471 research outputs found
Determining the Nonperturbative Collins-Soper Kernel From Lattice QCD
At small transverse momentum , transverse-momentum dependent parton
distribution functions (TMDPDFs) arise as genuinely nonperturbative objects
that describe Drell-Yan like processes in hadron collisions as well as
semi-inclusive deep-inelastic scattering. TMDPDFs naturally depend on the
hadron momentum, and the associated evolution is determined by the
Collins-Soper equation. For the corresponding
evolution kernel (or anomalous dimension) is nonperturbative and must be
determined as an independent ingredient in order to relate TMDPDFs at different
scales. We propose a method to extract this kernel using lattice QCD and the
Large-Momentum Effective Theory, where the physical TMD correlation involving
light-like paths is approximated by a quasi TMDPDF, defined using equal-time
correlation functions with a large-momentum hadron state. The kernel is
determined from a ratio of quasi TMDPDFs extracted at different hadron momenta.Comment: 9 pages, 2 figures; v2: extended the review of TMDPDF commonalities,
version submitted to PRD; v3: minor changes, journal versio
Resummation Improved Rapidity Spectrum for Gluon Fusion Higgs Production
Gluon-induced processes such as Higgs production typically exhibit large
perturbative corrections. These partially arise from large virtual corrections
to the gluon form factor, which at timelike momentum transfer contains Sudakov
logarithms evaluated at negative arguments . It has been
observed that resumming these terms in the timelike form factor leads to a much
improved perturbative convergence for the total cross section. We discuss how
to consistently incorporate the resummed form factor into the perturbative
predictions for generic cross sections differential in the Born kinematics,
including in particular the Higgs rapidity spectrum. We verify that this indeed
improves the perturbative convergence, leading to smaller and more reliable
perturbative uncertainties, and that this is not affected by cancellations
between resummed and unresummed contributions. Combining both fixed-order and
resummation uncertainties, the perturbative uncertainty for the total cross
section at NLONLL is about a factor of two smaller
than at NLO. The perturbative uncertainty of the rapidity spectrum at
NNLONNLL is similarly reduced compared to NNLO. We also
study the analogous resummation for quark-induced processes, namely Higgs
production through bottom quark annihilation and the Drell-Yan rapidity
spectrum. For the former the resummation leads to a small improvement, while
for the latter it confirms the already small uncertainties of the fixed-order
predictions.Comment: 30 pages + 17 pages in Appendices, 10 figures; v2: journal version;
references added, discussed individual partonic channels for Drell-Ya
Evolution of sperm morphology in a crustacean genus with fertilization inside an open brood pouch
Sperm is the most fundamental male reproductive feature. It serves the fertilization of eggs and evolves under sexual selection. Two components of sperm are of particular interest, their number and their morphology. Mode of fertilization is believed to be a key determinant of sperm length across the animal kingdom. External fertilization, unlike internal, favors small and numerous sperm, since sperm density is thinned out in the environment. Here, we study the evolution of sperm morphology in the genus Daphnia, where fertilization occurs in a receptacle, the brood pouch, where sperm can constantly be flushed out by a water current. Based on microscopic observations of sperm morphologies mapped on a phylogeny with 15 Daphnia and 2 outgroup species, we found that despite the internal fertilization mode, Daphnia have among the smallest sperm recorded, as would be expected with external fertilization. Despite being all relatively small compared to other arthropods, sperm length diverged at least twice, once within each of the Daphnia subgenera Ctenodaphnia and Daphnia. Furthermore, species in the latter subgenus also lost the ability of cell compaction by extracellular encapsulation and have very polymorphic sperm with long, and often numerous, filopodia. We discuss the different strategies that Daphnia evolved to achieve fertilization success in the females’ brood pouch
TMD Fragmentation Functions at NLO
We compute the unpolarized quark and gluon transverse-momentum dependent
fragmentation functions (TMDFFs) at next-to-next-to-next-to-leading order
(NLO) in perturbative QCD. The calculation is based on a relation between
the TMDFF and the limit of the semi-inclusive deep inelastic scattering cross
section where all final-state radiation becomes collinear to the detected
hadron. The required cross section is obtained by analytically continuing our
recent computation of the Drell-Yan and Higgs boson production cross section at
NLO expanded around the limit of all final-state radiation becoming
collinear to one of the initial states. Our results agree with a recent
independent calculation by Luo et al.Comment: 20 pages + appendices, 3 figures, 6 ancillary files; v2: journal
versio
Transverse momentum dependent PDFs at NLO
We compute the quark and gluon transverse momentum dependent parton
distribution functions at next-to-next-to-next-to-leading order (NLO) in
perturbative QCD. Our calculation is based on an expansion of the differential
Higgs boson and Drell-Yan production cross sections about their collinear
limit. This method allows us to employ cutting edge techniques for the
computation of cross sections to extract the universal building blocks in
question. The corresponding perturbative matching kernels for all channels are
expressed in terms of simple harmonic polylogarithms up to weight five. As a
byproduct, we confirm a previous computation of the soft function for
transverse momentum factorization at NLO. Our results are the last missing
ingredient to extend the subtraction methods to NLO and to obtain
resummed spectra at NLL accuracy both for gluon as well as
for quark initiated processes.Comment: 12 pages + appendices, 4 awesome figures, important ancillary files.
