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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.

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Figure 4

The $\pi^+\pi^-$ invariant-mass distribution for all selected $\pi\pi$ candidates with $p_T~<~100~\textrm{MeV}/c$.

Figure 5 a

The ratio $|B/A|$ of amplitudes of nonresonant $\pi^+\pi^-$ and $\rho^0$ mesons in the present STAR analysis.

Figure 5 b

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).

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Measurement of radiative Bhabha and quasi-real Compton scattering.

The L3 collaboration Acciarri, M. ; Adriani, O. ; Aguilar-Benitez, M. ; et al.

Phys.Lett.B 439 (1998) 183-196, 1998.

Inspire Record 473410 DOI 10.17182/hepdata.49339

We report on a study of radiative Bhabha and quasi-real Compton scattering at centre-of-mass energies between 50 GeV and 170 GeV, and 20 GeV and 140 GeV, respectively, using the L3 detector at LEP. The analysis is based on data corresponding to an integrated luminosity of 232.2 pb −1 . A total of 2856 radiative Bhabha and 4641 Compton scattering events are collected. Total and differential cross sections for both reactions are presented and found to be in good agreement with QED expectations. Our measurement of Compton scattering at the highest energies obtained so far is used to derive exclusion limits on the coupling λ for the on-shell production of an excited electron e ★ decaying into a γ e pair in the mass range 20 GeV

2 data tables match query

Table 1

Measured cross sections for radiative Bhabha scattering events.

Table 2

Measured cross sections for the quasi-real Compton scattering events.

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MEASUREMENT OF THE POLARIZATION ROTATION COEFFICIENT A IN THE P P SCATTERING AT 970-MEV. (IN RUSSIAN)

MEASUREMENT OF THE R PARAMETER IN P P SCATTERING AT 0.97-GEV AND ANALYSIS OF EXPERIMENTAL RESULTS ON TRIPLE SCATTERING. (IN RUSSIAN)

Investigation of the Energy Dependence of the Spin Spin Correlation in the Diproton Resonance Region

MEASUREMENT OF POLARIZATION PARAMETER P IN ELASTIC PI+ P SCATTERING IN THE ENERGY RANGE 500-MEV - 600-MEV. (IN RUSSIAN)

MEASUREMENT OF POLARIZATION PARAMETER P IN ELASTIC PI- P SCATTERING FOR ENERGIES OF 490-MeV AND 600-MeV. (IN RUSSIAN)

ANGULAR DEPENDENCE OF THE POLARIZATION CORRELATION PARAMETER A(00NN) AND THE ASYMMETRY PARAMETER A(000N) IN ELASTIC PROTON PROTON SCATTERING AT 690-MEV - 950-MEV

Polarization parameters A(000n) and A(00nn) in elastic proton proton scattering in the energy region 690-MeV to 890-MeV

STUDY OF POLARIZATION IN EXCLUSIVE PROTON - DEUTERON BREAKUP REACTION p D ---> p p n AT 1-GeV

Coherent diffractive photoproduction of $\rho^{0}$ mesons on gold nuclei at RHIC

Measurement of radiative Bhabha and quasi-real Compton scattering.