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Effect of stent position on flow characteristics in a cerebral aneurysm
The position of an intracranial stent in relation to the ostium of a cerebral aneurysm can
significantly affect the blood flow characteristics through the ostium and inside the aneurysm.
An idealised cerebral artery and aneurysm were simulated with a pulsatile flow. Simulation
results show that the effect on mass inflow between two stent positions is about 20%
whereas the difference in the porosity effect of the pattern at these two positions is around
3%. The remainder may be attributed to differences in flow velocity profile across the stent
into the aneurysm. The implications for clinical practice are an important consideration as the
surgeon may place the stent in any position between the two investigated and hence this will
lead to markedly different stent performance. Therefore, computational tools that take into
account the variability of stent placement will be valuable for assisting surgical planning
Renormalisation of heavy-light light ray operators
We calculate the renormalisation of different light ray operators with one
light degree of freedom and a static heavy quark. Both - and
-kernels are considered. A comparison with the light-light case suggests
that the mixing with three-particle operators is solely governed by the light
degrees of freedom. Additionally we show that conformal symmetry is already
broken at the level of the one loop counterterms due to the additional
UV-renormalisation of a cusp in the two contributing Wilson-lines. This general
feature can be used to fix the -renormalisation kernels up to a
constant. Some examples for applications of our results are given.Comment: 23 pages, 5 figures; v2: changed some wording, added a few references
and one appendix concerning some subtleties related to gauge fixing and ghost
terms; v3: clarified calculation in section 3.2., added an explicit
calculation in section 5.2, corrected a few typos and one figure, added a few
comments, results unchanged, except for typesetting matches version to appear
in JHE
Gauge links for transverse momentum dependent correlators at tree-level
In this paper we discuss the incorporation of gauge links in hadronic matrix
elements that describe the soft hadronic physics in high energy scattering
processes. In this description the matrix elements appear in soft correlators
and they contain non-local combinations of quark and gluon fields. In our
description we go beyond the collinear approach in which case also the
dependence on transverse momenta of partons is taken into consideration. The
non-locality in the transverse direction leads to a complex gauge link
structure for the full process, in which color is entangled, even at
tree-level. We show that at tree-level in a 1-parton unintegrated (1PU)
situation, in which only the transverse momentum of one of the initial state
hadrons is relevant, one can get a factorized expression involving transverse
momentum dependent (TMD) distribution functions. We point out problems at the
level of two initial state hadrons, even for relatively simple processes such
as Drell-Yan scattering.Comment: 25 pages, corrected typos and updated reference
Electroweak Gauge-Boson Production at Small q_T: Infrared Safety from the Collinear Anomaly
Using methods from effective field theory, we develop a novel, systematic
framework for the calculation of the cross sections for electroweak gauge-boson
production at small and very small transverse momentum q_T, in which large
logarithms of the scale ratio M_V/q_T are resummed to all orders. These cross
sections receive logarithmically enhanced corrections from two sources: the
running of the hard matching coefficient and the collinear factorization
anomaly. The anomaly leads to the dynamical generation of a non-perturbative
scale q_* ~ M_V e^{-const/\alpha_s(M_V)}, which protects the processes from
receiving large long-distance hadronic contributions. Expanding the cross
sections in either \alpha_s or q_T generates strongly divergent series, which
must be resummed. As a by-product, we obtain an explicit non-perturbative
expression for the intercept of the cross sections at q_T=0, including the
normalization and first-order \alpha_s(q_*) correction. We perform a detailed
numerical comparison of our predictions with the available data on the
transverse-momentum distribution in Z-boson production at the Tevatron and LHC.Comment: 34 pages, 9 figure
NNLO corrections to top-pair production at hadron colliders: the all-fermionic scattering channels
This is a second paper in our ongoing calculation of the
next-to-next-to-leading order (NNLO) QCD correction to the total inclusive
top-pair production cross-section at hadron colliders. In this paper we
calculate the reaction which was not considered
in our previous work on due to its phenomenologically
negligible size. We also calculate all remaining fermion-pair-initiated
partonic channels and that contribute to top-pair
production starting from NNLO. The contributions of these reactions to the
total cross-section for top-pair production at the Tevatron and LHC are small,
at the permil level. The most interesting feature of these reactions is their
characteristic logarithmic rise in the high energy limit. We compute the
constant term in the leading power behavior in this limit, and achieve
precision that is an order of magnitude better than the precision of a recent
theoretical prediction for this constant. All four partonic reactions computed
in this paper are included in our numerical program Top++. The calculation of
