Stephan Stetina | University of Washington (original) (raw)

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Papers by Stephan Stetina

We show that effective field theory techniques can be applied in the high temperature T regime of... more We show that effective field theory techniques can be applied in the high temperature T regime of plasmas to improve the accuracy of the physics of the hard scales (or scales of order T), and as a by-product, also that of the soft scales (or scales of order gT). At leading order in the coupling constant the hard scales of the plasma can be viewed as on-shell classical particles. Based on this observation, and without any reference to the state of the system, we derive an effective field theory describing the quantum fluctuations around an on-shell fermion with energy p, described as a set of high dimension operators over the on-shell energy p. When applied to systems close to thermal equilibrium , where for most on-shell particles p ∼ T , we show that the on-shell effective field theory (OSEFT) properly describes the HTL photon polarization tensor of QED, and its 1/T corrections. For the soft scales the first non-vanishing power correction turns out to be a perturbative correction to the HTL result.

PhysRevD.94.025017

We show that effective field theory techniques can be efficiently used to compute power correctio... more We show that effective field theory techniques can be efficiently used to compute power corrections to the hard thermal loops (HTL) in a high temperature T expansion. To this aim, we use the recently proposed on-shell effective field theory (OSEFT), which describes the quantum fluctuations around on-shell degrees of freedom. We provide the OSEFT Lagrangian up to third order in the energy expansion for QED, and use it for the computation of power corrections to the retarded photon polarization tensor for soft external momenta. Here soft denotes a scale of order eT, where e is the gauge coupling constant. We develop the necessary techniques to perform these computations, and study the contributions to the polarization tensor proportional to e^2*T^2, e^2*T and e^2*T^0. The first one describes the HTL contribution, the second one vanishes, while the third one provides corrections of order e^2 to the soft photon propagation. We check that the results agree with the direct calculation from QED, up to local pieces, as expected in an effective field theory.

We study two-fluid systems with nonzero fluid velocities and compute their sound modes, which ind... more We study two-fluid systems with nonzero fluid velocities and compute their sound modes, which indicate various instabilities. For the case of two zero-temperature superfluids we employ a microscopic field-theoretical model of two coupled bosonic fields, including an entrainment coupling and a non-entrainment coupling. We analyse the onset of the various instabilities systematically and point out that the dynamical two-stream instability can only occur beyond Landau's critical velocity, i.e., in an already energetically unstable regime. A qualitative difference is found for the case of two normal fluids, where certain transverse modes suffer a two-stream instability in an energetically stable regime if there is entrainment between the fluids. Since we work in a fully relativistic setup, our results are very general and of potential relevance for (super)fluids in neutron stars and, in the non-relativistic limit of our results, in the laboratory.

cite as: arXiv: 1502.00122/hep-ph, Oct 2014

The hydrodynamic description of a superfluid is usually based on a two-fluid picture. In this the... more The hydrodynamic description of a superfluid is usually based on a two-fluid picture. In this thesis, basic properties of such a relativistic two-fluid system are derived from the underlying microscopic physics of a complex scalar quantum field theory. To obtain analytic results of all non-dissipative hydrodynamic quantities in terms of field theoretic variables, calculations are first carried out in a lowtemperature and weak-coupling approximation. In a second step, the 2-particle-irreducible formalism is applied: This formalism allows for a numerical evaluation of the hydrodynamic parameters for all temperatures below the critical temperature. In addition, a system of two coupled superfluids is studied. As an application, the velocities of first and second sound in the presence of a superflow are calculated. The results show that first (second) sound evolves from a density (temperature) wave at low temperatures to a temperature (density) wave at high temperatures. This role reversal is investigated for ultra-relativistic and near-nonrelativistic systems for zero and nonzero superflow. The studies carried out in this thesis are of a very general nature as one does not have to specify the
system for which the microscopic field theory is an effective description. As a particular example, superfluidity in dense quark and nuclear matter in compact stars are discussed.

