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Gaseous detectors based on large-size GEM-foils are planned to be used for a variety of upgrades and new experiments using high-rate and high-intensity particle beams. An excellent quality control of GEM-foils is a mandatory prerequisite to select the best foils for the assembly of a GEM detector. The high voltage stability of the foils is here of uppermost importance. In particular discharges that occur at the same position need to be detected. A spark detection system has been developed to automatically detect and record the time and position of sparks. The system is based on a commercial web camera installed in a housing for the tests and a custom-made, LabVIEW-based software for control and operation. An automatic Spark-Detection System for GEM foils was designed, built and characterized. It is able to detect and record discharges in large-size GEM foils during the quality control procedure. The spark detection e ciency was estimated to be higher than 97 %, the position resolution was determined to be approximately 0.5 mm. With this system, the characterization of GEM foils can be standardized to a much greater degree than before.

The target asymmetry T , recoil asymmetry P , and beam-target double polarization observable H were determined in exclusive$$\\pi ^0$$π 0 and$$\\eta $$η photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the$$\\gamma p \\rightarrow \\pi ^0 p$$γ p → π 0 p and$$\\gamma p \\rightarrow \\eta p$$γ p → η p reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the$$\\eta n$$η n system at$$W = 1.68\\ \\textrm{GeV}$$W = 1.68 GeV . A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the$$S_{11}(1535)$$S 11 ( 1535 ) and$$S_{11}(1650)$$S 11 ( 1650 ) resonances within the$$S_{11}$$S 11 -partial wave.

The target asymmetry T, recoil asymmetry P, and beam-target double polarization observable H were determined in exclusive $$\\pi ^0$$ π0 and $$\\eta $$ η photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the $$\\gamma p \\rightarrow \\pi ^0 p$$ γp→π0p and $$\\gamma p \\rightarrow \\eta p$$ γp→ηp reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the $$\\eta n$$ ηn system at $$W = 1.68\\ \\textrm{GeV}$$ W=1.68GeV. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the $$S_{11}(1535)$$ S11(1535) and $$S_{11}(1650)$$ S11(1650) resonances within the $$S_{11}$$ S11-partial wave.

We compute the hadronic light-by-light scattering contribution to the muon g-2 from the up, down, and strange-quark sector directly using lattice QCD. Our calculation features evaluations of all possible Wick-contractions of the relevant hadronic four-point function and incorporates several different pion masses, volumes, and lattice-spacings. We obtain a value of aμHlbl=106.8(15.9)×10-11 (adding statistical and systematic errors in quadrature), which is consistent with current phenomenological estimates and a previous lattice determination. It now appears conclusive that the hadronic light-by-light contribution cannot explain the current tension between theory and experiment for the muon g-2.

We discuss the relation of a variety of different methods to determine energy levels in lattice QCD simulations: the generalised eigenvalue, the Prony, the generalised pencil of function and the Gardner methods. All three former methods can be understood as special cases of a generalised eigenvalue problem. We show analytically that the leading corrections to an energy E l in all three methods due to unresolved states decay asymptotically exponentially like exp ( - ( E n - E l ) t ) . Using synthetic data we show that these corrections behave as expected also in practice. We propose a novel combination of the generalised eigenvalue and the Prony method, denoted as GEVM/PGEVM, which helps to increase the energy gap E n - E l . We illustrate its usage and performance using lattice QCD examples. The Gardner method on the other hand is found less applicable to realistic noisy data.

We present results for the isovector electromagnetic form factors of the nucleon computed on the CLS ensembles with \\(N_f=2+1\\) flavors of \\(\\mathcal{O}(a)\\)-improved Wilson fermions and an \\(\\mathcal{O}(a)\\)-improved vector current. The analysis includes ensembles with four lattice spacings and pion masses ranging from 130 MeV up to 350 MeV and mainly targets the low-\\(Q^2\\) region. In order to remove any bias from unsuppressed excited-state contributions, we investigate several source-sink separations between 1.0 fm and 1.5 fm and apply the summation method as well as explicit two-state fits. The chiral interpolation is performed by applying covariant chiral perturbation theory including vector mesons directly to our form factor data, thus avoiding an auxiliary parametrization of the \\(Q^2\\) dependence. At the physical point, we obtain \\(\\mu=4.71(11)_{\\mathrm{stat}}(13)_{\\mathrm{sys}}\\) for the nucleon isovector magnetic moment, in good agreement with the experimental value and \\(\\langle r_\\mathrm{M}^2\\rangle~=~0.661(30)_{\\mathrm{stat}}(11)_{\\mathrm{sys}}\\,~\\mathrm{fm}^2\\) for the corresponding square-radius, again in good agreement with the value inferred from the \\(ep\\)-scattering determination [Bernauer et~al., Phys. Rev. Lett., 105, 242001 (2010)] of the proton radius. Our estimate for the isovector electric charge radius, \\(\\langle r_\\mathrm{E}^2\\rangle = 0.800(25)_{\\mathrm{stat}}(22)_{\\mathrm{sys}}\\,~\\mathrm{fm}^2\\), however, is in slight tension with the larger value inferred from the aforementioned \\(ep\\)-scattering data, while being in agreement with the value derived from the 2018 CODATA average for the proton charge radius.

