Introduction

The proton’s electromagnetic form factors, $G_{E}^{p}$ and $G_{M}^{p}$ are
fundamental properties that encode the spatial distribution of the proton’s
charge and current. Decades of measurements mapping out the proton’s form
factors were suddenly upended by new experiments at Jefferson Lab and
elsewhere that used polarization observables, rather than unpolarized cross
section measurements, to directly access the form factor ratio
$\mu_{p}G_{E}^{p}/G_{M}^{p}$. Whereas Rosenbluth separations of unpolarized
cross section data generally suggest that $\mu_{p}G_{E}^{p}/G_{M}^{p}$ is
approximately 1, the polarized measurements show an unambiguous downwards
slope as a function of squared four-momentum transfer $Q^{2}$. A subset of
world data
Litt:1969my,Berger:1971kr,Bartel:1973rf,Andivahis:1994rq,E94110:2004lsx,Qattan:2004ht,Gayou:2001qt,Punjabi:2005wq,ResonanceSpinStructure:2006oim,Zhan:2011ji,Crawford:2006rz,Puckett:2011xg,Puckett:2017flj,
showing this discrepancy is found in Fig. fig:GE_GM. As Rosenbluth separations
invariably produce large coupled uncertainties in $G_{E}$ and $G_{M}$, a more
modern technique is to perform a global fit directly to unpolarized cross
section measurements. An example, from Ref. A1:2013fsc, is shown as well,
along with uncertainty bands. The discrepancy is most prominent between
$Q^{2}$ values of approximately 2–7 GeV${}^{2}/c^{2}$ where there is a clear
gap between the polarized and unpolarized data. At higher $Q^{2}$, the large
uncertainties in the unpolarized data obscure whether the discrepancy persists
at even higher $Q^{2}$, though recent results from Hall A suggest that it does
Christy:2021snt.

[width=]GE_over_GM.pdf

Figure : A subset of world data on the proton’s form factor ratio
$\mu_{p}G_{E}^{p}/G_{M}^{p}$ show the discrepancy between Rosenbluth and
polarized measurements. Rosenbluth separations of unpolarized cross sections
(blue points) are taken from Refs. [Litt:1969my, Berger:1971kr, Bartel:1973rf,
Andivahis:1994rq, E94110:2004lsx, Qattan:2004ht]. Polarized measurements (red
points) are taken from Refs. [Gayou:2001qt, Punjabi:2005wq,
ResonanceSpinStructure:2006oim, Zhan:2011ji, Crawford:2006rz, Puckett:2011xg,
Puckett:2017flj]. A global fit to unpolarized cross sections along with
estimates of statistical and total uncertainty by Bernauer et al. [A1:2013fsc]
are shown as blue bands. For comparison, the $Q^{2}$ coverage of recent two-
photon exchange measurements by VEPP-3 [Rachek:2014fam], CLAS [CLAS:2016fvy],
and OLYMPUS [OLYMPUS:2016gso] are shown with arrows. The coverage of this
proposal is also shown, labeled ‘CLAS12.’

A leading hypothesis for the source of the discrepancy quickly emerged: a
radiative effect called hard two-photon exchange, which is neglected in
standard radiative corrections formulae, may be biasing the two measurement
techniques differently [Guichon:2003qm, Blunden:2003sp]. Radiative correction
procedures, such as that of Mo and Tsai [Mo:1968cg], or Maximon and Tjon
[Maximon:2000hk], treat two-photon exchange (TPE) in the so-called “soft-
limit,” an approximation in which one of the photons carries negligible four-
momentum. This approximation makes the TPE diagram tractable without model-
dependence. The TPE contribution beyond the soft-limit is referred to as “hard
two-photon exchange,” or hard TPE. It should be noted that two-photon exchange
has to be treated in order to cancel divergences in bremsstrahlung terms. The
set of diagrams treated by standard radiative corrections procedures are shown
in Fig. .

