arXiv subjects

Daisuke Satow

Publications and source records attributed to Daisuke Satow.

At least 19 recordsLinked to original sources

arXiv2020

Goldstino spectrum in an ultracold Bose-Fermi mixture with explicitly broken supersymmetry

We theoretically investigate a supersymmetric collective mode called Goldstino in a Bose-Fermi mixture. The explicit supersymmetry breaking, which is unavoidable in cold atom experiments, is considered. We derive the Gell-Mann--Oakes-Renner (GOR) relation for the Goldstino, which gives the relation between the energy gap at the zero momentum and the explicit breaking term. We also numerically evaluate the gap of Goldstino above the Bose-Einstein condensation temperature within the random phase approximation (RPA). While the gap obtained from the GOR relation coincides with that in the RPA for the mass-balanced system, there is a deviation from the GOR relation in the mass-imbalanced system. We point out the deviation becomes large when the Goldstino pole is close to the branch point, although it is parametrically a higher order with respect to the mass-imbalanced parameter. To examine the existence of the goldstino pole in realistic cold atomic systems, we show how the mass-imbalance effect appears in $^6$Li-$^7$Li, $^{40}$K-$^{41}$K, and $^{173}$Yb-$^{174}$Yb mixtures. Furthermore, we analyze the Goldstino spectral weight in a $^{173}$Yb-$^{174}$Yb mixture with realistic interactions and show a clear peak due to the Goldstino pole. As a possibility to observe the Goldstino spectrum in cold atom experiments, we discuss the effects of the Goldstino pole on the fermionic single-particle excitation as well as the relationship between the GOR relation and Tan's contact.

cond-mat.quant-gas↗

arXiv2019

Recent Progress in QCD Condensate Evaluations and Sum Rules

We review the recent status of the QCD sum rule approach to study the properties of hadrons in vacuum and in hot or dense matter. Special focus is laid on the progress made in the evaluation of the QCD condensates, which are the input of all QCD sum rule calculations, and for which much new information has become available through high precision lattice QCD calculations, chiral perturbation theory and experimental measurements. Furthermore, we critically examine common analysis methods for QCD sum rules and contrast them with potential alternative strategies. The status of QCD sum rule studies investigating the modification of hadrons at finite density as well as recent derivations of exact sum rules applicable to finite temperature spectral functions, are also reviewed.

hep-ph↗

arXiv2018

Gluon Spectrum in Quark-Gluon Plasma under Strong Magnetic Fields

We obtain the general expression of the gluon propagator at finite temperature ($T$) and in a magnetic field ($B$), for the case that the four transverse tensor structures appear in the gluon self-energy. By using this expression and a specific form of the one-loop gluon self-energy in the lowest Landau level approximation, we analyze the gluon spectrum in the strong magnetic field limit. As a result, we find that there exist two collective excitations of which the energies are of the order of $p\sim gT$ with $g$ being the coupling constant. One of the two excitations enjoys properties quite different from those of the collective excitations at $B=0$ which have been discussed by using the hard thermal loop approximation. We also discuss the static and dynamical screening effects, which are expected to be important for computation of transport coefficients in strong magnetic fields.

hep-ph↗

arXiv2017

Finite temperature sum rules in the vector channel at finite momentum

Exact sum rules for the longitudinal and transverse part of the vector channel spectral functions at nonzero momentum are derived in the first part of the paper. The sum rules are formulated for the finite temperature spectral functions, from which the vacuum component has been subtracted, and represent a generalization of previous work in which sum rules were derived only for the zero-momentum limit. In the second part of the paper, we demonstrate how the sum rules can be used as constraints in spectral fits to lattice data at various temperatures, with the latest dynamical lattice QCD data at zero momentum.

hep-ph↗

arXiv2017

Goldstino in supersymmetric Bose-Fermi mixtures in the presence of Bose Einstein condensate

We analyze the spectral properties of the Goldstino excitation in a Bose-Fermi mixture of cold atoms, whose masses and interaction strengths are tuned so that the hamiltonian is supersymmetric. We consider systems at zero temperature and assume that, in the weak coupling regime, the fermions form a Fermi sea, while the bosons form a Bose-Einstein condensate. We study the excitation spectrum within a simple extension of the random phase approximation, taking into account the mixing between the supercharge and the fermion caused by the condensate. This mixing affects the fermion spectrum strongly. We argue that the corresponding modification of the fermion spectrum, and the associated fermion distribution in momentum space, could be accessible experimentally, and potentially allow for a determination of the Goldstino properties.