v2: journal versio
Collinear expansion for color singlet cross sections
We demonstrate how to efficiently expand cross sections for color-singlet
production at hadron colliders around the kinematic limit of all final state
radiation being collinear to one of the incoming hadrons. This expansion is
systematically improvable and applicable to a large class of physical
observables. We demonstrate the viability of this technique by obtaining the
first two terms in the collinear expansion of the rapidity distribution of the
gluon fusion Higgs boson production cross section at next-to-next-to leading
order (NNLO) in QCD perturbation theory. Furthermore, we illustrate how this
technique is used to extract universal building blocks of scattering cross
section like the N-jettiness and transverse momentum beam function at NNLO.Comment: 39 pages, 1 awesome figure; v2: journal versio
Exploiting jet binning to identify the initial state of high-mass resonances
If a new high-mass resonance is discovered at the Large Hadron Collider,
model-independent techniques to identify the production mechanism will be
crucial to understand its nature and effective couplings to Standard Model
particles. We present a powerful and model-independent method to infer the
initial state in the production of any high-mass color-singlet system by using
a tight veto on accompanying hadronic jets to divide the data into two mutually
exclusive event samples (jet bins). For a resonance of several hundred GeV, the
jet binning cut needed to discriminate quark and gluon initial states is in the
experimentally accessible range of several tens of GeV. It also yields
comparable cross sections for both bins, making this method viable already with
the small event samples available shortly after a discovery. Theoretically, the
method is made feasible by utilizing an effective field theory setup to compute
the jet cut dependence precisely and model independently and to systematically
control all sources of theoretical uncertainties in the jet binning, as well as
their correlations. We use a 750 GeV scalar resonance as an example to
demonstrate the viability of our method.Comment: 6 pages, 2 figures, v2: journal versio
Towards Quasi-Transverse Momentum Dependent PDFs Computable on the Lattice
Transverse momentum dependent parton distributions (TMDPDFs) which appear in
factorized cross sections involve infinite Wilson lines with edges on or close
to the light-cone. Since these TMDPDFs are not directly calculable with a
Euclidean path integral in lattice QCD, we study the construction of
quasi-TMDPDFs with finite-length spacelike Wilson lines that are amenable to
such calculations. We define an infrared consistency test to determine which
quasi-TMDPDF definitions are related to the TMDPDF, by carrying out a one-loop
study of infrared logarithms of transverse position , which must agree between them. This agreement is a necessary
condition for the two quantities to be related by perturbative matching.
TMDPDFs necessarily involve combining a hadron matrix element, which nominally
depends on a single light-cone direction, with soft matrix elements that
necessarily depend on two light-cone directions. We show at one loop that the
simplest definitions of the quasi hadron matrix element, the quasi soft matrix
element, and the resulting quasi-TMDPDF all fail the infrared consistency test.
Ratios of impact parameter quasi-TMDPDFs still provide nontrivial information
about the TMDPDFs, and are more robust since the soft matrix elements cancel.
We show at one loop that such quasi ratios can be matched to ratios of the
corresponding TMDPDFs. We also introduce a modified "bent" quasi soft matrix
element which yields a quasi-TMDPDF that passes the consistency test with the
TMDPDF at one loop, and discuss potential issues at higher orders.Comment: 39 pages + appendices, 13 figures; v2: journal versio
Electronic structure of fully epitaxial Co2TiSn thin films
In this article we report on the properties of thin films of the full Heusler
compound Co2TiSn prepared by DC magnetron co-sputtering. Fully epitaxial,
stoichiometric films were obtained by deposition on MgO (001) substrates at
substrate temperatures above 600{\deg}C. The films are well ordered in the L21
structure, and the Curie temperature exceeds slightly the bulk value. They show
a significant, isotropic magnetoresistance and the resistivity becomes strongly
anomalous in the paramagnetic state. The films are weakly ferrimagnetic, with
nearly 1 \mu_B on the Co atoms, and a small antiparallel Ti moment, in
agreement with theoretical expectations. From comparison of x-ray absorption
spectra on the Co L3/L2 edges, including circular and linear magnetic
dichroism, with ab initio calculations of the x-ray absorption and circular
dichroism spectra we infer that the electronic structure of Co2TiSn has
essentially non-localized character. Spectral features that have not been
explained in detail before, are explained here in terms of the final state band
structure.Comment: 11 pages, 8 figure
Jettiness formulation of the M i NNLO PS method
We present a new formulation of the MiNNLOPS method to match NNLO QCD calculations with parton showers by using jettiness as a resummation variable. The full derivation for colour-singlet processes is presented using 0-jettiness starting from the NNLL′ resummation formula. We show phenomenological results for Drell-Yan and Higgs-boson production at the LHC and compare our predictions to ATLAS and CMS data. Differences to the original MiNNLOPS formulation using the transverse momentum of the colour singlet as resummation variable are discussed. We further present a comparison of MiNNLOPS predictions with Geneva. Finally, we extend the formulation of the MiNNLOPS method to 1-jettiness which is applicable to processes with a colour singlet plus one jet in the final state
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