It is well known that the hydrodynamics of a zero-temperature superfluid can be formulated in fi... more It is well known that the hydrodynamics of a zero-temperature superfluid can be formulated in
field-theoretic terms, relating for example the superfluid four-velocity to the gradient of the phase
of a Bose-condensed scalar field. At nonzero temperatures, where the phenomenology of a superfluid
is usually described within a two-fluid picture, this relationship is less obvious. For the
case of a uniform, dissipationless superfluid at small temperatures and weak coupling we discuss
this relationship within a j4 model. For instance, we compute the entrainment coefficient, which
describes the interaction between the superfluid and the normal-fluid components, and the velocities
of first and second sound in the presence of a superflow. Our study is very general, but can
also be seen as a step towards understanding the superfluid properties of various phases of dense
nuclear and quark matter in the interior of compact stars.

Physical Review D, 2014

Relativistic superfluidity at arbitrary temperature, chemical potential and (uniform) superflow i... more Relativistic superfluidity at arbitrary temperature, chemical potential and (uniform) superflow is discussed within a self-consistent field-theoretical approach. Our starting point is a complex scalar field with a φ4 interaction, for which we calculate the two-particle-irreducible effective action in the Hartree approximation. With this underlying microscopic theory, we can obtain the two-fluid picture of a superfluid, and compute properties such as the superfluid density and the entrainment coefficient for all temperatures below the critical temperature for superfluidity. We compute the critical velocity, taking into account the full self-consistent effect of the temperature and superflow on the quasiparticle dispersion. We also discuss first and second sound modes and how first (second) sound evolves from a density (temperature) wave at low temperatures to a temperature (density) wave at high temperatures. This role reversal is investigated for ultrarelativistic and near-nonrelativistic systems for zero and nonzero superflow. For nonzero superflow, we also observe a role reversal as a function of the direction of the sound wave.

The phase structure of dense QCD matter is studied based on the Ginzburg-Landau approach. In thre... more The phase structure of dense QCD matter is studied based on the Ginzburg-Landau approach. In three flavor massless quark matter, one can show that a novel entanglement between chiral condensate and diquark condensate via the axial anomaly gives rise to a critical point at moderate density. We further investigate the effect of nonzero strange quark mass by taking into account

Physical Review D, 2013

The hydrodynamic description of a superfluid is usually based on a two-fluid picture. We compute ... more The hydrodynamic description of a superfluid is usually based on a two-fluid picture. We compute the basic properties of the relativistic two-fluid system from the underlying microscopic physics of a relativistic φ4 complex scalar field theory. We work at nonzero but small temperature and weak coupling, and we neglect dissipation. We clarify the relationship between different formulations of the two-fluid model and how they are parametrized in terms of partly redundant current and momentum four-vectors. As an application, we compute the velocities of first and second sound at small temperatures and in the presence of a superflow. While our results are of a very general nature, we also comment on their interpretation as a step towards the hydrodynamics of the color-flavor locked state of quark matter, which, particularly in the presence of kaon condensation, appears to be a complicated multicomponent fluid.

Physical Review D, 2011

We discuss the phase structure of dense matter, in particular the nature of the transition betwee... more We discuss the phase structure of dense matter, in particular the nature of the transition between hadronic and quark matter. Calculations within a Ginzburg-Landau approach show that the axial anomaly can induce a critical point in this transition region. This is possible because in three-flavor quark matter with instanton effects a chiral condensate can be added to the color-flavor locked (CFL) phase without changing the symmetries of the ground state. In (massless) two-flavor quark matter such a critical point is not possible since the corresponding color superconductor (2SC) does not break chiral symmetry. We study the effects of a nonzero but finite strange quark mass which interpolates between these two cases. Since at ultra-high density the first reaction of CFL to a nonzero strange quark mass is to develop a kaon condensate, we extend previous Ginzburg-Landau studies by including such a condensate. We discuss the fate of the critical point systematically and show that the continuity between hadronic and quark matter can be disrupted by the onset of a kaon condensate. Moreover, we identify the mass terms in the Ginzburg-Landau potential which are needed for the 2SC phase to occur in the phase diagram.

