Breaking Terms Research Articles

Phase structure of the linear sigma model with the standard symmetry breaking term

The linear sigma model at finite isospin chemical potential μ and temperature T is systematically studied by means of the Cornwal–Jackiw–Tomboulis (CJT) effective potential calculated in the improved Hartree–Fock (HF) approximation, where the Goldstone theorem and the thermodynamic consistency are respected. It results that in the chiral limit, for μ=0 the chiral phase transition is second order as expected from the general universality arguments, and for μ≠0 the phase diagram for the pion condensation in the (T,μ) plane exhibits a tricritical point which is crossover from first-order to second-order phase transitions. In the physical world, where the chiral symmetry is explicitly broken, the pion condensation occurs at μ=mπ, the pion mass in vacuum, and its phase diagram is basically in agreement with those found from the chiral perturbation theory. The chiral symmetry gets restored at high values of T for fixed μ and of μ for fixed T.

The role of gauge symmetry in spintronics

In this work we employ a field theoretical approach to explain the nature of the non-conserved spin current in spintronics. In particular, we consider the usual U ( 1 ) gauge theory for the electromagnetism at classical level in order to obtain the broken continuity equation involving the spin current and spin-transfer torque. Inspired by the recent work of A. Vernes, B. L. Gyorffy and P. Weinberger where they obtain such an equation in terms of relativistic quantum mechanics, we formalize their result in terms of the well known currents of field theory such as the Bargmann–Wigner current and the chiral current. Thus, an interpretation of spintronics is provided in terms of Noether currents (conserved or not) and symmetries of the electromagnetism. In fact, the main result of the present work is that the non-conservation of the spin current is associated with the gauge invariance of physical observables where the breaking term is proportional to the chiral current. Moreover, we generalize their result by including the electromagnetic field as a dynamical field instead of an external one.

Bipolaron recombination in conjugated polymers

By using the Su-Schrieffer-Heeger model modified to include electron-electron interactions, the Brazovskii-Kirova symmetry breaking term and an external electric field, we investigate the scattering process between a negative and a positive bipolaron in a system composed of two coupled polymer chains. Our results show that the Coulomb interactions do not favor the bipolaron recombination. In the region of weak Coulomb interactions, the two bipolarons recombine into a localized excited state, while in the region of strong Coulomb interactions they can not recombine. Our calculations show that there are mainly four channels for the bipolaron recombination reaction: (1) forming a biexciton, (2) forming an excited negative polaron and a free hole, (3) forming an excited positive polaron and a free electron, (4) forming an exciton, a free electron, and a free hole. The yields for the four channels are also calculated.

From flavour to SUSY flavour models

If supersymmetry (SUSY) will be discovered, successful models of flavour not only have to provide an explanation of the flavour structure of the Standard Model fermions, but also of the flavour structure of their scalar superpartners. We discuss aspects of such “SUSY flavour” models, towards predicting both flavour structures, in the context of supergravity (SUGRA). We point out the importance of carefully taking into account SUSY-specific effects, such as 1-loop SUSY threshold corrections and canonical normalisation, when fitting the model to the data for fermion masses and mixings. This entangles the flavour model with the SUSY parameters and leads to interesting predictions for the sparticle spectrum. We demonstrate these effects by analyzing an example class of flavour models in the framework of an SU ( 5 ) Grand Unified Theory with a family symmetry with real triplet representations. For flavour violation through the SUSY soft breaking terms, the class of models realises a scheme we refer to as “Trilinear Dominance”, where flavour violation effects are dominantly induced by the trilinear terms.

Spontaneous U(1) Symmetry Breaking in Coupled Bose System

Adding a U(1) symmetry breaking term to Bogoliubov's truncated Hamiltonian HB for a weakly interacting coupled Bose system, by using the mean-field approximation rather than the c-number approximation, we find that, via a Feshbach resonance at zero temperature, the states of the coupled Bose system are generalized SU(1,1) ⊗ SU(1,1) coherent states. The Bose—Einstein condensation occurs in response to the spontaneous U(1) symmetry breaking.

