# EA Color in Nuclei - Paul Hoyer

Possible EIC issues which relate to CT: The measurement of GPD's in deeply inelastic exclusive processes is of central importance, and presupposes that the upper vertex involving the virtual photon is hard. In exclusive meson production this implies that the meson is transversely compact. Scattering on nuclei can test this, since it implies that there should be no nuclear suppression of the outgoing meson. (The GPD itself will have some A-dependence, which is a weaker effect and can be estimated by comparing with DVCS.)

There is evidence that mesons produced by real photons at large momentum transfer to the target are not compact, see

Duality in semiexclusive processes. Patrik Eden, Paul Hoyer, (Nordita) , Alexander Khodjamirian, (Karlsruhe U., TTP) . NORDITA-2001-46-HE, TTP-01-28, Oct 2001. 4pp. Published in JHEP 0110:040,2001. e-Print: hep-ph/0110297

Subprocess size in hard exclusive scattering. Paul Hoyer, (Helsinki U. & Helsinki Inst. of Phys.) , Jonathan T. Lenaghan, (Virginia U. & Bohr Inst.) , Kimmo Tuominen, Carsten Vogt, (Nordita) . HIP-2002-47-TH, NORDITA-2002-67-HE, Oct 2002. 14pp. Published in Phys.Rev.D70:014001,2004. e-Print: hep-ph/0210124

The (absence of) CT in real photoproduction at large t could also be studied at the EIC.

In a recent paper

The transverse shape of the electron. Paul Hoyer, Samu Kurki, . HIP-2009-30-TH, Nov 2009. 13pp. Temporary entry e-Print: arXiv:0911.3011 [hep-ph]

published as Phys. Rev. D 81, 013002 (2010) we noted that there is a qualitative difference between CT observed in mesons produced directly from virtual photons (\gamma^* \to \rho, measured in e+A \to e+\rho+A) as compared to hadrons recoiling from a t-channel photon (ep \to ep, measured in e+A \to e+p+(A-1)). In the former case, the meson size is determined by its overlap with the compact virtual photon wave function, which is exponentially suppressed for large sizes. In the latter case compact initial protons are selected by the phase variation \exp(ir_\perp \cdot p_perp) of the photon, which suppresses large r_\perp only by cancellations due to the oscillating phase. This systematics is in accord with data, and could be tested further at the EIC.