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Pions, nucleons and ∆

Dmitrij Siemens
Institute of Theoretical Physics II, Hadron and Particle Physics

My PhD Project

Pions, nucleons and ∆

My PhD project is supported by the DFG and is part of the SFB/TR 16 “Subnuclear Structure of Matter“, which is aiming at understanding the properties of strongly interacting hadrons. The major difficulty in this task lies in the observation that the well-established Standard Model is not easily applicable at low energies. This is due to the phenomenon called confinement which ensures that the elementary particles of the strong interaction, the so-called quarks and gluons, are bound in hadrons. Characterized by their quark content, one has two types of hadrons. Mesons are build out of two quarks whereas baryons contain three quarks.

I study interactions between the lightest mesons, the so-called pions (π), and the lightest baryons, the so called nucleons (N). The aim is to describe experimental data and to make predictions for two processes with the help of a theoretical framework called Chiral Perturbation Theory. This framework is based on the symmetry properties of Quantum Chromodynamics, the part of the Standard Model which describes the strong interactions and is formulated directly in terms of baryons and mesons.

The two processes I am interested in are πN → πN and πN → ππN. Whereas the first one is already well covered in the literature, the second one is barely studied. The reason for this is that the calculations for πN → ππN are much more complicated and challenging. One feature of Chiral Perturbation Theory is that it depends on the so called low-energy constants (LECs) which values are unknown and thus have to be fitted to experimental data. But once these constants are determined, the theory has predictive power. Because πN → πN is a subprocess of πN → ππN, they both have common LECs. Thus, is it advantageous to study both processes and to perform a combined determination of the LECs. The extracted values are also important in the broader context since they play an important role in nuclear forces.

In addition, both reactions allow to study in detail the role of the ∆ resonance, the lightest excitation of the nucleon. From experiments, it is well-known that the ∆ resonance plays an important role in low-energy hadron dynamics and thus the explicit inclusion of ∆ contributions in calculations is believed to significantly improve the description of low-energy data. This task requires state-of-the-art computational power and is one of the most challenging applications of Chiral Perturbation Theory so far.

My Internationalisation Strategy

The substantial ingredient of my PhD is the collaboration with internationally well-known physicists. I am part of a working group at RUB which closely collaborates with Ulf-G. Meißner, who is professor at the university in Bonn and also director of the theoretical nuclear physics institute at the Jüllich Research Center. The second main collaborator is Veronique Bernard from the Institut de physique nucléaire d'Orsay in Paris, France.

For my doctorate it would be of great importance to have a closer exchange with both Ulf Meißner and Veronique Bernard. Both are experts on my working field and the research they did years ago provides the motivation and foundation for my doctoral project. To improve the exchange with both of them, I plan to visit their home institutes and to participate in common workshops. This will allow me to discuss my work in person and, besides the scientific exchange, give me the chance to present my results at the institutes.

Other possible cooperation partners would be Daniel Phillips, who is professor at the Ohio University (USA), Gilberto Colangelo, who is professor at the university of Bern (Switzerland) and also Hans Bijnens, who is professor at the university of Lund (Sweden). The main idea of these visits would be to give talks on my project at the home institutes of the cooperation partners but also at other institutes close by. Of course, this would allow me to get into closer contact with the cooperation partners and their working groups, which would help me to increase my international reputation and even expand my international contacts for possible future joint publications. In addition, having discussions and attending lectures and talks at these institutes would broaden my knowledge in my research field and beyond.

There is also a range of interesting conferences, workshops and summer schools on the topic I am interested in. One particular is the Chiral Dynamics Workshop, to be held in Pisa (Italy) in 2015, which is one of the most important meetings in my research field. All cooperation partners listed above will be joining this workshop. This would be a great opportunity to get in contact with several experts and also to present my own achievements.


NSTAR 2015

In May 2015, I attended the 10th international workshop on the physics of excited nucleons (NSTAR2015) taking place at Osaka University in Japan. The workshop covered various topics related to the excitation of nucleons. Of greater interest was the status report on ongoing experimental measurements of the reaction πN → ππN at the proton collider J-PARC in Tokai, Japan. Being the only participant from RUB, I took the opportunity to give a talk and present my own work, which led to interesting discussions with groups working in my research field from Mainz and Valencia, Spain. The workshop was finalized by being published in JPS Conference Proceedings.

Chiral Dynamics 2015

In July 2015, the 8th international workshop on Chiral Dynamics took place at Pisa University in Italy. Being one of the most important meetings in my research field caused to gather experts from all over the world, who gave talks on various topics in my research area and beyond. My working group at RUB was represented by several talks including a contribution by myself on my recent progress. After my talk, my working group and a group of physicists from Bonn and Seattle, USA sat down to arrange a collaboration related to my work on the reaction πN→πN. Additionally, we agreed on me having a research stay at the Institute for Nuclear Theory in Seattle at the end of the year. I also had the chance to exchange with my collaboration partners from Paris and Bonn and to discuss further steps of my PhD project. The workshop was closed by being published in Proceedings of Science.

Institute for Nuclear Theory

From October to December 2015, I stayed at the Institute for Nuclear Theory (INT) at the University of Washington in Seattle. The INT is a leading institution for theoretical nuclear physics and hosts many international scientists during the year. In particular, it is known for frequently organizing workshops on a broad set of topics related to nuclear physics. In addition to participating in workshops, I also used my stay to work on a project related to the matching of Chiral Perturbation Theory to the Roy-Steiner (RS) analysis of pion-nucleon scattering. The RS analysis, which was carried out by a group around Martin Hoferichter, an assistant professor at the INT, provides a different approach to study the elastic scattering of pions and nucleons.

Our joint project was completed by being published in October 2016.

São Paulo

In February 2016, the Instituto de Física Teòrica - UNESP in São Paulo organized a school on Effective Field Theories covering many different fields from condensed matter to cosmology. Of particular interest were the lectures on inflation and gravitational waves. Although both topics seem not to be connected to nuclear physics, they nevertheless share common methods and provided a good insight for my future research interest.

HC2NP 2016

In September 2016, the 1st workshop dedicated to hadronic contributions to new physics searches took place on Tenerife, Spain. Considering the final stage of my PhD studies, I was able to present my main achievements in front of an international audience. Additionally, I met with my collaborators to discuss the completion of the project started at the INT.

University of Massachusetts

From October to December 2016, I stayed at the University of Massachusetts (UMass) at Amherst. During this stay, I worked on a project related to the Effective Field Theory for gravity in collaboration with a local group led by John Donoghue. This stay gave me the opportunity to work in the field of quantum gravity and apply the methods I acquired during my PhD studies on nuclear physics. In particular, I am very intrigued to direct my future research towards cosmological problems.

In addition, I also used the time to give seminar talks on my work regarding pion-nucleon physics at institutes nearby like the ACFI at UMass, the Jefferson Lab and the University of Connecticut. I also had short stays at the George Washington University in D.C. and at the Massachusetts Institute for Technology in Cambridge, where I met the members of the nuclear theory group and had extensive talks about my current and future research.