  Electronic interferometers using the chiral, one-dimensional (1D) edge
channels of the quantum Hall effect (QHE) can demonstrate a wealth of
fundamental phenomena. The recent observation of phase jumps in a Fabry-P\'erot
(FP) interferometer revealed anyonic quasiparticle exchange statistics in the
fractional QHE. When multiple integer edge channels are involved, FP
interferometers have exhibited anomalous Aharonov-Bohm (AB) interference
frequency doubling, suggesting putative pairing of electrons into 2e
quasiparticles. Here, we use a highly tunable graphene-based QHE FP
interferometer to observe the connection between interference phase jumps and
AB frequency doubling, unveiling how strong repulsive interaction between edge
channels leads to the apparent pairing phenomena. By tuning electron density
in-situ from filling factor {\nu}<2 to {\nu}>7, we tune the interaction
strength and observe periodic interference phase jumps leading to AB frequency
doubling. Our observations demonstrate that the combination of repulsive
interaction between the spin-split {\nu}=2 edge channels and charge
quantization is sufficient to explain the frequency doubling, through a
near-perfect charge screening between the localized and extended edge channels.
Our results show that interferometers are sensitive probes of microscopic
interactions and enable future experiments studying correlated electrons in 1D
channels using our highly tunable platform.
