Fermionic Background Research Articles

Aspects of the gauge boson-gaugino mixing in a supersymmetric scenario with Lorentz-symmetry violation

We write down an $\mathcal{N}=1$ supersymmetric extension for non-Abelian gauge theories in ($1+3$) dimensions with a Lorentz- and $CPT$-violating term of the Carroll-Field-Jackiw type. By including effects of the background (supersymmetric) fermion bilinears that accompany Lorentz-symmetry violation in a Carroll-Field-Jackiw scenario, we investigate both the gauge boson and gaugino dispersion relations in order to compute their respective masses in terms of the background structures. Such results open up a potential path towards a possible mechanism for gaugino-gauge boson conversion, an analog of the Primakoff effect, induced here not by an external magnetic field, but instead by the (Majorana) fermionic sector of the supersymmetry multiplet in the backstage of the Lorentz-symmetry violation.

Quantum vacuum phenomena in various backgrounds

In this paper, we will review, through various examples, some ideas connecting the physics of the quantum vacuum and the Casimir effect with the mechanism of symmetry breaking in different backgrounds. In the first example, we will discuss how the quantum vacuum energy is altered by the presence of nonlinearities in the underlying quantum field theory in a way that depends on the spatial dimensionality, as dictated by the Mermin–Wagner–Hohenberg–Coleman theorem. In the second example, we will explore how the combination of boundary effects and (discrete) chiral symmetry breaking can affect the thermodynamical behavior of a system of interacting fermions, and how this is reflected on the Casimir force. Even in the simplest setup of two parallel plates, two interesting things happen: first, the order of the transition through which discrete chiral symmetry may be broken/restored changes from second-order for infinitely large separation to first-order for a finite separation between the boundaries. Second, a peculiar behavior in the fermion condensate occurs, resulting in the appearance of two different phases (massless and massive) in the Casimir force. In the third example, we will also be concerned with the Casimir effect on an interacting fermion background over a string of finite length. Using a self-consistent method, we will show how a nontrivial behavior in the Casimir force arises, displaying a switch from an attractive to a repulsive regime, as a result of the competing effects due to the usual attractive Casimir force and a repulsive component coming from the condensate.

Non-universal Fermi polaron in quasi two-dimensional quantum gases

We consider an impurity problem in a quasi-two-dimensional Fermi gas, where a spin-down impurity is immersed in a Fermi sea of N spin-up atoms. Using a variational approach and an effective two-channel model, we obtain the energy for a wide range of interaction strength and for various different mass ratios between the impurity and the background fermion in the context of heteronuclear mixture. We demonstrate that in a quasi-two-dimensional Fermi gas there exists a transition of the ground state from polaron in the weakly interacting region to molecule in the strongly interacting region. The critical interaction strength of the polaron–molecule transition is non-universal and depends on the particle density of the background Fermi sea. We also investigate the excited repulsive polaron state, and find similar non-universal behavior.

Chiral gravitational effect in time-dependent backgrounds

Gravitational counterpart of the chiral magnetic effect, which is referred as the chiral gravitational effect, can also be of interest in a cosmological setup. In this study, we investigate this effect in the time-dependent chiral asymmetric fermion background and in the expanding spacetime by formulating the effective action of gravitational waves. We also analyze the anomaly equation to see how the backreaction from gravitational waves to thermal chiral plasma occurs. We find that the non-trivial time dependence of chiral chemical potential, which can be induced in some scenarios of baryogenesis, is the key ingredient of the chiral gravitational effect. It turns out that the “memory” of the effect is imprinted on the high frequency gravitational waves propagating in the plasma. Cosmological implications and potential effects on the gravitational wave observation are briefly discussed.

A systematic interpolatory method for an impurity in a one-dimensional fermionic background

We explore a new numerical method for studying one-dimensional quantum systems in a trapping potential. We focus on the setup of an impurity in a fermionic background, where a single distinguishable particle interacts through a contact potential with a number of identical fermions. We can accurately describe this system, for various particle numbers, different trapping potentials and arbitrary finite repulsion, by constructing a truncated basis containing states at both zero and infinite repulsion. The results are compared with matrix product states methods and with the analytical result for two particles in a harmonic well.

