arXiv subjects

Koichi Hattori

Publications and source records attributed to Koichi Hattori.

54 records · Page 3Linked to original sources

arXiv2017

Novel quantum phenomena induced by strong magnetic fields in heavy-ion collisions

The relativistic heavy-ion collisions create both hot quark-gluon matter and strong magnetic fields, and provide an arena to study the interplay between quantum chromodynamics and quantum electrodynamics. In recent years, it has been shown that such an interplay can generate a number of interesting quantum phenomena in hadronic and quark-gluon matter. In this short review, we first discuss some properties of the magnetic fields in heavy-ion collisions and then give an overview of the magnetic field-induced novel quantum effects. In particular, we focus on the magnetic effect on the heavy flavor mesons, the heavy-quark transports, and the phenomena closely related to chiral anomaly.

nucl-th↗

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

Heavy-Quark Diffusion Dynamics in Quark-Gluon Plasma under Strong Magnetic Fields

We discuss heavy-quark dynamics in the quark-gluon plasma under a strong magnetic field induced by colliding nuclei. By the use of the diagrammatic resummation techniques for Hard Thermal Loop and the external magnetic field, we show analytic results of heavy-quark diffusion coefficient and drag force which become anisotropic due to the preferred spatial orientation in the magnetic field. We argue that the anisotropic diffusion coefficient gives rise to an enhancement/suppression of the heavy-quark elliptic flow depending on the transverse momentum.

hep-ph↗

arXiv2016

Charge redistribution from anomalous magnetovorticity coupling

We investigate novel transport phenomena in a chiral fluid originated from an interplay between a vorticity and strong magnetic field, which induces a redistribution of vector charges in the system and an axial current along the magnetic field. The corresponding transport coefficients are obtained from an energy-shift argument for the chiral fermions in the lowest Landau level (LLL) due to a spin-vorticity coupling and also from diagrammatic computations on the basis of the linear response theory. Based on consistent results from the both methods, we observe that the transport coefficients are proportional to the anomaly coefficient and are independent of temperature and chemical potential. We therefore speculate that these transport phenomena are connected to quantum anomaly.

hep-th↗

arXiv2016

Heavy Quark Diffusion in Strong Magnetic Fields at Weak Coupling and Implications for Elliptic Flow

We compute the momentum diffusion coefficients of heavy quarks, $κ_\parallel$ and $κ_\perp$, in a strong magnetic field $B$ along the directions parallel and perpendicular to $B$, respectively, at the leading order in QCD coupling constant $α_s$. We consider a regime relevant for the relativistic heavy ion collisions, $α_s eB\ll T^2\ll eB$, so that thermal excitations of light quarks are restricted to the lowest Landau level (LLL) states. In the vanishing light-quark mass limit, we find $κ_\perp^{\rm LO}\propto α_s^2 T eB$ in the leading order that arises from screened Coulomb scatterings with (1+1)-dimensional LLL quarks, while $κ_\parallel$ gets no contribution from the scatterings with LLL quarks due to kinematic restrictions. We show that the first non-zero leading order contributions to $κ_\parallel^{\rm LO}$ come from the two separate effects: 1) the screened Coulomb scatterings with thermal gluons, and 2) a finite light-quark mass $m_q$. The former leads to $κ_\parallel^{\rm LO,\,gluon} \propto α_s^2 T^3$ and the latter to $κ_\parallel^{\rm LO,\,massive}\propto α_s (α_s eB)^{1/2} m_q^2$. Based on our results, we propose a new scenario for the large value of heavy-quark elliptic flow observed in RHIC and LHC. Namely, when $κ_\perp\ggκ_\parallel$, an anisotropy in drag forces gives rise to a sizable amount of the heavy-quark elliptic flow even if heavy quarks do not fully belong to an ellipsoidally expanding background fluid.

hep-ph↗

arXiv2016

Mesons in strong magnetic fields: (I) General analyses

We study properties of neutral and charged mesons in strong magnetic fields |eB|>> Lambda_QCD^2 with Lambda_QCD being the QCD renormalization scale. Assuming long-range interactions, we examine magnetic-field dependences of various quantities such as the constituent quark mass, chiral condensate, meson spectra, and meson wavefunctions by analyzing the Schwinger-Dyson and Bethe-Salpeter equations. Based on the density of states obtained from these analyses, we extend the hadron resonance gas (HRG) model to investigate thermodynamics at large B. As B increases the meson energy behaves as a slowly growing function of the meson's transverse momenta, and thus a large number of meson states is accommodated in the low energy domain; the density of states at low temperature is proportional to B^2. This extended transverse phase space in the infrared regime significantly enhances the HRG pressure at finite temperature, so that the system reaches the percolation or chiral restoration regime at lower temperature compared to the case without a magnetic field; this simple picture would offer a gauge invariant and intuitive explanation of the inverse magnetic catalysis.

