108 research outputs found

    Measurement of polarization-transfer to bound protons in carbon and its virtuality dependence

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    We measured the ratio Px/PzP_{x}/P_{z} of the transverse to longitudinal components of polarization transferred from electrons to bound protons in 12C^{12}\mathrm{C} by the 12C(e,ep)^{12}\mathrm{C}(\vec{e},e'\vec{p}) process at the Mainz Microtron (MAMI). We observed consistent deviations from unity of this ratio normalized to the free-proton ratio, (Px/Pz)12C/(Px/Pz)1H(P_{x}/P_{z})_{^{12}\mathrm{C}}/(P_{x}/P_{z})_{^{1}\mathrm{H}}, for both ss- and pp-shell knocked out protons, even though they are embedded in averaged local densities that differ by about a factor of two. The dependence of the double ratio on proton virtuality is similar to the one for knocked out protons from 2H^{2}\mathrm{H} and 4He^{4}\mathrm{He}, suggesting a universal behavior. It further implies no dependence on average local nuclear density

    Measurement of the Target-Normal Single-Spin Asymmetry in Quasielastic Scattering from the Reaction \u3csup\u3e3\u3c/sup\u3eHe\u3csup\u3e↑\u3c/sup\u3e(\u3cem\u3ee\u3c/em\u3e,\u3cem\u3ee\u3c/em\u3e′ )

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    We report the first measurement of the target single-spin asymmetry, Ay, in quasielastic scattering from the inclusive reaction 3He↑(e,e′ ) on a 3He gas target polarized normal to the lepton scattering plane. Assuming time-reversal invariance, this asymmetry is strictly zero for one-photon exchange. A nonzero Ay can arise from the interference between the one- and two-photon exchange processes which is sensitive to the details of the substructure of the nucleon. An experiment recently completed at Jefferson Lab yielded asymmetries with high statistical precision at Q2=0.13, 0.46, and 0.97  GeV2. These measurements demonstrate, for the first time, that the 3He asymmetry is clearly nonzero and negative at the 4σ–9σ level. Using measured proton-to-3He cross-section ratios and the effective polarization approximation, neutron asymmetries of −(1–3)% were obtained. The neutron asymmetry at high Q2 is related to moments of the generalized parton distributions (GPDs). Our measured neutron asymmetry at Q2=0.97  GeV2 agrees well with a prediction based on two-photon exchange using a GPD model and thus provides a new, independent constraint on these distributions

    Search for light massive gauge bosons as an explanation of the (g2)μ(g-2)_\mu anomaly at MAMI

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    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 paper, 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 (MAMI) is described. Exclusion limits in the mass range of 40 MeV up to 300 MeV with a sensitivity in the mixing parameter of down to ϵ2=8×107\epsilon^2 = 8\times 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 boson.Comment: 4 pages, 3 figure

    Measurement of the Beam-Recoil Polarization in Low-Energy Virtual Compton Scattering from the Proton

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    Double-polarization observables in the reaction epepγ\vec{e}p \rightarrow e'\vec{p'}\gamma{} have been measured at Q2=0.33(GeV/c)2Q^2=0.33 (GeV/c)^2. The experiment was performed at the spectrometer setup of the A1 Collaboration using the 855 MeV polarized electron beam provided by the Mainz Microtron (MAMI) and a recoil proton polarimeter. From the double-polarization observables the structure function PLTP_{LT}^\perp is extracted for the first time, with the value (15.4±3.3(stat.)2.4+1.5(syst.))GeV2(-15.4 \pm 3.3 (stat.)^{+1.5}_{-2.4} (syst.)) GeV^{-2}, using the low-energy theorem for Virtual Compton Sattering. This structure function provides a hitherto unmeasured linear combination of the generalized polarizabilities of the proton

    Observation of Lambda H-4 hyperhydrogen by decay-pion spectroscopy in electron scattering

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    At the Mainz Microtron MAMI, the first high-resolution pion spectroscopy from decays of strange systems was performed by electron scattering off a Be-9 target in order to study the ground-state masses of Lambda-hypernuclei. Positively charged kaons were detected by a short-orbit spectrometer with a broad momentum acceptance at zero degree forward angles with respect to the beam, efficiently tagging the production of strangeness in the target nucleus. In coincidence, negatively charged decay-pions were detected by two independent high-resolution spectrometers. About 10^3 pionic weak decays of hyperfragments and hyperons were observed. The pion momentum distribution shows a monochromatic peak at p_pi ~ 133 MeV/c, corresponding to the unique signature for the two-body decay of hyperhydrogen Lambda H-4 -> He-4 + pi-, stopped inside the target. Its binding energy was determined to be B_Lambda = 2.12 +- 0.01 (stat.) +- 0.09 (syst.) MeV with respect to the H-3 + Lambda mass
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