CompAZ: parametrization of the luminosity spectra for the photon collider

TL;DR

The paper addresses the need for accurate photon-photon luminosity spectra at the TESLA Photon Collider for reliable $\,\gamma\gamma$ studies. It proposes CompAZ, a simple analytical parametrization based on the Compton backscattering formula, augmented with nonlinear QED effects via $\tilde{x}$, angular correlations through $\rho$, electron rescattering, and two-photon scattering, with parameters fitted to full beam simulations by V. Telnov. The resulting model reproduces the high-energy photon energy distributions and average polarization across a range of electron-beam energies ($E_e$ in $\{100,\,250,\,400\}$ GeV) and enables fast Monte Carlo generation and cross-section calculations for $\gamma\gamma$ processes. While the model captures the high-energy region well, a fraction of events involving low-energy photons remains outside the parametrization, but the approach offers substantial speedups and flexibility over full simulations.

Abstract

A simple model, based on the analytical formula for the Compton scattering, is proposed to describe the realistic photon-energy spectra for the Photon Collider at TESLA. Parameters of the model are obtained from the full simulation of the beam by V.Telnov, which includes nonlinear corrections and contributions of higher order processes. Photon energy distribution and polarization, in the high energy part of the spectra, are well reproduced. Our model can be used for a Monte Carlo simulation of gamma-gamma events at various energies and for direct cross-section calculations.

Figures (13)

• Figure 1: Energy distribution for photons (left plot) and the $\gamma \gamma$ center-of-mass energy distribution (right plot) from full simulation of luminosity spectrum by V.Telnov TEL01 (solid line), compared to expectations for the simple Compton scattering (dashed line). For better comparison of shape, Compton spectra is scaled to the same height of the high energy peak.
• Figure 2: Two-dimensional energy distribution for two colliding photons, obtained from the full simulation by V.Telnov TEL01 (left plot) and the ratio of this distribution to the simple product of two one-dimensional energy spectra (right plot).
• Figure 3: Comparison of the photon energy distribution obtained from full simulation of luminosity spectrum by Telnov TEL01, with the fitted contributions of different processes considered in the described model, as indicated in the plot.
• Figure 4: Comparison of the photon energy distribution from the fitted parametrization with the distribution obtained from full simulation of luminosity spectra TEL01, for three electron beam energies, as indicated in the plot. Imposed cut on the energy of the second photon is 40, 150 and 260 GeV respectively.
• Figure 5: Normalization of the CompAZ parametrization of the photon energy distribution, relative to the distribution obtained from full simulation of luminosity spectrum TEL01, as a function of the electron beam energy. Also shown are normalizations of separate processes considered in the model.