arXiv subjects

M. Thiel

Publications and source records attributed to M. Thiel.

At least 37 records · Page 2Linked to original sources

arXiv2022

RPC based tracking system at CERN GIF++ facility

With the HL-LHC upgrade of the LHC machine, an increase of the instantaneous luminosity by a factor of five is expected and the current detection systems need to be validated for such working conditions to ensure stable data taking. At the CERN Gamma Irradiation Facility (GIF++) many muon detectors undergo such studies, but the high gamma background can pose a challenge to the muon trigger system which is exposed to many fake hits from the gamma background. A tracking system using RPCs is implemented to clean the fake hits, taking profit of the high muon efficiency of these chambers. This work will present the tracking system configuration, used detector analysis algorithm and results.

physics.ins-det↗

arXiv2022

Neutron polarisation transfer, $C_{x'}^n$, in $π^+$ photoproduction off the proton

We report a first measurement of the double-polarisation observable, $C_{x'}$, in $π^+$ photoproduction off the proton. The $C_{x'}$ double-polarisation observable represents the transfer of polarisation from a circularly polarised photon beam to the recoiling neutron. The MAMI circularly polarised photon beam impinged on a liquid deuterium target cell, with reaction products detected in the Crystal Ball calorimeter. Ancillary apparatus surrounding the target provided tracking, particle identification and determination of recoil nucleon polarisation. The $C_{x'}$ observable is determined for photon energies 800-1400 MeV, providing new constraints on models aiming to elucidate the spectrum and properties of nucleon resonances. This is the first determination of any polarisation observable from the beam-recoil group of observables for this reaction, providing a valuable constraint and systematic check of the current solutions of partial wave analysis based theoretical models.

nucl-ex↗

arXiv2022

Theoretical analysis of the extraction of neutron skin thickness from coherent π0 photoproduction off nuclei

Background: Coherent π0 photoproduction on heavy nuclei has been suggested as a reliable tool to infer neutron skin thicknesses. To this aim, various experiments have been performed, especially on 208Pb. Purpose: We analyze the sensitivity of that reaction process to the nucleonic density, and especially to the neutron skin thickness, for 12C, 40Ca and 208Pb, for which reliable data exist, and on 116,124Sn, for which measurements have been performed in Mainz. We study also the role played by the π0-nucleus final-state interaction. Method: A model of the reaction is developed at the impulse approximation considering either plane waves or distorted waves to describe the π0-nucleus scattering in the outgoing channel. Results: Our calculations are in good agreement with existing data, especially for 208Pb. The sensitivity of the theoretical cross sections to the choice of the nucleonic density is small, and below the experimental resolution. Conclusions: Coherent π0 photoproduction is mostly an isoscalar observable that bares no practical sensitivity to the neutron skin thickness. To infer that structure observable it should be coupled to other reaction measurements, such as electron scattering, or by comparing experiments performed on isotopes of the same chemical element.

nucl-th↗

arXiv2022

Target and beam-target asymmetries for the $γp \to π^0 π^0 p$ reaction

Background: Photoproduction of pion pairs allows to study sequential decays of nucleon resonances via excited intermediate states. Such decays are important e.g. for states which in the quark model have both oscillators excited and de-excite them in a two-step process. However, analyses of multi-meson final states is difficult and requires more than unpolarized cross section measurements. Purpose: Experimental study and model analysis in view of resonant contributions of target and beam-target polarization observables for the reaction $γp \to π^0 π^0 p$. Methods: Investigated were target (single) and beam-target (double) polarization asymmetries in dependence of several parameters. The experiments were performed at the Mainz Microtron (MAMI) using circularly polarized photon beams and transversally polarized solid-state butanol targets. The reaction products were analyzed with the Crystal Ball and TAPS detectors. Results: Studied were the polarization observables Py (unpolarized beam, target polarized in y direction) and Px (circularly polarized beam, target polarized in x direction), which are similar to T (target asymmetry) and F (beam-target asymmetry) for single pion production. The asymmetries were analyzed with three independent methods, revealing systematic uncertainties.