[width=0.95]diagrams.pdf

Figure : Diagrams treated in standard radiative corrections to elastic
electron scattering

The hypothesis that hard TPE could produce the proton form factor discrepancy
led to a renewed interest in attempting to quantify the effect experimentally.
One of the most straight-forward experimental signatures is to observe a
difference in cross sections in positron scattering relative to electron
scattering. The lowest order TPE correction to the elastic scattering cross
section is an interference term between one-photon exchange (OPE) and two-
photon exchange. This interference term is proportional to $e^{3}$, where $e$
is the lepton charge, and therefore changes sign between positron and electron
scattering. This can be written:

$\sigma\sim|\mathcal{M}_{1\gamma}|^{2}\pm
2\text{Re}\left[\mathcal{M}_{1\gamma}\mathcal{M}_{2\gamma}\right]+\ldots,$ (1)

where $\mathcal{M}_{1\gamma}$ and $\mathcal{M}_{2\gamma}$ are the OPE and TPE
matrix elements. The positron-proton to electron-proton cross section ratio,
denoted $R_{2\gamma}$, is therefore:

$R_{2\gamma}\equiv\frac{\sigma_{e^{+}p}}{\sigma_{e^{-}p}}=1+4\frac{\text{Re}\left[\mathcal{M}_{1\gamma}\mathcal{M}_{2\gamma}\right]}{|\mathcal{M}_{1\gamma}|^{2}}+\ldots$
(2)

This should be interpreted as a sketch of the principle of the measurement. In
practice, one must account for all of the other radiative effects entering at
the same order, most of which are lepton charge-symmetric but some (e.g.,
lepton-proton bremsstrahlung interference) are not. Three experiments were
proposed in response to the TPE hypothesis, and collected data in 2010–2013.
The results of those experiments were not conclusive, primarily because the
experiments lacked the kinematic reach necessary to probe the region of
$Q^{2}$ where the discrepancy is significant. Gathering data with the
necessary precision and kinematic reach to conclusively test if TPE is the
cause of the proton form factor discrepancy is the main motivation for this
proposal. In the following sections we briefly review recent theoretical,
phenomenological, and experimental efforts. For more details we refer to Ref.
[Afanasev:2017gsk]. For what follows, it is helpful to recall that in elastic
scattering at fixed beam energy, there is only one kinematic degree-of-
freedom. Therefore, the kinematics of any elastic scattering measurement can
be represented by just two numbers, for example beam energy and scattering
angle. We will frequently use the combination $Q^{2}$ and $\epsilon$, where
$Q^{2}$ is the familiar negative squared four-momentum transfer, given by: Q^2
= & 2EE’(1-cosθ)
→ 2E2mp(1-cosθ)mp\+ E(1-cosθ), where $E$ is the beam energy, $E^{\prime}$ is
the outgoing lepton energy, $m_{p}$ is the proton mass, and $\theta$ is the
lepton scattering angle; and $\epsilon$ is the virtual photon longitudinal
polarization parameter, given in elastic kinematics by:

$\epsilon\rightarrow\left[1+2(1+\tau)\tan^{2}\frac{\theta}{2}\right]^{-1},$
(3)

where $\tau\equiv\frac{Q^{2}}{4m_{p}^{2}}$. Increasing the lepton scattering
angle, $\theta$, increases $Q^{2}$ and decreases $\epsilon$.

##  Two-Photon Exchange Theory

One of the challenges preventing incorporation of hard TPE into standard
radiative corrections prescriptions is difficulty in calculating the TPE
diagram without adding significant model dependence. The TPE diagram has an
off-shell hadronic propagator, which requires some QCD input in order to be
evaluated. In general, there are two classes of approaches: hadronic methods
and partonic methods, which are described in the following sections. There are
also a handful of alternative theoretical approaches which fit in neither
category, some of which suggest that TPE cannot be the cause of the form
factor discrepancy (e.g. [Kuraev:2007dn]). Efforts to advance our
understanding of TPE and of radiative corrections more generally is an active
area of research, with applications beyond merely the discrepancy in the
proton form factors. TPE is a significant correction in scattering experiments
at low $Q^{2}$ to determine the proton radius. The TPE diagram shares many of
the same challenges with the $\gamma Z$ box diagram that is a major radiative
correction in parity-violating electron scattering, and with the $\gamma W$
box diagram in nuclear $\beta$-decay. An Ad-Hoc Workshop on Radiative
Corrections, held (remotely) in 2020 by CFNS at Stony Brook produced a white
paper [Afanasev:2020hwg] on current challenges in radiative corrections. This
was followed by a workshop at the ECT in Trento in 2022 with a second paper
forthcoming [radcorwp].

