Testing Strong Field QED Close to the Fully Nonperturbative Regime Using Aligned Crystals.

Processes occurring in the strong field regime of QED are characterized by background electromagnetic fields of the order of the critical field F_{cr}=m^{2}c^{3}/ℏ|e| in the rest frame of participating charges. It has been conjectured that if in their rest frame electrons and positrons experience field strengths of the order of F_{cr}/α^{3/2}≈1600F_{cr}, with α≈1/137 being the fine-structure constant, their effective coupling with radiation becomes of the order of unity. Here we show that channeling radiation by ultrarelativistic electrons with energies of the order of a few TeV on thin tungsten crystals allows us to test the predictions of QED close to this fully nonperturbative regime by measuring the angularly resolved single photon intensity spectrum. The proposed setup features the unique characteristics that essentially all electrons (1) undergo at most a single photon emission and (2) experience at the moment of emission and in the angular region of interest the maximum allowed value of the field strength, which at 2 TeV exceeds F_{cr} by more than 2 orders of magnitude in their rest frame.

Observation of Quasichanneling Oscillations.

We report on the first experimental observations of quasichanneling oscillations, recently seen in simulations and described theoretically. Although above-barrier particles penetrating a single crystal are generally seen as behaving almost as in an amorphous substance, distinct oscillation peaks nevertheless appear for particles in that category. The quasichanneling oscillations were observed at SLAC National Accelerator Laboratory by aiming 20.35 GeV positrons and electrons at a thin silicon crystal bent to a radius of R=0.15 m, exploiting the quasimosaic effect. For electrons, two relatively faint quasichanneling peaks were observed, while for positrons, seven quasichanneling peaks were clearly identified.

Observation of deflection of a beam of multi-GeV electrons by a thin crystal.

We report on an experiment performing channeling and volume reflection of a high-energy electron beam using a quasimosaic, bent silicon (111) crystal at the End Station A Test Beam at SLAC. The experiment uses beams of 3.35 and 6.3 GeV. In the channeling orientation, deflections of the beam of 400 µrad for both energies with about 22% efficiency are observed, while in the volume-reflection orientation, deflection of the beam by 120 µrad at 3.35 GeV and by 80 µrad at 6.3 GeV is observed with 86%-95% efficiency. Quantitative measurements of the channeling efficiency, surface transmission, and dechanneling length are taken. These are the first quantitative measurements of channeling and volume reflection using a primary beam of multi-GeV electrons.