0000000000061618
AUTHOR
Simon ŠIrca
Observation ofHΛ4Hyperhydrogen by Decay-Pion Spectroscopy in Electron Scattering
At the Mainz Microtron MAMI, the first high-resolution pion spectroscopy from decays of strange systems was performed by electron scattering off a ^9B
Comparing proton momentum distributions in A = 2 and 3 nuclei via 2H 3H and 3He (e,e′p) measurements
We report the first measurement of the $(e,e'p)$ reaction cross-section ratios for Helium-3 ($^3$He), Tritium ($^3$H), and Deuterium ($d$). The measurement covered a missing momentum range of $40 \le p_{miss} \le 550$ MeV$/c$, at large momentum transfer ($\langle Q^2 \rangle \approx 1.9$ (GeV$/c$)$^2$) and $x_B>1$, which minimized contributions from non quasi-elastic (QE) reaction mechanisms. The data is compared with plane-wave impulse approximation (PWIA) calculations using realistic spectral functions and momentum distributions. The measured and PWIA-calculated cross-section ratios for $^3$He$/d$ and $^3$H$/d$ extend to just above the typical nucleon Fermi-momentum ($k_F \approx 250$ …
Search for Effects Beyond the Born Approximation in Polarization Transfer Observables ine→pElastic Scattering
Intensive theoretical and experimental efforts over the past decade have aimed at explaining the discrepancy between data for the proton electric to magnetic form factor ratio, $$G_{E}/G_{M}$$, obtained separately from cross section and polarization transfer measurements. One possible explanation for this difference is a two-photon-exchange (TPEX) contribution. In an effort to search for effects beyond the one-photon-exchange or Born approximation, we report measurements of polarization transfer observables in the elastic $$H(\vec{e},e'\vec{p})$$ reaction for three different beam energies at a fixed squared momentum transfer $Q^2 = 2.5$ GeV$^2$, spanning a wide range of the virtual photon p…
High Resolution Spectroscopy ofBΛ12by Electroproduction
An experiment measuring electroproduction of hypernuclei has been performed in Hall A at Jefferson Lab on a $^{12}$C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a Ring Imaging CHerenkov detector (RICH) were added to the Hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed \lam{12}{B} spectrum shows for the first time identifiable strength in the core-excited region between the ground-state {\it s}-wave $\Lambda$ peak and the 11 MeV {\it p}-wave $\Lambda$ peak.
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…
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…
Measurement of the ExclusiveH3e(e,e′p)Reaction Below the Quasielastic Peak
New, high-precision measurements of the 3He(e,ep) reaction using the A1 Collaboration spectrometers at the Mainz microtron MAMI are presented. These were performed in antiparallel kinematics at energy transfers below the quasielastic peak, and at a central momentum transfer of 685 MeV/c. Cross sections and distorted momentum distributions were extracted and compared to theoretical predictions and existing data. The longitudinal and transverse behavior of the cross section was also studied. Sizable differences in the cross-section behavior from theoretical predictions based on the plane wave impulse approximation were observed in both the two- and three-body breakup channels. Full Faddeev-ty…
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.
Precise pion electroproduction in thep(e, e? ?+)n reaction at W=1125 MeV
The reactione+p →> e+π ++n at c.m. energyW=1125MeV and momentum transfer Q2=0.117GeV2/c2 has been measured. The transverse and longitudinal structure functions have been separated by varying the polarization of the virtual photon (Rosenbluth plot) with a 3 to 4% error. In addition the longitudinal-transverse interference term has been determined measuring the right-left asymmetry with an accuracy of 3%. The experimental data are compared to model calculations, and the sensitivity of the results to the axial and pion formfactors is discussed.
