20,596 research outputs found
Fermionic decays of scalar leptoquarks and scalar gluons in the minimal four color symmetry model
Fermionic decays of the scalar leptoquarks
and of the scalar gluons predicted by the four color symmetry
model with the Higgs mechanism of the quark-lepton mass splitting are
investigated. Widths and branching ratios of these decays are calculated and
analysed in dependence on coupling constants and on masses of the decaying
particles. It is shown that the decays are
dominant with the widths of order of a few GeV for TeV and with
the total branching ratios close to 1. In the case of the dominant
scalar leptoquark decays are S_1^{(+)}\to cl_j^+, S_1^{(-)}\to \nu_i\tilde b,
S_m\to b\l_j^+, S_m\to c\tilde \nu_j with the total branching ratios
,
and A
search for such decays at the LHC and Tevatron may be of interest.Comment: 11 pages, 1 figure, 1 table, to be published in Modern Physics
Letters
Summing up Subleading Sudakov Logarithms
We apply the strategy of regions within dimensional regularization to find
functions involved in evolution equations which govern the asymptotic dynamics
of the Abelian form factor and four-fermion amplitude in the SU(N) gauge theory
in the Sudakov limit up to the next-to-leading logarithmic approximation. The
results are used for the analysis of the dominant electroweak corrections to
the fermion-antifermion pair production in the annihilation at high
energy.Comment: 17 pages LaTeX, missprins corrected, references adde
Iteration of Planar Amplitudes in Maximally Supersymmetric Yang-Mills Theory at Three Loops and Beyond
We compute the leading-color (planar) three-loop four-point amplitude of N=4
supersymmetric Yang-Mills theory in 4 - 2 epsilon dimensions, as a Laurent
expansion about epsilon = 0 including the finite terms. The amplitude was
constructed previously via the unitarity method, in terms of two Feynman loop
integrals, one of which has been evaluated already. Here we use the
Mellin-Barnes integration technique to evaluate the Laurent expansion of the
second integral. Strikingly, the amplitude is expressible, through the finite
terms, in terms of the corresponding one- and two-loop amplitudes, which
provides strong evidence for a previous conjecture that higher-loop planar N =
4 amplitudes have an iterative structure. The infrared singularities of the
amplitude agree with the predictions of Sterman and Tejeda-Yeomans based on
resummation. Based on the four-point result and the exponentiation of infrared
singularities, we give an exponentiated ansatz for the maximally
helicity-violating n-point amplitudes to all loop orders. The 1/epsilon^2 pole
in the four-point amplitude determines the soft, or cusp, anomalous dimension
at three loops in N = 4 supersymmetric Yang-Mills theory. The result confirms a
prediction by Kotikov, Lipatov, Onishchenko and Velizhanin, which utilizes the
leading-twist anomalous dimensions in QCD computed by Moch, Vermaseren and
Vogt. Following similar logic, we are able to predict a term in the three-loop
quark and gluon form factors in QCD.Comment: 54 pages, 7 figures. v2: Added references, a few additional words
about large spin limit of anomalous dimensions. v3: Expanded Sect. IV.A on
multiloop ansatz; remark that form-factor prediction is now confirmed by
other work; minor typos correcte
Nonperturbative calculation of the anomalous magnetic moment in the Yukawa model
Within the covariant formulation of light-front dynamics, we calculate the
state vector of a fermion coupled to identical scalar bosons (the Yukawa
model). The state vector is decomposed in Fock sectors and we consider the
first three ones: a single fermion, a fermion coupled to one boson, and a
fermion coupled to two bosons. This last three-body sector generates nontrivial
and nonperturbative contributions to the state vector, and these contributions
are calculated with no approximations. The divergences of the amplitudes are
regularized using Pauli-Villars fermion and boson fields. Physical observables
can be unambiguously deduced using a systematic renormalization scheme we
developed. This renormalization scheme is a necessary condition in order to
avoid uncancelled divergences when Fock space is truncated. As an example, we
present preliminary numerical results for the anomalous magnetic moment of a
fermion in the Yukawa model.Comment: 7 pages, 7 figures. Contribution to the proceedings of the Workshop:
Light-Cone 2008, "Relativistic Nuclear and Particle Physics", Mulhouse,
France, July 7-11, 2008. To be published in the online journal "Proceedings
of Science" - Po
Free field representation for the O(3) nonlinear sigma model and bootstrap fusion
The possibility of the application of the free field representation developed
by Lukyanov for massive integrable models is investigated in the context of the
O(3) sigma model. We use the bootstrap fusion procedure to construct a free
field representation for the O(3) Zamolodchikov- Faddeev algebra and to write
down a representation for the solutions of the form-factor equations which is
similar to the ones obtained previously for the sine-Gordon and SU(2) Thirring
models. We discuss also the possibility of developing further this
representation for the O(3) model and comment on the extension to other
integrable field theories.Comment: 14 pages, latex, revtex v3.0 macro package, no figures Accepted for
publication in Phys. Rev.
