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%\section{Summaries of Experimental Activities}

\subsection{E01-001}

\begin{center} 
New Measurement of $(G_E/G_M)$ for the Proton
\end{center}

\begin{center}
J.~Arrington and R.~E.~Segel, for the E01-001 Collaboration.
\end{center}

Experiments using the polarization transfer technique have
reported~\cite{jones00,gayou02} that in the $1 < Q^2 <  5.6$~GeV$^2$ range
the ratio of proton electric to magnetic form factor falls off linearly with
increasing $Q^2$, with the results well fit by $\mu_p G_E / G_M = (1 -
0.135(Q^2 - 0.24))$~\cite{arrington03}. Uncertainties in the individual points
are 0.05 or less at moderate $Q^2$, and 0.10 or less for the larger $Q^2$
values.  These results appear to contradict experiments going back as far as
1970~\cite{litt70,berger71,price71,bartel71,walker94,andivahis94,christy03}
using the Rosenbluth technique that are more in agreement with "scaling"
($\mu_p G_E / G_M \approx 1$). Global analyses of the Rosenbluth data also
support this contention~\cite{arrington03,walker94,bosted94}. However, the
error bars reported in the results from the individual Rosenbluth experiments
are larger than those from the polarization transfer experiments, especially
at high $Q^2$.  The "Super Rosenbluth" experiment was undertaken with the goal
of doing Rosenbluth separations with uncertainties in $G_E/G_M$ comparable to
those achieved with polarization transfer.

\begin{figure}[htb]
\begin{center}
\psfig{file=e01001_kinematics.ps,angle=90,width=8cm}
\caption{\it The ($Q^2,\varepsilon$) points at which proton spectra were
taken.  The lines are fixed values of beam energy: 1.912 (red), 2.262 (black),
2.842 (green), 3.772 (blue), and 4.702 GeV (magenta). The $Q^2 = 0.50$~GeV$^2$
points served to monitor the luminosity.}
\label{fig:kinematics}
\end{center}
\end{figure} 

The experiment was run in May, 2002 using a 4 cm liquid hydrogen target and
the HRS spectrometers.  The separations were done with singles proton spectra.
This is a departure from previous p(e,e')p L-T separation experiments where
electrons were detected. Detecting the protons has several important
advantages~\cite{e01001}: (1) the momentum of the detected particles
is independent of $\varepsilon$ for a fixed $Q^2$ value, (2) the cross
sections vary slowly as a function of $\varepsilon$, and (3) the radiative
corrections are smaller than for electron detection.  Separations were
performed at three values of $Q^2$: 2.64, 3.20 and 4.10~GeV$^2$.
Figure~\ref{fig:kinematics} shows the points at which data were taken.  While
the left HRS spectrometer was obtaining spectra at the $Q^2$ of interest, the
right spectrometer was set at $Q^2 = 0.50$~GeV$^2$ and serving as a luminosity
monitor~\cite{e01001}.  At two kinematic settings, p(e,e'p) coincidence spectra
were taken with the right spectrometer detecting the electrons.  These data
are used to check efficiencies and spectral shapes.

The data have been replayed using the code ESPACE that generated ntuples that
were passed on to a stand-alone analysis code, RECON. RECON calculates the
angle and momentum for each event, applies cuts to reject events that scrape
at the Q3 exit, and applies particle identification and kinematic cuts to
identify protons in the region of the elastic peak. Corrections were applied
for tracking inefficiency and dead time, which were no more than a few percent
(except for computer dead time), and which are known to better than 0.5\%.

\begin{figure}[htb]
\begin{center}
\psfig{file=e01001_deltap.ps,width=8cm}
\caption{\it Missing momentum spectrum (in MeV) at $E_{beam} = 2.262$~GeV,
$\theta_p = 12.526^\circ$ ($Q^2 = 3.20$~GeV$^2$, $\varepsilon = 0.131$).  The
data from the LH2 target is shown in black, while the total simulated spectrum
is in red.  The spectrum is decomposed into contributions from the elastic peak
(blue), protons from the $\gamma + p \rightarrow \pi^0 + p$ reaction (green)
and protons coming from the solid endcaps of the target (magenta).}
\label{fig:deltap}
\end{center}
\end{figure} 

Figure ~\ref{fig:deltap} shows the missing momentum spectrum; measured proton
momentum minus the momentum calculated from the measured scattering angle, 
assuming elastic scattering kinematics.  The spectrum is dominated by the
elastic peak but there are two backgrounds that have to be subtracted:

\medskip
\noindent
1. Reactions, primarily quasi-elastic scattering, in the endcaps of the
target cell.  This background was determined by taking data at every kinematic
setting with a dummy target.

\medskip
\noindent
2. Protons from the $\gamma + $p $\rightarrow$ $\pi^0 + $p reaction initiated
by bremsstrahlung in the target's upstream endcap.  These protons were
significant only at forward angles and, because of the pion's finite mass,
their spectrum cuts off below the elastic peak. Nevertheless, they had to be
taken into account.  Their contribution was modeled using a calculated
bremsstrahlung spectrum and an $s^{-7}$ cross section dependence, scaled to
the measured background below the elastic peak.  This background was
insignificant at the $Q^2 = 0.50$~GeV$^2$ settings.

