πN scattering and electromagnetic corrections in the perturbative chiral quark model

We apply the perturbative chiral quark model to give predictions for the electromagnetic O(p²) low-energy couplings of the ChPT effective Lagrangian that define the electromagnetic mass shifts of nucleons and first-order (e²) radiative corrections to the πN scattering amplitude. We estimate the leading isospin-breaking correction to the strong energy shift of the π⁻p atom in the 1s state, which is relevant for the experiment “Pionic Hydrogen” at PSI.     

τ→μγ and μ→eγ as probes of neutrino mass models

We discuss the possibility of discriminating between different supersymmetric see-saw models by improving the experimental sensitivity to charged lepton flavour violating processes. Assuming a hierarchical neutrino mass spectrum, we classify see-saw models according to how the hierarchy Δm²Δm²_atm is generated, and study the predictions of each class for the branching ratios of τ→μγ and μ→eγ. The process τ→μγ is found to be a particularly promising tool to probe the fundamental see-saw parameters, and especially to identify the origin of the large atmospheric mixing angle. Predictions for μ→eγ are more model-dependent. We point out that, even with an improvement of the experimental sensitivities by three orders of magnitude, both τ→μγ and μ→eγ could escape detection in models where Δm²_atm is determined by one of the lightest right-handed neutrinos.     

Relativistic field-theoretical equations for the three-body multichannel πN and γN scattering reactions and propagator of the Δ-resonance

A new form of the relativistic three-body equations for the coupled πN and γN scattering reactions with three particle final states ππN and γπN is suggested. These equations are derived in the framework of the time-ordered three dimensional field theory. The solutions of the considered equations satisfy the unitarity condition and are exactly gauge invariant. The form of these three-body equations is does not depend on the choice of the model of Lagrangian and is also the same for the formulations with and without quark degrees of freedom.

The effective potentials of the suggested equations are defined by the vertex functions with two on-mass shell particles. It is emphasized that these input vertex functions can be constructed from experimental data. Special attention is given to the construction of the propagator of the Δ-resonance in the framework of the separable πN potential model. The strong dependence of the multichannel πN and γN cross sections on the form of the Δ-resonance propagator[2] is discussed.

The used formulation of the relativistic three-dimensional and three-body equations allows us to overcome a number of approximations which are usually used by practical calculations of the πN and γN scattering reactions.     

Charge symmetry breaking in the A = 3 system and electromagnetic penguins (of the second kind)

We discuss difficulties with the traditional understanding of the origins of the non-coulombic portion of the ³H---³He binding energy difference (ΔBE) and analyze the contribution to ΔBE arising from certain mixed QED × QCD effects which are known to be important to the generation of isomultiplet splittings in the hadron spectrum. This contribution is shown, on general grounds, to be of O(100 keV) and, in the context of the quark cluster model, dominated by a single quark magnetic term, barring unnatural cancellations. More accurate data on Σ, Σ^* and Σ_c isomultiplet splittings would help to pin down the model estimate more precisely. Despite the uncertainties, the A = 3 binding energy difference is noteworthy as an example of a nuclear observable for which quark level effects do not appear well represented by effective operators at the nucleon level.     

Search strategies for non-standard Higgs bosons at future ee colliders

Already in the simplest two-Higgs-doublet model with CP violation in the Higgs sector, the 3×3 mixing matrix for the neutral Higgs bosons can substantially modify their couplings, thereby endangering the “classical” Higgs search strategies. However, there are sum rules relating Yukawa and Higgs–Z couplings which ensure that the ZZ, and couplings of a given neutral 2HDM Higgs boson cannot all be simultaneously suppressed. This result implies that any single Higgs boson will be detectable at an e⁺e⁻ collider if the Z+Higgs, Higgs and Higgs production channels are all kinematically accessible and if the integrated luminosity is sufficient. We explore, as a function of Higgs mass, the luminosity required to guarantee Higgs boson detection, and find that for moderate tanβ values the needed luminosity is unlikely to be available for all possible mixing scenarios. Implications of the sum rules for Higgs discovery at the Tevatron and LHC are briefly discussed.     

