On the EMC effect

A simple model of deep inelastic scattering of nuclei is discussed. It is argued that at least a substantial part of the EMC effect reflects the physics on hadronic rather than quark level.Invited talk presented at Hamburg University on 4 December 1986.     

A realistic many-body approach to the EMC effect

The EMC effect in few nucleon systems and complex nuclei is calculated within a realistic many-body approach. The effect ofQ ²-rescaling is also investigated.Contributed paper to the symposium Mesons and Light Nuclei IV, Bechyn, Czechoslovakia, September 5–10, 1988.     

Origins of the EMC effect

The conventional nuclear (pion) and dynamical rescaling models of nuclear effects in deep inelastic scattering give similar results. We explain this, compare the two models, and find that dynamical rescaling mimics binding effects in the conventional model.     

Implications of the nuclear EMC effect

The discovery more than twenty years ago, by the EMC Collaboration, that the deep-inelastic-scattering DIS structure functions are influenced by the nuclear environment stunned the nuclear physics community and brought quarks and gluons into the field with great impact. A great length of time has passed, but despite a semi-infinite number of papers on the subject, there is no explanation that is universally accepted. Many models (related in one way or another to QCD) have been successful in reproducing data for deep inelastic scattering on nuclear targets, but fewer have described both the DIS and nuclear Drell-Yan experiments. Although there are some positive indications, no model has been used to predict correctly and unambiguously new independent phenomena. We review the history and discuss the best experimental prospects for future discovery.     

Partons and the EMC spin effect

We focus on the parton model and the role of the axial anomaly in polarised deep inelastic scattering. We show that the axial anomaly is relevant to each of the higher moments of the spin dependent structure functiong ₁ (x) and not just the first moment. This result implies that the factorisation of mass singularities is not sufficient to define the parton model in spin dependent QCD. (It is certainly a necessary condition.) We also need to consider the locality of the photon parton interaction. The anomaly is observed over allx in the EMCg ₁ (x) data.     

Thermodynamical analysis of the EMC effect

Assuming that the sea quark distribution vanishes for x > 0.3, we analyse the F₂^Fe(x, Q²) and F₂^D(x, Q²) structure functions measured by the European Muon Collaboration in the framework of a thermodynamical model of the valence quarks. The experimental ratio $\text{F}{}_2{}^{\mathrm{<mtext>Fe</mtext>}}\text{(x)}\text{F}{}_2{}^{\mathrm{<mtext>D</mtext>}}\text{(x)}$ is well reproduced over the whole x range by the ratio of two valence quark distributions at different temperatures T and confinement volumes V. We obtain T_D?T_Fe≈3 MeV and $\text{V}{}_{\mathrm{<mtext>Fe</mtext>}}\text{V}{}_{\mathrm{<mtext>D</mtext>}}\text{≈ 1.3}$.     

On the nature of the EMC effect

It is shown that none of the popular models suggested to explain the EMC effect seem satisfactory. A new point of view on the effect as a simple relativistic phenomenon is presented and is shown to be favoured by NA4 data.     

Single-particle Cherenkov effect in a bounded spatial region

The problem of the development of a beam instability in a bounded spatial region due to the single-particle Cherenkov effect is studied in the linear approximation. The threshold conditions for the onset of the convective and absolute instabilities of different longitudinal modes and their growth rates are determined with allowance for reflections from the boundaries of the system.     

Single-particle spin effect on fission fragment angular momentum

Independent isomeric yield ratios (IYR) of ¹²⁸Sb, ¹³⁰Sb, ¹³²Sb, ¹³¹Te, ¹³³Te, ¹³²I, ¹³⁴I, ¹³⁶I, ¹³⁵Xe, and ¹³⁸Cs have been determined in the fast neutron-induced fission of ²⁴³Am using the radiochemical and γ-ray spectrometric technique. From the IYR, fragment angular momenta (J _rms) have been deduced using the spin-dependent statistical model analysis. From the J _rms-values and experimental kinetic energy data deformation parameters (β) have been deduced using the pre-scission bending mode oscillation model and the statistical model. The J _rms- and β-values of fission fragments from the present and earlier work in the odd-Z fissioning systems ( ²³⁸Np ^* , ²⁴²Am ^* and ²⁴⁴Am ^* ) are compared with the literature data in the even-Z fissioning systems ( ^{230, 233}Th ^* , ^{233, 234, 236, 239}U ^* , ^{239, 240, 241, 242}Pu ^* , ²⁴⁴Cm(SF), ^{245, 246}Cm ^* , ²⁵⁰Cf ^* and ²⁵²Cf(SF)) to examine the role of single-particle (proton) spin effect. It was observed that i) in all the fissioning systems J _rms- and β-values of the fragments with spherical 82n shell and even-Z products are lower than the fragments away from the spherical neutron shell and odd-Z products, which indicate the effect of nuclear structure. ii) For both even-Z and odd-Z fission products J _rms-values increase with Z _F ²/A _F due to increase in Coulomb torque. iii) The J _rms- and β-values of even-Z fission products are comparable in all the fissioning systems. However, for odd-Z fission products they are slightly higher in the odd-Z fissioning systems compared to their adjacent even-Z fissioning systems. This is possible due to the contribution of the extra single-particle (proton) spin of the odd-Z fissioning systems to their odd-Z fragments. iv) The yield-weighted fragment angular momentum and elemental yields profile shows an anti-correlation in even-Z fissioning systems but not in the odd-Z fissioning systems.     

