# Central exclusive production as a probe of the gluonic component of the *η*′ and *η* mesons

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## Abstract

Currently, the long-standing issue concerning the size of the gluonic content of the *η*′ and *η* mesons remains unsettled. With this in mind we consider the central exclusive production (CEP) of *η*′, *η* meson pairs in the perturbative regime, applying the Durham pQCD-based model of CEP and the ‘hard exclusive’ formalism to evaluate the meson production subprocess. We calculate for the first time the relevant leading order parton-level processes \(gg \to q\overline{q}gg\) and *gg*→*gggg*, where the final-state *gg* and \(q\overline{q}\) pairs form a pseudoscalar flavour-singlet state. We observe that these amplitudes display some non-trivial and interesting theoretical properties, and we comment on their origin. Finally, we present a phenomenological study, and show that the cross sections for the CEP of *η*′, *η* meson pairs are strongly sensitive to the size of the gluon content of these mesons. The observation of these processes could therefore provide important and novel insight into this problem.

## Keywords

Transverse Momentum Distribution Amplitude Helicity Amplitude Transition Form Factor Central Exclusive Production## Notes

### Acknowledgements

We thank Mike Albrow, Erik Brucken, Victor Chernyak, Risto Orava, Kornelija Passek–Kumeric̆ki and Antoni Szczurek for useful discussions. This work was supported by the grant RFBR 11-02-00120-a and by the Federal Program of the Russian State RSGSS-4801.2012.2. WJS is grateful to the IPPP for an Associateship. VAK thanks the Galileo Galilei Institute for Theoretical Physics for hospitality and the INFN for partial support during the completion of this work.

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