Asymmetry parameter for nonmesonic hypernuclear decay

We give general expressions for the vector asymmetry in the angular distribution of protons in the nonmesonic weak decay of polarized hypernuclei. From these we derive an explicit expression for the calculation of the asymmetry parameter, a(Lambda), which is applicable to the specific cases of He-5(Lambda) and C-12(Lambda) described within the extreme shell model. In contrast to the approximate formula widely used in the literature, it includes the effects of three-body kinematics in the final states of the decay and correctly treats the contribution of transitions originating from single-proton states beyond the s-shell. This expression is then used for the corresponding numerical computation of a(Lambda) within several one-meson-exchange models. Besides the strictly local approximation usually adopted for the transition potential, we also consider the addition of the first-order nonlocality terms. We find values for a(Lambda) ranging from -0.62 to -0.24, in qualitative agreement with other theoretical estimates but in contradiction with some recent experimental determinations.

DECIPHERING THE QUARK-GLUON STRUCTURE OF THE PHOTON IN E-GAMMA COLLISIONS

We investigate the capability of an egamma collider to unravel the hadronic content of the photon. The experimental problem for probing the gluonic structure of the photon is that small-x triggers overwhelmingly select soft photons rather than soft gluons in hard photons. We show that the problem can be finessed in experiments where laser back-scattering is used to prepare a source of very hard photons. We illustrate their power for studying the parton distributions of the photon and, specifically, for separating the quark and gluon components in events where dijets, jet-gamma pairs, and heavy quark pairs are produced.

Periodic waves and solitons of two-photon propagation

We study the two-photon propagation (TPP) modelling equations. The one-phase periodic solutions are obtained in an effective form. Their modulation is investigated by means of the Whitham method. The theory developed is applied to the problem of creation of TPP solitons on the sharp front of a long pulse.

Asymmetry of the Aharonov-Bohm diffraction pattern and Ehrenfests theorem

The electron-diffraction pattern for two slits with magnetic flux confined to an inaccessible region between them is calculated. The Aharonov-Bohm effect gives a diffraction pattern that is asymmetric but has a symmetric envelope. In general, both the expected displacement and the kinetic momentum of the electron are nonzero as a consequence of the asymmetry. Nevertheless, Ehrenfests theorems and the conservation of momentum are satisfied. © 1992 The American Physical Society.

Searching for leptoquarks in electron-photon collisions

We study the production of composite scalar leptoquarks in eγ colliders, and we show that an e+e- machine operating in its eγ mode is the best way to look for these particles in e+e- collisions, due to the hadronic content of the photon.

Deciphering the quark-gluon structure of the photon in ey collisions

We investigate the capability of an ey collider to unravel the hadronic content of the photon. The experimental problem for probing the gluonic structure of the photon is that small-x triggers overwhelmingly select soft photons rather than soft gluons in hard photons. We show that the problem can be finessed in experiments where laser back-scattering is used to prepare a source of very hard photons. We illustrate their power for studying the parton distributions of the photon and, specifically, for separating the quark and gluon components in events where dijets, jet-y pairs, and heavy quark pairs are produced.

Niobium filtration of conventional and high-frequency x-ray generator beams for intraoral radiography. Effects on absorbed doses, image density and contrast, and photon spectra

We have studied the effects of niobium beam filtration on absorbed doses, on image density and contrast, and on photon spectra with conventional and high-frequency dental x-ray generators. Added niobium reduced entry and superficial absorbed doses in periapical radiography by 9% to 40% with film and digital image receptors, decreased the radiation necessary to produce a given image density on E-speed film and reduced image contrast on D- and E-speed films. As shown by increased half-value layers for aluminum, titanium, and copper and by pulse-height analyses of beam spectra, niobium increased average beam energy by 6% to 19%. Despite the benefits of adding niobium on patient dose reduction and on narrowing the beams' energy spectra, the beam can be overhardened. Adding niobium, therefore, strikes the best balance between radiation dose reduction and beam attenuation, with its risks of increased exposure times, motion blur, and diminished image contrast, when it is used at modest thicknesses (30 μm) and at lower kVp (70) settings. © 1995 Mosby-Year Book, Inc.

