5,612 research outputs found
Sum rules for helicity amplitudes from BRS invariance
The BRS invariance of the electroweak gauge theory leads to relationships
between amplitudes with external massive gauge bosons and amplitudes where some
of these gauge bosons are replaced with their corresponding Nambu-Goldstone
bosons. Unlike the equivalence theorem, these identities are exact at all
energies. In this paper we discuss such identities which relate the process
to and production. By using
a general form-factor decomposition for , and amplitudes, these identities are
expressed as sum rules among scalar form factors. Because these sum rules may
be applied order by order in perturbation theory, they provide a powerful test
of higher order calculations. By using additional Ward-Takahashi identities we
find that the various contributions are divided into separately gauge-invariant
subsets, the sum rules applying independently to each subset. After a general
discussion of the application of the sum rules we consider the one-loop
contributions of scalar-fermions in the Minimal Supersymmetric Standard Model
as an illustration.Comment: 37 pages, including 16 figure
Monte Carlo integration on GPU
We use a graphics processing unit (GPU) for fast computations of Monte Carlo
integrations. Two widely used Monte Carlo integration programs, VEGAS and
BASES, are parallelized on GPU. By using plus multi-gluon production
processes at LHC, we test integrated cross sections and execution time for
programs in FORTRAN and C on CPU and those on GPU. Integrated results agree
with each other within statistical errors. Execution time of programs on GPU
run about 50 times faster than those in C, and more than 60 times faster than
the original FORTRAN programs.Comment: 6 pages, 2 figure
Calculation of HELAS amplitudes for QCD processes using graphics processing unit (GPU)
We use a graphics processing unit (GPU) for fast calculations of helicity
amplitudes of quark and gluon scattering processes in massless QCD. New HEGET
({\bf H}ELAS {\bf E}valuation with {\bf G}PU {\bf E}nhanced {\bf T}echnology)
codes for gluon self-interactions are introduced, and a C++ program to convert
the MadGraph generated FORTRAN codes into HEGET codes in CUDA (a C-platform for
general purpose computing on GPU) is created. Because of the proliferation of
the number of Feynman diagrams and the number of independent color amplitudes,
the maximum number of final state jets we can evaluate on a GPU is limited to 4
for pure gluon processes (), or 5 for processes with one or more
quark lines such as and . Compared with the usual
CPU-based programs, we obtain 60-100 times better performance on the GPU,
except for 5-jet production processes and the processes for which
the GPU gain over the CPU is about 20
Probing the Weak Boson Sector in
We study possible deviations from the standard model in the reaction at a 500 GeV collider. As a photon source we use a
laser backscattered photon beam. We investigate the most general and vertices including operators up to
energy-dimension-six which are Lorentz invariant. These vertices require four
extra parameters; two are CP-conserving, and , and two are
CP-violating, and . We present analytical expressions of
the helicity amplitudes for the process for arbitrary
values of anomalous couplings. Assuming Standard Model values are actually
measured we present the allowed region in the () plane at the
90\% confidence level. We then show how the angular correlation of the
decay products can be used to extract detailed information on the anomalous
(especially CP-violating) and couplings.Comment: Latex, 25 pages, 12 figures (not included). One compressed postscript
file including all the figures available at
ftp://ftp.kek.jp/kek/preprints/TH/TH-420/kekth420.ps.g
Prospects of Measuring General Higgs Couplings at e^+e^- Linear Colliders
We examine how accurately the general HZV couplings, with V=Z,gamma, may be
determined by studying e^+e^- --> Hff-bar processes at future e^+e^- linear
colliders. By using the optimal-observable method, which makes use of all
available experimental information, we find out which combinations of the
various HZV coupling terms may be constrained most efficiently with high
luminosity. We also assess the benefits of measuring the tau-lepton helicities,
identifying the bottom-hadron charges, polarizing the electron beam and running
at two different collider energies. The HZZ couplings are generally found to be
well constrained, even without these options, while the HZ-gamma couplings are
not. The constraints on the latter may be significantly improved by beam
polarization.Comment: 28 pages (LaTeX), 5 figures (axodraw and eps
One-loop contributions of charginos and neutralinos to W-pair production in E+ E- collisions
