerical lattice simulations have, however, shown that the perturbatively constructed theory gives a surprisingly good description of the plasma all the way down to temperatures a few times the transition temperature. Near the critical temperature, the effective theory, however, ceases to give a valid description of the physics, since it fails to respect the approximate center symmetry of the full theory. The symmetry plays a key role in the dynamics near the phase transition, and thus one expects that the regime of validity of the dimensionally reduced theories can be significantly extended towards the deconfinement transition by incorporating the center symmetry in them. In the introductory part of the thesis, the status of dimensionally reduced effective theories of high temperature QCD is reviewed, placing emphasis on the phase structure of the theories. In the first research paper included in the thesis, the non-perturbative input required in computing the g^6 term in the weak coupling expansion of the pressure of QCD is computed in the effective theory framework at an arbitrary number of colors. The two last papers on the other hand focus on the construction of the center-symmetric effective theories, and subsequently the first non-perturbative studies of these theories are presented. Non-perturbative lattice simulations of a center-symmetric effective theory for SU(2) Yang-Mills theory show --- in sharp contrast to the perturbative setup --- that the effective theory accommodates a phase transition in the correct universality class of the full theory. This transition is seen to take place at a value of the effective theory coupling constant that is consistent with the full theory coupling at the critical temperature.Kun protoneista ja neutroneista koostuvaa ydinainetta kuumennetaan hyvin korkeaan lämpötilaan ~ 10^12 K, se läpikäy olomuodon muutoksen vahvasti vuorovaikuttavaksi kvarkkigluoniplasmaksi. Matalassa lämpötilassa vahva ydinvoima sitoo ydinhiukkasten rakennusosaset kvarkit ja gluonit tiukasti ydinhiukkasten sisälle, mutta olomuodon muutoksessa ydinhiukkasten rakenne sulaa ja kvarkit sekä gluonit vapautuvat vankeudestaan. Vahvan ydinvoiman teoria kvanttikromodynamiikka on tunnettu jo yli kolme vuosikymmentä, mutta silti teorian monimutkaisuuden takia tarkkojen ennusteiden tekeminen teoriasta on hyvin haastavaa ja uusia teoreettiesia tutkimusmenetelmiä tarvitaan. Eräs tällainen menetelmä on ulottuvuuksien supistaminen, jossa joitakin alkuperäisen neliulotteisen teorian tiettyjä tarkkaan rajattuja ominaisuuksia voidaan tutkia paljon helpommin yksinkertaisemmassa kolmiulotteisessa teoriassa. Asymptoottisen korkeassa lämpötilassa kvarkkien ja gluonien vuorovaikutuksen voimakkuus heikkenee, ja kolmiulotteinen teoria voidaan rakentaa käsittelemällä vain pieniä häiriöitä vapaan teorian ympärillä. Numeerisilla simulaatioilla on kuitenkin havaittu, että tämä efektiivinen teoria kuvaa tarkasti alkuperäistä teoriaa vielä yllättävänkin matalissa lämpötiloissa. Olomuodon muutoksen läheisyydessä kuvailu silti hajoaa, koska oletus pienistä häiriöistä vapaan teorian ympärillä on tässä tapauksessa perusteeton; oletus pienistä häiriöistä rikkoo alkuperäisen teorian keskussymmetrian, jolla on tärkeä merkitys olomuodon muutoksessa. Rakentamalla keskussymmetrian säilyttäviä efektiivisiä teorioita voidaan ulottuvuuksien supistamista kuitenkin käyttää vielä olomuodon muutoksen välittömässä läheisyydessä. Väitöskirja koostuu kolmesta julkaisusta ja johdanto-osasta. Johdanto-osassa esitetään katsaus korkean lämpötilan kvanttikromodynamiikkaa kuvaaviin ulottuvuudellisesti supistettuihin teorioihin. Ensimmäisessä julkaisussa ulottuvuuksien supistamista sovellettiin kvanttikromodynamiikan tilanyhtälön heikon kytkennän ekspansioon, jonka g^6-termiin tarvittava numeerinen syöte määritettiin simuloimalla ulottuvuudellisesti supistettua teoriaa. Jälkimmäiset julkaisut keskittyvät keskussymmetrian säilyttävien efektiivisten teorioiden rakentamiseen ja näiden ensimmäisiin numeerisiin analyyseihin. Numeeriset hilasimulaatiot osoittivat että, toisin kuin keskussymmetrian rikkovalla teorialla, SU(2)-mittaryhmän tapauksessa keskussymmetrian säilyttävällä efektiivisellä teorialla on faasitransitio, joka kuuluu samaan universaalisuusluokkaan alkuperäisen teorian kanssa

Cool Quark Matter

We generalize the state-of-the-art perturbative equation of state of cold quark matter to nonzero temperatures, needed in the description of neutron star mergers and core collapse processes. The new result is accurate to O(g(5)) in the gauge coupling, and is based on a novel framework for dealing with the infrared sensitive soft field modes of the theory. The zero Matsubara mode sector is treated via a dimensionally reduced effective theory, while the soft nonzero modes are resummed using the hard thermal loop approximation. This combination of known effective descriptions offers unprecedented access to small but nonzero temperatures, both in and out of beta equilibrium.Peer reviewe

Picturing perturbative parton cascades in QCD matter

Based on parametric reasoning, we provide a simple dynamical picture of how a perturbative parton cascade, in interaction with a QCD medium, fills phase space as a function of time.Comment: 9 pages, 3 figure