Differences in the Cooling Behavior of Strange Quark Matter Stars and Neutron Stars

The general statement that hypothetical strange (quark matter) stars cool more rapidly than neutron stars is investigated in greater detail. It is found that the direct Urca process could be forbidden not only in neutron stars but also in strange stars. In this case, strange stars are slowly cooling, and their surface temperatures are more or less indistinguishable from those of slowly cooling neutron stars. Furthermore the case of enhanced cooling is reinvestigated. It shows that strange stars cool significantly more rapidly than neutron stars within the first $\sim 30$ years after birth. This feature could become particularly interesting if continued observation of SN 1987A would reveal the temperature of the possibly existing pulsar at its center.Comment: 9 pages, LaTeX (aas-style file), 2 ps-figures. To be published in ApJ Letter

Thermal conductivity of quantum magnetic monopoles in the frustrated pyrochlore Yb2Ti2O7

We report low-temperature thermal conductivity $\kappa$ of pyrochlore Yb$_2$Ti$_2$O$_7$, which contains frustrated spin-ice correlations with significant quantum fluctuations. In the disordered spin-liquid regime, $\kappa(H)$ exhibits a nonmonotonic magnetic field dependence, which is well explained by the strong spin-phonon scattering and quantum monopole excitations. We show that the excitation energy of quantum monopoles is strongly suppressed from that of dispersionless classical monopoles. Moreover, in stark contrast to the diffusive classical monopoles, the quantum monopoles have a very long mean free path. We infer that the quantum monopole is a novel heavy particle, presumably boson, which is highly mobile in a three-dimensional spin liquid.Comment: 8 pages, 9 figure

Are strange stars distinguishable from neutron stars by their cooling behaviour?

The general statement that strange stars cool more rapidly than neutron stars is investigated in greater detail. It is found that the direct Urca process could be forbidden not only in neutron stars but also in strange stars. If so, strange stars would be slowly cooling and their surface temperatures would be more or less indistinguishable from those of slowly cooling neutron stars. The case of enhanced cooling is reinvestigated as well. It is found that strange stars cool significantly more rapidly than neutron stars within the first $\sim 30$ years after birth. This feature could become particularly interesting if continued observation of SN 1987A would reveal the temperature of the possibly existing pulsar at its centre.Comment: 10 pages, 3 ps-figures, to appear in the proceedings of the International Symposium on ''Strangeness in Quark Matter 1997``, April 14--18, Thera (Santorini), Hella

Magnetized strangelets at finite temperature

The main properties of magnetized strangelets, namely, their energy per baryon, radius and electric charge, are studied. Temperature effects are also taken into account in order to study their stability compared to the 56Fe isotope and non-magnetized strangelets using the liquid drop model. Massive quarks are considered with the aim to have a more realistic description for strangelets in the astrophysical context and the environment of heavy ion colliders, playing also an important role in the thermodynamical quantities of the quark gas. It is concluded that the presence of a magnetic field tends to stabilize more the strangelets, even when temperature effects are taken into account. Magnetized strangelets in a paired superconductor phase (magnetized color flavor locked phase) are also discussed. It is shown that they are more stable than ordinary magnetized strangelets for typical gap values of the order of O(100) MeV.Comment: 10 pages, 10 figures, discussion extended, new references adde

Strangeness in Neutron Stars

It is generally agreed on that the tremendous densities reached in the centers of neutron stars provide a high-pressure environment in which numerous novel particles processes are likely to compete with each other. These processes range from the generation of hyperons to quark deconfinement to the formation of kaon condensates and H-matter. There are theoretical suggestions of even more exotic processes inside neutron stars, such as the formation of absolutely stable strange quark matter, a configuration of matter even more stable than the most stable atomic nucleus, iron. In the latter event, neutron stars would be largely composed of pure quark matter, eventually enveloped in a thin nuclear crust. No matter which physical processes are actually realized inside neutron stars, each one leads to fingerprints, some more pronounced than others though, in the observable stellar quantities. This feature combined with the unprecedented progress in observational astronomy, which allows us to see vistas with remarkable clarity that previously were only imagined, renders neutron stars to nearly ideal probes for a wide range of physical studies, including the role of strangeness in dense matter.Comment: 15 pages, 6 figures, Presented at the 5th International Conference on Strangeness in Quark Matter (Strangeness 2000), Berkeley, California, USA, July 20-25, 200

The Principle of Non-Gravitating Vacuum Energy and some of its consequences

For Einstein's General Relativity (GR) or the alternatives suggested up to date the vacuum energy gravitates. We present a model where a new measure is introduced for integration of the total action in the D-dimensional space-time. This measure is built from D scalar fields $\varphi_{a}$. As a consequence of such a choice of the measure, the matter lagrangian $L_{m}$ can be changed by adding a constant while no gravitational effects, like a cosmological term, are induced. Such Non-Gravitating Vacuum Energy (NGVE) theory has an infinite dimensional symmetry group which contains volume-preserving diffeomorphisms in the internal space of scalar fields $\varphi_{a}$. Other symmetries contained in this symmetry group, suggest a deep connection of this theory with theories of extended objects. In general {\em the theory is different from GR} although for certain choices of $L_{m}$, which are related to the existence of an additional symmetry, solutions of GR are solutions of the model. This is achieved in four dimensions if $L_{m}$ is due to fundamental bosonic and fermionic strings. Other types of matter where this feature of the theory is realized, are for example: scalars without potential or subjected to nonlinear constraints, massless fermions and point particles. The point particle plays a special role, since it is a good phenomenological description of matter at large distances. de Sitter space is realized in an unconventional way, where the de Sitter metric holds, but such de Sitter space is supported by the existence of a variable scalar field which in practice destroys the maximal symmetry. The only space - time where maximal symmetry is not broken, in a dynamical sense, is Minkowski space. The theory has non trivial dynamics in 1+1 dimensions, unlike GR.Comment: 23 page

Naturalness Bounds on Dipole Moments from New Physics

Assuming naturalness that the quantum corrections to the mass should not exceed the order of the observed mass, we derive and apply model-independent bounds on the anomalous magnetic moments and electric dipole moments of leptons and quarks due to new physics.Comment: 4 pages, 2 figure

QCD corrections to decay-lepton polar and azimuthal angular distributions in e+e- -> t tbar in the soft-gluon approximation

QCD corrections to order alpha_s in the soft-gluon approximation to angular distributions of decay charged leptons in the process e+e- -> t tbar followed by semileptonic decay of t or tbar, are obtained in the e+e- centre-of-mass frame. As compared to distributions in the top rest frame, these have the advantage that they would allow direct comparison with experiment without the need to reconstruct the top rest frame. The results also do not depend on the choice of a spin quantization axis for t or tbar. Analytic expression for the triple distribution in the polar angle of t and polar and azimuthal angles of the lepton is obtained. Analytic expression is also derived for the distribution in the charged-lepton polar angle. Numerical values are discussed for total c.m. energies of 400 GeV, 800 GeV and 1500 GeV.Comment: 21 pages, Latex, 6 figures included in the submission. To appear in Pramana - Journal of Physics; expanded version of hep-ph/0011321, v

Strange Star Heating Events as a Model for Giant Flares of Soft Gamma-ray Repeaters

Two giant flares were observed on 5 March 1979 and 27 August 1998 from the soft gamma-ray repeaters SGR 0526-66 and SGR 1900+14, respectively. The striking similarity between these remarkable bursts strongly implies a common nature. We show that the light curves of the giant bursts may be easily explained in the model where the burst radiation is produced by the bare quark surface of a strange star heated, for example, by impact of a massive comet-like object.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Letter