Renormalization group invariant matrix elements of ΔS=2 and ΔI=3/2 four-fermion operators without quark masses

We introduce a new parameterization of four-fermion operator matrix elements which does not involve quark masses and thus allows a reduction of systematic uncertainties. In order to simplify the matching between lattice and continuum renormalization schemes, we express our results in terms of renormalization group invariant B-parameters which are renormalization-scheme and scale independent. As an application of our proposal, matrix elements of ΔI=3/2 and SUSY ΔS=2 operators have been computed. The calculations have been performed using the tree-level improved Clover lattice action at two different values of the strong coupling constant (β=6/g²=6.0 and 6.2), in the quenched approximation. Renormalization constants and mixing coefficients of lattice operators have been obtained non-perturbatively. Using lowest order χPT, we also obtain ππ|O₇|K^NDR_I=2=(0.11±0.02) GeV⁴ and ππ|O₈|K^NDR_I=2=(0.51±0.05) GeV⁴ at μ=2 GeV.     

π-electroproduction on hydrogen near threshold at four-momentum transfers of 0.2, 0.4 and 0.6 GeV

The reaction e + p → e + π⁰ + p was measured near the one pion threshold, detecting the final electron and proton in coincidence for values of q² = 0.2, 0.4 and 0.6 GeV². The slope of the cross section at threshold is determined. The data are compared with those of the e + p → e + π⁺ + n reaction, measured simultaneously, and with the results of pseudovector Born approximation and with dispersion theoretical calculations.     

Atmospheric neutrinos: LMA oscillations, Ue3 induced interference and CP-violation

We consider oscillations of the low energy (sub-GeV sample) atmospheric neutrinos in the three neutrino context. We present the semi-analytic study of the neutrino evolution and calculate characteristics of the e-like events (total number, energy spectra and zenith angle distributions) in the presence of oscillations. At low energies there are three different contributions to the number of events: the LMA contribution (from ν_e-oscillations driven by the solar oscillation parameters), the U_e3-contribution proportional to s₁₃², and the U_e3-induced interference of the two amplitudes driven by the solar oscillation parameters. The interference term is sensitive to the CP-violation phase. We describe in details properties of these contributions. We find that the LMA, the interference and U_e3 contributions can reach 5–6%, 2–3% and 1–2% correspondingly. An existence of the significant (>3–5%) excess of the e-like events in the sub-GeV sample and the absence of the excess in the multi-GeV range testifies for deviation of the 2–3 mixing from maximum. We consider a possibility to measure the deviation as well as the CP-violation phase in future atmospheric neutrino studies.     

CP test in J/ψ→γφφ decay

We propose to test CP symmetry in the decay J/ψ→γφφ, for which large data sample exists at BESII, and a data sample of 10¹⁰ J/ψ's will be collected with BESIII and CLEO-C program. We suggest some CP asymmetries in this decay mode for CP test. Assuming that CP violation is introduced by the electric- and chromo-dipole moment of charm quark, these CP asymmetries can be predicted by using valence quark models. Our work shows a possible way to get information about the electric- and chromo-dipole moment of charm quark, which is little known. Our results show that with the current data sample of J/ψ, electric- and chromo-dipole moment can be probed at order of 10⁻¹³ e cm. In the near future with a 10¹⁰ data sample, these moments can be probed at order of 10⁻¹⁴ e cm.     

The weak electroproduction of Σ in electron–proton scattering

We obtain differential cross sections for the reaction e⁻+p→Σ⁰+ν_e, for incident electron energies from 0.5 GeV to 6.0 GeV. This calculation is phenomenologically based and makes use of SU(3) and SU(2) relations. We obtain contributions of the individual form factors to the differential cross section and show that the vector current form factor dominates in the region of observability. We also obtain reaction rates for a standard set of conditions. Finally we compare this case with that for Λ production and discuss what might be learned from this reaction concerning the structure of the weak, strangeness changing, hadronic current.     

