arXiv subjects

Jiunn-Wei Chen

Publications and source records attributed to Jiunn-Wei Chen.

At least 73 records · Page 4Linked to original sources

arXiv2014

Novel Parity Violating Transport Coefficients in 2+1 Dimensions from Holography

We construct a 3+1 dimensional holographic model dual to a parity violating hydrodynamic system in 2+1 dimensions. Our model contains gravitational and electrodynamic Chern-Simons terms coupled to a neutral pseudo scalar $θ$, and a potential composed of quadratic and quartic terms in $θ$. The background is a charged black brane. We study the hydrodynamics to first order in spacetime derivatives near the probe limit of the pseudo scalar, by extracting the transport coefficients from the scalar, vector, and tensor modes of bulk perturbations. We study two mechanisms for breaking the parity of the boundary fluid: the parity is either spontaneously broken by the nonzero vev of the dual pseudo scalar operator, or by the pseudo scalar source on the boundary. The resulting transport coefficients have unusual behaviors compared to those in ordinary materials.

hep-th↗

arXiv2013

Atomic ionization of germanium due to neutrino magnetic moments

An ab initio calculation of atomic ionization of germanium (Ge) by neutrinos was carried out in the framework of multiconfiguration relativistic random phase approximation. The main goal is to provide a more accurate cross section formula than the conventional one, which is based on the free electron approximation, for searches of neutrino magnetic moments with Ge detectors whose threshold is reaching down to the sub-keV regime. Limits derived with both methods are compared, using reactor neutrino data taken with low threshold germanium detectors.

hep-ph↗

arXiv2013

Negative Off-Diagonal Conductivities in a Weakly Coupled Quark Gluon Plasma

We calculate the conductivity matrix of a weakly coupled quark-gluon plasma at the leading-log order. By setting all quark chemical potentials to be identical, the diagonal conductivities become degenerate and positive, while the off-diagonal ones become degenerate but negative (or zero when the chemical potential vanishes). This means a potential gradient of a certain fermion flavor can drive backward currents of other flavors. A simple explanation is provided for this seemingly counter intuitive phenomenon. It is speculated that this phenomenon is generic and most easily measured in cold atom experiments.

hep-ph↗

arXiv2013

Ionization of hydrogen by neutrino magnetic moment, relativistic muon, and WIMP

We studied the ionization of hydrogen by scattering of neutrino magnetic moment, relativistic muon, and weakly-interacting massive particle with a QED-like interaction. Analytic results were obtained and compared with several approximation schemes often used in atomic physics. As current searches for neutrino magnetic moment and dark matter have lowered the detector threshold down to the sub-keV regime, we tried to deduce from this simple case study the influence of atomic structure on the the cross sections and the applicabilities of various approximations. The general features being found will be useful for cases where practical detector atoms are considered.

hep-ph↗

arXiv2013

Fermi gases with imaginary mass imbalance and the sign problem in Monte Carlo calculations

Fermi gases in strongly coupled regimes, such as the unitary limit, are inherently challenging for many-body methods. Although much progress has been made with purely analytic methods, quantitative results require ab initio numerical approaches, such as Monte Carlo (MC) calculations. However, mass-imbalanced and spin-imbalanced gases are not accessible to MC calculations due to the infamous sign problem. It was recently pointed out that the sign problem, for finite spin imbalance, can be circumvented by resorting to imaginary polarizations and analytic continuation. Large parts of the phase diagram spanned by temperature and polarization then become accessible to MC calculations. We propose to apply a similar strategy to the mass-imbalanced case, which opens up the possibility to study the associated phase diagram with MC calculations. In particular, our analysis suggests that a detection of a (tri-)critical point in this phase diagram is possible. We also discuss calculations in the zero-temperature limit with our approach.

cond-mat.stat-mech↗

arXiv2013

Berry curvature and 4-dimensional monopole in relativistic chiral kinetic equation

We derive a relativistic chiral kinetic equation with manifest Lorentz covariance from Wigner functions of spin-1/2 massless fermions in a constant background electromagnetic field. It contains vorticity terms and a 4-dimensional Euclidean Berry monopole which gives axial anomaly. By integrating out the zero-th component of the 4-momentum p, we reproduce the previous 3-dimensional results derived from the Hamiltonian approach, together with the newly derived vorticity terms. The phase space continuity equation has an anomalous source term proportional to the product of electric and magnetic fields ($F\tilde{F} \sim E.B$). This provides a unified interpretation of the chiral magnetic and vortical effects, chiral anomaly, Berry curvature, and the Berry monopole in the framework of Wigner functions.

hep-th↗

arXiv2013

Parity violating quantum kinetic theory in (2+1)-dimensions

We study the kinetic theory for a (2+1)-dimensional fermionic system with special emphasis on the parity violating properties associated with the fermion mass. The Wigner function approach is used to derive hydrodynamical transport coefficients to the first spatial derivative order. As a first attempt, the collisions between fermions are neglected. The resulting system is dissipationless. The parity violating Hall electric conductivity has the same temperature and chemical potential dependence as the quantum field theory result at one-loop. Vorticity dependent transport properties, which were not considered before, also emerge naturally in this approach.

