445 research outputs found
New differential equations for on-shell loop integrals
We present a novel type of differential equations for on-shell loop
integrals. The equations are second-order and importantly, they reduce the loop
level by one, so that they can be solved iteratively in the loop order. We
present several infinite series of integrals satisfying such iterative
differential equations. The differential operators we use are best written
using momentum twistor space. The use of the latter was advocated in recent
papers discussing loop integrals in N=4 super Yang-Mills. One of our
motivations is to provide a tool for deriving analytical results for scattering
amplitudes in this theory. We show that the integrals needed for planar MHV
amplitudes up to two loops can be thought of as deriving from a single master
topology. The master integral satisfies our differential equations, and so do
most of the reduced integrals. A consequence of the differential equations is
that the integrals we discuss are not arbitrarily complicated transcendental
functions. For two specific two-loop integrals we give the full analytic
solution. The simplicity of the integrals appearing in the scattering
amplitudes in planar N=4 super Yang-Mills is strongly suggestive of a relation
to the conjectured underlying integrability of the theory. We expect these
differential equations to be relevant for all planar MHV and non-MHV
amplitudes. We also discuss possible extensions of our method to more general
classes of integrals.Comment: 39 pages, 8 figures; v2: typos corrected, definition of harmonic
polylogarithms adde
Water Consumption by Livestock Systems from 2002–2020 and Predictions for 2030–2050 under Climate Changes in the Czech Republic
The livestock system in Europe relies on a complex holistic equilibrium that is the outcome of an interplay of demand, market, crop production, livestock production, land use, water availability, and other factors. When modeling future scenarios of water consumption by livestock systems, the most suitable tools result from the interconnectivity of growth models, economic models, and climate models. We integrated the Environmental Policy Integrated Climate growth model (EPIC), animal-level model (RUMINANT), economic model (Global Biosphere Management Model, GLOBIOM), EURO-CORDEX climate models, and regression models. This study developed novel livestock production scenarios for individual regions of the Czech Republic with estimations of the categories of livestock that have been bred during the last 20 years and will be bred in the future and what their water consumption will be, both throughout the year and in particular seasons. First, the numbers of farm animals, namely, cattle, pigs, sheep, horses, goats, and poultry in 2002–2020 were evaluated, and their numbers were predicted for the following years until 2050. Second, livestock water consumption per region was determined based on the number of livestock individuals. Third, changes in the amount of water consumed by livestock per year in individual regions in 2050 compared to 2005 were estimated
On different lagrangian formalisms for vector resonances within chiral perturbation theory
We study the relation of vector Proca field formalism and antisymmetric
tensor field formalism for spin-one resonances in the context of the large N_C
inspired chiral resonance Lagrangian systematically up to the order O(p6) and
give a transparent prescription for the transition from vector to antisymmetric
tensor Lagrangian and vice versa. We also discuss the possibility to describe
the spin-one resonances using an alternative "mixed" first order formalism,
which includes both types of fields simultaneously, and compare this one with
the former two. We also briefly comment on the compatibility of the above
lagrangian formalisms with the high-energy constraints for concrete VVP
correlator.Comment: 34 pages, 3 figure
Generic multiloop methods and application to N=4 super-Yang-Mills
We review some recent additions to the tool-chest of techniques for finding
compact integrand representations of multiloop gauge-theory amplitudes -
including non-planar contributions - applicable for N=4 super-Yang-Mills in
four and higher dimensions, as well as for theories with less supersymmetry. We
discuss a general organization of amplitudes in terms of purely cubic graphs,
review the method of maximal cuts, as well as some special D-dimensional
recursive cuts, and conclude by describing the efficient organization of
amplitudes resulting from the conjectured duality between color and kinematic
structures on constituent graphs.Comment: 42 pages, 18 figures, invited review for a special issue of Journal
of Physics A devoted to "Scattering Amplitudes in Gauge Theories", v2 minor
corrections, v3 added reference
The All-Loop Integrand For Scattering Amplitudes in Planar N=4 SYM
We give an explicit recursive formula for the all L-loop integrand for
scattering amplitudes in N=4 SYM in the planar limit, manifesting the full
Yangian symmetry of the theory. This generalizes the BCFW recursion relation
for tree amplitudes to all loop orders, and extends the Grassmannian duality
for leading singularities to the full amplitude. It also provides a new
physical picture for the meaning of loops, associated with canonical operations
for removing particles in a Yangian-invariant way. Loop amplitudes arise from
the "entangled" removal of pairs of particles, and are naturally presented as
an integral over lines in momentum-twistor space. As expected from manifest
Yangian-invariance, the integrand is given as a sum over non-local terms,
rather than the familiar decomposition in terms of local scalar integrals with
rational coefficients. Knowing the integrands explicitly, it is straightforward
to express them in local forms if desired; this turns out to be done most
naturally using a novel basis of chiral, tensor integrals written in
momentum-twistor space, each of which has unit leading singularities. As simple
illustrative examples, we present a number of new multi-loop results written in
local form, including the 6- and 7-point 2-loop NMHV amplitudes. Very concise
expressions are presented for all 2-loop MHV amplitudes, as well as the 5-point
3-loop MHV amplitude. The structure of the loop integrand strongly suggests
that the integrals yielding the physical amplitudes are "simple", and
determined by IR-anomalies. We briefly comment on extending these ideas to more
general planar theories.Comment: 46 pages; v2: minor changes, references adde
Basics of Generalized Unitarity
We review generalized unitarity as a means for obtaining loop amplitudes from
on-shell tree amplitudes. The method is generally applicable to both
supersymmetric and non-supersymmetric amplitudes, including non-planar
contributions. Here we focus mainly on N=4 Yang-Mills theory, in the context of
on-shell superspaces. Given the need for regularization at loop level, we also
review a six-dimensional helicity-based superspace formalism and its
application to dimensional and massive regularizations. An important feature of
the unitarity method is that it offers a means for carrying over any identified
tree-level property of on-shell amplitudes to loop level, though sometimes in a
modified form. We illustrate this with examples of dual conformal symmetry and
a recently discovered duality between color and kinematics.Comment: 37 pages, 10 figures. Invited review for a special issue of Journal
of Physics A devoted to "Scattering Amplitudes in Gauge Theories", R.
