Conformal Anomaly as a Source of Soft Photons in Heavy Ion Collisions

We introduce a novel photon production mechanism stemming from the conformal anomaly of $\mathrm{QCD}\times \mathrm{QED}$ and the existence of strong (electro)magnetic fields in heavy ion collisions. Using the hydrodynamical description of the bulk modes of QCD plasma, we show that this mechanism leads to the photon production yield that is comparable to the yield from conventional sources. This mechanism also provides a significant positive contribution to the azimuthal anisotropy of photons, $v_2$, as well as to the radial “flow.” We compare our results to the data from the PHENIX Collaboration.

Figure 1

The coupling of the conformal anomaly to the external magnetic field resulting in photon production.

Figure 2

The azimuthal anisotropy $v_2$ of the direct photons for different values of bulk viscosity corresponding to $C_{\zeta }$ in the range of $2.5–5$ calculated for minimum bias Au-Au collisions. The dashed line represents the results with $C_{\zeta }=4$. The black dots are the data from the PHENIX Collaboration [25] for minimum bias Au-Au collisions at $\sqrt{s}=200\mathrm{GeV}$.

Figure 3

The transverse momentum spectra of the produced direct photons for $C_{\zeta }=2.5$ calculated for minimum bias Au-Au collisions; see text for details.