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 ¼ 0:22 ðGeV=cÞ 2 is reported here. The measured asymmetry is A LR ¼ ðÀ17:23 AE 0:82 stat AE 0:89 syst Þ Â 10 À6 . The standard model prediction assuming no strangeness is A 0 ¼ ðÀ15:87 AE 1:22Þ Â 10 À6 . 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 s E ¼ 0:050 AE 0:038 AE 0:019 and G s M ¼ À0:14 AE 0:11 AE 0:11. DOI: 10.1103/PhysRevLett.102.151803 PACS numbers: 13.40.Gp, 11.30.Er, 12.15.Ày, 14.20.Dh Sea quarks are an important ingredient to describe nucleon properties in terms of fundamental QCD degrees of freedom. Strange quark-antiquark pairs might play a relevant role and affect, e.g., the electromagnetic properties of the nucleon. The contribution of strange quarks to the charge radius and magnetic moment in the nucleon ground state is of specific interest since this is a pure sea quark effect. The strange quark contribution to the electromagnetic form factors of the nucleon can be expressed in terms of the strange electric and magnetic form factors G s E and G s M . There are various theoretical approaches for estimating the strange form factors [1,2], such as quark soliton models [3][4][5], chiral quark models [6], quenched lattice calculations [7], or two-component models [8]. Parity violating electron scattering provides a direct experimental approach [9][10][11].A measurement of parity violation necessarily involves a weak interaction probe of the nucleon. This provides additional information allowing a measurement of G s E and G s M . Within the standard model of electroweak interaction, it is known that electromagnetic and weak currents are related. Assuming isospin symmetry, the weak vector form factors G p E;M of the proton, describing the vector coupling to the Z 0 boson, can be expressed in terms of the electromagnetic nucleon form factors G p;n E;M and the strange form factors G s E;M . The interference between tree level electromagnetic and weak amplitudes leads to a parity violating asymmetry in the elastic scattering cross section of left-and righthanded electrons (LR) L , R :This asymmetry can be written as a sum of three terms, A LR ¼ A V þ A S þ A A . A V represents the vector coupling on the proton vertex without strangeness contribution, A S contains the strange quark vector contribution, and A A represents the axial coupling to the proton vertex [11]: PRL 102, 151803 (2009)

We report on a new measurement of the beam transverse single spin asymmetry in electron-proton elastic scattering, A ep ⊥ , at five beam energies from 315.1 MeV to 1508.4 MeV and at a scattering angle of 30 • < θ < 40 • . The covered Q 2 values are 0.032, 0.057, 0.082, 0.218, 0.613 (GeV/c) 2 . The measurement clearly indicates significant inelastic contributions to the two-photon-exchange (TPE) amplitude in the low-Q 2 kinematic region. No theoretical calculation is able to reproduce our result. Comparison with a calculation based on unitarity, which only takes into account elastic and πN inelastic intermediate states, suggests that there are other inelastic intermediate states such as ππN, KΛ and ηN. Covering a wide energy range, our new high-precision data provide a benchmark to study those intermediate states.PACS numbers: 13.60. Fz, 11.30.Er, 13.40.Gp As a probe of hadron structure, electron scattering has two advantages: the structurelessness of the electron and the smallness of the electromagnetic coupling (α ≈ 1/137). The small coupling allows to expand the scattering amplitude in powers of α and to interpret experiments within the one-photon-exchange (Born) approximation. This leading order approximation enables a straightforward extraction of the electromagnetic form factors with the Rosenbluth separation technique [1]. For a precise extraction of the form factors it is necessary to include higher order quantum corrections [2,3]. Importantly, most of those corrections do not alter the Rosenbluth formula in that they contribute an overall factor to the cross section.The contribution that is expected to break this pattern [4,5] is the two-photon-exchange (TPE) diagram depicted in Fig. 1. For a long time the TPE effects have eluded direct experimental searches [6][7][8]. The situation changed when a striking discrepancy between the Rosenbluth separation [9, 10] and the polarization transfer [11?-13] data on the proton form factor ratio µ p G E /G M was observed. To evaluate the TPE corrections one needs to model the doubly-virtual Compton scattering (VVCS) in the most general kinematics. This involves calculating the two-current correlator with inclusive hadronic intermediate states. The full account of the inclusive intermediate states contribution can be made in the limited nearforward kinematics [15]. Beyond the forward kinematics, it is only possible to account for the elastic [16][17][18][19] or the pion-nucleon (πN) [20] intermediate state contributions.The theoretical framework for calculating the TPE contributions plays an important role in evaluating the two-boson-exchange corrections to precision low-energy tests of the Standard Model (SM) in the electroweak sector. The proton polarizability contribution to the fine structure of light muonic atoms stems from the TPE diagram and is a substantial ingredient [21] in the proton radius puzzle, the 7σ discrepancy in the value of the proton charge radius extracted from hydrogen spectroscopy [22] and electron-proton (ep) scattering [23] on one hand, an...