PROCEDURE FOR GENERATING EQUATIONS IN LINEAR SIGMA MODEL

A procedure for implementation of the generating equations in the linear sigma model of low energy QCD is presented. For any explicit symmetry breaking term, this procedure computes the masses of scalar and pseudoscalar mesons as well as various three-point and four-point interaction vertices that are needed in calculation of different decay widths and scattering amplitudes.

QCD Phase Diagram According to the Center Group

We study an effective theory for QCD at finite temperature and density which contains the leading center symmetric and center symmetry breaking terms. The effective theory is studied in a flux representation where the complex phase problem is absent and the model becomes accessible to Monte Carlo techniques also at finite chemical potential. We simulate the system by using a generalized Prokof'ev-Svistunov worm algorithm and compare the results to a low temperature expansion. The phase diagram is determined as a function of temperature, chemical potential, and quark mass. The shape and quark mass dependence of the phase boundaries are as expected for QCD. The transition into the deconfined phase is smooth throughout, without any discontinuities or critical points.

Affleck-Dine baryogenesis, condensate fragmentation and gravitino dark matter in gauge-mediation with a large messenger mass

We study the conditions for successful Affleck-Dine baryogenesis and the origin of gravitino dark matter in GMSB models. AD baryogenesis in GMSB models is ruled out by neutron star stability unless Q-balls are unstable and decay before nucleosynthesis. Unstable Q-balls can form if the messenger mass scale is larger than the flat-direction field Φ when the condensate fragments. We provide an example based on AD baryogenesis along a d = 6 flat direction for the case where m3/2 ≈ 2GeV, as predicted by gravitino dark matter from Q-ball decay. Using a phenomenological GMSB potential which models the Φ dependence of the SUSY breaking terms, we numerically solve for the evolution of Φ and show that the messenger mass can be sufficiently close to the flat-direction field when the condensate fragments. We compute the corresponding reheating temperature and the baryonic charge of the condensate fragments and show that the charge is large enough to produce late-decaying Q-balls which can be the origin of gravitino dark matter.

(Extra)ordinary gauge/anomaly mediation

We study anomaly mediation models with gauge mediation effects from messengers which have a general renormalizable mass matrix with a supersymmetry-breaking spurion. Our models lead to a rich structure of supersymmetry breaking terms in the visible sector. We derive sum rules among the soft scalar masses for each generation. Our sum rules for the first and second generations are the same as those in general gauge mediation, but the sum rule for the third generation is different because of the top Yukawa coupling. We find the parameter space where the tachyonic slepton problem is solved. We also explore the case in which gauge mediation causes the anomalously small gaugino masses. Since anomaly mediation effects on the gaugino masses exist, we can obtain viable mass spectrum of the visible sector fields.

Slepton Mass Matrices, µ→eγ Decay and EDM in SUSYS4Flavor Model

We discuss slepton mass matrices in the $S_4$ flavor model with SUSY SU(5) GUT. By considering the gravity mediation within the framework of supergravity theory, we estimate the SUSY breaking terms in the slepton mass matrices, which contribute to the $\mu \rightarrow e + \gamma$ decay. We obtain a lower bound for the ratio of $\mu\rightarrow e\gamma$ as $10^{-13}$ if $m_{\text{SUSY}}$ and $m_{1/2}$ are below 500GeV. The off diagonal terms of slepton mass matrices also contribute to EDM of leptons. The predicted electron EDM is around $10^{-29}-10^{-28}$cm. Our predictions are expected to be tested in the near future experiment.