###  Hadronic Methods

In hadronic methods, the off-shell propagator is expanded as a sum of on-shell
intermediate hadronic states. This series is, in principle, infinite,
containing all baryons with quantum numbers accessible from photon-nucleon
coupling. In practice, the higher mass resonances contribute less and less
with mass, and the expansion can be truncated to get an approximate result.
Additional model dependence comes from the assumptions about the transition
form factors for each intermediate state. Early works used the direct
evaluation of loop integrals [Blunden:2003sp, Blunden:2005ew], but the
flexibility of hadronic calculations have greatly improved by using dispersive
methods [Gorchtein:2006mq, Borisyuk:2008es, Borisyuk:2015xma, Blunden:2017nby,
Ahmed:2020uso]. The most advanced dispersive hadronic calculation incorporates
contributions from the nucleon as well as all $N*$ and $\Delta$ resonances
with masses up to 1.8 GeV [Ahmed:2020uso]. Hadronic approaches should perform
better at low momentum transfer, and calculations are typically not performed
above $Q^{2}>3$ GeV${}^{2}/c^{2}$. In that limit, however, the results of
hadronic calculations suggest that hard TPE would drive the apparent
$\mu_{p}G_{E}/G_{M}$ extracted from unpolarized cross sections up and away
from the results of polarization transfer [Blunden:2003sp, Arrington:2007ux].
Nevertheless, hadronic calculations do show some variance based on different
model assumptions used and which intermediate states are included.

###  Partonic Methods

Partonic methods model the hard interaction of the two exchanged photons with
individual quarks, but then embed those quarks inside the proton using a model
of the proton’s partonic structure—either its distribution amplitudes (DAs) or
generalized parton distributions (GPDs). This approach is more accurate at
high momentum transfers where the factorization between the hard scattering
and the soft structure are on firmer ground, ideally above $Q^{2}>5$
GeV${}^{2}/c^{2}$. The accuracy is only as good, however, as our understanding
of the proton’s multi-dimensional structure. Examples of partonic estimates
include Refs. [Chen:2004tw, Afanasev:2005mp, Borisyuk:2008db, Kivel:2009eg,
Kivel:2012vs].

##  Phenomenology

There is a class of phenomenological two-photon exchange estimates that use
experimental data to estimate the magnitude of the two-photon exchange
amplitude, making the assumption that hard TPE is the sole cause of the proton
form factor discrepancy [Chen:2007ac, Borisyuk:2010ep, Guttmann:2010au,
A1:2013fsc, Schmidt:2019vpr]. This amounts to determining how much of a hard
TPE effect would be necessary to fully resolve the discrepancy. If the
measured TPE differed substantially from these phenomenological predictions,
this would be evidence that factors other than hard TPE are responsible for
the discrepancy. However, even modestly different assumptions lead to a wide
range of phenomenological estimates, which implies that even the magnitude of
the discrepancy is not well-constrained.

tpe_pred_QSq250.pdf

Figure : Predictions for $R_{2\gamma}$, the ratio of positron-proton to
electron-proton elastic cross sections, shown as a function of $\epsilon$, for
a fixed $Q^{2}=2.50$ GeV${}^{2}/c^{2}$. The phenomenological prediction comes
from Bernauer et al. (Mainz A1) [A1:2013fsc]. The hadronic prediction is the
$N+\Delta$ calculation of Blunden et al. (2017) [Blunden:2017nby]. The
partonic calculations come from Kivel and Vanderhaeghen (2019) [Kivel:2009eg].
The calculation labelled $N/\Delta$-cancelled is by Kuraev et al. (2008)
[Kuraev:2007dn], whose authors conclude that TPE _cannot_ be the only cause of
the proton form factor discrepancy. The predictions assume the Maximon and
Tjon [Maximon:2000hk] definition of soft TPE.

tpe_pred_eps742.pdf

Figure : Predictions for $R_{2\gamma}$, the ratio of positron-proton to
electron-proton elastic cross sections, shown as a function of $Q^{2}$, for a
fixed $\epsilon=0.742$. The calculations are the same as in Fig. .

