16,456 research outputs found
The Flavour of Inflation
A new class of particle physics models of inflation based on the phase
transition associated with the spontaneous breaking of family symmetry is
proposed. The Higgs fields responsible for the breaking of family symmetry, the
flavons, are natural inflaton candidates or waterfall fields in hybrid
inflation. This opens up a rich vein of possible inflation models, all linked
to the physics of flavour, with several interesting cosmological implications.Comment: Talk given at the 16th International Conference on Supersymmetry and
the Unification of Fundamental Interactions (SUSY08), Seoul, Korea (June
16-21, 2008). To be published in the Conference Proceeding
âYo No Cruse La Frontera, La Frontera Me Cruzoâ (I Didnât Cross the Border, the Border Crossed Me)
The song, âSomos Mas Americanosâ is a work of art, and every single one of its lyrics is tied to a history about my experiences, my familyâs experiences, and the experiences of countless others. Lyrics like those in âSomos Mas Americanosâ touch peopleâs souls and pierce their hearts because it speaks to the socioeconomic, racial hierarchy, racially biased politics, and laws against immigrant workers like myself in the U.S. Further than that, it educates listeners about the U.S./Mexican War and the Treaty of Guadalupe Hidalgo, where MĂ©xico lost its territory to the U.S., which explains the heated controversy surrounding the U.S.âMexican border today
Galactic PeV neutrinos from dark matter annihilation
The IceCube Neutrino Observatory has observed highly energetic neutrinos in
excess of the expected atmospheric neutrino background. It is intriguing to
consider the possibility that such events are probing physics beyond the
standard model. In this context, (PeV) dark matter particles
decaying to neutrinos have been considered while dark matter annihilation has
been dismissed invoking the unitarity bound as a limiting factor. However, the
latter claim was done ignoring the contribution from dark matter substructure,
which for PeV Cold Dark Matter would extend down to a free streaming mass of
(M). Since the unitarity bound is less stringent
at low velocities, (v), then, it is
possible that these cold and dense subhalos would contribute dominantly to a
dark-matter-induced neutrino flux and easily account for the events observed by
IceCube. A Sommerfeld-enhanced dark matter model can naturally support such
scenario. Interestingly, the spatial distribution of the events shows features
that would be expected in a dark matter interpretation. Although not
conclusive, 9 of the 37 events appear to be clustered around a region near the
Galactic Center while 6 others spatially coincide, within the reported angular
errors, with 5 of 26 Milky Way satellites. However, a simple estimate of the
probability of the latter occurring by chance is . More events are
needed to statistically test this hypothesis. PeV dark matter particles are
massive enough that their abundance as standard thermal relics would overclose
the Universe. This issue can be solved in alternative scenarios, for instance
if the decay of new massive unstable particles generates significant entropy
reheating the Universe to a slightly lower temperature than the freeze-out
temperature, ~GeV.Comment: 14 pages, 3 figures, accepted for publication in Physical Review D;
added: new IceCube data, Fig. 3 and related discussio
Are There Rotation Measure Gradients Across AGN Jets?
We report on multi-frequency polarimetry Very Long Baseline Interferometry
observations of active galactic nuclei using the VLBA. These observations are
used to construct images of the Faraday Rotation Measure (RM) in J1613+342, Mrk
501, 3C 371, and BL Lac. Despite having resolved the jets in total intensity
and polarization for three of these sources no RM gradients are found. This is
in contrast to the large fraction of sources with RM gradients now claimed in
the literature, and invoked as evidence in support of helical magnetic fields.
We propose objective criteria for establishing what constitutes an RM gradient.
