Neutron Charge Research Articles

Experimental study on the electric polarizability of the neutron

Neutron transmission cross sections of lead and bismuth were measured for 143 keV and with high precision for 1970 eV neutrons which were selected from the reactor neutron spectrum by means of a novel dual combination of resonance scatterers. These data and highly accurate values of the coherent scattering lengths at zero energy and of transmission cross sections at 1.26, 5.19, 18.8 and 132 eV were the basis to determine the potential scattering radii, the neutron electron and the electric polarization scattering lengths as well as to test the used resonance corrections by which the influence of nuclear reactions on the results is eliminated. We obtained for the electric polarizability of the neutron:α n=(0.8±1.0) 10−3 fm3 and for the neutron electron scattering length:b ne=(−1.32±0.04) 10−3 fm. In conclusion we have shown that these results favour the cloudy bag model for the description of the neutron's charge structure.

Effects of d-state quarks on the nucleon electric form factors.

Considering the d orbital excitation of a quark in the bag, we calculate the nucleon electric form factors in the cloudy bag model. In these calculations, we have taken into account the ..pi..NN, ..pi delta..N, and ..pi gamma.. form factors though neglecting the c.m. correction. It turns out that the neutron charge form factor is very sensitive to the d-state quark admixture in the overall region of the momentum transfer but the proton charge form factor remains unchanged. Taking the d-state quark admixture in the intermediate-state baryons, we can obtain the nucleon rms radii in remarkable agreement with the experimental values. We also investigate the roles of ..delta.. particles in the nucleon charge form factors.

The generalized hedgehog and the projected chiral soliton model

The linear chiral soliton model with quark fields and elementary pion and sigma fields is solved in order to describe static properties of the nucleon and the delta resonance. To this end a Fock state of the system is constructed which consists of three valence quarks in a 1s orbit with a generalized hedgehog spin-flavour configuration cos η¦u↓〉 − sin η¦d↑〉 . Coherent states are used to provide a quantum description for the mesonic parts of the total wave function. The corresponding classical pion field also exhibits a generalized hedgehog structure. In a pure mean field approximation the variation of the total energy results in the ordinary hedgehog form ( η = 45°). In a quantized approach, however, the generalized hedgehog baryon is projected onto states with good spin and isospin and then noticeable deviations from the simple hedgehog form occur (η ≅ 20°), if the relevant degrees of freedom of the wave functions are varied after the projection. Various nucleon properties are calculated. These include proton and neutron charge radii, and the magnetic moment of the proton for which good agreement with experiment is obtained. The absolute value of the neutron magnetic moment comes out too large, similarly as the axial vector coupling constant and the pion-nucleon-nucleon coupling constant. However, due to the generalization of the hedgehog, the Goldberger-Treiman relation and a corresponding virial theorem are fulfilled. Variation of the quark-meson coupling parameter g and the sigma mass m σ shows that the g A is always about 40% too large compared to experiment. The concepts and results of the projections are compared with the semiclassical collective quantization method. It is demonstrated that noticeable deviations occur for the delta-nucleon splitting, the isovector squared charge radius and the axial vector coupling constant.

A neutron optical imaging system for the search for a neutron charge

The authors describe the design and performance of an optical instrument for neutrons of wavelengths larger than 1.2 nm, containing cylindrical neutron lenses and multislits. The achromatic optics resemble a slide projector with a magnification of one, and will be used in a neutron beam deflection experiment aimed at reducing the upper limit of a possible electric charge of the free neutron.

Time resolved neutron yield measurements on FT device

A system of four BF 3 proportional chambers for time resolved neutron yield measurements is operative on the FT (Frascati Torus) device. The determination of plasma parameters from neutron yield measurements is a relatively new field of investigation subjected to continuous improvements, also in view of feasible application to the control of future fusion reactors. The absolute calibration of neutron detectors in the presence of extended neutron sources and large scattering masses, such as tokamak devices, presents the major problem. Techniques employed for BF 3 chamber calibration are presented. In thermonuclear plasmas the neutron yield is related to ion temperature through the density. On FT device, neutron yield measurements and charge exchange analysis are the only routine methods of monitoring ion temperature. The method and results for this measurement are presented. Neutron yield measurements in fusion devices give quantative and qualitative informations about ion velocity and nonthermonuclear neutron sources. The results of neutron yield measurements in different working regimes of the FT device are presented.