Predictive AdS/QCD model for mass spectra of mesons with three flavors

The predictive soft-wall AdS/QCD model with a modified 5D metric at the infrared region is constructed to obtain a nontrivial dilaton solution for three flavor quarks $u$, $d$ and $s$. Such a model is shown to incorporate both the chiral symmetry breaking and linear confinement. After considering some high-order terms including the $\mathrm{U}(1{)}_{L}\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1{)}_{R}$ chiral symmetry breaking term, we find that the resulting predictions for the SU(3) octet and singlet resonance states of pseudoscalar, scalar, vector and axial-vector mesons agree well with the experimentally confirmed resonance states. Contributions from the instanton effects given by the determinant term are also discussed. It is observed that the chiral symmetry breaking phenomena of $\mathrm{SU}(3{)}_{L}\ifmmode\times\else\texttimes\fi{}\mathrm{SU}(3{)}_{R}$ and $\mathrm{U}(1{)}_{L}\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1{)}_{R}$ can be well described in this model, while the SU(3) flavor symmetry breaking effect due to quark mass difference in the source term is not enough to explain all of the current experimental data.

Observable gravity waves from supersymmetric hybrid inflation. II.

It is shown that a tensor-to-scalar ratio close to $r=0.03$, which can be observed by Planck, is realized in supersymmetric hybrid inflation models with TeV-scale soft supersymmetry breaking terms. This extends our previous analysis, which also found $r\ensuremath{\lesssim}0.03$ but employed intermediate-scale soft terms. Other cosmological observables such as the scalar spectral index are in good agreement with the WMAP data.

Higgs portal to visible supersymmetry breaking

We propose a supersymmetric extension of the standard model whose Higgs sector induces a spontaneous supersymmetry breaking by itself. Unlike the minimal extension, the current Higgs mass bound can be evaded even at the tree-level without the help of the soft breaking terms due to the usual hidden sector, as is reminiscent of the next to minimal case. We also have a possibly light pseudo-goldstino in our visible sector in addition to extra Higgs particles, both of which stem from supersymmetry breaking dynamics. In such a setup of visible supersymmetry breaking, we may see a part of supersymmetry breaking dynamics rather directly in future experiments.

Staggered chiral random matrix theory

We present a random matrix theory for the staggered lattice QCD Dirac operator. The staggered random matrix theory is equivalent to the zero-momentum limit of the staggered chiral Lagrangian and includes all taste breaking terms at their leading order. This is an extension of previous work which only included some of the taste breaking terms. We will also present some results for the taste breaking contributions to the partition function and the Dirac eigenvalues.

Are three flavors special?

It has become clearer recently that the regular pattern of three flavor nonets describing the low spin meson multiplets seems to require some modification for the case of the spin 0 scalar mesons. One picture, which has had some success, treats the scalars in a chiral Lagrangian framework and considers them to populate two nonets. These are, in turn, taken to result from the mixing of two ``bare'' nonets, one of which is of quark-antiquark type and the other of two-quark--two-antiquark type. Here we show that such a mixing is, before chiral symmetry breaking terms are included, only possible for three flavors. In other cases, the two types of structure cannot have the same chiral symmetry transformation property. Specifically, our criterion would lead one to believe that scalar and pseudoscalar states containing charm would not have ``four quark'' admixtures. This work is of potential interest for constructing chiral Lagrangians based on exact chiral symmetry which is then broken by well-known specific terms. It may also be of interest in studying some kinds of technicolor theories.

Phase structure of the linear sigma model with the non-standard symmetry breaking term

The phase structure of the linear sigma model at finite isospin chemical potential μ and temperature T is systematically studied in the non-standard case of symmetry breaking by means of the Cornwall–Jackiw–Tomboulis effective potential. The latter quantity is calculated in the improved Hartree–Fock approximation which preserves the Goldstone theorem and the thermodynamic consistency. It results that the charged pions are condensed for μ equal to the pion mass in vacuum and the pion condensation corresponds to a first-order phase transition for 0 ⩽ T ⩽ 175.813 MeV, whereas at higher temperature it becomes a second-order phase transition. Moreover, the chiral-symmetry restoration, which is a first-order phase transition, occurs for 138.464 MeV ⩽ T ⩽ 147.168 MeV. The phase diagrams for both pion and chiral condensates are obtained.