Figures  and  compare several examples of theoretical predictions
[Blunden:2017nby, Kivel:2009eg, Kuraev:2007dn] (colored curves) against a
phenomenological estimate [A1:2013fsc] (black curve) by the Mainz A1
collaboration, which assumes that the entire proton form factor discrepancy
can be attributed to hard TPE, among other assumptions. This curve is a useful
reference; theoretical predictions that are similar generally indicate that
TPE could resolve the form factor discrepancy, while theoretical predictions
that deviate suggest that other factors must be involved. In general
predictions show that $R_{2\gamma}$ increases as $\epsilon$ decreases. The
hadronic calculation of Blunden et al. (2017) [Blunden:2017nby] is below the
phenomenological curve at $Q^{2}=2.5$ GeV${}^{2}/c^{2}$, but with a similar
slope. The partonic calculation of Kivel and Vanderhaeghen (2009)
[Kivel:2009eg], particularly the calculation labelled COZ is above the
phenomenological estimate at $Q^{2}=2.5$ GeV${}^{2}/c^{2}$, but with a
shallower slope. The prediction of Kuraev et al. (2008) [Kuraev:2007dn], whose
authors criticize the TPE hypothesis, is below the phenomenological estimate.
When viewed as a function of $Q^{2}$, the phenomenological prediction
continues to grow, which reflects the fact that experimental form factor data
indicate that the discrepancy does not abate at higher $Q^{2}$. Neither the
hadronic nor partonic calculations continue to grow with $Q^{2}$. A region of
particular importance for new measurements is the region of $2<Q^{2}<5$
GeV${}^{2}/c^{2}$. In this region, neither the hadronic calculations nor the
partonic calculations are expected to perform accurately. The best bet for
constraining hard TPE in this range is experimental data.

##  Previous Measurements

Hard TPE can be probed experimentally through a number of observables. The
positron-proton to electron-proton cross section ratio probes the real part of
the TPE amplitude. One can also probe the imaginary part of the TPE amplitude
through measurements of single-spin asymmetries in which the polarization
vector is normal to the scattering plane. In this section, we review only the
former, since the leading TPE correction to the unpolarized cross section
(i.e., the effect on extractions of proton form factors) comes from the real
part of TPE. Comparisons of the positron-proton and electron-proton elastic
cross sections to look for signs of TPE were carried out at a number of
laboratories in the 1960s, including at Stanford [Yount:1962, Browman:1965zz,
Mar:1968qd], Cornell [Anderson:1966, Cassiday:1967, Anderson:1968zzc], DESY
[Bartel:1967dsa], and Orsay [Bouquet:1968yqa]. The kinematics and
uncertainties of these data are shown in Fig.  as gray circles. From Figs.
and , the best kinematics for observing a hard TPE signature are at
low-$\epsilon$, i.e., backward scattering angles, where the effect size needs
to be larger to produce the necessary bias to Rosenbluth plots, and at large
$Q^{2}$, where the discrepancy is larger. The data from the 1960s, by
contrast, are predominantly at either high-$\epsilon$ or low $Q^{2}$.
Furthermore, the typical uncertainties are of order 5–10%, i.e., too large to
resolve the expected effect size. Unsurprisingly, the 1960s experiments did
not observe any significant hard TPE effects.

prev_data_kin.pdf

Figure : A kinematic map in the $\epsilon$-$Q^{2}$ plane of previous
measurements of $R_{2\gamma}$. Each data point is represented by a circle
whose diameter is proportional to its uncertainty. Data from the 1960s
[Yount:1962, Browman:1965zz, Mar:1968qd, Anderson:1966, Cassiday:1967,
Anderson:1968zzc, Bartel:1967dsa, Bouquet:1968yqa] are shown as light gray
circles. Data from VEPP-3 [Rachek:2014fam] are shown as blue circles, data
from CLAS [CLAS:2016fvy] are shown as yellow circles, while data from OLYMPUS
[OLYMPUS:2016gso] are shown as red circles.

vepp3.pdf

Figure : Results from the two-photon exchange experiment conducted at the
VEPP-3 storage ring at the Budker Institute, Novosibirsk, Russia, taken from
Ref. [Rachek:2014fam]. The left panel shows the results for a beam energy of
1.6 GeV, while the right panel shows the results for a beam energy of 1 GeV.
The filled black circles show the VEPP-3 results, while the other data points
come from previous measurements: open squares from Ref. [Browman:1965zz],
closed upward-pointing triangles from Ref. [Anderson:1968zzc], closed
downward-pointing triangles from Ref. [Anderson:1966], and closed diamonds
from Ref. [Bartel:1967dsa]. Theoretical and phenomenological predictions are
shown as curves for comparison: the dash-dotted teal curve is from Ref.
[Borisyuk:2008es], the red curve is from Ref. [Blunden:2005ew], the thick blue
curve is from Ref. [A1:2013fsc], the gray dashed curve is from Ref. [Tomasi-
Gustafsson:2009cfi], the magenta dashed curve is from Ref. [Arrington:2004is],
and the gray dotted curve is from Ref. [Qattan:2011ke].

