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Register a new userLUXE: A new experiment to study non-perturbative QED in $e^{-}$-laser and $gamma$-laser collisions
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Ruth Jacobs
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The LUXE experiment (Laser Und XFEL Experiment) is a new experiment in planning at DESY Hamburg using the electron beam of the European XFEL. LUXE is intended to study collisions between a high-intensity optical laser and up to 16.5 GeV electrons from the Eu.XFEL electron beam, or, alternatively, high-energy secondary photons. The physics objective of LUXE are processes of Quantum Electrodynamics (QED) at the strong-field frontier, where QED is non-perturbative. This manifests itself in the creation of physical electron-positron pairs from the QED vacuum. LUXE intends to measure the positron production rate in a new physics regime at an unprecedented laser intensity. Parasitically, the high-intensity Compton photon beam of LUXE can be used to search for physics beyond the Standard Model.
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The vast majority of QED results are obtained in relatively weak fields and so in the framework of perturbation theory. However, forthcoming laser facilities providing extremely high fields can be used to enter not-yet-studied regimes. Here, a scheme is proposed that might be used to reach a supercritical regime of radiation reaction or even the fully non-perturbative regime of quantum electrodynamics. The scheme considers the collision of a 100 GeV-class electron beam with a counterpropagating ultraintense electromagnetic pulse. To reach these supercritical regimes, it is unavoidable to use a pulse with ultrashort duration. Using two-dimensional particle-in-cell simulations, it is therefore shown how one can convert a next-generation optical laser to an ultraintense ($Iapprox 2.9times 10^{24} text{ W} , text{cm}^{-2}$) attosecond (duration $approx$ 150 as) pulse. It is shown that if the perturbation theory persists in extremely fields, the spectrum of secondary particles can be found semi-analytically. In contrast, a comparison with experimental data may allow differentiating the contribution of high-order radiative corrections if the perturbation theory breaks.
A nonlinear interaction between photons is observed in a process that involves charge sources. To observe this process in a vacuum, there are a growing number of theoretical and experimental studies. This process may contain exotic contribution from new physics beyond the Standard Model of particle physics, and is probed by experiments using a high-power laser or a high-field magnet, and more recently using an X-ray Free-Electron Laser (XFEL). Here, we review the present status of our experiments testing various vacuum processes. We describe four experiments with a focus on those using an XFEL: (i) photon-photon scattering in the x-ray region, (ii) laser-induced birefringence and diffraction of x rays, (iii) vacuum birefringence induced by a high-field magnet, and (iv) a dedicated search for axion-like particles using the magnet and x rays.
The MEG experiment took data at the Paul Scherrer Institute in the years 2009--2013 to test the violation of the lepton flavour conservation law, which originates from an accidental symmetry that the Standard Model of elementary particle physics has, and published the most stringent limit on the charged lepton flavour violating decay ${mu}^+ rightarrow {rm e}^+ gamma$: BR(${mu}^+ rightarrow {rm e}^+ gamma$) $<4.2 times 10^{-13}$ at 90% confidence level. The MEG detector has been upgraded in order to reach a sensitivity of $6times10^{-14}$. The basic principle of MEG II is to achieve the highest possible sensitivity using the full muon beam intensity at the Paul Scherrer Institute ($7times10^{7}$ muons/s) with an upgraded detector. The main improvements are better rate capability of all sub-detectors and improved resolutions while keeping the same detector concept. In this paper, we present the current status of the preparation, integration and commissioning of the MEG II detector in the recent engineering runs.
We compute the non-perturbative contribution of semileptonic tensor operators $(bar q sigma^{mu u} q)(bar ell sigma_{mu u} ell)$ to the purely leptonic process $mu to e gamma$ and to the electric and magnetic dipole moments of charged leptons by matching onto chiral perturbation theory at low energies. This matching procedure has been used extensively to study semileptonic and leptonic weak decays of hadrons. In this paper, we apply it to observables that contain no strongly interacting external particles. The non-perturbative contribution to $mu to e $ processes is used to extract the best current bound on lepton-flavor-violating semileptonic tensor operators, $Lambda_text{BSM} gtrsim 450$ TeV. We briefly discuss how the same method applies to dark-matter interactions.
The analysis of a combined data set, totaling 3.6 times 10^14 stopped muons on target, in the search for the lepton flavour violating decay mu^+ -> e^+ gamma is presented. The data collected by the MEG experiment at the Paul Scherrer Institut show no excess of events compared to background expectations and yield a new upper limit on the branching ratio of this decay of 5.7 times 10^-13 (90% confidence level). This represents a four times more stringent limit than the previous world best limit set by MEG.
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