Scattering process between polaron and exciton in conjugated polymers

Scattering process between a negative polaron and an exciton in a polymer chain is investigated by using the Su-Schrieffer-Heeger model modified to include electron-electron interactions, the Brazovskii-Kirova symmetry breaking term, and an external electric field. It is found that the scattering process is spin dependent. If the polaron and the exciton have parallel spins, the polaron can easily pass through the exciton as if it "do not see" the exciton. If the polaron and the exciton have antiparallel spins, there exist strong repulsion between them. The polaron may be bounced back, be dissociated or pass through the exciton depending on the strength of the external electric field. In any of these cases, the polaron cannot break the exciton.

Low-lying even-parity meson resonances and spin-flavor symmetry

A study is presented of the $s-$wave meson-meson interactions involving members of the $\rho-$nonet and of the $\pi-$octet. The starting point is an SU(6) spin-flavor extension of the SU(3) flavor Weinberg-Tomozawa Lagrangian. SU(6) symmetry breaking terms are then included to account for the physical meson masses and decay constants, while preserving partial conservation of the axial current in the light pseudoscalar sector. Next, the $T-$matrix amplitudes are obtained by solving the Bethe Salpeter equation in coupled-channel with the kernel built from the above interactions. The poles found on the first and second Riemann sheets of the amplitudes are identified with their possible Particle Data Group (PDG) counterparts. It is shown that most of the low-lying even parity PDG meson resonances, specially in the $J^P=0^+$ and $1^+$ sectors, can be classified according to multiplets of the spin-flavor symmetry group SU(6). The $f_0(1500)$, $f_1(1420)$ and some $0^+(2^{++})$ resonances cannot be accommodated within this SU(6) scheme and thus they would be clear candidates to be glueballs or hybrids. Finally, we predict the existence of five exotic resonances ($I \ge 3/2$ and/or $|Y|=2$) with masses in the range 1.4--1.6 GeV, which would complete the $27_1$, $10_3$, and $10_3^*$ multiplets of SU(3)$\otimes$SU(2).

Random matrix model at nonzero chemical potentials with anomaly effects

Phase diagram of the chiral random matrix model with U(1)A breaking term is studied with the quark chemical potentials varied independently at zero temperature, by taking the chiral and meson condensates as the order parameters. Although, without the U(1)A breaking term, chiral transition of each flavor can happen separately responding to its chemical potential, the U(1)A breaking terms mix the chiral condensates and correlate the phase transitions. In the three flavor case, we find that there are mixings between the meson and chiral condensates due to the U(1)A anomaly, which makes the meson condensed phase more stable. Increasing the hypercharge chemical potential ($\mu_Y$) with the isospin and quark chemical potentials ($\mu_I$, $\mu_q$) kept small, we observe that the kaon condensed phase becomes the ground state and at the larger $\mu_Y$ the pion condense phase appears unexpectedly, which is caused by the competition between the chiral restoration and the meson condensation. The similar happens when $\mu_Y$ and $\mu_I$ are exchanged, and the kaon condensed phase becomes the ground state at larger $\mu_I$ below the full chiral restoration.

Supertopcolor

We consider a supersymmetric QCD with soft supersymmetry breaking terms as the dynamics for the electroweak symmetry breaking. We find various advantages compared to the non-supersymmetric models, such as a natural incorporation of the dynamical top-quark mass generation (the topcolor mechanism), the existence of a boson-pair condensation (the composite Higgs fields) and a large anomalous dimension of the composite operator to cure the flavor-changing-neutral-current and the S-parameter crises of the technicolor theories. The knowledge of the weakly coupled description (the Seiberg duality) enables us to perform perturbative computations in strongly coupled theories. Working in a large flavor theory where perturbative calculations are reliable in the dual description, one can find a stable vacuum with chiral symmetry breaking. The top/bottom quarks and also the Higgsinos obtain masses through a dynamically generated superpotential.