Talks by Stephan Stetina

We show that effective field theory techniques can be applied in the high temperature T regime of... more We show that effective field theory techniques can be applied in the high temperature T regime of plasmas to improve the accuracy of the physics of the hard scales (or scales of order T), and as a by-product, also that of the soft scales (or scales of order gT). At leading order in the coupling constant the hard scales of the plasma can be viewed as on-shell classical particles. Based on this observation, and without any reference to the state of the system, we derive an effective field theory describing the quantum fluctuations around an on-shell fermion with energy p, described as a set of high dimension operators over the on-shell energy p. When applied to systems close to thermal equilibrium , where for most on-shell particles p ∼ T , we show that the on-shell effective field theory (OSEFT) properly describes the HTL photon polarization tensor of QED, and its 1/T corrections. For the soft scales the first non-vanishing power correction turns out to be a perturbative correction to the HTL result.

PhysRevD.94.025017

We show that effective field theory techniques can be efficiently used to compute power correctio... more We show that effective field theory techniques can be efficiently used to compute power corrections to the hard thermal loops (HTL) in a high temperature T expansion. To this aim, we use the recently proposed on-shell effective field theory (OSEFT), which describes the quantum fluctuations around on-shell degrees of freedom. We provide the OSEFT Lagrangian up to third order in the energy expansion for QED, and use it for the computation of power corrections to the retarded photon polarization tensor for soft external momenta. Here soft denotes a scale of order eT, where e is the gauge coupling constant. We develop the necessary techniques to perform these computations, and study the contributions to the polarization tensor proportional to e^2*T^2, e^2*T and e^2*T^0. The first one describes the HTL contribution, the second one vanishes, while the third one provides corrections of order e^2 to the soft photon propagation. We check that the results agree with the direct calculation from QED, up to local pieces, as expected in an effective field theory.

We study two-fluid systems with nonzero fluid velocities and compute their sound modes, which ind... more We study two-fluid systems with nonzero fluid velocities and compute their sound modes, which indicate various instabilities. For the case of two zero-temperature superfluids we employ a microscopic field-theoretical model of two coupled bosonic fields, including an entrainment coupling and a non-entrainment coupling. We analyse the onset of the various instabilities systematically and point out that the dynamical two-stream instability can only occur beyond Landau's critical velocity, i.e., in an already energetically unstable regime. A qualitative difference is found for the case of two normal fluids, where certain transverse modes suffer a two-stream instability in an energetically stable regime if there is entrainment between the fluids. Since we work in a fully relativistic setup, our results are very general and of potential relevance for (super)fluids in neutron stars and, in the non-relativistic limit of our results, in the laboratory.

cite as: arXiv: 1502.00122/hep-ph, Oct 2014

The hydrodynamic description of a superfluid is usually based on a two-fluid picture. In this the... more The hydrodynamic description of a superfluid is usually based on a two-fluid picture. In this thesis, basic properties of such a relativistic two-fluid system are derived from the underlying microscopic physics of a complex scalar quantum field theory. To obtain analytic results of all non-dissipative hydrodynamic quantities in terms of field theoretic variables, calculations are first carried out in a lowtemperature and weak-coupling approximation. In a second step, the 2-particle-irreducible formalism is applied: This formalism allows for a numerical evaluation of the hydrodynamic parameters for all temperatures below the critical temperature. In addition, a system of two coupled superfluids is studied. As an application, the velocities of first and second sound in the presence of a superflow are calculated. The results show that first (second) sound evolves from a density (temperature) wave at low temperatures to a temperature (density) wave at high temperatures. This role reversal is investigated for ultra-relativistic and near-nonrelativistic systems for zero and nonzero superflow. The studies carried out in this thesis are of a very general nature as one does not have to specify the
system for which the microscopic field theory is an effective description. As a particular example, superfluidity in dense quark and nuclear matter in compact stars are discussed.

