Ab initio simulations of accretion disks instability

We show that accretion disks, both in the subcritical and supercritical accretion rate regime, may exhibit significant amplitude luminosity oscillations. The luminosity time behavior has been obtained by performing a set of time-dependent 2D SPH simulations of accretion disks with different values of alpha and accretion rate. In this study, to avoid any influence of the initial disk configuration, we produced the disks injecting matter from an outer edge far from the central object. The period of oscillations is 2 - 50 s respectively for the two cases, and the variation amplitude of the disc luminosity is 10^38 - 10^39 erg/s. An explanation of this luminosity behavior is proposed in terms of limit cycle instability: the disk oscillates between a radiation pressure dominated configuration (with a high luminosity value) and a gas pressure dominated one (with a low luminosity value). The origin of this instability is the difference between the heat produced by viscosity and the energy emitted as radiation from the disk surface (the well-known thermal instability mechanism). We support this hypothesis showing that the limit cycle behavior produces a sequence of collapsing and refilling states of the innermost disk region.Comment: 11 pages, 15 Postscript figures, uses natbib.sty, accepted for publication in MNRA

Implications of Lorentz violation on Higgs-mediated lepton flavor violation

The lepton flavor violating decay of the Higgs boson $H\to l_Al_B$ is studied within two qualitatively different extensions of the Yukawa sector: one renormalizable and the other nonrenormalizable; both incorporating Lorentz violation in a model-independent fashion. These extensions are characterized by Yukawa-like matrices, the former by a constant Lorentz 2-tensor $Y^{AB}_{\mu \nu}$, whereas the latter by a constant Lorentz vector $Y^{AB}_\mu$. It is found that the experimental constraints on the decays $l_A\to l_B\gamma$ severely restrict lepton flavor violating Higgs signals in the renormalizable scenario. In this context, it is found that $BR(H\to \mu^\pm e^\mp)$ and $BR(H\to \tau^\pm \mu^\mp)$ cannot be larger than $10^{-18}$ and $10^{-11}$, respectively. In the nonrenormalizable scenario, transitions mediated by the Higgs or the $Z$ gauge boson are induced at tree level, and we find mild restrictions on lepton flavor violation. Using the experimental limits on the three-body decays $l_A \to l_B \bar{l}_Cl_C$ to constraint the vector $Y^{AB}_\mu$, it is found that the branching ratio for the decays $H\to \mu^\pm e^\mp$ is of about $4\times 10^{-9}$, more important, a branching ratio of $7\times 10^{-4}$ is found for the $\tau^\pm \mu^\mp$ mode. Accordingly, the decay $H \to \tau^\pm \mu^\mp$ could be at the reach of future measurements. The lepton flavor violating decays of the $Z$ gauge boson were also studied. In the renormalizable scenario, it was found the undetectable branching ratios $BR(Z\to \mu^\pm e^\mp)<5.7\times 10^{-21}$ and $BR(Z\to \tau^\pm \mu^\mp)<2.0\times 10^{-12}$. In the nonrenormalizable scenario, it was found that $BR(Z\to \mu^\pm e^\mp)<0.67\times 10^{-12}$ and $BR(Z\to \tau^\pm \mu^\mp)<1.12\times 10^{-7}$. Although the latter branching ratio is relatively large, it still could not be within the range of future measurements.Comment: Updated to essentially match published versio

Gauge invariant electromagnetic properties of fermions induced by CPT violation in the Standard Model Extension

Low-energy Lorentz-invariant quantities could receive contributions from a fundamental theory producing small Lorentz-violating effects. Within the Lorentz-violating extension of quantum electrodynamics, we investigate, perturbatively, the contributions to the one-loop $ff\gamma$ vertex from the $CPT$-violating axial coupling of a vector background field to fermions. We find that the resulting vertex function has a larger set of Lorentz structures than the one characterizing the usual, Lorentz invariant, parametrization of the $ff\gamma$ vertex. We prove gauge invariance of the resulting one-loop expression through a set of gauge invariant nonrenormalizable operators introducing new-physics effects at the first and second orders in Lorentz violation, and which generate tree-level contributions to the $ff\gamma$ vertex. Whereas loop contributions involving parameters that violate Lorentz invariance at the first order are $CPT$-odd, those arising at the second order are $CPT$-even, so that contributions to low-energy physics are restricted to emerge for the first time at the second order. In this context, we derive a contribution to anomalous magnetic moment of fermions, which we use to set a bound on Lorentz violation.Comment: 13 pages, 1 figure, 1 table, a couple of references were adde

