The problem
Fast electron or muon scattering reveals that the quark momentum pattern inside a nucleus differs from the pattern expected by combining separate protons and neutrons. Why does binding these particles change their internal quark structure? This is the EMC effect. One explanation emphasizes the nuclear environment felt by many protons and neutrons; another emphasizes pairs that briefly approach each other closely.[1] [2]
In energetic electron or muon scattering, a nucleus’s quark-sensitive response differs from the response expected by adding independent protons and neutrons. What microscopic effects of nuclear binding cause this EMC modification, and why does its size depend on the nucleus?
Protons and neutrons are collectively called nucleons. The measured scattering rates, normalized to the beam and target, give cross sections. Quark distributions are inferred from them with a description of nucleon motion, nuclear binding and the scattering process. A mean-field mechanism changes nucleon structure through the average surrounding nuclear environment. A short-range-correlation mechanism emphasizes closely spaced pairs with large relative momenta. Their ability to describe similar data leaves a causal distinction to test with additional observables.
For example, helium-3 contains two protons and one neutron, while tritium contains one proton and two neutrons. Comparing their measured scattering ratios constrains how proton and neutron contributions differ. The revised 2026 global analysis uses these and other data in a specified nuclear framework. Its off-shell corrections describe modifications associated with a bound nucleon not satisfying the free-particle energy–momentum relation. The fitted corrections are inferred functions within that framework; they are not direct observations identifying one microscopic cause.[1] [2]
Scope
The microscopic origin and nuclear dependence of the modification of bound-nucleon quark distributions.
What remains open
Why it matters
A mechanism should connect the measured nuclear response to interactions among nucleons and their internal quarks. It can then make distinguishable predictions, for example for spin-dependent responses or nuclei with different proton–neutron balance. Those comparisons can test microscopic accounts that agree on existing unpolarized data; fitting one dataset alone cannot provide that test.[1]
Definitions
4 terms
Nucleon
A proton or neutron, the particles bound together in an atomic nucleus. Scattering probes their internal quark structure as well as their nuclear environment.[1] [2]
Mean field
An average nuclear environment felt by a nucleon. A mean-field explanation of the EMC effect links that environment to changes in the nucleon’s internal quark structure; it is one proposed microscopic account.[1]
Short-range correlation
Closely spaced nucleons with large relative momenta. Correlation-based EMC accounts emphasize these pairs; fitting existing data does not by itself measure their separate causal contribution.[1] [2]
Off-shell correction
In the cited nuclear fit, a correction associated with a bound nucleon’s four-momentum squared differing from its free mass squared. The inferred correction depends on the chosen framework and is not a direct observation of one microscopic mechanism.[2]
Sources
- Reflections on the Origin of the EMC Effect
Notes on Reflections on the Origin of the EMC Effect
Selected primary-author comparison of mean-field and short-range-correlation accounts and discriminating spin/isovector observables. The author explicitly favors one account; that preference is not treated as consensus or a settled cause.
- Isospin dependence of nuclear EMC effect from global QCD analysis
Notes on Isospin dependence of nuclear EMC effect from global QCD analysis
Primary preprint revised July 27, 2026 fits light-nucleus scattering with framework-dependent off-shell corrections. Selected v2 introduction/fit/results read. Served HTML has internal August 24 date; landing version history supplies the revision date, with no whole-version byte identity claimed.