EMC effect

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The EMC effect is the surprising observation that the cross section for deep inelastic scattering from an atomic nucleus is different from that of the same number of free protons and neutrons (collectively referred to as nucleons). From this observation, it can be inferred that the quark momentum distributions in nucleons bound inside nuclei are different from those of free nucleons. This effect was first observed in 1983 at CERN by the European Muon Collaboration, [1] hence the name "EMC effect". It was unexpected, since the average binding energy of protons and neutrons inside nuclei is insignificant when compared to the energy transferred in deep inelastic scattering reactions that probe quark distributions. While over 1000 scientific papers have been written on the topic and numerous hypotheses have been proposed, no definitive explanation for the cause of the effect has been confirmed. [2] Determining the origin of the EMC effect is one of the major unsolved problems in the field of nuclear physics.

Contents

Background

Protons and neutrons, collectively referred to as nucleons, are the constituents of atomic nuclei, and nuclear matter such as that in neutron stars. Protons and neutrons themselves are composite particles made up of quarks and gluons, a discovery made at SLAC in the late 1960s using deep inelastic scattering (DIS) experiments (1990 Nobel Prize).

In the DIS reaction, a probe (typically an accelerated electron) scatters from an individual quark inside a nucleon. By measuring the cross section of the DIS process, the distribution of quarks inside the nucleon can be determined. These distributions are effectively functions of a single variable, known as Bjorken-x, which is a measure of the fraction of the momentum of the quark stricken by the electron.

Experiments using DIS from protons by electrons and other probes have allowed physicists to measure the proton's quark distribution over a wide range of Bjorken-x, i.e. the probability of finding a quark with momentum fraction x in the proton. Experiments using deuterium and helium-3 targets have similarly allowed physicists to determine the quark distribution of the neutron.

Experimental history

Fig 1. The original figure from the paper by the EMC Collaboration. In the absence of the EMC effect, the data would not have a falling slope as a function of Bjorken-x. In more recent experiments, the ratio was below 1 for x [?] 0.08 EMC Effect.png
Fig 1. The original figure from the paper by the EMC Collaboration. In the absence of the EMC effect, the data would not have a falling slope as a function of Bjorken-x. In more recent experiments, the ratio was below 1 for x ≲ 0.08
Fig 2: Another figure from the original EMC paper, showing predictions for the scaled DIS cross section ratio based on Fermi effects. These predictions do not match the experimental data. EMC Effect predictions.png
Fig 2: Another figure from the original EMC paper, showing predictions for the scaled DIS cross section ratio based on Fermi effects. These predictions do not match the experimental data.

In 1983, the European Muon Collaboration published results from an experiment conducted at CERN in which the DIS reaction was measured for high-energy muon scattering from iron and deuterium targets. It was expected that the cross section for DIS from iron divided by that from deuterium, and scaled by a factor of 28 (the iron-56 nucleus has 28 times more nucleons than deuterium) would be approximately 1. Instead, the data (Fig. 1) showed a decreasing slope in the region of 0.3 < x < 0.7 , reaching a minimum of 0.85 at the largest values of x .

This decreasing slope is a hallmark of the EMC effect. The slope of this cross section ratio between 0.3 < x < 0.7 is often referred to as the "size of the EMC effect" for a given nucleus.

Since that landmark discovery, the EMC effect has been measured over a wide range of nuclei, at several different laboratories, and with multiple different probes. Notable examples include:

Possible explanations

The EMC effect is surprising because of the difference in energy scales between nuclear binding and deep inelastic scattering. Typical binding energies for nucleons in nuclei are on the order of 10 megaelectron volts (MeV). Typical energy transfers in DIS are on the order of several gigaelectron volts (GeV). Nuclear binding effects were therefore believed to be insignificant when measuring quark distributions.

A number of hypotheses for the cause of the EMC effect have been offered. While many older hypotheses, such as Fermi motion (see Fig. 2), nuclear pions, and others have been ruled out by electron scattering or Drell–Yan data, modern hypotheses generally fall into two viable categories: mean-field modification, and short-range correlated pairs. [7] [8]

Mean-field modification

The mean-field modification hypothesis suggests that the nuclear environment leads to a modification of nucleon structure. As an illustration, consider that the average density inside a nuclear matter is approximately 0.16 nucleons per fm 3. If nuclei were hard spheres, their radius would be approximately 1.1 fm, leading to a density of only 0.13 nucleons per fm3, assuming ideal close-packing.