hep-ex↗

arXiv2022

Low-$Q^2$ elastic electron-proton scattering using a gas jet target

In this paper, we describe an experiment measuring low-$Q^2$ elastic electron-proton scattering using a newly developed cryogenic supersonic gas jet target in the A1 three-spectrometer facility at the Mainz Microtron. We measured the proton electric form factor within the four-momentum transfer range of $0.01\le Q^2 \le 0.045(\text{GeV/c})^2$. The experiment showed consistent results with the existing measurements. The data we collected demonstrated the feasibility of the gas jet target and the potential of future scattering experiments using high-resolution spectrometers with this gas jet target.

nucl-ex↗

arXiv2022

New Measurements of the Beam-Normal Single Spin Asymmetry in Elastic Electron Scattering Over a Range of Spin-0 Nuclei

We report precision determinations of the beam normal single spin asymmetries ($A_n$) in the elastic scattering of 0.95 and 2.18~GeV electrons off $^{12}$C, $^{40}$Ca, $^{48}$Ca, and $^{208}$Pb at very forward angles where the most detailed theoretical calculations have been performed. The first measurements of $A_n$ for $^{40}$Ca and $^{48}$Ca are found to be similar to that of $^{12}$C, consistent with expectations thus demonstrating the validity of theoretical calculations for nuclei with Z~$\leq20$. We also report $A_n$ for $^{208}$Pb at two new momentum transfers (Q$^2$) extending the previous measurement. Our new data confirm the surprising result previously reported, with all three data points showing significant disagreement with the results from the $Z\leq 20$ nuclei. These data confirm our basic understanding of the underlying dynamics that govern $A_n$ for nuclei containing $\lesssim 50$ nucleons, but point to the need for further investigation to understand the unusual $A_n$ behaviour discovered for scattering off $^{208}$Pb.

nucl-ex↗

arXiv2022

Precision Determination of the Neutral Weak Form Factor of $^{48}$Ca

We report a precise measurement of the parity-violating asymmetry $A_{\rm PV}$ in the elastic scattering of longitudinally polarized electrons from $^{48}{\rm Ca}$. We measure $A_{\rm PV} =2668\pm 106\ {\rm (stat)}\pm 40\ {\rm (syst)}$ parts per billion, leading to an extraction of the neutral weak form factor $F_{\rm W} (q=0.8733$ fm$^{-1}) = 0.1304 \pm 0.0052 \ {\rm (stat)}\pm 0.0020\ {\rm (syst)}$ and the charge minus the weak form factor $F_{\rm ch} - F_{\rm W} = 0.0277\pm 0.0055$. The resulting neutron skin thickness $R_n-R_p=0.121 \pm 0.026\ {\rm (exp)} \pm 0.024\ {\rm (model)}$~fm is relatively thin yet consistent with many model calculations. The combined CREX and PREX results will have implications for future energy density functional calculations and on the density dependence of the symmetry energy of nuclear matter.

nucl-ex↗

arXiv2022

Measurement of the helicity dependence for single $π^{0}$ photoproduction from the deuteron

The helicity-dependent single $π^{0}$ photoproduction cross section on the deuteron and the angular dependence of the double polarisation observable $E$ for the quasi-free single $π^0$ production off the proton and the neutron have been measured for the first time from the threshold region up to the photon energy 1.4 GeV. The experiment was performed at the tagged photon facility of the MAMI accelerator and used a circularly polarised photon beam and longitudinally polarised deuteron target. The reaction products were detected using the large acceptance Crystal Ball/TAPS calorimeter, which covered 97% of the full solid angle. Comparing the cross section from the deuteron with the sum of free nucleon cross sections provides a quantitative estimate of the effects of the nuclear medium on pion production. In contrast, comparison of $E$ helicity asymmetry data from quasi-free protons off deuterium with data from a free proton target indicates that nuclear effects do not significantly affect this observable. As a consequence, it is deduced that the helicity asymmetry $E$ on a free neutron can be reliably extracted from measurements on a deuteron in quasi-free kinematics.

nucl-ex↗

arXiv2022

Measurement of Compton scattering at MAMI for the extraction of the electric and magnetic polarizabilities of the proton

