13,902 research outputs found
An Model for Lepton Mass Matrices with Nearly Minimal Texture
We propose a simple extension of the electroweak standard model based on the
discrete symmetry that is capable of realizing a nearly minimal
Fritzsch-type texture for the Dirac mass matrices of both charged leptons and
neutrinos. This is achieved with the aid of additional and
symmetries, one of which can be embedded in . Five complex scalar
singlet fields are introduced in addition to the SM with right-handed
neutrinos. Although more general, the modified texture of the model retains the
successful features of the minimal texture without fine-tuning; namely, it
accommodates the masses and mixing of the leptonic sector and relates the
emergence of large leptonic mixing angles with the seesaw mechanism. For large
deviations of the minimal texture, both quasidegenerate spectrum or inverted
hierarchy are allowed for neutrino masses.Comment: 11pp, 2 figures. v2: vev alignment addressed, additional analysis
performed; to appear in PR
Count-Based Exploration with the Successor Representation
In this paper we introduce a simple approach for exploration in reinforcement
learning (RL) that allows us to develop theoretically justified algorithms in
the tabular case but that is also extendable to settings where function
approximation is required. Our approach is based on the successor
representation (SR), which was originally introduced as a representation
defining state generalization by the similarity of successor states. Here we
show that the norm of the SR, while it is being learned, can be used as a
reward bonus to incentivize exploration. In order to better understand this
transient behavior of the norm of the SR we introduce the substochastic
successor representation (SSR) and we show that it implicitly counts the number
of times each state (or feature) has been observed. We use this result to
introduce an algorithm that performs as well as some theoretically
sample-efficient approaches. Finally, we extend these ideas to a deep RL
algorithm and show that it achieves state-of-the-art performance in Atari 2600
games when in a low sample-complexity regime.Comment: This paper appears in the Proceedings of the 34th AAAI Conference on
Artificial Intelligence (AAAI 2020
Static and dynamic properties of vortices in anisotropic magnetic disks
We investigate the effect of the magnetic anisotropy () on the static
and dynamic properties of magnetic vortices in small disks. Our micromagnetic
calculations reveal that for a range of there is an enlargement of the
vortex core. We analyze the influence of on the dynamics of the vortex
core magnetization reversal under the excitation of a pulsed field. The
presence of , which leads to better resolved vortex structures, allows us
to discuss in more details the role played by the in-plane and perpendicular
components of the gyrotropic field during the vortex-antivortex nucleation and
annihilation.Comment: 4 pages, 4 figure
The Quest for an Intermediate-Scale Accidental Axion and Further ALPs
The recent detection of the cosmic microwave background polarimeter
experiment BICEP2 of tensor fluctuations in the B-mode power spectrum basically
excludes all plausible axion models where its decay constant is above
GeV. Moreover, there are strong theoretical, astrophysical, and cosmological
motivations for models involving, in addition to the axion, also axion-like
particles (ALPs), with decay constants in the intermediate scale range, between
GeV and GeV. Here, we present a general analysis of models
with an axion and further ALPs and derive bounds on the relative size of the
axion and ALP photon (and electron) coupling. We discuss what we can learn from
measurements of the axion and ALP photon couplings about the fundamental
parameters of the underlying ultraviolet completion of the theory. For the
latter we consider extensions of the Standard Model in which the axion and the
ALP(s) appear as pseudo Nambu-Goldstone bosons from the breaking of global
chiral (Peccei-Quinn (PQ)) symmetries, occuring accidentally as low
energy remnants from exact discrete symmetries. In such models, the axion and
the further ALP are protected from disastrous explicit symmetry breaking
effects due to Planck-scale suppressed operators. The scenarios considered
exploit heavy right handed neutrinos getting their mass via PQ symmetry
breaking and thus explain the small mass of the active neutrinos via a seesaw
relation between the electroweak and an intermediate PQ symmetry breaking
scale. We show some models that can accommodate simultaneously an axion dark
matter candidate, an ALP explaining the anomalous transparency of the universe
for -rays, and an ALP explaining the recently reported 3.55 keV gamma
line from galaxies and clusters of galaxies, if the respective decay constants
are of intermediate scale.Comment: 43pp, 4 figures. v2: version accepted for publication in JHE
Discrepancies between empirical and theoretical models of the flaring solar chromosphere and their possible resolution
Models of the solar chromosphere during flaring deduced theoretically or empirically are compared. Marked discrepancies are noted and various reasons are offered to explain their existence. A means is presented for testing theoretical heating models (electron heating) by analyzing the net energy loss rates in (observed) empirical atmospheres and inverting the flare energy equation to deduce the parameters of the supposed heating mechanism
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