Nuclear matter is dense, and the close proximity of nucleons may allow quarks in different nucleons to interact directly, leading to nucleon modification. Mean-field models predict that all nucleons experience some degree of structure modification, and they are consistent with the observation that the EMC effect increases with nuclear size, scales with local density, and saturates for very large nuclei. Furthermore, mean-field models also predict a large "polarized EMC effect": a large modification of the spin-dependent g1 structure function for nuclei relative to that of their constituent protons and neutrons. [9] This prediction will be tested experimentally using measurements of a polarized Li-7 target as part of the Jefferson Lab CLAS-12 program.[ citation needed ]

Short-range correlations (SRC)

Rather than all nucleons experiencing some modification, the short-range correlations hypothesis predicts that most nucleons at any one time are unmodified, but some are substantially modified. The most heavily modified nucleons are those in temporary short-range correlated (SRC) pairs. It has been observed that approximately 20% of nucleons (in medium and heavy nuclei) at any given moment are part of short-lived pairs with significant spatial overlap with a partner nucleon.

The nucleons in these pairs then recoil apart with large back-to-back momenta of several hundred MeV/c – larger than the nuclear Fermi momentum – making them the highest-momentum nucleons in the nucleus. In the short-range correlations (SRC) hypothesis, the EMC effect emerges from large modification of these high-momentum SRC nucleons.

This explanation is supported by the observation that the size of the EMC effect in different nuclei correlates linearly with the density of SRC pairs. [10] [11] This hypothesis predicts increasing modification as a function of nucleon momentum, which was tested using recoil-tagging techniques in experiments at Jefferson Lab. The results showed definitive evidence in favor of SRC. [4]

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A muon is an elementary particle similar to the electron, with an electric charge of −1 e and a spin of 1/2, but with a much greater mass. It is classified as a lepton. As with other leptons, the muon is not thought to be composed of any simpler particles; that is, it is a fundamental particle.

<span class="mw-page-title-main">Neutron</span> Subatomic particle with no charge

The neutron is a subatomic particle, symbol
n
 or
n0
, which has a neutral charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave similarly within the nucleus, they are both referred to as nucleons. Nucleons have a mass of approximately one atomic mass unit, or dalton, symbol Da. Their properties and interactions are described by nuclear physics. Protons and neutrons are not elementary particles; each is composed of three quarks.

<span class="mw-page-title-main">Proton</span> Subatomic particle with positive charge

A proton is a stable subatomic particle, symbol
p
, H+, or 1H+ with a positive electric charge of +1 e (elementary charge). Its mass is slightly less than the mass of a neutron and 1,836 times the mass of an electron (the proton-to-electron mass ratio). Protons and neutrons, each with masses of approximately one atomic mass unit, are jointly referred to as "nucleons" (particles present in atomic nuclei).

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The nuclear force is a force that acts between hadrons, most commonly observed between protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nuclear force almost identically. Since protons have charge +1 e, they experience an electric force that tends to push them apart, but at short range the attractive nuclear force is strong enough to overcome the electrostatic force. The nuclear force binds nucleons into atomic nuclei.

<span class="mw-page-title-main">Deep inelastic scattering</span> Type of collision between subatomic particles

In particle physics, deep inelastic scattering is the name given to a process used to probe the insides of hadrons, using electrons, muons and neutrinos. It was first attempted in the 1960s and 1970s and provided the first convincing evidence of the reality of quarks, which up until that point had been considered by many to be a purely mathematical phenomenon. It is an extension of Rutherford scattering to much higher energies of the scattering particle and thus to much finer resolution of the components of the nuclei.

<span class="mw-page-title-main">Drell–Yan process</span> Process in high-energy hadron–hadron scattering

The Drell–Yan process occurs in high energy hadron–hadron scattering. It takes place when a quark of one hadron and an antiquark of another hadron annihilate, creating a virtual photon or Z boson which then decays into a pair of oppositely-charged leptons. Importantly, the energy of the colliding quark-antiquark pair can be almost entirely transformed into the mass of new particles. This process was first suggested by Sidney Drell and Tung-Mow Yan in 1970 to describe the production of lepton–antilepton pairs in high-energy hadron collisions. Experimentally, this process was first observed by J. H. Christenson et al. in proton–uranium collisions at the Alternating Gradient Synchrotron.

