Hadron & Nuclear Physics – Physics JGU Mainz

Johannes Gutenberg University Mainz > Faculty 08 > Physics > Physics Research > Research Areas > Hadron & Nuclear Physics

What is the shape and size of the proton, of other hadrons, and of nuclei? We endeavor to answer these questions using a variety of experimental techniques. They range from form factor measurements in electron scattering using our unique electron accelerators MAMI and MESA via electron-positron collisions to charge radius measurements using muonic atoms. At the same time, we are developing the theoretical methods and performing the calculations needed to correct and interpret these measurements.

The physics program of the A1 collaboration with its 50 members from 20 countries is focused on researching hadron structure. The spectrometer setup is well-suited for exploring fundamental properties of the nucleon, such as charge distributions by elastic form factor measurements and generalized polarizabilities by virtual Compton scattering. Meson production gives access to the resonance structure of the nucleon and meson properties.

A photon incident on a nucleon couples to the nucleon electromagnetic current, causing it to radiate mesons if the photon energy is high enough. Such reactions, induced by circularly and linearly polarized real photons up to energies of 1.5 GeV, are the international A2 collaboration’s subject of study at MAMI.

Protons are one of the main building blocks of the visible universe. Together with neutrons, they make up the nuclei of every atom. Yet questions still exist about some of the proton’s most fundamental properties, such as its size, internal structure, and intrinsic spin. In December 2020, the CERN Research Board approved the first phase (“phase-1”) of a new experiment that will help settle some of these questions. AMBER (Apparatus for Meson and Baryon Experimental Research) will be the next-generation successor of the laboratory’s COMPASS experiment.

The Institute of High Energy Physics of the Chinese Academy of Sciences in Beijing operates an electron-positron collider called BEPC-II (Beijing electron-positron collider II). It is designed for center-of-mass energies between 2 and 5 GeV, which allows the study of charmonium with very high statistics. The design luminosity is 10³³cm^-2 s^-1. Since 2009, measurements at various collision energies are being performed using the BESIII (Beijing Spectrometer III) detector.

The MAGIX experiment (MAinz Gas-Internal Target Experiment), currently under construction, will consist of two magnetic spectrometers with a relative momentum resolution of order 10⁻⁴, provided a spatial resolution in the focal plane of the spectrometers of 100 μm is achieved. MAGIX will operate in beam-dump mode with solid targets and in ERL mode with a windowless internal gas-jet target.

This project is conducting a series of muonic X-ray measurements in medium- and high-Z nuclei at PSI, utilizing a high-purity germanium detector array, in-beam muon detectors, and a modern digital data-acquisition system. A novel hydrogen target for muon transfer was developed, enabling measurements with as little as a few micrograms of target material.

The P2 experiment is a magnetic spectrometer with the main goal of measuring the electroweak mixing angle at very low momentum transfer, which becomes accessible by measuring a parity-violating left-right asymmetry in elastic electron-proton scattering.

Protons and neutrons – collectively called nucleons – belong to the family of hadrons. They are composed of quarks and bound by the strong force that is mediated via gluons. The force is active between two quarks and shows an unusual behavior: It is very small when the quarks are at close distance, increases as the distance grows, and then remains constant even if the quarks are moved further and further away from each other.

How do quarks and gluons combine to form hadrons? What bound systems exist and what are their binding energies and their excitations? We elucidate these questions using electron-positron and proton-antiproton collisions as well as electron and photon scattering experiments. We study the excited states of the nucleon and of unconventional hadrons containing more than three quarks, particularly if one of them is a charm quark.

The Institute of High Energy Physics of the Chinese Academy of Sciences in Beijing operates an electron-positron collider called BEPC-II (Beijing electron-positron collider II). It is designed for center-of-mass energies between 2 and 5 GeV, which allows the study charmonium with very high statistics. The design luminosity is 10³³ cm^-2 s^-1. Since 2009, measurements at various collision energies are being performed using the BESIII (Beijing Spectrometer III) detector.

A detailed understanding of hadrons and nuclei forms the basis of many precision measurements designed to test the Standard Model of elementary particle physics or determine fundamental constants. The P2 experiment at MESA aims to measure the weak mixing angle – a fundamental parameter of the Standard Model – in parity-violating electron-proton scattering, which requires a detailed understanding of the proton. Hadronic contributions lead to the largest uncertainties in predictions of the muon g-factor, one of the most precisely calculated and measured quantities in the Standard Model. We provide experimental, phenomenological and lattice QCD input to quantify these hadronic contributions and shrink their uncertainties, leading to ever more stringent tests of the Standard Model.

We study how stars forge the elements and how matter behaves under extreme conditions, from the nuclear reactions powering stellar evolution to the dense matter inside neutron stars. Find out more about our nuclear astrophysics research in the section “Stellar & Galactic Physics” on our webpage “Astro-, Astroparticle & Neutrinophysics”.

Experimental & Theoretical Research Groups

Neutron physics studies the properties, behavior, and interactions of neutrons, the electrically neutral component of atomic nuclei. We investigate free neutrons with high precision to determine their lifetime, their magnetic interaction, the possible existence of an electric dipole moment and the correlations between the neutron’s decay products. We generate free neutrons either from nuclear fission in nuclear reactors like the TRIGA reactor or from spallation sources like at PSI.

The tSPECT experiment aims to measure the free neutron lifetime by suspending very low-energy neutrons, called ultracold neutrons (UCNs), in a fully magnetic trap without collisions with material walls. Based on the interaction of the neutron magnetic moment with a magnetic field gradient, we suspend UCNs in free space. UCNs that minimize their potential energy in a low magnetic field region can be stored there to measure their decay rate through neutron β-decay. With a combination of permanent and superconducting magnets, we create a suitable trapping region inside the cold bore of a cryostat.

With ultra cold neutrons exposed to very well characterized low magnetic and high electric fields we search for the electric dipole moment of the neutron. This property, if finite, breaks the symmetry between matter and anti-matter and can provide one of the indispensable ingredients to the generation of the matter-anti-matter asymmetry we observe in the Universe.

At the scales of interest for hadron and nuclear physics, the strength of the strong interaction does not allow for perturbative expansions. Instead, calculations can be performed using discretized space-time and Monte-Carlo methods on supercomputers. The Mainz lattice group performs pioneering calculations for hadron spectroscopy and structure as well as for hadronic corrections to precision observables, particularly the magnetic moment of the muon.

Flavour physics describes the transitions between different generations (flavours) of quarks and leptons via the weak interaction. To learn more about our research in “Flavour Physics” please visit the webpage “High Energy Particle Physics”.

Experimental & Theoretical Research Groups