arXiv subjects

K. Frahm

Publications and source records attributed to K. Frahm.

7 recordsLinked to original sources

arXiv2026

Energy, time, and position resolution measurements of an array of large tapered LYSO crystals

We report on the performance of six custom-made tapered LYSO crystals of unprecedented volume, which constitute a sector of the 19 radiation length electromagnetic calorimeter planned for the PIONEER experiment. The longitudinal response uniformity of each crystal was measured using radioactive sources before characterizing the energy and time resolution of the crystals in an array using a 20 to 80 MeV positron beam at the Paul Scherrer Institute. The array demonstrated an energy resolution better than 2 % for energies above 40 MeV and a time resolution better than 130 ps for energies above 30 MeV. The spatial resolution was measured in the central region of the array to be 4.9 mm at 70 MeV, and was extrapolated to 5.4 mm across a 30 mm radius region using Geant4 simulation. The measured properties satisfy the key design parameters of the PIONEER calorimeter for the measurement of rare pion decays.

physics.ins-det↗

arXiv2025

European Strategy for Particle Physics Update -- PIONEER: a next generation rare pion decay experiment

PIONEER is a rapidly developing effort aimed to perform a pristine test of lepton flavour universality (LFU) and of the unitarity of the first row of the CKM matrix by significantly improving the measurements of rare decays of the charged pion. In Phase I, PIONEER aims to measure the charged-pion branching ratio to electrons vs.\ muons $R_{e/\mu}$ to 1 part in $10^4$, improving the current experimental result $R_{e/\mu}\,\text{(exp)} =1.2327(23)\times10^{-4}$ by a factor of 15. This precision on $R_{e/\mu}$ will match the theoretical accuracy of the SM prediction allowing for a test of LFU at an unprecedented level, probing non-SM explanations of LFU violation through sensitivity to quantum effects of new particles up to the PeV mass scale. Phase II and III will aim to improve the experimental precision of the branching ratio of pion beta decay, $\pi^+\to \pi^0 e^+ \nu (\gamma)$, currently at $1.036(6)\times10^{-8}$, by a factor of three and six, respectively. The improved measurements will be used to extract $V_{ud}$ in a theoretically pristine manner. The ultimate precision of $V_{ud}$ is expected to reach the 0.05\,\% level, allowing for a stringent test of CKM unitarity. The PIONEER experiment will also improve the experimental limits by an order of magnitude or more on a host of exotic decays that probe the effects of heavy neutrinos and dark sector physics. This input to the 2026 update of the European Strategy for Particle Physics Strategy describes the physics motivation and the conceptual design of the PIONEER experiment, and is prepared based on the PIONEER proposal submitted to and approved with high priority by the PSI program advisory committee (PAC). Using intense pion beams, and state-of-the-art instrumentation and computational resources, the PIONEER experiment is aiming to begin data taking by the end of this decade.

hep-ex↗

arXiv2024

Measurements of a LYSO crystal array from threshold to 100 MeV

We report measurements of ten custom-made high-homogeneity LYSO crystals. The investigation is motivated by the need for a compact, high-resolution, and fast electromagnetic calorimeter for a new rare pion decay experiment. Each $2.5\times 2.5 \times 18$ cm$^3$ crystal was first characterized for general light yield properties and then its longitudinal response uniformity and energy resolution were measured using low-energy gamma sources. The ten crystals were assembled as an array and subjected to a 30 - 100 MeV positron beam with excellent momentum definition. The energy and timing resolutions were measured as a function of energy, and the spatial resolution was determined at 70 MeV. An additional measurement using monoenergetic 17.6 MeV gammas produced through a p-Li resonance was later made after the photosensors used in positron testing were improved. As an example of the results, the energy resolution at 70 MeV of 1.52 $\pm$ 0.03% is more than two times better than reported results using previous generation LYSO crystals.

physics.ins-det↗

arXiv1999

Large Petermann factor in chaotic cavities with many scattering channels

The quantum-limited linewidth of a laser cavity is enhanced above the Schawlow-Townes value by the Petermann factor K, due to the non-orthogonality of the cavity modes. The average Petermann factor $ $ in an ensemble of cavities with chaotic scattering and broken time-reversal symmetry is calculated non-perturbatively using random-matrix theory and the supersymmetry technique, as a function of the decay rate $Γ$ of the lasing mode and the number of scattering channels N. We find for $N\gg 1$ that for typical values of $Γ$ the average Petermann factor $ \propto \sqrt{N}\gg 1$ is parametrically larger than unity.

chao-dyn↗

arXiv1995

Quantum Transport in Disordered Wires: Equivalence of One-Dimensional Sigma Model and Dorokhov-Mello-Pereyra-Kumar Equation

The two known non-perturbative theories of localization in disordered wires, the Fokker-Planck approach due to Dorokhov, Mello, Pereyra, and Kumar, and the field-theoretic approach due to Efetov and Larkin, are shown to be equivalent for all symmetry classes. The equivalence had been questioned as a result of field-theoretic calculations of the average conductance by Zirnbauer [PRL 69, 1584 (1992)], which disagreed with the Fokker-Planck approach in the symplectic symmetry class. We resolve this controversy by pointing to an incorrect implementation of Kramers degeneracy in these calculations, and we derive modified expressions for the first two conductance moments which agree well with existing numerical simulations from the metallic into the localized regime. ***Submitted to Physical Review B.***

cond-mat↗

arXiv1995

$h/2e$--Oscillations for Correlated Electron Pairs in Disordered Mesoscopic Rings

The full spectrum of two interacting electrons in a disordered mesoscopic one--dimensional ring threaded by a magnetic flux is calculated numerically. For ring sizes far exceeding the one--particle localization length $L_1$ we find several $h/2e$--periodic states whose eigenfunctions exhibit a pairing effect. This represents the first direct observation of interaction--assisted coherent pair propagation, the pair being delocalized on the scale of the whole ring.

cond-mat↗

arXiv1995

Scaling in Interaction-Assisted Coherent Transport

The pair localization length $L_2$ of two interacting electrons in one--dimensional disordered systems is studied numerically. Using two direct approaches, we find $L_2 \propto L_1^α$, where $L_1$ is the one-electron localization length and $α\approx 1.65$. This demonstrates the enhancement effect proposed by Shepelyansky, but the value of $α$ differs from previous estimates ($α=2$) in the disorder range considered. We explain this discrepancy using a scaling picture recently introduced by Imry and taking into account a more accurate distribution than previously assumed for the overlap of one-electron wavefunctions.

cond-mat↗