Contribution of boundness and motion of nucleons to the EMC effect

The kinematical corrections to the structure function of the nucleon in the nucleus due to the boundness and motion of nucleons arise from the excitation of the doorway states for one-nucleon transfer reactions in the deep inelastic scattering on nuclei.     

Gauge symmetry and the EMC spin effect

We emphasise the EMC spin effect as a problem of symmetry and discuss the renormalisation of theC=+1 axial tensor operators. This involves the generalisation of the Adler-Bell-Jackiw anomaly to each of these operators. We find that the contribution of the axial anomaly to the spin dependent structure functiong ₁(x, Q ²) scales at O(α_s). This means that the anomaly can be a largex effect ing ₁. Finally we discuss the jet signature of the anomaly.     

Shadowing,dipions and the EMC effect

The EMC effect is thought to be a compound phenomenon that appears when nucleons are bound in a nucleus. The shadowing effect shows up in a very low-x range. It is possible that in low- and medium-x ranges there exist weak-binding regions caused by the nucleon distortion in the nucleus as well as by exchange pions among the nucleons and other pions in weak-binding regions. Both these types of pions will be in the dipion state because of the Coulomb field. The dipion effect contributes to the structure function of the nucleus. Nuclear binding energy and Fermi motion in medium- and higher-x ranges cause a shift of the ratio of the nucleus structure function to the deuteron one. Calculations show that better agreement with recent data is obtained if the above three effects are mixed.     

Short range correlations and the EMC effect

This Letter shows quantitatively that the magnitude of the EMC effect measured in electron deep inelastic scattering at intermediate x(B), 0.35≤x(B)≤0.7, is linearly related to the short range correlation (SRC) scale factor obtained from electron inclusive scattering at x(B)≥1. The observed phenomenological relationship is used to extract the ratio of the deuteron to the free pn pair cross sections and F(2)(n)/F(2)(p), the ratio of the free neutron to free proton structure functions. We speculate that the observed correlation is because both the EMC effect and SRC are dominated by the high virtuality (high momentum) nucleons in the nucleus.     

Quark-hadron duality and the nuclear EMC effect

Recent data on polarized proton knockout reactions off ⁴He nuclei suggest a small but nonzero modification of proton electromagnetic form factors in medium. Using model-independent relations derived on the basis of quark-hadron duality, we relate the medium modification of the form factors to the modification at large x of the deep-inelastic structure function of a bound proton. This places strong constraints on models of the nuclear EMC effect which assume a large deformation of the intrinsic structure of the nucleon in medium.     

The contribution of reflections to the Lorentz-Lorenz effect

We use the static limit and delta isobar dominance to calculate the contribution to the optical potential of pionic atoms of those terms in which a pion or a rho meson are exchanged any number of times between two nucleons, which constitute the ρ² term of the Lorentz-Lorenz effect. By suppressing the spin and isospin degrees of freedom, we are able to express the Lorentz-Lorenz parameter ξ in analytical form. In the general case when we include fully the spin and isospin variables, we obtain the Lorentz-Lorenz parameter ξ in numerical form. Our results show that ξ can not be larger than one and more likely is close to zero.     

Intrinsic laser linewidth contribution to the Weisskopf-Heitler effect

The existence of an additional peak in the power spectrum of the fluorescence light of a weakly driven two-level atom is predicted if a realistic density operator of the laser system is used in the calculations.     

Studies of the EMC effect in the QCD framework with the EMC iron and hydrogen structure functions

The QCD evolution of the measured structure functions F₂ in iron and hydrogen have been analysed to show that the momentum distribution of the valence quarks is lower in iron than in hydrogen and that the opposite is true for the gluon and sea quark distributions in the region of 0.1<x<0.65.     

Carrier mobility contribution to the piezoresistive effect in SmS

A study is reported of the contribution to the piezoresistive effect in samarium-monosulfide-based materials by pressure-induced variation of carrier mobility. As follows from calculations and experimental data, the piezoresistance coefficient for hydrostatic pressure cannot exceed 7×10⁻³ MPa⁻¹ at T=300 K. Fiz. Tverd. Tela (St. Petersburg) 41, 1963–1964 (November 1999)