Proton Asymmetry Research Articles

Laser-phase directional control of photofragments in dissociative ionization ofH2+using two-color intense laser pulses

Exact non-Born-Oppenheimer numerical solutions of the time-dependent Schr\odinger equation for the one-dimensional ${\mathrm{H}}_{2}^{+}$ molecule in an intense, two-color $(\ensuremath{\omega}+2\ensuremath{\omega})$ laser field, with relative phase $\ensuremath{\varphi},$ have been obtained. Both electron and proton kinetic energy spectra show spatial, correlated, asymmetric distributions. The calculated spectra exhibit the same unusual correlations as seen in experiments, in which both positively charged nuclear fragments and negatively charged photoelectrons are preferentially emitted in the same direction. It is found that, for the most asymmetric combined electric field $(\ensuremath{\varphi}=0),$ the electron is ionized in a ``counterintuitive'' direction: i.e., more electrons follow the direction of the maximum electric field. This unexpected behavior of electrons is not specific to molecules: we show that the same effect occurs in atoms. The above asymmetries of photoemission of electrons are interpreted in the framework of a quasistatic tunneling model and it is shown that for $\ensuremath{\varphi}=0$ the electron asymmetry is induced by the Coulomb attraction from the parent ion. Proton asymmetries found in the dissociation $\mathrm{H}+p$ channel depend strongly on the initialization: proton asymmetry for low vibrational state initialization is opposite that for high vibrational states. In the latter case, for $\ensuremath{\varphi}=0,$ protons follow the direction of the maximum field. Coulomb explosion spectra are shown to arise from an enhanced ionization mechanism, which is phase dependent also.

Asymmetric weak decay of polarized hypernuclei

We measured the asymmetric weak decay of the polarized Λ 5He hypernuclei. The polarization was derived by the observed asymmetry of the mesonic decay pions for the first time. The asymmetry parameter of the nonmesonic decay has been evaluated using the obtained polarization and the asymmetry of the decay protons. The positive asymmetry parameter contradicts theoretical prediction based on meson exchange model. The present data indicates the limitation of the model in ΛN weak interaction, in particular short range phenomena such the nonmesonic decay of hypernuclei.

Monopole vibrations in asymmetric nuclei: A Fermi liquid approach

Isoscalar and isovector monopole responses of a neutron–proton asymmetric finite system are studied within a semiclassical approach based on the Vlasov kinetic equation. Both the isoscalar and the isovector vibration are considered simultaneously using a self-consistency condition with the phenomenological surface pressure. We focus on the influence of the neutron excess on the strength distribution of the monopole vibrations in nuclei near the stable line. It is found that the effects of the neutron–proton asymmetry on the monopole strength function are essentially due to the Landau (one-body) damping. The isoscalar monopole strength is concentrated on one giant peak. The neutron–proton asymmetry decreases the collectivity of isovector monopole resonance. This decrease is due to the strengthening of the Landau damping. Surface effects are essentially implicated in the isovector monopole resonance.

Absolute calibration of a double-cell polarized 3He gas target

We introduce a new method for determining the polarization of the atoms in a polarized 3He target. The new method makes use of special properties of the reaction 3He(d,p) 4He. We demonstrate that for protons produced by this reaction at θ=0°, the polarization observables T 20 and C z, z are related according to 2 T 20+3C z,z=−2 . Using this result, we have made an absolute measurement of the target polarization in the Wisconsin double-cell polarized 3He target. The calibration was carried out by observing the 3He(d,p) 4He reaction with vector- and tensor-polarized deuterons of known polarization. The analyzing power T 20(0°) at energy E d=2.1 MeV was first measured with the result T 20(0°)=−0.530±0.012. The corresponding spin correlation coefficient is C z, z (0°)=−0.417±0.006. The target polarization, P t, was then determined by measuring the 0° proton asymmetry with vector-polarized deuterons. During the measurement, the relative target polarization was monitored by measuring the circular polarization P ⊙ of the 668 nm emission light from the optical pumping cell. From the measured target polarization we obtain a calibration constant K= P t/ P ⊙=11.57±0.43. This result is in good agreement with the calibrations based on other techniques.