Roiban(ed), M. Spradlin(ed), A. Volovich(ed
On the background in the reaction and mixed event simulation
In this paper we evaluate sources of background for the , with the detected through its decay channel, to
compare with the experiment carried out at ELSA. We find background from
followed by decay of a into two ,
recombining one and one , and from the reaction with subsequent decay of the into two photons. This
background accounts for the data at invariant masses beyond 700
MeV, but strength is missing at lower invariant masses which was attributed to
photon misidentification events, which we simulate to get a good reproduction
of the experimental background. Once this is done, we perform an event mixing
simulation to reproduce the calculated background and we find that the method
provides a good description of the background at low invariant
masses but fakes the background at high invariant masses, making background
events at low invariant masses, which are due to misidentification
events, responsible for the background at high invariant masses which is due to
the and reactions.Comment: 10 pages, 5 figure
QCD in the nuclear medium and effects due to Cherenkov gluons
The equations of in-medium gluodynamics are proposed. Their classical lowest
order solution is explicitly shown for a color charge moving with constant
speed. For nuclear permittivity larger than 1 it describes emission of
Cherenkov gluons resembling results of classical electrodynamics. The values of
the real and imaginary parts of the nuclear permittivity are obtained from the
fits to experimental data on the double-humped structure around the away-side
jet obtained at RHIC. The dispersion of the nuclear permittivity is predicted
by comparing the RHIC, SPS and cosmic ray data. This is important for LHC
experiments. Cherenkov gluons may be responsible for the asymmetry of dilepton
mass spectra near rho-meson, observed in the SPS experiment with excess in the
low-mass wing of the resonance. This feature is predicted to be common for all
resonances. The "color rainbow" quantum effect might appear according to higher
order terms of in-medium QCD if the nuclear permittivity depends on color.Comment: 29 p., 4 figs; for "Phys. Atom. Nucl." volume dedicated to 80th
birthday of L.B. Okun; minor corrections on pp. 11 and 13 in v
Uncovering Dangerous Cheats: How Do Avian Hosts Recognize Adult Brood Parasites?
BACKGROUND: Co-evolutionary struggles between dangerous enemies (e.g., brood parasites) and their victims (hosts) lead to the emergence of sophisticated adaptations and counter-adaptations. Salient host tricks to reduce parasitism costs include, as front line defence, adult enemy discrimination. In contrast to the well studied egg stage, investigations addressing the specific cues for adult enemy recognition are rare. Previous studies have suggested barred underparts and yellow eyes may provide cues for the recognition of cuckoos Cuculus canorus by their hosts; however, no study to date has examined the role of the two cues simultaneously under a consistent experimental paradigm. METHODOLOGY/PRINCIPAL FINDINGS: We modify and extend previous work using a novel experimental approach--custom-made dummies with various combinations of hypothesized recognition cues. The salient recognition cue turned out to be the yellow eye. Barred underparts, the only trait examined previously, had a statistically significant but small effect on host aggression highlighting the importance of effect size vs. statistical significance. CONCLUSION: Relative importance of eye vs. underpart phenotypes may reflect ecological context of host-parasite interaction: yellow eyes are conspicuous from the typical direction of host arrival (from above), whereas barred underparts are poorly visible (being visually blocked by the upper part of the cuckoo's body). This visual constraint may reduce usefulness of barred underparts as a reliable recognition cue under a typical situation near host nests. We propose a novel hypothesis that recognition cues for enemy detection can vary in a context-dependent manner (e.g., depending on whether the enemy is approached from below or from above). Further we suggest a particular cue can trigger fear reactions (escape) in some hosts/populations whereas the same cue can trigger aggression (attack) in other hosts/populations depending on presence/absence of dangerous enemies that are phenotypically similar to brood parasites and costs and benefits associated with particular host responses
Photoproduction of {\omega} Mesons off the Proton
The differential cross sections and unpolarized spin-density matrix elements
for the reaction were measured using the CBELSA/TAPS
experiment for initial photon energies ranging from the reaction threshold to
2.5 GeV. These observables were measured from the radiative decay of the
meson, . The cross sections cover the full
angular range and show the full extent of the -channel forward rise. The
overall shape of the angular distributions in the differential cross sections
and unpolarized spin-density matrix elements are in fair agreement with
previous data. In addition, for the first time, a beam of linearly-polarized
tagged photons in the energy range from 1150 MeV to 1650 MeV was used to
extract polarized spin-density matrix elements.
These data were included in the Bonn-Gatchina partial wave analysis (PWA).
The dominant contribution to photoproduction near threshold was found
to be the partial wave, which is primarily due to the sub-threshold
resonance. At higher energies, pomeron-exchange was found to
dominate whereas -exchange remained small. These -channel contributions
as well as further contributions from nucleon resonances were necessary to
describe the entire dataset: the , , and partial waves
were also found to contribute significantly.Comment: 8 pages, 5 figures, accepted for publication in Physics Letters
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