A precise measurement of the differential cross-sections $dσ/dΩ$ and the linearly polarized photon beam asymmetry $Σ_3$ for Compton scattering on the proton below pion threshold has been performed with a tagged photon beam and almost $4π$ detector at the Mainz Microtron. The incident photons were produced by the recently upgraded Glasgow-Mainz photon tagging facility and impinged on a cryogenic liquid hydrogen target, with the scattered photons detected in the Crystal Ball/TAPS set-up. Using the highest statistics Compton scattering data ever measured on the proton along with two effective field theories (both covariant baryon and heavy-baryon) and one fixed-$t$ dispersion relation model, constraining the fits with the Baldin sum rule, we have obtained the proton electric and magnetic polarizabilities with unprecedented precision: \begin{align*} &{}α_{E1} = 10.99 \pm 0.16 \pm 0.47 \pm 0.17 \pm 0.34 &{}β_{M1} = 3.14 \pm 0.21 \pm 0.24 \pm 0.20 \pm 0.35 \end{align*} in units of $10^{-4}$\,fm$^3$ where the errors are statistical, systematic, spin polarizability dependent and model dependent.

nucl-ex↗

arXiv2022

Systematic treatment of hypernuclear data and application to the hypertriton

A database is under construction to provide a complete collection of published basic properties of hypernuclei such as Λ binding energies, lifetimes, or excitation energies. From these values, averages with related errors are computed in a systematic way. For each property, the overall experimental situation is depicted in form of an ideogram showing the combined probability density function of the measurements. The database is accessible via a dynamic website at https://hypernuclei.kph.uni-mainz.de with an user interface offering customizable visualizations, selections, or unit conversions. The capabilities of the database are demonstrated for the puzzling and disputed data situation of the hypertriton.

physics.data-an↗

arXiv2021

Measurement of the α-particle monopole transition form factor challenges theory: a low-energy puzzle for nuclear forces?

We perform a systematic study of the $α$-particle excitation from its ground state $0_1^+$ to the $0_2^+$ resonance. The so-called monopole transition form factor is investigated via an electron scattering experiment in a broad $Q^2$-range (from $0.5$ to $5.0$ fm$^{-2}$). The precision of the new data dramatically superseeds that of older sets of data, each covering only a portion of the $Q^2$-range. The new data allow the determination of two coefficients in a low-momentum expansion leading to a new puzzle. By confronting experiment to state-of-the-art theoretical calculations we observe that modern nuclear forces, including those derived within chiral effective field theory which are well tested on a variety of observables, fail to reproduce the excitation of the $α$-particle.

nucl-ex↗

arXiv2021

Operation and characterization of a windowless gas jet target in high-intensity electron beams

A cryogenic supersonic gas jet target was developed for the MAGIX experiment at the high-intensity electron accelerator MESA. It will be operated as an internal, windowless target in the energy-recovering recirculation arc of the accelerator with different target gases, e.g., hydrogen, deuterium, helium, oxygen, argon, or xenon. Detailed studies have been carried out at the existing A1 multi-spectrometer facility at the electron accelerator MAMI. This paper focuses on the developed handling procedures and diagnostic tools, and on the performance of the gas jet target under beam conditions. Considering the special features of this type of target, it proves to be well suited for a new generation of high-precision electron scattering experiments at high-intensity electron accelerators.

physics.ins-det↗

arXiv2021

Single $π^0$ Production Off Neutrons Bound in Deuteron with Linearly Polarized Photons

The quasifree $\overrightarrowγ d\toπ^0n(p)$ photon beam asymmetry, $Σ$, has been measured at photon energies, $E_γ$, from 390 to 610 MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the first time. The data were collected in the A2 hall of the MAMI electron beam facility with the Crystal Ball and TAPS calorimeters covering pion center-of-mass angles from 49 to 148$^\circ$. In this kinematic region, polarization observables are sensitive to contributions from the $Δ(1232)$ and $N(1440)$ resonances. The extracted values of $Σ$ have been compared to predictions based on partial-wave analyses (PWAs) of the existing pion photoproduction database. Our comparison includes the SAID, MAID, and Bonn-Gatchina analyses; while a revised SAID fit, including the new $Σ$ measurements, has also been performed. In addition, isospin symmetry is examined as a way to predict $π^0n$ photoproduction observables, based on fits to published data in the channels $π^0p$, $π^+n$, and $π^-p$.

nucl-ex↗

arXiv2021

Measurement of the Generalized Polarizabilities of the Proton at Intermediate $Q^2$

