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تسجيل مستخدم جديدUsing coherent dipion photoproduction to image gold nuclei
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تأليف
Spencer R. Klein
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Vector meson photoproduction offers the opportunity to image target nuclei. The two-dimensional Fourier transform $dsigma_{rm coherent}/dt$ of coherent vector meson photoproduction gives the two-dimensional distribution of interaction sites in the target. Since vector meson photoproduction occurs, at lowest order, via two-gluon exchange, this is sensitive to gluon shadowing. We present an analysis of $pi^+pi^-$ photoproduction using data from the STAR detector and a study of $dsigma_{rm coherent}/dt$, with an emphasis on probing the nuclear shape and its systematic uncertainties.
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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 i
on. 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 $gamma prightarrow omega p$ cross section, a classical Glauber calculation and the $omegarightarrowpi^+pi^-$ 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 $dsigma/dt$ 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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e attractive interaction of two $rho$-mesons. Such a state would be expected to decay strongly to final states with charged pions, due to the dominant decay $rho to pi^+ pi^-$, whereas decay to two neutral pions would likely be suppressed. Here, we measure for the first time the reaction $gamma p to pi^0 pi^0 p$, using the CLAS detector at Jefferson Lab for incident beam energies between 3.6-5.4~GeV. Differential cross sections, $dsigma / dt$, for $f_2(1270)$ photoproduction are extracted with good precision, due to low backgrounds, and are compared with theoretical calculations.
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