cond-mat.quant-gas↗

arXiv2017

Bulk Viscosity of Quark-Gluon Plasma in Strong Magnetic Fields

We investigate the viscosities of the quark-gluon plasma in strong magnetic fields within the leading-log and lowest Landau level (LLL) approximations. We first show that the bulk viscosity in the direction parallel to the magnetic field is the only component that has a contribution from the quarks occupying the LLL. We then compute the bulk viscosity from the Kubo formula and find an intriguing quark-mass dependence as a consequence of a competition between the suppression of the bulk viscosity by conformal symmetry and an enhancement of the mean-free path by chirality conservation, which governs the behavior in the massless limit. The quark contribution to the viscosity along the magnetic field becomes larger than the one in the absence of a magnetic field. We also briefly estimate the other transport coefficients by considering the contribution of gluons. We show that the shear viscosities are suppressed compared to their values in the absence of a magnetic field.

hep-ph↗

arXiv2017

Longitudinal Conductivity in Strong Magnetic Field in Perturbative QCD: Complete Leading Order

We compute the longitudinal electrical conductivity in the presence of strong background magnetic field in complete leading order of perturbative QCD, based on the assumed hierarchy of scales $α_s eB\ll (m_q^2,T^2)\ll eB$. We formulate an effective kinetic theory of lowest Landau level quarks with the leading order QCD collision term arising from 1-to-2 processes that become possible due to 1+1 dimensional Landau level kinematics. In small $m_q/T\ll 1$ regime, the longitudinal conductivity behaves as $σ_{zz}\sim e^2(eB)T/(α_s m_q^2\log(m_q/T))$, where the quark mass dependence can be understood from the chiral anomaly with the axial charge relaxation provided by a finite quark mass $m_q$. We also present parametric estimates for the longitudinal and transverse "color conductivities" in the presence of strong magnetic field, by computing dominant damping rates for quarks and gluons that are responsible for color charge transportation. We observe that the longitudinal color conductivity is enhanced by strong magnetic field, which implies that the sphaleron transition rate in perturbative QCD is suppressed by strong magnetic field due to the enhanced Lenz's law in color field dynamics.

hep-ph↗

arXiv2016

Electrical Conductivity of Quark-Gluon Plasma in Strong Magnetic Fields

We compute the electrical conductivity of quark-gluon plasma in a strong magnetic field $B$ with quantum field theory at finite temperature using the lowest Landau level approximation. We provide the one-loop result arising from 1-to-2 scattering processes whose kinematics are satisfied by the (1+1) dimensional fermion dispersion relation. Due to the chirality conservation, the conductivity diverges in the massless limit, and is sensitive to the value of the current quark mass. As a result, we find that the conductivity along the direction of the magnetic field is quite large compared with the value at $B=0$, mainly because of the small value of the current quark mass. We show that the resummation of the ladder diagrams for the current-current correlator gives rise to only sub-leading contributions beyond the leading-log order, and thus verify our one-loop result at the leading-log accuracy. We also discuss possible implications for the relativistic heavy-ion collisions.

hep-ph↗

arXiv2016

Exact vector channel sum rules at finite temperature and their applications to lattice QCD data analysis

We derive three exact sum rules for the spectral function of the electromagnetic current with zero spatial momentum at finite temperature. Two of them are derived in this paper for the first time. We explicitly check that these sum rules are satisfied in the weak coupling regime and examine which sum rule is sensitive to the transport peak in the spectral function at low energy or the continuum at high energy. Possible applications of the three sum rules to lattice computations of the spectral function and transport coefficients are also discussed: We propose an ansatz for the spectral function that can be applied to all three sum rules and fit it to available lattice data of the Euclidean vector correlator above the critical temperature. As a result, we obtain estimates for both the electrical conductivity $σ$ and the second order transport coefficient $τ_J$.

