[ee] proposal abstract

[Stepan Stepanyan]

Pawel,

Couple of points - first, sentence where you refer to CLAS12
E12-12-001 better to be as "There is already approved experiment
to run on CLAS12, E12-12-001". We do know for sure that
CLAS12 measurement will the the first.

Second point, drop everything starting from sentence "Recent
calculation suggest ...". Abstract is not a place to discuss this.
Instead add couple of sentence on experiment, required beam
time, detector setting ...

Regards, Stepan

On 3/28/13 2:10 AM, Pawel Nadel-Turonski wrote:
> Hello Everyone,
>
> I started looking at the text of the new proposal, but before we discuss
> it in detail, I thought that for the discussion tomorrow it could be
> good to focus on the abstract. I include a draft below to serve as a
> starting point for how we should adjust the emphasis for the SoLID
> proposal. Once we have this in place I hope that updating the rest will
> go relatively quickly.
>
> Cheers,
>
>       Pawel
>
> Abstract draft:
> We propose to measure exclusive $e^+e^-$ production with SoLID using an
> 11 GeV polarized beam and a $LH_2$ target to study the reaction $\gamma
> p \to \gamma^* p^\prime \to e^+ e^- p^\prime$, known as Timelike Compton
> Scattering (TCS), which is the timelike equivalent of (spacelike) DVCS.
> Both the differential cross section and moments of the weighted cross
> section will be measured as a function of the four-momentum transfer
> $-t$, the outgoing photon virtuality $Q^{\prime 2}$ (up to 9 GeV$^2$),
> and the skewness $\eta$. The latter reflects the difference between the
> initial and final momentum fraction carried by the struck quark, and
> corresponds to $\xi$ in DVCS. A first measurement of TCS at 12 GeV will
> be performed as part of the approved CLAS12 experiemnt E12-12-001. This
> proposed SoLID measurement will add two essential features. First, the
> different acceptance of the SoLID detector, which is more uniform in the
> azimuthal angle $\varphi$, will provide an important experimental cross
> check, resulting in reduced systematic uncertainties on, for instance,
> the real part of the Compton form factor $\mathcal{H}$, to which TCS
> provides a straightforward access. The higher luminosity of SoLID will
> also allow collecting an order of magnitude more statistics in the
> region of large $Q^{\prime 2}$ and $\eta$, making it possible to study
> the dependence on these variables in sufficiently narrow bins, which is
> important for understanding the effects of higher-twist and NLO
> corrections (in $\alpha_s$). Recent calculations suggest that the latter
> may be sizeable, and larger for TCS than DVCS, but are expected to be
> small at larger values of $\eta$ (above 0.3-0.4), and increase rapidly
> as $\eta$ approaches 0.1. Since $\eta = \tau / (2 - \tau) = Q^{\prime 2}
> / (4ME_\gamma - Q^{\prime 2})$, where $\tau$ is the TCS equivalent of
> Bjorken $x$, $M$ is the proton mass, and $E_\gamma$ is the photon
> energy, the region of high $Q^{\prime 2}$, where both higher-twist and
> NLO corrections are expected to be small, provides a natural reference
> point - but one that requires a high luminosity for precision studies.
> On the other hand, the NLO corrections are almost entirely due to
> gluons. Once they are understood, this sensitivity at lower values of
> $\eta$ could provide a new tool for studying gluons at 12 GeV.
>
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