Transverse Beam Spin Asymmetries in Forward-Angle Elastic Electron-Proton Scattering

Armstrong, David; Arvieux, J.; Asaturyan, R.; Averett, T.; Bailey, S. L.; Batigne, G.; Beck, Kenneth M.; Beise, E. J.; Benesch, J.; Bimbot, L.; Bírchall, J.; Biselli, A. S.; Bosted, P.; Boukobza, E.; Breuer, H.; Carlini, R.; Carr, R.; Chant, N. S.; Chao, Yu‐Chiu; Chattopadhyay, S.; Clark, Robert Beck; Covrig, S.; Cowley, A.A.; Dale, D.; Davis, C. A.; Falk, W.R.; Finn, J. M.; Forest, Tony A.; Franklin, G.; Furget, C.; Gaskell, D.; Grames, Joseph; Griffioen, K. A.; Grimm, K.; Guillon, B.; Güler, H.; Hannelius, L.; Hasty, R.; Allen, A. Hawthorne; Horn, T.; Johnston, K.; Jones, M.; Kammel, P.; Kazimi, R.; P, King; Kolarkar, A.; Korkmaz, E.; Korsch, W.; Kox, S.; Kühn, J.; Lachniet, J.; Lee, L.; Lenoble, J.; Liatard, E.; Liu, J.; Loupias, B.; Lung, A.; Marchand, D.; Martin, J. W.; McFarlane, K. W.; McKee, D.; McKeown, R. D.; Merchez, F.; Mkrtchyan, H.; Moffit, B.; Morlet, M.; Nakagawa, I.; Nakahara, K.; Neveling, R.; S, Ong; Page, S. A.; Papavassiliou, V.; Pate, S. F.; Phillips, S. K.; Pitt, M. L.; Poelker, M.; Porcelli, T. A.; Qúeḿener, G.; Quinn, B.; Ramsay, W. D.; Rauf, A. W.; Réal, J. S.; Roche, J.; Roos, P.G.; Rutledge, G.; Secrest, J. A.; Šimičević, N.; Smith, G. R.; Spayde, D. T.; Stepanyan, S.; Stutzman, Marcy; Sulkosky, V.; Tadevosyan, V.; Tieulent, R.; Wiele, J. Van de; Oers, W. T. H. van; Voutier, E.; Vulcan, W.; Warren, G.; Wells, S. P.; Williamson, S. E.; Wood, S. A.; Yan, Chao; Yun, J.; Zeps, V.

doi:10.1103/physrevlett.99.092301

Cited by 37 publications

(26 citation statements)

References 24 publications

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“…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.…”

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confidence: 40%

“…There are more recent measurements of the A4 experiment at forward angles at Q 2 ¼ 0.11; 0.23 ðGeV=cÞ 2 [39] with hydrogen, the G0 experiment at Q 2 ¼ 0.15; 0.25 ðGeV=cÞ 2 at forward angles angles with hydrogen [23], and at Q 2 ¼ 0.22; 0.63 ðGeV=cÞ 2 at backward angles with hydrogen and deuterium [25], the measurements of Happex at GeV energies, which include besides the hydrogen target heavier nuclei [24], and the preliminary measurements of the Q weak experiment in the elastic electron-proton scattering and in the inelastic scattering on hydrogen, aluminum, and carbon [41]. There is a very recent measurement of the A1 experiment at MAMI of A ⊥ in the reaction 12 Cð ⃗e; e 0 Þ 12 C [42,43].…”

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confidence: 99%

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New Measurements of the Beam Normal Spin Asymmetries at Large Backward Angles with Hydrogen and Deuterium Targets

Ríos¹,

Aulenbacher²,

Baunack³

et al. 2017

Phys. Rev. Lett.

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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...

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confidence: 40%

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confidence: 99%

New Measurements of the Beam Normal Spin Asymmetries at Large Backward Angles with Hydrogen and Deuterium Targets

Ríos¹,

Aulenbacher²,

Baunack³

et al. 2017

Phys. Rev. Lett.

