Speaker
Friederike Bock
(Ruprecht-Karls-Universitaet Heidelberg (DE))
Description
Unlike hadrons, direct photons are produced in all stages of a
nucleus-nucleus collision and therefore test our understanding of the
space-time evolution of the produced medium. Of particular interest
are
so-called thermal photons expected to be produced in a quark-gluon
plasma and the subsequent hadron gas.
The transverse momentum spectrum
of thermal photons carries information about the temperature of the
emitting medium. The effect of Doppler blueshift on photons spectra
from later and colder stages of a
collision, however, potentially
complicates the extraction of the temperature.
In this presentation,
direct-photon spectra in the range $1 < p_T < 12$ GeV/c from Pb-Pb
collisions at
$\sqrt{s_{NN}} = 2.76$ TeV will be shown. The results
were obtained with two independent methods:
by measuring photons with
the electromagnetic calorimeter PHOS and by measuring $e^+e^-$ pairs
from
external conversions of photons in the detector material. The
measured direct-photon spectra will be
compared with predictions from
state-of-the-art hydrodynamic models. In addition, direct-photon
production
in p-Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV will be
discussed. In the standard hydrodynamical
modeling of nucleus-nucleus
collisions, thermal photons mostly come from the early hot stage of the
collision. As collective hydrodynamic flow needs time to build up, the
azimuthal anisotropy of
thermal photons quantified with Fourier
coefficient $v_2$ is expected to be smaller than the one for
hadrons. However, the PHENIX experiment and ALICE experiment observed
$v_2$ values of direct-photons
similar in magnitude to the pion
$v_2$. These unexpected observations constitute the so called
"direct-photon flow puzzle" as they challenge the standard
hydrodynamic picture of nucleus-nucleus
collisions and/or the standard
photon emissions rates in the quark-gluon plasma and the hadron
gas.
We will present the inclusive photon $v_2$ and $v_3$ in Pb-Pb
collisions at $\sqrt{s_{NN}} = 2.76$
TeV in the range $1 < p_T < 5$
GeV/c and discuss implications for the $v_2$ and $v_3$ of
direct-photons.
| On behalf of collaboration: | ALICE |
|---|
Author
Friederike Bock
(Ruprecht-Karls-Universitaet Heidelberg (DE))