A field-cycling NMR relaxometry investigation of proton tunnelling in a partially disordered system of hydrogen bonds

Double-proton transfer in the hydrogen bonds of the benzoic acid dimer is described by an asymmetric double-well potential. At low temperature, the hydrogen bond dynamics are dominated by phonon-assisted tunnelling and the correlation time for proton transfer is independent of temperature. We report proton transfer measurements on samples that incorporate a small concentration of thioindigo dye molecules as a substitutional impurity. The potential energy surface, particularly the energy asymmetry of the double-well potential, of dimers within a sphere of influence of the guest molecule is perturbed. These dimers exhibit a distribution of energy asymmetries and correlation times. The system is analogous to disordered systems such as glasses and provides insight into low-frequency excitations that are invoked to explain the molecular dynamics in those systems. Field-cycling NMR relaxometry was used to make a direct measurement of the spectral density of the sample at low temperatures and to identify the contribution made by the proton transfer dynamics of the hydrogen bonds in the vicinity of the guest molecules. The mean proton transfer rate and asymmetry of these dimers have been measured and are compared with the values characteristic of dimers that are remote from impurity centres. The proficiency of field-cycling NMR relaxometry, and the advantages over conventional narrow band spin–lattice relaxation measurements, is discussed.

The spin-dependent structure function g1(x) of the proton from polarized deep-inelastic muon scattering

We present a new measurement of the virtual photon proton asymmetry A 1 p from deep inelastic scattering of polarized muons on polarized protons in the kinematic range 0.0008 < x < 0.7 and 0.2 < Q 2 < 100 GeV 2 . With this, the statistical uncertainty of our measurement has improved by a factor of 2 compared to our previous measurements. The spin-dependent structure function g 1 p is determined for the data with Q 2 > 1 GeV 2 . A perturbative QCD evolution in next-to-leading order is used to determine g 1 p ( x ) at a constant Q 2 . At Q 2 = 10 GeV 2 we find, in the measured range, ∫ 0.003 0.7 g 1 P (x) d x=0.139±0.006 ( stat ) ±0.008 ( syst ) ±0.006( evol ) . The value of the first moment Г 1 P = ∫ 0 1 g 1 p (x) d x of g 1 p depends on the approach used to describe the behaviour of g 1 p at low x . We find that the Ellis-Jaffe sum rule is violated. With our published result for Γ 1 d we confirm the Bjorken sum rule with an accuracy of ≈ 15% at the one standard deviation level.

Weak decay of hypernuclei

The nonmesonic weak decay of $\ensuremath{\Lambda}$ hypernuclei is studied in a shell model framework. A complete strangeness-changing weak $\ensuremath{\Lambda}N\ensuremath{\rightarrow}\mathrm{NN}$ transition potential, based on one boson exchange, is constructed by including the exchange of the pseudoscalar mesons $\ensuremath{\pi}$, $K$, $\ensuremath{\eta}$ as well as the vector mesons $\ensuremath{\rho},\ensuremath{\omega}$, and ${K}^{*}$, whose weak-coupling constants are obtained from soft meson theorems and SU(6)${}_{w}.$ General expressions for nucleons in arbitrary shells are obtained. The transition matrix elements include realistic $\ensuremath{\Lambda}N$ short-range correlations and $\mathrm{NN}$ final state interactions based on the Nijmegen baryon-baryon potential. The decay rates are found to be especially sensitive to the inclusion of the strange mesons $K$ and ${K}^{*}$ even though the role of kaon exchange is found to be reduced with recent couplings obtained from one-loop corrections to the leading order in chiral perturbation theory. With the weak couplings used in this study the rates remain dominated by the pion-exchange mechanism since the contributions of heavier mesons either cancel each other or are suppressed by form factors and short-range correlations. The total decay rate therefore remains in agreement with present measurements. However, the partial rates which are even more sensitive to the inclusion of heavier mesons cannot be reconciled with the data. The proton asymmetry changes by 50% once heavier mesons are included and agrees with the available data.

