Abstract
The elliptic flow of inclusive and direct photons was measured at mid-rapidity in two centrality classes 0–20% and 20–40% in Pb–Pb collisions at sNN=2.76 TeV by ALICE. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the detector material with the e+e− pairs reconstructed in the central tracking system. The results of the two methods were combined and the direct-photon elliptic flow was extracted in the transverse momentum range 0.9<pT<6.2GeV/c. A comparison to RHIC data shows a similar magnitude of the measured direct-photon elliptic flow. Hydrodynamic and transport model calculations are systematically lower than the data, but are found to be compatible.
Highlights
The theory of the strong interaction, Quantum ChromoDynamics (QCD), predicts a transition from ordinary hadronic matter to a new state where quarks and gluons are no longer confined to hadrons [1,2]
We present the first measurement of the directphoton flow in Pb–Pb collisions at the LHC and compare our findings to RHIC results and to predictions of hydrodynamic as well as transport models
They can be split into two groups: contributions related to the contamination and dependence of reconstruction, identification and smearing efficiency on the angle with respect to the event plane, and uncertainties related to the flatness of the event plane calculation, the event plane resolution and the contribution of non-flow effects
Summary
The theory of the strong interaction, Quantum ChromoDynamics (QCD), predicts a transition from ordinary hadronic matter to a new state where quarks and gluons are no longer confined to hadrons [1,2]. Since the observed direct-photon flow is the convolution of all stages of the collision, including the contribution from the initial stage when the flow pattern has not yet developed, the calculations predict much smaller azimuthal anisotropy for thermal photons than for hadrons [16,17]. In the second class of models [45,46,47], a new azimuthally asymmetric source of direct photons is considered like jet-matter interactions or synchrotron radiation in the field of colliding nuclei. These theoretical efforts considerably reduce the discrepancy, but consistent reproduction of both the direct-photon spectra and flow is still missing. We present the first measurement of the directphoton flow in Pb–Pb collisions at the LHC and compare our findings to RHIC results and to predictions of hydrodynamic as well as transport models
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