Hadron Physics

Hadrons are bound states of quarks and gluons, formed by the strong interaction. The ones best known to us are the proton and the neutron, the building blocks of nuclei and hence the world around us. Many more such states, most of them decaying extremely quickly, have been discovered in the past and their properties are filling a whole book compiled every other year by the Particle Data Group. Most of these states can be explained by some combination of three quarks or a quark and an antiquark, including excitations corresponding to different radial , orbital or spin configurations of quarks. With the advent of more powerful accelerators and detectors, however, some new and unexpected states were identified, generally called exotic because they do not fit the predictions of the simple quark model. Our group contributes to a better understanding of the hadron spectrum and the search for exotic particles, a hot topic in particle physics. With the COMPASS experiment at CERN, we study light mesons containing up, down and strange quarks, while we are looking for the production of heavier exotic states containing charm quarks with the ALICE detector. With the successor of COMPASS, AMBER, we plan to perform unprecedented measurements of strange mesons using an RF-separated beam.    

In Quantum Chromodynamics, the interaction between quarks is mediated by the exchange of gluons, which couple to the strong charge (color), in much the same was as the interaction between electrically charged particles is mediated by the exchange of photons. In stark contrast to photons, however, gluons also carry color and can therefore couple to themselves. This self-interaction generates a wealth of new phenomena for strongly interacting particles, the most important ones being confinement and asymptotic freedom. A very fundamental quantity related to the confinement of quarks inside a hadron is its charge-radius. For protons, it was long assumed that this quantity is well-known experimentally. Recently, a discrepancy between different measurements of this quantity has led to discussions on fundamental issues concerning the methods used, issues of data analysis and also involving new physics. With AMBER, we plan to perform a new high-precision measurement of the proton charge radius by elastic scattering of high-energy muons off protons in the next years.

Spectroscopy at COMPASS

At COMPASS we study the internal structure and the excitation spectrum of hadrons. One hot topic is the study of spin-exotic final states. Using data taken by the COMPASS experiment, we look deeper into diffractively produced final-states with a pion beam, and perform high-level event selections, partial-wave and amplitude analyses on the data. All that is done with bash and C++ code, exploring various libraries like ROOT, CORAL and PHAST. We also develop Monte-Carlo programs and perform the simulations on computer clusters. 

Prior knowledge on any of that is not required for a bachelor or master thesis in our group and we would be happy to guide you on your journey through hadron physics at COMPASS if you join our analyis.

Also, every student can get the possibility to visit CERN and participate in the data taking or the preparation of the new AMBER experiment, the successor to COMPASS.

Proton Radius Measurement

One of the flagship experiments of the new AMBER collaboration is the proton radius measurement. In 2010 and 2013, the CREMA collaboration has measured significantly different proton radii than previous experiments. This discrepancy became known as the proton radius puzzle.

At AMBER, the proton radius will be measured with a scattering experiment between high-energy muons and protons. In this experiment, the muon and the recoil proton will be measured. A test measurement 2018 served as a successful proof-of-principle experiment. A pilot run is set to take data in October 2021 and the main experiment is expected to run in the period 2022-2024.

Current tasks are the simulation of the experiment using Monte Carlo programs (based on Geant4) and the analysis of data from a first pilot run. During beam time at the AMBER experiment or its predecessor COMPASS, students have the possibility to visit CERN and participate in the data taking.

Contact Person

Here you can find possible thesis topics.

Spectroscopy with ALICE

χc1(3872)  was the first observed exotic meson in the charmonium mass range, also known as X(3872). From data its quantum numbers JPC = 1++ and its mass were precisely determined. Its internal structure, however, is still unknown and controversially debated as meson molecules or compact tetraquarks.

Ultra Relativistic Heavy-Ion Collisions (URHIC) offer a new approach to systematically investigate the structures of exotic states. For LHC energies, model calculations predict a much higher production of  χc1(3872) for the molecule scenario than for the tetraquark scenario. At LHC, ALICE is the dedicated URHIC experiment. With its recently upgraded detectors, ALICE will be able to record enough data such that χc1(3872) could come within reach in proton-proton and nucleus-nucleus collisions. With such measurements, we will be able to confirm or exclude the prediction of phenomenological models and thus contribute to determine the structure of this exotic meson.

Currently, our group works on testing the prepared code for LHC-RUN3 on proton-proton data taken by the ALICE experiment during LHC-Run 2. We try to extract signals in the decay channel J/ψπ-π+. Both  χc1(3872) and ψ(2S) decay into this final state.

Tasks range from the analysis of data from Run 2 and 3 to the Monte-Carlo simulations of signal and background using the ALICE analysis frameworks AliPhysics and O2. You will also get the chance to visit CERN and participate in the data taking.

Contact Person

Here you can find possible thesis topics.

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