0000000001017634
AUTHOR
L. Nungesser
Lowest- Q2 measurement of the γp → Δ reaction: Probing the pionic contribution
To determine nonspherical angular momentum amplitudes in hadrons at long ranges (low Q^2), data were taken for the p(\vec{e},e'p)\pi^0 reaction in the Delta region at Q^2=0.060 (GeV/c)^2 utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios at W=1232 MeV are: M_{1+}^{3/2} = (40.33 +/- 0.63_{stat+syst} +/- 0.61_{model}) (10^{-3}/m_{\pi^+}),Re(E_{1+}^{3/2}/M_{1+}^{3/2}) = (-2.28 +/- 0.29_{stat+syst} +/- 0.20_{model})%, and Re(S_{1+}^{3/2}/M_{1+}^{3/2}) = (-4.81 +/- 0.27_{stat+syst} +/- 0.26_{model})%. These disagree with predictions of constituent quark models but are in reaso…
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 $e p \to e p \gamma$ reaction. The article focuses on selected aspects of the analysis. Method: The experiment extracted the $P_{LL} -P_{TT} / \epsilon$ 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 ca…
Noise and radiation damage in silicon photomultipliers exposed to electromagnetic and hadronic radiation
For the electron arm tracking system in the KAOS spectrometer at the Mainz Microtron MAMI a detector based on 2 m long scintillating fibres read out by silicon photomultipliers (SiPM) is planned. Because of the detector's close proximity to the intense electron beam a study of noise and radiation damage in SiPM has been performed. A sample of devices was exposed directly to a 14 MeV electron beam and to a mixed radiation field in the experimental area. First noticeable effects are a large increase in the dark count rate and a severe loss of the gain uniformity.
Particle tracking in kaon electroproduction with cathode-charge sampling in multi-wire proportional chambers
Abstract Wire chambers are routinely operated as tracking detectors in magnetic spectrometers at high-intensity continuous electron beams. Especially in experiments studying reactions with small cross-sections the reaction yield is limited by the background rate in the chambers. One way to determine the track of a charged particle through a multi-wire proportional chamber (MWPC) is the measurement of the charge distribution induced on its cathodes. In practical applications of this read-out method, the algorithm to relate the measured charge distribution to the avalanche position is an important factor for the achievable position resolution and for the track reconstruction efficiency. An al…
Search at the Mainz Microtron for light massive gauge bosons relevant for the muon g-2 anomaly.
A massive, but light, Abelian U(1) gauge boson is a well-motivated possible signature of physics beyond the standard model of particle physics. In this Letter, the search for the signal of such a U(1) gauge boson in electron-positron pair production at the spectrometer setup of the A1 Collaboration at the Mainz Microtron is described. Exclusion limits in the mass range of 40 MeV/c^{2} to 300 MeV/c^{2}, with a sensitivity in the squared mixing parameter of as little as ε^{2}=8×10^{-7} are presented. A large fraction of the parameter space has been excluded where the discrepancy of the measured anomalous magnetic moment of the muon with theory might be explained by an additional U(1) gauge …
The frontiers of the virtual photons program at MAMI
The most recent results and the future physics program of the high precision electron-scattering experiment at MAMI are briefly outlined. The A1 high-resolution spectrometers facility allows for a unique quality of virtual photon experiments. High precision form factor measurements, few-baryon systems highresolution structure studies and the innovative way in the search of dark photons illustrate the interplay between such diverse fields as precision atomic physics, nuclear astrophysics and astroparticle physics, where hadron physics plays a central and connecting role.
Determination of quadrupole strengths in the γ∗p→Δ(1232) transition at Q2=0.20(GeV/c)2
Abstract We report new precise p ( e → , e ′ p ) π 0 measurements at the peak of the Δ + ( 1232 ) resonance at Q 2 = 0.20 ( GeV / c ) 2 performed at the Mainz Microtron (MAMI). The new data are sensitive to both the electric (E2) and the Coulomb (C2) quadrupole amplitudes of the γ ∗ N → Δ transition. They yield precise quadrupole to dipole amplitude ratios: CMR = ( − 5.09 ± 0.28 stat + sys ± 0.30 model ) % and EMR = ( − 1.96 ± 0.68 stat + sys ± 0.41 model ) % for M 1 + 3 / 2 = ( 39.57 ± 0.75 stat + sys ± 0.40 model ) ( 10 −3 / m π + ) . The new results are in disagreement with Constituent Quark Model predictions and in qualitative agreement with models that account for mesonic contributions…
First measurement of proton's charge form factor at very low $Q^2$ with initial state radiation
We report on a new experimental method based on initial-state radiation (ISR) in e-p scattering, in which the radiative tail of the elastic e-p peak contains information on the proton charge form factor ($G_E^p$) at extremely small $Q^2$. The ISR technique was validated in a dedicated experiment using the spectrometers of the A1-Collaboration at the Mainz Microtron (MAMI). This provided first measurements of $G_E^p$ for $0.001\leq Q^2\leq 0.004 (GeV/c)^2$.
