185 research outputs found
On the Bound States in a Non-linear Quantum Field Theory of a Spinor Field with Higher Derivatives
We consider a model of quantum field theory with higher derivatives for a
spinor field with quartic selfinteraction. With the help of the Bethe-Salpeter
equation we study the problem of the two particle bound states in the "chain"
approximation. The existence of a scalar bound state is established.Comment: 14 pages, no figures, LaTe
Causative factors for hyperopia regression in excimer keratorefractive surgery
Регресът и хипокорекцията са специфични усложнения в кераторефрактивната хирургия, които трудно се диференцират, пряко зависят от роговичната биомеханика и обуславят икономически значима тенденция към повторна корекция. Научен интерес буди въпросът кои са и от какво естество са факторите, предразполагащи към неочаквани или нетрайни рефрактивни резултати. Не по-малко важен въпрос е дали регресът е хирургична усложнение или само проблем на селекцията на пациенти, на избора на неподходяща процедура или на лазерна платформа. Настоящото изследване предлага напълно нов подход в измерването на регpеса, както и статистически обосновани резултати по отношение на възможните причини за него.Regression and hypocorrection are specific complications in refractive surgery that frequently lead to unexpected or perishable refractive results. They develop due to unforeseen alteration in corneal biomechanics and are hard to differentiate and presume. Their ethiology and pathogenesis are still vague and no statistical evidence on predisposing factors is present. Once more familiar, the regression might be controlled by patient selection, procedure planning or laser platform opting. This study offers a completely new approach to the measurement of regression and statistically sound results in terms of the possible reasons and biomarkers
Mössbauer and magnetic study of Co x Fe3−x O4 nanoparticles
Magnetic nanoparticles of cobalt ferrites Co x Fe3−x O4 (x = 1 or 2) have been obtained either by mechanical milling or thermal treatment of pre-prepared layered double hydroxide carbonate x-LDH–CO3. Mechanical milling of the 1-LDH–CO3 leads to the large-scale preparation of nearly spherical nanoparticles of CoFe2O4, the size of which (5 to 20 nm) is controlled by the treatment time. Core-shell structure with surface spin-canting has been considered for the nanoparticles formed to explain the observed hysteresis loop shift (from ZFC–FC) in the magnetic properties. Annealing treatment of the 2-LDH–CO3 below 673 K results in the formation of nearly spherical pure Co2FeO4 nanoparticles. At 673 K and above, the LDH decomposition leads to the formation of a mixture of both spinels phases Co2FeO4 and CoFe2O4, the amount of the latter increases with annealing temperature. Unusually high magnetic hardness characterized by a 22 kOe coercive field at 1.8 K has been observed, which reflects the high intrinsic anisotropy for Co2FeO4
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
On the renormalization of multiparton webs
We consider the recently developed diagrammatic approach to soft-gluon
exponentiation in multiparton scattering amplitudes, where the exponent is
written as a sum of webs - closed sets of diagrams whose colour and kinematic
parts are entangled via mixing matrices. A complementary approach to
exponentiation is based on the multiplicative renormalizability of intersecting
Wilson lines, and their subsequent finite anomalous dimension. Relating this
framework to that of webs, we derive renormalization constraints expressing all
multiple poles of any given web in terms of lower-order webs. We examine these
constraints explicitly up to four loops, and find that they are realised
through the action of the web mixing matrices in conjunction with the fact that
multiple pole terms in each diagram reduce to sums of products of lower-loop
integrals. Relevant singularities of multi-eikonal amplitudes up to three loops
are calculated in dimensional regularization using an exponential infrared
regulator. Finally, we formulate a new conjecture for web mixing matrices,
involving a weighted sum over column entries. Our results form an important
step in understanding non-Abelian exponentiation in multiparton amplitudes, and
pave the way for higher-loop computations of the soft anomalous dimension.Comment: 60 pages, 15 figure
Calculation of the Two-Loop Heavy-Flavor Contribution to Bhabha Scattering
We describe in detail the calculation of the two-loop corrections to the QED
Bhabha scattering cross section due to the vacuum polarization by heavy
fermions. Our approach eliminates one mass scale from the most challenging part
of the calculation and allows us to obtain the corrections in a closed
analytical form. The result is valid for arbitrary values of the heavy fermion
mass and the Mandelstam invariants, as long as s,t,u >> m_e^2.Comment: 43 pages, 8 figures; added reference
Loop amplitudes in gauge theories: modern analytic approaches
This article reviews on-shell methods for analytic computation of loop
amplitudes, emphasizing techniques based on unitarity cuts. Unitarity
techniques are formulated generally but have been especially useful for
calculating one-loop amplitudes in massless theories such as Yang-Mills theory,
QCD, and QED.Comment: 34 pages. Invited review for a special issue of Journal of Physics A
devoted to "Scattering Amplitudes in Gauge Theories." v2: typesetting macro
error fixe
General properties of multiparton webs: proofs from combinatorics
Recently, the diagrammatic description of soft-gluon exponentiation in
scattering amplitudes has been generalized to the multiparton case. It was
shown that the exponent of Wilson-line correlators is a sum of webs, where each
web is formed through mixing between the kinematic factors and colour factors
of a closed set of diagrams which are mutually related by permuting the gluon
attachments to the Wilson lines. In this paper we use replica trick methods, as
well as results from enumerative combinatorics, to prove that web mixing
matrices are always: (a) idempotent, thus acting as projection operators; and
(b) have zero sum rows: the elements in each row in these matrices sum up to
zero, thus removing components that are symmetric under permutation of gluon
attachments. Furthermore, in webs containing both planar and non-planar
diagrams we show that the zero sum property holds separately for these two
sets. The properties we establish here are completely general and form an
important step in elucidating the structure of exponentiation in non-Abelian
gauge theories.Comment: 38 pages, 10 figure
Factorization Properties of Soft Graviton Amplitudes
We apply recently developed path integral resummation methods to perturbative
quantum gravity. In particular, we provide supporting evidence that eikonal
graviton amplitudes factorize into hard and soft parts, and confirm a recent
hypothesis that soft gravitons are modelled by vacuum expectation values of
products of certain Wilson line operators, which differ for massless and
massive particles. We also investigate terms which break this factorization,
and find that they are subleading with respect to the eikonal amplitude. The
results may help in understanding the connections between gravity and gauge
theories in more detail, as well as in studying gravitational radiation beyond
the eikonal approximation.Comment: 35 pages, 5 figure
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