[Hallc] [EXTERNAL] Backward-angle Exclusive pi0 Production above the Resonance Region

[Garth Huber]

Dear Colleagues,

We are inviting you to provide comments and hopefully join our new experimental
proposal before submitting it to PAC 48.

The proposal can be downloaded from:
    https://urldefense.proofpoint.com/v2/url?u=http-3A__lichen.phys.uregina.ca_-7Ehuberg_pizero_PAC48-5Fpi0-5Fproposal.pdf&d=DwICaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=QxP5KTzkAiDxkNbQw9W17g&m=qho3NbSI7EBBkswZ3AGkQdLk9WPnOJoYXx5FFNauLV8&s=X9I-PYwiZz-_pOelaR9oNtYBdqYF7iBd9ST7rWaai08&e= 

In a short summary, this is standard hall C L/T measurement: standard HMS+SHMS
configuration, standard unpolarized LH2 target and standard accelerator
gradient (2.2 GeV/pass).  The proposed run time is 674 PAC hours.  Technically,
the experiment is as straightforward as an L/T-separation experiment ever gets.

The unique aspect of this measurement is we aim to probe the nucleon structure
at the backward-angle (t=t_max, u'=0) through exclusive process: 1H(e,e'p)pi0
above the resonance region (W>2), x=0.36.  In this interaction picture, the
recoil nucleon absorbs most of the induced momentum and is detected in the
spectrometer at forward angles, while the scattered electron will be detected
in other spectrometer (typically the HMS).  The backward pi0 (CM frame) is
produced 180 degree opposite to the q-vector (central proton momentum) almost
at rest, and is identified from the missing mass. You could call it a "knocking
a proton out of a proton process".

We aim to perform L/T separation at Q^2 range of 2-5 GeV^2, and two additional
measured for check the W and Q^2 scaling.

The objectives of the measurement are as follows:

1. Demonstrate the existence of a backward-angle (t=t_max) peak in the
exclusive 1H(e,e'p)pi0 cross section above the resonance region.  The
kinematics have been selected to facilitate comparison to forward angle
exclusive pi0 data from E12-13-010, and the ideas behind the experiment have
been presented previously at NPS Collaboration meetings.

2. Once the backward angle peaks are confirmed, the separated cross sections
sigma_T and sigma_L at Q^2 =2, 3, 4 and 5 GeV^2 will be used to confirm the
dominance of sigma_T over sigma_L, one of the predictions of the backward angle
collinear factorization regime (Transition Distribution Amplitudes=TDA).

3. Perform a sigma_T~1/Q^n scaling test for 2<Q^2<6.25 GeV^2, which would
further validate or refute the TDA factorization, and hopefully identify the
scale at which the factorization regime is reached.

If you have suggestions for improvement, please send them to us no later than
June 17, and please include the relevant line number in your comment.

We are looking forward to have your support for this proposal.

Many thanks,

Bill (Wenliang) Li, billee at jlab.org
Justin Stevens, jrstevens at jlab.org
Garth Huber, huberg at uregina.ca

-- 
Dr. Garth Huber, Dept. of Physics, Univ. of Regina, Regina, SK S4S0A2, Canada.
tel: 1-306-585-4240. fax: 1-306-585-5659.