# Direct Photon Production in High-Energy Heavy Ion Collisions within the Integrated Hydrokinetic Model

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Photon Emission Sources

#### 2.1. Prompt Photons

#### 2.2. Pre-Equilibrium and Thermal Photons

- The photons radiating from a meson gas. Here we base our consideration on the Reference [65]. We account for the hadronic reactions with $\pi $, K and $\rho $ mesons and ${K}^{*}$ resonances, where photons are produced. The corresponding emission rates are taken from [65] with one additional prescription for the chemically frozen evolution regime: in this case the contribution from each hadronic reaction has to be multiplied by $exp\left(\frac{\sum {\mu}_{i}(\tau ,x)}{T(\tau ,x)}\right)$, where ${\mu}_{i}(\tau ,x)$ represent the individual chemical potentials of hadrons participating in the reaction.
- The photons coming from in-medium $\rho $ mesons. Here we follow the results of the Reference [66]. The emission rate for these photons incorporates baryon (antibaryon) effects and in our analysis depends on the local baryon chemical potential ${\mu}_{B}(\tau ,x)$ (for chemical equilibrium expansion), and, in addition, on individual chemical potentials of hadrons involved in reaction (for chemically frozen evolution).
- The photons emitted in reactions with the three mesons $\pi $, $\rho $ and $\omega $ [19], such as $\pi \rho \to \omega \gamma $, $\pi \omega \to \rho \gamma $, and $\rho \omega \to \pi \gamma $. Again, for the chemically frozen regime the corresponding contributions should be multiplied by $exp\left(\frac{\sum {\mu}_{i}(\tau ,x)}{T(\tau ,x)}\right)$.
- The photons originating from $\pi \pi $ bremmstrahlung [66].

#### 2.3. Hadronization Photons

## 3. Results and Discussion

## 4. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**The photon spectra calculated within the iHKM for the RHIC $200A$ GeV and the LHC $2.76A$ TeV A+A collisions, in comparison with the corresponding experimental data [25,28]. The events from the centrality class c = 0–20% are analyzed. The solid lines are related to the simulations where the hadronization emission is taken into account, while the dashed lines represent the simulations where HE was ignored.

**Figure 2.**The total direct-photon ${p}_{T}$ spectra in the top RHIC energy Au+Au collisions, obtained by the summation of the different sets of components: the photon radiation calculated within the iHKM with chemically equilibrated evolution at the final stage of the expansion and with the Hadronization Emission (HE) contribution taken into account; the same as in previous case, but without the HE contribution; the photon radiation from the HKM with the chemically frozen evolution at the hadronic stage together with the contribution from the HE. The results for the three centrality classes, c = 0–20%, c = 20–40%, and c = 40–60% are shown. The spectra for the c = 0–20% centrality class are multiplied by 100, and the spectra for c = 20–40% are multiplied by 10. The experimental points are taken from [25].

**Figure 3.**The photon ${v}_{2}\left({p}_{T}\right)$ and ${v}_{3}\left({p}_{T}\right)$ flow harmonics behavior calculated in iHKM for the case of the top RHIC energy Au+Au collisions with the centrality c = 20–40% in comparison with the measured experimental data [71]. The solid lines are related to the simulations where the hadronization emission is taken into account, while the dashed lines represent the simulations where HE was ignored.

**Figure 4.**The dependency of the photon ${v}_{2}$ coefficients on ${p}_{T}$ calculated in iHKM for the case of the LHC Pb+Pb collisions at $\sqrt{{s}_{NN}}=2.76$ TeV with the centrality c = 0–40% in comparison with the measured experimental data [73]. The results obtained with and without taking into account the hadronization emission are demonstrated, as well as the results including only thermal photon component (without prompt photon contribution).

**Table 1.**The different contributions to the total direct-photon yield in iHKM for the top RHIC energy Au+Au collisions. The data for the three collision centrality classes together with the corresponding mean charged particle multiplicities $\langle \frac{d{N}_{ch}}{d\eta}\rangle $ are presented.

Centrality, % | $\langle \frac{{\mathit{d}\mathit{N}}_{\mathit{c}\mathit{h}}}{\mathit{d}\mathit{\eta}}\rangle $ | Prompt $\mathit{\gamma}$ | Thermal $\mathit{\gamma}$ | Hadroniz. $\mathit{\gamma}$ | Total $\mathit{\gamma}$ | $\mathit{a}\xb7{\langle \frac{{\mathit{d}\mathit{N}}_{\mathit{c}\mathit{h}}}{\mathit{d}\mathit{\eta}}\rangle}^{1.25}$ |
---|---|---|---|---|---|---|

0–20 | 567 | 0.2471 | 0.5000 | 0.5499 | 1.2970 | 1.2970 |

20–40 | 236 | 0.0905 | 0.1727 | 0.2314 | 0.4946 | 0.4337 |

40–60 | 98 | 0.0265 | 0.0456 | 0.0751 | 0.1472 | 0.1446 |

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Sinyukov, Y.; Shapoval, V. Direct Photon Production in High-Energy Heavy Ion Collisions within the Integrated Hydrokinetic Model. *J* **2022**, *5*, 1-14.
https://doi.org/10.3390/j5010001

**AMA Style**

Sinyukov Y, Shapoval V. Direct Photon Production in High-Energy Heavy Ion Collisions within the Integrated Hydrokinetic Model. *J*. 2022; 5(1):1-14.
https://doi.org/10.3390/j5010001

**Chicago/Turabian Style**

Sinyukov, Yuri, and Volodymyr Shapoval. 2022. "Direct Photon Production in High-Energy Heavy Ion Collisions within the Integrated Hydrokinetic Model" *J* 5, no. 1: 1-14.
https://doi.org/10.3390/j5010001