the NNLO corrections to the two remaining partonic reactions,
and , is ongoing.Comment: 1+16 pages; 3 figure
The Ï-meson light-cone distribution amplitudes from lattice QCD
We present the results of a lattice study of the normalization constants and second moments of the light-cone distribution amplitudes of longitudinally and transversely polarized mesons. The calculation is performed using two flavors of dynamical clover fermions at lattice spacings between and , different lattice volumes up to and pion masses down to . Bare lattice results are renormalized non-perturbatively using a variant of the RI'-MOM scheme and converted to the scheme. The necessary conversion coefficients, which are not available in the literature, are calculated. The chiral extrapolation for the relevant decay constants is worked out in detail. We obtain for the ratio of the tensor and vector coupling constants and the values of the second Gegenbauer moments and at the scale for the longitudinally and transversely polarized mesons, respectively. The errors include the statistical uncertainty and estimates of the systematics arising from renormalization. Discretization errors cannot be estimated reliably and are not included. In this calculation the possibility of decay at the smaller pion masses is not taken into account
Probing the low transverse momentum domain of Z production with novel variables
The measurement of the low transverse momentum region of vector boson
production in Drell-Yan processes has long been invaluable to testing our
knowledge of QCD dynamics both beyond fixed-order in perturbation theory as
well as in the non-perturbative region. Recently the D\O\ collaboration have
introduced novel variables which lead to improved measurements compared to the
case of the standard QT variable. To complement this improvement on the
experimental side, we develop here a complete phenomenological study dedicated
in particular to the new \phi* variable. We compare our study, which contains
the state-of-the-art next-to-next-to-leading resummation of large logarithms
and a smooth matching to the full next-to-leading order result, to the
experimental data and find excellent agreement over essentially the entire
range of \phi*, even without direct inclusion of non-perturbative effects. We
comment on our findings and on the potential for future studies to constrain
non-perturbative behaviour.Comment: 20 pages, 7 figures. Version accepted for publication in JHEP. A
figure with comparison to RESBOS has been adde
Bessel-Weighted Asymmetries in Semi Inclusive Deep Inelastic Scattering
The concept of weighted asymmetries is revisited for semi-inclusive deep
inelastic scattering. We consider the cross section in Fourier space, conjugate
to the outgoing hadron's transverse momentum, where convolutions of transverse
momentum dependent parton distribution functions and fragmentation functions
become simple products. Individual asymmetric terms in the cross section can be
projected out by means of a generalized set of weights involving Bessel
functions. Advantages of employing these Bessel weights are that they suppress
(divergent) contributions from high transverse momentum and that soft factors
cancel in (Bessel-) weighted asymmetries. Also, the resulting compact
expressions immediately connect to previous work on evolution equations for
transverse momentum dependent parton distribution and fragmentation functions
and to quantities accessible in lattice QCD. Bessel weighted asymmetries are
thus model independent observables that augment the description and our
understanding of correlations of spin and momentum in nucleon structure.Comment: Matches published version, JHEP style, 36 pages and 2 figures, minor
correction
Reducing Bit-Vector Polynomials to SAT using Gröbner Bases
We address the satisfiability of systems of polynomial equations over bit-vectors. Instead of conventional bit-blasting, we exploit word-level inference to translate these systems into non-linear pseudo-boolean constraints. We derive the pseudo-booleans by simulating bit assignments through the addition of (linear) polynomials and applying a strong form of propagation by computing Gröbner bases. By handling bit assignments symbolically, the number of Gröbner basis calculations, along with the number of assignments, is reduced. The final Gröbner basis yields expressions for the bit-vectors in terms of the symbolic bits, together with non-linear pseudo-boolean constraints on the symbolic variables, modulo a power of two. The pseudo-booleans can be solved by translation into classical linear pseudo-boolean constraints (without a modulo) or by encoding them as propositional formulae, for which a novel translation process is described
NLO QCD corrections to tW' and tZ' production in forward-backward asymmetry models
We consider Z' and W' models recently proposed to explain the top
forward-backward asymmetry at the Tevatron. We present the next-to-leading
order QCD corrections to associated production of such vector bosons together
with top quarks at the Large Hadron Collider, for centre-of-mass energies of 7
and 8 TeV. The corrections are significant, modifying the total production
cross-section by 30-50%. We consider the effects of the corrections on the top
and vector-boson kinematics. The results are directly applicable to current
experimental searches, for both the ATLAS and CMS collaborations.Comment: 62 pages, 13 figures, 36 tables. v3 Updated to correspond to Journal
version and incorporate supplementary materia
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