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Whether all nucleons are slightly modified or only those in close pairs are strongly modified is unknown.","posed_since":"1983","precise":"The per-nucleon ratio $F2^{A}(x)/F2^{d}(x)$ of deep-inelastic structure functions falls below 1 for $0.3 < x < 0.7$, with slope correlated with $a_{2}$, the number of short-range correlated (SRC) pairs per nucleon. Determine the mechanism: mean-field modification of all nucleons, strong modification of nucleons in high-virtuality SRC pairs, or another effect. Answer: a model reproducing the A- and x-dependence that is also consistent with tagged and flavor-separated measurements.","problem_ref":null,"references":"","settled_by":"Spectator-tagged deep-inelastic scattering on deuterium at several recoil momenta, combined with flavor-separated EMC measurements.","status_note":"The linear EMC-SRC correlation (CLAS, Nature 2019) supports the SRC picture but did not exclude mean-field models as of 2026.","title":"Physical origin of the EMC 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