Role of magnetic field in photon excess in heavy ion collisions

The synchrotron photon spectrum in heavy ion collisions is computed taking into account the spatial and temporal structure of the magnetic field. It is found that a significant fraction of photon excess in heavy ion collisions in the region $k_{\perp }=1\text{–}3$ GeV can be attributed to the synchrotron radiation. Azimuthal anisotropy of the synchrotron photon spectrum is characterized by the Fourier coefficients $v_2=4/7$ and $v_4=1/10$ that are independent of photon momentum and centrality.

Figure 1

Spectrum of synchrotron photons averaged over the azimuthal angle versus photon transverse momentum $k_{\perp }$ at rapidity $y=0$ and centrality $0\%\text{–}20$% $(b=4.3\mathrm{fm}$ [27]). Solid line: $T=400$ MeV; dashed line: $T=200$ MeV. Data are from [1]; they represent the direct photon $k_T$ spectra after subtraction of the Ncoll scaled $p+p$ contribution (Fig. 8 there).

Figure 2

Spectrum of synchrotron photons averaged over the azimuthal angle versus photon transverse momentum $k_{\perp }$ at rapidity $y=0$ and centrality $40\%\text{–}60$% $(b=10.2$ fm [27]). Solid line: $T=400\mathrm{MeV}$; dashed line: $T=200$ MeV. Data are from [1]; they represent the direct photon $k_T$ spectra after subtraction of the Ncoll scaled $p+p$ contribution (Fig. 8 there).

Figure 3

Time evolution of the photon spectrum (emitted by $u$ and $\overline{u}$ quarks) from $t=1$ fm (the lowest line) to $t=10$ fm (the highest line) in time increments of 1 fm. $T=400$ MeV, $0\%\text{–}20\%$ centrality, $y=0$.