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…
Ground-state binding energy of HΛ4 from high-resolution decay-pion spectroscopy
Abstract A systematic study on the Λ ground state binding energy of hyperhydrogen H Λ 4 measured at the Mainz Microtron MAMI is presented. The energy was deduced from the spectroscopy of mono-energetic pions from the two-body decays of hyperfragments, which were produced and stopped in a 9Be target. First data, taken in the year 2012 with a high resolution magnetic spectrometer, demonstrated an almost one order of magnitude higher precision than emulsion data, while being limited by systematic uncertainties. In 2014 an extended measurement campaign was performed with improved control over systematic effects, increasing the yield of hypernuclei and confirming the observation with two indepen…
Measurement of GEp/GMp via polarization transfer at Q2 = 0.4 GeV /c2
The polarization transfer from longitudinally polarized electrons to protons in the elastic scattering p( e, e' p) has been measured around Q2 = 0.4 (GeV/c)2 with the three-spectrometer facility at the Mainz microtron MAMI. From this polarization transfer the ratio G Ep/(G Mp/μp) has been determined. The ratio is found to be slightly less than unity in agreement with recent results from other laboratories and from the Rosenbluth separation of cross-sections measured with unpolarized electrons.
Rosenbluth Separation of the π^{0} Electroproduction Cross Section.
We present deeply virtual $\pi^0$ electroproduction cross-section measurements at $x_B$=0.36 and three different $Q^2$--values ranging from 1.5 to 2 GeV$^2$, obtained from experiment E07-007 that ran in the Hall A at Jefferson Lab. The Rosenbluth technique was used to separate the longitudinal and transverse responses. Results demonstrate that the cross section is dominated by its transverse component, and thus is far from the asymptotic limit predicted by perturbative Quantum Chromodynamics. An indication of a non-zero longitudinal contribution is provided by the interference term $\sigma_{LT}$ also measured. Results are compared with several models based on the leading twist approach of G…
Unpolarized and polarized elementary kaon electroproduction cross sections measured at MAMI
Present and future research into the electroproduction of kaons plays an im- portant role at Mainz Microtron MAMI. With the Kaos spectrometer employed for kaon detection in the multi-spectrometer facility, cross section measurements of the exclusive p(e;e 0 K + ); 0 reactions at low momentum transfers have been performed. Isobar and Regge-plus-resonance models were compared with the data. These measurements have clearly discriminated between e ective Lagrangian models for photo- and electroproduc- tion of strangeness. New experiments with polarized beam at low four-momentum transfer are addressing the imaginary part of the longitudinal-transverse response in this process, that can be separa…
New Measurements of the Transverse Beam Asymmetry for Elastic Electron Scattering from Selected Nuclei
We have measured the beam-normal single-spin asymmetry $A_n$ in the elastic scattering of 1-3 GeV transversely polarized electrons from $^1$H and for the first time from $^4$He, $^{12}$C, and $^{208}$Pb. For $^1$H, $^4$He and $^{12}$C, the measurements are in agreement with calculations that relate $A_n$ to the imaginary part of the two-photon exchange amplitude including inelastic intermediate states. Surprisingly, the $^{208}$Pb result is significantly smaller than the corresponding prediction using the same formalism. These results suggest that a systematic set of new $A_n$ measurements might emerge as a new and sensitive probe of the structure of heavy nuclei.
