117295580
Some remarks on tree-level vacuum stability in two Higgs doublet models
It is proved that the minimum of a general two Higgs doublet models' potential is stable at tree level. A relation between stability and flavour changing neutral currents at tree level is shown.
18964730
Probing wrong-sign Yukawa couplings at the LHC and a future linear collider
We consider the two-Higgs-doublet model as a framework in which to evaluate the viability of scenarios in which the sign of the coupling of the observed Higgs boson to down-type fermions (in particular, b-quark pairs) is opposite to that of the Standard Model (SM), while at the same time all other tree-level couplings are close to the SM values. We show that, whereas such a scenario is consistent with current LHC observations, both future running at the LHC and a future e +e − linear collider could determine the sign of the Higgs coupling to b-quark pairs. Discrimination is possible for two reasons. First, the interference between the b-quark and the t-quark loop contributions to the ggh coupling changes sign. Second, the charged-Higgs loop contribution to the γγh coupling is large and fairly constant up to the largest charged-Higgs mass allowed by tree-level unitarity bounds when the b-quark Yukawa coupling has the opposite sign from that of the SM (the change in sign of the interference terms between the b-quark loop and the W and t loops having negligible impact).
118483060
Can we build a sensible theory with broken charge and colour symmetries
Charge and colour symmetries are broken using the vevs of scalar fields, but it is shown the theory possesses a photon-like massless particle and electroweak interactions almost identical to those of the Standard Model. We consider experimental signatures of this theory.
117960699
The CP-conserving 2HDM after the 8 TeV run
We confront the most common CP-conserving 2HDM with the LHC data analysed so far while taking into account all previously available experimental data. A special allowed corner of the parameter space is analysed - the so-called wrong-sign scenario where the Higgs coupling to down-type quarks changes sign relative to the Standard Model while the coupling to the massive vector bosons does not.
119205906
Discrete and continuous symmetries in multi-Higgs-doublet models
We consider the Higgs sector of multi-Higgs-doublet models in the presence of simple symmetries relating the various fields. We construct basis-invariant observables which may in principle be used to detect these symmetries for any number of doublets. A categorization of the symmetries into classes is required, which we perform in detail for the case of two and three Higgs doublets.
119493491
One-loop stau masses in the effective potential approach
We calculate the one-loop contributions to the tau slepton masses in the Minimal Supersymmetric Standard Model in the effective potential approach. For the majority of parameter space under study, those corrections are shown to elevate the value of the lightest stau mass.
11014726
Initial conditions of the universe: how much isocurvature is allowed?
We investigate the constraints imposed by current data on correlated mixtures of adiabatic and nonadiabatic primordial perturbations. We discover subtle flat directions in parameter space that tolerate large (approximately 60%) fractions of nonadiabatic fluctuations. In particular, larger values of the baryon density and a spectral tilt are allowed. The cancellations in the degenerate directions are explored and the role of priors is elucidated.
15105689
Effective Electric and Magnetic Local Actions for Electromagnetism with two Gauge Fields
A simple mechanism of dynamical symmetry breaking of electromagnetism with two gauge fields (U(1) × U(1)) is considered. By considering the action variations with respect to the gauge connections F = dA and G = dC we obtain an electric and a magnetic effective actions (U(1)) that have an extra coupling to the magnetic and electric currents (respectivelly) that decouple from the classical theory, meaning that it does not contribute to the standard equations of motion obtained by varying A and C, i.e. Maxwell equations. The extra gauge field C(or A) effectivelly is fixed by the physical effective field A(or C) constituting a non-trivial configuration C = C(A) (or A = A(C)) such that the field discontinuities (Dirac string or WuYang non-trivial fiber-bundle) are encoded in the extra gauge field C (or A). Our construction is only compactible with local current densities. work supported by SFRH/BPD/17683/2004 and POCTI P-FIS-57547/2004; also at Centra
119052706
Constraints on a primordial magnetic field
We derive an upper limit of B{sub 0}{lt}3.4{times}10{sup -9}({Omega}{sub 0}h{sup 2}{sub 50}){sup 1/2} G on the present strength of any primordial homogeneous magnetic field. The microwave background anisotropy created by cosmological magnetic fields is calculated in the most general flat and open anisotropic cosmologies containing expansion-rate and 3-curvature anisotropies. Our limit is derived from a statistical analysis of the 4-year Cosmic Background Explorer (COBE) data for anisotropy patterns characteristic of homogeneous anisotropy averaged over all possible sky orientations with respect to the COBE receiver. The limits we obtain on homogeneous magnetic fields are stronger than those imposed by nucleosynthesis. {copyright} {ital 1997} {ital The American Physical Society}
119305993
The kSZ effect as a test of general radial inhomogeneity in LTB cosmology
The apparent accelerating expansion of the Universe, determined from observations of distant supernovae, and often taken to imply the existence of dark energy, may alternatively be explained by the effects of a giant underdense void if we relax the assumption of homogeneity on large scales. Recent studies have made use of the spherically-symmetric, radially-inhomogeneous Lemaitre-Tolman-Bondi (LTB) models to derive strong constraints on this scenario, particularly from observations of the kinematic Sunyaev-Zel'dovich (kSZ) effect which is sensitive to large scale inhomogeneity. However, most of these previous studies explicitly set the LTB 'bang time' function to be constant, neglecting an important freedom of the general solutions. Here we examine these models in full generality by relaxing this assumption. We find that although the extra freedom allowed by varying the bang time is sufficient to account for some observables individually, it is not enough to simultaneously explain the supernovae observations, the small-angle CMB, the local Hubble rate, and the kSZ effect. This set of observables is strongly constraining, and effectively rules out simple LTB models as an explanation of dark energy.
16906939
Effect of Pions in Cosmic Rays
The effects of pions for vacuum polarization in background magnetic fields are considered. The effects of quark condensates is also briefly addresses. Although these effects are out of the measurement accuracy of laboratory experiments they may be relevant for gamma‐ray burst propagation. In particular, for emissions from the center of the galaxy, we show that the mixing between the neutral pion and photons results in a deviation of the gamma‐ray spectrum from the standard power‐law in the TeV range.