The elastic peak was modeled using the Monte Carlo code SIMC adopted to the
HRS spectrometers.  The code incorporates radiative corrections taken from the
work of Mo and Tsai~\cite{mo69}, with additional terms from~\cite{walker94}
(the exact implementation is described in Ref.~\cite{ent01}).  The breakdown
of the spectrum into its three components is shown in Fig.~\ref{fig:deltap}. 
The p(e,p) scattering cross sections were obtained from the normalization
required to match the elastic peak calculated by the Monte Carlo code to that
observed.

\begin{figure}[htb]
\begin{center}
\psfig{file=e01001_sigr.ps,height=11cm,width=8cm}
\caption{\it Reduced cross sections plotted as a function of $\varepsilon$.  The
solid line is the best linear fit to the data and the dotted line is the best
fit with the slope fixed to match the results of the polarization transfer
experiments~\cite{jones00,gayou02}.}
\label{fig:sigr}
\end{center}
\end{figure} 

Preliminary results for the reduced cross sections, arbitrarily normalized to
$\sigma_R \approx 1$, are shown as a function of $\varepsilon$ for the three
$Q^2$ values in Fig~\ref{fig:sigr}.  The uncorrelated errors on the individual
points are about 0.6\%, and there is an additional systematic uncertainty on
the $\varepsilon$-dependence (\textit{i.e.} the Rosenbluth slope) of
$\sim$0.6\%.  It is expected that further analysis will reduce these
uncertainties. The current systematics are limited by the background
subtraction and efficiency of the tracking cuts.  Because we do not apply
tight cuts on the `quality' of the VDC information, we have a very high
tracking efficiency, but long, non-gaussian tails in the reconstructed target
quantities. Therefore, there is some uncertainty in the efficiency of the cut
on the elastic peak, and we are forced to apply a very loose cut around the
elastic peak to avoid losing too much of the data. We are working on improving
the tracking, or else verifying that we can apply tight cuts on the track
quality without introducing unacceptable uncertainties in the tracking
efficiency. This will improve the uncertainty related to the tracking and the
cuts, and will also allow us to use a tighter cut on the elastic peak,
significantly reducing the size of the background corrections.

In the one-photon approximation the reduced cross section can be written
$\sigma_R = \tau G_M^2 + \varepsilon G_E^2$, where
$\varepsilon$ is the polarization of the virtual photon.  The slope of this
linear function is proportional to $(G_E/G_M)^2$.  Shown in Fig~\ref{fig:sigr}
are the best fit to the present data using the form factor ratios obtained
from a fit to all previous Rosenbluth data~\cite{arrington03} and using those
obtained in the polarization transfer measurements~\cite{jones00,gayou02}.
Clearly the present results are consistent with the previous Rosenbluth
separation results and are not at all consistent with the polarization
transfer determinations.

\medskip
\noindent
Accepting the E01-001 results leaves but two alternatives:

\medskip
\noindent
1. There is a flaw in the polarization transfer experiments or formalism.  A
continuation of the polarization transfer experiments in Hall C has been
approved~\cite{e01109} and this will provide a check on the Hall A results.
Also, a proposal to make an equivalent measurement of the form factor ratios
using a polarized proton target~\cite{pr01105}, which was deferred pending the
results of E01-001, may be revived now that the discrepancy has been confirmed.

\medskip
\noindent
2. The Rosenbluth data are not being interpreted correctly.  The possibility
that two-photon exchange corrections could explain the discrepancy is being
intensively pursued and
calculations~\cite{guichon03,blunden03,afanasev_priv,rekalo03} have shown that
the contributions to the elastic cross section from this mechanism could be
significant, and may have the potential to bring the two techniques into
agreement. Considerable experimental and theoretical efforts are underway to
determine if this is indeed the case.


\begin{thebibliography}{9}

\bibitem{jones00} M. K. Jones et al., Phys. Rev. Lett. {\bf 84}, 1398 (2000).
\bibitem{gayou02} O. Gayou et al., Phys. Rev. Lett. {\bf 88}, 092301 (2002).
\bibitem{arrington03} J. Arrington, Phys. Rev. C {\bf 68}, 034325 (2003).
\bibitem{litt70} J. Litt et al., Phys. Lett. B {\bf 31}, 40 (1970).
\bibitem{berger71} Ch. Berger et al., Phys. Lett. B {\bf 35}, 87 (1971).
\bibitem{price71} L. E. Price et al., Phys. Rev. D {\bf 4}, 45 (1971).
\bibitem{bartel71} W. Bartel et al., Nucl. Phys. {\bf B58}, 429 (1971).
\bibitem{walker94} R. C. Walker et al., Phys. Rev. D {\bf 49}, 5671 (1994).
\bibitem{andivahis94} L. Andivahis et al., Phys. Rev. D {\bf 50}, 5491 (1994).
\bibitem{christy03} M. E. Christy, to be submitted to Phys. Rev. C.
\bibitem{bosted94} P. E. Bosted et al., Phys. Rev. C {\bf 51}, 409 (1995).
\bibitem{e01001} R. E. Segel, J. Arrington et al., proposal E01-001.  
\bibitem{mo69} L. W. Mo and Y. S. Tsai, Rev. Mod. Phys. 41. 205 (1969). 
\bibitem{ent01} R. Ent et al., Phys. Rev. C {\bf 64}, 054610 (2001).
\bibitem{e01109} C. F. Perdrisat, et al., proposal E01-109.
\bibitem{pr01105} G. Warren et al., proposal PR01-105.
\bibitem{guichon03} P. A. M. Guichon and M. Vanderhaeghen, Phys. Rev. Lett. {\bf 91}, 142303 (2003).
\bibitem{blunden03} P. G. Blunden, W. Melnitchouk and J. A. Tjon, Phys. Rev. Lett. {\bf 91}, 142304 (2003).
\bibitem{afanasev_priv} A. Afanasev, Workshop on Nucleon Form Factors \& Parity Violation, Athens, (2003) and private communication.
\bibitem{rekalo03} M. P. Rekalo and E. Tomasi-Gustafsson, nucl-th/0307066

\end{thebibliography}

\end{document}