[width=0.49]clas_q2_085.pdf [width=0.49]clas_q2_145.pdf

Figure : Results from the two-photon exchange experiment performed with CLAS,
taken from Ref. [CLAS:2016fvy]. The left and right panels show the
$Q^{2}=0.85$ GeV2 and $Q^{2}=1.45$ GeV2 settings, respectively. The VEPP-3
data (blue points) come from Ref. [Rachek:2014fam] (Fig. ), while the previous
data (green points) come from Refs. [Yount:1962, Browman:1965zz, Mar:1968qd,
Anderson:1966, Cassiday:1967, Anderson:1968zzc, Bartel:1967dsa,
Bouquet:1968yqa]. The predictions labeled “Zhou & Yang” come from Ref.
[Zhou:2014xka], while the prediction of Blunden et al. comes from Ref.
[Blunden:2005ew], and the “Point-like proton” prediction comes from Ref.
[Arrington:2011dn]

The emergence of the form factor discrepancy and the inadequacy of previous
$R_{2\gamma}$ data motivated three new experiments to measure hard TPE at the
percent level. An experiment at the VEPP-3 storage ring, at the Budker
Institute in Novosibirsk, Russia, used alternating beams of electrons and
positrons, scattering from a gaseous hydrogen target, and measured scattered
leptons and recoiling protons in non-magnetic detectors [Rachek:2014fam]. An
experiment was conducted in Hall B at Jefferson Lab, as part of the eg5 run
period, which in the absence of a positron beam used a tertiary beam of
$e^{-}e^{+}$ pairs with a broad energy range and the CLAS detector
[CLAS:2013mza, CLAS:2014xso, CLAS:2016fvy]. Finally, the OLYMPUS Experiment
[OLYMPUS:2013lem, OLYMPUS:2016gso], conducted at the DORIS storage ring at
DESY, in Hamburg, Germany, used alternating electron and positron beams and a
gaseous hydrogen target, but, unlike at Novosibirsk, reconstructed events with
a toroidal magnetic spectrometer. The fact that none of the three experiments
used the same experimental technique gave each set of results different
sensitivities to systematic effects. The kinematics of the data from these new
experiments is also shown in Fig. . The results of the VEPP-3 TPE experiment
are shown in Fig. . The experiment had runs with a 1.6 GeV and 1.0 GeV beam
energy. The VEPP-3 data show the largest effect of the three contemporary
experiments. $R_{2\gamma}$ grows with decreasing $\epsilon$ and increasing
$Q^{2}$, as would be expected given the form factor discrepancy, and reaches a
few percent above unity. Because VEPP-3 had non-magnetic detectors, the
experiment lacked a way to momentum analyze out-going particles. Elastic
scattering events were separated from inelastic background through
calorimetric energy measurements and angular correlations. The experiment also
lacked an independent way to determine the relative $e^{+}$ and $e^{-}$
luminosities. Instead the data are reported relative to $R_{2\gamma}$ at a
“luminosity normalization point” (LNP). Therefore the results should be
interpreted as $R_{2\gamma}/R_{2\gamma}^{\text}{LNP}$. When comparing to
theoretical predictions, the VEPP-3 data should be rescaled to the theory
prediction for $R_{2\gamma}$ at the LNP. The results of the CLAS TPE
experiment are shown in Fig. . Because of the wide energy spread of the
$e^{+}e^{-}$ pair beam, the experiment published results in two-dimensional
$\epsilon$-$Q^{2}$ bins. The incoming beam particle energy was reconstructed
from the kinematics of the outgoing lepton and proton, while the lepton charge
was determined from curvature direction in the CLAS toroidal magnetic field.
Two different binning schemes were used: one in which the bin centers had
approximately constant $Q^{2}$ and varied in $\epsilon$, and another in which
the bin centers had approximately constant $\epsilon$ and varied in $Q^{2}$.
The two sets of results are not statistically independent as the bins overlap.
Because a $e^{+}e^{-}$ pair beam was used, there was no need for an
independent luminosity normalization between the two lepton charges. From the
CLAS results, the lowest $\epsilon$, highest $Q^{2}$ data point is
approximately 3% above unity ($\pm 1.4\%$), but all other data points are
consistent with no TPE effect, given the large uncertainties, which are
primarily driven by statistics.

olympus.pdf

Figure : Results from the OLYMPUS experiment, performed at DESY in Hamburg,
Germany, taken from Ref. [OLYMPUS:2016gso].

