Abstract
The PHENIX experiment discovered a large excess of low-pT direct photons in Au+Au collisions at 200 GeV compared to reference p+p collisions, which has been attributed to thermal radiation from the medium produced in the collisions. At the same time the excess photons show a large azimuthal anisotropy, expressed as Fourier coefficients v2 and v3. These surprising results have not yet been fully described by theoretical models. We will present the results obtained from real photons in the electromagnetic calorimeter and photons converted on the outer shell of the Hadron Blind Detector. PHENIX has also developed a new technique to identify conversion photons without assuming the radius where the conversion happened. This method greatly increases the available statistics and reduces systematic uncertainties.
Highlights
The main goal of colliding heavy ions is to study the the properties of Quark Gluon Plasma (QGP), and direct photons have long been considered a golden probe for that
Once photons from hadronic decays are subtracted, the so-called direct photons remain, which can come from hard scattering, thermal radiation of the QGP, thermal radiation of the hadron gas (HG), or other sources like Bremsstrahlung
The detectors involved in these measurements are the tracking system to measure the trajectories and momenta of charged particles, the electromagnetic calorimeter (EMCal) to determine the energy photons and electrons, and the Ring Imaging Cerenkov detector (RICH) to identify electrons
Summary
The main goal of colliding heavy ions is to study the the properties of Quark Gluon Plasma (QGP), and direct photons have long been considered a golden probe for that. Direct photons are “color blind” probes, which means they will not interact strongly with the medium leave the medium unscathed. Direct photons are produced at almost all known or conjectured stages of the collision. By measuring direct photons one has access to the information about the entire evolution of the colliding system. Once photons from hadronic decays are subtracted, the so-called direct photons remain, which can come from hard scattering, thermal radiation of the QGP, thermal radiation of the hadron gas (HG), or other sources like Bremsstrahlung. More differential measurements may be needed to help disentangle the multiple sources and separate the QGP thermal signal from the rest
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