Compton sources for the observation of elastic photon-photon scattering events

We present the design of a photon-photon collider based on conventional Compton gamma sources for the observation of elastic $\gamma \gamma$ scattering. Two symmetric electron beams, generated by photocathodes and accelerated in linacs, produce two primary gamma rays through Compton backscattering with two high energy lasers. The elastic photon-photon scattering is analyzed by start-to-end simulations from the photocathodes to the detector. A new Monte Carlo code has been developed ad hoc for the counting of the QED events. Realistic numbers of the secondary gamma yield, obtained by using the parameters of existing or approved Compton devices, a discussion of the feasibility of the experiment and of the nature of the background are presented.

Figure 1

Scheme of the $\gamma \gamma$ interaction. Two lasers (in red) impinge on two electron beams (in green) in two interaction points (Compton IP), generating primary gamma rays (in violet). The primary gamma rays interact in the $\gamma \gamma$ IP, generating secondary gammas.

Figure 2

Energies $E_3$ and $E_4$ and angle ${\theta }_4$ as a function ${\theta }_3$ for: (a) $E_1=E_2=0.5\mathrm{MeV}$, (b) $E_1=0.707\mathrm{MeV}$ and $E_2=0.353\mathrm{MeV}$, and (c) $E_1=1\mathrm{MeV}$ and $E_2=0.25\mathrm{MeV}$.

Figure 3

Semipolarized (linear polarization) and unpolarized total cross section of $\gamma \gamma$ scattering as a function of center of mass energy.

Figure 4

Differential cross section of $\gamma \gamma$ scattering vs angle for unpolarized photons at various values of the center of mass energy: (a) $\sqrt{s}=0.75\mathrm{MeV}$, (b) $\sqrt{s}=1\mathrm{MeV}$, (c) $\sqrt{s}=1.25\mathrm{MeV}$, (d) $\sqrt{s}=1.5\mathrm{MeV}$, (e) $\sqrt{s}=1.75\mathrm{MeV}$, and (f) $\sqrt{s}=2\mathrm{MeV}$.

Figure 5

Single shot number of events $N$ as function of the rms transverse dimension of the electrons and of the distance $L$ between Compton and $\gamma \gamma$ IP.

Figure 6

Electron normalized emittances ${ϵ}_{n,x}$, ${ϵ}_{n,y}$ and rms transverse ${\sigma }_x$, ${\sigma }_y$ dimensions along the beam line coordinate $z$.

Figure 7

Focusing stage of the electron beam. Rms transverse ${\sigma }_x$, ${\sigma }_y$ dimensions vs the beam line coordinate $z.Q_1$, $Q_2$, $Q_3$ represent the focusing gradients of the quadrupoles. In the inner window, the detail of the focus.

Figure 8

Electron phase space. (a) and (c) at the end of the linac. (b) and (d) at the Compton interaction point.

Figure 9

Compton gamma photons in the space domain. In blue, the whole gamma beam. In light blue, the photon beam after collimation and cut.

Figure 10

Compton spectra of (a) natural, (b) collimated at ${\theta }_{\text{acc}}=0.01\mathrm{rad}$.

Figure 11

Superposition of the beams in space at various instants (a) and instantaneous (b) and time averaged (c) center of mass energy dispersion. (1) $t=0\mathrm{ps}$. (2) $t=0.2\mathrm{ps}$. (3) $t=0.4\mathrm{ps}$. (4) $t=0.6\mathrm{ps}$. (5) $t=0.8\mathrm{ps}$. (6) $t=1\mathrm{ps}$.

Figure 12

Distribution of the $\gamma \gamma$ events in the laboratory as a function of the energy of the secondary particles $E=E_{3,4}$ and of the zenith angle $\theta$.

Figure 13

$\gamma \gamma$ event rate.

Figure 14

Comparison between $\gamma \gamma$, Breit-Wheeler and TPP cross sections.