1,071 research outputs found
Gravitationally enhanced depolarization of ultracold neutrons in magnetic-field gradients
Trapped ultracold neutrons (UCN) have for many years been the mainstay of experiments to search for the electric dipole moment (EDM) of the neutron, a critical parameter in constraining scenarios of new physics beyond the Standard Model. Because their energies are so low, UCN preferentially populate the lower region of their physical enclosure, and do not sample uniformly the ambient magnetic field throughout the storage volume. This leads to a substantial increase in the rate of depolarization, as well as to shifts in the measured frequency of the stored neutrons. Consequences for EDM measurements are discussed
Determination of polarized parton distribution functions with recent data on polarization asymmetries
Global analysis has been performed within the next-to-leading order in
Quantum Chromodynamics (QCD) to determine polarized parton distributions with
new experimental data in spin asymmetries. The new data set includes JLab,
HERMES, and COMPASS measurements on spin asymmetry A_1 for the neutron and
deuteron in lepton scattering. Our new analysis also utilizes the double-spin
asymmetry for pi^0 production in polarized pp collisions, A_{LL}^{pi^0},
measured by the PHENIX collaboration. Because of these new data, uncertainties
of the polarized PDFs are reduced. In particular, the JLab, HERMES, and COMPASS
measurements are valuable for determining Delta d_v(x) at large x and Delta
qbar(x) at x~0.1. The PHENIX pi^0 data significantly reduce the uncertainty of
Delta g(x). Furthermore, we discuss a possible constraint on Delta g(x) at
large x by using the HERMES data on g_1^d in comparison with the COMPASS ones
at x~0.05.Comment: 11 pages, REVTeX, 13 eps files, Phys. Rev. D in pres
Search for the disappearance of muon antineutrinos in the NuMI neutrino beam
We report constraints on muon antineutrino oscillation parameters that were obtained by using the two MINOS detectors to measure the 7% antineutrino component of the NuMI neutrino beam. In the Far Detector, we select 130 events in the charged-current muon antineutrino sample, compared to a prediction of 136.4 +/- 11.7(stat) ^{+10.2}_{-8.9}(syst) events under the assumption |dm2bar|=2.32x10^-3 eV^2, snthetabar=1.0. A fit to the two-flavor oscillation approximation constrains |dm2bar|<3.37x10^-3 eV^2 at the 90% confidence level with snthetabar=1.0
Light-front CQM calculations of baryon electromagnetic form factors
The parameter-free predictions for the and electromagnetic transition form factors, obtained within our
light-front constituent quark model using eigenfunctions of a baryon mass
operator which includes a large amount of configuration mixing, are reported.
The effects due to small components in the baryon wave functions, such as S'-
and D-wave, are also investigated.Comment: to appear in the Proceedings of the International Workshop on Hadron
Dynamics with the new DAPHNE and CEBAF facilities, Frascati, Italy, 11-14
November 199
Fully Unintegrated Parton Correlation Functions and Factorization in Lowest Order Hard Scattering
Motivated by the need to correct the potentially large kinematic errors in
approximations used in the standard formulation of perturbative QCD, we
reformulate deeply inelastic lepton-proton scattering in terms of gauge
invariant, universal parton correlation functions which depend on all
components of parton four-momentum. Currently, different hard QCD processes are
described by very different perturbative formalisms, each relying on its own
set of kinematical approximations. In this paper we show how to set up
formalism that avoids approximations on final-state momenta, and thus has a
very general domain of applicability. The use of exact kinematics introduces a
number of significant conceptual shifts already at leading order, and tightly
constrains the formalism. We show how to define parton correlation functions
that generalize the concepts of parton density, fragmentation function, and
soft factor. After setting up a general subtraction formalism, we obtain a
factorization theorem. To avoid complications with Ward identities the full
derivation is restricted to abelian gauge theories; even so the resulting
structure is highly suggestive of a similar treatment for non-abelian gauge
theories.Comment: 44 pages, 69 figures typos fixed, clarifications and second appendix
adde
A Review of Target Mass Corrections
With recent advances in the precision of inclusive lepton--nuclear scattering
experiments, it has become apparent that comparable improvements are needed in
