1,264 research outputs found
N-String Vertices in String Field Theory
We give the general form of the vertex corresponding to the interaction of an
arbitrary number of strings. The technique employed relies on the ``comma"
representation of String Field Theory where string fields and interactions are
represented as matrices and operations between them such as multiplication and
trace. The general formulation presented here shows that the interaction vertex
of N strings, for any arbitrary N, is given as a function of particular
combinations of matrices corresponding to the change of representation between
the full string and the half string degrees of freedom.Comment: 22 pages, A4-Latex (latex twice), FTUV IFI
Half-String Approach to Closed String Field Theory
In this letter we present an operator formalism for Closed String Field
Theory based on closed half-strings. Our results indicate that the restricted
polyhedra of the classical non-polynomial string field theory, can be
represented as traces of infinite matrices, with operator insertions that
reparametrise the half-strings.Comment: 13 pages, Latex, OUTP 93-10-
Witten's cubic vertex in the comma theory (I)
It is shown that Witten's interaction 3-vertex is a solution to the comma overlap equations; hence establishing the equivalence between the conventional and the 'comma' formulation of interacting string theory at the level of vertices
Charm-quark mass from weighted finite energy QCD sum rules
The running charm-quark mass in the scheme is determined from
weighted finite energy QCD sum rules (FESR) involving the vector current
correlator. Only the short distance expansion of this correlator is used,
together with integration kernels (weights) involving positive powers of ,
the squared energy. The optimal kernels are found to be a simple {\it pinched}
kernel, and polynomials of the Legendre type. The former kernel reduces
potential duality violations near the real axis in the complex s-plane, and the
latter allows to extend the analysis to energy regions beyond the end point of
the data. These kernels, together with the high energy expansion of the
correlator, weigh the experimental and theoretical information differently from
e.g. inverse moments FESR. Current, state of the art results for the vector
correlator up to four-loop order in perturbative QCD are used in the FESR,
together with the latest experimental data. The integration in the complex
s-plane is performed using three different methods, fixed order perturbation
theory (FOPT), contour improved perturbation theory (CIPT), and a fixed
renormalization scale (FMUPT). The final result is , in a wide region of stability against changes in the
integration radius in the complex s-plane.Comment: A short discussion on convergence issues has been added at the end of
the pape
Quark masses in QCD: a progress report
Recent progress on QCD sum rule determinations of the light and heavy quark
masses is reported. In the light quark sector a major breakthrough has been
made recently in connection with the historical systematic uncertainties due to
a lack of experimental information on the pseudoscalar resonance spectral
functions. It is now possible to suppress this contribution to the 1% level by
using suitable integration kernels in Finite Energy QCD sum rules. This allows
to determine the up-, down-, and strange-quark masses with an unprecedented
precision of some 8-10%. Further reduction of this uncertainty will be possible
with improved accuracy in the strong coupling, now the main source of error. In
the heavy quark sector, the availability of experimental data in the vector
channel, and the use of suitable multipurpose integration kernels allows to
increase the accuracy of the charm- and bottom-quarks masses to the 1% level.Comment: Invited review paper to be published in Modern Physics Letters
Neural Collaborative Filtering
In recent years, deep neural networks have yielded immense success on speech
recognition, computer vision and natural language processing. However, the
exploration of deep neural networks on recommender systems has received
relatively less scrutiny. In this work, we strive to develop techniques based
on neural networks to tackle the key problem in recommendation -- collaborative
filtering -- on the basis of implicit feedback. Although some recent work has
employed deep learning for recommendation, they primarily used it to model
auxiliary information, such as textual descriptions of items and acoustic
features of musics. When it comes to model the key factor in collaborative
filtering -- the interaction between user and item features, they still
resorted to matrix factorization and applied an inner product on the latent
features of users and items. By replacing the inner product with a neural
architecture that can learn an arbitrary function from data, we present a
general framework named NCF, short for Neural network-based Collaborative
Filtering. NCF is generic and can express and generalize matrix factorization
under its framework. To supercharge NCF modelling with non-linearities, we
propose to leverage a multi-layer perceptron to learn the user-item interaction
function. Extensive experiments on two real-world datasets show significant
improvements of our proposed NCF framework over the state-of-the-art methods.
Empirical evidence shows that using deeper layers of neural networks offers
better recommendation performance.Comment: 10 pages, 7 figure
Theoretical Study of the Phosphoryl Transfer Reaction from ATP to Dha Catalyzed by DhaK from Escherichia coli
Protein kinases, representing one of the largest protein families involved in almost all aspects of cell life, have become one of the most important targets for the development of new drugs to be used in, for instance, cancer treatments. In this article an exhaustive theoretical study of the phosphoryl transfer reaction from adenosine triphosphate (ATP) to dihydroxyacetone (Dha) catalyzed by DhaK from Escherichia coli (E. coli) is reported. Two different mechanisms, previously proposed for the phosphoryl transfer from ATP to the hydroxyl side chain of specific serine, threonine, or tyrosine residues, have been explored based on the generation of free energy surfaces (FES) computed with hybrid QM/MM potentials. The results suggest that the substrate-assisted phosphoryl and proton-transfer mechanism is kinetically more favorable than the mechanism where an aspartate would be activating the Dha. Although the details of the mechanisms appear to be dramatically dependent on the level of theory employed in the calculations (PM3/MM, B3LYP:PM3/MM, or B3LYP/MM), the transition states (TSs) for the phosphoryl transfer step appear to be described as a concerted step with different degrees of synchronicity in the breaking and forming bonds process in both explored mechanisms. Residues of the active site belonging to different subunits of the protein, such as Gly78B, Thr79A, Ser80A, Arg178B, and one Mg2+ cation, would be stabilizing the transferred phosphate in the TS. Asp109A would have a structural role by posing the Dha and other residues of the active site in the proper orientation. The information derived from our calculations not only reveals the role of the enzyme and the particular residues of its active site, but it can assist in the rational design of new more specific inhibitors
Chiral condensates from tau decay: a critical reappraisal
The saturation of QCD chiral sum rules is reanalyzed in view of the new and
complete analysis of the ALEPH experimental data on the difference between
vector and axial-vector correlators (V-A). Ordinary finite energy sum rules
(FESR) exhibit poor saturation up to energies below the tau-lepton mass. A
remarkable improvement is achieved by introducing pinched, as well as
minimizing polynomial integral kernels. Both methods are used to determine the
dimension d=6 and d=8 vacuum condensates in the Operator Product Expansion,
with the results: {O}_{6}=-(0.00226 \pm 0.00055) GeV^6, and O_8=-(0.0053 \pm
0.0033) GeV^8 from pinched FESR, and compatible values from the minimizing
polynomial FESR. Some higher dimensional condensates are also determined,
although we argue against extending the analysis beyond dimension d = 8. The
value of the finite remainder of the (V-A) correlator at zero momentum is also
redetermined: \Pi (0)= -4 \bar{L}_{10}=0.02579 \pm 0.00023. The stability and
precision of the predictions are significantly improved compared to earlier
calculations using the old ALEPH data. Finally, the role and limits of
applicability of the Operator Product Expansion in this channel are clarified.Comment: Replaced versio
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