hep-ph↗

arXiv2015

Spectral properties of the Goldstino in supersymmetric Bose-Fermi mixtures

We evaluate the spectral properties of the Goldstino in a Bose-Fermi mixture of cold atoms and molecules whose dynamics is governed by a supersymmetric hamiltonian. Model independent results are obtained from sum rules obeyed by the spectral function. We carry out specific calculations, at zero and finite temperature, using the Random Phase Approximation, and obtain in particular analytic expressions for the dispersion relation at small momentum. These explicit calculations allow us to pin down more precisely the features of the Goldstino that can be attributed to the supersymmetry alone, together with its spontaneous breaking. The anomalous large effect of the Fermi sea at moderate and large momenta is emphasized.

cond-mat.quant-gas↗

arXiv2015

Chiral symmetry breaking and confinement effects on dilepton and photon production around $T_c$

Production rates of dileptons and photons from the quark-gluon (QGP) phase are calculated taking into account effects of confinement and spontaneous chiral symmetry breaking (χSB) not far from the transition temperature $T_c$. We find that the production rates of dileptons with large momenta and of photons originating from the QGP around $T_c$ are suppressed by the χSB effect. We also discuss to what extent information about details of the chiral transition, such as its characteristic temperature range and the steepness of the crossover, are reflected in these quantities.

hep-ph↗

arXiv2015

Dilepton and photon production in the presence of a nontrivial Polyakov loop

We calculate the production of dileptons and photons in the presence of a nontrivial Polyakov loop in QCD. This is applicable to the semi-Quark Gluon Plasma (QGP), at temperatures above but near the critical temperature for deconfinement. The Polyakov loop is small in the semi-QGP, and near unity in the perturbative QGP. Working to leading order in the coupling constant of QCD, we find that there is a mild enhancement, ~ 20%, for dilepton production in the semi-QGP over that in the perturbative QGP. In contrast, we find that photon production is strongly suppressed in the semi-QGP, by about an order of magnitude, relative to the perturbative QGP. In the perturbative QGP photon production contains contributions from 2->2 scattering and collinear emission with the Landau- Pomeranchuk-Migdal (LPM) effect. In the semi-QGP we show that the two contributions are modified differently. The rate for 2->2 scattering is suppressed by a factor which depends upon the Polyakov loop. In contrast, in an SU(N) gauge theory the collinear rate is suppressed by 1/N, so that the LPM effect vanishes at infinite N. To leading order in the semi-QGP at large N, we compute the rate from 2->2 scattering to the leading logarithmic order and the collinear rate to leading order.

hep-ph↗

arXiv2014

Nonlinear electromagnetic response in quark-gluon plasma

We perform the first systematic study of the nonlinear electromagnetic currents induced by external electromagnetic field in quark-gluon plasma, both in the two cases that the inhomogeneity of electromagnetic field is small (large) so that the collision effect is important (negligible). In the former case, we list and classify possible components of the currents in a systematic way, and make an order estimate of each component by using the Boltzmann equation in the relaxation time approximation. In the latter case, we explicitly calculate the quadratic current by using the Vlasov equation, and find that the current generated by the chiral magnetic effect and the quadratic current can have the same order of magnitude by using the Kadanoff-Baym equation. We also demonstrate this property by using a possible configuration of electromagnetic field realized in heavy ion collision.

hep-ph↗

arXiv2014

Chiral Magnetic Effect at Weak Coupling with Relaxation Dynamics

We provide a resolution of an old issue in weak coupling computation of the Chiral Magnetic Effect (CME) current, where a free chiral fermion theory gives two different results depending on the order of the two limits, $ω\rightarrow 0$ (frequency) and $k\rightarrow 0$ (spatial momentum). We first argue based on hydrodynamics that in any reasonable interacting theory of chiral fermions the non-commutativity between the two limits should be absent, and we demonstrate this at weak coupling regime in two different frameworks: kinetic theory in the relaxation time approximation, and diagrammatic computation with resummation of damping rate. In the latter computation, we also show that the "pinch" singularity, which would make the summation of ladder diagrams necessary as in the P-even correlation function, is absent in the relevant P-odd correlation function. The correct value of CME current is reproduced even in the presence of relaxation dynamics in both computations.