Phenomenological determination of polarized quark distributions in the nucleon

We present a fit to spin asymmetries which gives polarized quark distributions. These functions are closely related to the ones given by the newest Martin, Roberts and Stirling fit for unpolarized structure functions. The integrals of polarized distributions are discussed and compared with the corresponding quantities obtained from neutron and hyperon $\beta$-decay data. We use the combination of proton and neutron spin asymmetries in order to determine the coefficients of our polarized quark distributions. Our fit shows that phenomenologically there is no need for taking gluonic degrees of freedom into account.

The proton asymmetry in neutron decay

Results of recoil-order, Coulomb and model-independent order-α radiative correction calculations for the proton asymmetry of neutron decay are presented. These corrections are important for the precise interpretations of the present and future proton asymmetry measurements. In addition to the case of the Standard Model, general model families are also discussed.

The x-dependent helicity distributions for valence quarks in nucleons

We derive simple relations between the polarized and unpolarized valence quark distributions in a light-cone SU(6) quark-spectator model for nucleons. The explicit x-dependent Wigner rotation effect for the light-flavor quarks is calculated. It is shown that the mass difference between the scalar and vector spectators could reproduce the up and down valence quark asymmetry that accounts for the observed ratio F 2 n F 2 p . The proton, neutron, and deuteron polarization asymmetries, A 1 p , A 1 n , and A 1 d , are in agreement with the available data by taking into account the Wigner rotation effect. The calculated x-dependent helicity distributions of the up and down valence quarks are compared with the recent results from semi-inclusive hadron asymmetries in polarized deep inelastic scattering by the Spin Muon Collaboration.

Measurements of the proton and deuteron spin structure function g2 and asymmetry A2.

We have measured proton and deuteron virtual photon-nucleon asymmetries A{sup p}{sub 2} and A{sup d}{sub 2} and structure functions g{sup p}{sub 2} and g{sup d}{sub 2} over the range 0.03 < x < 0.8 and 1.3 < Q{sup 2} < 10 (GeV/c){sup 2} by inelastically scattering polarized electrons off polarized ammonia targets. Results for A{sub 2} are significantly smaller than the positivity limit (sqrt)R for both targets. Within experimental precision the g{sub 2} data are well described by the twist-2 contribution, g{sup ww}{sub 2}. Twist-3 matrix elements have been extracted and are compared to theoretical predictions.

A Novel and Perfectly Aligned Highly Electro−Optic Organic Cocrystal of a Merocyanine Dye and 2,4-Dihydroxybenzaldehyde

A Novel and Perfectly Aligned Highly Electro−Optic Organic Cocrystal of a Merocyanine Dye and 2,4-Dihydroxybenzaldehyde

Scale dependence of polarized deep inelastic scattering asymmetries.

We compare the $Q^{2}$ dependence of the polarized deep inelastic scattering proton asymmetry, driven by the leading order Altarelli Parisi evolution equations, to those arising from fixed order $\alpha_{s}$ and $\alpha_{s}^{2}$ approximations. It is shown that the evolution effects associated with gluons, which are not properly taken into account by the leading order approximation, cannot be neglected in the analysis of the most recent experimental data.

Measurable parameters of neutron decay

The effect of the general vector, axialvector, scalar and tensor interactions on the theoretical description of neutron decay is investigated. Results of least squares adjustment computations for the weak coupling constants are presented (neutron decay and several nuclear beta decay observables have been employed). A simple calculation method, which makes it possible to get analytical formulae for many important distributions and quantities of neutron decay, is described. The sensitivities of the electron and proton spectra and asymmetries and of two combined electron-proton asymmetries to the determination of the most important phenomenological parameters are studied by statistical methods.

