Browsing by Subject "FINITE-TEMPERATURE"

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  • Gorda, Tyler; Kurkela, Aleksi; Paatelainen, Risto; Säppi, Saga; Vuorinen, Aleksi (2021)
    High-order perturbative calculations for thermodynamic quantities in QCD are complicated by the physics of dynamical screening that affects the soft, long-wavelength modes of the system. Here, we provide details for the evaluation of this soft contribution to the next-to-next-to-next-to-leading order ((NLO)-L-3) pressure of high-density, zero-temperature quark matter (QM), complementing our accompanying Letter [T. Gorda et al., Phys. Rev. Lett. 127, 162003 (2021)]. Our calculation requires the determination of the pressure of the hard-thermal-loop effective theory to full two-loop order at zero temperature, which we go through in considerable detail. In addition to this, we comprehensively discuss the structure of the weak-coupling expansion of the QM pressure, and lay out a roadmap towards the evaluation of the contributions missing from a full (NLO)-L-3 result for this quantity.
  • Gould, Oliver; Hirvonen, Joonas (2021)
    The standard vacuum bounce formalism suffers from inconsistencies when applied to thermal bubble nucleation, for which ad hoc workarounds are commonly adopted. Identifying the length scales on which nucleation takes place, we demonstrate how the construction of an effective description for these scales naturally resolves the problems of the standard vacuum bounce formalism. Further, by utilising high temperature dimensional reduction, we make a connection to classical nucleation theory. This offers a clear physical picture of thermal bubble nucleation, as well as a computational framework which can then be pushed to higher accuracy. We demonstrate the method for three qualitatively different quantum field theories.
  • De Romeri, Valentina; Karamitros, Dimitrios; Lebedev, Oleg; Toma, Takashi (2020)
    Sterile neutrinos are one of the leading dark matter candidates. Their masses may originate from a vacuum expectation value of a scalar field. If the sterile neutrino couplings are very small and their direct coupling to the inflaton is forbidden by the lepton number symmetry, the leading dark matter production mechanism is the freeze-in scenario. We study this possibility in the neutrino mass range up to 1 GeV, taking into account relativistic production rates based on the Bose-Einstein statistics, thermal masses and phase transition effects. The specifics of the production mechanism and the dominant mode depend on the relation between the scalar and sterile neutrino masses as well as on whether or not the scalar is thermalized. We find that the observed dark matter abundance can be produced in all of the cases considered. We also revisit the freeze-in production of a Higgs portal scalar, pointing out the importance of a fusion mode, as well as the thermalization constraints.
  • Gorda, Tyler; Kurkela, Aleksi; Romatschke, Paul; Säppi, Matias; Vuorinen, Aleksi (2018)
    At high baryon chemical potential mu(B), the equation of state of QCD allows a weak-coupling expansion in the QCD coupling alpha(s). The result is currently known up to and including the full next-to-next-to-leading order alpha(2)(s). Starting at this order, the computations are complicated by the modification of particle propagation in a dense medium, which necessitates nonperturbative treatment of the scale alpha(1/2)(s) mu(B). We apply a hard-thermal-loop scheme for capturing the contributions of this scale to the weak-coupling expansion, and we use it to determine the leading-logarithm contribution to next-to-next-to-next-to-leading order: alpha(3)(s) ln(2) alpha(s). This result is the first improvement to the equation of state of massless cold quark matter in 40 years. The new term is negligibly small and thus significantly increases our confidence in the applicability of the weak-coupling expansion.
  • Andersen, Jens O.; Gorda, Tyler; Helset, Andreas; Niemi, Lauri; Tenkanen, Tuomas V.; Tranberg, Anders; Vuorinen, Aleksi; Weir, David J. (2018)
    We perform a nonperturbative study of the electroweak phase transition (EWPT) in the two Higgs doublet model (2HDM) by deriving a dimensionally reduced high-temperature effective theory for the model, and matching to known results for the phase diagram of the effective theory. We find regions of the parameter space where the theory exhibits a first-order phase transition. In particular, our findings are consistent with previous perturbative results suggesting that the primary signature of a first-order EWPT in the 2HDM is m(A0) > m(H0) + m(Z).
