The MESA accelerator combined with the MAGIX spectrometers provides unique opportunities for precission experiments in hadron and nuclear physics. The large beam current of an energy recovery accelerator allows for the use of a windowless gas target, leading to large luminosity at still very low resolution losses due to multiple scattering and other target effects. In addition, a gas target allows for the use of polarized target nuclei at modest luminosities.
The MAGIX physics program is centered on precision and resolution at the energy range below the pion production threshold. On this pages, a few examples are given to illustrate the possibilities of this setup:
The existence of a dark photon as light but massive exchange particle, which couples to the sector of dark matter, is a well motivated extension of the standard model of particle physics. This particle could explain several astrophysical observations, like the observed positron excess at higher energies, but could also solve well known puzzles like the discrepancy of the anomalous magnetic moment of the muon.
An extensive program for the search for dark photon is running at several accelerator laboratories, e.g. MAMI and JLab. But also MESA with MAGIX can contribute to this search in the low mass region. The high resolution of the spectrometers result in a similar high resolution for a dark photon, which would be produced radiatively in electron scattering off a heavy target nucleus and would decay in an electron pair.
Fig. 1 shows the exclusion limits which can be reached with the MAGIX spectrometers detecting the lepton pair for several different beam energies.
The magnetic and electric form factors have to be separated experimentally. This can be done by two different methods:
The double polarization measurement is superior to extract the magnetic form factor at low virtualities, is however limited e.g. by the detection of the polarization of the proton, which requires a large momentum.
The MAGIX setup however is optimized for very low momenta, and by using a polarized target and the polarized beam of MESA, the double polarization can be measured to very low virtualities, necessary for example to determine the magnetic radius of the proton, which is defined as the slope of the magnetic form factor at photon virtuality zero.
Fig. 2 shows the result of a simulation for such an experiment at MAGIX. MAGIX can reach virtualities down to q2=-0.005 GeV2/c2, with six settings. Fig. 3 shows the expected error bars on the ratio of the magnetic and electric form factor. As can be seen, much lower values of the four-momentum transfer can be reached at the low energy accelerator MESA.
In recent years, considerable progress has been made from theory side to describe nuclei starting from first principles. These so called ab inito calculations use for example Chiral effective field theory to describe the interaction between nucleons inside a nucleus.
Up to now, these theories are only tested with static properties of the nucleus, manly the excitation spectrum and ground state masses. In electroproduction, however, e.g. by single nucleon knock-out, much more multi-dimensional information on the wave function of the nucleons inside the nucleus can be determined.
The problem is, however, that chiral effective theories are very limited in their convergence in the energy range, making it up to know nearly impossible to determine e.g. polarization observables of such processes. MAGIX has the advantage of being optimized for low energies, and with a windowless gas target very low energy recoil particles can be detected with a detector close to the target.
Starting with light nuclei, e.g. 2Ha and 3,4He also important precision measurements of polarizability corrections are possible, which are needed to determine the charge radii of light nuclei to compare with the radii extracted by spectroscopy of muonic atoms, giving an important contribution to the well known proton radius puzzle.
The capture of an α particle by a carbon nucleus is one of the most important reactions to describe the nucleo-synthesis in the burning of a star, especially to model the ratio of carbon and oxygen in the star. The measurement of this cross section is however very demanding, since the cross section drops dramatically at very low energies. Between the lowest up to now reached center of mass energy of 1MeV and the point of helium burning at 0.3 MeV the cross section drops by nearly six orders of magnitude (see fig. 3).
MAGIX can address this problem my measuring the time reversed reaction 12C + α→ 16O + γ. The cross section of this reaction is one order of magnitude larger. The photon is produced by electron scattering with very low four-momentum transfer, where the photon is “quasi-real”. The center of mass energy can be determined very accurately by the electron detection by a high resolution spectrometer. By the use of a windowless gas target MAGIX has the advantage that the recoil α particle can be detected by silicon strip detectors in coincidence with the spectrometers and with sufficient resolution to identify the reaction. The cross section is nevertheless small, first simulations showed however that the luminosity of MESA is sufficient for a significant experiment.
The experiments on direct detection of dark matter concentrate up to now on the detection of recoil signals from elastic scattering of dark matter particles from the galactic “Dark Matter Wind” with the detector material and have seen no confirmed signal up to now.
These kind of experiments is limited to the lower mass range by the minimal recoil energy of the target nucleus required to produce a detectable signal. One aproach to overcome this limit is to produce dark matter particles with an accelerator. In this case, the recoil can be tuned by the accelerator energy and is considerable larger, requiring only much simpler detectors as for the direct detection experiments.
The reaction mechanism might be the already mentioned production via a dark photon which decays to light dark matter particles. In that case, stoping a beam of electrons allways produces a collimated beam of dark matter particles. Such an experiment can be installed at MESA. A detector, e.g. on the basis of standard scintillator bars, has to be placed well shielded behind the beam dump of the P2 experiments. It has been shown that the high external current of MESA can be used for this experiment leading to a sufficient large number of particles-on-target.