Ballistic Protons in Incoherent Exclusive Vector Meson Production as a Measure of Rare Parton Fluctuations at an Electron-Ion Collider

We argue that the proton multiplicities measured in Roman pot detectors at an electron ion collider can be used to determine centrality classes in incoherent diffractive scattering. Incoherent diffraction probes the fluctuations in the interaction strengths of multiparton Fock states in the nuclear wave functions. In particular, the saturation scale that characterizes this multiparton dynamics is significantly larger in central events relative to minimum bias events. As an application, we study the centrality dependence of incoherent diffractive vector meson production. We identify an observable which is simultaneously very sensitive to centrality triggered parton fluctuations and insensitive to details of the model.

Figure 1

Diffractive DIS kinematics.

Figure 2

Scattered proton longitudinal and transverse momentum in the target rest frame in diffractive $\mathrm{J}/\mathrm{\Psi }$ production. Here we choose $Q^2=1{\mathrm{GeV}}^2$.

Figure 3

EIC acceptance for protons, as a function of $p_T$, in a Roman pot detector. Figure reprinted with permission from Ref. [19]. The reaction simulated is Deeply Virtual Compton Scattering (DVCS) in $e+p$ scattering. The region of strong overlap between three curves demarcates the $p_T$ acceptance. Because of the different configuration of magnetic fields in $e+A$ scattering, there will be a shift in the acceptance to lower $p_T$ than shown here.

Figure 4

Ratio of two vector meson incoherent diffractive cross sections in central events relative to minimum bias events as a function of $Q^2$. See text for details.