856 research outputs found
Threshold Resummation for W-Boson Production at RHIC
We study the resummation of large logarithmic perturbative corrections to the
partonic cross sections relevant for the process pp -> W^+- X at the BNL
Relativistic Heavy Ion Collider (RHIC). At RHIC, polarized protons are
available, and spin asymmetries for this process will be used for precise
measurements of the up and down quark and anti-quark distributions in the
proton. The corrections arise near the threshold for the partonic reaction and
are associated with soft-gluon emission. We perform the resummation to
next-to-leading logarithmic accuracy, for the rapidity-differential cross
section. We find that resummation leads to relatively moderate effects on the
cross sections and spin asymmetries.Comment: 25 pages, 15 figures as eps files. One reference added and typo
correcte
Structure of Real and Virtual Photons from ZEUS
Measurements sensitive to the structure of both real and virtual photons are
presented and compared to theoretical models with various photon parton
distribution functions (PDFs). Measurements for real photons show a tendency
for the available photon PDFs to be too small to describe the data. For virtual
photons, the photon PDF is seen to decrease with increasing photon virtuality.
In order to describe the data, resolved photon processes are required up to a
photon virtuality of at least 4.5 GeV^2
A_{LT} in the Nucleon-Nucleon Polarized Drell-Yan Process
We present a leading order (LO) estimate for the longitidinal-transverse spin
asymmetry (A_{LT}) in the nucleon-nucleon polarized Drell-Yan process at RHIC
and HERA- energies in comparison with A_{LL} and A_{TT}. A_{LT}
receives contribution from g_1, the transversity ditribution h_1, and the
twist-3 distributions g_T and h_L. For the twist-3 contribution we use the bag
model prediction evolved to a high energy scale by the large-N_c evolution
equation. We found that A_{LT} (normalized by the asymmetry in the parton
level) is much smaller than the corresponding A_{TT}. Twist-3 contribution
given by the bag model also turned out to be negligible.Comment: Talk presented at the KEK-Tanashi International Conference "Physics
of Hadrons and Nuclei", Dec. 14-17, 1998, Tokyo. 4 pages in Latex. 1 figure
included as eps files using epsbox.st
Calculating resonance positions and widths using the Siegert approximation method
Here we present complex resonance states (or Siegert states), that describe
the tunneling decay of a trapped quantum particle, from an intuitive point of
view which naturally leads to the easily applicable Siegert approximation
method that can be used for analytical and numerical calculations of complex
resonances of both the linear and nonlinear Schr\"odinger equation. Our
approach thus complements other treatments of the subject that mostly focus on
methods based on continuation in the complex plane or on semiclassical
approximations.Comment: 15 pages, 1 figure, contains MATLAB source code; new version with
additional illustration
Modeling of electron emission processes accompanying Radon--decays within electrostatic spectrometers
Electrostatic spectrometers utilized in high-resolution beta-spectroscopy
studies such as in the Karlsruhe Tritium Neutrino (KATRIN) experiment have to
operate with a background level of less than 10^(-2) counts per second. This
limit can be exceeded by even a small number of Rn-219 or Rn-220 atoms being
emanated into the volume and undergoing alpha-decay there. In this paper we
present a detailed model of the underlying background-generating processes via
electron emission by internal conversion, shake-off and relaxation processes in
the atomic shells of the Po-215 and Po-216 daughters. The model yields electron
energy spectra up to 400 keV and electron multiplicities of up to 20 which are
compared to experimental data.Comment: 7 figure
A_{LT} in the polarized Drell-Yan process at RHIC and HERA energies
We present a leading order (LO) estimate for the longitidinal-transverse spin
asymmetry (A_{LT}) in the nucleon-nucleon polarized Drell-Yan process at RHIC
and HERA- energies in comparison with A_{LL} and A_{TT}. A_{LT}
receives contribution from g_1, the transversity ditribution h_1, and the
twist-3 distributions g_T and h_L. For the twist-3 contribution we use the bag
model prediction evolved to a high energy scale by the large-N_c evolution
equation. We found that A_{LT} (normalized by the asymmetry in the parton
level) is much smaller than the corresponding A_{TT}. Twist-3 contribution
given by the bag model also turned out to be negligible.Comment: 12 pages in Latex. 5 figures included as eps files using epsbox.sty.
Minor corrections for typos and notations incorporate
Next-to-Leading order approximation of polarized valon and parton distributions
Polarized parton distributions and structure functions of the nucleon are
analyzed in the improved valon model. The valon representation provides a model
to represent hadrons in terms of quarks, providing a unified description of
bound state and scattering properties of hadrons. Polarized valon distributions
are seen to play an important role in describing the spin dependence of parton
distributions in the leading order (LO) and next-to-leading order (NLO)
approximations. In the polarized case, a convolution integral is derived in the
framework of the valon model. The Polarized valon distribution in a proton and
the polarized parton distributions inside the valon are necessary to obtain the
polarized parton distributions in a proton. Bernstein polynomial averages are
used to extract the unknown parameters of the polarized valon distributions by
fitting to the available experimental data. The predictions for the NLO
calculations of the polarized parton distributions and proton structure
functions are compared with the LO approximation. It is shown that the results
of the calculations for the proton structure function, , and its first
moment, , are in good agreement with the experimental data for a
range of values of . Finally the spin contribution of the valons to the
proton is calculated.Comment: 22 pages, 7 figures. Published in Journal of High Energy Physics
(JHEP
Diffractive production of high pt photons at HERA
We study the diffractive production of high pt photons at HERA. We have
implemented the process as a new hard sub-process in the HERWIG event generator
in order to prepare the ground for a future measurement.Comment: 4 pages, 4 figures. Contribution to the 1999 UK Phenomenology
Workshop on Collider Physics, Durham, U
BFKL versus HERA
The BFKL equation and the kT-factorization theorem are used to obtain
predictions for F2 in the small Bjorken-x region over a wide range of Q**2. The
dependence on the parameters, especially on those concerning the infrared
region, is discussed. After a background fit to recent experimental data
obtained at HERA and at Fermilab (E665 experiment), we find that the predicted,
almost Q**2 independent BFKL slope lambda >= 0.5 appears to be too steep at
lower Q**2 values. Thus there seems to be a chance that future HERA data can
distinguish between pure BFKL and conventional field theoretic renormalization
group approaches.Comment: 26 pages, 6 eps figures, LaTeX2e using epsfig.sty and amssymb.st
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