10,463 research outputs found
Multi-PeV Signals from a New Astrophysical Neutrino Flux Beyond the Glashow Resonance
The IceCube neutrino discovery was punctuated by three showers with ~
1-2 PeV. Interest is intense in possible fluxes at higher energies, though a
marked deficit of ~ 6 PeV Glashow resonance events implies a spectrum
that is soft and/or cutoff below ~few PeV. However, IceCube recently reported a
through-going track event depositing 2.6 0.3 PeV. A muon depositing so
much energy can imply 10 PeV. We show that extending the
soft spectral fit from TeV-PeV data is unlikely to yield such an
event. Alternatively, a tau can deposit this much energy, though requiring
~10x higher. We find that either scenario hints at a new flux,
with the hierarchy of and energies suggesting a window
into astrophysical neutrinos at ~ 100 PeV if a tau. We address
implications, including for ultrahigh-energy cosmic-ray and neutrino origins.Comment: 6 pages, 4 figures + 3 pages Supplementary Material; updated to agree
with version published in Physical Review Letter
Is the principle of least action a must?
The least action principle occupies a central part in contemporary physics.
Yet, as far as classical field theory is concerned, it may not be as essential
as generally thought. We show with three detailed examples of classical
interacting field theories that it is possible, in cases of physical interest,
to derive the correct field equations for all fields from the action (which we
regard as defining the theory), some of its symmetries, and the conservation
law of energy-momentum (this last regarded as ultimately coming from
experiment)Comment: RevTeX, no figures, Eq. 3 corrected, a reference added, added sundry
clarifying comments, no change in the results, 23 page
Merging -means with hierarchical clustering for identifying general-shaped groups
Clustering partitions a dataset such that observations placed together in a
group are similar but different from those in other groups. Hierarchical and
-means clustering are two approaches but have different strengths and
weaknesses. For instance, hierarchical clustering identifies groups in a
tree-like structure but suffers from computational complexity in large datasets
while -means clustering is efficient but designed to identify homogeneous
spherically-shaped clusters. We present a hybrid non-parametric clustering
approach that amalgamates the two methods to identify general-shaped clusters
and that can be applied to larger datasets. Specifically, we first partition
the dataset into spherical groups using -means. We next merge these groups
using hierarchical methods with a data-driven distance measure as a stopping
criterion. Our proposal has the potential to reveal groups with general shapes
and structure in a dataset. We demonstrate good performance on several
simulated and real datasets.Comment: 16 pages, 1 table, 9 figures; accepted for publication in Sta
Extra-matrix Mg\u3csup\u3e2+\u3c/sup\u3e Limits Ca\u3csup\u3e2+\u3c/sup\u3e Uptake and Modulates Ca\u3csup\u3e2+\u3c/sup\u3e Uptake-independent Respiration and Redox State in Cardiac Isolated Mitochondria
Cardiac mitochondrial matrix (m) free Ca2+ ([Ca2+]m) increases primarily by Ca2+ uptake through the Ca2+ uniporter (CU). Ca2+ uptake via the CU is attenuated by extra-matrix (e) Mg2+ ([Mg2+]e). How [Ca2+]m is dynamically modulated by interacting physiological levels of [Ca2+]e and [Mg2+]e and how this interaction alters bioenergetics are not well understood. We postulated that as [Mg2+]e modulates Ca2+ uptake via the CU, it also alters bioenergetics in a matrix Ca2+–induced and matrix Ca2+–independent manner. To test this, we measured changes in [Ca2+]e, [Ca2+]m, [Mg2+]e and [Mg2+]m spectrofluorometrically in guinea pig cardiac mitochondria in response to added CaCl2 (0–0.6 mM; 1 mM EGTA buffer) with/without added MgCl2 (0–2 mM). In parallel, we assessed effects of added CaCl2 and MgCl2 on NADH, membrane potential (ΔΨm), and respiration. We found that \u3e0.125 mM MgCl2 significantly attenuated CU-mediated Ca2+ uptake and [Ca2+]m. Incremental [Mg2+]e did not reduce initial Ca2+uptake but attenuated the subsequent slower Ca2+ uptake, so that [Ca2+]m remained unaltered over time. Adding CaCl2 without MgCl2 to attain a [Ca2+]m from 46 to 221 nM enhanced state 3 NADH oxidation and increased respiration by 15 %; up to 868 nM [Ca2+]m did not additionally enhance NADH oxidation or respiration. Adding MgCl2 did not increase [Mg2+]m but it altered bioenergetics by its direct effect to decrease Ca2+ uptake. However, at a given [Ca2+]m, state 3 respiration was incrementally attenuated, and state 4 respiration enhanced, by higher [Mg2+]e. Thus, [Mg2+]e without a change in [Mg2+]m can modulate bioenergetics independently of CU-mediated Ca2+ transport
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