Background: Generalized polarizabilities (GPs) are important observables to describe the nucleon structure, and measurements of these observables are still scarce. Purpose: This paper presents details of a virtual Compton scattering (VCS) experiment, performed at the A1 setup at the Mainz Microtron by studying the $e p \to e p γ$ reaction. The article focuses on selected aspects of the analysis. Method: The experiment extracted the $P_{LL} -P_{TT} / ε$ and $P_{LT}$ structure functions, as well as the electric and magnetic GPs of the proton, at three new values of the four-momentum transfer squared $Q^2$: 0.10, 0.20 and 0.45 GeV$^2$. Results: We emphasize the importance of the calibration of experimental parameters. The behavior of the measured $e p \to e p γ$ cross section is presented and compared to the theory. A detailed investigation of the polarizability fits reveals part of their complexity, in connection with the higher-order terms of the low-energy expansion. Conclusions: The presented aspects are elements which contribute to minimize the systematic uncertainties and improve the precision of the physics results.

nucl-ex↗

arXiv2020

Comparison of recoil polarization in the $^{12}{\rm C}(\vec{e},{e}'\vec{p})$ process for protons extracted from $s$ and $p$ shell

We present first measurements of the double ratio of the polarization transfer components $(P^{\prime}_{\!x} \!/ P^{\prime}_{\!z} )_p/ (P^{\prime}_{\!x} \!/ P^{\prime}_{\!z} )_s$ for knock-out protons from $s$ and $p$ shells in $^{12}{\rm C}$ measured by the $^{12}{\rm C}(\vec{e},{e}'\vec{p}\,)$ reaction in quasi-elastic kinematics. The data are compared to theoretical predictions in relativistic distorted-wave impulse approximation. Our results show that differences between $s$- and $p$-shell protons, observed when compared at the same initial momentum (missing momentum) largely disappear when the comparison is done at the same proton virtuality. We observe no density-dependent medium modifications for protons from $s$ and $p$ shells with the same virtuality in spite of the large differences in the respective nuclear densities.

nucl-ex↗

arXiv2020

Spin polarizabilities of the proton by measurement of Compton double-polarization observables

The Compton double-polarization observable $Σ_{2z}$ has been measured for the first time in the $Δ(1232)$ resonance region using a circularly polarized photon beam incident on a longitudinally polarized target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using dispersion relation and chiral perturbation theory calculations, with the former resulting in: $γ_{E1E1} = -3.18 \pm 0.52$, $γ_{M1M1} = 2.98 \pm 0.43$, $γ_{E1M2} = -0.44 \pm 0.67$ and $γ_{M1E2} = 1.58 \pm 0.43$, in units of $10^{-4}~\mathrm{fm}^{4}$.

nucl-ex↗

arXiv2020

Helicity-dependent cross sections for the photoproduction of $π^0$ pairs from nucleons

The double-polarization observable $E$ and helicity-dependent cross sections $σ_{1/2}$, $σ_{3/2}$ have been measured for the photoproduction of $π^0$ pairs off quasi-free protons and neutrons at the Mainz MAMI accelerator with the Crystal Ball/TAPS setup. A circularly polarized photon beam was produced by bremsstrahlung from longitudinally polarized electrons and impinged on a longitudinally polarized deuterated butanol target. The reaction products were detected with an almost $4π$ covering calorimeter. The results reveal for the first time the helicity- and isospin-dependent structure of the $γN\rightarrow Nπ^0π^0$ reaction. They are compared to predictions from reaction models in view of nucleon resonance contributions and also to a refit of one model that predicted results for the proton and for the neutron target. The comparison of the prediction and the refit demonstrate the large impact of the new data.

nucl-ex↗

arXiv2020

Beam-Normal Single Spin Asymmetry in Elastic Electron Scattering off $^{28}$Si and $^{90}$Zr

We report on a new measurement of the beam-normal single spin asymmetry $A_{\mathrm{n}}$ in the elastic scattering of 570 MeV transversely polarized electrons off $^{28}$Si and $^{90}$Zr at $Q^{2}=0.04\, \mathrm{GeV}^2/c^2$. The studied kinematics allow for a comprehensive comparison with former results on $^{12}$C. No significant mass dependence of the beam-normal single spin asymmetry is observed in the mass regime from $^{12}$C to $^{90}$Zr.

nucl-ex↗