We present a comprehensive analysis of the γW interference radiative correction to the neutron β-decay matrix element. Within a dispersion relations approach, we compute the axial-vector part of the ...γW box amplitude □γWA in terms of the isoscalar part of the FγW3 interference structure function. Using the latest available phenomenology for FγW3 from the nucleon elastic, resonance, deep-inelastic, and Regge regions, we find the real part of the box correction to be □γWA = 3.90 (9) × 10−3. This improved correction gives a theoretical estimate of the CKM matrix element |Vud|2 = 0.94805 (26), which represents a 4σ violation of unitarity.
We review the role of two-photon exchange (TPE) in electron–hadron scattering, focusing in particular on hadronic frameworks suitable for describing the low and moderate
Q
2
region relevant to most ...experimental studies. We discuss the effects of TPE on the extraction of nucleon form factors and their role in the resolution of the proton electric to magnetic form factor ratio puzzle. The implications of TPE on various other observables, including neutron form factors, electroproduction of resonances and pions, and nuclear form factors, are summarized. Measurements seeking to directly identify TPE effects, such as through the angular dependence of polarization observables, nonlinear
ε
contributions to the cross sections, and via
e
+
p
to
e
−
p
cross section ratios, are also outlined. In the weak sector, we describe the role of TPE and
γ
Z
interference in parity-violating electron scattering, and assess their impact on the extraction of the strange form factors of the nucleon and the weak charge of the proton.
► We review two-photon exchange (TPE) in electron–hadron scattering. ► TPE resolves the proton electric to magnetic form factor discrepancy. ► TPE can be tested in other observables such as
e
+
p
to
e
−
p
ratios.► Photon-
Z
boson exchange impacts extraction of the proton weak charge.
We examine the two-photon exchange corrections to elastic electron-nucleon scattering within a dispersive approach, including contributions from both nucleon and Δ intermediate states. The dispersive ...analysis avoids off-shell uncertainties inherent in traditional approaches based on direct evaluation of loop diagrams, and guarantees the correct unitary behavior in the high energy limit. Using empirical information on the electromagnetic nucleon elastic and NΔ transition form factors, we compute the two-photon exchange corrections both algebraically and numerically. Finally, results are compared with recent measurements of e+ p to e- p cross section ratios from the CLAS, VEPP-3 and OLYMPUS experiments.
We present a new formulation of one of the major radiative corrections to the weak charge of the proton-that arising from the axial-vector hadron part of the γZ box diagram, ℜe□(γZ)(A). This ...formulation, based on dispersion relations, relates the γZ contributions to moments of the F(3)(γZ) interference structure function. It has a clear connection to the pioneering work of Marciano and Sirlin, and enables a systematic approach to improved numerical precision. Using currently available data, the total correction from all intermediate states is ℜe□(γZ)(A)=0.004 4(4) at zero energy, which shifts the theoretical estimate of the proton weak charge from 0.071 3(8) to 0.070 5(8). The energy dependence of this result, which is vital for interpreting the Q(weak) experiment, is also determined.
We present a new dispersive formulation of the γZ box radiative corrections to weak charges of bound protons and neutrons in atomic parity violation measurements on heavy nuclei such as 133Cs and ...213Ra. We evaluate for the first time a small but important additional correction arising from Pauli blocking of nucleons in a heavy nucleus. Overall, we find a significant shift in the γZ correction to the weak charge of 133Cs, approximately 4 times larger than the current uncertainty on the value of sin2 θ(W), but with a reduced error compared to earlier estimates.
In this paper, we use a recently developed dispersive approach to compute the two-photon exchange (TPE) correction to elastic electron-proton scattering, including contributions from hadronic ...$J^P=1/2^\pm$ and $3/2^\pm$ resonant intermediate states below~1.8~GeV. For the transition amplitudes from the proton ground state to the resonant excited states we employ new exclusive meson electroproduction data from CLAS at $Q^2 \lesssim 5$~GeV$^2$, and we explore the effects of both fixed and dynamic widths for the resonances. Among the resonant states, the $N(1520)~\!3/2^-$ becomes dominant for $Q^2 \gtrsim 2$~GeV$^2$, with a sign opposite to the comparably sized $\Delta(1232)~\!3/2^+$ contribution, leading to an overall increase in the size of the TPE correction to the cross section relative to the nucleon only contribution at higher $Q^2$ values. The results are in good overall agreement with recent $e^+ p$ to $e^- p$ cross section ratio and polarization transfer measurements, and provide compelling evidence for a resolution of the electric to magnetic form factor ratio discrepancy.
We calculate the effects on the elastic electron-proton scattering cross section of the two-photon exchange contribution with an intermediate Delta resonance. The Delta two-photon exchange ...contribution is found to be smaller in magnitude than the previously evaluated nucleon contribution, with an opposite sign at backward scattering angles. The sum of the nucleon and Delta two-photon exchange corrections has an angular dependence compatible with both the polarization-transfer and the Rosenbluth methods of measuring the nucleon electromagnetic form factors.
We calculate the beam (Bn) and target (An) normal single-spin asymmetries in electron–proton elastic scattering from two-photon exchange amplitudes with resonance intermediate states of spin-parity ...1/2± and 3/2± and mass W ≲1.8 GeV. The latest CLAS exclusive meson electroproduction data are used as input for the transition amplitudes from the proton to the excited resonance states. For Bn, the spin 3/2 resonances dominate by an order of magnitude over the spin 1/2 states. In general we observe cancellations between the negative contributions of the Δ(1232) and N(1520) across both beam energy and scattering angle, and the positive contributions of the Δ(1700) and N(1720), leading to a rather large overall uncertainty band in the total Bn. At forward angles and beam energies Elab < 1 GeV, where the Δ(1232) dominates, the calculated Bn tend to overshoot the A4 and SAMPLE data. The calculated Bn compare well with the measured values from the A4 and Qweak experiments with Elab > 1 GeV.
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