arXiv subjects

P. G. Blunden

Publications and source records attributed to P. G. Blunden.

At least 19 recordsLinked to original sources

arXiv2023

A novel approach to the global analysis of proton form factors in elastic electron-proton scattering

We present a novel method for the global analysis of elastic electron-proton scattering when combining data sets from experiments with different overall normalization uncertainties. The method is a modification of one employed by the NNPDF collaboration in the fitting of parton distribution data. This method is an alternative to the 'penalty trick' method traditionally employed in global fits to proton electric and magnetic form factors, while avoiding the biases inherent in that approach. We discuss issues that arise when extending the method to nonlinear models. For data with $Q^2>1$ GeV$^2$ we find relatively minor differences to traditional model fits when the normalization uncertainties from different experiments are correctly accounted for. We discuss implications of this method for the well-known discrepancy between the form factor ratio $G_E/G_M$ extracted from the Rosenbluth and polarization transfer techniques.

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arXiv2023

A Direct Measurement of Hard Two-Photon Exchange with Electrons and Positrons at CLAS12

One of the most surprising discoveries made at Jefferson Lab has been the discrepancy in the determinations of the proton's form factor ratio $μ_p G_E^p/G_M^p$ between unpolarized cross section measurements and the polarization transfer technique. Over two decades later, the discrepancy not only persists but has been confirmed at higher momentum transfers now accessible in the 12-GeV era. The leading hypothesis for the cause of this discrepancy, a non-negligible contribution from hard two-photon exchange, has neither been conclusively proven or disproven. This state of uncertainty not only clouds our knowledge of one-dimensional nucleon structure but also poses a major concern for our field's efforts to map out the three-dimensional nuclear structure. A better understanding of multi-photon exchange over a wide phase space is needed. We propose making comprehensive measurements of two-photon exchange over a wide range in momentum transfer and scattering angle using the CLAS12 detector. Specifically, we will measure the ratio of positron-proton to electron-proton elastic scattering cross sections, using the proposed positron beam upgrade for CEBAF. The experiment will use 2.2, 4.4, and 6.6 GeV lepton beams incident on the standard CLAS12 unpolarized hydrogen target. Data will be collected by the CLAS12 detector in its standard configuration, except for a modified trigger to allow the recording of events with beam leptons scattered into the CLAS12 central detector. The sign of the beam charge, as well as the polarity of the CLAS12 solenoid and toroid, will be reversed several times in order to suppress systematics associated with local detector efficiency and time-dependent detector performance. The proposed high-precision determination of two-photon effects will be...

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arXiv2023

Normal single-spin asymmetries in electron-proton scattering: two-photon exchange with intermediate state resonances

We calculate the beam ($B_n$) and target ($A_n$) normal single-spin asymmetries in electron-proton elastic scattering from two-photon exchange amplitudes with resonance intermediate states of spin-parity $1/2^\pm$ and $3/2^\pm$ and mass $W \lesssim 1.8$ 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 $B_n$, 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 $B_n$. At forward angles and beam energies $E_\textrm{lab}<1$ GeV, where the $Δ(1232)$ dominates, the calculated $B_n$ tend to overshoot the A4 and SAMPLE data. The calculated $B_n$ compare well with the measured values from the A4 and $Q_{\textrm{weak}}$ experiments with $E_\textrm{lab}>1$ GeV.

hep-ph↗

arXiv2020

Electroweak axial structure functions and improved extraction of the $V_{ud}$ CKM matrix element

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 $\Box^{γW}_{A}$ in terms of the isoscalar part of the $F_3^{γW}$ interference structure function. Using the latest available phenomenology for $F_3^{γW}$ from the nucleon elastic, resonance, deep-inelastic, and Regge regions, we find the real part of the box correction to be $\Box^{γW}_A = 3.90(9) \times 10^{-3}$. This improved correction gives a theoretical estimate of the CKM matrix element $|V_{ud}|^2=0.94805(26)$, which represents a 4$σ$ violation of unitarity.

hep-ph↗

arXiv2020

Two-photon exchange from intermediate state resonances in elastic electron-proton scattering

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 $Δ(1232)~\!3/2^+$ 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.

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arXiv2017

A dispersive approach to two-photon exchange in elastic electron-proton scattering

We examine the two-photon exchange corrections to elastic electron-proton scattering within a dispersive approach, including contributions from both Nucleon and Delta 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 N-Delta transition form factors, we compute the two-photon exchange corrections both algebraically and numerically. Results are compared with recent measurements of e+p to e-p cross section ratios from the CLAS, VEPP-3 and OLYMPUS experiments, as well as with polarization transfer observables.

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arXiv2017

Two-photon exchange in elastic electron-proton scattering

We review recent theoretical and experimental progress on the role of two-photon exchange (TPE) in electron-proton scattering at low to moderate momentum transfers. We make a detailed comparison and analysis of the results of competing experiments on the ratio of e+p to e-p elastic scattering cross sections, and of the theoretical calculations describing them. A summary of the current experimental situation is provided, along with an outlook for future experiments.

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arXiv2015

Quark-hadron duality constraints on γZ box corrections to parity-violating elastic scattering

We examine the interference γZ box corrections to parity-violating elastic electron--proton scattering in the light of the recent observation of quark-hadron duality in parity-violating deep-inelastic scattering from the deuteron, and the approximate isospin independence of duality in the electromagnetic nucleon structure functions down to Q^2 \approx 1 GeV^2. Assuming that a similar behavior also holds for the γZ proton structure functions, we find that duality constrains the γZ box correction to the proton's weak charge to be \Re e\, \square_{γZ}^V = (5.4 \pm 0.4) \times 10^{-3} at the kinematics of the Q_{\text{weak}} experiment. Within the same model we also provide estimates of the γZ corrections for future parity-violating experiments, such as MOLLER at Jefferson Lab and MESA at Mainz.

