Two-photon Events Research Articles

The Quantum Electrodynamics of Synergistic Two-photon Processes

Abstract In this paper it is demonstrated how the phenomena of cooperative Raman scattering and cooperative two-photon absorption may be described in a unified manner, through canonically transforming the system Hamiltonian. The unique features associated with the virtual photon coupling between centres are discussed. In particular it is illustrated how in the near-zone, close proximity leads to two-body two-photon events involving interaction-induced dipoles. In the far-zone, it is shown how the coupling between molecules may be understood in terms of a radiative dipole effect. Attention is focused upon the behaviour of this coupling for distributive and cooperative mechanisms in fluids, and the relative importance of each process discussed, particularly with regard to their very different range dependences. A detailed analysis of the coupling reveals that in two-photon absorption the distributive mechanism dominates, whilst in Raman scattering cooperative energy transfer is favoured. Finally recent prog...

Multiphoton interactions in rutile

Laser-induced photoluminescence and photoconductivity in rutile crystal are described. Nd:YAG laser photons were employed which have energies of 1.16 eV (1.06 μm). This energy is just slightly more than ⅓ of the TiO2 band gap. Intensities between 105 and 106 W/cm2 were employed. The photoconductivity is shown to be produced by competing single- and two-photon events. The first-order and second-order photoconductivity cross sections are found to be 3.6 × 10−26 cm2 and 1.54 × 10−50 cm4 sec, respectively. A thermoluminescence study revealed traps with thermal ionization energies between 0.4 and 0.9 eV below the conduction band. Traps with photoionization energies ~1 and ~2 eV below the conduction band are believed to be responsible for the observed results and may or may not be due to alumina impurities known to exist in the samples.

Observation of the two-photon decay of a light penetrating particle

Coincident two-photon events, emerging from a 2 m long decay region, and pointing back to the SIN 590 MeV proton beam dump were detected in a thin-foil optical spark chamber. There was a significant excess of photons at small angles (<7°): 19 were observed, whereas (2.0 ± 0.5) were measured with cosmic rays, and additional (2.5 ± 1.0) were accelerator background. The visible γ-ray energies were estimated by multiple scattering to be 〉 E γ1 〈 = (83 ± 20) MeV, 〉 E γ2 〈 = (84 ± 15) MeV. The invariant 2γ mass is < 1 MeV. The 2γ event rate did not change significantly, when a 20 cm thick iron wall was placed in front of the decay region, but vanished with the wall put at its end. Presumably a light boson χ 0 comes from the beam dump, penetrates the shielding, and decays: χ 0 → 2 γ The measured rate of (14.5 ± 5.0) events in 129 Coulomb fixes a combination of production cross section and life-time. If the new boson χ 0 was the axion, one can solve for the Higgs parameter X = 3.0 ± 0.3, and infer τ a ≈ 7 ms, and m a = (250 ± 25) keV.

Two-photon exchange in p⊆)p→ℓ +ℓ −X and a comparison with QCD

A thorough study of lepton-pair production from two-photon annihilation in p⊆)p collisions is presented. The differential cross section is calculated over a large range of energies (27⩽√ s⩽800 GeV as a function of the dilepton mass M as well as the dilepton transverse momentum Q T and the Feynman variable x F. No kinematical approximations (such as the equivalent photon approximation) are made. For Q T ≈ 0 the two-photon mechanism represents an important fraction of the pp→e +e −X cross section already at ISR energies, whereas at ISABELLE energies it dramatically dominates in the interval 0⩽ Q T⪅1 GeV. At ISR energies these conclusions follow from a direct comparison of the two-photon contribution with pp→e +e −X data. For the ISABELLE energy range the expected O( α s ) QCD contribution to pp→ℓ +ℓ −X, corrected for soft gluon radiation to all orders (in leading bilogarithmic approximation), was taken as a reference. At larger Q T and ISR energies the γγ contribution is negligible, whereas at √ s = 800 GeV γγ/QCD≅ 10–20% almost everywhere. Furthermore, two-photon candidate events from the ISR are shown to be in reasonable agreement with theory. A decomposition of the γγ cross section into contributions from both proton vertices being elastic, inelastic and of mixed configuration is given. The results provide important clues for a future isolation of the two-photon mechanism.

A study of double tagged two-photon reactions at spear

We have identified 262 doubly tagged two-photon events. A subset of the data shows an enhancement of 21 events in the inclusive two-photon mass squared distribution between 0.8 and 2.2 GeV 2. If these events result from spin 2 resonance production then Γ γγ = 9.5 ± 3.9 ± 2.4 keV (statistical and systematic). From another subset of 58 events in which the final state could be classified we determine the two-photon hadron to muon cross section ratio R γγ = 1.1 ± 0.3 ± 0.3.