Scintillating fiber detectors for the HypHI project at GSI
WOS: 000270326800009
Search for Light Gauge Bosons of the Dark Sector at the Mainz Microtron
A new exclusion limit for the electromagnetic production of a light U(1) gauge boson {\gamma}' decaying to e^+e^- was determined by the A1 Collaboration at the Mainz Microtron. Such light gauge bosons appear in several extensions of the standard model and are also discussed as candidates for the interaction of dark matter with standard model matter. In electron scattering from a heavy nucleus, the existing limits for a narrow state coupling to e^+e^- were reduced by nearly an order of magnitude in the range of the lepton pair mass of 210 MeV/c^2 < m_e^+e^- < 300 MeV/c^2. This experiment demonstrates the potential of high current and high resolution fixed target experiments for the search fo…
STATUS OF STRANGENESS ELECTRO-PRODUCTION AT MAMI
At the Institut für Kernphysik in Mainz, Germany, the microtron MAMI has been upgraded to 1.5 GeV electron beam energy and can now be used to study strange hadronic systems. The magnetic spectrometer KAOS from GSI was dismantled and re-installed in the spectrometer facility operated by the A1 collaboration. The spectrometer's primary purpose is to study strangeness electro-production. Its compact design and its capability to detect negative and positive charged particles simultaneously under forward scattering angles complements the existing spectrometers. In 2008, an important milestone has been reached by the successful measurement of kaon production off a liquid hydrogen target. The ide…
High-Precision Determination of the Electric and Magnetic Form Factors of the Proton
New precise results of a measurement of the elastic electron-proton scattering cross section performed at the Mainz Microtron MAMI are presented. About 1400 cross sections were measured with negative four-momentum transfers squared up to Q^2=1 (GeV/c)^2 with statistical errors below 0.2%. The electric and magnetic form factors of the proton were extracted by fits of a large variety of form factor models directly to the cross sections. The form factors show some features at the scale of the pion cloud. The charge and magnetic radii are determined to be r_E=0.879(5)(stat.)(4)(syst.)(2)(model)(4)(group) fm and r_M=0.777(13)(stat.)(9)(syst.)(5)(model)(2)(group) fm.
Design criteria for multi-layered scintillating fibre arrays with inclined columns
Multi-layered scintillating fibre arrays read-out are commonly used as high resolution charged particle hodoscopes. Fibres of a column along the geometrical trajectory of incident particles are typically grouped to one pixel of a multi-channel read-out device. In some applications the incident particles will cross the detection plane with large angles w.r.t. the normal to the layers. Then, the packing of the fibres needs to be adapted to the incident particles and the columns need to be inclined. In this paper possible fibre array geometries are shown, relevant design criteria for detectors are discussed, and the effect of diverging particles incident on fibre arrays was studied using a Mon…
In-beam tests of scintillating fibre detectors at MAMI and at GSI
The performance of scintillating fibre detectors was studied with electrons at the spectrometer facility of the Mainz microtron MAMI, as well as in a C-12 beam of 2 AGeV energy and in a beam of different particle species at GSI. Multi-anode photomultipliers were used to read out one or more bundles of 128 fibres each in different geometries. For electrons a time resolution of FWHM = 1 ns was measured in a single detector plane with a detection efficiency epsilon > 99%. A time resolution of 310 ps (FWHM) between two planes of fibres was achieved for carbon ions, leading to a FWHM = 220 ps for a single detector. The hit position residual was measured with a width of FWHM = 0.27 mm. The var…
New Insight in the $Q^2$-Dependence of Proton Generalized Polarizabilities
Virtual Compton scattering on the proton has been investigated at three yet unexplored values of the four-momentum transfer $Q^2$: 0.10, 0.20 and 0.45 GeV$^2$, at the Mainz Microtron. Fits performed using either the low-energy theorem or dispersion relations allowed the extraction of the structure functions $P_{LL} -P_{TT} / \epsilon$ and $P_{LT}$, as well as the electric and magnetic generalized polarizabilities $\alpha_{E1}(Q^2)$ and $\beta_{M1}(Q^2)$. These new results show a smooth and rapid fall-off of $\alpha_{E1}(Q^2)$, in contrast to previous measurements at $Q^2$ = 0.33 GeV$^2$, and provide for the first time a precise mapping of $\beta_{M1}(Q^2)$ in the low-$Q^2$ region.