hep-ph↗

arXiv2016

D mesons in a magnetic field

We investigate the mass spectra of open heavy flavor mesons in an external constant magnetic field within QCD sum rules. Spectral ansätze on the phenomenological side are proposed in order to properly take into account mixing effects between the pseudoscalar and vector channels, and the Landau levels of charged mesons. The operator product expansion is implemented up to dimension-5 operators. As a result, we find for neutral D mesons a significant positive mass shift that goes beyond simple mixing effects. In contrast, charged D mesons are further subject to Landau level effects, which together with the mixing effects almost completely saturate the mass shifts obtained in our sum rule analysis.

hep-ph↗

arXiv2015

Photon and dilepton spectra from nonlinear QED effects in supercritical magnetic fields induced by heavy-ion collisions

We discuss properties of photons in extremely strong magnetic fields induced by the relativistic heavy-ion collisions. We investigate the vacuum birefringence, the real-photon decay, and the photon splitting which are all forbidden in the ordinary vacuum, but become possible in strong magnetic fields. These effects potentially give rise to anisotropies in photon and dilepton spectra.

hep-ph↗

arXiv2015

QCD Kondo effect: quark matter with heavy-flavor impurities

We show that the Kondo effect occurs in light quark matter which contains heavy quarks as impurities. We consider a scattering between a heavy-flavor impurity and a light quark near a Fermi surface which is mediated by gluon-exchange interactions. We find that the scattering amplitude has a logarithmic infrared divergence originating from imperfect cancellation between quark-impurity and hole-impurity scatterings in a loop integral, implying the presence of a strongly coupled regime near the Fermi surface. Renormalization group method is used to find the Kondo scale where a running coupling constant hits a Landau pole. Following an illustration by a simple contact-interaction model, we examine gluon-exchange interactions on the basis of high density QCD.

hep-ph↗

arXiv2015

Euler-Heisenberg-Weiss action for QCD+QED

We derive an analytic expression for one-loop effective action of QCD+QED at zero and finite temperatures by using the Schwinger's proper time method. The result is a nonlinear effective action not only for electromagnetic and chromo-electromagnetic fields but also the Polyakov loop, and thus reproduces the Euler-Heisenberg action in QED, QCD, and QED+QCD, and also the Weiss potential for the Polyakov loop at finite temperature. As applications of this "Euler-Heisenberg-Weiss" action in QCD+QED, we investigate quark pair productions induced by QCD+QED fields at zero temperature and the Polyakov loop in the presence of strong electromagnetic fields. Quark one-loop contribution to the effective potential of the Polyakov loop explicitly breaks the center symmetry, and is found to be enhanced by the magnetic field, which is consistent with the inverse magnetic catalysis observed in lattice QCD simulation.

hep-ph↗

arXiv2015

Charmonium Spectroscopy in Strong Magnetic Fields by QCD Sum Rules: S-Wave Ground States

We investigate quarkonium mass spectra in external constant magnetic fields by using QCD sum rules. We first discuss a general framework of QCD sum rules necessary for properly extracting meson spectra from current correlators computed in the presence of strong magnetic fields, that is, a consistent treatment of mixing effects caused in the mesonic degrees of freedom. We then implement operator product expansions for pseudoscalar and vector heavy-quark current correlators by taking into account external constant magnetic fields as operators, and obtain mass shifts of the lowest-lying bound states $η_c$ and $J/ψ$ in the static limit with their vanishing spatial momenta. Comparing results from QCD sum rules with those from hadronic effective theories, we find that the dominant origin of mass shifts comes from a mixing between $η_c$ and $J/ψ$ with a longitudinal spin polarization, accompanied by other subdominant effects such as mixing with higher excited states and continua.

hep-ph↗

arXiv2014

QCD sum rules for magnetically induced mixing between $η_c$ and $J/ψ$

We investigate the properties of charmonia in strong magnetic fields by using QCD sum rules. We show how to implement the mixing effects between $η_c$ and $J/ψ$ on the basis of field-theoretical approaches, and then show that the sum rules are saturated by the mixing effects with phenomenologically determined parameters. Consequently, we find that the mixing effects are the dominant contribution to the mass shifts of the charmonia in strong magnetic fields.

