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Christoph Langer

Publications and source records attributed to Christoph Langer.

11 recordsLinked to original sources
arXiv2025

STORI2024: Tests of Amorphous Carbon-coated Storage Cells for a Polarized Gas Target at LHCb and Further Results

As the LHC beams cannot be polarized, introducing a dense polarized gas target at the LHCb experiment at CERN, to be operated concurrently with beam-beam collisions, will facilitate fixed-target interactions to explore a new energy regime of spin physics measurements. Unfortunately, typical surface coatings, such as water, Teflon, or aluminum, commonly used to avoid polarization losses, are prohibited due to restrictions imposed by vacuum and beam policies. Using the former atomic beam source for the polarized target at ANKE/COSY (Forschungszentrum J\"ulich), an accompanying Lamb-shift polarimeter and a storage cell chamber inside a superconducting magnet, provide a perfect test stand to investigate the properties of a storage cell coated with amorphous carbon. A significant recombination rate, ranging from $93\%$ to $100\%$, as well as preservation of polarization during recombination surpassing $74\%$, were observed. We successfully produced H$_2$ molecules with a nuclear polarization of P$\sim 0.59$. In addition, we could produce polarized H$_3^+$ ions for the first time and observed the shift of the axis of rotation within HD molecules.

physics.ins-det↗
arXiv2021

Introduction to Causal Fermion Systems

This paper is dedicated to give a concise introduction to the theory of causal fermion systems. After putting the theory of causal fermion systems into the historical context, we recall fundamental physical preliminaries. Afterwards, we enter the underlying ideas of the principle of the fermionic projector and clarify its connection to causal fermion systems. We finally outline the main structures of the theory of causal fermion systems.

math-ph↗
arXiv2021

Existence of Minimizers for Causal Variational Principles on Compact Subsets of Momentum Space in the Homogeneous Setting

We prove the existence of minimizers in the class of negative definite measures on compact subsets of momentum space in the homogeneous setting under several side conditions (constraints). The method is to employ Prohorov's theorem. Given a minimizing sequence of negative definite measures, we show that, under suitable side conditions, a unitarily equivalent subsequence thereof is bounded. By restricting attention to compact subsets, from Prohorov's theorem we deduce the existence of minimizers in the class of negative definite measures.

math-ph↗
arXiv2021

Causal Variational Principles in the Infinite-Dimensional Setting: Existence of Minimizers

We provide a method for constructing (possibly non-trivial) measures on non-locally compact Polish subspaces of infinite-dimensional separable Banach spaces which, under suitable assumptions, are minimizers of causal variational principles in the non-locally compact setting. Moreover, for non-trivial minimizers the corresponding Euler-Lagrange equations are derived. The method is to exhaust the underlying Banach space by finite-dimensional subspaces and to prove existence of minimizers of the causal variational principle restricted to these finite-dimensional subsets of the Polish space under suitable assumptions on the Lagrangian. This gives rise to a corresponding sequence of minimizers. Restricting the resulting sequence to countably many compact subsets of the Polish space, by considering the resulting diagonal sequence we are able to construct a regular measure on the Borel algebra over the whole topological space. For continuous Lagrangians of bounded range it can be shown that, under suitable assumptions, the obtained measure is a (possibly non-trivial) minimizer under variations of compact support. Under additional assumptions, we prove that the constructed measure is a minimizer under variations of finite volume and solves the corresponding Euler-Lagrange equations. Afterwards, we extend our results to continuous Lagrangians vanishing in entropy. Finally, assuming that the obtained measure is locally finite, topological properties of spacetime are worked out and a connection to dimension theory is established.

math-ph↗
arXiv2020

Causal Variational Principles in the $σ$-Locally Compact Setting: Existence of Minimizers

