503 research outputs found
Heavy Quark Fluorescence
Heavy hadrons containing heavy quarks (for example, Upsilon-mesons) feature a
scale separation between the heavy quark mass (about 4.5 GeV for the b-quark)
and the QCD scale (about 0.3 GeV}) that controls effective masses of lighter
constituents. Therefore, as in ordinary molecules, the de-excitation of the
lighter, faster degrees of freedom leaves the velocity distribution of the
heavy quarks unchanged, populating the available decay channels in
qualitatively predictable ways. Automatically an application of the
Franck-Condon principle of molecular physics explains several puzzling results
of Upsilon(5S) decays as measured by the Belle collaboration, such as the high
rate of Bs*-anti Bs* versus Bs*-anti Bs production, the strength of three-body
B-anti B + pion decays, or the dip in B momentum shown in these decays. We
argue that the data is showing the first Sturm-Liouville zero of the
Upsilon(5S) quantum mechanical squared wavefunction, and providing evidence for
a largely b-anti b composition of this meson.Comment: 4 pages, 4 figures, Figure 2 updated and some typos corrected. To be
published in Physical Review Letter
Improved Semileptonic Form Factor Calculations in Lattice QCD
We investigate the computational efficiency of two stochastic based
alternatives to the Sequential Propagator Method used in Lattice QCD
calculations of heavy-light semileptonic form factors. In the first method, we
replace the sequential propagator, which couples the calculation of two of the
three propagators required for the calculation, with a stochastic propagator so
that the calculations of all three propagators are independent. This method is
more flexible than the Sequential Propagator Method but introduces stochastic
noise. We study the noise to determine when this method becomes competitive
with the Sequential Propagator Method, and find that for any practical
calculation it is competitive with or superior to the Sequential Propagator
Method. We also examine a second stochastic method, the so-called ``one-end
trick", concluding it is relatively inefficient in this context. The
investigation is carried out on two gauge field ensembles, using the
non-perturbatively improved Wilson-Sheikholeslami-Wohlert action with N_f=2
mass-degenerate sea quarks. The two ensembles have similar lattice spacings but
different sea quark masses. We use the first stochastic method to extract
-improved, matched lattice results for the semileptonic form
factors on the ensemble with lighter sea quarks, extracting f_+(0)
Charmonium spectroscopy and mixing with light quark and open charm states from nF=2 lattice QCD
We study the charmonium spectrum including higher spin and gluonic
excitations. We determine an upper limit on the mixing of the eta_c ground
state with light pseudoscalar flavour-singlet mesons and investigate the mixing
of charmonia near open charm thresholds with pairs of (excited) D and anti-D
mesons. For charm and light valence quarks and nF=2 sea quarks, we employ the
non-perturbatively improved Sheikholeslami-Wohlert (clover) action. Excited
states are accessed using the variational technique, starting from a basis of
suitably optimised operators. For some aspects of this study, the use of
improved stochastic all-to-all propagators was essential.Comment: 23 pages, v2: references updated, correction of an ambiguous
statement, minor typos corrected, some figures update
Supervised and Unsupervised Learning of Audio Representations for Music Understanding
In this work, we provide a broad comparative analysis of strategies for
pre-training audio understanding models for several tasks in the music domain,
including labelling of genre, era, origin, mood, instrumentation, key, pitch,
vocal characteristics, tempo and sonority. Specifically, we explore how the
domain of pre-training datasets (music or generic audio) and the pre-training
methodology (supervised or unsupervised) affects the adequacy of the resulting
audio embeddings for downstream tasks.
We show that models trained via supervised learning on large-scale
expert-annotated music datasets achieve state-of-the-art performance in a wide
range of music labelling tasks, each with novel content and vocabularies. This
can be done in an efficient manner with models containing less than 100 million
parameters that require no fine-tuning or reparameterization for downstream
tasks, making this approach practical for industry-scale audio catalogs.