hep-ph↗

arXiv2013

Neutral-pion reactions induced by chiral anomaly in strong magnetic fields

We investigate decay and production of neutral pions in strong magnetic fields. In the presence of strong magnetic fields, transition between a neutral pion and a virtual photon becomes possible through the triangle diagram relevant for the chiral anomaly. We find that the decay mode of a neutral pion into two photons cannot persist in the dominant mode in strong magnetic fields, and that decay into a dilepton instead dominates over the other modes. We also investigate the effects of magnetic fields on prompt virtual photons created in ultrarelativistic heavy-ion collisions. There is no anisotropy in the spectrum at the stage of creation of prompt virtual photons, but after traversing the strong magnetic field that is induced perpendicularly to the reaction plane, virtual photons turn into neutral pions, leading to an anisotropic spectrum of dileptons as a feasible signature in the measurement.

hep-ph↗

arXiv2013

Vacuum birefringence in strong magnetic fields: (II) Complex refractive index from the lowest Landau level

We compute the refractive indices of a photon propagating in strong magnetic fields on the basis of the analytic representation of the vacuum polarization tensor obtained in our previous paper. When the external magnetic field is strong enough for the fermion one-loop diagram of the polarization tensor to be approximated by the lowest Landau level, the propagating mode in parallel to the magnetic field is subject to modification: The refractive index deviates from unity and can be very large, and when the photon energy is large enough, the refractive index acquires an imaginary part indicating decay of a photon into a fermion-antifermion pair. We study dependences of the refractive index on the propagating angle and the magnetic-field strength. It is also emphasized that a self-consistent treatment of the equation which defines the refractive index is indispensable for accurate description of the refractive index. This self-consistent treatment physically corresponds to consistently including the effects of back reactions of the distorted Dirac sea in response to the incident photon.

hep-ph↗

arXiv2012

Vacuum birefringence in strong magnetic fields: (I) Photon polarization tensor with all the Landau levels

Photons propagating in strong magnetic fields are subject to a phenomenon called the "vacuum birefringence" where refractive indices of two physical modes both deviate from unity and are different from each other. We compute the vacuum polarization tensor of a photon in a static and homogeneous magnetic field by utilizing Schwinger's proper-time method, and obtain a series representation as a result of double integrals analytically performed with respect to proper-time variables. The outcome is expressed in terms of an infinite sum of known functions which is plausibly interpreted as summation over all the Landau levels of fermions. Each contribution from infinitely many Landau levels yields a kinematical condition above which the contribution has an imaginary part. This indicates decay of a sufficiently energetic photon into a fermion-antifermion pair with corresponding Landau level indices. Since we do not resort to any approximation, our result is applicable to the calculation of refractive indices in the whole kinematical region of a photon momentum and in any magnitude of the external magnetic field.

hep-ph↗

arXiv2012

Effects of extremely strong magnetic field on photon HBT interferometry

We discuss potential attainability by the Hanbury Brown and Twiss (HBT) interferometry of photons to probe the spacetime geometry of the primordial plasma created by ultrarelativistic heavy-ion collisions. A possible effect to distort the HBT image is due to an interaction between emitted photon and an extremely strong magnetic field induced by the colliding heavy nuclei in the peripheral collisions. We examine the effects of variation of the refraction index in the strong magnetic field that is called the vacuum birefringence.

nucl-th↗

arXiv2012

Distortion of the HBT image by mean field interaction

We study effects of a mean-field interaction on the spacetime geometry of the hadron source measured by utilizing the Hanbury Brown and Twiss (HBT) interferometry in the ultrarelativistic heavy-ion collisions. We show how a modification of a pion amplitude, caused in the freeze-out process, is incorporated into the correlation function of the interferometry within a semiclassical method. Profiles of the distorted images are illustrated. To make a quantitative estimate of the effects, we construct a mean-field-interaction model on the basis of the pion-pion scattering amplitude, and then investigate to what extent the effects of the mean-field interaction acts to efficiently modify the HBT radii.

nucl-th↗

arXiv2010

HBT pion interferometry with phenomenological mean field interaction

To extract information on hadron production dynamics in the ultrarelativistic heavy ion collision, the space-time structure of the hadron source has been measured using Hanbury Brown and Twiss interferometry. We study the distortion of the source images due to the effect of a final state interaction. We describe the interaction, taking place during penetrating through a cloud formed by evaporating particles, in terms of a one-body mean field potential localized in the vicinity of the source region. By adopting the semiclassical method, the modification of the propagation of an emitted particle is examined. In analogy to the optical model applied to nuclear reactions, our phenomenological model has an imaginary part of the potential, which describes the absorption in the cloud. In this work, we focus on the pion interferometry and mean field interaction obtained using a phenomenological $ππ$ forward scattering amplitude in the elastic channels. The p-wave scattering with $ρ$ meson resonance leads to an attractive mean field interaction, and the presence of the absorptive part is mainly attributed to the formation of this resonance. We also incorporate a simple time dependence of the potential reflecting the dynamics of the evaporating source. Using the obtained potential, we examine how and to what extent the so-called HBT Gaussian radius is varied by the modification of the propagation.

nucl-th↗