A1 Collaboration Research Articles

Combined analysis of the γn→K0Σ0 and γn→K+Σ− reactions

The recently released data on differential cross sections for $\gamma n \to K^0\Sigma^0$ from the A2 and BGOOD Collaborations are used to examine the theoretical model constructed in our previous work [Phys. Rev. D \textbf{105}, 094017 (2022)] for $\gamma n \to K^+\Sigma^-$, and it is found that the model predictions are able to qualitatively reproduce the A2 data but fail to describe the BGOOD data. Then, a combined analysis of the $\gamma n \to K^0\Sigma^0$ and $\gamma n \to K^+\Sigma^-$ reactions is performed to revise the theoretical model. Due to the inconsistency problem, the A2 and BGOOD data are included in fits separately. In the case of including the A2 data, both the data for $\gamma n \to K^0\Sigma^0$ and $\gamma n \to K^+\Sigma^-$ can be fairly well described, and the contributions from the $N(1710)1/2^+$, $N(1880)1/2^+$, $N(1900)3/2^+$, and $\Delta(1920)3/2^+$ resonances are found to dominate the reactions in the lower energy region. While in the case of including the BGOOD data, although most of the data for the $\gamma n \to K^+ \Sigma^-$ reaction can be described with the exception of some noticeable discrepancies on beam asymmetries at lower energies, the BGOOD data for $\gamma n \to K^0\Sigma^0$ can be only qualitatively described, and the contributions from the $N(1710)1/2^+$, $N(1900)3/2^+$, and $\Delta(1910)1/2^+$ resonances are found to dominate the reactions in the lower energy region. In both cases, the $t$-channel $K^\ast(892)$ exchange is found to play a crucial role at forward angles in the higher energy region. Further precise measurements of data for $\gamma n \to K^0\Sigma^0$ are called on to disentangle the discrepancies between the data sets from the A2 and BGOOD Collaborations.

Understanding the systematic differences in extractions of the proton electric form factors at low Q2

Systematic differences exist between values of the proton's electric form factors in the low-$Q^2$ region extracted by different experimental and theoretical groups, though they are all making use of basically the same electron-proton scattering data. To try understand the source of these differences, we make use of the analytically well-behaved rational (N=1, M=1) function, a predictive function that can be reasonably used for extrapolations at $Q^{2} \rightarrow 0$. First, we test how well this deceptively simple two-parameter function describes the extremely complex and state-of-the-art dispersively improved chiral effective field theory calculations. Second, we carry out a complete re-analysis of the 34 sets of eletron-proton elastic scattering cross-section data of the Mainz A1 Collaboration with its unconstrained 31 normalization parameters up to $Q^{2} = 0.5~{\rm (GeV/c)^{2}}$. We find that subtle shifts in the normalization parameters can result in relatively large changes in the extracted physical qualities. In conclusion, we show that by simply using a well-behaved analytic function, the apparent discrepancy between recent form-factor extractions can be resolved.

Transverse charge density and the radius of the proton

A puzzling discrepancy exists between the values of the proton charge radius obtained using different experimental techniques: elastic electron-proton scattering and spectroscopy of electronic and muonic hydrogen. The proton radius is defined through the slope of the electric form factor, $G_E(Q^2)$, at zero four-momentum transfer, which is inaccessible in scattering experiments. We propose a novel method for extracting the proton radius from scattering data over a broad $Q^2$ range rather than attempting to directly determine the slope of $G_E$ at $Q^2 = 0$. This method relates the radius of the proton to its transverse charge density, which is the two-dimensional Fourier transform of the Dirac form factor, $F_1(Q^2)$. We apply our method to reanalyze the extensive data obtained by the A1 Collaboration [J. C. Bernauer et al., Phys. Rev. Lett. 105, 242001 (2010)] and extract a radius value, $r_E = 0.889(5)_{\text{stat}}(5)_{\text{syst}}(4)_{\text{model}}~\text{fm}$, that is consistent with the original result. We also provide new parametrizations for the Dirac and Pauli form factors and the transverse charge and magnetization densities of the proton. Our reanalysis shows that the proton radius discrepancy cannot be explained by issues with fitting and extrapolating the A1 data to $Q^2 = 0$.

