The STAR Collaboration reports on the photoproduction of $\pi^+\pi^-$ pairs in gold-gold collisions at a center-of-mass energy of 200 GeV/nucleon-pair. These pion pairs are produced when a nearly-real photon emitted by one ion scatters from the other ion. We fit the $\pi^+\pi^-$ invariant mass spectrum with a combination of $\rho$ and $\omega$ resonances and a direct $\pi^+\pi^-$ continuum. This is the first observation of the $\omega$ in ultra-peripheral collisions, and the first measurement of $\rho-\omega$ interference at energies where photoproduction is dominated by Pomeron exchange. The $\omega$ amplitude is consistent with the measured $\gamma p\rightarrow \omega p$ cross section, a classical Glauber calculation and the $\omega\rightarrow\pi^+\pi^-$ branching ratio. The $\omega$ phase angle is similar to that observed at much lower energies, showing that the $\rho-\omega$ phase difference does not depend significantly on photon energy. The $\rho^0$ differential cross section $d\sigma/dt$ exhibits a clear diffraction pattern, compatible with scattering from a gold nucleus, with 2 minima visible. The positions of the diffractive minima agree better with the predictions of a quantum Glauber calculation that does not include nuclear shadowing than with a calculation that does include shadowing.
The $\pi^+\pi^-$ invariant-mass distribution for all selected $\pi\pi$ candidates with $p_T~<~100~\textrm{MeV}/c$.
The ratio $|B/A|$ of amplitudes of nonresonant $\pi^+\pi^-$ and $\rho^0$ mesons in the present STAR analysis.
The ratio $|B/A|$ of amplitudes of nonresonant $\pi^+\pi^-$ and $\rho^0$ mesons in the previous STAR analysis, Phys. Rev. C 77 034910 (2008).
Measurements of the deuteron elastic magnetic structure function B(Q2) are reported at squared four-momentum transfer values 1.20≤Q2≤2.77 (GeV/c)2. Also reported are values for the proton magnetic form factor GMp(Q2) at 11 Q2 values between 0.49 and 1.75 (GeV/c)2. The data were obtained using an electron beam of 0.5 to 1.3 GeV. Electrons backscattered near 180° were detected in coincidence with deuterons or protons recoiling near 0° in a large solid-angle double-arm spectrometer system. The data for B(Q2) are found to decrease rapidly from Q2=1.2 to 2 (GeV/c)2, and then rise to a secondary maximum around Q2=2.5 (GeV/c)2. Reasonable agreement is found with several different models, including those in the relativistic impulse approximation, nonrelativistic calculations that include meson-exchange currents, isobar configurations, and six-quark configurations, and one calculation based on the Skyrme model. All calculations are very sensitive to the choice of deuteron wave function and nucleon form factor parametrization. The data for GMp(Q2) are in good agreement with the empirical dipole fit.
The measured cross section have been devided by those obtained using the dipole form for the proton form factors: G_E=1/(1+Q2/0.71)**2, G_E(Q2)=G_M(Q2)/mu,where Q2 in GeV2, mu=2.79.
Axis error includes +- 0.0/0.0 contribution (?////Errors given are the statistical errors and systematic uncertainties add ed in quadreture).
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We have measured absolute differential cross sections and analyzing powers for neutron-proton elastic scattering for momentum transfer 0.01 < | t |< 0.08 (GeV/ c ) 2 at several energies between 378 and 1135 MeV. The ionization chamber IKAR filled with methane was used as both a gas target and recoil detector. For the analyzing-power measurements the scattered neutron was detected in scintillation counters in coincidence with the recoil proton detected in IKAR. Special care was taken to ensure a precise absolute normalization of the cross sections, with overall systematic uncertainties of 4–7%.
TOTAL SYSTEMATIC UNCERTAINTIES IN D(SIG)/D(T)=6.4 PCT.
TOTAL SYSTEMATIC UNCERTAINTIES IN D(SIG)/D(T)=5.4 PCT.
TOTAL SYSTEMATIC UNCERTAINTIES IN D(SIG)/D(T)=6.5 PCT.
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The differential cross section in free n-p forward elastic scattering has been measured for incident neutron energies of 378, 481, 582, 683, 784, 884, and 1085 MeV and for momentum transfer 0.01<‖t‖<0.08 (GeV/c)2. The experiment used a recoil-detector ionization chamber which served at the same time as a gas target. Special care has been taken to obtain a precise absolute normalization.
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X ERROR D(THETA) =0.5000MRAD AND D(P)/P = 0.700PCT.
X ERROR D(THETA) =0.5000MRAD AND D(P)/P = 0.700PCT.
X ERROR D(THETA) =0.5000MRAD AND D(P)/P = 0.700PCT.
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MODEL DEPENDENT ESTIMATION.
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