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Register a new userUsing coherent dipion photoproduction to image gold nuclei
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Authors
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 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 $gamma prightarrow omega p$ 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.
Coherent photoproduction of two neutral pions on the deuteron and ^3He is calculated. The isoscalar and isovector parts of the elementary photoproduction amplitude was determined by fitting the measured total cross section on protons and neutrons. The dependence of the cross section on the isospin of the target nucleus is discussed.
Coherent photoproduction of $eta$-mesons off $^3$He, i.e. the reaction $gamma ^3{He}rightarrow eta ^3{He}$, has been investigated in the near-threshold region. The experiment was performed at the Glasgow tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS detector. Angular distributions and the total cross section were measured using the $etarightarrow gammagamma$ and $etarightarrow 3pi^0rightarrow 6gamma$ decay channels. The observed extremely sharp rise of the cross section at threshold and the behavior of the angular distributions are evidence for a strong $eta {^3{He}}$ final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of $pi^0$-proton back-to-back emission in the $gamma ^3{He}rightarrow pi^0 pX$ reaction revealed a very complicated structure of the background and could not support previous conclusions.
The quark structure of the $f_2(1270)$ meson has, for many years, been assumed to be a pure quark-antiquark ($qbar{q}$) resonance with quantum numbers $J^{PC} = 2^{++}$. Recently, it was proposed that the $f_2(1270)$ is a molecular state made from the 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.
The photoproduction of $eta^{}$-mesons from different nuclei has been measured using the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in Bonn. Recent results on the in-medium properties of the $eta^{}$-meson, derived from the transparency ratio measurements, are presented. The absorption of the $eta^{}$-meson in nuclear matter is compared to the properties of the other mesons ($eta$ and $omega$).
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