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References

  1. 1 2 3 J.J. Aubert; et al. (1983). "The ratio of the nucleon structure functions F2N for iron and deuterium". Phys. Lett. B. 123B (3–4): 275–278. Bibcode:1983PhLB..123..275A. doi:10.1016/0370-2693(83)90437-9.
  2. ↑ D. Higinbotham, G.A Miller, O. Hen, and K. Rith, CERN Courier, April, 26 2013
  3. ↑ Gomez, J.; Arnold, R.G.; Bosted, P.E.; Chang, C.C.; Katramatou, A.T.; Petratos, G.G.; et al. (1994-05-01). "Measurement of the A dependence of deep-inelastic electron scattering". Physical Review D. 49 (9): 4348–4372. Bibcode:1994PhRvD..49.4348G. doi:10.1103/PhysRevD.49.4348. PMID   10017440.
  4. 1 2 B. Schmookler; et al. (CLAS Collaboration) (2019-02-19). "Modified structure of protons and neutrons in correlated pairs" (PDF). Nature. 566 (7744): 354–358. arXiv: 2004.12065 . Bibcode:2019Natur.566..354C. doi:10.1038/s41586-019-0925-9. PMID   30787453. S2CID   67772892.
  5. ↑ Segarra, E. P.; Schmidt, A.; Kutz, T.; Higinbotham, D. W.; Piasetzky, E.; Strikman, M.; Weinstein, L. B.; Hen, O. (2020-03-06). "Neutron Valence Structure from Nuclear Deep Inelastic Scattering". Physical Review Letters. 124 (9): 092002. arXiv: 1908.02223 . doi: 10.1103/PhysRevLett.124.092002 .
  6. ↑ Seely, J.; Daniel, A.; Gaskell, D.; Arrington, J.; Fomin, N.; Solvignon, P.; et al. (2009-11-13). "New Measurements of the European Muon Collaboration Effect in Very Light Nuclei". Physical Review Letters. 103 (20): 202301. arXiv: 0904.4448 . Bibcode:2009PhRvL.103t2301S. doi:10.1103/PhysRevLett.103.202301. PMID   20365978. S2CID   119305632.
  7. ↑ Hen, Or; Miller, Gerald A.; Piasetzky, Eli; Weinstein, Lawrence B. (2017-11-13). "Nucleon-nucleon correlations, short-lived excitations, and the quarks within". Reviews of Modern Physics. 89 (4): 045002. arXiv: 1611.09748 . Bibcode:2017RvMP...89d5002H. doi:10.1103/RevModPhys.89.045002. S2CID   53706086.
  8. ↑ Norton, P. R. (2003). "The EMC effect". Reports on Progress in Physics. 66 (8): 1253–1297. Bibcode:2003RPPh...66.1253N. doi:10.1088/0034-4885/66/8/201. ISSN   0034-4885. S2CID   250899760.
  9. ↑ Cloët, I.C.; Bentz, W.; Thomas, A.W. (2006). "EMC and polarized EMC effects in nuclei". Physics Letters B. 642 (3): 210–217. arXiv: nucl-th/0605061 . Bibcode:2006PhLB..642..210C. doi:10.1016/j.physletb.2006.08.076. S2CID   119517750.
  10. ↑ Weinstein, L.B.; Piasetzky, E.; Higinbotham, D.W.; Gomez, J.; Hen, O.; Shneor, R. (2011-02-04). "Short Range Correlations and the EMC Effect". Physical Review Letters. 106 (5): 052301. arXiv: 1009.5666 . Bibcode:2011PhRvL.106e2301W. doi:10.1103/PhysRevLett.106.052301. PMID   21405385. S2CID   26201601.
  11. ↑ Hen, O.; Piasetzky, E.; Weinstein, L.B. (2012-04-26). "New data strengthen the connection between short range correlations and the EMC effect". Physical Review C. 85 (4): 047301. arXiv: 1202.3452 . Bibcode:2012PhRvC..85d7301H. doi:10.1103/PhysRevC.85.047301. S2CID   119249929.
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