  In heavy-ion collisions, momentum-dependent pair correlations can be
characterized by a principal component analysis (PCA), in which subleading
modes are expected to reveal new information on flow fluctuations. However, we
find that, as currently measured, these modes can be dominated by multiplicity
fluctuations, which serve as an unwanted background. Here, we propose new PCA
observables that are robust against multiplicity fluctuations and isolate novel
sources of flow fluctuations, thus being suited to provide fresh insight into
the initial stages of the system at small length scales.
