  We present a Bayesian method to reconstruct event-by-event multiplicity
fluctuations and rapidity correlations in p+Pb collisions at zero impact
parameter from minimum-bias data, without assuming any model of the collision
dynamics. We test it on Monte Carlo simulations with the Angantyr model, then
apply it to ATLAS data on the distribution of charged multiplicity and
transverse energy in p+Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV.
Fluctuations in $b=0$ collisions are quantum fluctuations which originate
mostly from the proton wave function, and therefore have the potential to
constrain the subnucleonic structure of the proton. The Angantyr model is found
to overestimate fluctuations. In addition, we find that as the rapidity
increases (towards the Pb-going side), not only the multiplicity density
increases, but also its relative event-by-event fluctuation. This
counter-intuitive phenomenon is also observed in simulations with Angantyr, and
with the QCD dipole model, where its origin can be traced back to the branching
process through which gluons are produced.