Cranking in hedgehog models with vector mesons

A cranking calculation is performed in a massive SU(2) × SU(2) × U(1) model with valence quarks and the σ, π, ϱ, A and ω mesons. The nucleon moment of inertia, the N-Δ mass splitting and the proton and neutron charge distributions are obtained. The general framework and the specific ansatz for the cranked fields can be used in any hedgehog model with vector mesons. A possible role of η and δ mesons is also discussed.

Chiral model of the nucleon: Projecting the hedgehog as a coherent state.

The construction of baryon states with good spin and isospin is studied in a chiral model. This is a linear \ensuremath{\sigma} model, describing quarks interacting with pions and \ensuremath{\sigma} mesons. It has previously been investigated in the mean-field approximation (MFA), using the hedgehog ansatz. The ansatz corresponds to a mixture of spin-isospin eigenstates. A coherent state is used to provide a quantum description for the mesonic part of the wave function. The hedgehog baryon is projected onto states with good spin and isospin. The Peierls-Yoccoz projection is used, in analogy with the treatment of deformed nuclei. As well as N and \ensuremath{\Delta}, a J=T=(5/2) baryon state is obtained. The wave functions are varied after projection. For the N and \ensuremath{\Delta}, the resulting field distributions are fairly similar to those from the MFA. Various nucleon properties are calculated. These include proton and neutron charge radii, for which reasonable agreement with experiment is obtained. Results for other properties are similar to those obtained with an approximate projection method, and indicate the need to extend the present approach to include vector mesons and the effects of vacuum polarization. Comparisons are made with wave functions used in the cloudy bag and Skyrme models.

Color hyperfine versus pion dynamics in the N(939)- Δ(1232) baryons

The E2 M1 ratio of electromagnetic N → Δ (1232) transitions, the neutron charge form factor and nucleon magnetic moments are calculated in a harmonic-oscillator basis of mixed SU(6) symmetric three-quark states, which diagonalize the color hyperfine and pion exchange interactions. Pionic effects are found to be small (i.e. 10–15%) corrections provided the zero-range part of the one-pion-exchange potential is omitted, which we advocate.

Determination of the neutron and proton effective charges in the quadrupole operator of nuclear collective models.

We discuss the reliability of the determination of the neutron and proton effective charges in nuclear collective models from experimental effective neutron and proton matrix elements for the ${0}_{1}^{+}$ to ${2}_{1}^{+}$ transition

Neutron and proton polarisation charges from E2 mirror transitions

Abstract A review of sd-shell data relevant to a determination of the E2 effective charge is presented. A best fit is obtained assuming a linear dependence of the isoscalar effective charge on mass number, consistent with the predictions of the schematic model. Theory and experiment also agree in yielding a ratio of neutron to proton polarisation charges close to two.

Study of the mixed symmetry J pi =2(+) states in 156Gd with inelastic electron scattering.

The study of electric quadrupole transitions from the ground state into symmetric states and into states of mixed symmetry in the deformed nucleus $^{156}\mathrm{Gd}$ by inelastic electron scattering has been used to derive effective neutron and proton boson charges for the $E2$ operator in the framework of the interacting-boson model. The values for ${e}_{\ensuremath{\pi}}$ and ${e}_{\ensuremath{\nu}}$ differ at most by 30% in the rotational limit.

