Exotic phases of finite temperature gauge theories

We calculate the phase diagrams at high temperature of SU(N) gauge theories with massive fermions by minimizing the one-loop effective potential. Considering fermions in the adjoint (Adj) representation at various N we observe a variety of phases when N_f2 Majorana flavours and periodic boundary conditions are applied to fermions. Also the confined phase is perturbatively accessible. For N=3, we add Fundamental (F) representation fermions with antiperiodic boundary conditions to adjoint QCD to show how the Z(3)-symmetry breaks in the confined phase.     

Natural suppression of flavour-changing neutral currents in supersymmetric gauge theories

Induced flavour-changing neutral currents (FCNC) in supersymmetric unified theories are investigated both in models with the standard SU(2)_L × U(1) gauge symmetry and in models with an extra ?(1) gauge symmetry. Supersymmetric extension of the natural flavour conservation laws for neutral currents is obtained by adding a condition regarding the assumed type of supersymmetry breaking. This condition ensures no direct flavour-changing couplings of neutral gauge-Higgs fermions and at the same time is necessary and sufficient for the natural suppression of the induced FCNC. It is found that in the class of models satisfying the new condition the contribution of the scalar partners of quarks to the induced strangeness-changing neutral current is comparable to that of the quarks in $\text{K}{}_{\mathrm{L}}\text{→ π}\text{π}$, while it is negligibly small in K_L ? K_S mass difference.     

A technocolor model with softly broken flavor symmetry

I describe an explicit technicolor model in which the interactions that give rise to the quark and lepton masses are generated by the exchange of massive gauge bosons as in extended technocolor models but in which the gauge bosons couple the quarks and leptons not directly to the technifermions, but to heavy fermions. Quark and lepton masses arise from box diagrams. The masses of the heavy fermions in this model come both from bare fermion mass terms and from the renormalizable interactions of a set of spinless fields. The model has a softly broken flavor symmetry that realizes the GIM suppression of FCNC effects as in a CTSM model.     

Loop induced flavor changing neutral decays of the top quark in a general two-Higgs-doublet model

Decays of the top quark induced by flavor changing neutral currents (FCNC) are known to be extremely rare events within the Standard Model. This is so not only for the decay modes into gauge bosons, but most notably in the case of the Higgs channels, e.g., t→H_SM+c, with a branching fraction of 10⁻¹³ at most. Therefore, detection of FCNC top quark decays in a future high-energy, and high-luminosity, machine like the LHC or the LC would be an indisputable signal of new physics. In this paper we show that within the simplest extension of the SM, namely the general two-Higgs-doublet model, the FCNC top quark decays into Higgs bosons, t→(h⁰,H⁰,A⁰)+c, can be the most favored FCNC modes — comparable or even more efficient than the gluon channel t→g+c. In both cases the optimal results are obtained for Type II models. However, only the Higgs channels can have rates reaching the detectable level (10⁻⁵), with a maximum of order 10⁻⁴ which is compatible with the charged Higgs bounds from radiative B-meson decays. We compare with the previous results obtained in the Higgs sector of the MSSM.     

Γ(Z → b ): A signature of hard mass terms for a heavy top

We calculate analytically the weak radiative corrections to the weak neutral current gauge boson-bottom fermion vertex, keeping the mass m_t of the internal fermion line for the relevant diagrams. We find, to order α, a hard mass-term dependence m_t²/M_W² of the amplitude, for large m_t values. Its origin comes from the unphysical charged Higgs coupling to fermions in the renormalizable gauge or, equivalently, from the longitudinal charged gauge boson couplings. The diagonal Z⁰ decay width to b-quarks decreases, due to these weak radiative corrections, by 0.6%–2.5% when the top mass m_t varies from 45 to 200 GeV.     

New physics contribution to neutral trilinear gauge boson couplings

We study the one-loop new physics effects to the CP even triple neutral gauge boson vertices γ ^⋆ γ Z, γ ^⋆ Z Z, Z ^⋆ Z γ and Z ^⋆ ZZ in the context of Little Higgs models. We compute the contribution of the additional fermions in Little Higgs models in the framework of direct product groups where [SU(2)×U(1)]² gauge symmetry is embedded in SU(5) global symmetry and also in the framework of the simple group where SU(N)×U(1) gauge symmetry breaks down to SU(2) _L ×U(1). We calculate the contribution of the fermions to these couplings when T parity is invoked. In addition, we re-examine the MSSM contribution at the chosen point of SPS1a′ and compare with the SM and Little Higgs models.     