Incoherent bremsstrahlung in nuclei

We have studied the processes A(e, e′γ)X in nuclei, or incoherent bremsstrahlung, and determined expressions for the cross section in terms of the same nuclear response functions R_L, R_T, which appear in inclusive electron scattering (e, e′) in nuclei. Calculations of the cross sections are carried out using a Fermi gas model, complemented by the local-density approximation, to evaluate the response functions. We have carried out a study which shows that the reaction can be used to determine reliably the response functions from experimental data. On the other hand we have compared the incoherent bremsstrahlung with the coherent one in order to see the limits to the tagging technique, which produces monochromatic photons based on the assumption of the dominance of the coherent process. We observe that at energies E_γ < 1 GeV the dominance of the coherent process extends to relatively large scattering angles, making the present technique completely safe. However, as the energy of the electron increases, the region of dominance of the coherent process is reduced to smaller scattering angles. These results should be of use when extending the tagging technique to planned or future electron facilities.     

Measurements of fragment mass, charge and kinetic energy distributions in thermal neutron fission of U with a back-to-back ΔE-E detector system

A simultaneous measurement of mass (M), charge (Z) and kinetic energy (E_k) distributions of fragments in thermal-neutron fission of ²³⁵U has been carried out using a back-to-back ΔE-E detector system. A pair of gridded ionization chambers filled with P-5 gas measured the energy losses ΔE₁, ΔE₂ of the complementary fragments in the gas, and the residual fragment energies were measured with a pair of semiconductor detectors. The four-parameter data were analysed to obtain fragment Z-distributions using the mass-momentum conservation relations to obtain M and then using the dependence of ΔE on E/M and Z. The charge resolution for the light-fragment group was determined at the gas pressures of 44, 150, 270 Torr and the best Z-resolution was obtained at 270 Torr. The data at this latter gas pressure were analysed to obtain the variances σ²_z of the fragment charge distributions for different E_k windows and this was compared with the measured variances σ²_A of the fragment mass distribution as a function of E_k as obtained by twoparameter measurements of the correlated fragment energies. The dependence of the measured variances σ²_A, σ²_Z on E_k is considered with a view to learning about the degree of neutron-proton correlations in the nucleon exchanges taking place during the mass division in fission, in analogy with that used in the case of heavy-ion deep inelastic collisions (DIC). The variation of σ²_A/σ²_z with E_k observed in the present work, which is somewhat similar to that seen in heavy-ion DIC, implies that the fragment mass division in fission is governed by nucleon-exchange processes, where the degree of neutron-proton correlations is dependent on the total kinetic energy E_k. While for very large E_k the neutron-proton motion in the exchange processes is found to be uncorrelated, for lower E_k values the motion is seen to be correlated. Implications of these results regarding the applicability of fission models such as statistical model are pointed out.     

Impurity effects on adhesive energies

Some simple arguments are made about the effect of impurities on adhesive interactions at solid junctions. A universal adhesive force relation is derived for brittle adhesion. The adhesive binding energy, ΔE, is an important parameter in brittle adhesion forces. ΔE has also been shown by others to be important when there is plastic flow. We found that impurity effects on ΔE are determined by the segregation energies to the junction and to the free surfaces. At low temperatures, if it is energetically more favorable for impurities to segregate to the surfaces than to the junction, then the impurities will decrease ΔE. The converse is also true. For example, for self junctions which are in registry, ΔE is decreased if surface segregation is exothermic and increased if it is endothermic. These segregation energy relationships are consistent with the results of a number of experiments on the effects of impurities on adhesion forces and grain boundary embrittlement.     

The particle emitter size dependence on energy in ee annihilations into hadrons

The nearly energy independent hadron emitter dimension r, measured in e⁺e⁻ annihilation in the energy range 10 to 91 GeV via the Bose–Einstein correlation of two identical charged pions, is shown to be well accounted for by choosing the hadron jets as independent pion sources. To this end the known normalised factorial cumulant moments dependence on particle sources is adapted to the Bose–Einstein correlation formalism to yield a relation between r and these sources. This approach is also able to account for the measured r values obtained for the Z⁰ decays into two and three hadron jets. Finally the estimated r value of the hadronic (9.46) decay via three gluons is expected to be higher by about 6 to 11% over that predicted for its one photon hadronic decay mode.     