On the photoproduction reactions $$\varvec{\gamma d\rightarrow \pi NN}$$

A review of our works providing a theoretical description of incoherent pion photoproduction on the deuteron is presented. The existing $\gamma d\to\pi NN$ data are analysed, especially those obtained more recently by the CLAS Collaboration at JLab, the A2 Collaboration at MAMI at Mainz, and the PION@MAX-lab Collaboration at Lund. A procedure, which accounts for the final state interactions (FSI), is applied to extract $\gamma n\to\pi N$ differential cross sections from the deuteron data. The role of FSI is discussed. We also comment on the results of other works in connection with some discrepancies seen in comparing the model with experiment. The electromagnetic properties of baryon $N^\ast$ resonances, improved using the extracted $\gamma n\to\pi N$ differential cross sections, are presented. A model description of the cross section data from charged-pion photoproduction reactions $\gamma d\to\pi^{\pm}NN$ near threshold is also given. The $\gamma d\to\pi^-pp$ data are used to extract the $E_{0+}$ multipole and total crsoss sections of the reaction $\gamma n\to\pi^-p$ near threshold.

The origin of the proton radius puzzle

The dimension of the proton, the basic building block of matter, is still object of controversy. The most precise electron-proton scattering data at low transferred momenta are re-analyzed and the extraction of the proton radius is discussed. A recent experiment from the JLAB-CLAS collaboration gives a small value for the radius (The symbol R_E^alpha stands for the root-mean-square charge radius of the proton sqrt{langle r_E^2rangle }, obtained by the experimental or theoretical Collaboration alpha .) R_E^mathrm{CLAS}= (0.831pm 0.007_mathrm{stat}pm 0.012_mathrm{syst}) fm (Xiong et al. in Nature 575:147, 2019), in contrast with previous electron scattering experiments, in particular with the MAINZ experiment (Bernauer et al. (A1 Collaboration), Phys. Rev. C 90:015206, 2014) that concluded R_E^mathrm{MAINZ}= (0.879pm 0.005_mathrm{stat}pm 0.004_mathrm{syst}pm 0.002_mathrm{model}pm 0.004_mathrm{group}) fm. The experimental results are re-analyzed in terms of different fits of the cross section and of its discrete derivative with analyticity constraints. The uncertainty on the derivative is two orders of magnitude larger than the error on the measured observable, i.e., the cross section. The systematic error associated with the radius is evaluated taking into account the uncertainties from different sources, as the extrapolation to the static point, the choice of the class of fitting functions, and the range of the data sample.

Development of an accurate DWIA model of coherent π0 —photoproduction to study neutron skins in medium heavy nuclei

Despite decades of studies which have seen the nuclear charge distribution being measured with increasing precision, the neutron distribution remains elusive. The difference between the neutron and proton distributions is often expressed as the difference of their root mean square radii: the neutron skin thickness. Recently, the A2 collaboration at MaMi has measured the skin thickness in lead through coherent pion photoproduction [1] with a very high precision. However, they do not include theoretical uncertainties, which can be significant for this process.A new reaction code in the distorted wave impulse approximation (DWIA) is developed to help the (ongoing) analysis of the recent measurement by the A2 collaboration at MaMi of the coherent pion photoproduction cross section on 116,120,124Sn isotopes [2] and to properly quantify the theoretical uncertainties.

