Re-entrant melting and freezing in a model system of charged colloids

We studied the phase behavior of charged and sterically stabilized colloids using confocal microscopy in a less polar solvent (dielectric constant 5.4). Upon increasing the colloid volume fraction we found a transition from a fluid to a body centered cubic crystal at 0.0415+/-0.0005, followed by re-entrant melting at 0.1165+/-0.0015. A second crystal of different symmetry, random hexagonal close-packed, was formed at a volume fraction around 0.5, similar to that of hard spheres. We attribute the intriguing phase behavior to particle interactions that depend strongly on volume fraction, mainly due to changes in the colloid charge. In this low polarity system the colloids acquire charge through ion adsorption. The low ionic strength leads to fewer ions per colloid at elevated volume fractions and consequently a density-dependent colloid charge.Comment: 25 pages, 5 figures 1 tabl

New Grand Unified Models with Intersecting D6-branes, Neutrino Masses, and Flipped SU(5)

We construct new supersymmetric SU(5) Grand Unified Models based on Z4 x Z2 orientifolds with intersecting D6-branes. Unlike constructions based on Z2 x Z2 orientifolds, the orbifold images of the three-cycles wrapped by D6-branes correspond to new configurations and thus allow for models in which, in addition to the chiral sector in 10 and 5-bar representations of SU(5), only, there can be new sectors with (15 + 15-bar) and (10 + 10-bar) vector-pairs. We construct an example of such a globally consistent, supersymmetric model with four-families, two Standard Model Higgs pair-candidates and the gauge symmetry U(5) x U(1) x Sp(4). In a N = 2 sector, there are 5 x (15 + 15-bar) and 1 x (10 + 10-bar) vector pairs, while another N = 1 sector contains one vector-pair of 15-plets. The N = 2 vector pairs can obtain a large mass dynamically by parallel D6-brane splitting in a particu- lar two-torus. The 15-vector-pairs provide, after symmetry breaking to the Standard Model (via parallel D-brane splitting), triplet pair candidates which can in principle play a role in generating Majorana- type masses for left-handed neutrinos, though the necessary Yukawa couplings are absent in the specific construction. Similarly, the 10- vector-pairs can play the role of Higgs fields of the flipped SU(5), though again there are phenomenological difficulties for the specific construction.Comment: 38 pages, 2 figures Minor corrections, references adde

Proton decay via dimension-six operators in intersecting D6-brane models

We analyze the proton decay via dimension six operators in supersymmetric SU(5)-Grand Unified models based on intersecting D6-brane constructions in Type IIA string theory orientifolds. We include in addition to 10* 10 10* 10 interactions also the operators arising from 5-bar* 5-bar 10* 10 interactions. We provide a detailed construction of vertex operators for any massless string excitation arising for arbitrary intersecting D-brane configurations in Type IIA toroidal orientifolds. In particular, we provide explicit string vertex operators for the 10 and 5-bar chiral superfields and calculate explicitly the string theory correlation functions for above operators. In the analysis we chose the most symmetric configurations in order to maximize proton decay rates for the above dimension six operators and we obtain a small enhancement relative to the field theory result. After relating the string proton decay rate to field theory computations the string contribution to the proton lifetime is tau^{ST}_p =(0.5-2.1) x 10^{36} years, which could be up to a factor of three shorter than that predicted in field theory.Comment: 46 pages, 2 figures references added, minor correction

Spacetime Instanton Corrections in 4D String Vacua - The Seesaw Mechanism for D-Brane Models

We systematically investigate instanton corrections from wrapped Euclidean D-branes to the matter field superpotential of various classes of N=1 supersymmetric D-brane models in four dimensions. Both gauge invariance and the counting of fermionic zero modes provide strong constraints on the allowed non-perturbative superpotential couplings. We outline how the complete instanton computation boils down to the computation of open string disc diagrams for boundary changing operators multiplied by a one-loop vacuum diagram. For concreteness we focus on E2-instanton effects in Type IIA vacua with intersecting D6-branes, however the same structure emerges for Type IIB and heterotic vacua. The instantons wrapping rigid cycles can potentially destabilise the vacuum or generate perturbatively absent matter couplings such as proton decay operators, mu-parameter or right-handed neutrino Majorana mass terms. The latter allow the realization of the seesaw mechanism for MSSM-like intersecting D-brane models.Comment: 40 pages, 3 tables, 7 figures; v2: typos corrected, references added; v3: minor sign adjustments, some comments added; v4: published versio