The results of the OLYMPUS experiment are shown in Fig. . OLYMPUS used a
large-acceptance toroidal magnetic spectrometer, similar to CLAS, as well as a
storage ring and gaseous target like the VEPP-3 experiment, and so shared
similarities with both experiments. Data were collected in 2012–13 at DESY,
and published in 2017, with a single fixed beam energy of 2 GeV. OLYMPUS
determined the relative $e^{+}$ and $e^{-}$ luminosities in an entirely
different way to the VEPP-3 experiment. OLYMPUS used a pair of forward
calorimeters to compare rates of symmetric Møller/Bhabha scattering and
forward elastic $e^{\pm}p$ scattering in each bunch crossing
[Schmidt:2017jby]. While OLYMPUS had the largest total data set in terms of
integrated luminosity times acceptance, the experiment had to overcome a
number of systematic hurdles, the most significant of which was unacceptable
detector performance in the electrons-outbending toroid polarity setting. Both
the OLYMPUS and CLAS experiments planned frequent reversals of all magnet
polarities to suppress systematic differences between electron and positron
detection. In OLYMPUS, with the detector only able to operate in one torus
polarity, a great deal of effort needed to be expended to understand the small
scale details of the detector efficiency over the entire acceptance. The
OLYMPUS results show an $R_{2\gamma}$ that dips below 1 at high $\epsilon$,
but then shows and upward trend at lower $\epsilon$ and higher $Q^{2}$. This
dip is a feature of some phenomenological estimates [A1:2013fsc,
Schmidt:2019vpr] (but not all [Chen:2007ac, Guttmann:2010au]), and is not
present in the hadronic predictions of Blunden et al. [Blunden:2017nby,
Ahmed:2020uso].

##  Current Status of Hard TPE

The current state of knowledge of hard TPE is an uncomfortable one. There is a
clear effect producing some kind of bias between determinations of the
proton’s form factors based on unpolarized cross sections, and those based on
polarization observables. Hard TPE, as a neglected radiative correction is a
natural candidate for the source of the effect. The effect is difficult to
calculate, and the calculations we have come with model dependence. A recent
round of experiments attempted to measure the hard TPE effect and found
underwhelming results. This is in part because none of the experiments had the
kinematic reach to probe hard TPE in kinematics where the proton form factor
discrepancy is large. As seen in Fig. , the recent experiments were able to
cover the region up to $Q^{2}=2$ GeV${}^{2}/c^{2}$, whereas the discrepancy is
large between 2 and 7 GeV${}^{2}/c^{2}$ (or even higher, given the recent
results from Hall A [Christy:2021snt]). Without data covering this region, we
do not know whether hard TPE is truly the cause of the discrepancy, or if a
different effect is in play (for example, the treatment of other radiative
corrections, multi-photon emissions, etc.). In this introduction, we have
discussed hard TPE in the context of the simplest reaction, elastic electron-
proton scattering; however, radiative corrections are also critical for the
interpretation of more complicated reactions, such as deeply virtual Compton
scattering or deeply exclusive meson production. The effects of hard TPE may
also produce bias in these reactions, and continue to cloud our understanding
of nucleon structure. The future positron facility planned at Jefferson Lab,
combined with the high luminosity and large acceptance of the CLAS12 detector
would open up the possibility of a high-statistics measurement of hard TPE
covering a wide swath of kinematic space. Such an experiment would be a
decisive test of whether or not hard TPE is the cause of the form factor
discrepancy, build a firm bridge between the kinematic regimes suited for
partonic and hadronic calculations, and provide valuable constraints for our
progressing understanding of radiative corrections. This proposal is developed
from Section 5.4 Projected measurements at CLAS12 (pp. 40–21) of letter of
intent submitted to PAC 46 in 2018
(https://www.jlab.org/exp_prog/proposals/18/LOI12-18-004.pdfLOI 12-18-004
[positronLOI]). That letter covered a wide range of physics that would be made
possible by a positron beam at CEBAF. It was developed by the Jefferson Lab
Positron Working Group into a white paper, published as a topical issue of the
European Physical Journal A [Alamanos:2022wwn]. A preliminary concept for the
experiment proposed here was published in that issue [Bernauer:2021vbn]. A
summary of the general physics case for a positron beam at Jefferson Lab, as
well as an overview of how it can be implemented can be found in Ref.
[Accardi:2020swt].