Double polarisation observables G and E and helicity dependent cross section for single π 0 photoproduction off proton and neutron at MAMI

Meson photoproduction, like other photon-induced reactions, allow to study the excitation spectra of the nucleons and, in combination with the use of polarised beam and/or target, allow to determine the properties of the nucleon resonances by accessing many different polarisation observables with great accuracy. The A2 Collaboration, located at the MAMI facility in Mainz, used elliptically polarised photons on longitudinally polarised proton and deuteron targets, for energies up to 1.6 GeV. From the experimental data, it was possible to extract the double polarisation observables E and G on single π 0 photoproduction off the proton and the neutron. New precise results on the helicity-dependent total and differential cross sections on the deuteron were collected too.

Parametrization and applications of the low- Q2 nucleon vector form factors

We present the proton and neutron vector form factors in a convenient parametric form that is optimized for momentum transfers $\lesssim$ few GeV$^2$. The form factors are determined from a global fit to electron scattering data and precise charge radius measurements. A new treatment of radiative corrections is applied. This parametric representation of the form factors, uncertainties and correlations provides an efficient means to evaluate many derived observables. We consider two classes of illustrative examples: neutrino-nucleon scattering cross sections at GeV energies for neutrino oscillation experiments and nucleon structure corrections for atomic spectroscopy. The neutrino-nucleon charged current quasielastic (CCQE) cross section differs by 3-5% compared to commonly used form factor models when the vector form factors are constrained by recent high-statistics electron-proton scattering data from the A1 Collaboration. Nucleon structure parameter determinations include: the magnetic and Zemach radii of the proton and neutron, $[r_M^p, r_M^n] = [ 0.739(41)(23), 0.776(53)(28)]$ fm and $[r_Z^p, r_Z^n] = [ 1.0227(94)(51), -0.0445(14)(3)]$ fm; the Friar radius of nucleons, $[(r^p_F)^3, (r^n_F)^3] = [2.246(58)(2), 0.0093(6)(1)]$ fm$^3$; the electric curvatures, $[\langle r^4 \rangle^p_E, \langle r^4 \rangle^n_E ] = [1.08(28)(5), -0.33(24)(3)]$ fm$^4$; and bounds on the magnetic curvatures, $[ \langle r^4 \rangle^p_M, \langle r^4 \rangle^n_M ] = [ -2.0(1.7)(0.8), -2.3(2.1)(1.1)]$ fm$^4$. The first and dominant uncertainty is propagated from the experimental data and radiative corrections, and the second error is due to the fitting procedure.

Photoproduction of mesons and Compton scattering on the proton: Selected results from the A2 Collaboration at MAMI

The A2 Collaboration performs a manifold research program using real photons in the Crystal Ball/TAPS experiment at the MAMI accelerator facility in Mainz. The experiments take advantage of high- intensity unpolarized, linearly or circularly polarized photon beams, and unpolarized or polarized targets. The detector setup provides almost complete coverage in solid angle and is well suited for the detection of multi particle final states. In order to probe the internal structure of the nucleon, the spectrum of baryon resonances is studied via measurements of unpolarized cross sections and various polarization observables in single and double meson photoproduction. The program aiming to determine the scalar and spin polarizabilities of the nucleons with high precision is performed with the Compton scattering experiments. In 2017, the focal plane detector used in the tagging system of the Crystal Ball/TAPS experiment was completely renewed, allowing new measurements with unprecedentedly high precision. This paper presents recent selected results from the A2 Collaboration at MAMI.

Partial wave analysis for KΣ photoproduction on the nucleon valid from threshold up to W = 2.8 GeV

We have simultaneously analyzed the available kaon photoproduction data for the , , , and channels by using an isobar model constructed from the Feynman diagrammatic approach for the background terms and multipoles formulation for the resonance terms. The unknown coupling constants in the background terms as well as the helicity amplitudes and the kaon branching ratios of the resonance terms were extracted by fitting the calculated observables to nearly 8000 data points from the latest experimental measurements, including the recent and data from MAMI A2 collaboration. During the fitting process we used the data listed in the Review of Particle Properties of Particle Data Group as a guidance for the resonance properties. Since in the previous study the kaon branching ratio of the N(1710)P11 state was found to be relatively large and, as a consequence, a sizable spike appeared in the K+Σ− cross section near the threshold, in the present work we propose two isobar models, i.e. with and without this state. We have found that both models can nicely fit the data. Extensive comparisons between the result of model calculation and experimental data are presented in this paper. To investigate the role of each resonance in the present work the significance of each resonance is calculated and discussed. The present work indicates that the N(1895)S11 and Δ(1900)S31 as the important resonances in the KΣ photoproduction. We also present two examples of application of the presented isobar models, i.e. investigation of the nucleon resonance N(1535)S11 coupling to the KΣ channels and the contribution of KΣ channels to the Gerasimov–Drell–Hearn sum rule.