Search for positron annihilation with a single-photon emission

Annihilation of a thermalised positron in a solid can result in the emission of a single photon, accompanied by the recoil of a 'spectator' electron. This decay mode, if observed, would provide a unique method for measuring short-range electron correlations. Both theoretical and experimental results of this process are reported. The theoretical calculations were carried out for different electron densities. For Li the predicted ratio for one-photon events to the familiar two-photon events was in the range of 2*10-9 to 1*10-8. An experimental measurement of this ratio provides an upper limit of 4*10-6.

Search for Doubly RadiativenpCapture

We have made a search for doubly radiative $\mathrm{np}$ capture $n+p\ensuremath{\rightarrow}{\ensuremath{\gamma}}_{1}+{\ensuremath{\gamma}}_{2}+^{2}\mathrm{H}$. It has been suggested that this process might contribute to the total $\mathrm{np}$-capture cross section and thus explain the long-standing discrepancy in $\mathrm{np}$ capture (${\ensuremath{\sigma}}_{\mathrm{expt}}=334.2\ifmmode\pm\else\textpm\fi{}0.5$ mb and ${\ensuremath{\sigma}}_{\mathrm{th}}=302.5\ifmmode\pm\else\textpm\fi{}4.0$ mb). Previous $\mathrm{np}$-capture experiments have been insensitive to the energy distribution of the capture $\ensuremath{\gamma}$ rays. In this experiment we searched for two-photon events by looking for $\ensuremath{\gamma}$ rays in coincidence from a ${\mathrm{H}}_{2}$O target exposed to a beam of thermal neutrons using two 5\ifmmode\times\else\texttimes\fi{}5-cm NaI detectors. Analysis of the coincidence spectra indicates two-photon $\mathrm{np}$-capture events have not been observed. An upper limit at the 3 standard deviation confidence level of ${\ensuremath{\sigma}}_{2\ensuremath{\gamma}}\ensuremath{\le}1.0$ mb can be placed on the doubly radiative $\mathrm{np}$-capture cross section. Therefore the two-photon cross section does not explain the discrepancy between the theoretical and experimental $\mathrm{np}$-capture cross sections.

Towards an Accurate Wave Function for Positronium Hydride

A variational wave function for the ground state of positronium hydride is presented. The wave function is considerably more accurate than any previously reported. The only simplifications used are (a) only kinetic and Coulombic potential energy terms are included in the Hamiltonian, (b) the motion of the proton is ignored, and (c) the nucleus-positron distance is excluded from the wave function. The last assumption is the major source of error, but it enables us to evaluate all integrals exactly. Several trial wave functions are examined, the one giving the lowest energy being an open-shell exponential function times a 12-term power series through second-degree terms in the five remaining interparticle coordinates. Positronium hydride is found to be stable with respect to separation into hydrogen and positronium by at least 0.657 eV. The annihilation rate for two-photon events is found to be 2.095 ${\mathrm{nsec}}^{\ensuremath{-}1}$ for the trial wave function giving the lowest energy, and 2.265 ${\mathrm{nsec}}^{\ensuremath{-}1}$ for the trial wave function which most nearly satisfies the positron-electron cusp condition. A search for bound excited states of PsH and a bound state for the system ${e}^{+}\mathrm{He}$ was unsuccessful.

Measurement of the Longitudinal Polarization of Positrons Emitted by Sodium-22

Measurement of the preponderance of $1^{3}S_{1,0}$ states of positronium over $1^{1}S_{0,0}$ states formed in a gas at moderately high field $H$ (10 to 15 kilogauss), yields directly the preponderance of positron spins opposed to H over those parallel, just prior to capture, the magnitude and sign of the Paschen-Back effect may be inferred from the field value and the known triplet-singlet energy separation at zero field. By using suitable gases, the two-photon annihilation events from $1^{3}S_{1,0}$ are differentiated from those from $1^{1}S_{0,0}$ by requiring strict angular correlation at 180\ifmmode^\circ\else\textdegree\fi{}; positronium atoms in the latter state, being only about ${10}^{\ensuremath{-}10}$ sec old at annihilation, have retained enough of the kinetic energy at formation to yield a broad angular correlation, while the former states, being on the average 30 times older at annihilation, yield a narrow component. The relative efficiency for counting the triplet/singlet states can in practice be made \ensuremath{\sim}1.5, in the presence of all other two-quantum annihilation events. Positrons emitted into the hemisphere about $+z$, plus those scattered by the source material or source-backing into that hemisphere, are found to be polarized along $+z$ by an amount 0.15. Taking, for those positrons ultimately slowed and counted, that ${〈\frac{v}{c}〉}_{\mathrm{effective}}$ at emission is 0.75, it is implied that their longitudinal polarization at emission is at least $0.4〈\frac{v}{c}〉$, this lower limit to apply if there be no back-scattering, no depolarization during the slowing process, and no de-alignment on the part of the positronium. The sodium-22 positrons are unmistakably right-handed. The sought-for connection between failure of parity conservation and polarization of betas from an unpolarized source is thus experimentally established.