Probing Gluonic Saturation in Deeply Virtual Meson Production beyond Leading Power

Exclusive diffractive meson production represents a golden channel for investigating gluonic saturation inside nucleons and nuclei. In this Letter, we settle a systematic framework to deal with beyond leading power corrections at small <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>x</a:mi></a:math>, including the saturation regime, and obtain the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msup><c:mrow><c:mi>γ</c:mi></c:mrow><c:mrow><c:mo>*</c:mo></c:mrow></c:msup><c:mo stretchy="false">→</c:mo><c:mi>M</c:mi><c:mo stretchy="false">(</c:mo><c:mi>ρ</c:mi><c:mo>,</c:mo><c:mi>ϕ</c:mi><c:mo>,</c:mo><c:mi>ω</c:mi><c:mo stretchy="false">)</c:mo></c:mrow></c:math> impact factor with both incoming photon and outgoing meson carrying arbitrary polarizations. This is of particular interest since the saturation scale at modern colliders, although entering a perturbative regime, is not large enough to prevent higher-twist effects to be sizable.