Hunting for fermionic instabilities in charged AdS black holes

Fermions scattering on a black hole background cannot develop an instability sourced by superradiance. However, in a global (or planar) AdS4-Reissner-Nordström background fermions can violate the AdS2 fermionic mass stability bound as measured by a near horizon observer at zero temperature. This suggests that AdS-Reissner-Nordström black holes might still be unstable to Dirac perturbations. Motivated by this observation we search for linear mode instabilities of Dirac fields in these backgrounds but find none. This is in contrast with the scalar field case, where a violation of the near-horizon Breitenlöhner-Freedman stability bound in the AdS-Reissner-Nordström background triggers the already known scalar condensation near-horizon linear instability (in the planar limit this is Gubser’s instability that initiated the holographic superconductor programme). We consider both the standard and alternative AdS/CFT quantizations (that preserve the conformal invariance of AdS). These are reflective boundary conditions that have vanishing energy flux at the asymptotic boundary.

Tunneling dynamics of a Bose–Fermi superfluid mixture

We study the tunneling dynamics of a Bose–Fermi superfluid mixture where bosons are confined in a double-well potential and fermions in a harmonic potential symmetrically positioned at the center of the double-well. By focusing on the deep Bose–Einstein condensate side of the inter-fermion Feshbach resonance where the fermions can be well described as molecular bosons, we employ a mean-field method and numerically investigate the dynamical evolution of bosons under different parameters and initial conditions. We find that the interaction with background fermions can enrich the tunneling dynamics of the bosons, and specifically, lead to a re-entrant macroscopic quantum self-trapping state by varying different parameters. Our results can be observed in Bose–Fermi superfluid mixture by tuning interactions via Feshbach resonance and confinement-induced resonance.

Recent results for UPC at the LHC

Ultraperipheral heavy ion collisions are a source of photons which can collide with each other producing a pair of particles. This work will be focused on analysis of the light-by-light scattering. Here contribution from fermionic boxes, resonance scattering, VDM-Regge model, two-gluon exchange and pionic background will be compared. Each of these processes dominate at different ranges of two-photon invariant masses. Our calculated nuclear cross section is in good agreement with recently measured ATLAS and CMS data. Predictions including ALICE and LHCb experimental cuts for the next run at the LHC will be shown.

Neutrino effective potential in a fermion and scalar background

We consider the neutrino self-energy in a background composed of a scalar particle and a fermion using a simple model for the coupling of the form $\lambda\bar f_R\nu_L\phi$. The results are useful in the context of Dark Matter-neutrino interaction models in which the scalar and/or fermion constitute the dark-matter. The corresponding formulas for models in which the scalar particle couples to two neutrinos via a coupling of the form $\lambda^{(\nu\nu\phi)}\bar\nu^c_R\nu_L\phi$ are then obtained as a special case. In the presence of these interactions there can be new contributions to the neutrino effective potential and index of refraction in the context of neutrino collective oscillations in a supernova and in the Early Universe hot plasma before neutrino decoupling. The formulas obtained here can be used to estimate those effects and/or put limits on the model parameters based on the contribution that such interactions can have in those contexts. A particular feature of the results is that the contribution to the neutrino self-energy or effective potential in a neutrino background due to the $\nu\nu\phi$ coupling is proportional to the antineutrino-neutrino asymmetry $(n_{\bar\nu} - n_{\nu})$, in contrast to the standard $Z$ contribution which is proportional to $(n_{\nu} - n_{\bar\nu})$. Therefore the two contributions tend to cancel. If the cancellation is significant, it is conceivable that the $O(1/m^4_\phi)$ terms give the dominant contribution. Alternatively, a limit can be set by requiring that the contribution of the $\nu\nu\phi$ interaction to the neutrino effective potential does not cancel the standard contribution in an appreciable way.