Hypernuclear production cross section in the reaction of 6Li + 12C at 2 A GeV
WOS: 000358624800021
Hypernuclear spectroscopy of products from Li-6 projectiles on a carbon target at 2 A GeV
WOS: 000322848900009
Measurement of propagation time dispersion in a scintillator
One contribution to the time resolution of a scintillation detector is the signal time spread due to path length variations of the detected photons from a point source. In an experimental study a rectangular scintillator was excited by means of a fast pulsed ultraviolet laser at different positions along its longitudinal axis. Timing measurements with a photomultiplier tube in a detection plane displaced from the scintillator end face showed a correlation between signal time and tube position indicating only a small distortion of photon angles during transmission. The data is in good agreement with a Monte Carlo simulation used to compute the average photon angle with respect to the detecti…
Silicon Detector Telescope for proton detection in electron scattering reactions at MAMI
Abstract A new Silicon Detector Telescope has been constructed and installed within the experimental facility of the A1 collaboration at Mainz Microtron, with the goal to detect low-energy protons. It consists of seven silicon layers for energy and angle measurement and a plastic scintillator for triggering purposes. The detector subtends a solid angle up to 88 msr, depending on the distance from the target and covers the proton kinetic energy range of 25–41 MeV with the mean energy resolution σ E = 0.47 MeV , operating at 500 kHz. Digital signal processing methods applied for energy reconstruction have been important for keeping the acceptable energy resolution at high counting rates. The…
Production of hypernuclei in peripheral HI collisions: The HypHI project at GSI
ECT Workshop on Strange Hadronic Matter -- SEP 26-30, 2011 -- Trento, ITALY
Recoil Polarization and Beam-Recoil Double Polarization Measurement ofηElectroproduction on the Proton in the Region of theS11(1535)Resonance
The beam-recoil double polarization P{sub x{sup '}}{sup h} and P{sub z{sup '}}{sup h} and the recoil polarization P{sub y{sup '}} were measured for the first time for the p(e-vector,e{sup '}p-vector){eta} reaction at a four-momentum transfer of Q{sup 2}=0.1 GeV{sup 2}/c{sup 2} and a center of mass production angle of {theta}=120 deg. at the Mainz Microtron MAMI-C. With a center of mass energy range of 1500 MeV<W<1550 MeV the region of the S{sub 11}(1535) and D{sub 13}(1520) resonance was covered. The results are discussed in the framework of a phenomenological isobar model (Eta-MAID). While P{sub x{sup '}}{sup h} and P{sub z{sup '}}{sup h} are in good agreement with the model, P{sub y{sup '…
Front-End Electronics for the KAOS Spectrometer at MAMI
A new front-end electronics system has been developed for the electron arm tracking detectors in the Kaos spectrometer at the Mainz microtron MAMI. The signals of multi-anode photomultipliers are collected by 96-channel front-end boards, digitized by double-threshold discriminators and the signal time is picked up by F1 TDC chips. The system was designed to process more than 4,000 channels and to cope with the high electron flux in the spectrometer and the high count rate requirement of the detectors. A subset of 288 channels was installed and successfully used in the 2008 data taking period of the Kaos spectrometer.