We study the one-loop effects of charginos and neutralinos on the helicity
amplitudes for \eeww in the minimal supersymmetric standard model. The
calculation is tested by using two methods. First, the sum rule for the form
factors between \eeww and the process where the external bosons are
replaced by the corresponding Goldstone bosons is employed to test
the analytic expression and the accuracy of the numerical program. Second, the
decoupling property in the large mass limit is used to test the overall
normalization of the amplitudes. These two tests are most effectively carried
out when the amplitudes are expanded in terms of the modified minimal
subtraction () couplings of the standard model. The resulting
perturbation expansion is valid at collider energies below and around the
threshold of the light supersymmetric particles. We find that the corrections
to the cross section of the longitudinally polarized -pair production can be
as large as -1.4% at the threshold of the light chargino-pair production for
large scattering angles. We also study the effects of the CP-violating phase in
the chargino and neutralino sectors on the helicity amplitudes. We find that
the resulting CP-violating asymmetries can be at most 0.1%.Comment: 30 pages, 25 figures, Final verision, To appear in Physical Review D,
Several sentences are improve
Origin of the second coherent peak in the dynamical structure factor of an asymmetric spin-ladder
Appearance of the second coherent peak in the dynamical structure factor of
an asymmetric spin ladder is suggested. The general arguments are confirmed by
the first order (with respect to the asymmetry) calculation for a spin ladder
with singlet-rung ground state. Basing on this result a new interpretation is
proposed for the inelastic neutron scattering data in the spin gap compound
CuHpCl.Comment: 11 page
Consequences of a Possible Di-Gamma Resonace at TRISTAN
If high mass di-gamma events observed at LEP are due to the production of a
di-gamma resonance via its leptonic coupling, its consequences can be observed
at TRISTAN. We find that a predicted decay branching rate is too small to
account for the observed events if the resonance spin is zero, due to a strong
cancellation in the decay amplitudes. Such a cancellation is absent if the
resonance has a spin two. We study the consequences of a tensor production in
the processes , and at TRISTAN
energies. Complete helicity amplitudes with tensor boson exchange contributions
are given, and the signal can clearly be identified from various distributions.
TRISTAN experiments are also sensitive to the virtual tensor boson exchange
effects, which reduce to the contact interaction terms in the high mass limit.Comment: 23 pages in revtex, 7 figures (not included) available upon request,
KEK-TH-35
Jet-disturbed molecular gas near the Seyfert 2 nucleus in M51
Previous molecular gas observations at arcsecond-scale resolution of the
Seyfert 2 galaxy M51 suggest the presence of a dense circumnuclear rotating
disk, which may be the reservoir for fueling the active nucleus and obscures it
from direct view in the optical. However, our recent interferometric CO(3-2)
observations show a hint of a velocity gradient perpendicular to the rotating
disk, which suggests a more complex structure than previously thought. To image
the putative circumnuclear molecular gas disk at sub-arcsecond resolution to
better understand both the spatial distribution and kinematics of the molecular
gas. We carried out CO(2-1) and CO(1-0) line observations of the nuclear region
of M51 with the new A configuration of the IRAM Plateau de Bure Interferometer,
yielding a spatial resolution lower than 15 pc. The high resolution images show
no clear evidence of a disk, aligned nearly east-west and perpendicular to the
radio jet axis, as suggested by previous observations, but show two separate
features located on the eastern and western sides of the nucleus. The western
feature shows an elongated structure along the jet and a good velocity
correspondence with optical emission lines associated with the jet, suggesting
that this feature is a jet-entrained gas. The eastern feature is elongated
nearly east-west ending around the nucleus. A velocity gradient appears in the
same direction with increasingly blueshifted velocities near the nucleus. This
velocity gradient is in the opposite sense of that previously inferred for the
putative circumnuclear disk. Possible explanations for the observed molecular
gas distribution and kinematics are that a rotating gas disk disturbed by the
jet, gas streaming toward the nucleus, or a ring with another smaller counter-
or Keplarian-rotating gas disk inside.Comment: 5 pages, 4 figures, to appear in A&A Letters Special Issue for the
new extended configuration at the IRAM PdB
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