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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)

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The (anti-Proton ANnihiliation at DArmstadt) experiment will be one of the four flagship experiments at the new international accelerator complex FAIR (Facility for Antiproton and Ion Research) in Darmstadt, Germany. will address fundamental questions of hadron physics and quantum chromodynamics using high-intensity cooled antiproton beams with momenta between 1.5 and 15 GeV/c and a design luminosity of up to 2 × 1032 cm−2 s−1. Excellent particle identification (PID) is crucial to the success of the physics program. Hadronic PID in the barrel region of the target spectrometer will be performed by a fast and compact Cherenkov counter using the detection of internally reflected Cherenkov light (DIRC) technology. It is designed to cover the polar angle range from 22° to 140° and will provide at least 3 standard deviations (s.d.) π/K separation up to 3.5 GeV/c, matching the expected upper limit of the final state kaon momentum distribution from simulation. This documents describes the technical design and the expected performance of the Barrel DIRC detector. The design is based on the successful BaBar DIRC with several key improvements. The performance and system cost were optimized in detailed detector simulations and validated with full system prototypes using particle beams at GSI and CERN. The final design meets or exceeds the PID goal of clean π/K separation with at least 3 s.d. over the entire phase space of charged kaons in the Barrel DIRC.

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...