BFKL resummation effects in gamma* gamma* to rho rho Page: 2 of 23
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Figure 1: The process *(qi) t*(q2) -- pL(k1) p2(k2). The box represents the BFKL
Green's function, and the t-channel gluons couple to the quark lines in all possible ways.
The next generation of c+c- colliders will offer the possibility of clean testing of QCD
dynamics. By selecting events in which two vector mesons are produced with a large
rapidity gap in between, through the scattering of two highly virtual photons, one accesses
a kinematical regime in which a perturbative approach is justified. If additionally one
selects events with comparable photon virtualities, the perturbative Regge dynamics of
QCD should dominate and allow the use of resummation techniques of the BFKL  type.
In this paper we study these effects in the case of the reaction (see Fig. 1)
t*(qi) *(q2) -> pL(k1) pL(k2) (1)
in the forward region, i.e. -t = -(qi - k1)2 = -tmi, where the virtualities Qi = -q,
Q2 = -q2 of the scattered photons play the role of the hard scales.
This is an interesting process to study, first because the experimental signature is
simple: double tagged events in high energy c+c- collisions , with two longitudinally
polarized rho mesons in the final state. The non-zero virtualities of the photons allow
studying also the forward region. Second, the theoretical calculation can be performed
analytically and gives a simple answer which makes it easy to investigate BFKL effects.
This is partly because in the forward region we are studying, the amplitude for transversely
polarized photons vanishes identically .
No data exist yet for the process (1) at sufficiently high energies, but some experimental
data exist for Q2 small  which may be analyzed  in terms of generalized distribution
amplitudes . Related reactions have been calculated, both at the Born level and using
BFKL resummation. The similar process with real incoming photons, -y- -> V1V2 and
large t where the hard scale is provided by the momentum transfer instead of the photon
virtualities, was studied in [7,8], and the process yp -> VX at large t, has been studied
both for heavy  and light  vector mesons. The advantage of the latter process is that
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Enberg, R.; Pire, B.; Szymanowski, L. & Wallon, S. BFKL resummation effects in gamma* gamma* to rho rho, article, August 11, 2005; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc892501/m1/2/: accessed March 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.