Generation of the relic neutrino asymmetry in a hot plasma of the early universe

The neutrino asymmetry in the early universe plasma, [Formula: see text], is calculated both before and after the electroweak phase transition (EWPT). In the Standard Model, before EWPT, the leptogenesis is well known to be driven by the abelian anomaly in a massless hypercharge field. The generation of the neutrino asymmetry in the Higgs phase after EWPT, in its turn, has not been considered previously because of the absence of any quantum anomaly in an external electromagnetic field for such electroneutral particles as neutrino, unlike the Adler–Bell–Jackiw anomaly for charged left and right polarized massless electrons in the same electromagnetic field. Using the neutrino Boltzmann equation, modified by the Berry curvature term in the momentum space, we establish the violation of the macroscopic neutrino current in plasma after EWPT and exactly reproduce the nonconservation of the lepton current in the symmetric phase before EWPT arising in quantum field theory due to the nonzero lepton hypercharge and corresponding triangle anomaly in an external hypercharge field. In the last case, the nonconservation of the lepton current is derived through the kinetic approach without a computation of corresponding Feynman diagrams. Then, the new kinetic equation is applied for the calculation of the neutrino asymmetry accounting for the Berry curvature and the electroweak interaction with background fermions in the Higgs phase. Such an interaction generates a neutrino asymmetry through the electroweak coupling of neutrino currents with electromagnetic fields in plasma, which is [Formula: see text]. It turns out that this effect is especially efficient for maximally helical magnetic fields.

Nonconservation of lepton current and asymmetry of relic neutrinos

The neutrino asymmetry in the early universe plasma, $n_\nu - n_{\bar \nu}$, is calculated both before and after the electroweak phase transition (EWPT). The leptogenesis before EWPT within the standard model is well known to be driven by the abelian anomaly in a massless hypercharge field. The generation of the neutrino asymmetry in the Higgs phase after EWPT, in its turn, has not been considered previously because of the absence of any quantum anomaly in an external electromagnetic field for such electroneutral particles as neutrino, unlike the Adler anomaly for charged left and right polarized massless electrons in the same electromagnetic field. Using the neutrino Boltzmann equation, modified by the Berry curvature term in the momentum space, we establish the violation of the macroscopic neutrino current in plasma after EWPT and exactly reproduce the nonconservation of the lepton current in the symmetric phase before EWPT arising in quantum field theory due to the non-zero lepton hypercharge and corresponding triangle anomaly in an external hypercharge field. In the last case the violation of the lepton current is derived through the kinetic approach without the computation of the corresponding Feynman diagrams. Then the new kinetic equation is applied for the calculation of the neutrino asymmetry accounting for the the Berry curvature and the electroweak interaction with background fermions in the Higgs phase, including the stage after the neutrino decoupling at the absence of neutrino collisions in plasma. This asymmetry is found to be rather small to be observed. Thus, a difference of the relic neutrino and antineutrino densities, if exists, should be acquired mainly in the symmetric phase before EWPT.

Scalar mass stability bound in a simple Yukawa-theory from renormalization group equations

Functional renormalization group (FRG) equations are constructed for a simple Yukawa-model with discrete chiral symmetry, including also the effect of a nonzero composite fermion background beyond the conventional scalar condensate. The evolution of the effective potential of the model, generically depending on two invariants, is explored with the help of power series expansions. Systematic investigation of the effect of a class of irrelevant operators on the lower (stability) bound allows a non-perturbative extension of the maximal cutoff value consistent with any given mass of the scalar field.

Renormalisation group determination of scalar mass bounds in a simple Yukawa-model

The scalar mass is determined in the simplest cut-off regularized Yukawa-model in the whole range of stability of the scalar potential. Two versions of the Functional Renormalisation Group (FRG) equations are solved in the Local Potential Approximation (LPA), where also the possible existence of a composite fermionic background is taken into account. The close agreement of the results with previous studies taking into account exclusively the effect of the scalar condensate, supports a rather small systematic truncation error of FRG due to the omission of higher dimensional operators.

Energy source for the magnetic field growth in magnetars driven by the electron-nucleon interaction

We study the magnetic field generation in a neutron star within the model based on the magnetic field instability in the nuclear matter owing to the electron-nucleon parity violating interaction. We suggest that the growing magnetic field takes the energy from thermal background fermions in the neutron star matter. The system of kinetic equations for the spectra of the magnetic helicity density and magnetic energy density as well as the chiral imbalance are solved numerically accounting for this energy source. We obtain that, for the initial conditions corresponding to a typical neutron star, the large scale magnetic field $\sim 10^{15}\thinspace\text{G}$ is generated during $(10^4-10^5)\thinspace\text{yr}$. We suggest that the proposed model describes strong magnetic fields observed in magnetars.

