16091296
Dielectron cross-section measurements in nucleus-nucleus reactions at 1-A/GeV
We present measured dielectron production cross sections for Ca+Ca, C+C, He+Ca, and d+Ca reactions at 1.0 A GeV . Statistical uncertainties and systematic effects are smaller than in previous dilepton spectrometer (DLS) nucleus-nucleus data. For pair mass M{le}0.35 GeV/c{sup 2} we obtain (1) the Ca+Ca cross section is larger than the previous DLS measurement and current model results, (2) the mass spectra suggest large contributions from {pi}{sup 0} and {eta} Dalitz decays, and (3) d{sigma}/dM{proportional_to}A{sub P}A{sub T}. For M{gt}0.5 GeV/c{sup 2} the Ca+Ca to C+C cross section ratio is significantly larger than the ratio of A{sub P}A{sub T} values. {copyright} {ital 1997} {ital The American Physical Society}
6986761
Threshold behavior of electron pair production in p-Be collisions.
We report on the measurements of electron pair production in /ital p/-Becollisions at 1.0, 2.1, and 4.9 GeV beam kinetic energies. The invariant-massand transverse-momentum spectra are presented, along with the total crosssections. A rapid decrease in the integrated cross section is observed as thebeam energy is reduced. Comparison of the excitation function for dielectron(/ital e//sup +/e/sup /minus//) and dipion (..pi../sup +/..pi../sup /minus//)production suggests that the dominant production mechanism at beam kineticenergies 2.1 and 4.9 GeV may be ..pi../sup +/..pi../sup /minus// annihilation.
44598663
Measurement of dielectron production in niobium-niobium collisions at 1.05 GeV/nucleon.
A dielectron signal for pair masses greater than [ital M][sub [pi]] is observed in Nb+Nb collisions at 1.05 GeV/nucleon beam kinetic energy. The invariant mass spectrum and the associated multiplicity distribution are presented. Comparisons are made with previous cross section measurements in [ital p]+Be and Ca+Ca.
119395992
Dilepton production from p-p to CaCa at the Bevalac
Abstract The DLS collaboration has recently completed a high statistics study of dilepton production at the Bevalac. In particular, we have measured dielectrons (e + e − ) from p - p and p - d collisions to understand the basic dilepton production mechanisms in the energy range from 1.05 – 4.9 GeV. These data can be used to determine the basic processes which contribute to nucleon-nucleon dilepton production such as hadronic bremsstrahlung, vector meson processes, and hadronic Dalitz decay. The data show that a simple elastic bremsstrahlung calculation is insufficient to explain the data. Theoretical models are compared with the data. A new high statistics study of Ca  Ca at 1.05 A·GeV has been made to study the collectivity of A - A collisions.
117522082
First results on dilepton production at the Bevalac
We report on preliminary results of direct electron pair measurements in p+Be at 4.9 GeV and 2.1 GeV and Ca+Ca at 1.95 GeVA collisions at the Bevalac. The results are compared to existing data in p+Be at 12.1 GeV and ..pi../sup -/p at 15.9 and 16.9 GeV.
120239957
Threshold behavior of direct electron production in proton‐beryllium collisions
We report measurements of direct electron pair production in p‐Be interactions at 1.0, 2.1 and 4.9 GeV kinetic beam energies. The invariant mass and pt spectra along with the total cross sections are presented. A rapid decrease in the total cross section is observed at the lowest energy.
6448147
Analysis of the vertices DDV and D*DV with light-cone QCD sum rules
In this article, we study the vertices DDV and D*DV with the light-cone QCD sum rules. The strong coupling constants gDDV and fD*DV play an important role in understanding the final-state re-scattering effects in the hadronic B decays. They are related to the basic parameters β and λ respectively in the heavy quark effective Lagrangian; our numerical values are smaller than the existing estimations.
119313543
How are Feynman graphs resummed by the Loop Vertex Expansion
The purpose of this short letter is to clarify which set of pieces of Feynman graphs are resummed in a Loop Vertex Expansion, and to formulate a conjecture on the $\phi^4$ theory in non-integer dimension.