Bounding the Z′tcZ^\prime tc coupling from D0−D0ˉD^{0}-\bar{D^{0}} mixing and single top production at the ILC

In the present work the $Z^\prime tc$ coupling is bounded by using the current experimental data on the $D^{0}-\bar{D^{0}}$ meson-mixing system. It is found that the strength associated to this coupling is less than $5.75\times 10^{-2}$. The single top production through the $e^+e^-\to Z^\prime\to tc$ process at the $Z^\prime$ boson resonance is studied and we found that around $10^7$ $tc$ events will be expected at the International Linear Collider. For the $Z^\prime\to tc$ decay, we predict a branching ratio of $10^{-2}$.Comment: Revised version. Now, 11 pages and 6 figure

Z'tc coupling from D0-D0 mixing

We bound the $Z^\prime tc$ coupling using the $D^{0}-\bar{D^{0}}$ meson mixing system. We obtained such coupling which is less than $5.75\times 10^{-2}$. We have studied the $Z^\prime$ boson resonance considering single top production in the $e^+e^-\to Z^\prime\to tc$ process. We obtained the number of events which is expected to be less than $10^7$ at the International Linear Collider scenario. We get a branching ratio of the order of $10^{-2}$ for the $Z^\prime\to tc$ decay.Comment: 4 pages, 5 figure

Decays Z→γγZ\to\gamma\gamma and Z→ggZ\to gg in the Standard Model Extension

The $Z\to \gamma \gamma$ and $Z\to gg$ decays are studied in the context of the renormalizable version of the Standard Model Extension. The $CPT$-odd $\bar{\psi}\gamma_5 \gamma^\mu b_\mu\psi$ bilinear interaction, which involves the constant background field $b_\alpha$ and which has been a subject of interest in literature, is considered. It is shown that the $Z\to \gamma \gamma$ and $Z\to gg$ decays, which are strictly zero in the standard model, can be generated radiatively at the one-loop level. It is found that these decays are gauge invariant and free of ultraviolet divergences, and that the corresponding decay widths only depend on the spatial component of the background field $b$.Comment: A few references correcte

Response to. comment on optic nerve sheath diameter ultrasound evaluation in intensive care unit: possible role and clinical aspects in neurological critical patients' daily monitoring

Comment on "Optic Nerve Sheath Diameter Ultrasound Evaluation in Intensive Care Unit: Possible Role and Clinical Aspects in Neurological Critical Patients' Daily Monitoring"

Spacelab 3 flight experiment No. 3AFT23: Autogenic-feedback training as a preventive method for space adaptation syndrome

Space adaptation syndrome is a motion sickness-like disorder which affects up to 50 percent of all people exposed to microgravity in space. This experiment tested a physiological conditioning procedure (Autogenic-Feedback Training, AFT) as an alternative to pharmacological management. Four astronauts participated as subjects in this experiment. Crewmembers A and B served as treatment subjects. Both received preflight training for control of heart rate, respiration rate, peripheral blood volume, and skin conductance. Crewmembers C and D served as controls (i.e., did not receive training). Crewmember A showed reliable control of his own physiological responses, and a significant increase in motion sickness tolerance after training. Crewmember B, however, demonstrated much less control and only a moderate increase in motion sickness tolerance was observed after training. The inflight symptom reports and physiological data recordings revealed that Crewmember A did not experience any severe symptom episodes during the mission, while Crewmember B reported one severe symptom episode. Both control group subjects, C and D (who took antimotion sickness medication), reported multiple symptom episodes on mission day 0. Both inflight data and crew reports indicate that AFT may be an effective countermeasure. Additional data must be obtained inflight (a total of eight treatment and eight control subjects) before final evaluation of this treatment can be made

The anapole moment in scalar quantum electrodynamics

The anapole moment of a charged scalar particle is studied in a model independent fashion, using the effective Lagrangian technique, as well as radiatively within the context of scalar quantum electrodynamics (SQED). It is shown that this gauge structure is characterized by a non renormalizable interaction, which is radiatively generated at the one--loop. It is found that the resulting anapole moment for off-shell particles, though free of ultraviolet divergences, is gauge dependent and thus it is not a physical observable. We also study some of its kinematical limits. In particular, it is shown that its value comes out to be zero when all particles are on--shell.Comment: 4 pages, 1 figur