Thu·Jul 28, 2011Thursday·July 28, 2011

1. 01*

   Exclusive Measurement of the eta --> pi+ pi- gamma Decay

   P. Adlarson🇸🇪 · C. Adolph🇩🇪 · W. Augustyniak🇵🇱 · W. Bardan🇵🇱 · M. Bashkanov🇩🇪 · T. Bednarski🇵🇱 · F.S. Bergmann🇩🇪 · M. Berłowski🇵🇱 · H. Bhatt🇮🇳 · K.-T. Brinkmann🇩🇪 · M. Büscher🇩🇪 · H. Calén🇸🇪 and 110 other authors

   An exclusive measurement of the decay eta --> pi+ pi- gamma has been performed at the WASA facility at COSY. The eta mesons were produced in the fusion reaction pd --> 3He X at a proton beam momentum of 1.7 GeV/c. Efficiency corrected differential distributions have been extracted based on 13340\pm140 events after background subtraction. The measured pion angular distribution is consistent with a relative p-wave of the two-pion system, whereas the measured photon energy spectrum was found at variance with the simplest gauge invariant matrix element of eta --> pi+ pi- gamma. A parameterization of the data can be achieved by the additional inclusion of the empirical pion vector form factor multiplied by a first-order polynomial in the squared invariant mass of the pi+ pi- system.

   nucl-exhep-exPLB(2012)·66 citations
2. 02*

   Direct photons in Au+Au collisions measured with the PHENIX detector at RHIC

   Richard Petti🇺🇸

   A major goal of experiments in heavy-ion physics is the characterization of the quark gluon plasma (QGP) produced in the collision of heavy ions at high energy. Direct photons are a particularly good probe of the produced medium because they do not interact strongly and so can escape the medium unmodified, carrying information about when the photon was produced. It is expected that direct photon contributions from different sources (QGP radiation, hard scattering, hadron gas radiation) dominate at different transverse momentum ranges. Low momentum direct photons are dominated by thermal radiation (both from the QGP and hadron gas), while high momentum direct photons dominantly come from hard parton scatterings in the initial collision. We present a summary of techniques to measure direct photons with the PHENIX detector, with a focus on low momentum direct photons through their external conversion to dilepton pairs.

   nucl-exJ.Phys.Conf.Ser.(2011)·8 citations
3. 03*

   Resonance interpretation of the bump structure in the $\gamma p\to K^{+}\Lambda (1520)$ differential cross section

   Ju-Jun Xie🇪🇸 · J. Nieves🇪🇸

   We investigate the $\Lambda (1520)$ photoproduction in the $\vec{\gamma }p\to K^{+}\Lambda (1520)$ reaction within the effective Lagrangian method near threshold. In addition to the "background" contributions from the contact, $t-$channel $K$ exchange, and $s-$channel nucleon pole terms, which were already considered in previous works, the contribution from the nucleon resonance $N^{\ast }(2080)$ (spin-parity $J^P=3/2^{-}$) is also considered. We show that the inclusion of the nucleon resonance $N^{\ast }(2080)$ leads to a fairly good description of the new LEPS differential cross-section data, and that these measurements can be used to determine some of the properties of this latter resonance.

   ↳ nucl-thhep-exhep-phnucl-exAIP Conf.Proc.(2012)·0 citations
4. 04*

   Precision calculation of threshold pi^- d scattering, pi N scattering lengths, and the GMO sum rule

   V. Baru🇩🇪 · C. Hanhart🇩🇪 · M. Hoferichter🇩🇪 · B. Kubis🇩🇪 · A. Nogga🇩🇪 · D. R. Phillips🇺🇸

   We use chiral perturbation theory (ChPT) to calculate the ${\pi }^{-}d$ scattering length with an accuracy of a few percent, including isospin-violating corrections both in the two- and three-body sector. In particular, we provide the technical details of a recent letter, where we used data on pionic deuterium and pionic hydrogen atoms to extract the isoscalar and isovector pion-nucleon scattering lengths $a^{+}$ and $a^{-}$. We study isospin-breaking contributions to the three-body part of $a_{{\pi }^{-}d}$ due to mass differences, isospin violation in the $\pi N$ scattering lengths, and virtual photons. This last class of effects is ostensibly infrared enhanced due to the smallness of the deuteron binding energy. However, we show that the leading virtual-photon effects that might undergo such enhancement cancel, and hence the standard ChPT counting provides a reliable estimate of isospin violation in $a_{{\pi }^{-}d}$ due to virtual photons. Finally, we discuss the validity of the Goldberger-Miyazawa-Oehme sum rule in the presence of isospin violation, and use it to determine the charged-pion-nucleon coupling constant.

   ↳ nucl-thhep-phnucl-exNPA(2011)·165 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.