On the corrections of pole dominance in B→π/v

In the framework combining the heavy quark expansion and chiral symmetry for light quarks, we identify a class of corrections to the dominant vector meson pole diagram for B→π/v, in the vicinity of the zero pion recoil point. We also give a qualitative estimate of these effects, using higher states exchanges with couplings constrained by current algebra and PCAC.     

The NN → NNη reaction close to threshold

The pp → ppη and np → npη reactions at energies near the η production threshold are studied in a non-relativistic one boson exchange model, where the N^* (1535 MeV) S11 resonance is excited through the exchange of π, η, and ω mesons and subsequently decays into an ηN pair. Energy integrated cross sections and energy spectra of the out going η's are reported. Providing NN and ηN final state interactions are taken into account coherently, the model reproduces both the scale and energy dependence of the cross section for the pp → ppη reactions up to 100 MeV. Final state interaction corrections due to the nucleon-nucleon and meson-nucleon forces influence strongly the scale and shape of the cross sections. The shape of the energy spectra of the outgoing η's provides a clear signature of the ηN force.     

Charge dependence of nuclear forces in ΔJ = 0, ΔT = ±1 beta decays

By assuming the validity of the conserved vector current theory, the Fermi nuclear matrix elements M_F for the beta decays of ²⁰F, ⁴¹Ar, ⁴⁴Sc, ⁵²Mn and ⁵⁶Co which violate the ΔT = 0 selection rule are calculated theoretically in the jj-coupling, shell model. In these calculations, M_F is obtained by considering a short-range, charge-dependent, internucleon potential in addition to the usual Coulomb potential. Experimental values of M_F have been obtained by measuring the asymmetry parameter A appearing in the β-γ circular polarization correlation. Comparison between the theoretical and experimental values yields the result that the deviation of nuclear forces from exact charge independence is about a few per cent, while the deviation from charge symmetry is consistent with a value an order of magnitude smaller. If the conserved vector current theory is not valid, additional contributions to the Fermi nuclear matrix elements can arise from mesonic exchange currents. It is noted that such effects could be as large as those due to charge-dependent forces and would therefore cloud the above interpretation.     

Relations between electromagnetic form factors of baryons

The inclusion of two-body exchange currents in the constituent quark model leads to new relations between the electromagnetic properties of octet and decuplet baryons. In particular, the N → Δ quadrupole transition form factor can be expressed in terms of the neutron charge form factor.     

On the pp → ppη(η′) reactions near threshold

The production rate for η′ in pp → ppη′ at rest is calculated in a covariant one boson exchange model, previously applied to study π⁰ and η production in NN collisions. The transition amplitudes for the elementary BN → η′N processes with B being the meson exchanged (B = π, σ, η, , ω and a₀) are taken to be the sum of s- and u-channels with a nucleon in the intermediate states, and an a₀ meson pole in a t-channel. The couplings of the η′ to hadrons are a factor 0.4 weaker than the respective η-hadron couplings, as suggested by a quark model and a singlet-octet mixing angle θ = −23°. The model reproduces near threshold cross sections for the quasielastic processes π⁻p → nη(η′) and pp → ppη(η′) reactions.     

Sneutrino–Higgs mixing in WW and ZZ production in supersymmetry with R-parity violation

We consider new s-channel scalar exchanges in e⁺e⁻→ZZ, W⁺W⁻ in supersymmetry with a small lepton number violation. We show that a small bilinear R-parity violating term which leads to sneutrino–Higgs mixing can give rise to a significant scalar resonance enhancement in e⁺e⁻→ZZ, W⁺W⁻. We use the LEP2 measurements of the WW and ZZ cross-sections to place useful constraints on this scenario. We also find, under conservative assumptions on the relevant parameter space involved, that such an exchange of the sneutrino-like admixture in e⁺e⁻→ZZ, W⁺W⁻ may be accessible to a 500 GeV e⁺e⁻ collider.