Topological superfluid in one-dimensional ultracold atomic system with spin-orbit coupling

We propose a one-dimensional Hamiltonian H _1D which supports Majorana fermions when d _{x² ? y²}-wave superfluid appears in the ultracold atomic system and obtain the phase diagrams both for the time-reversal-invariant (TRI) case and time-reversal-symmetry-breaking (TRSB) case. From the phase diagrams, we find that the Majorana doublets and the single Majorana fermions exist in the topological superfluid (TSF) regions for the TRI case and the TRSB case, respectively, and we can reach these regions by tuning the chemical potential μ and spin-orbit coupling α _R . Importantly, the spin-orbit coupling has been realized in ultracold atoms by the recent experimental achievement of synthetic gauge field, therefore, our one-dimensional ultra-cold atomic system described by H _1D is a promising platform to find the mysterious Majorana fermions.     

Breaking E6 via trinification in F‐theory

We consider E₆ GUT model in F‐theory approach where E₆ is broken via trinification to the Standard Model (SM) gauge group using non‐abelian fluxes. Including the gauge singlet wave function we found hierarchically small values for both the μ term as well as Dirac neutrino masses.     

A Condensed Matter Interpretation of SM Fermions and Gauge Fields

We present the bundle (Aff(3)⊗ℂ⊗Λ)(ℝ³), with a geometric Dirac equation on it, as a three-dimensional geometric interpretation of the SM fermions. Each (ℂ⊗Λ)(ℝ³) describes an electroweak doublet. The Dirac equation has a doubler-free staggered spatial discretization on the lattice space (Aff(3)⊗ℂ)(ℤ³). This space allows a simple physical interpretation as a phase space of a lattice of cells. We find the SM SU(3) _c ×SU(2) _L ×U(1) _Y action on (Aff(3)⊗ℂ⊗Λ)(ℝ³) to be a maximal anomaly-free gauge action preserving E(3) symmetry and symplectic structure, which can be constructed using two simple types of gauge-like lattice fields: Wilson gauge fields and correction terms for lattice deformations. The lattice fermion fields we propose to quantize as low energy states of a canonical quantum theory with ℤ₂-degenerated vacuum state. We construct anticommuting fermion operators for the resulting ℤ₂-valued (spin) field theory. A metric theory of gravity compatible with this model is presented too.     

Pseudo-observables in axial gauge

We have given a first application of the axial gauge à la Dams and Kleiss to the standard model (SM) physics at the LHC. We have focused on the issue of providing a well-behaved signal definition in presence of potentially strong gauge cancellations at high energies. As a first illustration, we have analyzed the production of WZ vector-boson pairs, which gives rise to four final-state fermions. Purely leptonic finalstates, pp→lν̄_ll^′l^′̄, have been numerically investigated in the region of high center-of-mass energies and large scattering angles, which are particularly sensitive to gauge dependences. We have found that the axial gauge is the appropriate framework to recover a meaningful separation of signal and irreducible background over the full energy domain.     

Anomaly driven signatures of new invisible physics at the Large Hadron Collider

Many extensions of the Standard Model (SM) predict new neutral vector bosons at energies accessible by the Large Hadron Collider (LHC). We study an extension of the SM with new chiral fermions subject to non-trivial anomaly cancellations. If the new fermions have SM charges, but are too heavy to be created at LHC, and the SM fermions are not charged under the extra gauge field, one would expect that this new sector remains completely invisible at LHC. We show, however, that a non-trivial anomaly cancellation between the new heavy fermions may give rise to observable effects in the gauge boson sector that can be seen at the LHC and distinguished from backgrounds.     

Two loop electroweak corrections from heavy fermions to b→s+γ

Applying an effective Lagrangian method and an on-shell scheme, we analyze the electroweak corrections to the rare decay b→, s+γ from some special two loop diagrams in which a closed heavy fermion loop is attached to the virtual charged gauge bosons or Higgs. At the decoupling limit where the virtual fermions in the inner loop are much heavier than the electroweak scale, we verify the final results satisfying the decoupling theorem explicitly when the interactions among Higgs and heavy fermions do not contain the nondecoupling couplings. Adopting the universal assumptions on the relevant couplings and mass spectrum of new physics, we find that the relative corrections from those two loop diagrams to the SM theoretical prediction on the branching ratio of B → Xsγ can reach 5% as the energy scale of new physics ANp=200 GeV.     

Towards the effective potential of the littlest Higgs model

We compute the relevant parameters of the combined Higgs and φ scalar effective potential in the littlest Higgs (LH) model. These parameters are obtained as the sum of two kinds of contributions. The first one is the one-loop radiative corrections coming from fermions and gauge bosons. The second one is obtained at tree level from the higher-order effective operators needed for the ultraviolet completion of the model. Finally, we analyze the restrictions that the requirement of reproducing the standard electroweak symmetry breaking of the SM set on the LH model parameters.     

Chiral gauge theories in the 1/N expansion

We study the large-N limit of various (SU(N) gauge theories with chiral fermion content. Assuming that the leading N → ∞ behavior is given by a sum of planar diagrams, we find that the gauge interactions must fail to confine color in some models. Other models, assuming both a planar diagram limit and confinement, must contain massless composite fermions.     