We prove the existence of minimizers of causal variational principles on second countable, locally compact Hausdorff spaces. Moreover, the corresponding Euler-Lagrange equations are derived. The method is to first prove the existence of minimizers of the causal variational principle restricted to compact subsets for a lower semi-continuous Lagrangian. Exhausting the underlying topological space by compact subsets and rescaling the corresponding minimizers, we obtain a sequence which converges vaguely to a regular Borel measure of possibly infinite total volume. It is shown that, for continuous Lagrangians of compact range, this measure solves the Euler-Lagrange equations. Furthermore, we prove that the constructed measure is a minimizer under variations of compact support. Under additional assumptions, it is proven that this measure is a minimizer under variations of finite volume. We finally extend our results to continuous Lagrangians decaying in entropy.

math-ph↗
arXiv2019

Neutron-induced cross sections -- from raw data to astrophysical rates

Neutron capture cross sections are one of the most important nuclear inputs to models of stellar nucleosynthesis of the elements heavier than iron. The activation technique and the time-of-flight method are mostly used to determine the required data experimentally. Recent developments of experimental techniques allow for new experiments on radioactive isotopes. Monte-Carlo based analysis methods give new insights into the systematic uncertainties of previous measurements. We present an overview over the state-of-the-art experimental techniques, a detailed new evaluation of the $^{197}$Au(n,$γ$) cross section in the keV-regime and the corresponding re-evaluation of 63 more isotopes, which have been measured in the past relative to the gold cross section.

astro-ph.IM↗
arXiv2018

Treatment of isomers in nucleosynthesis codes

The decay properties of long-lived excited states (isomers) can have a significant impact on the destruction channels of isotopes under stellar conditions. In sufficiently hot environments, the population of isomers can be altered via thermal excitation or de-excitation. If the corresponding lifetimes are of the same order of magnitude as the typical time scales of the environment, the isomers have to be the treated explicitly. We present a general approach to the treatment of isomers in stellar nucleosynthesis codes and discuss a few illustrative examples. The corresponding code is available online at http://exp-astro.de/isomers/

astro-ph.SR↗
arXiv2016

Coulomb dissociation of $^{20,21}$N

Neutron-rich light nuclei and their reactions play an important role for the creation of chemical elements. Here, data from a Coulomb dissociation experiment on $^{20,21}$N are reported. Relativistic $^{20,21}$N ions impinged on a lead target and the Coulomb dissociation cross section was determined in a kinematically complete experiment. Using the detailed balance theorem, the $^{19}\mathrm{N}(\mathrm{n},γ)^{20}\mathrm{N}$ and $^{20}\mathrm{N}(\mathrm{n},γ)^{21}\mathrm{N}$ excitation functions and thermonuclear reaction rates have been determined. The $^{19}\mathrm{N}(\mathrm{n},γ)^{20}\mathrm{N}$ rate is up to a factor of 5 higher at $T<1$\,GK with respect to previous theoretical calculations, leading to a 10\,\% decrease in the predicted fluorine abundance.

nucl-ex↗
arXiv2015

First Measurement of the $^{96}$Ru(p,$γ$)$^{97}$Rh Cross Section for the p-Process with a Storage Ring

This work presents a direct measurement of the $^{96}$Ru($p, γ$)$^{97}$Rh cross section via a novel technique using a storage ring, which opens opportunities for reaction measurements on unstable nuclei. A proof-of-principle experiment was performed at the storage ring ESR at GSI in Darmstadt, where circulating $^{96}$Ru ions interacted repeatedly with a hydrogen target. The $^{96}$Ru($p, γ$)$^{97}$Rh cross section between 9 and 11 MeV has been determined using two independent normalization methods. As key ingredients in Hauser-Feshbach calculations, the $γ$-ray strength function as well as the level density model can be pinned down with the measured ($p, γ$) cross section. Furthermore, the proton optical potential can be optimized after the uncertainties from the $γ$-ray strength function and the level density have been removed. As a result, a constrained $^{96}$Ru($p, γ$)$^{97}$Rh reaction rate over a wide temperature range is recommended for $p$-process network calculations.

nucl-ex↗