Gluon Sivers function from forward exclusive <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>χ</mml:mi></mml:mrow><mml:mrow><mml:mi>c</mml:mi><mml:mn>1</mml:mn></mml:mrow></mml:msub></mm

Exclusive production of a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>χ</a:mi><a:mrow><a:mi>c</a:mi><a:mn>1</a:mn></a:mrow></a:msub></a:math> axial vector quarkonia in photon-proton scattering at high energies requires a <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>C</c:mi></c:math>-odd <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi>t</e:mi></e:math>-channel exchange. In the limit of vanishing momentum transfer this occurs either via the exchange of a photon, the Primakoff process, where the spin of the proton does not change. For axial-vector meson production, as a consequence of the Landau-Yang theorem, the Primakoff cross section is finite as <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>t</g:mi><g:mo stretchy="false">→</g:mo><g:mn>0</g:mn></g:math>. Alternatively, a <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mi>C</j:mi></j:math>-odd spin dependent Odderon can be exchanged, which involves a spin flip of the proton. The resulting cross section is related to the square of the collinear trigluon correlator or the <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:msub><l:mi>k</l:mi><l:mo>⊥</l:mo></l:msub></l:math>-moment of the gluon Sivers function. Using two models for the gluon Sivers function from the literature we compute the ratio of Sivers to Primakoff cross sections and the angular coefficient <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:msub><n:mi>λ</n:mi><n:mi>θ</n:mi></n:msub></n:math> governing the angular distribution of the <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:msub><p:mi>χ</p:mi><p:mrow><p:mi>c</p:mi><p:mn>1</p:mn></p:mrow></p:msub><p:mo stretchy="false">→</p:mo><p:mi>J</p:mi><p:mo>/</p:mo><p:mi>ψ</p:mi><p:mo>+</p:mo><p:mi>γ</p:mi></p:math> decay as functions of <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mi>x</s:mi></s:math>. We point out that these observables constrain the magnitude of the gluon Sivers function at small <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mi>x</u:mi></u:math> which could be accessed in electron-proton scattering and ultraperipheral proton-proton and nucleus-proton collisions.