The role of Spectator Fragments at an electron Ion collider

Abstract

Efficient detection of spectator fragments is key to the main topics at an electron-ion collider (eIC). Any process which leads to emission of fragments or γ's breaks coherence in diffractive processes. Therefore this is equivalent to non-detection of rapidity gaps in pp collisions. For example, in coherent photoproduction of vector mesons their 4-momentum transfer distribution would image the "gluon charge" in the nucleus in the same way that Hofstadter measured its charge structure using elastic scattering of ∼100 MeV electrons. Whereas he could measure the ∼4 MeV energy loss by the electron due to excitation of nuclear energy levels (Figure 1), even the energy spread of the incident beam would prevent such an inclusive selection of quasielastic events at an eIC. The only available tool is fragment detection. Since, in our example, one finds that ∼ 100% of deexcitations go through γ's or 1 neutron, rarely to 2 neutron and never to protons(due to Coulomb barrier suppression), the eIC design should emphasize their detection. Also in incoherent diffraction fragmentation involves emission of a neutron.

Leading twist nuclear shadowing phenomena in hard processes with nuclei

“…It was suggested that high precision measurement of the transverse momenta of J/ψ would allow one to determine the t dependence of the γ+A → J/ψ+A amplitude and, hence, determine the transverse distribution of gluons in nuclei using a Fourier transform of the amplitude. Unfortunately the processes γ +A → J/ψ+A * , where A * denotes excited nuclear states which decay into the ground state and a photon (photons), dominate the cross section beyond the first minimum making it very difficult to observe the coherent channel beyond the first minimum [236]. At the same time, if the precision measurements of the t dependence at t ∼ 0 are feasible, it would be possible to check the change of the t slope as compared to the impulse approximation, which we find to be of the order of 10 − 15% depending on x, see the discussion of b 2 in Sec.…”