High Energy Particle Physics – Physics JGU Mainz

Johannes Gutenberg University Mainz > Faculty 08 > Physics > Physics Research > Research Areas > High Energy Particle Physics

The Standard Model of particle physics provides a comprehensive and predictive framework for all known elementary particles and their interactions. Various precision measurements of Standard Model processes at the LHC have been and currently are being performed by scientists from Mainz using data collected with the ATLAS experiment. These include measurements of the mass of the W and the Z bosons, the effective electroweak mixing angle, as well as diboson production, benefiting from the excellent performance of the LHC (in terms of the delivered integrated luminosity) and of the ATLAS experiment.

Effective field theories are a powerful tool for driving the theoretical precision necessary for understanding Standard Model measurements, bridging Standard Model measurements and searches for new physics, and deepening our understanding of quantum field theories overall. At Mainz, researchers are engaged in soft-collinear effective theory calculations for collider physics observables and in using effective field theories to characterize new physics. Effective field theories are also studied to connect measurements at high energy colliders and observations from astrophysics, flavour physics, and low-energy precision laboratory probes.

ATLAS is a general-purpose particle physics experiment at the Large Hadron Collider (LHC) at CERN. It is designed to exploit the full discovery potential of the LHC, expanding the boundaries of scientific knowledge as it uses precision measurements to seek answers to fundamental questions such as: What are the fundamental forces of nature? What is dark matter made of? What is the fundamental physics of the early universe?

The discovery of the Higgs boson in 2012 opened an entirely new window into the physics of the universe and many fundamental questions of particle physics. Measuring the couplings of the Higgs boson to all elementary particles with increasing precision not only represents a stringent test of the Higgs mechanism, but also probes new particles by detecting deviations due to loop corrections. Similarly, the Higgs boson might couple to dark sector particles, which can be probed by measuring its decay width into invisible final states. In addition, an improved understanding of the Higgs potential based on measurements of the Higgs self-coupling is of crucial importance for both the physics of the early universe, where the electroweak phase transition may have played a decisive role in creating the matter-antimatter asymmetry, as well as the ultimate fate of the universe, as dictated by the stability of the Higgs vacuum. Researchers in Mainz are pursuing all of these topics with measurements using the ATLAS detector at the LHC, by preparing for new experiments at a future Higgs factory, proposing new models to expand our interpretative power of Higgs physics data, analyzing connections between Higgs physics and early universe physics, and investigating fundamental questions about the origin of the electroweak scale.

A new high-energy collider is needed to perform precision studies of the physics of the Higgs boson, aiming to address the mysteries of the early universe and potentially open a portal to the dark sector.

Mainz physicists are pursuing R&D on high-precision calorimetry to fully exploit the physics potential of such a future collider. Together with partners from the DRD Calo Collaboration and in close collaboration with the PRISMA+ detector laboratory, a particular focus is placed on the design and construction of scintillator-based high-granularity sampling calorimeters.

Various observations challenge the Standard Model of particle physics: the existence of dark matter, which is crucial for structure formation in the early universe, the matter-antimatter asymmetry that formed shortly after the Big Bang, and the tiny neutrino masses. Mainz theorists are devising new models to explain these open questions, perform phenomenological studies to understand the impact of experimental results or to motivate novel experimental searches, and advance our theoretical methods to get reliable predictions.

Mainz experimentalists are searching for new particles and interactions with the ATLAS experiment, including dark matter particles and messenger particles to the dark sector. At CERN, the SHiP and NA62 experiments are searching for exotic long-lived particles. These are complemented by direct searches for dark matter in our galaxy as well as searches at the MESA accelerator in Mainz.

The NA62 experiment at CERN is dedicated to the ultra-rare decay K⁺ → π⁺νν̄, whose branching fraction is below 10⁻¹⁰ – the smallest branching ratio ever measured for any particle decay. In addition to this flagship channel, NA62 measures a broad spectrum of K⁺ and π⁰ decay modes including searches for dark sector particles, all in the pursuit of Physics beyond the Standard Model.

The SHiP (Search for Hidden Particles) experiment at CERN is designed to detect new particles that belong to a hypothetical Hidden Sector and interact only feebly with the particles of the Standard Model. Currently under construction, SHiP will begin operation in 2032, making use of a new proton beamdump facility with unprecedented intensity. The experiment will look for the decays of massive, long‑lived, weakly interacting states – such as Dark Photons, Heavy Neutral Leptons, Axion‑like particles, and many other candidates – that can be produced in the dump.

Flavour physics describes the transitions between different generations (flavours) of quarks and leptons via the weak interaction. Within the Standard Model (SM), quark flavour physics is governed by the CKM matrix, while transitions between charged leptons are quasi-forbidden and are an immediate sign of new physics. Precision studies of flavour-changing neutral currents, CP violation, and lepton flavour violation are sensitive probes of new physics at energy scales not accessible to colliders. As such, flavour physics plays a central role in the search for new phenomena through indirect effects, complementary to direct searches at high-energy colliders.

The Belle II detector at the SuperKEKB accelerator in Tsukuba (Japan) is an experiment built to search for new physics beyond the Standard Model by measuring rare decays of elementary particles, such as bottom and charm quarks or tau leptons. Belle II will address the problem of finding evidence for the existence of new unknown particles that can provide a possible explanation for the predominance of matter compared to antimatter and answer other open fundamental questions about our understanding of the universe.

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.

Mu3e is a planned particle physics experiment at the Paul Scherrer Institute that searches for the decay of an antimyron (Mu) into an electron and two positrons (3e): μ+→e+γ∗/Z∗→e+e+e−

This decay is extremely unlikely in the Standard Model of particle physics, as it changes the lepton family number (conversion of a lepton into one of a different family). This is only possible in a higher-order loop process and under neutrino oscillation within the loop. In several theories for physics beyond the Standard Model, especially those involving supersymmetry, this decay is significantly more frequent. The search for this decay allows some of these theories to be tested, even if they are not accessible to direct observation (e.g., at the LHC).