Abstract
The elastic scattering of two real photons in vacuum is one of the most elusive of the fundamentally new processes predicted by quantum electrodynamics. This explains why, although it was first predicted more than eighty years ago, it has so far remained undetected. Here we show that in present-day facilities, the elastic scattering of two real photons can become detectable far off axis in an asymmetric photon-photon collider setup. This may be obtained within one day of operation time by colliding 1 mJ extreme ultraviolet pulses with the broadband gamma-ray radiation generated in nonlinear Compton scattering of ultrarelativistic electron beams with terawatt-class optical laser pulses operating at a 10 Hz repetition rate. In addition to the investigation of elastic photon-photon scattering, this technique allows us to unveil or constrain new physics that could arise from the coupling of photons to yet undetected particles, therefore opening new avenues for searches of physics beyond the standard model.
Highlights
In classical electrodynamics, light beams in vacuum pass through each other unaffected [1]
Following the seminal calculations of photon-photon scattering by Euler [24] and the full QED results by Karplus and Neuman [2,25,26,27], proposals to detect the elastic photon-photon scattering of synchrotron radiation [28] or of free electron laser (FEL) radiation [29,30] were suggested
There exist two main challenges to detecting the elastic scattering of real photons: (i) the smallness of the photonphoton scattering cross section, and (ii) the fact that photons are identical particles such that, e.g., the backscattering of two photons colliding head-on is indistinguishable from the case of no scattering
Summary
Experiments could only place an upper bound on the photon-photon scattering cross section [55,56,57,58]. A substantial number of scattering events can be triggered by requiring both that photon collisions occur frame with photponffiffieffiffinffiffieffiffirffiffigffiffiyffiffiffiiffiffinffiffiffitffihffiffiffieffiffifficffiffieffiffinffiffitffiffier-of-momentum εCM 1⁄4 εγεωð1 − cos θÞ=2 approaching (CM) mec, where the elastic photon-photon scattering cross section σ is close to its maximum [2], and that at least one of the colliding beams has a large photon density. In the considered asymmetric setup, the high energy of gamma photons allows us to attain εCM ≲ mec, while XUV beams provide a relatively large photon number density. After passing through the collimator, photons collide with an XUV pulse propagating in the xz plane with a 10° crossing angle, and scattered photons with jψsj > maxðjψγjÞ ≈ 0.1 mrad are subsequently detected (see Fig. 1). 1, where nεω is the XUV pulse photon density, τ is the duration of the XUV beam, and the sum is taken over all photons of the gamma-ray beam
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