Chiral asymmetry and gravitation theory

In this letter we apply an alternative approach, recently developed, to the description of massless particles of arbitrary spin to the case of spin-two particles. This provides a non-geometrical approach to the theory of linearized gravitation. Within this method the chiral components of a spinor field are treated as independent field variables. The free field Lagrangian is built up from the requirement of chiral invariance. This formulation is parallel to the neutrino theory and leads to a formulation that generalizes, to particles of spin-two, the two-component neutrino theory. At the free field level the analog of curvature tensor, spin connection tensor, and metric tensor are independent quantities. By introducing left-right asymmetric linear interactions of these chiral components we get the linearized gravitation theory.

Testing anomalous Higgs couplings in triple photon production at the Tevatron collider

We derive bounds on Higgs and gauge-boson anomalous interactions using the CDF data for the process pp̄ → γγγ + X. We use a linearly realized SU L(2) X U Y(1) invariant effective Lagrangian to describe the bosonic sector of the Standard Model, keeping the fermionic couplings unchanged. All dimension-six operators that lead to anomalous Higgs interactions involving γ and Z are considered. We also show the sensitivity that can be achieved for these couplings at Fermilab Tevatron upgrades. © 1998 Published by Elsevier Science B.V. All rights reserved.

Photon-photon and pomeron-pomeron processes in peripheral heavy ion collisions

We estimate the cross sections for the production of resonances, pion pairs, and a central cluster of hadrons in peripheral heavy-ion collisions through two-photon and double-pomeron exchange, at energies that will be available at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). The effect of the impact parameter in the diffractive reactions is introduced, and by imposing the condition for realistic peripheral collisions we verify that in the case of very heavy ions the pomeron-pomeron contribution is indeed smaller than the electromagnetic one. However, they give a non-negligible background in the collision of light ions. This diffractive background will be more important at RHIC than at LHC.

New limits on doubly charged bileptons from CERN LEP data and the search at future electron-positron and electron-photon colliders

We show that the accumulated CERN LEP-II data taken at √s = 130-206 GeV can establish more restrictive bounds on doubly charged bilepton couplings and masses than any other experiment so far. We also analyze the discovery potential of a prospective linear collider operating in both e+e- and e γ modes.

Two-photon final states in peripheral heavy ion collisions

We discuss processes leading to two photon final states in peripheral heavy ion collisions at RHIC. Due to the large photon luminosity we show that the continuum subprocess γγ→ γγ can be observed with a large number of events. We study this reaction when it is intermediated by a resonance made of quarks or gluons and discuss its interplay with the continuum process, verifying that in several cases the resonant process overwhelms the continuum one. It also investigated the possibility of observing a scalar resonance (the σ meson) in this process. Assuming for the σ the mass and total decay width values recently reported by the E791 Collaboration we show that RHIC may detect this particle in its two photon decay mode if its partial photonic decay width is of the order of the ones discussed in the literature.

The flavor asymmetry in the polarized proton sea

We perform a careful study on the effect of the Pauli blocking to the light polarized antiquark structure of the proton sea. We develop the formal expressions for the polarized antiquark distributions, highlighting the role played by quark statistics and the vacuum structure. The ratio involving the polarized antiquarks is calculated. In particular, it is found that Δd̄(x)/Δū(x) should be negative and x independent.

Simple analytical expression for vector hypernuclear asymmetry in nonmesonic decay ofΛ 5He andΛ 12C

We present general explicit expressions for a shell-model calculation of the vector hypernuclear parameter in nonmesonic weak decay. We use a widely accepted effective coupling Hamiltonian involving the exchange of the complete pseudoscalar and vector meson octets (π, η, K, ρ, ω, K*). In contrast to the approximated formula widely used in the literature, we correctly treat the contribution of transitions originated from single-proton states beyond the s-shell. Exact and simple analytical expressions are obtained for the particular cases of Λ 5He and Λ 12C, within the one-pion-exchange model. Numerical computations of the asymmetry parameter, aΛ, are presented. Our results show a qualitative agreement with other theoretical estimates but also a contradiction with recent experimental determinations. Our simple analytical formulas provide a guide in searching the origin of such discrepancies, and they will be useful for helping to solve the hypernuclear weak decay puzzle.