5,271 research outputs found

### Tau polarization effects in the CNGS tau-neutrino appearance experiments

We studied tau polarization effects on the decay distributions of tau
produced in the CNGS tau-neutrino appearance experiments. We show that energy
and angular distributions for the decay products in the laboratory frame are
significantly affected by the tau polarization. Rather strong azimuthal
asymmetry about the tau momentum axis is predicted, which may have observable
consequences in experiments even with small statistics.Comment: 5 pages, 6 eps figures, espcrc2.sty; Proceedings of the 4th
International Workshop on Neutrino-Nucleus Interactions in the Few GeV Region
(NuInt05), September 26-29, 2005, Okayama, Japa

### Recent measurements of CP violation at the B factories

Recent measurements of time dependent CP asymmetries at the B factories have
led to substantial progress in our understanding of CP violation. In this
article, I review some of these experimental results and discuss their
implications in the Standard Model and their sensitivity to New Physics.Comment: 17 pages, 13 figures, to be published in Modern Physics Letters A. To
be published in Modern Physics Letters

### Probing the CP nature of the Higgs bosons by top-pair production at photon linear colliders

We study effects of heavy Higgs bosons on the top-pair production process at
photon linear colliders. The interference patterns between the resonant
Higgs-production amplitudes and the continuum QED amplitudes are examined. The
patterns tell us not only the CP nature of the Higgs bosons but also the phase
of the $\gamma\gamma$--Higgs vertex which gives new information about the Higgs
couplings to new charged particles. We point out that it is necessary to use
circularly polarized photon beams to produce efficiently heavy Higgs bosons
whose masses exceed the electron beam energy, and show that the above
interference patterns of the production amplitudes can be studied by observing
top decay angular distributions. Analytic expressions for the helicity
amplitudes for the sequential process $\gamma\gamma \to t \bar{t} \to (bW^+)
(\bar{b}W^-) \to (b f_1 \bar{f}_2) (\bar{b} f_3 \bar{f}_4)$ are presented in
terms of the generic $\gamma\gamma \to t \bar{t}$ production amplitudes.Comment: 34 pages, LATEX file with 6 PS figures, comments adde

### Pseudoscalar Form Factors in Tau-Neutrino Nucleon Scattering

We investigate the pseudoscalar transition form factors of nucleon for
quasi-elastic scattering and \Delta resonance production in tau-neutrino
nucleon scattering via the charged current interactions. Although the
pseudoscalar form factors play an important role for the \tau production in
neutrino-nucleon scattering, these are not known well. In this article, we
examine their effects in quasi-elastic scattering and \Delta resonance
production and find that the cross section, Q^{2} distribution, and spin
polarization of the produced \tau^{\pm} leptons are quite sensitive to the
pseudoscalar form factors.Comment: 7 pages, 3 figure

### Polarization effects in tau production by neutrino

We studied polarization effects in tau production by neutrino-nucleon
scattering. Quasi-elastic scattering, $Delta$ resonance production and deep
inelastic scattering processes are taken into account for the CERN-to-Gran
Sasso projects. We show that the tau produced by neutrino has high degree of
polarization, and its spin direction depends non-trivially on the energy and
the scattering angle of tau in the laboratory frame.Comment: 6 pages, 5 eps figures, espcrc2.sty; Proceedings of the 3rd
International Workshop on Neutrino-Nucleus Interactions in the Few GeV Region
(NuInt04), March 17-21, 2004, Gran Sasso, Italy; minor changes, typos in Eq.
(6) correcte

### Analyticity, crossing and the absorptive parts of the one-loop contributions to the quark-quark-gluon gauge boson four-point function

Starting from the known one-loop result for the $e^{+}e^{-}$-annihilation
process $e^{+}e^{-}\stackrel{\gamma,Z} {\longrightarrow} q\bar{q}g$ with
massless quarks we employ analyticity and crossing to determine the absorptive
parts of the corresponding one-loop contributions in Deep Inelastic Scattering
(DIS) and in the Drell-Yan process (DY). Whereas the ${\cal O}(\alpha_s^2)$
absorptive parts generate a non-measurable phase factor in the
$e^{+}e^{-}$-annihilation channel one obtains measurable phase effects from the
one-loop contributions in the deep inelastic and in the Drell-Yan case. We
compare our results with the results of previous calculations where the
absorptive parts in DIS and in the DY process were calculated directly in the
respective channels. We also present some new results on the dispersive and
absorptive contributions of the triangle anomaly graph to the DIS process.Comment: 23 pages, 5 figures, typos corrected. Version to appear in Phys. Rev.

### 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 $W^{+}$ 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

### Light bottom squark and gluino confront electroweak precision measurements

We address the compatibility of a light sbottom (mass 2\sim 5.5 \gev) and a
light gluino (mass 12\sim 16 \gev) with electroweak precision measurements.
Such light particles have been suggested to explain the observed excess in the
$b$ quark production cross section at the Tevatron. The electroweak observables
may be affected by the sbottom and gluino through the SUSY-QCD corrections to
the $Zbb$ vertex. We examine, in addition to the SUSY-QCD corrections, the
electroweak corrections to the gauge boson propagators from the stop which are
allowed to be light from the SU(2)$_L$ symmetry. We find that this scenario is
strongly disfavored from electroweak precision measurements unless the heavier
sbottom mass eigenstate is lighter than 180\gev and the left-right mixing in
the stop sector is sufficiently large. This implies that one of the stops
should be lighter than about 98\gev.Comment: 4 pages, revtex, 2 figures. Reference added, version to appear in
Phys.Rev.Let

### Probing the Weak Boson Sector in $\gamma e\rightarrow Ze$

We study possible deviations from the standard model in the reaction $\gamma
e\rightarrow Ze$ at a 500 GeV $e^+e^-$ collider. As a photon source we use a
laser backscattered photon beam. We investigate the most general $\gamma
Z\gamma$ and $\gamma ZZ$ vertices including operators up to
energy-dimension-six which are Lorentz invariant. These vertices require four
extra parameters; two are CP-conserving, $h^\gamma_1$ and $h^Z_1$, and two are
CP-violating, $h^\gamma_2$ and $h^Z_2$. We present analytical expressions of
the helicity amplitudes for the process $\gamma e\rightarrow Ze$ for arbitrary
values of anomalous couplings. Assuming Standard Model values are actually
measured we present the allowed region in the ($h^\gamma_1,h^Z_1$) plane at the
90\% confidence level. We then show how the angular correlation of the $Z$
decay products can be used to extract detailed information on the anomalous
(especially CP-violating) $\gamma Z\gamma$ and $\gamma ZZ$ 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

### 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 ($gg\to 4g$), or 5 for processes with one or more
quark lines such as $q\bar{q}\to 5g$ and $qq\to qq+3g$. 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 $gg\to 4g$ processes for which
the GPU gain over the CPU is about 20

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