Four-dimensional integration by parts with differential renormalization as a method of evaluation of Feynman diagrams
It is shown how strictly four-dimensional integration by parts combined with
differential renormalization and its infrared analogue can be applied for
calculation of Feynman diagrams.Comment: 6 pages, late
Predicting scattering properties of ultracold atoms: adiabatic accumulated phase method and mass scaling
Ultracold atoms are increasingly used for high precision experiments that can
be utilized to extract accurate scattering properties. This calls for a
stronger need to improve on the accuracy of interatomic potentials, and in
particular the usually rather inaccurate inner-range potentials. A boundary
condition for this inner range can be conveniently given via the accumulated
phase method. However, in this approach one should satisfy two conditions,
which are in principle conflicting, and the validity of these approximations
comes under stress when higher precision is required. We show that a better
compromise between the two is possible by allowing for an adiabatic change of
the hyperfine mixing of singlet and triplet states for interatomic distances
smaller than the separation radius. A mass scaling approach to relate
accumulated phase parameters in a combined analysis of isotopically related
atom pairs is described in detail and its accuracy is estimated, taking into
account both Born-Oppenheimer and WKB breakdown. We demonstrate how numbers of
singlet and triplet bound states follow from the mass scaling.Comment: 14 pages, 9 figure
Nonperturbative renormalization in light-front dynamics and applications
We present a general framework to calculate the properties of relativistic
compound systems from the knowledge of an elementary Hamiltonian. Our framework
provides a well-controlled nonperturbative calculational scheme which can be
systematically improved. The state vector of a physical system is calculated in
light-front dynamics. From the general properties of this form of dynamics, the
state vector can be further decomposed in well-defined Fock components. In
order to control the convergence of this expansion, we advocate the use of the
covariant formulation of light-front dynamics. In this formulation, the state
vector is projected on an arbitrary light-front plane \omega \cd x=0 defined
by a light-like four-vector . This enables us to control any violation
of rotational invariance due to the truncation of the Fock expansion. We then
present a general nonperturbative renormalization scheme in order to avoid
field-theoretical divergences which may remain uncancelled due to this
truncation. This general framework has been applied to a large variety of
models. As a starting point, we consider QED for the two-body Fock space
truncation and calculate the anomalous magnetic moment of the electron. We show
that it coincides, in this approximation, with the well-known Schwinger term.
Then we investigate the properties of a purely scalar system in the three-body
approximation, where we highlight the role of antiparticle degrees of freedom.
As a non-trivial example of our framework, we calculate the structure of a
physical fermion in the Yukawa model, for the three-body Fock space truncation
(but still without antifermion contributions). We finally show why our approach
is also well-suited to describe effective field theories like chiral
perturbation theory in the baryonic sector.Comment: 17 pages, 19 figures "Relativistic Description of Two- and Three-Body
Systems in Nuclear Physics", ECT*, October 19-23 200
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