Introduction to QCD slides - P.Hoyer.pdf - High Energy Physics Group
Introduction to QCD slides - P.Hoyer.pdf - High Energy Physics Group
Introduction to QCD slides - P.Hoyer.pdf - High Energy Physics Group
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No exclusive amplitudes for charged particles (III)<br />
Two charged particles at different positions x, y must be connected by a<br />
pho<strong>to</strong>n string <strong>to</strong> be gauge invariant:<br />
Paul <strong>Hoyer</strong> Mugla 2010<br />
¯ψ(y)exp<br />
�<br />
ie<br />
� y<br />
x<br />
dzνA ν �<br />
(z) ψ(x)<br />
The pho<strong>to</strong>n (gauge) field serves as a connection, which “informs” about the<br />
choice of gauge at each point in space.<br />
In perturbation theory, the missing string causes an infrared singularity at the<br />
one pho<strong>to</strong>n correction level.<br />
But we previously saw that there is no problem with the <strong>to</strong>tal e + e – cross<br />
section, which includes the µ + µ – final state?!<br />
To see how the IR singularity cancels in the <strong>to</strong>tal cross section we may use<br />
the optical theorem.<br />
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