5,812 research outputs found
Exclusive Photoproduction of Large Momentum-Transfer K and K* Mesons
The reactions gamma p -> K+ Lambda and gamma p -> K* Lambda are analyzed
within perturbative QCD, allowing for diquarks as quasi-elementary constituents
of baryons. The diquark-model parameters and the quark-diquark distribution
amplitudes of proton and Lambda are taken from previous investigations of
electromagnetic baryon form factors and Compton-scattering off protons.
Unpolarized differential cross sections and polarization observables are
computed for different choices of the K and K* distribution amplitudes. The
asymptotic form of the K distribution amplitude (proportional to x1 x2) is
found to provide a satisfactory description of the K photoproduction data.Comment: 32 pages, 7 figures available as tared, compressed and uuencoded
PS-file
Hygroscopicity of secondary organic aerosols formed by oxidation of cycloalkenes, monoterpenes, sesquiterpenes, and related compounds
A series of experiments has been conducted in the Caltech indoor smog chamber facility to investigate the water uptake properties of aerosol formed by oxidation of various organic precursors. Secondary organic aerosol (SOA) from simple and substituted cycloalkenes (C5-C8) is produced in dark ozonolysis experiments in a dry chamber (RH~5%). Biogenic SOA from monoterpenes, sesquiterpenes, and oxygenated terpenes is formed by photooxidation in a humid chamber (~50% RH). Using the hygroscopicity tandem differential mobility analyzer (HTDMA), we measure the diameter-based hygroscopic growth factor (GF) of the SOA as a function of time and relative humidity. All SOA studied is found to be slightly hygroscopic, with smaller water uptake than that of typical inorganic aerosol substances. The aerosol water uptake increases with time early in the experiments for the cycloalkene SOA, but decreases with time for the biogenic SOA. This behavior could indicate competing effects between the formation of more highly oxidized polar compounds (more hygroscopic), and formation of longer-chained oligomers (less hygroscopic). All SOA also exhibit a smooth water uptake with RH with no deliquescence or efflorescence. The water uptake curves are found to be fitted well with an empirical three-parameter functional form. The measured pure organic GF values at 85% RH are between 1.09–1.16 for SOA from ozonolysis of cycloalkenes, 1.01–1.04 for sesquiterpene photooxidation SOA, and 1.06–1.11 for the monoterpene and oxygenated terpene SOA. The GF of pure SOA (GForg) in experiments in which inorganic seed aerosol is used is determined by assuming volume-weighted water uptake (Zdanovskii-Stokes-Robinson or ''ZSR'' approach) and using the size-resolved organic mass fraction measured by the Aerodyne Aerosol Mass Spectrometer. Knowing the water content associated with the inorganic fraction yields GForg values. However, for each precursor, the GForg values computed from different HTDMA-classified diameters agree with each other to varying degrees. Lack of complete agreement may be a result of the non-idealities of the solutions that are not captured by the ZSR method. Comparing growth factors from different precursors, we find that GForg is inversely proportional to the precursor molecular weight and SOA yield, which is likely a result of the fact that higher-molecular weight precursors tend to produce larger and less hygroscopic oxidation products
Hard exclusive photoproduction of mesons
We calculate the differential cross section and single-polarization
observables for the reaction within perturbative QCD,
treating the proton as a quark-diquark system. The phenomenological couplings
of gauge bosons to (spatially extended) diquarks and the quark-diquark
distribution amplitude of the proton are adopted from previous investigations
of baryon form factors and two-photon processes. Going beyond leading order, we
take into account hadron-mass effects by means of a systematic expansion in the
small parameter (hadron mass/ photon energy). With the -meson
distribution amplitude taken from the literature our predictions for the
differential cross section at | t | \agt 4 \text{GeV}^2 seem to provide a
reasonable extrapolation of the low-t data and are also comparable in magnitude
with the results of a two-gluon exchange model in which the gluons are
considered as a remnant of the pomeron. For momentum transfers of a few GeV
hadron-mass effects appear still to be sizeable.Comment: 37 pages, 7 figures, uses RevTeX styl
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Manufacturing of 42SiCr-pipes for quenching and partitioning by longitudinal HFI-Welding