Search for T-violation in K+ decays

In this paper, we analyze the elastic scattering of the muon neutrino (νμ) beam on the polarized proton target (PPT) in a presence of induced couplings, and predict how the existence of relative phases between the complex vector (weak magnetism) and axial (induced pseudoscalar) form factors of the proton with left-chirality νμ affects the azimuthal dependence of the differential cross section. The neutrinos are assumed to be Dirac fermions with non-zero mass and CPT symmetry is conserved. We show that the azimuthal asymmetry of recoil protons depends on the neutrino mass, but contributions are very tiny (∼10−5). Analysis of the differential cross section in the case of pure vector and axial couplings at zero νμ mass limit and zero momentum transfer shows that the T-violating phase βVA generates the T-odd, P-even triple correlation and it could be detected by measuring the asymmetry between the (0,π) and (π,2π) angles. It should be clearly stressed that the considered T-odd observable is not a genuine CP-violating quantity as it can also be produced by the T-invariant contributions due to the final state interactions (FSI). Their magnitude must be precisely estimated and subtracted from the measured observable to extract information on the possible time reversal violation (TRV). We also indicate the possibility of using the PPT in the neutrino telescope.

Study of Λ → NN process in polarized Λ5He

Strangeness changing weak hyperon nucleon interaction is studied by observing the weak nonmesonic decay (NM-decay) of Λ hypernuclei ( ΛN → NN). Observation of its branching ratios gave spin-isospin structure of the NM-decay. Recently we carried out the experiment to observe asymmetry of protons from the weak NM-decay of the polarized Λ 5He hypernucleus. The asymmetry is due to interference between amplitudes with different parities. It is shown that asymmetry parameter elucidates the dominant part of the NM-decay mechanism.

Weak decay of polarized Λ hypernuclei

The non mesonic decay of Λ hypernuclei is investigated in a relativistic nuclear model which includes, apart from the conventional pion mechanism, the exchange of the heavier mesons. The non mesonic decay rate and the proton asymmetry are in agreement with present experimental data. It is found that these observables are dominated by one pion exchange, since short range ΛN correlations supress the contribution from heavier mesons. The neutron to proton induced ratio, more sensitive to the isospin structure at the vertices, is increased by about 30%. However, the theoretical result is still far off the present experimental value.

Spin polarization and weak decay of polarized Λ hypernuclei

Weak nonmesonic decay of Λ hypernuclei is almost unique way to study strangeness changing weak ΛN → NN process. The study of branching ratio gave spin-isospin structure of the the nonmesonic decay. A recent experiment, which observed asymmetry of protons from the polarized Λ hypernuclei, gives the interference term of amplitudes between different parities. It is shown that the asymmetry parameter elucidates the dominant part of the nonmesonic decay mechanism.

Non-standard physics and nucleon strangeness in low-energy PV electron scattering

Contributions from physics beyond the Standard Model, strange quarks in the nucleon, and nuclear structure effects to the left-right asymmetry measured in parity-violating (PV) electron scattering from12C and the proton are discussed. It is shown how lack of knowledge of the distribution of strange quarks in the nucleon, as well as theoretical uncertainties associated with higher-order dispersion amplitudes and nuclear isospin-mixing, enter the extraction of new limits on the electroweak parametersS andT from these PV observables. It is found that a series of elastic PV electron scattering measurements using4He could significantly constrain thes-quark electric form factor if other theoretical issues are resolved. Such constraints would reduce the associated form factor uncertainty in the carbon and proton asymmetries below a level needed to permit extraction of interesting low-energy constraints onS andT from these observables. For comparison, the much smaller scale ofs-quark contributions to the weak charge measured in atomic PV is quantified. It is likely that only in the case of heavy muonic atoms could nucleon strangeness enter the weak charge at an observable level.

Primakoff scattering for polarized photons or polarized protons

We present a way to measure the axial coupling of the proton for the neutral strangeness current in coherent π 0 production induced by photon-proton scattering. By means of the γ− Z− π 0 triangle anomaly, the parity violating asymmetries for polarized photon or polarized proton Primakoff effect filter the couplings so as to leave the proton axial coupling only. We calculate the relevant observables induced by the electroweak interference and give results for regions of energy and Q 2 of possible experimental interest. The polarized proton asymmetry is predicted to be 10 −6-10 −5 when Q 2 ∼ 0.1−0.5 GeV 2.