Virtual Photon Emission Research Articles

Experimental Separation of Virtual Photon Exchange and Electron Transfer in Interatomic Coulombic Decay of Neon Dimers

We investigate the interatomic Coulombic decay (ICD) of neon dimers following photoionization with simultaneous excitation of the ionized atom (shakeup) in a multiparticle coincidence experiment. We find that, depending on the parity of the excited state, which determines whether ICD takes place via virtual dipole photon emission or overlap of the wave functions, the decay happens at different internuclear distances, illustrating that nuclear dynamics heavily influence the electronic decay in the neon dimer.

Virtual photon emission from a quark-gluon plasma

We present phenomenological formulas for virtual photon emission rates from a thermalized quark-gluon plasma (QGP) that include bremsstrahlung and annihilation with scattering (AWS) mechanisms along with the Landau-Pomeranchuk-Migdal (LPM) effects. For this purpose we follow the approach of generalized emission functions (GEF) for virtual photon emission, we showed earlier for a fixed temperature and strong coupling constant. In the present work, we extend the LPM calculations for several temperatures and strong coupling strengths, photon energies (q{sub 0}), photon mass (Q{sup 2}), and quark energies (p{sub 0}). We generalize the dynamical scaling variables, x{sub T},x{sub L}, for bremsstrahlung and AWS processes that are now functions of variables p{sub 0},q{sub 0},Q{sup 2},T,{alpha}{sub s}. The GEF introduced earlier, g{sub T}{sup b},g{sub T}{sup a},g{sub L}{sup b},g{sub L}{sup a}, are also generalized for any temperatures and coupling strengths. From this, the imaginary part of the photon polarization tensor as a function of photon mass and energy has been calculated as a one-dimensional integral over these GEF and parton distribution functions in the plasma. By fitting these polarization tensors obtained from GEF method, we obtained a phenomenological formula for virtual photon emission rates as a function of (q{sub 0},Q{sup 2},T,{alpha}{sub s}) that includes bremsstrahlung and AWSmore » mechanisms with LPM effects.« less

Electromagnetically Induced Exciton Mobility in a Photonic Band Gap

It is suggested that an exciton in the engineered vacuum of a photonic-band-gap-quantum-well heterostructure exhibits electromagnetically induced anomalous quantum dynamics. The exciton is dressed by coherent emission and reabsorption of virtual photons near the photonic band edge and captured in momentum space, lowering its energy by 1-10 meV and lowering its effective mass by 4-5 orders of magnitude. The photonic band gap simultaneously enables strong coupling to confined optical modes and long exciton lifetime.

Exciton dressing and capture by a photonic band edge

We demonstrate electromagnetically induced anomalous quantum dynamics of an exciton in a photonic band gap (PBG)-quantum well (QW) heterostructure. Within the engineered electromagnetic vacuum of the PBG material, the exciton can propagate through the QW by the emission and reabsorption of virtual photons in addition to the conventional electronic hopping mechanism. When the exciton wave vector and recombination energy nearly coincide with a photonic band edge, the exciton kinetic energy is lowered by $1--10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ through coherent radiative hopping. This capture of the exciton by the photonic band edge is accompanied by strong electromagnetic dressing in which exciton's renormalized effective mass is 4--5 orders of magnitude smaller than in the absence of the PBG environment. This dressed exciton exhibits a long radiative lifetime characteristic of a photon-atom bound state and is robust to phonon-assisted recombinative decay. By inheriting properties of the PBG electromagnetic vacuum, the bound electron-hole pair becomes a stable, ultramobile quantum excitation.

Radiative corrections to chiral amplitudes in quasiperipheral kinematics

Chiral amplitudes for two-jet processes in quasiperipheral kinematics are calculated at the Born and one-loop correction levels. The amplitudes of subprocesses describing the interaction of a virtual and a real photon with the creation of a charged fermion pair for various chiral states are considered in detail. Similar results are presented for the Compton subprocess with a virtual photon. Contributions of the emission of virtual, soft, and hard real additional photons are explicitly taken into account. The relevant cross sections expressed in terms of impact factors are in agreement with the structure function approach in the leading logarithmic approximation. Contributions of the next-to-leading terms are represented in analytic form. Estimation of accuracy is discussed.

Rare radiative leptonic decays B d,s 0 → ℓ+ℓ− γ

The rare radiative leptonic decays $$\bar B_{d,s}^0 \to \ell ^ + \ell ^ - \gamma $$ are studied. The contributions to the respective amplitude from the emission of photons from the quark loop, bremsstrahlung from leptons, and weak-annihilation effects are taken into account in relevant calculations. Results are presented for the partial widths and distributions of leptons in the final state. It is shown that the previously disregarded contributions of vector resonances associated with virtual-photon emission from the light valence quark of the B meson have a significant effect on the mass spectra of dileptons.

Rare radiative leptonic decaysBd,s→ℓ+ℓ−γ

Long-distance QCD effects in $B_{d,s}\to\ell^+\ell^-\gamma$ decays are analyzed. We show that the contribution of the light vector-meson resonances related to the virtual photon emission from valence quarks of the $B$-meson, which was not considered in previous analyses, strongly influences the dilepton differential distrubution. Taking into account photon emission from the $b$-quark loop, weak annihilation, and Bremsstrahlung from leptons in the final state, we give predictions for dilepton spectrum in $B_{d, s}\to \ell^+\ell^-\gamma$ decays within the Standard model.

