Abstract
We present the first measurement of the direct-photon elliptic flow vγ,dir2 in Pb-Pb collisions at = 2.76 TeV with data taken by the ALICE experiment at the LHC. The measurement provides evidence for a non-zero vγ,dir2 for 1 < pT < 3GeV/c with a magnitude similar to the observed charged pion . In order to explain the large inverse slope parameter Teff of the low pT direct-photon spectrum observed at LHC and RHIC, recent hydrodynamical descriptions of the direct-photon production include a substantial portion of thermal photons from the hot plasma phase. As a consequence of the early production time, vγ,dir2 is expected to be small compared to hadrons. A large vγ,dir2 might lend support for a significant direct-photon emission from late stages of the system evolution where hadron flow has developed.
Highlights
A unique tool for the study of the collision evolution in nucleus-nucleus collisions is the measurement of photons
The magnitude of decay photons is comparable to hadrons but the values are shifted in transverse momentum pT depending on the mass of the mother particles
3 GeV/c the inclusive photon v2γ,inc is significantly smaller than v2γ,bg which can only be explained by a direct-photon contribution with smaller v2γ,dir compared to v2γ,bg
Summary
A unique tool for the study of the collision evolution in nucleus-nucleus collisions is the measurement of photons. Besides photons from hadron decays direct photons are emitted at every stage of the system evolution. The direct-photon spectrum can be calculated from the measured inclusive photon spectra by subtraction of all contributions from hadron decays. Yield a significant the direct-photon contribution spectrum in of direct photons the 0–40 % most to the inclusive central Pb-Pb cpohlolitsoionnsspaetct√ras.NFNig=ur2e.176shToewVs. At transverse momenta above 4 GeV/c the signal is described by binary scaled NLO (pQCD)
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