Meson transition form factor measurements with A2

Meson transition form factors (TFF) describe the electromagnetic structure of mesons and hence provide an important tool for the understanding of the meson-photon interactions. The A2 experiment at MAMI provides a high yield of light mesons produced in photo-induced reactions on protons, which makes the experiment ideal for precision measurements of meson TFFs via studies of meson decays. The A2 collaboration has recently published results of TFFs obtained from measurements of Dalitz decays of the π0 and η mesons as well as the ω → π0e+e− decay. For the study of the η′ Dalitz decay, data has been collected and is being analysed. As a ongoing endeavour, A2 is performing a dedicated measurement of the π0 TFF with statistics increased by several factors compared to the previous A2 result.

Measurement of the proton scalar polarizabilities at MAMI

The electric (αE1) and magnetic (βM1) scalar polarizabilities describe the response of the nucleon to an applied electric or magnetic field. They are not only fundamental properties related to the internal structure and dynamics of the nucleon, but they are important also in other areas of physics, such as atomic structure. The values of αE1 and βM1 quoted by the Particle Data Group were determined using data on the unpolarized differential cross-section of the Compton scattering $\gamma p \to \gamma p$. The measurement of the beam asymmetry Σ3, provides an alternative approach to the extraction of the scalar polarizabilities, with different sensitivity and systematics compared to the unpolarized crosssection. This asymmetry was measured recently for the first time below the pion photoproduction threshold by the A2 Collaboration with the Crystal Ball/TAPS experiment at MAMI (Mainz, Germany). A new high precision measurement of both a unpolarized cross-section and beam asymmetry Σ3a is ongoing at MAMI and the polarizabilities αE1 and βM1 will be extracted with unprecedented precision.

Recent Results on Meson Decays from A2

Light meson decays are used to investigate topics related to fundamental aspects of particle physics. Precision measurements of meson Dalitz decays give input to theoretical evaluations of the Hadronic Light-by-Light contribution (HLbL) to the anomalous magnetic moment of the muon. The A2 collaboration, using the Crystal Ball/TAPS setup at MAMI, has recently published several high precision results on transition form factors which are related to HLbL. Pseudoscalar η’ decays allow for studies of topics like ππ scattering lengths, effective field theories and fundamental symmetries. In 2014 the collaboration had a dedicated experimental campaign with one of its main goals to measure the dynamics of η’→ π0π0η with high precision. A brief overview of the experimental setup, physics motivations, analyses and results are given.

Radiative corrections to elastic-electron proton scattering and uncertainty in proton charge radius

Higher-order QED radiative corrections to elastic electron-proton scattering are discussed. It is shown that they are relevant for high-precision experiments on proton form factor measurements. Analytic result are obtained for next-to-leading second order corrections to the electron line. Light pair corrections are taken into account. The role of the hadronic contribution to vacuum polarization is discussed. Numerical results are given for the conditions of the experiment on proton form factors performed by A1 Collaboration. Preliminary results are also shown for the set-up with reconstruction of momentum transfer from the recoil proton momentum.

