Estimation of the Inverse Compton Scattering Background in MeV Gamma-Gamma Collider

TL;DR

The MeV Gamma-Gamma Collider aims to study elastic light-by-light scattering and real-photon Breit-Wheeler production in the MeV regime. The authors introduce GBET, a Monte Carlo tool that directly simulates two consecutive inverse Compton scatterings to preserve full particle-level information and improve physical fidelity over luminosity-spectrum chaining. Benchmarking against CAIN and theory shows GBET accurately reproduces Compton photon yields and luminosities, while providing detailed background rates across photons and leptons, including N_gamma ≈ 1.5e12/s and L_gamma_gamma ≈ 5.1e28 cm^-2 s^-1. The study also quantifies background contributions from Møller scattering and Breit-Wheeler production, offering concrete numbers to guide detector design and beam-laser optimization, and it notes limitations such as neglecting nonlinear Compton effects and polarization for future work.

Abstract

The MeV Gamma-Gamma Collider would provide a direct experimental platform for elastic light-by-light scattering ($γγ\to γγ$) and the Breit-Wheeler process with two real photons ($γγ\to e^+e^-$). A Monte Carlo code, the Genie Background Evaluation Tool (GBET), has been developed to fully simulate two successive inverse Compton scatterings in the linear regime, including $e^- + \text{laser} \to e^-+ γ$ and $e^- + γ\rightarrow e^- + γ$. GBET overcomes the inherent information loss in traditional luminosity-spectrum-based chain simulations, preserving full particle-level information and achieving higher physical fidelity. The effectiveness of the code is verified by benchmarking against the simulation results of CAIN. GBET shows that the event rate of background photons generated by the first inverse Compton scattering is $6.24 \times 10^7$/s, with energies below 18 eV; the second inverse Compton scattering generates background electrons at 51.99/s and photons at 0.99/s, both with energies below 11 MeV. In addition, Møller scattering contributes background electrons at 0.56/s with energies around 200 MeV. The count rates of background electrons and positrons originating from the Breit-Wheeler process are 1312.2/s and 1314.3/s, respectively, with energy distributions ranging from 511 to 720 keV.

Figures (11)

• Figure 1: Schematic diagram of colliding beams in the Gamma-Gamma Collider. The laser pulse and electron beam are scattered at the CP to produce Compton photons. The Compton photons and electrons are brought to the IP and produce $e^-\gamma$, $\gamma\gamma$ and $e^-e^-$ collisions. The detector surrounding the beams consists of a scintillator array composed of CsI and plastic scintillators (PS), with inner and outer radii of $R_1 = 15 \, \text{cm}$ and $R_2 = 22 \, \text{cm}$, respectively, and lengths of $L_1 = 31.92 \, \text{cm}$ and $L_2 = 46.809 \, \text{cm}$. The entire detector is located inside a vacuum chamber. The angular range of the detector is set to $\pi / 6$ to $5\pi / 6$, particles in this angular range will be detected.
• Figure 2: Cross section of inverse Compton scattering. Variation trend of total cross section with CM energy $\sqrt{s_{e\gamma}}$ : The CM energy for the first inverse Compton scattering is around 0.511 MeV, with a total cross section of $6.628 \times 10^{-29}$$\mathrm{m^2}$. The CM energy for the second inverse Compton scattering ranges from 0.511 MeV to 24 MeV, with the corresponding total cross sections over a range from $6.628 \times 10^{-29}$$\mathrm{m^2}$ to $1.855 \times 10^{-31}$$\mathrm{m^2}$.
• Figure 3: Energy and angular distribution of scattered particles on both sides after inverse Compton scattering (in the Thomson regime) of electrons ($E_e\approx200~\mathrm{MeV}$) and lasers ($E_L\approx 1.18~\mathrm{eV}$): (a) Electron energy spectrum. The electron energy is mainly concentrated around 199 MeV. Electrons lose hundreds of keV of energy in a single scattering, and some electrons will undergo multiple scatterings; (b) Electron scattering angle distribution. The electron scattering angle is in the order of $\mu$rad, so its propagation direction remains nearly unchanged; (c) Gamma photon energy spectrum. The energies of the gamma photons are on the order of 100 keV, with the Compton edge is about 720 keV. The overall energy spectrum shows a continuous wide spectrum distribution, with Compton photon yield on one side is approximately $1.48 \times 10^{12}$/s; (d) Gamma photon scattering angle distribution. The gamma photon scattering angle is on the order of mrad and is concentrated around 2.5 mrad. The (a) and (d) only show the main distribution areas of the data. $L$: Left; $R$: Right. This notation is used consistently throughout the paper.
• Figure 4: The angular distribution of scattered particles relative to the z-axis: Both electrons and gamma photons tend to be emitted in the forward direction.The peak angle relative to the electron’s direction of motion (±z axis) is 2.5 mrad. (a) and (b) show the main distribution areas of scattered electrons and scattered gamma photons on both sides.
• Figure 5: Luminosity distributions and electron scattering count after inverse Compton scattering. (a) Gamma-gamma luminosity $\mathcal{L}_{\gamma\gamma}$. The maximum CM energy of the Compton photons is approximately 1.5 MeV, and the total luminosity can reach $5.10 \times 10^{28} \, \mathrm{cm^{-2} \, s^{-1}}$; (b) Electron-electron luminosity $\mathcal{L}_{ee}$. The CM energy of the electron pairs is concentrated above 396 MeV. The inverse Compton scattering process induces only small energy loss and negligible deflection in the electrons, thereby preserving the electron-electron luminosity at a high value of approximately $1.18 \times 10^{28} \, \mathrm{cm^{-2} \, s^{-1}}$; (c) Electron-gamma luminosity $\mathcal{L}_{e\gamma}$. The electron-gamma luminosity exhibits a peak near 25 MeV in the CM energy spectrum, with a total luminosity of $2.38 \times 10^{28} \, \mathrm{cm^{-2} \, s^{-1}}$; (d) Electron scattering count $N_{\text{scat}}$. A substantial proportion of electrons undergo multiple scattering events, some even more than ten times.