653 research outputs found
The economical 3-3-1 model revisited
We show that the economical 3-3-1 model poses a very high new physics scale
of the order of 1000~TeV due to the constraint on the flavor-changing neutral
current. The implications of the model for neutrino masses, inflation,
leptogenesis, and superheavy dark matter are newly recognized. Alternatively,
we modify the model by rearranging the third quark generation differently from
the first two quark generations, as well as changing the scalar sector. The
resultant model now predicts a consistent new physics at TeV scale unlike the
previous case and may be fully probed at the current colliders. Particularly,
due to the minimal particle contents, the models under consideration manifestly
accommodate dark matter candidates and neutrino masses, with novel and distinct
production mechanisms. The large flavor-changing neutral currents that come
from the ordinary and exotic quark mixings can be avoided due to the
approximate symmetry.Comment: 21 pages; english writing improved, dark matter stability stated, and
references added; matches journal versio
Two Bronze Medals for Switzerland at the 46th International Chemistry Olympiad in Hanoi, Vietnam
Dispersion Relations in String Theory
We analyze the analytic continuation of the formally divergent one-loop
amplitude for scattering of the graviton multiplet in the Type II Superstring.
In particular we obtain explicit double and single dispersion relations,
formulas for all the successive branch cuts extending out to plus infinity, as
well as for the decay rate of a massive string state of arbitrary mass 2N into
two string states of lower mass. We compare our results with the box diagram in
a superposition of -like field theories. The stringy effects are traced
to a convergence problem in this superposition.Comment: 17 pages, COLUMBIA-YITP-UCLA/93/TEP/45 (figures fixed up
Structures of the Neisseria meningitides methionineābinding protein MetQ in substrate-free form and bound to L- and D-methionine isomers
The bacterial periplasmic methionineābinding protein MetQ is involved in the import of methionine by the cognate MetNI methionine ABC transporter. The MetNIQ system is one of the few members of the ABC importer family that has been structurally characterized in multiple conformational states. Critical missing elements in the structural analysis of MetNIQ are the structure of the substrateāfree form of MetQ, and detailing how MetQ binds multiple methionine derivatives, including both Lā and Dāmethionine isomers. In this study, we report the structures of the Neisseria meningitides MetQ in substrateāfree form and in complexes with Lāmethionine and with Dāmethionine, along with the associated binding constants determined by isothermal titration calorimetry. Structures of the substrateāfree (N238A) and substrateābound N. meningitides MetQ are related by a āVenusāfly trapā hingeātype movement of the two domains accompanying methionine binding and dissociation. Lāmethionine and Dāmethionine bind to the same site on MetQ, and this study emphasizes the important role of asparagine 238 in ligand binding and affinity. A thermodynamic analysis demonstrates that ligandāfree MetQ associates with the ATP bound form of MetNI ~40 times more tightly than does liganded MetQ, consistent with the necessity of dissociating methionine from MetQ for transport to occur
Structures of the Neisseria meningitides methionineābinding protein MetQ in substrate-free form and bound to L- and D-methionine isomers
The bacterial periplasmic methionineābinding protein MetQ is involved in the import of methionine by the cognate MetNI methionine ABC transporter. The MetNIQ system is one of the few members of the ABC importer family that has been structurally characterized in multiple conformational states. Critical missing elements in the structural analysis of MetNIQ are the structure of the substrateāfree form of MetQ, and detailing how MetQ binds multiple methionine derivatives, including both Lā and Dāmethionine isomers. In this study, we report the structures of the Neisseria meningitides MetQ in substrateāfree form and in complexes with Lāmethionine and with Dāmethionine, along with the associated binding constants determined by isothermal titration calorimetry. Structures of the substrateāfree (N238A) and substrateābound N. meningitides MetQ are related by a āVenusāfly trapā hingeātype movement of the two domains accompanying methionine binding and dissociation. Lāmethionine and Dāmethionine bind to the same site on MetQ, and this study emphasizes the important role of asparagine 238 in ligand binding and affinity. A thermodynamic analysis demonstrates that ligandāfree MetQ associates with the ATP bound form of MetNI ~40 times more tightly than does liganded MetQ, consistent with the necessity of dissociating methionine from MetQ for transport to occur
Artificial intelligence approaches for materials-by-design of energetic materials: state-of-the-art, challenges, and future directions
Artificial intelligence (AI) is rapidly emerging as an enabling tool for
solving various complex materials design problems. This paper aims to review
recent advances in AI-driven materials-by-design and their applications to
energetic materials (EM). Trained with data from numerical simulations and/or
physical experiments, AI models can assimilate trends and patterns within the
design parameter space, identify optimal material designs (micro-morphologies,
combinations of materials in composites, etc.), and point to designs with
superior/targeted property and performance metrics. We review approaches
focusing on such capabilities with respect to the three main stages of
materials-by-design, namely representation learning of microstructure
morphology (i.e., shape descriptors), structure-property-performance (S-P-P)
linkage estimation, and optimization/design exploration. We provide a
perspective view of these methods in terms of their potential, practicality,
and efficacy towards the realization of materials-by-design. Specifically,
methods in the literature are evaluated in terms of their capacity to learn
from a small/limited number of data, computational complexity,
generalizability/scalability to other material species and operating
conditions, interpretability of the model predictions, and the burden of
supervision/data annotation. Finally, we suggest a few promising future
research directions for EM materials-by-design, such as meta-learning, active
learning, Bayesian learning, and semi-/weakly-supervised learning, to bridge
the gap between machine learning research and EM research
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