[Gpd_wg] Joint N*/GPD informal meeting Friday 11-Sep, 10 AM, F224/225

[Christian Weiss]

Dear Colleague,


As you surely know, an extensive experimental program is planned
to study the excitation of nucleon resonances at high photon
virtualities with CLAS12 at JLab. The recently approved experiment

   "Nucleon Resonance Studies with CLAS12"
   http://www.physics.sc.edu/~gothe/research/pub/nstar12-12-08.pdf

will for the first time determine the N* electrocouplings in the
region Q2 = 5-10 GeV2 for almost all well-established resonances
in the pi-N and pi-pi-N channels, providing unique information on
resonance structure at yet unexplored distance scales.
A complementary program is under way to study high-Q2 exclusive
processes in the DIS region, W > 2 GeV, where they can be described
by a partonic mechanism (GPDs) and give new insight into the
short-range structure of the ground state. It is clear that there
are many issues of potentially common interest to both programs,
such as

  ** the possible use of the partonic mechanism to constrain the
     non-resonant background in hadronic reaction models used
     to extract N* parameters;

  ** the use of finite-energy sum rules and duality concepts in
     exclusive channels to relate/combine the different descriptions
     of high-Q2 exclusive production in the resonance (low W)
     and DIS (high W) regions.


Following suggestions by several people we are planning an informal
meeting between researchers in N* electroproduction and hard exclusive 
processes, to exchange information, identify issues of common interest,
and begin their discussion:

    "N* excitation and hard exclusive processes"
     Friday, 11-Sep, 10:00 -- 13:00, CC F224/225

This first meeting will in particular focus on the phenomenology of
the production processes in both regions (status of reaction models
for N* extraction, status of partonic description); a follow-up meeting
will be devoted to the physical interpretation of high-Q2 resonance
excitation data, the connection with dynamical models and, possibly,
lattice QCD calculations. The tentative agenda for the Friday meeting is:

   10:00--10:40   V. Mokeev (JLab): Status of reaction models for
                  N* excitation analysis and extension to high Q2

   10:40--11:00   Discussion

   11:00--11:30   P. Kroll (Wuppertal U.): Status of partonic
                  description of exclusive pion production

   11:30--12:00   Discussion

   12:00--12:20   C. Weiss (JLab): Comments on finite-energy sum rules
                  and duality in exclusive electroproduction processes

   12:20--13:00   Discussion


The agenda is very much open; if you would like to make a topical
contribution please bring it to our attention. In particular, we would
like to encourage people with expertise in duality and other
"cross-over" issues to come forward and share their knowledge with us.
The meeting is intended to be INFORMAL; we want to learn and identify
interesting issues and will leave ample time for discussion.


A summary of the status of the CLAS N* analysis and the reaction models
used to analyze the data, including references, can be found at:
   http://www.jlab.org/~mokeev/react_models_highq2/highq2.html


People interested in participating in this meeting remotely should
kindly contact us. There will be a speakerphone in the room (we will
provide the dial-in information), and the slides can be viewed over
the web.


We hope to see you at the meeting this Friday and are looking forward
to interesting discussions!


    V. Mokeev, R. Gothe, Ch. Weiss


Christian Weiss
Theory Center
Jefferson Lab
12000 Jefferson Ave
Newport News, VA 23606, USA
Phone: +1.757.269.7013
Fax:   +1.757.269.7002
E-mail: weiss at jlab.org