It is well known that the hydrodynamics of a zero-temperature superfluid can be formulated in fi... more It is well known that the hydrodynamics of a zero-temperature superfluid can be formulated in
field-theoretic terms, relating for example the superfluid four-velocity to the gradient of the phase
of a Bose-condensed scalar field. At nonzero temperatures, where the phenomenology of a superfluid
is usually described within a two-fluid picture, this relationship is less obvious. For the
case of a uniform, dissipationless superfluid at small temperatures and weak coupling we discuss
this relationship within a j4 model. For instance, we compute the entrainment coefficient, which
describes the interaction between the superfluid and the normal-fluid components, and the velocities
of first and second sound in the presence of a superflow. Our study is very general, but can
also be seen as a step towards understanding the superfluid properties of various phases of dense
nuclear and quark matter in the interior of compact stars.

Physical Review D, 2014

Relativistic superfluidity at arbitrary temperature, chemical potential and (uniform) superflow i... more Relativistic superfluidity at arbitrary temperature, chemical potential and (uniform) superflow is discussed within a self-consistent field-theoretical approach. Our starting point is a complex scalar field with a φ4 interaction, for which we calculate the two-particle-irreducible effective action in the Hartree approximation. With this underlying microscopic theory, we can obtain the two-fluid picture of a superfluid, and compute properties such as the superfluid density and the entrainment coefficient for all temperatures below the critical temperature for superfluidity. We compute the critical velocity, taking into account the full self-consistent effect of the temperature and superflow on the quasiparticle dispersion. We also discuss first and second sound modes and how first (second) sound evolves from a density (temperature) wave at low temperatures to a temperature (density) wave at high temperatures. This role reversal is investigated for ultrarelativistic and near-nonrelativistic systems for zero and nonzero superflow. For nonzero superflow, we also observe a role reversal as a function of the direction of the sound wave.

The phase structure of dense QCD matter is studied based on the Ginzburg-Landau approach. In thre... more The phase structure of dense QCD matter is studied based on the Ginzburg-Landau approach. In three flavor massless quark matter, one can show that a novel entanglement between chiral condensate and diquark condensate via the axial anomaly gives rise to a critical point at moderate density. We further investigate the effect of nonzero strange quark mass by taking into account

Physical Review D, 2013

The hydrodynamic description of a superfluid is usually based on a two-fluid picture. We compute ... more The hydrodynamic description of a superfluid is usually based on a two-fluid picture. We compute the basic properties of the relativistic two-fluid system from the underlying microscopic physics of a relativistic φ4 complex scalar field theory. We work at nonzero but small temperature and weak coupling, and we neglect dissipation. We clarify the relationship between different formulations of the two-fluid model and how they are parametrized in terms of partly redundant current and momentum four-vectors. As an application, we compute the velocities of first and second sound at small temperatures and in the presence of a superflow. While our results are of a very general nature, we also comment on their interpretation as a step towards the hydrodynamics of the color-flavor locked state of quark matter, which, particularly in the presence of kaon condensation, appears to be a complicated multicomponent fluid.

Physical Review D, 2011

We discuss the phase structure of dense matter, in particular the nature of the transition betwee... more We discuss the phase structure of dense matter, in particular the nature of the transition between hadronic and quark matter. Calculations within a Ginzburg-Landau approach show that the axial anomaly can induce a critical point in this transition region. This is possible because in three-flavor quark matter with instanton effects a chiral condensate can be added to the color-flavor locked (CFL) phase without changing the symmetries of the ground state. In (massless) two-flavor quark matter such a critical point is not possible since the corresponding color superconductor (2SC) does not break chiral symmetry. We study the effects of a nonzero but finite strange quark mass which interpolates between these two cases. Since at ultra-high density the first reaction of CFL to a nonzero strange quark mass is to develop a kaon condensate, we extend previous Ginzburg-Landau studies by including such a condensate. We discuss the fate of the critical point systematically and show that the continuity between hadronic and quark matter can be disrupted by the onset of a kaon condensate. Moreover, we identify the mass terms in the Ginzburg-Landau potential which are needed for the 2SC phase to occur in the phase diagram.