Slow and fast quarks in nucleons

Using light-front dynamics, and the notion of the running effective quark mass, we propose a model of the three-quark nucleon wave function, which accounts simultaneously both for the low and the high energy experimental data. Our wave function is a product of a spinor part, inferred from the QCD sum rules, and a gaussian scalar factor, taken for simplicity. Including all three Ioffe currents we get from one wave function both the negative neutron charge radius 〈 r 2〉 neutron=-0.108 fm 2 and the decreasing d/u ratio in the proton for x Bj increasing from 0.5 to 1.

Moiré effects in neutron interferometry and their use in the experimental search for an electric charge of the neutron

The operation of neutron interferometers is analyzed with particular emphasis on the Moiré effects present. The high sensitivity of neutron interferometers to deflection of the incident and propagating beams is shown to make these instruments a promising tool in the experimental search for an electric charge on the neutron. The corresponding estimates for interferometers of different types are given. It is shown that by using diffraction grating interferometers for very cold neutrons, the present-day upper limit for the neutron charge can be lowered by several orders of magnitude.

Nucleon charge form factors and chiral quark models

Calculations of the proton and neutron charge form factors G E p,n( q 2) are presented, based on chiral bag as well as confining Dirac potential models with chiral pion-quark coupling. Special emphasis is placed on a detailed treatment of the charged pion cloud contribution to the nucleon current. The role of a finite extension of the pion-quark vertex in truncating the summation over intermediate quark bag states is studied. Quark core radii (including recoil corrections) are constrained by a simultaneous calculation of the nucleon axial form factor. The proton charge form factor is well reproduced for |q 2| 1 2 ≲ 0.7 GeV with quark core rms radii between 0.5–0.6 fm. About 1 3 of the proton charge is carried by the pion cloud in this model. The neutron charge form factor is obtained with the correct sign and overall q 2 dependence but needs further refinements, probably at the level of the isoscalar form factor.

AN IMPROVED METHOD TO SEARCH FOR THE NEUTRON CHARGE

We describe improvements on the optical bench to measure a neutron charge. These will lower the present limit by more than one order of magnitude.

The Paris potential in the deuteron problem: Elastic ed scattering and the slope of the neutron charge form factor

The neutron charge form factor G rmEn ( Q 2) for low values of Q 2⪅1 fm −2 and its slope G′ En(0) are calculated from the deuteron charge structure function for the Paris potential and the results of elastic ed scattering experiments. It is for the first time that the value of the slope G′ En(0) = (0.0171 ± 0.003) fm 2, calculated in totally nonrelativistic fashion agrees with experiment on thermal neutron scattering off atomic electronic shells.

Diquarks in nucleons

Under the assumption that a scalar diquark coexists in part of the time with a single quark within a nucleon, which is favored to describe the higher twist effects of both γ and ν induced nucleon structure functions consistently, we consider how its presence affects nucleon properties. It is shown that the sum rule and the magnetic moment constrain its contribution strongly and the asymmetry is quite sensitive to it. In particular, theR=σ L /σ T ratio can be explained in terms of the large transverse momentum of the diquark which arises from the consideration on the neutron charge radius value.

A relativistic potential model

The neutron charge radius is calculated in a relativistic potential model. One finds a numerical value in better agreement with the measured result than in the conventional MIT bag model. In a refined version of the model we show the result to be highly dependent on the quark mass. We also comment on the electromagnetic decay Δ+→pγ.

Electromagnetic radii and quark structure of the nucleon

Electromagnetic radii and the empirical dipole radius of the nucleon are considered in conjunction with probability densities of constituent quarks. Using as input the neutron charge radius and the dipole radius, constraints on the remaining radii lead to deviations of about 15% from values taken from literature. Some modifications of current values are proposed especially concerning proton radii. Implications of radial quark structure of the nucleon are indicated.

Nucleon RMS radii from chiral quark model

We can reproduce both the proton and neutron charge rms radii by calculating all relevant parameters including order ( ƒ πR) −2 terms of the chiral quark bag model. The pion decay constant ƒ π is determined by the Goldberger-Treiman relation. The neutron charge radius is almost independent of the value of bag radius R.