The possibility of new fermions with ΔI = 0 mass

In the Glashow-Weinberg-Salam model the fermions have $\text{ΔI =}\text{1}\text{2}$ masses from the breaking of the weak SU(2) gauge symmetry. In many enlarged models, such as those from grand unified and/or supersymmetric theories, there are additional fermions with undetermined ΔI = 0 (SU(2) invariant) masses. We study these new fermions. They induce flavour changing neutral currents. We show that the mixing angles of ΔI = 0 fermions of mass order M with normal $\text{ΔI =}\text{1}\text{2}$ fermions of mass order m ? M are order η or η², where η = m/M. If M > 150 δ, δ being a model-dependent mixing parameter of order a normal fermion mass, the amplitudes of all FCNC processes are below the experimental sizes and limits. Thus for δ?0.1 GeV, M can be as low as 20 GeV, close to the present experimental lower bound. δ is fixed, and less than 0.1 GeV for all relevant cases, if we assume the mass hierarchy of the known fermions is not the result of a particular choice of ΔI = 0 mass parameters. If produced ΔI = 0 mass fermions will be noticeable by the mass degeneracy within their isospin multiplets. There will be an enhanced ratio of neutral to charged decays over the normal fermions. Standard GUT predictions are changed little.     

Flavor changing neutral currents and a Z′

We consider a non-universal Z′ that affects primarily the third generation fermions as an example of new physics associated with the top-quark. We first discuss constraints on the mass and coupling strength of such a Z′. We then turn our attention to the flavor changing neutral currents (FCNC) present in the model. We discuss the experimental constraints and their implications. We propose an ansatz to understand the smallness of the FCNC in terms of the CKM matrix.     

Search for top quark FCNC couplings in Z′ models at the LHC and CLIC

The top quark is the heaviest particle to date discovered, with a mass close to the electroweak symmetry breaking scale. It is expected that the top quark would be sensitive to the new physics at the TeV scale. One of the most important aspects of the top quark physics can be the investigation of the possible anomalous couplings. Here, we study the top quark flavor changing neutral current (FCNC) couplings via the extra gauge boson Z′ at the Large Hadron Collider (LHC) and the Compact Linear Collider (CLIC) energies. We calculate the total cross sections for the signal and the corresponding Standard Model (SM) background processes. For an FCNC mixing parameter x=0.2 and the sequential Z′ mass of 1 TeV, we find the single top quark FCNC production cross sections 0.38(1.76) fb at the LHC with $\sqrt{s_{pp}}=7(14)$ TeV, respectively. For the resonance production of sequential Z′ boson and decays to single top quark at the Compact Linear Collider (CLIC) energies, including the initial state radiation and beamstrahlung effects, we find the cross section to be 27.96(0.91) fb at $\sqrt{s_{e^{+}e^{-}}}=1(3)$ TeV, respectively. We make the analysis to investigate the parameter space (mixing-mass) through various Z′ models. It is shown that the results benefit from the flavor tagging.     

Induced fermionic interactions in theories with general nonlinearly realized SSB

We consider a model with nonlinear SSB, which can be considered as a limiting case of the electroweak SM whenM _H→∞. It possesses a chain of hidden local gauge symmetries yielding a series of heavy gauge boson triplets, which can be interpreted as effects of the strong self-interactions of the scalar sector and are able to infect via mixing low energy quantities. The theory is non-renormalizable and, therefore, new Lagrangian terms are induced at each loop order. We investigate these quantum-induced interactions (which are of non-standard type) of fermions and vector bosons, and show that they can be expressed in additional Lagrangian terms which obey the symmetry of the original theory.     

Flavour-changing neutral currents in models with extra<Emphasis Type="Italic">Z′</Emphasis> boson

New neutral gauge bosonsZ′ are the features of many models addressing the physics beyond the standard model. Together with the existence of new neutral gauge bosons, models based on extended gauge groups (rank > 4) often predict new charged fermions also. A mixing of the known fermions with new states, with exotic weak-isospin assignments (left-handed singlets and right-handed doublets) will induce tree-level flavour-changing neutral interactions mediated byZ exchange, while if the mixing is only with new states with ordinary weak-isospin assignments, the flavour-changing neutral currents are mainly due to the exchange of the new neutral gauge bosonZ′. We review flavour-changing neutral currents in models with extraZ′ boson. Then we discuss some flavour-changing processes forbidden in the standard model and new contributions to standard model processes.     

Gauge group breaking by Wilson loops

We examine the breaking of gauge symmetries by Wilson loops in the Hosotani-Toms model by determining the background gauge field which minimises the one-loop effective potential for massless Dirac fermions. For anti-periodic fermions, all gauge groups remain unbroken. For periodic fermions, the groups G₂, F₄ and E₈ are broken by quantum corrections due to fermions in any irreducible representation, whereas E₆, E₇ and the classical groups only break if the fermion representation is in the same congruency class as the adjoint.