6241544
Wind Accretion and State Transitions in Cygnus X-1
We present the results of a spectroscopic monitoring program (from 1998 to 2002) of the Hα emission strength in HDE 226868, the optical counterpart of the black hole binary Cyg X-1. The feature provides an important probe of the mass-loss rate in the base of the stellar wind of the supergiant star. We derive an updated ephemeris for the orbit based on radial velocities measured from He I λ6678. We list net equivalent widths for the entire Hα emission/absorption complex, and we find that there are large variations in emission strength over both long (years) and short (hours to days) time spans. There are coherent orbital phase-related variations in the profiles when the spectra are grouped by Hα equivalent width. The profiles consist of (1) a P Cygni component associated with the wind of the supergiant, (2) emission components that attain high velocity at the conjunctions and that probably form in enhanced outflows both toward and away from the black hole, and (3) an emission component that moves in antiphase with the supergiant's motion. We argue that the third component forms in accreted gas near the black hole and that the radial velocity curve of the emission is consistent with a mass ratio of MX/Mopt ≈ 0.36 ± 0.05. We find that there is a general anticorrelation between the Hα emission strength and X-ray flux (from the Rossi X-Ray Timing Explorer All Sky Monitor) in the sense that when the Hα emission is strong (Wλ < -0.5 A) the X-ray flux is weaker and the spectrum harder. On the other hand, there is no correlation between Hα emission strength and X-ray flux when Hα is weak. We argue that this relationship is not caused by wind X-ray absorption nor by the reduction in Hα emissivity by X-ray heating. Instead, we suggest that the Hα variations track changes in wind density and strength near the photosphere. The density of the wind determines the size of X-ray ionization zones surrounding the black hole, and these in turn control the acceleration of the wind in the direction of the black hole. During the low/hard X-ray state, the strong wind is fast and the accretion rate is relatively low, while during the high/soft state, the weaker, highly ionized wind attains only a moderate velocity and the accretion rate increases. We argue that the X-ray transitions from the normal low/hard to the rare high/soft state are triggered by episodes of decreased mass-loss rate in the supergiant donor star.
15122700
Analysis of the Y(2175) as a tetraquark state with QCD sum rules
Abstract In this article, we take the point of view that Y ( 2175 ) be a tetraquark state which consists of color octet constituents, and calculate its mass and decay constant within the framework of the QCD sum rule approach. We release standard criterion in the QCD sum rules approach and take more phenomenological analysis, the value of the mass of Y ( 2175 ) is consistent with experimental data; there may be some tetraquark components in the state Y ( 2175 ) . If we retain standard criterion, larger mass than experimental data can be obtained, the current J μ ( x ) can interpolate a tetraquark state with larger mass, or Y ( 2175 ) has some components with larger mass. The dominating contribution comes from the perturbative term, which is in contrast to the sum rules with interpolating currents constructed from diquark pairs. The tetraquark states may consist of color octet constituents rather than diquark pairs.
17728532
The Off-Nuclear Starbursts in NGC 4038/4039 (The Antennae Galaxies)
Imaging of the Antennae galaxies (NGC 4038/4039) with the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope reveals large concentrations of star-forming activity away from both nuclei of the two merging galaxies. These images confirm earlier findings based on Infrared Space Observatory (ISO) data with lower angular resolution. The short-wavelength emission shows numerous compact sources identified as stellar clusters. At the longer wavelengths, bright, more amorphous and filamentary features correlate well with the known distributions of denser gas, warm dust, and H II regions. There are also fainter, more diffuse components at all wavelengths that permeate the entire region and extend into the two tidal tails. Nonstellar dust emission dominates the 5.8 and 8.0 μm images, accounting for as much as 79% of the light at 5.8 μm and 95% at 8 μm, averaged over the entire galaxy. Assuming that the nonstellar emission traces star formation, the IRAC data provide a view into the total underlying star-forming activities unaffected by obscuration. Using the flux ratio of nonstellar to stellar emission as a guide, we map the local star formation rate in the Antennae and compare that to similar measurements in both normal and infrared-luminous galaxies. This rate in the active regions is found to be as high as those seen in starburst and some ultraluminous infrared galaxies on a "per unit mass" basis. The two galactic centers actually have lower star-forming rates than the off-nuclear regions despite the presence of abundant dense gas and dust, suggesting that the latter is a necessary but not sufficient condition for ongoing star formation.