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arXiv2014

Hadronic gamma-Z box corrections in M\oller scattering

The possibility of measuring the parity-violating asymmetry in M\oller scattering with sufficient accuracy to determine sin^2 theta_W to 0.1% offers a complementary path to the discovery of new physics to that followed at high energy colliders. We present a new calculation of the gamma-Z box contribution to parity-violating electron-proton scattering, which constitutes an important uncertainty in computing the background to this process. We show that while the gamma-Z correction grows rapidly with energy, it can be relatively well constrained by data from parity-violating inelastic scattering and parton distribution functions.

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arXiv2013

Q^2-evolution of the electric and magnetic polarizabilities of the proton

The generalized Baldin sum rule at finite four-momentum transfer Q^2 is evaluated utilizing a structure function parameterization fit to recent experimental data. The most recent measurements on F_1 from Hall C at Jlab, as well as the F_2 structure function data from Hall B at Jlab and SLAC, were used in constructing our parameterization. We find that at Q^2 below 1 GeV^2 the dominant contribution to the electric and magnetic polarizabilities of the nucleon comes from the resonance region.

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arXiv2013

Constrained gamma-Z interference corrections to parity-violating electron scattering

We present a comprehensive analysis of gamma-Z interference corrections to the weak charge of the proton measured in parity-violating electron scattering, including a survey of existing models and a critical analysis of their uncertainties. Constraints from parton distributions in the deep-inelastic region, together with new data on parity-violating electron scattering in the resonance region, result in significantly smaller uncertainties on the corrections compared to previous estimates. At the kinematics of the Qweak experiment, we determine the gamma-Z box correction to be ReBox^V_{gamma-Z} = (5.57 +- 0.36) x 10^{-3}. The new constraints also allow precise predictions to be made for parity-violating deep-inelastic asymmetries on the deuteron.

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arXiv2012

Gamma-Z box corrections to weak charges of heavy nuclei in atomic parity violation

We present a new dispersive formulation of the gamma-Z box radiative corrections to weak charges of bound protons and neutrons in atomic parity violation (APV) measurements on heavy nuclei such as 133-Cs and 213-Ra. 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 gamma-Z correction to the weak charge of 133-Cs, approximately 4 times larger than the current uncertainty on the value of sin^2(theta_W), but with a reduced error compared to earlier estimates.

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arXiv2011

Review of two-photon exchange in electron scattering

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 measurements, nonlinear epsilon 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 gamma-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.

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arXiv2011

New formulation of gamma-Z box corrections to the weak charge of the proton

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, $\Re{\rm e}\, \Box_{γZ}^{\rm A}$. 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 $\Re{\rm e}\, \Box_{γZ}^{\rm A} = 0.0044(4)$ at zero energy, which shifts the theoretical estimate of the proton weak charge from $0.0713(8)$ to $0.0705(8)$. The energy dependence of this result, which is vital for interpreting the Q$_{\rm weak}$ experiment, is also determined.

hep-ph↗

arXiv2010

gamma-Z corrections to forward-angle parity-violating e-p scattering

We use dispersion relations to evaluate the gamma-Z box contribution to parity-violating electron scattering in the forward limit arising from the axial-vector coupling at the electron vertex. The calculation makes full use of the critical constraints from recent JLab data on electroproduction in the resonance region as well as high energy data from HERA. At the kinematics of the Qweak experiment, this gives a correction of 0.0047{+0.0011}{-0.0004}$ to the Standard Model value 0.0713(8) of the proton weak charge. While the magnitude of the correction is highly significant, the uncertainty is within the anticipated experimental uncertainty of +- 0.003.

hep-ph↗

arXiv2009

Two-photon exchange corrections to the pion form factor

We compute two-photon exchange corrections to the electromagnetic form factor of the pion, taking into account the finite size of the pion. Compared to the soft-photon approximation for the infrared divergent contribution which neglects hadron structure effects, the corrections are found to be < 1% for small Q^2 (Q^2 < 0.1 GeV^2), but increase to several percent for Q^2 > 1 GeV^2 at extreme backward angles.

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arXiv2009

Interpretation of the neutron quantum gravitational states in terms of isospectral potentials

The recently observed quantum states of neutrons bound in a gravitational field are analyzed in the framework of one-parameter isospectral hamiltonians. Potentials isospectral to the usual Newton potential are explicitly constructed for the first time, then constrained using measured properties of the neutron gravitational states. The corresponding wave functions and the neutron fluxes are also calculated and analyzed in a simple model, including the ground state and the excited state contributions. The constructed isospectral potentials are discussed as candidates for a possible modification of Newton's law at a submillimetre scale. Our results indicate that significant deviations from the Newtonian gravity at submillimetre distances could be compatible with experiment.

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arXiv2009

Detailed analysis of two-boson exchange in parity-violating e-p scattering

We present a comprehensive study of two-boson exchange (TBE) corrections in parity-violating electron-proton elastic scattering. Within a hadronic framework, we compute contributions from box (and crossed box) diagrams in which the intermediate states are described by nucleons and Delta baryons. The Delta contribution is found to be much smaller than the nucleon one at backward angles (small epsilon), but becomes dominant in the forward scattering limit (epsilon -> 1), where the nucleon contribution vanishes. The dependence of the corrections on the input hadronic form factors is small for Q^2 < 1 GeV^2, but becomes significant at larger Q^2. We compute the nucleon and Delta TBE corrections relevant for recent and planned parity-violating experiments, with the total corrections ranging from -1% for forward angles to 1-2% at backward kinematics.

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