Measurement of Strange-Quark Contributions to the Nucleon's Form Factors atQ2=0.230 (GeV/c)2
We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a Q2 of 0.230 (GeV/c)(2) and a scattering angle of theta (e) = 30 degrees - 40 degrees. Using a large acceptance fast PbF2 calorimeter with a solid angle of delta omega = 0.62 sr, the A4 experiment is the first parity violation experiment to count individual scattering events. The measured asymmetry is A(phys)=(-5.44+/-0.54(stat)+/-0.26(sys))x10(-6). The standard model expectation assuming no strangeness contributions to the vector form factors is A(0) = (-6.30+/-0.43) x 10(-6). The difference is a direct measurement of the strangeness contribution t…
Measurement of the Neutron Electric to Magnetic Form Factor Ratio atQ2=1.58 GeV2Using the ReactionHe→3(e→,e′n)pp
A measurement of beam helicity asymmetries in the reaction $^{3}\stackrel{\ensuremath{\rightarrow}}{\mathrm{He}}(\stackrel{\ensuremath{\rightarrow}}{e},{e}^{\ensuremath{'}}n)pp$ is performed at the Mainz Microtron in quasielastic kinematics to determine the electric to magnetic form factor ratio of the neutron ${G}_{E}^{n}/{G}_{M}^{n}$ at a four-momentum transfer ${Q}^{2}=1.58\text{ }\text{ }{\mathrm{GeV}}^{2}$. Longitudinally polarized electrons are scattered on a highly polarized $^{3}\mathrm{He}$ gas target. The scattered electrons are detected with a high-resolution magnetic spectrometer, and the ejected neutrons are detected with a dedicated neutron detector composed of scintillator ba…
Beam-Recoil Polarization Measurement of π0 Electroproduction on the Proton in the Region of the Roper Resonance
The helicity-dependent recoil proton polarizations P_{x}^{'} and P_{z}^{'} as well as the helicity-independent component P_{y} have been measured in the p(e[over →],e^{'}p[over →])π^{0} reaction at four-momentum transfer Q^{2}≃0.1 GeV^{2}, center-of-mass proton emission angle θ_{p}^{*}≃90°, and invariant mass W≃1440 MeV. This first precise measurement of double-polarization observables in the energy domain of the Roper resonance P_{11}(1440) by exploiting recoil polarimetry has allowed for the extraction of its scalar electroexcitation amplitude at an unprecedentedly low value of Q^{2}, establishing a powerful instrument for probing the interplay of quark and meson degrees of freedom in t…
Initial state radiation experiment at MAMI
In an attempt to contribute further insight into the discrepancy between the Lamb shift and elastic scattering determinations of the proton charge radius, a new experiment at MAMI is underway, aimed at measuring proton form-factors at very low momentum transfers by using a new technique based on initial state radiation. This paper reports on the conclusions of the pilot measurement performed in 2010, whose main goal was to check the feasibility of the proposed experiment and to recognize and overcome any obstacles before running the full experiment. The modifications to the experimental apparatus are then explained which significantly improved the quality of data collected in the full scale…
First measurements of Λ and hyperons in elementary electroproduction at MAMI
Abstract Since 2008 the magnetic spectrometer Kaos , dedicated to the detection of charged kaons, is operating at the 1.5 GeV electron beam of MAMI at the Institut fur Kernphysik in Mainz, Germany. The strangeness programme performed in 2008–9 is addressing some important issues in the field of elementary kaon photoelectro-production reactions. The identification of Λ and Σ 0 hyperons in the missing mass spectra from kaon production off a liquid hydrogen target demonstrates the capability of the extended facility to perform strangeness electro-production spectroscopy at low momentum transfers Q 2 0.5 ( GeV / c ) 2 . Systematic uncertainties in the cross-section extraction from the data are …
"Table 1" of "Lowest Q**2 measurement of the gamma* p --> delta reaction: Probing the pionic contribution."
Measured value of SIG(C=T) + EPS*SIG(C=L) as a function of the pion angle relative to the virtual photon direction.
"Table 5" of "Lowest Q**2 measurement of the gamma* p --> delta reaction: Probing the pionic contribution."
Unpolarized cross sections as a function of the azimuthal pion angle relative to the virtual photon direction from which the extracted results were taken (to be published).
"Table 3" of "Lowest Q**2 measurement of the gamma* p --> delta reaction: Probing the pionic contribution."
Measured value of SIG(C=LT) as a function of the pion angle relative to thevirtual photon direction.
"Table 4" of "Lowest Q**2 measurement of the gamma* p --> delta reaction: Probing the pionic contribution."
Measured value of SIG(C=LTP) as a function of the pion angle relative to the virtual photon direction.
"Table 2" of "Lowest Q**2 measurement of the gamma* p --> delta reaction: Probing the pionic contribution."
Measured value of SIG(C=TT) as a function of the pion angle relative to thevirtual photon direction.
"Table 7" of "Lowest Q**2 measurement of the gamma* p --> delta reaction: Probing the pionic contribution."
Unpolarized cross sections as a function of the azimuthal pion angle relative to the virtual photon direction from which the extracted results were taken (to be published).
"Table 6" of "Lowest Q**2 measurement of the gamma* p --> delta reaction: Probing the pionic contribution."
Unpolarized cross sections as a function of the azimuthal pion angle relative to the virtual photon direction from which the extracted results were taken (to be published).