Boiling the vacuum
A group of scientists has set out to shed light on an unexplored area of physics using a high-energy electron beam, a laser, a vacuum chamber and a set of detectors. Federico Meloni, scientist at Germany’s national accelerator lab DESY and since December professor at the University of Bern, explains what the “LUXE” experiment is all about.
Quantum electrodynamics (or QED), which describes the interactions of charged particles and light, is one of the best tested theories in nature. In fact, it is one of the most accurate physical theories we have: its predictions and measurements agree to an almost unbelievable level. However, the predictive power of QED relies on perturbative calculations and, when particles interact with strong electromagnetic fields, these calculations are expected to break down. The so-called Schwinger critical field – the point where an electric field reaches a strength at which the vacuum itself becomes unstable and starts to produce electron–positron pairs out of the field energy – marks the entrance in the regime of strong-field QED. Until recently, it was impossible to reach such a field in a lab, but the advent of high-power lasers has created an opportunity to explore this new region of physics.
Charting this unknown domain of QED would be interesting in itself – any physical effect that nobody has ever studied is interesting for physicists. However, understanding it better would also have implications for other areas of science. It can expand our understanding of astrophysical phenomena involving neutron stars or the event horizons of black holes, improve our understanding of plasmas relevant in fusion research and wakefield acceleration, and allow us to gain new insights in the beam dynamics of future colliders that collide electrons, positrons or muons.
“We don’t know much about QED in strong fields,” says DESY scientist and particle physicist Federico Meloni, who has recently started a joint appointment as professor at the University of Bern. He is part of a group of some 100 scientists from over ten countries who have set out to fill this knowledge gap with an idea for a new experiment…
Enter LUXE. LUXE, short for “Laser Und XFEL Experiment” (laser and XFEL experiment) is a planned experiment that would be installed at a new beamline of the European XFEL, the X-ray free-electron laser facility that runs from the DESY campus in Hamburg in Germany to the neighbouring village of Schenefeld. The European XFEL accelerates electrons to up to 17.5 GeV before sending them through a set of magnets that make them produce X-rays for all kinds of experiments from materials science to biochemistry. LUXE would sit downstream of the linear accelerator and use electrons directly from this accelerator in a section of underground complex that is currently empty. And it wouldn’t be a particle physics experiment if there wasn’t some synergy that can be used: the European XFEL is interested in widening its scope to scientific experiments in areas like fusion research, which would need high-power lasers … as does LUXE. Two birds, one stone.
It’s such a high-power laser that would approach the Schwinger regime. “We know that this intensity itself cannot be reached; it would have to reach a field strength of more than 1018 Volts per metre – this can’t be produced in any lab,” Meloni, who is deputy spokesperson of the LUXE experiment, explains. “The nearest we can get to it is with high-power lasers that can reach about 1014V/m, but the relativistic contraction of lengths of a high energy beam can help us reach these extreme regimes!” By using the high-energy particles from the XFEL beam they plan to surpass the Schwinger threshold in order to then study and compare the way particles are produced, the way they behave and how they combine to form new ones. “We know Compton scattering very well and we want to use this knowledge to learn how the phenomenon varies in the presence of strong fields,” he says.
The team plans to use a diverse range of particle detectors from different branches of physics – tracking detectors, Cherenkov counters, and calorimeters borrowed from particle physics, for example, or scintillating screens also used in photon science experiments like those done regularly at the European XFEL. “We want redundancy, so we will try to measure the same observables with different detection systems. We also want high precision to characterise the phenomenology as function of beam and laser parameters in order to see possible deviations.” Most of these detection systems have been tested at various light sources and test facilities around the world, some have undergone design revisions, but are now validated and soon ready to go.
Meloni, who comes from northern Italy, is the first professor in a joint appointment of a Swiss university with DESY. However, it’s not his first affiliation with a Swiss institution. After his studies and PhD at the University of Milan working on data analysis and vertex reconstruction at the ATLAS experiment at CERN, he spent four postdoc years in Bern designing, constructing, testing and calibrating the upgrade of the readout of the ATLAS pixel tracking detector as well as searching for supersymmetry. He joined DESY in 2018 as part of the local ATLAS group and used to divide his time between working on ATLAS, LUXE and future colliders. Now teaching has been added to this list: “I am really looking forward to this new aspect,” he says. “My first graduate-level course will start soon!”
The LUXE scientists are engaged in intense discussions with the European XFEL management towards a decision on the construction of ELBEX, the new beamline needed by the experiment – the XFEL council will ultimately select if the project goes ahead. The team hopes that the synergies with fusion research will encourage the council and DESY to soon turn this programme into reality.
Barbara Warmbein