nucl-th↗

arXiv2013

Imaginary polarization as a way to surmount the sign problem in ab initio calculations of spin-imbalanced Fermi gases

From ultracold atoms to quantum chromodynamics, reliable ab initio studies of strongly interacting fermions require numerical methods, typically in some form of quantum Monte Carlo calculation. Unfortunately, (non)relativistic systems at finite density (spin polarization) generally have a sign problem, such that those ab initio calculations are impractical. It is well-known, however, that in the relativistic case imaginary chemical potentials solve this problem, assuming the data can be analytically continued to the real axis. Is this feasible for nonrelativistic systems? Are the interesting features of the phase diagram accessible in this manner? By introducing complex chemical potentials, for real total particle number and imaginary polarization, the sign problem is avoided in the nonrelativistic case. To give a first answer to the above questions, we perform a mean-field study of the finite-temperature phase diagram of spin-1/2 fermions with imaginary polarization.

cond-mat.stat-mech↗

arXiv2013

Shear and Bulk Viscosities of a Gluon Plasma in Perturbative QCD: Comparison of Different Treatments for the gg<->ggg Process

The leading order contribution to the shear and bulk viscosities, ηand ζ, of a gluon plasma in perturbative QCD includes the gg -> gg (22) process, gg <-> ggg (23) process and multiple scattering processes known as the Landau-Pomeranchuk-Migdal (LPM) effect. Complete leading order computations for ηand ζwere obtained by Arnold, Moore and Yaffe (AMY) and Arnold, Dogan and Moore (ADM), respectively, with the inelastic processes computed by an effective g <-> gg gluon splitting. We study how complementary calculations with 22 and 23 processes and a simple treatment to model the LPM effect compare with the results of AMY and ADM. We find that our results agree with theirs within errors. By studying the contribution of the 23 process to η, we find that the minimum angle θamong the final state gluons in the fluid local rest frame has a distribution that is peaked at θ\sim \sqrt{α_{s}}, analogous to the near collinear splitting asserted by AMY and ADM. However, the average of θis much bigger than its peak value, as its distribution is skewed with a long tail. The same θbehavior is also seen if the 23 matrix element is taken to the soft gluon bremsstrahlung limit in the center-of-mass (CM) frame. This suggests that the soft gluon bremsstrahlung in the CM frame still has some near collinear behavior in the fluid local rest frame. We also generalize our result to a general SU(N_c) pure gauge theory and summarize the current viscosity computations in QCD.

hep-ph↗

arXiv2012

Shear and Bulk Viscosities of a Weakly Coupled Quark Gluon Plasma with Finite Chemical Potential and Temperature---Leading-Log Results

We calculate the shear (eta) and bulk (zeta) viscosities of a weakly coupled quark gluon plasma at the leading-log order with finite temperature T and quark chemical potential mu. We find that the shear viscosity to entropy density ratio eta/s increases monotonically with mu and eventually scales as (mu/T)^2 at large mu. In contrary, zeta/s is insensitive to mu. Both eta/s and zeta/s are monotonically decreasing functions of the quark flavor number N_f when N_f \geq 2. This property is also observed in pion gas systems. Our perturbative calculation suggests that QCD becomes the most perfect (i.e. with the smallest eta/s) at mu=0 and N_f = 16 (the maximum N_f with asymptotic freedom). It would be interesting to test whether the currently smallest eta/s computed close to the phase transition with mu=0 and N_f = 0 can be further reduced by increasing N_f.

hep-ph↗

arXiv2012

Near Threshold Proton-Proton Fusion in Effective Field Theory

The astrophysical S-factor for proton-proton fusion, S_11(E), is obtained with the nuclear matrix element analytically calculated in pionless effective field theory. To the third order, the zero-energy result S_11(0) and the first energy derivative S'_11(0) are found to be (3.99 \pm 0.14)* 10^-25 MeV b and S_11(0)*(11.3 \pm 0.1) MeV^-1, respectively; both consistent with the current adopted values. The second energy derivative is also calculated for the first time, and the result S"_11(0) = S_11(0)*(170 \pm 2) MeV^-2 contributes at the level of 0.5% to the fusion rate at the solar center, which is smaller than 1% as previously estimated.

nucl-th↗

arXiv2012

A Holographic Model For Hall Viscosity

We have modified the holographic model of Saremi and Son by using a charged black brane, instead of a neutral one, such that when the bulk pseudo scalar potential is made of quadratic and quartic terms, parity can still be broken spontaneously in the boundary theory. In our model, the 3+1 dimensional bulk has a pseudo scalar coupled to the gravitational Chern-Simons term in the anti de Sitter charged black brane back ground. Parity could be broken spontaneously in the bulk by the pseudo scalar hairy solution and give rise to non-zero Hall viscosity at the boundary theory.

hep-th↗

arXiv2011

How Perfect a Gluon Plasma Can Be in Perturbative QCD?