  • Ghiglieri, Jacopo; Kurkela, Aleksi; Strickland, Michael; Vuorinen, Aleksi (2020)
    In this review article, we discuss the current status and future prospects of perturbation theory as a means of studying the equilibrium thermodynamic and near-equilibrium transport properties of deconfined QCD matter. We begin with a brief introduction to the general topic, after which we review in some detail the foundations and modern techniques of the real- and imaginary-time formalisms of thermal field theory, covering e.g. the different bases used in the real-time formalism and the resummations required to deal with soft and collinear contributions. After this, we discuss the current status of applications of these techniques, including topics such as electromagnetic rates, transport coefficients, jet quenching, heavy quarks and quarkonia, and the Equations of State of hot quark-gluon plasma as well as cold and dense quark matter. Finally, we conclude with our view of the future directions of the field, i.e. how we anticipate perturbative calculations to contribute to our collective understanding of strongly interacting matter in the coming years. (C) 2020 The Author(s). Published by Elsevier B.V.
  • Niemi, Lauri; Schicho, Philipp; Tenkanen, Tuomas V. (2021)
    We investigate the electroweak phase transition in the real-singlet extension of the Standard Model at two-loop level, building upon existing one-loop studies. We calculate the effective potential in the high-temperature approximation and detail the required resummations at two-loop order. In typical strongtransition scenarios, we find deviations of order 20%-50% from one-loop results in transition strength and critical temperature for both one- and two-step phase transitions. For extremely strong transitions, the discrepancy with one-loop predictions is even larger, presumably due to sizable scalar couplings in the tree-level potential. Along the way, we obtain a dimensionally reduced effective theory applicable for nonperturbative lattice studies of the model.
  • Gorda, Tyler; Kurkela, Aleksi; Paatelainen, Risto; Sappi, Saga; Vuorinen, Aleksi (2021)
    Accurate knowledge of the thermodynamic properties of zero-temperature, high-density quark matter plays an integral role in attempts to constrain the behavior of the dense QCD matter found inside neutronstar cores, irrespective of the phase realized inside the stars. In this Letter, we consider the weak-coupling expansion of the dense QCD equation of state and compute the next-to-next-to-next-to-leading-order contribution arising from the non-Abelian interactions among long-wavelength, dynamically screened gluonic fields. Accounting for these interactions requires an all-loop resummation, which can be performed using hard-thermal-loop (HTL) kinematic approximations. Concretely, we perform a full two-loop computation using the HTL effective theory, valid for the long-wavelength, or soft, modes. We find that the soft sector is well behaved within cold quark matter, contrary to the case encountered at high temperatures, and find that the new contribution decreases the renormalization-scale dependence of the equation of state at high density.
  • D'Onofrio, Michela; Rummukainen, Kari (2016)
    With the physical Higgs mass the standard model symmetry restoration phase transition is a smooth cross-over. We study the thermodynamics of the cross-over using numerical lattice Monte Carlo simulations of an effective SU(2) x U(1) gauge + Higgs theory, significantly improving on previously published results. We measure the Higgs field expectation value, thermodynamic quantities like pressure, energy density, speed of sound and heat capacity, and screening masses associated with the Higgs and Z fields. While the cross-over is smooth, it is very well defined with a width of only similar to 5 GeV. We measure the cross-over temperature from the maximum of the susceptibility of the Higgs condensate, with the result T-c = 159.5 +/- 1.5 GeV. Outside of the narrow cross-over region the perturbative results agree well with nonperturbative ones.
  • Gorda, Tyler; Helset, Andreas; Niemi, Lauri; Tenkanen, Tuomas V. I.; Weir, David J. (2019)
    Due to the infrared problem of high-temperature field theory, a robust study of the electroweak phase transition (EWPT) requires use of non-perturbative methods. We apply the method of high-temperature dimensional reduction to the two Higgs doublet model (2HDM) to obtain three-dimensional effective theories that can be used for non-perturbative simulations. A detailed derivation of the mapping between the full four-dimensional and the effective three-dimensional theories is presented. The results will be used in future lattice studies of the 2HDM. In the limit of large mass mixing between the doublets, existing lattice results can be recycled. The results of such a study are presented in a companion paper.
  • Gould, Oliver; Rajantie, Arttu; Xie, Cheng (2018)
    With increasing temperatures, Schwinger pair production changes from a quantum tunneling to a classical, thermal process, determined by a worldline sphaleron. We show this and calculate the corresponding rate of pair production for both spinor and scalar quantum electrodynamics, including the semiclassical prefactor. For electron-positron pair production from a thermal bath of photons and in the presence of an electric field, the rate we derive is faster than both perturbative photon fusion and the zero temperature Schwinger process. We work to all-orders in the coupling and hence our results are also relevant to the pair production of (strongly coupled) magnetic monopoles in heavy-ion collisions.