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Title: Physical origin of the EMC effect
Plain: Per nucleon, a heavy nucleus has fewer quarks carrying 30 to 70 percent of a nucleon's momentum than deuterium has. Whether all nucleons are slightly modified or only those in close pairs are strongly modified is unknown.
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.
Settled by: Spectator-tagged deep-inelastic scattering on deuterium at several recoil momenta, combined with flavor-separated EMC measurements.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: The linear EMC-SRC correlation (CLAS, Nature 2019) supports the SRC picture but did not exclude mean-field models as of 2026.
Posed since: 1983