科研之友
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摘要
Understanding various fundamental properties of nucleons and nuclei is among the most important scientific goals at the upcoming Electron-Ion Collider (EIC). With the unprecedented opportunity provided by the next-generation machine, the EIC might provide definitive answers to many standing puzzles and open questions in modern nuclear physics. We investigate one of the golden measurements proposed at the EIC, which is to obtain the spatial gluon density distribution within a lead (Pb) nucleus. The proposed experimental process is the exclusive J/psi vector-meson production off the Pb nucleus: e + Pb -> e' + J/psi + Pb'. The Fourier transformation of the momentum transfer vertical bar t vertical bar distribution of the coherent diffraction is the transverse gluon spatial distribution. In order to measure it, the experiment has to overcome an overwhelmingly large background arising from the incoherent diffractive production, where the nucleus Pb' mostly breaks up into fragments of particles in the far-forward direction close to the hadron-going beam rapidity. We systematically study the rejection of incoherent J/psi production by vetoing products from these nuclear breakups-protons, neutrons, and photons-which is based on the BeAGLE event generator and the most up-to-date EIC Far-forward Interaction Region design. The achieved vetoing efficiency, the ratio between the numbers of vetoed events and total incoherent events, ranges from about 80% to 99% depending on vertical bar t vertical bar. Assuming a 5% smearing applied to the reconstructed vertical bar t vertical bar resolution in the Sartre model, this vetoing efficiency can suppress the incoherent background to at least the first minimum of the coherent vertical bar t vertical bar distribution. Experimental and accelerator machine challenges as well as potential improvements are discussed.
