Prospects for the production and detection of Breit-Wheeler tunneling positrons in Experiment 320 at the FACET-II accelerator

TL;DR

This work targets the first statistically significant observation of Breit-Wheeler tunneling positrons in the strong-field regime by analyzing Experiment 320 at FACET-II. It combines a high-granularity ALPIDE-based pixel tracker with advanced tracking algorithms (seed, CKF, KF with smoothing) and dual simulation pipelines (FullSim and FastSim) to overcome enormous beam-induced backgrounds while preserving a clean NBW positron signal. The study shows that, for realistic $a_0$ values, single NBW positrons can be reconstructed with energy resolution around $\sim$ $1.3\%$ and momentum resolution of $<2\%$, achieving a signal-to-background ratio sufficient to claim a measurement within a few hours of data-taking. The proposed detector and analysis framework thus enable a robust spectral characterization of NBW in the deep tunneling regime and provide a path forward for SF-QED experiments at LUXE-like facilities.

Abstract

The SLAC Experiment 320 collides 10 TW-class laser pulses with the high-quality, 10 GeV electron beam from the FACET-II RF LINAC. This setup is expected to produce a sizable number of $e^+e^-$ pairs via nonlinear Breit-Wheeler mechanism in the strong-field tunneling regime, with an estimated yield of ~0.01-0.1 pairs per collision. This small signal rate typically comes along with large backgrounds originating, e.g., from dumping the high-charge primary beam, secondaries induced by the beam halo, as well as photons and low-energy electrons produced in the electron-laser collision itself. These backgrounds may reach densities of O(100) charged particles per cm^2 (and even more neutral particles) at the surface of the sensing elements, making it a tremendous challenge for an unambiguous detection of single particles. In this work, we demonstrate how detectors and methods adapted from high-energy physics experiments, can enable this measurement. The solution presented is based on a highly granular, multi-layer, radiation-hard pixel detector paired with powerful particle-tracking algorithms. Using a detailed simulation of the existing experimental setup, we show how the false-positive rate due to background processes can be reduced by more than an order of magnitude relative to the expected signal after full reconstruction. Furthermore, we show that the high spatial tracking resolution achievable (<10 microns) allows for positron momentum measurements with a resolution of <2%, enabling spectral characterization of the nonlinear Breit-Wheeler process. Based on our extensive simulation, with a conservatively large background assumption, we show that it is possible to measure single Breit-Wheeler positrons in the coming data taking campaign of E320. That would be the first statistically significant observation and characterization of this elusive process in the (deep) tunneling regime.

Figures (38)

• Figure 1: A schematic illustration of the E320 experimental setup from the IP chamber to the dump, focusing on the key elements related to the NBW signal positron detection (the NCS electrons and NCS photons are also detected by E320).
• Figure 2: The expected NBW positron production rate versus the laser intensity, $a_0$. The relevant electron beam parameters used are: Gaussian profile with an average electron energy of $\mathcal{E}=10$ GeV, a transverse beam radius of $r=40~\mu{\rm m}$ (rms) and a longitudinal length of $20~\mu{\rm m}$ (rms). The laser pulse parameters are: duration of 50 fs (intensity FWHM) and a focal spot size of $2~\mu{\rm m}$ (FWHM).
• Figure 3: Left: an ALPIDE "stave" Abelevetal:2014dnaAGLIERIRINELLA2017583 including nine ($3 \times 1.5~{\rm cm}^2$) ALPIDE sensors flip-chip mounted on a flexible printed circuit that is glued to a carbon fibre cooling sheet and a mechanical support frame. Right: a schematic layout of a single stave.
• Figure 4: The default detector arrangement, where the staves are placed vertically in the closest approach to the vacuum exit window. Top: a full view around the vacuum exit window as seen from the accelerator's back wall. Bottom: a close-up view of the detector from the opposite side (the accelerator's aisle).
• Figure 5: The alternative detector arrangement, where the staves can be tilted and the detector is pushed further downstream the vacuum exit window, to allow larger acceptance. The tilt angle on the picture is chosen arbitrarily for illustration purposes.