In the pipe manufacturing and pipe processing industry, the demand for cost-effective pipes with high strength and good ductility is increasing. In the present study, the inductive longitudinal welding process was combined with a Quenching and Partitioning (Q&P) treatment to manufacture pipes with enhanced mechanical properties. The aim of the Q&P process is to establish a martensitic structure with increased retained austenite content. This allows for the beneficial use of both phases: the strength of martensite as well as the ductility of retained austenite. A 42SiCr steel, developed for Q&P processes, was joined at the longitudinal seam by a high-frequency induction (HFI) welding process and was subsequently heat-treated. The applied heat treatments included normalizing, austenitizing, quenching, and two Q&P strategies (Q&P-A/Q&P-B) with distinct quenching (Tq = 200/150â—¦ C) and partitioning temperatures (Tp = 300/250â—¦ C). Investigations of the microstructures revealed that Q&P tubes exhibit increased amounts of retained austenite in the martensitic matrix. Differences between the weld junction and the base material occurred, especially regarding the morphology of the martensite; the martensite found in the weld junction is finer and corresponds more to the lath-type morphology, compared to the base material in the circumference. In all zones of the welded tube circumference, retained austenite has been found in similar distributions. The mechanical testing of the individual tubes demonstrated that the Q&P treatments offer increased strength compared to all other states and significantly improved ductility compared to the quenched condition. Therefore, the approach of Q&P treatment of HFI-welded tubes represents a route for the mass production of high-strength tubular products with improved ductility
One-loop self-energy correction to the 1s and 2s hyperfine splitting in H-like systems
The one-loop self-energy correction to the hyperfine splitting of the 1s and
2s levels in H-like low-Z atoms is evaluated to all orders in Z\alpha. The
results are compared to perturbative calculations. The residual higher-order
contribution is evaluated. Implications to the specific difference of the
hyperfine structure intervals 8\Delta \nu_2 - \Delta \nu_1 in He^+ are
investigated.Comment: 17 pages, RevTeX, 3 figure
Skewed parton distributions and the scale dependence of the transverse size parameter
We discuss the scale dependence of a skewed parton distribution of the pion
obtained from a generalized light-cone wave function overlap formula. Using a
simple ansatz for the transverse momentum dependence of the light-cone wave
function and restricting ourselves to the case of a zero skewedness parameter,
the skewed parton distribution can be expressed through an ordinary parton
distribution multiplied by an exponential function. Matching the generalized
and ordinary DGLAP evolution equations of the skewed and ordinary parton
distributions, respectively, we derive a constraint for the scale dependence of
the transverse size parameter, which describes the width of the pion wave
function in transverse momentum space. This constraint has implications for the
Fock state probability and valence distribution. We apply our results to the
pion form factor.Comment: 10 pages, 4 figures; version to appear in Phys. Rev. D; Refs. added,
new discussion of results for pion form factor in view of new dat
Timelike form factors at high energy
The difference between the timelike and spacelike meson form factors is
analysed in the framework of perturbative QCD with Sudakov effects included. It
is found that integrable singularities appear but that the asymptotic behavior
is the same in the timelike and spacelike regions. The approach to asymptotia
is quite slow and a rather constant enhancement of the timelike value is
expected at measurable large . This is in agreement with the trend
shown by experimental data.Comment: 17 pages, report DAPNIA/SPhN 94 0
Signals of Disoriented Chiral Condensate
If a disoriented chiral condensate is created over an extended space-time
region following a rapid cooling in hadronic or nuclear collisions, the
misalignment of the condensate with the electroweak symmetry breaking can
generate observable effects in the processes which involve both strong and
electromagnetic interactions. We point out the relevance of the dilepton decay
of light vector mesons as a signal for formation of the disoriented condensate.
We predict that the decay \rho^0 to dileptons will be suppressed and/or the
\rho resonance peak widens, while the decay \omega to dileptons will not be
affected by the condensate.Comment: 13 pages in LaTeX, UCB-PTH-94/05, LBL-3533
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