Furthermore, although we note the absence of simple, monotonic gradients,
comparison with simulations could reveal systematic changes in the RM which may
be masked by a varying jet orientation.Comment: 5 pages, 4 figures, accepted to Astrophysical Journal Letter
Tracking sea bed topography in the Jurassic. The Lotena Group in the Sierra de la Vaca Muerta (Neuquén Basin, Argentina)
The Lotena Group is a clastic to evaporitic unit up to 650 m thick that accumulated during the Middle to Late Jurassic in the NeuquĂ©n Basin, western Argentina. Extensive field work carried out in the Sierra de la Vaca Muerta and Arroyo Covunco areas, including the measurement of seven detailed stratigraphic sections and geo- logical mapping allow the discrimination of six unconformity-bounded units or sequences. The first sequence is composed of red beds and evaporites belonging to the TĂĄbanos Formation that unconformably overlies strata of the Lower to Middle Jurassic Cuyo Group. Sequences 2 to 5 are shallow marine and display a basal sandstone interval attributed to confined shelfal sandstone lobes. These grade vertically into unconfined shelfal sandstone lobes, and terminate with carbonate deposits. The basal interval is restricted to the thickest areas of each sequence, a relationship attributed to structural relief. Sequence 6 has a very irregular shape and strongly trun- cates the underlying deposits. It is composed almost entirely of massive carbonate strata that were deposited by density currents. Facies analysis and stratigraphic mapping suggest periodic recycling of previous accumula- tions. Stratigraphic evidence suggests that the Lotena Group in the Sierra de la Vaca Muerta and adjacent areas probably accumulated over a tectonically unstable basement. Sequences 1, 2 and 3 display evidence of accumu- lation in an extensional tectonic setting, while sequences 4, 5 and 6 experienced a northward shift of their depocentres associated with extensive erosional truncation of the marginal areas, suggesting that accumulation was affected by early stages of growth of the Covunco anticline.Fil: Zavala, Carlos Alberto. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - BahĂa Blanca. Instituto Argentino de OceanografĂa. Universidad Nacional del Sur. Instituto Argentino de OceanografĂa; Argentin
Signatures of Relativistic Helical Motion in the Rotation Measures of AGN Jets
Polarization has proved an invaluable tool for probing magnetic fields in
relativistic jets. Maps of the intrinsic polarization vectors have provided the
best evidence to date for uniform, toroidally dominated magnetic fields within
jets. More recently, maps of the rotation measure (RM) in jets have for the
first time probed the field geometry of the cool, moderately relativistic
surrounding material. In most cases, clear signatures of toroidal magnetic
field are detected, corresponding to gradients in RM profiles transverse to the
jet. However, in many objects these profiles also display marked asymmetries
which are difficult to explain in simple helical jet models. Furthermore, in
some cases the RM profiles are strongly frequency and/or time dependent. Here
we show that these features may be naturally accounted for by including
relativistic helical motion in the jet model. In particular, we are able to
reproduce bent RM profiles observed in a variety of jets, frequency dependent
RM profile morphologies and even the time dependence of the RM profiles of
knots in 3C 273. Finally, we predict that some sources may show reversals in
their RM profiles at sufficiently high frequencies, depending upon the the
ratio of the components of jet sheath velocity transverse and parallel to the
jet. Thus, multi-frequency RM maps promise a novel way in which to probe the
velocity structure of relativistic outflows.Comment: 5 pages, 4 figures, submitted to ApJ
An analysis of overall network architecture reveals an infinite-period bifurcation underlying oscillation arrest in the segmentation clock
Unveiling the mechanisms through which the somitogenesis regulatory network
exerts spatiotemporal control of the somitic patterning has required a
combination of experimental and mathematical modeling strategies. Significant
progress has been made for the zebrafish clockwork. However, due to its
complexity, the clockwork of the amniote segmentation regulatory network has
not been fully elucidated. Here, we address the question of how oscillations
are arrested in the amniote segmentation clock. We do this by constructing a
minimal model of the regulatory network, which privileges architectural
information over molecular details. With a suitable choice of parameters, our
model is able to reproduce the oscillatory behavior of the Wnt, Notch and FGF
signaling pathways in presomitic mesoderm (PSM) cells. By introducing
positional information via a single Wnt3a gradient, we show that oscillations
are arrested following an infinite-period bifurcation. Notably: the
oscillations increase their amplitude as cells approach the anterior PSM and
remain in an upregulated state when arrested; the transition from the
oscillatory regime to the upregulated state exhibits hysteresis; and an
opposing distribution of the Fgf8 and RA gradients in the PSM arises naturally
in our simulations. We hypothesize that the interaction between a limit cycle
(originated by the Notch delayed-negative feedback loop) and a bistable switch
(originated by the Wnt-Notch positive cross-regulation) is responsible for the
observed segmentation patterning. Our results agree with previously unexplained
experimental observations and suggest a simple plausible mechanism for
spatiotemporal control of somitogenesis in amniotes.Comment: 11 pages, 5 figures, added references, added figures, extended
supporting material, revised arguments in the discussion, corrected typo
Clustering in the Phase Space of Dark Matter Haloes. II. Stable Clustering and Dark Matter Annihilation
We present a model for the structure of the particle phase space average
density () in galactic haloes, introduced recently as a novel measure
of the clustering of dark matter. Our model is based on the stable clustering
hypothesis in phase space, the spherical collapse model, and tidal disruption
of substructures, which is calibrated against the Aquarius simulations. Using
this model, we can predict the behaviour of in the numerically
unresolved regime, down to the decoupling mass limit of generic WIMP models.
This prediction can be used to estimate signals sensitive to the small scale
structure of dark matter. For example, the dark matter annihilation rate can be
estimated for arbitrary velocity-dependent cross sections in a convenient way
using a limit of to zero separation in physical space. We illustrate
our method by computing the global and local subhalo annihilation boost to that
of the smooth dark matter distribution in a Milky-Way-size halo. Two cases are
considered, one where the cross section is velocity independent and one that
approximates Sommerfeld-enhanced models. We find that the global boost is
, which is at the low end of current estimates (weakening
expectations of large extragalactic signals), while the boost at the solar
radius is below the percent level. We make our code to compute
publicly available, which can be used to estimate various observables that
probe the nanostructure of dark matter haloes.Comment: 12 pages, 7 figures, version published in MNRAS (minor corrections),
publicly available code in IDL at http://spaces.perimeterinstitute.ca/p2sad
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