[ee] proposal abstract

[Pawel Nadel-Turonski]

Stepan

Very good points! However, although, as you say, we cannot know for sure 
until CLAS12 will start taking data until its construction is complete, 
the PAC will probably have to assume that proton running should start on 
schedule and we will get our data in 2016 and treat SoLID as a follow-up 
experiment. This would force us to address the issue of complementarity. 
Of course, we could indeed make the abstract shorter, moving mentioning 
the details only in the introduction. But for now I was hoping that this 
longer abstract would serve as a starting point for today's discussions 
- as it already has. :)

Regards,

     Pawel

On 3/28/2013 7:37 AM, Stepan Stepanyan wrote:
> 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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