Measurement of excitation spectra in the ${}^{12}$C$(p,d)$ reaction near the $eta$ emission threshold

Excitation spectra of $^{11}$C were measured in the $^{12}$C$(p,d)$ reaction near the $eta$ emission threshold. A proton beam extracted from the synchrotron SIS-18 at GSI with an incident energy of 2.5 GeV impinged on a carbon target. The momenta of deuterons emitted at 0 degrees were precisely measured with the fragment separator FRS operated as a spectrometer. In contrast to theoretical predictions on the possible existence of deeply bound $eta$ mesic states in carbon nuclei, no distinct structures were observed associated with the formation of bound states. The spectra were analyzed to set stringent constraints on the formation cross section and on the hitherto barely-known $eta$-nucleus interaction.