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…
Overview of the electromagnetic production of strange mesons at MAMI
Abstract The Mainz Microtron MAMI provides a continuous-wave unpolarized or spin-polarized electron beam with energies up to 1.6 GeV and high degrees of polarization. Electro-production of strange mesons is performed in the multi-spectrometer facility with the Kaos spectrometer for kaon detection and a high-resolution spectrometer for electron detection in plane or out of plane. Differential cross section measurements of exclusive p ( e , e ′ K + ) Λ , Σ 0 reactions at low four-momentum transfers in the nucleonʼs third resonance region have been done, followed by a measurement of the beam helicity asymmetry for p ( e → , e ′ K + ) Λ . These studies are important for the understanding of the…
Experimental investigations of the hypernucleus $_Λ^4$H
International audience; Negatively charged pions from two-body decays of stopped _Lambda^4H hypernuclei were studied in 2012 at the Mainz Microtron MAMI, Germany. The momenta of the decay-pions were measured with unprecedented precision by using high-resolution magnetic spectrometers. A challenge of the experiment was the tagging of kaons from associated K^+∧ production off a Be target at very forward angles. In the year 2014, this experiment was continued with a better control of the systematic uncertainties, with better suppression of coincident and random background, improved particle identification, and with higher luminosities. Another key point of the progress was the improvemen…
Polarization transfer to bound protons measured by quasi-elastic electron scattering on $^{12}$C
We report the measurements of the transverse ($P'x$) and longitudinal ($P'z$) components of the polarization transfer to a bound proton in carbon via the quasi-free $^{12}{\rm C}(\vec e,e'\vec p)$ reaction, over a wide range of missing momenta. We determine these polarization-transfers separately for protons knocked out from the $s$- and $p$-shells. The electron-beam polarization was measured to determine the individual components with systematic uncertainties which allow a detailed comparison with theoretical calculations.
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…
Final State Interaction Effects in 3He(e ,e'p)
Abstract Asymmetries in quasi-elastic 3 He ( e → , e ′ p ) have been measured at a momentum transfer of 0.67 (GeV/ c ) 2 and are compared to a calculation which takes into account relativistic kinematics in the final state and a relativistic one-body current operator. With an exact solution of the Faddeev equation for the 3 He -ground state and an approximate treatment of final state interactions in the continuum good agreement is found with the experimental data.
Large recoil momenta in the D(e,e′p)n reaction
Abstract The D(e,e′p)n reaction cross section has been measured for recoil momenta ranging from 17 MeV/c up to 950 MeV/c at momentum transfers between 600 MeV/c and 700 MeV/c. At recoil momenta above 400 MeV/c, the gross features of the cross section are only reproduced if virtual nucleon excitations are included in the calculations.
Strange hadronic physics in electroproduction experiments at the Mainz Microtron
Abstract Present and future research into the electroproduction of strangeness plays an important role at Mainz Microtron MAMI. With the Kaos spectrometer for kaon detection operated in the multi-spectrometer facility first cross section measurements of the exclusive p ( e , e ′ K + ) Λ , Σ 0 reactions at low-momentum transfers have been performed. These measurements have clearly discriminated between effective Lagrangian models for photo- and electroproduction of strangeness. Recently, the Kaos spectrometer was upgraded to a double-arm spectrometer for the measurement of elementary cross sections at very forward scattering angles and for the missing mass spectroscopy of hypernuclear states…
Deeply virtual compton scattering off the neutron.
The present experiment exploits the interference between the Deeply Virtual Compton Scattering (DVCS) and the Bethe-Heitler processes to extract the imaginary part of DVCS amplitudes on the neutron and on the deuteron from the helicity-dependent D$({\vec e},e'\gamma)X$ cross section measured at $Q^2$=1.9 GeV$^2$ and $x_B$=0.36. We extract a linear combination of generalized parton distributions (GPDs) particularly sensitive to $E_q$, the least constrained GPD. A model dependent constraint on the contribution of the up and down quarks to the nucleon spin is deduced.