the accuracy of the theoretical analysis tools. In particular, when extracting
parton distribution functions in the large-x region, it is crucial to correct
the data for effects associated with the nonzero mass of the target. We present
here a comprehensive review of these target mass corrections (TMC) to structure
functions data, summarizing the relevant formulas for TMCs in electromagnetic
and weak processes. We include a full analysis of both hadronic and partonic
masses, and trace how these effects appear in the operator product expansion
and the factorized parton model formalism, as well as their limitations when
applied to data in the x->1 limit. We evaluate the numerical effects of TMCs on
various structure functions, and compare fits to data with and without these
corrections.Comment: 41 pages, 13 figures; minor updates to match published versio
Dilepton production in proton-proton collisions at BEVALAC energies
The dilepton production in elementary reactions at
BEVALAC energies GeV is investigated. The calculations
include direct decays of the vector mesons , , and , Dalitz decays of the -, -, -, -, and -mesons, and of the baryon resonances
. The subthreshold vector meson production cross sections in
collisions are treated in a way sufficient to avoid double counting with the
inclusive vector meson production. The vector meson dominance model for the
transition form factors of the resonance Dalitz decays is
used in an extended form to ensure correct asymptotics which are in agreement
with the quark counting rules. Such a modification gives an unified and
consistent description of both radiative decays and meson decays.
The effect of multiple pion production on the experimental efficiency for the
detection of the dilepton pairs is studied. We find the dilepton yield in
reasonable agreement with the experimental data for the set of intermediate
energies whereas at the highest energy GeV the number of
dilepton pairs is likely to be overestimated experimentally in the mass range
MeV.Comment: 25 pages (IOP style), 5 figures, revised manuscript accepted for
publication in JP
Electroproduction, photoproduction, and inverse electroproduction of pions in the first resonance region
Methods are set forth for determining the hadron electromagnetic structure in
the sub--threshold timelike region of the virtual-photon ``mass'' and
for investigating the nucleon weak structure in the spacelike region from
experimental data on the process at low energies. These
methods are formulated using the unified description of photoproduction,
electroproduction, and inverse electroproduction of pions in the first
resonance region in the framework of the dispersion-relation model and on the
basis of the model-independent properties of inverse electroproduction.
Applications of these methods are also shown.Comment: The revised published version; Revtex4, 18 pages, 6 figure
Charged current weak electroproduction of Delta resonance
We study the weak production of (i.e. and ) in the intermediate
energy range corresponding to the Mainz and TJNAF electron accelerators. The
differential cross sections are found to be of the order of cm/sr, over a range of angles which increases with energy. The
possibility of observing these reactions with the high luminosities available
at these accelerators, and studying the weak N- transition form factors
through these reactions is discussed. The production cross section of
N in the kinematic region of production is also estimated
and found to be small.Comment: 19 pages, REVTEX, 4 figure
Structure Functions of the Nucleon and their Interpretation
The current status of measurements of the nucleon structure functions and
their understanding is reviewed. The fixed target experiments E665, CCFR and
NMC and the HERA experiments H1 and ZEUS are discussed in some detail. The
extraction of parton momentum distribution functions from global fits is
described, with particular attention paid to much improved information on the
gluon momentum distribution. The status of alpha_s measurements from deep
inelastic data is reviewed. Models and non-perturbative approaches for the
parton input distributions are outlined. The impact on the phenomenology of QCD
of the data at very low values of the Bjorken x variable is discussed in
detail. Recent advances in the understanding of the transition from deep
inelastic scattering to photoproduction are summarised. Some brief comments are
made on the recent HERA measurements of the ep NC and CC cross-sections at very
high Q2.Comment: 196 pages, 79 figures, uses ijmpa.sty and psfig.tex (included
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