Within the class of unsupervised learning strategies, we show that the domain
of the training dataset can significantly impact the performance of
representations learned by the model. We find that restricting the domain of
the pre-training dataset to music allows for training with smaller batch sizes
while achieving state-of-the-art in unsupervised learning -- and in some cases,
supervised learning -- for music understanding.
We also corroborate that, while achieving state-of-the-art performance on
many tasks, supervised learning can cause models to specialize to the
supervised information provided, somewhat compromising a model's generality
Complex Langevin and other approaches to the sign problem in quantum many-body physics
We review the theory and applications of complex stochastic quantization to
the quantum many-body problem. Along the way, we present a brief overview of a
number of ideas that either ameliorate or in some cases altogether solve the
sign problem, including the classic reweighting method, alternative
Hubbard-Stratonovich transformations, dual variables (for bosons and fermions),
Majorana fermions, density-of-states methods, imaginary asymmetry approaches,
and Lefschetz thimbles. We discuss some aspects of the mathematical
underpinnings of conventional stochastic quantization, provide a few
pedagogical examples, and summarize open challenges and practical solutions for
the complex case. Finally, we review the recent applications of complex
Langevin to quantum field theory in relativistic and nonrelativistic quantum
matter, with an emphasis on the nonrelativistic case.Comment: 51 pages, 19 figures, review articl
Charm quark system at the physical point of 2+1 flavor lattice QCD
We investigate the charm quark system using the relativistic heavy quark
action on 2+1 flavor PACS-CS configurations previously generated on lattice. The dynamical up-down and strange quark masses are set to
the physical values by using the technique of reweighting to shift the quark
hopping parameters from the values employed in the configuration generation. At
the physical point, the lattice spacing equals GeV and the
spatial extent fm. The charm quark mass is determined by the
spin-averaged mass of the 1S charmonium state, from which we obtain m_{\rm
charm}^{\msbar}(\mu = m_{\rm charm}^{\msbar}) = 1.260(1)(6)(35) GeV, where the
errors are due to our statistics, scale determination and renormalization
factor. An additional systematic error from the heavy quark is of order
, which is estimated to be a percent
level if the factor analytic in is of order unity. Our
results for the charmed and charmed-strange meson decay constants are
MeV, MeV, again up to the heavy quark
errors of order . Combined with the CLEO
values for the leptonic decay widths, these values yield , , where the last error is on
account of the experimental uncertainty of the decay widths.Comment: 16 pages, 12 figure
Excitations of single-beauty hadrons
In this work we study the predominantly orbital and radial excitations of
hadrons containing a single heavy quark. We present meson and baryon mass
splittings and ratios of meson decay constants (e.g., and
) resulting from quenched and dynamical two-flavor
configurations. Light quarks are simulated using the chirally improved (CI)
lattice Dirac operator at valence masses as light as MeV.
The heavy quark is approximated by a static propagator, appropriate for the
quark on our lattices ( GeV). We also include some preliminary
calculations of the kinetic corrections to the states, showing,
in the process, a viable way of applying the variational method to three-point
functions involving excited states. We compare our results with recent
experimental findings.Comment: 23 pages, 18 figures, 17 tables; slight title change (Ed. killjoy);
reference added; version to appear in Phys Rev
Core reconstruction in pseudopotential calculations
A new method is presented for obtaining all-electron results from a
pseudopotential calculation. This is achieved by carrying out a localised
calculation in the region of an atomic nucleus using the embedding potential
method of Inglesfield [J.Phys. C {\bf 14}, 3795 (1981)]. In this method the
core region is \emph{reconstructed}, and none of the simplifying approximations
(such as spherical symmetry of the charge density/potential or frozen core
electrons) that previous solutions to this problem have required are made. The
embedding method requires an accurate real space Green function, and an
analysis of the errors introduced in constructing this from a set of numerical
eigenstates is given. Results are presented for an all-electron reconstruction
of bulk aluminium, for both the charge density and the density of states.Comment: 14 pages, 5 figure
- …