EMC effect

Last updated

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 nucleon's momentum carried by the struck quark (within the Breit frame).

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]

Related Research Articles

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

The neutron is a subatomic particle, symbol
n
 or
n0
, that has no electric 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. 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 approximately 1836 times the mass of an electron (the proton-to-electron mass ratio). Protons and neutrons, each with a mass of approximately one atomic mass unit, are jointly referred to as nucleons (particles present in atomic nuclei).

The European Muon Collaboration (EMC) was formed in 1973 to study the interactions of high energy muons at CERN. These experiments were motivated by the interest in determining the quark structure of the nucleon following the discovery of high levels of deep inelastic scattering at SLAC.

<span class="mw-page-title-main">Nuclear force</span> Force that acts between the protons and neutrons of atoms

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.

This is a timeline of subatomic particle discoveries, including all particles thus far discovered which appear to be elementary given the best available evidence. It also includes the discovery of composite particles and antiparticles that were of particular historical importance.

Helium (2He) has nine known isotopes, but only helium-3 (3He) and helium-4 (4He) are stable. All radioisotopes are short-lived; the longest-lived is 6He with half-life 806.92(24) milliseconds. The least stable is 10He, with half-life 260(40) yoctoseconds, though 2He may have an even shorter half-life.

<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.

<span class="mw-page-title-main">MINERνA</span> Neutrino scattering experiment at Fermilab in Illinois, USA

Main Injector Experiment for ν-A, or MINERνA, is a neutrino scattering experiment which uses the NuMI beamline at Fermilab. MINERνA seeks to measure low energy neutrino interactions both in support of neutrino oscillation experiments and also to study the strong dynamics of the nucleon and nucleus that affect these interactions.

<span class="mw-page-title-main">Quark–gluon plasma</span> Phase of quantum chromodynamics (QCD)

Quark–gluon plasma is an interacting localized assembly of quarks and gluons at thermal and chemical (abundance) equilibrium. The word plasma signals that free color charges are allowed. In a 1987 summary, Léon Van Hove pointed out the equivalence of the three terms: quark gluon plasma, quark matter and a new state of matter. Since the temperature is above the Hagedorn temperature—and thus above the scale of light u,d-quark mass—the pressure exhibits the relativistic Stefan-Boltzmann format governed by temperature to the fourth power and many practically massless quark and gluon constituents. It can be said that QGP emerges to be the new phase of strongly interacting matter which manifests its physical properties in terms of nearly free dynamics of practically massless gluons and quarks. Both quarks and gluons must be present in conditions near chemical (yield) equilibrium with their colour charge open for a new state of matter to be referred to as QGP.

<span class="mw-page-title-main">Fermilab E-906/SeaQuest</span> Particle physics experiment

Fermilab E-906/SeaQuest is a particle physics experiment which will use Drell–Yan process to measure the contributions of antiquarks to the structure of the proton or neutron and how this structure is modified when the proton or neutron is included within an atomic nucleus.

The rms charge radius is a measure of the size of an atomic nucleus, particularly the proton distribution. The proton radius is about one femtometre = 10−15 metre. It can be measured by the scattering of electrons by the nucleus. Relative changes in the mean squared nuclear charge distribution can be precisely measured with atomic spectroscopy.

The proton spin crisis is a theoretical crisis precipitated by a 1987 experiment by the European Muon Collaboration (EMC), which tried to determine the distribution of spin within the proton.

<span class="mw-page-title-main">Light-front quantization applications</span> Quantization procedure in quantum field theory

The light-front quantization of quantum field theories provides a useful alternative to ordinary equal-time quantization. In particular, it can lead to a relativistic description of bound systems in terms of quantum-mechanical wave functions. The quantization is based on the choice of light-front coordinates, where plays the role of time and the corresponding spatial coordinate is . Here, is the ordinary time, is a Cartesian coordinate, and is the speed of light. The other two Cartesian coordinates, and , are untouched and often called transverse or perpendicular, denoted by symbols of the type . The choice of the frame of reference where the time and -axis are defined can be left unspecified in an exactly soluble relativistic theory, but in practical calculations some choices may be more suitable than others. The basic formalism is discussed elsewhere.

In quantum field theory, a sum rule is a relation between a static quantity and an integral over a dynamical quantity. Therefore, they have a form such as:

<span class="mw-page-title-main">Volker Burkert</span> German-American physicist

Volker D. Burkert is a German physicist, academic and researcher. He is a Principal Staff Scientist at the Thomas Jefferson National Accelerator Facility at Jefferson Lab (JLab) in Newport News, Virginia (USA). He has made major contributions to the design of the CEBAF Large Acceptance Spectrometer (CLAS) that made it suitable for high luminosity operation in experiments studying spin-polarized electron scattering.

Joel Marshall Moss is an American experimental nuclear physicist.

The nucleon magnetic moments are the intrinsic magnetic dipole moments of the proton and neutron, symbols μp and μn. The nucleus of an atom comprises protons and neutrons, both nucleons that behave as small magnets. Their magnetic strengths are measured by their magnetic moments. The nucleons interact with normal matter through either the nuclear force or their magnetic moments, with the charged proton also interacting by the Coulomb force.

<span class="mw-page-title-main">Shape of the atomic nucleus</span>

The shape of the atomic nucleus depends on the variety of factors related to the size and shape of its nucleon constituents and the nuclear force holding them together. The origins of nuclear shape begin with the spacial extent of almost nearly all stable and a great many unstable nuclei has been determined mainly by electron and muon scattering experiments as well as spectroscopic experiments. An important factor in the internal structure of the nucleus is the nucleon-nucleon potential, which ultimately governs the distance between individual nucleons, while a dip in the charge density of some light nuclide structures a lesser density of nucleonic matter. A surprising non-spherical expectation for the shape of the nucleus originated in 1939 in the spectroscopic analysis of the quadrupole moments while the prolate spheroid shape of the nucleon arises from analysis of the intrinsic quadruple moment. The simple spherical approximation of nuclear size and shape provides at best a textbook introduction to nuclear size and shape. The unusual cosmic abundance of alpha nuclides has inspired geometric arrangements of alpha particles as a solution to nuclear shapes, although the atomic nucleus generally assumes a prolate spheroid shape. Nuclides can also be discus-shaped, triaxial or pear-shaped.

Color transparency is a phenomenon observed in high-energy particle physics, where hadrons created in a nucleus propagate through that nucleus with less interaction than expected. It suggests that hadrons are first created with a small size in the nucleus, and then grow to their nominal size. Here, color refers to the color charge, the property of quarks and gluons that determines how strongly they interact through the nuclear strong force.

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 . Bibcode:2020PhRvL.124i2002S. doi: 10.1103/PhysRevLett.124.092002 . PMID   32202871.
  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.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.