The shear viscosity to entropy density ratio, η/s, characterizes how perfect a fluid is. We calculate the leading order η/s of a gluon plasma in perturbation using the kinetic theory. The leading order contribution only involves the elastic gg -> gg (22) process and the inelastic gg<->ggg (23) process. The Hard-Thermal-Loop (HTL) treatment is used for the 22 matrix element, while the exact matrix element in vacuum is supplemented by the gluon Debye mass insertion for the 23 process. Also, the asymptotic mass is used for the external gluons in the kinetic theory. The errors from not implementing HTL and the Landau-Pomeranchuk-Migdal effect in the 23 process, and from the uncalculated higher order corrections, are estimated. Our result for η/s lies between that of Arnold, Moore and Yaffe (AMY) and Xu and Greiner (XG). Our result shows that although the finite angle contributions are important at intermediate α_s (α_s \sim 0.01-0.1), the 22 process is still more important than 23 when α_s < 0.1. This is in qualitative agreement with AMY's result. We find no indication that the proposed perfect fluid limit η/s \simeq 1/(4π) can be achieved by perturbative QCD alone.

hep-ph↗

arXiv2011

$d+id$ Holographic Superconductors

A holographic model of $d+id$ superconductors based on the action proposed by Benini, Herzog, and Yarom [arXiv:1006.0731] is studied. This model has a charged spin two field in an AdS black hole spacetime. Working in the probe limit, the normalizable solution of the spin two field in the bulk gives rise to a $d+id$ superconducting order parameter at the boundary of the AdS. We calculate the fermion spectral function in this\ superconducting background and confirm the existence of fermi arcs for non-vanishing Majorana couplings. By changing the relative strength $γ$ of the $d$ and $id$ condensations, the position and the size of the fermi arcs are changed. When $γ=1$, the spectrum becomes isotropic and the spectral function is s-wave like. By changing the fermion mass, the fermi momentum is changed. We also calculate the conductivity for these holographic $d+id$ superconductors where time reversal symmetry has been broken spontaneously. A non-vanishing Hall conductivity is obtained even without an external magnetic field.

hep-th↗

arXiv2010

On the Quark Mass Dependence of Two Nucleon Observables

We study the implications of lattice QCD determinations of the S-wave nucleon-nucleon scattering lengths at unphysical light quark masses. It is found that with the help of nuclear effective field theory (NEFT), not only the quark mass dependence of the effective range parameters, but also the leading quark mass dependence of all the low energy deuteron matrix elements can be obtained. The quark mass dependence of deuteron charge radius, magnetic moment, polarizability and the deuteron photodisintegration cross section are shown based on the NPLQCD lattice calculation of the scattering lengths at 354 MeV pion mass and the NEFT power counting scheme of Beane, Kaplan and Vuorinen. Further improvement can be obtained by performing the lattice calculation at smaller quark masses. Our result can be used to constrain the time variation of isoscalar combination of u and d quark mass m_q, to help the anthropic principle study to find the m_q range which allows the existence of life, and to provide a weak test of the multiverse conjecture.

nucl-th↗

arXiv2010

Minimum Shear Viscosity over Entropy Density at Phase Transition? --- A Counterexample

The ratio eta/s, shear viscosity (eta) to entropy density (s), reaches its local minimum at the (second order) phase transition temperature in a wide class of systems. It was suspected that this behavior might be universal. However, a counterexample is found in a system of two weakly self-interacting real scalar fields with one of them condensing at low temperatures while the other remains in the symmetric phase. There is no interaction between the two fields. The resulting eta/s is monotonically decreasing in temperature despite the phase transition.

hep-ph↗

arXiv2010

Shear Viscosity in Weakly Coupled N-Component Scalar Field Theories

The rich phenomena of the shear viscosity (eta) to entropy density (s) ratio, eta/s, in weakly coupled N-component scalar field theories are studied. eta/s can have a "double dip" behavior due to resonances and the phase transition. If an explicit goldstone mass term is added, then eta/s can either decrease monotonically in temperature or, as seen in many other systems, reach a minimum at the phase transition. We also show how to go beyond the original variational approach to make the Boltzmann equation computation of eta systematic.

hep-ph↗

arXiv2010

Model analysis of thermal UV-cutoff effects on the chiral critical surface at finite temperature and chemical potential

We study the effects of temporal UV-cutoff on the chiral critical surface in hot and dense QCD using a chiral effective model. Recent lattice QCD simulations indicate that the curvature of the critical surface might change toward the direction in which the first order phase transition becomes stronger on increasing the number of lattice sites. To investigate this effect on the critical surface in an effective model approach, we use the Nambu-Jona-Lasinio model with finite Matsubara frequency summation. We find that qualitative feature of the critical surface does not alter appreciably as we decrease the summation number, which is unlike the case what is observed in the recent lattice QCD studies. This may either suggest the dependence of chemical potential on the coupling strength or due to some additional interacting terms such as vector interactions which could play an important role at finite density.

hep-ph↗