10,045 research outputs found
On the renormalization of quasi parton distribution
Recent developments showed that light-cone parton distributions can be
studied by investigating the large momentum limit of the hadronic matrix
elements of spacelike correlators, which are known as quasi parton
distributions. Like a light-cone parton distribution, a quasi parton
distribution also contains ultraviolet divergences and therefore needs
renormalization. The renormalization of non-local operators in general is not
well understood. However, in the case of quasi quark distribution, the bilinear
quark operator with a straight-line gauge link appears to be multiplicatively
renormalizable by the quark wave function renormalization in the axial gauge.
We first show that the renormalization of the self energy correction to the
quasi quark distribution is equivalent to that of the heavy-light quark vector
current in heavy quark effective theory at one-loop order. Assuming this
equivalence at two-loop order, we then show that the multiplicative
renormalizability of the quasi quark distribution is true at two-loop order.Comment: 14 pages, 4 figure
QCD Factorization for Spin-Dependent Cross Sections in DIS and Drell-Yan Processes at Low Transverse Momentum
Based on a recent work on the quantum chromodynamic (QCD) factorization for
semi-inclusive deep-inelastic scattering (DIS), we present a set of
factorization formulas for the spin-dependent DIS and Drell-Yan cross sections
at low transverse momentum.Comment: 12 pages, two figures include
Software Tools and Approaches for Compound Identification of LC-MS/MS Data in Metabolomics.
The annotation of small molecules remains a major challenge in untargeted mass spectrometry-based metabolomics. We here critically discuss structured elucidation approaches and software that are designed to help during the annotation of unknown compounds. Only by elucidating unknown metabolites first is it possible to biologically interpret complex systems, to map compounds to pathways and to create reliable predictive metabolic models for translational and clinical research. These strategies include the construction and quality of tandem mass spectral databases such as the coalition of MassBank repositories and investigations of MS/MS matching confidence. We present in silico fragmentation tools such as MS-FINDER, CFM-ID, MetFrag, ChemDistiller and CSI:FingerID that can annotate compounds from existing structure databases and that have been used in the CASMI (critical assessment of small molecule identification) contests. Furthermore, the use of retention time models from liquid chromatography and the utility of collision cross-section modelling from ion mobility experiments are covered. Workflows and published examples of successfully annotated unknown compounds are included
From Dust To Planetesimal: The Snowball Phase ?
The standard model of planet formation considers an initial phase in which
planetesimals form from a dust disk, followed by a phase of mutual
planetesimal-planetesimal collisions, leading eventually to the formation of
planetary embryos. However, there is a potential transition phase (which we
call the "snowball phase"), between the formation of the first planetesimals
and the onset of mutual collisions amongst them, which has often been either
ignored or underestimated in previous studies. In this snowball phase, isolated
planetesimals move on Keplerian orbits and grow solely via the direct accretion
of sub-cm sized dust entrained with the gas in the protoplanetary disk. Using a
simplified model in which planetesimals are progressively produced from the
dust, we consider the expected sizes to which the planetesimals can grow before
mutual collisions commence and derive the dependence of this size on a number
of critical parameters, including the degree of disk turbulence, the
planetesimal size at birth and the rate of planetesimal creation. For systems
in which turbulence is weak and the planetesimals are created at a low rate and
with relatively small birth size, we show that the snowball growth phase can be
very important, allowing planetesimals to grow by a factor of 10^6 in mass
before mutual collisions take over. In such cases, the snowball growth phase
can be the dominant mode to transfer mass from the dust to planetesimals.
Moreover, such growth can take place within the typical lifetime of a
protoplanetary gas disk. A noteworthy result is that ... ...(see the paper).
For the specific case of close binaries such as Alpha Centauri ... ... (see the
paper). From a more general perspective, these preliminary results suggest that
an efficient snowball growth phase provides a large amount of "room at the
bottom" for theories of planet formation.Comment: Accepted for publication in the Astrophysical Journal. 15 pages, 4
figures, 1 tabl
On Resource Pooling and Separation for LRU Caching
Caching systems using the Least Recently Used (LRU) principle have now become
ubiquitous. A fundamental question for these systems is whether the cache space
should be pooled together or divided to serve multiple flows of data item
requests in order to minimize the miss probabilities. In this paper, we show
that there is no straight yes or no answer to this question, depending on
complex combinations of critical factors, including, e.g., request rates,
overlapped data items across different request flows, data item popularities
and their sizes. Specifically, we characterize the asymptotic miss
probabilities for multiple competing request flows under resource pooling and
separation for LRU caching when the cache size is large.
Analytically, we show that it is asymptotically optimal to jointly serve
multiple flows if their data item sizes and popularity distributions are
similar and their arrival rates do not differ significantly; the
self-organizing property of LRU caching automatically optimizes the resource
allocation among them asymptotically. Otherwise, separating these flows could
be better, e.g., when data sizes vary significantly. We also quantify critical
points beyond which resource pooling is better than separation for each of the
flows when the overlapped data items exceed certain levels. Technically, we
generalize existing results on the asymptotic miss probability of LRU caching
for a broad class of heavy-tailed distributions and extend them to multiple
competing flows with varying data item sizes, which also validates the Che
approximation under certain conditions. These results provide new insights on
improving the performance of caching systems
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