Unruh effect for neutrinos interacting with accelerated matter

We study the evolution of neutrinos in a background matter moving with a linear acceleration. The Dirac equation for a massive neutrino electroweakly interacting with background fermions is obtained in a comoving frame where matter is at rest. We solve this Dirac equation for ultrarelativistic neutrinos. The neutrino quantum states in matter moving with a linear acceleration are obtained. We demonstrate that the neutrino electroweak interaction with an accelerated matter leads to the vacuum instability which results in the neutrino-antineutrino pairs creation. We rederive the temperature of the Unruh radiation and find the correction to the Unruh effect due to the specific neutrino interaction with background fermions. As a possible application of the obtained results we discuss the neutrino pairs creation in a core collapsing supernova. The astrophysical upper limit on the neutrino masses is obtained.

Aspects of CPT-even Lorentz-symmetry violating physics in a supersymmetric scenario

Background fermion condensates in a landscape dominated by global SUSY are reassessed in connection with a scenario where Lorentz symmetry is violated in the bosonic sector (actually, the photon sector) by a $CPT$-even $k_F$-term. An effective photonic action is discussed that originates from the supersymmetric background fermion condensates. Also, the photino mass emerges in terms of a particular condensate contrary to what happens in the $k_{AF}$-violation. Finally, the interparticle potential induced by the effective photonic action is investigated and a confining profile is identified.

Mapping between the classical and pseudoclassical models of a relativistic spinning particle in external bosonic and fermionic fields. II

The exact solution of a system of bilinear identities derived in the first part of our work [1] for the case of real Grassmann-odd tensor aggregate of the type (S,Vμ,Tμν⁎,Aμ,P) is obtained. The consistency of the solution with a corresponding system of bilinear identities including both the tensor variables and their derivatives (S˙,V˙μ,T˙μν⁎,A˙μ,P˙) is considered. The alternative approach in solving of the algebraic system based on introducing complex tensor quantities is discussed. This solution is used in constructing the mapping of the interaction terms of spinning particle with a background (Majorana) fermion field ΨMαi(x). A way of the extension of the obtained results for the case of the Dirac spinors (ψDα,θDα) and a background Dirac field ΨDαi(x), is suggested. It is shown that for the construction of one-to-one correspondence between the most general spinors and the tensor variables, we need a four-fold increase of the number of the tensor ones. A connection with the higher-order derivative Lagrangians for a point particle and in particular, with the Lagrangian suggested by A.M. Polyakov, is proposed.

Are preformed Cooper pairs the cause for the pseudogap in superconductors?

A weak-coupling scenario wherein bosonic preformed electron pairs emerge upon cooling from two-electron correlations can explain the pseudogap phase consisting of segments where a Bogoliubov-like energy-momentum relation gapped spectrum alternates with a normal ungapped one. Bose–Einstein condensation (BEC) of preformed pairs interacting with the background fermions leads to either - or -wave-like superconducting gaps, the result being sensitive to the magnitude of the total number density of pairs at which BEC occurs and becomes possible already for a moderately anisotropic -wave pairing of fermions repelling each other via isotropic coulombic forces. The present model is compatible with the coexistence of pseudogap and of superconductivity phenomena.

Preformed Cooper Pairs as Origin of Pseudogap in Cuprates

A weak-coupling scenario wherein bosonic preformed electron pairs emerge upon cooling from two-electron correlations can explain the pseudogap phase consisting of segments where a Bogoliubov-like energy-momentum relation gapped spectrum alternates with a normal ungapped one. Bose-Einstein condensation (BEC) of preformed pairs interacting with the background fermions leads to either d- or s wave-like superconducting gaps, the result being sensitive to the magnitude of the total number density of pairs nB at which BEC occurs and becomes possible already for a moderately anisotropic s wave pairing of fermions repelling each other via isotropic coulombic forces. The present model is compatible with the coexistence of both pseudogap and of superconductivity phenomena.

Renormalization of QED Near Decoupling Temperature

We study the effective parameters of QED near decoupling temperatures and show that the QED perturbative series is convergent, at temperatures below the decoupling temperature. The renormalization constant of QED acquires different values if a system cools down from a hotter system to the electron mass temperature or heats up from a cooler system to the same temperature. At T = m, the first order contribution to the electron self-mass, δm/m is 0.0076 for a heating system and 0.0115 for a cooling system and the difference between two values is equal to 1/3 of the low temperature value and 1/2 of the high temperature value around T~m. This difference is a measure of hot fermion background at high temperatures. With the increase in release of more fermions at hotter temperatures, the fermion background contribution dominates and weak interactions have to be incorporated to understand the background effects.