D6-brane Splitting on Type IIA Orientifolds

We study the open-string moduli of supersymmetric D6-branes, addressing both the string and field theory aspects of D6-brane splitting on Type IIA orientifolds induced by open-string moduli Higgsing (i.e., their obtaining VEVs). Specifically, we focus on the Z_2 x Z_2 orientifolds and address the symmetry breaking pattern for D6-branes parallel with the orientifold 6-planes as well as those positioned at angles. We demonstrate that the string theory results, i.e., D6-brane splitting and relocating in internal space, are in one to one correspondence with the field theory results associated with the Higgsing of moduli in the antisymmetric representation of Sp(2N) gauge symmetry (for branes parallel with orientifold planes) or adjoint representation of U(N) (for branes at general angles). In particular, the moduli Higgsing in the open-string sector results in the change of the gauge structure of D6-branes and thus changes the chiral spectrum and family number as well. As a by-product, we provide the new examples of the supersymmetric Standard-like models with the electroweak sector arising from Sp(2N)_L x Sp(2N)_R gauge symmetry; and one four-family example is free of chiral Standard Model exotics.Comment: 44 pages, 7 figures; The anomaly-free models in Subsections 4.2 and 4.3 presented, references added, typos fixe

To whom should I be kind? A randomized trial about kindness for strong and weak social ties on mental wellbeing and its specific mechanisms of change

The current study examines the role of social ties in performing kind acts to enhance university students’ wellbeing. Due to facing multifaceted challenges, university students form a group that is particularly vulnerable in terms of their mental health. Interventions harnessing prosocial behaviour have the potential to increase students’ wellbeing, strengthen personal competencies, and broaden social networks. The first aim of the trial (N = 222) was to explore whether a 4-week acts-of-kindness intervention targeting either (1) strong social ties, (2) weak social ties or (3) unspecified receivers (treatment-as-usual) differ in their impact on students’ mental wellbeing, positive relations, depressive symptoms, anxiety, and perceived stress. The second aim was to examine whether kindness for strong versus weak social ties have different underlying working mechanisms (i.e., positive emotions versus self-esteem) and who benefits most from these instructions (i.e., those with high or low levels of self-esteem and positive relations). Results demonstrated that the most significant improvements in mental wellbeing were found in the kindness for strong social ties condition compared to the other conditions. No mediation effects of positive emotions and self-esteem were found. Moderation analyses revealed that participants who performed kind acts for weak social ties reported significantly less positive effects on mental wellbeing, but only when their levels of self-esteem at baseline were medium or high. Independent of group allocation, participants’ mental wellbeing increased throughout the intervention, but so did the experience of depressive symptoms, anxiety, and perceived stress. More research is needed to examine the timing of kindness interventions and investigate how they can improve mental wellbeing and psychological distress in acute phases of academic stress in university students

Comparison of conventional and higher-resolution reduced-FOV diffusion-weighted imaging of breast tissue

Objective: Reduced FOV-diffusion-weighted imaging (rFOV-DWI) allows for acquisition of a tissue region without back-folding, and may have better fat suppression than conventional DWI imaging (c-DWI). The aim was to compare the ADCs obtained with c-DWI bilateral-breast imaging with single-breast rFOV-DWI. Materials and Methods: Breasts of 38 patients were scanned at 3 T. The mean ADC values obtained for 38 lesions, and fibro-glandular (N = 35) and adipose (N = 38) tissue ROIs were compared between c-DWI and higher-resolution rFOV-DWI (Wilcoxon rank test). Also, the ADCs were compared between the two acquisitions for an oil-only phantom and a combined water/oil phantom. Furthermore, ghost artifacts were assessed. Results: No significant difference in mean ADC was found between the acquisitions for lesions (c-DWI: 1.08 × 10–3 mm2/s, rFOV-DWI: 1.13 × 10–3 mm2/s) and fibro-glandular tissue. For adipose tissue, the ADC using rFOV-DWI (0.31 × 10–3 mm2/s) was significantly higher than c-DWI (0.16 × 10–3 mm2/s). For the oil-only phantom, no difference in ADC was found. However, for the water/oil phantom, the ADC of oil was significantly higher with rFOV-DWI compared to c-DWI. Discussion: Although ghost artifacts were observed for both acquisitions, they appeared to have a greater impact for rFOV-DWI. However, no differences in mean lesions’ ADC values were found, and therefore this study suggests that rFOV can be used diagnostically for single-breast DWI imaging.</p

Instructions for Authors

Nonadherence to medical therapy is frequently encountered in patients with inflammatory bowel disease (IBD). We aimed to identify predictors for future (non)adherence in IBD