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 '…
Prospects for hypernuclear physics at Mainz: From KAOS@MAMI to PANDA@FAIR
Abstract At the Mainz Microtron hypernuclei are produced by ( e , e ′ K ) reactions. A dedicated kaon spectrometer located at 0° with respect to the electron beam is used to detect kaons emitted in forward direction thus tagging events involving strangeness production. By measuring the momenta of pions from two body weak decays using high resolution magnetic spectrometers one gains direct access to the ground state masses of the produced hyperfragments. At FAIR the PANDA Collaboration intends to produce double-hypernuclei by numbers with an antiproton beam and study their high resolution γ -spectroscopy thus providing for the first time precise information on the level structure of these nu…
Neutral Pion Threshold Production atQ2=0.05GeV2/c2and Chiral Perturbation Theory
New data are presented on the $p({e,e}^{\ensuremath{'}}p){\ensuremath{\pi}}^{0}$ reaction at threshold at a four-momentum transfer of ${Q}^{2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.05{\mathrm{GeV}}^{2}{/c}^{2}$. The data were taken with the three-spectrometer setup of the A1 Collaboration at the Mainz Microtron MAMI. The complete center of mass solid angle was covered up to a center of mass energy of 4 MeV above threshold. Combined with measurements at three different values of the virtual photon polarization $\ensuremath{\epsilon}$, the structure functions ${\ensuremath{\sigma}}_{T}$, ${\ensuremath{\sigma}}_{L}$, ${\ensuremath{\sigma}}_{\mathrm{TT}}$, and ${\ensuremath{\sigma…
Kaon Tagging at 0° Scattering Angle for High-Resolution Decay-Pion Spectroscopy
At the Mainz Microtron hypernuclei can be studied by (e,e'K) reactions. By detecting the kaon which is emitted in forward direction, with the KAOS spectrometer placed at 0 scattering angle, reactions involving open strangeness production are tagged. High-resolution magnetic spectrometers are then used to coincidentally detect the mono- energetic decay-pions from mesonic two-body weak decays of light hypernuclei at rest. As a pioneering experiment has confirmed, the KAOS spectrometer is exposed to a large flux of background particles, mostly positrons from bremsstrahlung pair production. In order to increase the e ciency of kaon identification the KAOS spectrometer was modi- fied to suppress…
Evidence for narrow Δ0(1232) states in the 12 C(e,e'pπ−}11 C Reaction
The reaction 12C(e,e'Δ0)11C →12C(e,e'pπ−)11C was investigated at the Mainz Microtron MAMI in a triple coincidence measurement using the three spectrometer setup of the A1 Collaboration. The good missing mass resolution of σm= 0.27 MeV/c2 allowed to select the events belonging to the ground state of 11C. Cutting on these events the excitation energy spectra of 12CΔ0 show evidence for two peaks of about 4 MeV width (FWHM) at 282 MeV and 296 MeV with a significance of about 4.5 standard deviations. The peaks are interpreted in a simple weak coupling model as bound Δ0 states in 12CΔ0.
Spectroscopy ofLiΛ9by electroproduction
Background: In the absence of accurate data on the free two-body hyperon-nucleon interaction, the spectra of hypernuclei provides information on the details of the effective hyperon-nucleon interaction.Purpose: To obtain a high-resolution binding-energy spectrum for the ${}^{9}\mathrm{Be}(e,{e}^{\ensuremath{'}}{K}^{+})_{\ensuremath{\Lambda}}^{9}\mathrm{Li}$ reaction.Method: Electroproduction of the hypernucleus $_{\ensuremath{\Lambda}}^{9}\mathrm{Li}$ has been studied for the first time with sub-MeV energy resolution in Hall A at Jefferson Lab on a $^{9}\mathrm{Be}$ target. In order to increase the counting rate and to provide unambiguous kaon identification, two superconducting septum magn…
Reinterpretation of Classic Proton Charge Form Factor Measurements
In 1963, a proton radius of $0.805(11)~\mathrm{fm}$ was extracted from electron scattering data and this classic value has been used in the standard dipole parameterization of the form factor. In trying to reproduce this classic result, we discovered that there was a sign error in the original analysis and that the authors should have found a value of $0.851(19)~\mathrm{fm}$. We additionally made use of modern computing power to find a robust function for extracting the radius using this 1963 data's spacing and uncertainty. This optimal function, the Pad\'{e} $(0,1)$ approximant, also gives a result which is consistent with the modern high precision proton radius extractions.
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 …