The Form Factors of the Nucleons: Present Status - Fisica
The Form Factors of the Nucleons: Present Status - Fisica
The Form Factors of the Nucleons: Present Status - Fisica
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Indication <strong>of</strong> two hard photon exchange?<br />
<strong>The</strong> striking and significant difference between <strong>the</strong> Rosenbluth data<br />
for G Ep/G Mp, and <strong>the</strong> recoil polarization from P t/P ℓ, for Q 2 larger<br />
than 2-3 GeV 2 , has triggered a re-examination <strong>of</strong> all older models<br />
<strong>of</strong> <strong>the</strong> nucleon.<br />
Rosenbluth data give μ pG Ep/G Mp values hovering around 1, polarization<br />
data give μ pG Ep/G Mp~1-0.14(Q 2 -0.04), up to approximately 6 GeV 2 ,<br />
followed by slightly slower decrease with Q 2 .<br />
A hypo<strong>the</strong>sis favored by a majority <strong>of</strong> <strong>the</strong>orists, but not all<br />
<strong>the</strong>orists or experimentalists is that <strong>the</strong> difference arises due to<br />
<strong>the</strong> hard two-photon exchange, a term neglected in radiative<br />
corrections until recently .<br />
Here note that a two-photon contribution tends, in first<br />
approximation, to decrease <strong>the</strong> Rosenbluth values <strong>of</strong> G Ep/G Mp, but<br />
leaves <strong>the</strong> polarization values unchanged.<br />
2/17/2013 C.F. Perdrisat SAEMP Trento 2013 34
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