Meson decay studies from MAMI A2

Decays of the light mesons π0, η, ω, and η’ provide a unique laboratory to test fundamental aspects of hadron physics. Precision studies of such diverse topics as the light quark mass ratio, π−π scattering lengths, and searches for physics beyond the Standard Model are possible. Additionally, Dalitz decays of light mesons provide a way of measuring the electromagnetic meson transition formfactors in the time-like region. The A2 tagged photon facility at the Mainz Microtron provides a high yield of light mesons produced by photo-induced reactions on protons, which makes the experiment ideal for high precision measurements of meson decays. Presented here are the contributions made by the A2 collaboration to such studies.

Eta ^{prime }rightarrow eta pi pi decays in unitarized resonance chiral theory

We study the hadronic eta ^{prime }rightarrow eta pi pi decays within the framework of U(3)_{L}otimes U(3)_{R} Chiral Perturbation Theory including resonance states and the complete one-loop corrections. The amplitude is projected in partial waves and unitarized by means of the N / D method resumming both the important S- and D-wave pi pi and the subleading S-wave pi eta final-state interactions. The participating scalar multiplet mass and coupling strengths are determined from fits to the Dalitz plot experimental data recently released by the A2 collaboration. As a byproduct of our analysis, the associated Dalitz-plot slope parameters are found to be a=-,0.072(7)_{mathrm{stat}}(8)_{mathrm{syst}},,b=-,0.052(1)_{mathrm{stat}}(2)_{mathrm{syst}}, d=-,0.051(8)_{mathrm{stat}}(6)_{mathrm{syst}}, which lie in the ballpark of the current experimental and theoretical determinations.

N* Photoexcitation Results from Mainz

The A2 Collaboration at MAMI performs a manifold research program with real photons with the Crystal Ball/TAPS detector at the MAMI accelerator facility in Mainz. In order to probe the internal structure of the nucleon, the spectrum of baryon resonances is studied via measurements of unpolarized cross sections and various polarization observables in single and double meson photoproduction. In this paper, recent results from the baryon spectroscopy program performed by the A2 Collaboration at MAMI are presented.

High-statistics measurement of the η→3π0 decay at the Mainz Microtron

The largest, at the moment, statistics of 7x10^6 eta->3pi^0 decays, based on 6.2x10^7 eta mesons produced in the gamma p -> eta p reaction, has been accumulated by the A2 Collaboration at the Mainz Microtron, MAMI. It allowed a detailed study of the eta->3pi^0 dynamics beyond its conventional parametrization with just the quadratic slope parameter alpha and enabled, for the first time, a measurement of the second-order term and a better understanding of the cusp structure in the neutral decay. The present data are also compared to recent theoretical calculations that predict a nonlinear dependence along the quadratic distance from the Dalitz-plot center.

Experimental study of the γp→π0ηp reaction with the A2 setup at the Mainz Microtron

The data available from the A2 Collaboration at MAMI were analyzed to select the γp→π0ηp reaction on an event-by-event basis, which allows for partial-wave analyses of three-body final states to obtain more reliable results, compared to fits to measured distributions. These data provide the world's best statistical accuracy in the energy range from threshold to Eγ=1.45 GeV, allowing a finer energy binning in the measurement of all observables needed for understanding the reaction dynamics. The results obtained for the measured observables are compared to existing models, and the impact from the new data is checked by the fit with the revised Mainz model.11 MoreReceived 7 March 2018DOI:https://doi.org/10.1103/PhysRevC.97.055212©2018 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle & resonance productionPhotonuclear reactionsPhysical SystemsProtonsNuclear Physics

Neutron Detection in the A2 Collaboration Experiment on Neutral Pion Photo-production on Neutron

Neutron detection is of crucial importance for the neutral pion photo-production study on a neutron target that now is in progress at MAMI. Two electro-magnetic calorimeters, based on NaI and BaF2 crystals, are used in the A2 experiment. While these calorimeters are optimized for pion decay photon detection, they have a reason able efficiency for neutron detection also. The paper describes the method, which has been used to measure this efficiency using the same data taken for pion photo-production study on deuterium target with tagged photon been of 800 MeV maximal energy. The detection efficiency is a rising function of neutron momentum that reaches 40% near 1 GeV/c.