hep-ph↗

arXiv2014

Ultra-soft fermionic excitation at finite chemical potential

It has been suggested previously that an ultra-soft fermionic excitation develops, albeit with a small spectral weight, in a system of massless fermions and scalar bosons with Yukawa interaction at high temperature ($T$). In this paper we study how this excitation is modified at finite chemical potential ($μ$). We relate the existence of the ultra-soft mode to symmetries, in particular charge conjugation, and a supersymmetry of the free system which is spontaneously broken by finite temperature and finite density effects, as argued earlier by Lebedev and Smilga. A non vanishing chemical potential breaks both symmetries explicitly, and maximally at zero temperature where the mode ceases to exist. A detailed calculation indicates that the ultra-soft excitation persists as long as $T\gtrsim μ$.

hep-ph↗

arXiv2013

Neutrino spectral density at electroweak scale temperature

Motivated by the scenario of resonant leptogenesis in which lepton number creation in the electroweak-scale is relevant, we investigate the spectral properties and possible collective nature of the standard model neutrinos at electroweak scale temperature (T). We adopt the R_xi gauge fixing, which includes the unitary gauge as a limiting case, and allows us to study the broken as well as the restored phases of the gauge symmetry in a unified way. We show that the spectral density of the neutrino has a three-peak structure in the low-momentum region due to the scattering with the thermally excited particles (i.e., Landau damping) when T becomes comparable to the weak-boson masses in the plasma. The three peaks are identified with a novel ultrasoft mode, the usual quasiparticle, and antiplasmino modes. Varying the gauge-fixing parameter, we show that the three-peak structure appears independently of the gauge fixing and thus has a physical significance. We discuss possible implications of the neutrino spectral density obtained in the present work on particle cosmology, in particular in the context of resonant leptogenesis.

hep-ph↗

arXiv2013

Diagrammatic and Kinetic Equation Analysis of Ultrasoft Fermionic Sector in Quark-Gluon Plasma

At so high temperature (T) that the coupling constant (g) is small and the masses of the particles are negligible, different scheme has to be applied in each energy scale in the analysis of the quark-gluon plasma (QGP). In the soft energy region (p gT), the simple perturbative expansion called the hard thermal loop (HTL) approximation can be applied, and that approximation expects the existence of the bosonic and fermionic collective excitations called plasmon and plasmino. On the other hand, in the ultrasoft energy region (p g^2T), the HTL approximation is inapplicable due to infrared singularity, so the question whether there are any excitation modes in that energy region has not been studied well. In this thesis, we analyze the quark spectrum whose energy is ultrasoft in QGP, using the resummed perturbation theory which enables us to successfully regularize the infrared singularity. Since the Yukawa model and QED are simpler than QCD but have some similarity to QCD, we also work in these models. As a result, we establish the existence of a novel fermionic mode in the ultrasoft energy region, and obtain the expressions of the pole position and the strength of that mode. We also show that the Ward-Takahashi identity is satisfied in the resummed perturbation theory in QED/QCD. Furthermore, we derive the linearized and generalized Boltzmann equation for the ultrasoft fermion excitations in the Kadanoff-Baym formalism, and show that the resultant equation is equivalent to the self-consistent equation in the resummed perturbation theory. We also derive the equation which determines the n-point functions with external lines for a pair of fermions and (n-2) bosons with ultrasoft momenta, by considering the non-linear response regime using the gauge symmetry. We also derive the Ward-Takahashi identity from the conservation law of the electromagnetic current in the Kadanoff-Baym formalism.

hep-ph↗

arXiv2013

Ultrasoft fermion mode and off-diagonal Boltzmann equation in quark-gluon plasma at high temperature

We derive the generalized Boltzmann equation (GBE) near equilibrium from the Kadanoff-Baym equation for quark excitation with ultrasoft momentum (~g^2T, g: coupling constant, T: temperature) in quantum chromodynamics (QCD) at extremely high T, and show that the equation is equivalent to the self-consistent equation derived in the resummed perturbation scheme used to analyze the quark propagator. We obtain the expressions of the dispersion relation, the damping rate, and the strength of a quark excitation with ultrasoft momentum by solving the GBE. We also show that the GBE enables us to obtain the equation determining the n-point function containing a pair of quarks and (n-2) gluon external lines whose momenta are ultrasoft.

hep-ph↗