15,049 research outputs found
Molecular Force Fields with Gradient-Domain Machine Learning: Construction and Application to Dynamics of Small Molecules with Coupled Cluster Forces
We present the construction of molecular force fields for small molecules
(less than 25 atoms) using the recently developed symmetrized gradient-domain
machine learning (sGDML) approach [Chmiela et al., Nat. Commun. 9, 3887 (2018);
Sci. Adv. 3, e1603015 (2017)]. This approach is able to accurately reconstruct
complex high-dimensional potential-energy surfaces from just a few 100s of
molecular conformations extracted from ab initio molecular dynamics
trajectories. The data efficiency of the sGDML approach implies that atomic
forces for these conformations can be computed with high-level
wavefunction-based approaches, such as the "gold standard" CCSD(T) method. We
demonstrate that the flexible nature of the sGDML model recovers local and
non-local electronic interactions (e.g. H-bonding, proton transfer, lone pairs,
changes in hybridization states, steric repulsion and interactions)
without imposing any restriction on the nature of interatomic potentials. The
analysis of sGDML molecular dynamics trajectories yields new qualitative
insights into dynamics and spectroscopy of small molecules close to
spectroscopic accuracy
Deep-Inelastic Final States in a Space-Time Description of Shower Development and Hadronization
We extend a quantum kinetic approach to the description of hadronic showers
in space, time and momentum space to deep-inelastic collisions, with
particular reference to experiments at HERA. We follow the history of hard
scattering events back to the initial hadronic state and forward to the
formation of colour-singlet pre-hadronic clusters and their decays into
hadrons. The time evolution of the space-like initial-state shower and the
time-like secondary partons are treated similarly, and cluster formation is
treated using a spatial criterion motivated by confinement and a
non-perturbative model for hadronization. We calculate the time evolution of
particle distributions in rapidity, transverse and longitudinal space. We also
compare the transverse hadronic energy flow and the distribution of observed
hadronic masses with experimental data from HERA, and find encouraging results.
The techniques developed in this paper may be applied in the future to more
complicated processes such as eA, pp, pA and AA collisions.Comment: 44 pages plus 14 postscript figure
Identifying Heavy-Flavor Jets Using Vectors of Locally Aggregated Descriptors
Jets of collimated particles serve a multitude of purposes in high energy
collisions. Recently, studies of jet interaction with the quark-gluon plasma
(QGP) created in high energy heavy ion collisions are of growing interest,
particularly towards understanding partonic energy loss in the QGP medium and
its related modifications of the jet shower and fragmentation. Since the QGP is
a colored medium, the extent of jet quenching and consequently, the transport
properties of the medium are expected to be sensitive to fundamental properties
of the jets such as the flavor of the parton that initiates the jet.
Identifying the jet flavor enables an extraction of the mass dependence in
jet-QGP interactions. We present a novel approach to tagging heavy-flavor jets
at collider experiments utilizing the information contained within jet
constituents via the \texttt{JetVLAD} model architecture. We show the
performance of this model in proton-proton collisions at center of mass energy
GeV as characterized by common metrics and showcase its
ability to extract high purity heavy-flavor jet sample at various jet momenta
and realistic production cross-sections including a brief discussion on the
impact of out-of-time pile-up. Such studies open new opportunities for future
high purity heavy-flavor measurements at jet energies accessible at current and
future collider experiments.Comment: 18 pages, 6 figures and 3 tables. Accepted by JINS
Hadrons in the Nuclear Medium
Quantum Chromodynamics, the microscopic theory of strong interactions, has
not yet been applied to the calculation of nuclear wave functions. However, it
certainly provokes a number of specific questions and suggests the existence of
novel phenomena in nuclear physics which are not part of the the traditional
framework of the meson-nucleon description of nuclei. Many of these phenomena
are related to high nuclear densities and the role of color in nucleonic
interactions. Quantum fluctuations in the spatial separation between nucleons
may lead to local high density configurations of cold nuclear matter in nuclei,
up to four times larger than typical nuclear densities. We argue here that
experiments utilizing the higher energies available upon completion of the
Jefferson Laboratory energy upgrade will be able to probe the quark-gluon
structure of such high density configurations and therefore elucidate the
fundamental nature of nuclear matter. We review three key experimental
programs: quasi-elastic electro-disintegration of light nuclei, deep inelastic
scattering from nuclei at , and the measurement of tagged structure
functions. These interrelated programs are all aimed at the exploration of the
quark structure of high density nuclear configurations.
The study of the QCD dynamics of elementary hard processes is another
important research direction and nuclei provide a unique avenue to explore
these dynamics. We argue that the use of nuclear targets and large values of
momentum transfer at would allow us to determine whether the physics of the
nucleon form factors is dominated by spatially small configurations of three
quarks.Comment: 52 pages IOP style LaTex file and 20 eps figure
The Challenge of Machine Learning in Space Weather Nowcasting and Forecasting
The numerous recent breakthroughs in machine learning (ML) make imperative to
carefully ponder how the scientific community can benefit from a technology
that, although not necessarily new, is today living its golden age. This Grand
Challenge review paper is focused on the present and future role of machine
learning in space weather. The purpose is twofold. On one hand, we will discuss
previous works that use ML for space weather forecasting, focusing in
particular on the few areas that have seen most activity: the forecasting of
geomagnetic indices, of relativistic electrons at geosynchronous orbits, of
solar flares occurrence, of coronal mass ejection propagation time, and of
solar wind speed. On the other hand, this paper serves as a gentle introduction
to the field of machine learning tailored to the space weather community and as
a pointer to a number of open challenges that we believe the community should
undertake in the next decade. The recurring themes throughout the review are
the need to shift our forecasting paradigm to a probabilistic approach focused
on the reliable assessment of uncertainties, and the combination of
physics-based and machine learning approaches, known as gray-box.Comment: under revie
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