M. Morlet

Measurement of strange-quark contributions to the nucleon's form factors at Q(2) = 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 to the vector form factors of the proton. The extracted value is G(s)(E) + 0.225G(s)(M) = 0.039+/-0.034 or F(s)(1) + 0.130F(s)(2) = 0.032+/-0.028.

Evidence for Strange-Quark Contributions to the Nucleon's Form Factors at Q^2 = 0.108 (GeV/c)^2

We report on a measurement of the parity violating asymmetry in the elastic scattering of polarized electrons off unpolarized protons with the A4 apparatus at MAMI in Mainz at a four momentum transfer value of Q(2)=0.108 (GeV/c)(2) and at a forward electron scattering angle of 30 degrees p)=[-1.36+/-0.29(stat)+/-0.13(syst)]x10(-6). The expectation from the standard model assuming no strangeness contribution to the vector current is A(0)=(-2.06+/-0.14)x10(-6). We have improved the statistical accuracy by a factor of 3 as compared to our previous measurements at a higher Q2. We have extracted the strangeness contribution to the electromagnetic form factors from our data to be G(s)(E)+0.106G(s)(M)=0.071+/-0.036 at Q(2)=0.108 (GeV/c)(2). We again find the value for G(s)(E)+0.106G(s)(M) to be positive, this time at an improved significance level of two sigma.

Measurement of Strange Quark Contributions to the Vector Form Factors of the Proton at Q2 = 0.22 (GeV/c)2

A new measurement of the parity violating asymmetry in elastic electron scattering on hydrogen at backward angles and at a four momentum transfer of Q;{2} = 0.22 (Ge V / c);{2} is reported here. The measured asymmetry is A_{LR} = (-17.23 +/- 0.82_{stat} +/- 0.89_{syst}) x 10;{-6}. The standard model prediction assuming no strangeness is A_{0} = (-15.87 +/- 1.22) x 10;{-6}. In combination with previous results from measurements at forward angles, it is possible to disentangle for the first time the strange form factors at this momentum transfer, G_{E};{s} = 0.050 +/- 0.038 +/- 0.019 and G_{M};{s} = -0.14 +/- 0.11 +/- 0.11.

201 MeV proton excitation of giant resonances in 208Pb: Macroscopic and microscopic analysis

Abstract The region of the giant resonances in 208 Pb has been investigated by inelastic scattering of 201 MeV protons. To test the analysis, angular distributions were measured for the low-lying 3 − , 5 − , 2 + and 4 + collective states. The giant isoscalar quadrupole resonance (ISGQR) is split into two structures, one at 9.0 MeV with a full width at half-maximum Γ = 1.0 MeV, the other one at 10.6 MeV ( Γ = 2.0 MeV), with fine structures at 8.9, 9.3, 10.1, 10.6 and 11 MeV. A macroscopic analysis using the distorted-wave Born approximation (DWBA) leads for the low-lying collective levels, as well as for the ISGQR, to transition probabilities too small by a factor of two, compared with those obtained in other reactions. Microscopic analysis using the distorted-wave impulse approximation (DWIA), with three different sets of random phase approximation (RPA) transition densities, is in very good agreement with the data. At forward angles, in the 12 to 16 MeV excitation energy region, a strong resonance at 13.5 MeV ( Γ = 3.6 MeV) is accounted for by the Coulomb excitation of the isovector giant dipole resonance (IVGDR); at larger angles the results are compatible with the excitation of the isoscalar monopole resonance (ISGMR) located at 13.9 MeV ( Γ = 2.6 MeV). A resonance located at 21.5 MeV ( Γ = 5.7 MeV) appears as the superposition of an isovector quadrupole resonance (IVGQR) excited by Coulomb interaction and a resonance of multipolarity L = 1 ΔT = 0 (ISGDR “squeezing mode”).

Systematics of the excitation of M1 resonances in medium heavy nuclei by 200 MeV proton inelastic scattering

Abstract In a series of seventeen nuclei ranging from 51 V to 14 Ce, broad resonance structures are observed at energies between 8 and 10 MeV, nearly mass independent. These resonances have very forward peaked angular distributions which imply that they are populated by an angular momentum transfer of zero. This together with the observed excitation energies suggests an M1 character for these resonances. In 51 V, 58 Ni, 60 Ni, 62 Ni, a sharp peak located at an excitation energy above the threshold for neutron emission is interpreted as a part of the T 0 + 1 component of the M1 resonance. Cross sections are given for all the M1 resonances. For 58 Ni, 90 Zr, 92 Mo, 120 Sn and 140 Ce, an “attenuation” factor for the cross sections is extracted in a DWIA calculation assuming simple shell-model structures for these resonances.

Proton-nucleus elastic scattering at 156 MeV

Abstract Cross sections for the elastic scattering of 156 MeV protons on eleven targets ranging from 12 C to 209 Bi were measured and an optical model analysis has been performed. The effect of different optical potentials in DWIA inelastic scattering calculations is shown by some examples.

On the nature of low-lying collective 1+ states in the heavy deformed nuclei 154Sm, 156Gd and 164Dy and in the f72 shell nucleus 46Ti

Abstract High resolution inelastic proton scattering at 201 MeV on 154Sm, 156Gd and 164Dy is used to clarify the nature of low-lying magnetic dipole states previously observed in inelastic electron scattering. An upper limit on the spin contribution to these collective magnetic dipole excitations is presented. The roles of spin and convection currents are also studied by inelastic proton and electron scattering to the low-lying 1+ state at 4.319 MeV in the f 7 2 - shell nucleus 46Ti. A substantial orbital contribution is observed in all cases.

Excitation of giant resonances by inelastic proton scattering at 155 MeV

Abstract The inelastic proton spectra in the giant resonance region have been studied with 155 MeV protons on 27 Al, 40 Ca, Fe, 89 Y, 115 In, Sn, 165 Ho, 181 Ta, 197 Au, 208 Pb and 209 Bi. For all nuclei a strong resonance at E = (63.0 ± 1.5) A − 1 3 MeV is seen, with an angular distribution in agreement with L = 2 DWBA calculations. The resonance exhausts between 50 % ( 40 Ca) to 14 % of the energy weighted sum rule. Fine structure similar to that reported in electron scattering experiments is observed for 208 Pb. In addition some other possible excitations are discussed.

Quasifree proton-proton scattering on 3He

Abstract Quasifree proton-proton scattering cross sections in the 3 He(p, 2p) 2 H reaction have been measured at 155 MeV incident energy for low momenta of the recoiling deuteron and are compared with theoretical calculations in the DWIA for different 3 He radial wave functions. The results are very sensitive to the radial form of the wave functions and the best agreement is obtained with an exponential wave function with short-range correlations.

POLDER: a tensor polarimeter for intermediate energy deuterons

A new deuteron tensor polarimeter (POLDER), based upon the charge exchange reaction 1H(→d, 2p)n, has been constructed and calibrated in the energy range 175–500 MeV for all polarisation signals. The large figures of merit obtained make POLDER a unique instrument in this energy domain. The operation of this polarimeter has been checked in practice in a test experiment on 40Ca(→d, →d′)40Ca elastic scattering performed at Ed = 380 MeV.