Using the funding made available to the Cluster of Excellence "Precision Physics, Fundamental Interactions and Structure of Matter" (PRISMA+), Johannes Gutenberg University Mainz (JGU) is currently constructing a new electron accelerator on the Gutenberg Campus. The MESA accelerator and the key experiments will extend across several stories below ground including a new experimental hall provided by the new research building "Center for Fundamental Physics". In 2024, an electron beam is expected to be generated with MESA for the first time.
In linear accelerators or "linacs" like MESA, electrically charged particles are accelerated with the help of alternating electromagnetic fields. In the various experimentation stations, these particles are made to collide with the atomic nuclei of the targets, from which they are scattered to detectors that measure their impulse or energy, as well as other physical parameters. This enables researchers to explore the structure of complex particles. It is planned to use the MESA accelerator to measure, among other things, the radius of the proton, something that has baffled over recent years. Furthermore, accelerators also make it possible to observe the short-lived particles that are sometimes generated by the collisions. Thus, for example, MESA will contribute to the search for dark photons. Once discovered, these could provide information on the structure of dark matter, which makes up a large proportion of our universe.
MESA is an ERL or energy recovery linac that will be highly efficient because of its very low energy consumption. The accelerator will be able to generate a beam of extremely high intensity that would otherwise require massive amounts of power so that a great number of particles can be focused on a tiny area of the target. It will thus be possible to produce a correspondingly high rate of particle collisions within a short period of time. This will aid in the hunt for very rare events such as the consequences of the decay of dark photons and of other, as yet, unidentified particles. In addition, the quality of the beam produced by MESA will be second to none; all electrons will be provided with the exact same kinetic energy of 155 MeV in the accelerator. These are optimal conditions for important precision experiments.
The MESA accelerator will offer ideal conditions in which scientists will be able to explore the limits of the Standard Model of particle physics. The Standard Model describes the properties and interactions of elementary particles - in other words, the smallest components of matter, such as electrons, neutrinos and quarks. The Standard Model provides explanations for many of the natural processes that occur in the universe - however, there is a multitude of questions that still remain to be answered, such as:
Precision experiments at the MESA accelerator play a crucial role answering these questions. Several key experiments are currently under development, two of them, P2 and Magix, have already entered the advanced development phase.
P2 is an experiment designed for the precise measurement of an important natural constant - the mixing angle of the electroweak interaction. Measuring the mixing angle is essential, because it will provide us with information on the fundamental properties of the interactions between elementary particles. In addition, if the weak mixing angle can be precisely measured, it provides insight into other aspects of physics, such as dark matter or hitherto unknown forces in nature. P2's target relative accuracy of 0.1% exceeds existing measurements in low energy processes by more than one order of magnitude. This increase in accuracy regarding low energy processes is especially important in the search for clues beyond the standard model and can thus supply new evidence beyond the standard model. The P2 experiment can measure the effects of new particles in a range of 50 MeV to 6.4 TeV and is complementary to the direct search for such particles at the LHC.
MAGIX is a multi-purpose spectrometer which allows the precise measurement of the proton's form factors at the lowest impulse transfer rates. This will contribute decisively to the clarification of the existing contradictions in the experimental determination of the proton radius (the so-called proton radius puzzle) and to the search for clues for dark matter. With its unique combination of a spectrometer system and a windowless gas target, MAGIX is an absolute pioneer in experimental design. The experiment takes place in the energy-recovering arm of MESA. A gas stream with supersonic speed is introduced into the beam, which then collides with a small fraction of the accelerated electrons. By means of two magnetic spectrometers, which can be rotated on a circular path around the collision point, it is possible to measure the angle and impulse of the deflected electrons.
The generation of dark matter particles and the detection of their decay signatures is the objective of the beam dump experiment BDX@MESA. The accelerator's beam dump is used as a target for the production of dark matter particles. In this case, these would be highly energized, so that the mass sensitivity should reach the MeV range, as long as the accelerator's full collision rate (MESA >1022 electrons at the target) is used. Mainz scientists are cooperating with the BDX collaboration, which analyzes beam dump experiments from electron accelerators worldwide, looking for the exclusion limits of dark matter. A future beam dump experiment with MESA will increase the sensitivity to light dark matter significantly. This experiment, which will be running parallel to the P2 data collection, profits from the extremely high intensity of the MESA accelerator.
MESA is a particular kind of particle accelerator known as an ERL, or energy recovery linac, a system that has only recently been developed. With this technology, the accelerated electron's energy can be won back and two thirds of the energy conventional accelerators require can be saved. MESA will be the first accelerator worldwide using this superconductive, energy-recovery technology for research purposes and can top the efficiency of existing accelerators many times over. In ERL mode, up to 1,035 electrons per second will hit one square centimeter of a target - even more than in the Large Hadron Collider (LHC) in Geneva. MESA's role as a pioneer means it will additionally serve as a testing environment for other research facilities, such as the planned LHeC (Large Hadron Electron Collider) in Geneva.
There are two modes of operation that will be used in the various experiments. In the extracted beam mode, which will be used primarily for the P2 experiment and the BDX@MESA, so-called spin-polarized electrons are directed at the target in the P2 experiment.
The use of the MESA accelerator's energy-recovery mode is the first chance worldwide to operate a high intensity ERL beam in combination with an internal gas target. This leads to extremely clean conditions for precision experiments with the multipurpose spectrometer MAGIX. Because of this, the electrons transfer their energy to the SRF system before they actually leave the system - the principle is basically similar to that of a car with a hybrid drive, in which regenerative brakes charge the main battery.
The electron beam is produced at the particle source and accelerated with 100,000 volts, which is equivalent to around half the speed of light. Using magnets, the beam is transported to the next station, the pre-accelerator, and fit to its requirements.
The pre-accelerator is made up of four high frequency sectors. It accelerates the low energy electron beam to the MESA accelerator's injection energy by transmitting an impulse to the electrons when necessary.
In order to be able to control the electron movement in the accelerator, a large number of electromagnets are needed. With their help, the electron beam is both concentrated and steered.
These are the backbone of the MESA accelerator and are used to supply the electrons with more and more energy by accelerating them in an electromagnetic field. In order to keep the acceleration efficient, the structures need to be cooled to 2 Kelvin (-271°C) using liquid helium.
This is where the SRF system is cooled down. The design is similar to the design of a thermos flask: the GSRF-system swims in liquid helium inside an internal vessel, and a vacuum is created between this vessel and the external tank in order to prevent warmth from entering.
Cooling the superconducting cavities down to low temperatures using nitrogen (typically around 77 K) and liquid helium (down to 2 K) is an important step towards building the MESA particle accelerator at the PRISMA+ Cluster of Excellence.
After successfil cooldown of the MESA cryomodules the next experiments will be the high power radio-frequency tests including the measurements of the quality factors of the used superconducting cavities. As x-ray and gamma-rays can occur in high field operation, prior to the planned activities the radiation shielding of the module using lead and concrete blocks had to be completed.
We already know quite a bit about the smallest building blocks of matter and of the forces that shape the universe. However, we also know that there is a lot more to discover. Many questions are yet open, such as that of dark matter: what does it consist of, and what is dark energy? Why does the universe contain more matter than antimatter? And what role do the mysterious ghost particles - or neutrinos - play in the early universe?
Financed with funds from the Cluster of Excellence PRISMA+ - Precision Physics, Fundamental Interaction and Structure of Matter, Johannes Gutenberg University Mainz is constructing a new electron accelerator.