Symmetry (physics)

About: Symmetry (physics) is a research topic. Over the lifetime, 26435 publications have been published within this topic receiving 500189 citations. The topic is also known as: symmetry (physics) & physical symmetry.

Expanding Relativistic Shells and Gamma-Ray Burst Temporal Structure

Abstract: Many models of gamma-ray bursts (GRBs) involve a shell expanding at extreme relativistic speeds. The shell of material expands in a photon-quiet phase for a period t0 and then becomes gamma-ray active, perhaps due to inhomogeneities in the interstellar medium or the generation of shocks. Based on kinematics, we relate the envelope of the emission of the event to the characteristics of the photon-quiet and photon-active phases. We initially assume local spherical symmetry wherein, on average, the same conditions prevail over the shell's surface within angles the order of Γ–1, where Γ is the Lorentz factor for the bulk motion. The contribution of the curvature to the temporal structure is comparable to the contribution from the overall expansion. As a result, GRB time histories from a shell should have an envelope similar to "FRED" (fast rise, exponential decay) events in which the rise time is related to the duration of the photon-active phase and the fall time is related to the duration of the photon-quiet phase. This result depends only on local spherical symmetry and, since most GRBs do not have such envelopes, we introduce the "shell symmetry" problem: the observed time history envelopes of most GRBs do not agree with that expected for a relativistic expanding shell.Although FREDs have the signature of a relativistic shell, they may not be due to a single shell, as required by some cosmological models. Some FREDs have precursors in which the peaks are separated by more than the expansion time required to explain FRED shape. Such a burst is most likely explained by a central engine; that is, the separation of the multiple peaks occurs because the central site produced multiple releases of energy on timescales comparable to the duration of the event. Alternatively, there still could be local spherical symmetry of the bulk material, but with a low "filling factor"; that is, only a few percent of the viewable surface (which is already very small, 4πΓ–2) ever becomes gamma-ray active.Long complex bursts present a myriad of problems for the models. The duration of the event at the detector is ~t0/(2Γ2). The long duration cannot be due to large t0, since it requires too much energy to sweep up the interstellar medium. Nor can it be due to small Σ if the time variation is due to ambient objects, since the density of such objects is unreasonable (~1018Γ–4 pc–3 for typical parameters). Long events must explain why they almost always violate local spherical symmetry or why they have low filling factors.Both precursor and long complex events are likely to be "central engines" that produce multiple releases of energy over ~100 s. One promising alternative scenario is one in which the shell becomes thicker than the radius of the curvature within Γ–1. Then it acts as a parallel slab, eliminating the problems associated with local spherical symmetry.

Viral self-assembly as a thermodynamic process.

Abstract: The protein shells, or capsids, of nearly all spherelike viruses adopt icosahedral symmetry. In the present Letter, we propose a statistical thermodynamic model for viral self-assembly. We find that icosahedral symmetry is not expected for viral capsids constructed from structurally identical protein subunits and that this symmetry requires (at least) two internal "switching" configurations of the protein. Our results indicate that icosahedral symmetry is not a generic consequence of free energy minimization but requires optimization of internal structural parameters of the capsid proteins.

Flavour physics from an approximate U(2)^3 symmetry

Abstract: The quark sector of the Standard Model exhibits an approximate U(2)3 flavour symmetry. This symmetry, broken in specific directions dictated by minimality, can explain the success of the Cabibbo-Kobayashi-Maskawa picture of flavour mixing and CP violation, confirmed by the data so far, while allowing for observable deviations from it, as expected in most models of ElectroWeak Symmetry Breaking. Building on previous work in the specific context of supersymmetry, we analyze the expected effects and we quantify the current bounds in a general Effective Field Theory framework. As a further relevant example we then show how the U(2)3 symmetry and its breaking can be implemented in a generic composite Higgs model and we make a first analysis of its peculiar consequences. We also discuss how some partial extension of U(2)3 to the lepton sector can arise, both in general and in composite Higgs models. An optimistic though conceivable interpretation of the considerations developed in this paper gives reasons to think that new physics searches in the flavour sector may be about to explore an interesting realm of phenomena.