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Misha,
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<div class="">thank for this very clear lecture. </div>
<div class="">You derive the Karplus curve representing the singularity of the box diagram, in the equal mass case. </div>
<div class="">Here is a paper about the box diagram in pion photoproduction: <a href="http://inspirehep.net/record/120443?ln=en" class="">http://inspirehep.net/record/120443?ln=en</a></div>
<div class="">The author claims that the singularities are in the physical region, t < 1.1 GeV^2 for s > 1.6^2 GeV^2</div>
<div class="">(for one particular box, other boxes are outside the physical region as in your example)</div>
<div class="">See Fig 2 and the discussion above. </div>
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<div class="">That means that fixed t dispersion relation (as I used them for FESR) can be ill defined for t < 1.1 GeV^2</div>
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<div class="">There is no explicit calculation. I suppose the curve on Fig comes from a similar demonstration as you did. </div>
<div class="">If somebody does this exercise, I’d happy to have a confirmation of the result. </div>
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<div class="">*** The next Gribov Lecture is Friday March 10 at 9am (US time - 3pm in Europe) *** </div>
<div class="">Misha could you confirm this schedule ?</div>
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<div class="">Cheers,</div>
<div class="">Vincent</div>
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<div class="">Vincent Mathieu</div>
<div class="">Indiana University Research Fellow</div>
<div class="">INFN-Genova Associate</div>
<div class="">UMons Associate</div>
<div class=""><a href="mailto:mathieuv@indiana.edu" class="">mathieuv@indiana.edu</a></div>
<div class="">Office Phone: +1 812 855 2900</div>
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<div class="">Joint Physics Analysis Center:</div>
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