Charge symmetry violation in the parton distributions of the nucleon

We point out that charge symmetry violation in both the valence and sea quark distributions of the nucleon has a non-perturbative source. We calculate this non-perturbative charge symmetry violation using the meson cloud model, which has earlier been successfully applied to both the study of SU(2) flavour asymmetry in the nucleon sea and quark-antiquark asymmetry in the nucleon. We find that the charge symmetry violation in the valence quark distribution is well below 1%, which is consistent with most low energy tests but significantly smaller than the quark model prediction about 5%-10%. Our prediction for the charge symmetry violation in the sea quark distribution is also much smaller than the quark model calculation.Comment: RevTex, 26 pages, 6 PostScript figure

Janus monolayers of transition metal dichalcogenides.

Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers or stacked van der Waals heterostructures. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields or, as theoretically proposed, with an asymmetric out-of-plane structural configuration. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS2 monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements

Symmetry-out asymmetry-in: the role of DMRT2

Dissertation presented to obtain the PhD degree in BiologyIt may come as a surprise that vertebrates are not entirely bilaterally symmetric. In fact, at the internal level they are asymmetric with the organs acquiring a biased disposition along the left-right (LR) axis. Problems in the establishment of both symmetric and asymmetric features may give rise to several human developmental disorders. Thus, it is very important to study in more detail how LR embryonic development is initiated and maintained. The built of the axial skeleton and its associated skeletal muscles is dependent on somite formation and differentiation. The somites are formed as pairs and are bilaterally symmetric between both sides of the LR axis. Interestingly, it has been suggested that asymmetric somite formation may account for problems at the level of the axial skeleton. At the same time that the somites are being formed, asymmetric signals are being transferred only to the left side of the axis being responsible to set up the correct asymmetric internal organs distribution. In fact, an incorrect transfer of information leads to serious problems in organ situs. It has become clear that several mechanisms are necessary to initiate the LR asymmetry pathway however, something that only recently has been acknowledge is that symmetry is not a default state and there are in fact mechanisms responsible for promoting it. It is not completely understood how, but there are indications that there is a crosstalk between the mechanisms necessary to regulate symmetry vs. asymmetry. Surprisingly, a zebrafish transcription factor named Dmrt2a/ Terra, a gene that belongs to a family mainly associated with sex determination, as been shown to play a role in regulating these two contradictory features. Not only is it necessary for bilateral symmetric somite formation but also for the correct establishment of asymmetry in the left side of the axis. Given this, it is extremely important to study the signalling cascade where it operates and to identify its downstream targets.(...)Apoio financeiro da Fundação para a Ciência e a Tecnologia e do FSE no âmbito do Quadro Comunitário de Apoio, BD nº SFRH/BD/24861/200

Size effects in radiospectroscopy spectra of ferroelectric nanopowders

The theoretical and experimental investigation of ferroelectric nanopowders is performed. The manifestation in radiospectroscopy spectra of size driven ferroelectric-paraelectric phase transition at some critical particle average size was the main goal of the consideration. In theoretical part the size effect for the materials with ferroelectric tetragonal phase and cubic paraelectric phase was considered allowing for the spontaneous polarization inhomogeneity inside a particle and distribution of particle sizes. In ESR the transformation of the spectra from tetragonal symmetry to cubic symmetry with decreasing of nanoparticle sizes was calculated. Measurements of Fe3+ ESR spectra in nanopowder of BaTiO3 were carried out at room temperature. The decrease of intensity of tetragonal symmetry ESR lines of Fe3+ and appearance of cubic symmetry line with asymmetry of the shoulders was observed with the average sizes decrease with complete disappearance of tetragonal spectrum at average size less or equal 40 nm. The comparison of the theory with experiment was carried out. The value of critical size Rc = 40 nm was extracted from ESR data. The asymmetry and broadening of right hand side shoulder of ESR cubic symmetry line was shown to be related to contribution of paramagnetic centers in the vicinity of the particles surface. The deconvolution of the cubic line allowed to show, that this region size is about 3 nm.Comment: 10 pages, 8 figure

Inflationary Baryogenesis in a Model with Gauged Baryon Number

We argue that inflationary dynamics may support a scenario where significant matter-antimatter asymmetry is generated from initially small-scale quantum fluctuations that are subsequently stretched out over large scales. This scenario can be realised in extensions of the Standard Model with an extra gauge symmetry having mixed anomalies with the electroweak gauge symmetry. Inflationary baryogenesis in a model with gauged baryon number is considered in detail.Comment: 11 pages, V3 - major revisio

Symmetry Energy I: Semi-Infinite Matter

Energy for a nucleus is considered in macroscopic limit, in terms of nucleon numbers. Further considered for a nuclear system is the Hohenberg-Kohn energy functional, in terms of proton and neutron densities. Finally, Skyrme-Hartree-Fock calculations are carried out for a half-infinite particle-stable nuclear-matter. In each case, the attention is focused on the role of neutron-proton asymmetry and on the nuclear symmetry energy. We extend the considerations on the symmetry term from an energy formula to the respective term in the Hohenberg-Kohn functional. We show, in particular, that in the limit of an analytic functional, and subject to possible Coulomb corrections, it is possible to construct isoscalar and isovector densities out of the proton and neutron densities, that retain a universal relation to each other, approximately independent of asymmetry. In the so-called local approximation, the isovector density is inversely proportional to the symmetry energy in uniform matter at the local isoscalar density. Generalized symmetry coefficient of a nuclear system is related, in the analytic limit of a functional, to an integral of the isovector density. We test the relations, inferred from the Hohenberg-Kohn functional, in the Skyrme-Hartree-Fock calculations of half-infinite matter. Within the calculations, we obtain surface symmetry coefficients and parameters characterizing the densities, for the majority of Skyrme parameterizations proposed in the literature. The volume-to-surface symmetry-coefficient ratio and the displacement of nuclear isovector relative to isoscalar surfaces both strongly increase as the slope of symmetry energy in the vicinity of normal density increases.Comment: 87 pages, 18 figures; discussion of Kohn-Sham method added, comparison to results in literature broadene

Return of the grand unified theory baryogenesis: Source of helical hypermagnetic fields for the baryon asymmetry of the universe

It has been considered that baryogenesis models without a generation of $B$-$L$ asymmetry such as the GUT baryogenesis do not work since the asymmetry is washed out by the electroweak sphalerons. Here, we point out that helical hypermagnetic fields can be generated through the chiral magnetic effect with a chiral asymmetry generated in such baryogenesis models. The helical hypermagnetic fields then produce baryon asymmetry mainly at the electroweak symmetry breaking, which remains until today. Therefore, the baryogenesis models without $B$-$L$ asymmetry can still be the origin of the present baryon asymmetry. In particular, if it can produce chiral asymmetry mainly carried by right-handed electrons of order of $10^{-3}$ in terms of the chemical potential to temperature ratio, the resultant present-day baryon asymmetry can be consistent with our Universe, although simple realizations of the GUT baryogenesis are hard to satisfy the condition. We also argue the way to overcome the difficulty in the GUT baryogenesis. The intergalactic magnetic fields with $B_0\sim 10^{-16 \sim 17}$ G and $\lambda_0 \sim 10^{-2\sim3}$ pc are the smoking gun of the baryogenesis scenario as discussed before.Comment: 10 pages; v2: comments and references added, matches version published in PR