New measurements of the beam normal single spin asymmetry in the electron elastic and quasielastic scattering on the proton and deuteron, respectively, at large backward angles and at hQ 2 i ¼ 0.22 ðGeV=cÞ The exchange of two hard virtual photons in the elastic electron-nucleon scattering beyond the one-photon exchange Born approximation has been the subject of recent investigation [1,2]. Two complementary methods, a determination from a measurement of a differential cross section using unpolarized electrons (Rosenbluth separation) and the measurement of the polarization transfer to the proton final state, gave significantly different results forThe two-photon exchange has been addressed as the explanation for such a discrepancy. Other observables in which the two-photon exchange physics plays a role are: neutron form factors, resonance electroproduction, the pion form factor and the elastic electron-nucleus cross section, in particular deuteron and 3 He [1]. The exchange of two virtual photons implies the excitation of nucleon intermediate states offering the possibility of testing models of hadronic structure [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. In contrast to previous measurements at forward [23,24] and close to right angles [25], the A4 measurements presented in this work have been performed at large backward angles. The A4 measurements explore a new important parameter region of the virtualities of the exchanged virtual photons, involved in the theoretical calculation of the nucleon intermediate states, see Fig. 2 from Ref. [11]. Moreover, in contrast to the SAMPLE measurement [26], at similar backward scattering angle, the A4 measurements have improved the precision by a factor ∼4. The A4 measurements have been performed at larger beam energies, probing the πN and Δ resonance inelastic intermediate states.The two-photon exchange physics affects the parity violating asymmetry through the normalization to the electromagnetic amplitude [27] and it is of special relevance in precision measurements of the weak charge of the proton at low Q 2 like Q weak [28] and P2 [29]. It is also useful in the theoretical determination of the γZ box diagrams which also takes part in the proton weak charge radiative corrections [30][31][32][33].Two-photon exchange effects can be observed in radiative corrections to the cross section, the ratio in e − p and e þ p elastic scattering cross sections, the depolarization tensor, and the target and beam normal single spin asymmetries [34][35][36][37]. Six generalized form factors depending on two kinematic variables parametrize the elastic electron-proton scatteringG M ðs; Q 2 Þ,G E ðs; Q 2 Þ, [10]. There are also partonic calculations which resort to generalized parton distributions of the nucleon and are able to resolve the discrepancy at large Q 2 [12,16]. The T-odd observables target and beam normal spin asymmetries arise from the interference of the one-photon and two-photon exchange amplitudes, see Fig. 1 in Ref. [39]. The target normal spi...

Sufficient energy resolution is the key issue for the calorimetry in particle and nuclear physics. The calorimeter of the A4 parity violation experiment at MAMI is a segmented calorimeter where the energy of an event is determined by summing the signals of neighboring channels. In this case, the precise matching of the individual modules is crucial to obtain a good energy resolution. We have developed a calibration procedure for our total absorbing electromagnetic calorimeter which consists of 1022 lead fluoride (PbF2) crystals. This procedure reconstructs the single-module contributions to the events by solving a linear system of equations, involving the inversion of a 1022×1022-matrix. The system has shown its functionality at beam energies between 300 and 1500 MeV and represents a new and fast method to keep the calorimeter permanently in a well-calibrated state

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