We compute the hadronic light-by-light scattering contribution to the muon \\(g-2\\) from the up, down, and strange-quark sector directly using lattice QCD. Our calculation features evaluations of all possible Wick-contractions of the relevant hadronic four-point function and incorporates several different pion masses, volumes, and lattice-spacings. We obtain a value of \\(a_\\mu^{\\text{Hlbl}} = 106.8(14.7) \\times 10^{-11}\\) (adding statistical and systematic errors in quadrature), which is consistent with current phenomenological estimates and a previous lattice determination. It now appears conclusive that the hadronic light-by-light contribution cannot explain the current tension between theory and experiment for the muon \\(g-2\\).

The neutral pion generates the leading pole contribution to the hadronic light-by-light tensor, which is given in terms of the nonperturbative transition form factor \\(\\mathcal{F}_{\\pi^0\\gamma\\gamma}(q_1^2,q_2^2)\\). Here we present an ab-initio lattice calculation of this quantity in the continuum and at the physical point using twisted-mass lattice QCD. We report our results for the transition form factor parameterized using a model-independent conformal expansion valid for arbitrary space-like kinematics and compare it with experimental measurements of the single-virtual form factor, the two-photon decay width, and the slope parameter. We then use the transition form factors to compute the pion-pole contribution to the hadronic light-by-light scattering in the muon \\(g-2\\), finding \\(a_\\mu^{\\pi^0\\text{-pole}} = 56.7(3.2) \\times 10^{-11}\\).

Euclidean time windows in the integral representation of the hadronic vacuum polarization contribution to the muon \\(g-2\\) serve to test the consistency of lattice calculations and may help in tracing the origins of a potential tension between lattice and data-driven evaluations. In this paper, we present results for the intermediate time window observable computed using O(\\(a\\)) improved Wilson fermions at six values of the lattice spacings below 0.1\\,fm and pion masses down to the physical value. Using two different sets of improvement coefficients in the definitions of the local and conserved vector currents, we perform a detailed scaling study which results in a fully controlled extrapolation to the continuum limit without any additional treatment of the data, except for the inclusion of finite-volume corrections. To determine the latter, we use a combination of the method of Hansen and Patella and the Meyer-Lellouch-L\"uscher procedure employing the Gounaris-Sakurai parameterization for the pion form factor. We correct our results for isospin-breaking effects via the perturbative expansion of QCD+QED around the isosymmetric theory. Our result at the physical point is \\(a_\\mu^{\\mathrm{win}}=(237.30\\pm0.79_{\\rm stat}\\pm1.22_{\\rm syst})\\times10^{-10}\\), where the systematic error includes an estimate of the uncertainty due to the quenched charm quark in our calculation. Our result displays a tension of 3.9\\(\\sigma\\) with a recent evaluation of \\(a_\\mu^{\\mathrm{win}}\\) based on the data-driven method.

Following the publication of the new measurement of the anomalous magnetic moment of the muon, the discrepancy between experiment and the theory prediction from the \\(g-2\\) theory initiative has increased to \\(4.2\\,\\). Recent lattice QCD calculations predict values for the hadronic vacuum polarization contribution that are larger than the data-driven estimates, bringing the Standard Model prediction closer to the experimental measurement. Euclidean time windows in the time-momentum representation of the hadronic vacuum polarization contribution to the muon \\(g-2\\) can help clarify the discrepancy between the phenomenological and lattice predictions. We present our calculation of the intermediate distance window contribution using \\(N_f=2+1\\) flavors of O\\((a)\\) improved Wilson quarks. We employ ensembles at six lattice spacings below \\(0.1\\,\\)fm and pion masses down to the physical value. We present a detailed study of the continuum limit, using two discretizations of the vector current and two independent sets of improvement coefficients. Our result at the physical point displays a tension of \\(3.9\\,\\) with a recent evaluation of the intermediate window based on the data-driven method.