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“…Compared to the contribution from the excited states, the contribution from the ground state is negligible (see [170] 0.300 140 < θ lab < 150 -87 ± 6 7 G0 Backward [81] 0.362 108 -176.5 ± 9.4 5 G0 Backward [81] 0.687 108 -21 ± 24 87 Forward Angle Scattering G0 Forward [171] 3.000 52 < θ lab < 76.5 -4.06 ± 0.99 ± 0.63 29 G0 Forward [171] 3.000 52 < θ lab < 76. Together with the G0-backward angle beam normal spin asymmetry measurement, the Q weak result confirms the importance of using multiple pions in the excited intermediate state of the two-photon exchange for beam energies above pionthreshold.…”

Section: The Determination Of the Weak Charge Of The Protonmentioning

confidence: 99%

A 3% Measurement of the Beam Normal Single Spin Asymmetry in Forward Angle Elastic Electron-Proton Scattering using the Qweak Setup

Waidyawansa

2013

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The beam normal single spin asymmetry generated in the scattering of transversely polarized electrons from unpolarized nucleons is an observable of the imaginary part of the two-photon exchange process. Moreover, it is a potential source of false asymmetry in parity violating electron scattering experiments. The Q weak experiment uses parity violating electron scattering to make a direct measurement of the weak charge of the proton. The targeted 4% measurement of the weak charge of the proton probes for parity violating new physics beyond the Standard Model. The beam normal single spin asymmetry at Q weak kinematics is at least three orders of magnitude larger than 5 ppb precision of the parity violating asymmetry. To better understand this parity conserving background, the Q weak Collaboration has performed elastic scattering measurements with fully transversely polarized electron beam on the proton and aluminum. This dissertation presents the analysis of the 3% measurement (1.3% statistical and 2.6% systematic) of beam normal single spin asymmetry in electronproton scattering at a Q 2 of 0.025 (GeV/c) 2 . It is the most precise existing measurement of beam normal single spin asymmetry available at the time. A measurement of this precision helps to improve the theoretical models on beam normal single spin asymmetry and thereby our understanding of the doubly virtual Compton scattering process.

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“…At the time when the interest on the two-photon exchange was revived by the polarization-Rosenbluth discrepancy, the techniques as well as expertise for measuring asymmetries of part per million (ppm) had been developed at several facilities like MIT-Bates, JLab and MAMI, aiming at measuring parity-violating asymmetries in electron scattering [56]. With these facilities, investigations of transverse beam spin asymmetries in various kinematic regions have been performed with transversely polarized electrons scattering off different targets [57][58][59][60][61][62], including A4 measurements [63,64] with both hydrogen and deuterium targets at the A4 experiment. In this letter, we report new results of the beam transverse spin asymmetry A ep ⊥ in ep elastic scattering at forward angles over a wide energy range, measured with a full azimuthal-angle detector at the A4 experiment.…”

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Study of Two-Photon Exchange via the Beam Transverse Single Spin Asymmetry in Electron-Proton Elastic Scattering at Forward Angles over a Wide Energy Range

Gou¹,

Arvieux²,

Aulenbacher³

et al. 2020

Phys. Rev. Lett.

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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...

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Transverse Beam Spin Asymmetries in Forward-Angle Elastic Electron-Proton Scattering

Cited by 37 publications

References 24 publications

New Measurements of the Beam Normal Spin Asymmetries at Large Backward Angles with Hydrogen and Deuterium Targets

New Measurements of the Beam Normal Spin Asymmetries at Large Backward Angles with Hydrogen and Deuterium Targets

A 3% Measurement of the Beam Normal Single Spin Asymmetry in Forward Angle Elastic Electron-Proton Scattering using the Qweak Setup

Study of Two-Photon Exchange via the Beam Transverse Single Spin Asymmetry in Electron-Proton Elastic Scattering at Forward Angles over a Wide Energy Range

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