Radiative corrections to radiativeπe2decay

The lowest order radiative corrections (RC) to width and spectra of radiative $\pi e2$ decay are calculated. We take into account virtual photon emission contribution as well as soft and hard real photon emission one. The result turns out to be consistent with the standard Drell-Yan picture for the width and spectra in the leading logarithmical approximation which permits us to generalize it to all orders of perturbation theory. Explicit expressions of nonleading contributions are obtained. The contribution of short distance is found to be in agreement with Standard Model predictions. It is presented as a general normalization factor. We check the validity of the Kinoshita-Lee-Nauenberg theorem about cancellation in the total width of the mass singularities at zero limit of electron mass. We discuss the results of the previous papers devoted to this problem. The Dalitz plot distribution is illustrated numerically.

Radiative muon pair production in high-energy electron-positron annihilation process

The lowest order radiative correction to the differential cross section of process of muon pair production with the emission of a hard photon at high energy electron-positron annihilation are calculated. Taking into account the emission of additional soft and hard photon, the cross section can be put in the form of the Drell-Yan process in leading logarithmical approximation. Applying the crossing transformation, we obtain the cross section of the radiative electron-muon high-energy scattering process. Virtual and soft photon emission contributions of the nonleading form are tabulated for several typical kinematical points. The limit of the small invariant mass of a muon pair is in agreement with our previous analysis.

The radiative return at $\phi$ - and B-factories: small-angle photon emission at next-to-leading order

The radiative return offers the unique possibility for a measurement of the cross section of electron--positron annihilation into hadrons over a wide range of energies. The large luminosity of present phi- and B-factories easily compensates for the additional factor of alpha due to the emission of a hard photon. Final states with photons at large angles can be easily identified. The rate for events with collinear photons, however, is enhanced by a large logarithm and allows, in particular at lower energies, for a complementary measurement. The Monte Carlo generator PHOKHARA, which includes next to leading order corrections from virtual and real photon emission, has been extended from large photon angles into the collinear region, using recent results for the virtual corrections. In addition, the present version includes final state radiation for muon and pion pair production and final states with four pions. Implications for the experimental analysis at three typical energies, 1.02, 4 and 10.6 GeV, are presented: the magnitude of these new corrections is studied, possibilities for the separation of initial and final state radiation are proposed, and the differences with respect to the previous treatment based on structure functions are investigated.

(Quasi)elastic electron-muon large-angle scattering within the two-loop approximation: Vertex contributions

We consider quasielastic large-angle electron-muon scattering at high energies with radiative corrections up to the two-loop level. The lowest order radiative corrections arising from the one-loop virtual photon emission and a real soft emission are presented within a power accuracy. Two-loop corrections are supposed to be of three gauge-invariant classes. One of them, the so-called vertex contribution, is given in the logarithmic approximation. The relation to the renormalization group approach is discussed.

Photon and dilepton emission rates from high density quark matter

We compute the rates of real and virtual photon (dilepton) emission from dense QCD matter in the color-flavor locked (CFL) phase, focusing on results at moderate densities (3-5 times the nuclear saturation density) and temperatures $T\simeq80$ MeV. We pursue two approaches to evaluate the electromagnetic (e.m.) response of the CFL ground state: (i) a direct evaluation of the photon self energy using quark particle/-hole degrees of freedom, and (ii) a Hidden Local Symmetry (HLS) framework based on generalized mesonic excitations where the $\rho$ meson is introduced as a gauge boson of a local SU(3) color-flavor group. The $\rho$ coupling to generalized two-pion states induces a finite width and allows to address the issue of vector meson dominance (VMD) in the CFL phase. We compare the calculated emissivities (dilepton rates) to those arising from standard hadronic approaches including in-medium effects. For rather large superconducting gaps (several tens of MeV at moderate densities), as suggested by both perturbative and nonperturbative estimates, the dilepton rates from CFL quark matter turn out to be very similar to those obtained in hadronic many-body calculations, especially for invariant masses above $M\simeq0.3$ GeV. A similar observation holds for (real) photon production.

Vector mesons and baryon resonances in nuclear matter

We review the current understanding of time-like virtual photon emission from QCD matter. The phenomenology of dilepton emission is discussed and basic theoretical concepts are introduced. The experimental findings are presented, grouped into production of lepton pairs in elementary processes, production off cold nuclear matter and emission from heavy-ion collisions. The review emphasizes the role of dilepton emission as tool for studying exotic phases of QCD matter. Open questions and a route to probe the QCD phase diagram with dileptons are outlined.

QED correction to the radiative tail from the elastic peak in deep inelastic scattering

We calculate the deep inelastic scattering (DIS) cross section under kinematical requirements when a final hadron state is a single proton. The process in the lowest order is known as the radiative tail from the elastic peak. We take into account the correction, coming from the emission of virtual and soft real photons, to the amplitude of this process as well as the one induced by the emission of an additional hard photon and a hard pair. The resulting contributions of these channels are presented in the leading and, partly, next-to-leading logarithmic approximations. Some general expressions for the photon contributions are given. Numerical results are presented for kinematical conditions of the current experiments in DIS.

Radiative large-angle Bhabha scattering in collinear kinematics

The process of large-angle high energy electron–positron scattering with emission of one hard photon almost collinear to one of the charged particles' momenta is considered. The differential cross section with radiative corrections due to emission of virtual and soft real photons calculated to a power accuracy is presented. Emission of two hard photons and total expressions for radiative correction are given in leading logarithmical approximation. The latter are illustrated by numeric estimates. A relation of the results with structure function formalism is discussed.

Collinear radiative electron-positron scattering in the leading logarithmic approximation

Radiative corrections to the cross section of the radiative large-angle Bhabha scattering process at high energies are calculated. The kinematics in which a hard photon is emitted along one of the momentum directions of the charged fermions is considered. Contributions coming from the emission of virtual, soft, and hard additional photons are taken into account in the leading logarithmic approximation.

Radiative tail in π e2 decay and some remarks on μ-e universality

The result of lowest-order perturbation theory calculations of the photon and positron spectra in radiative pion(e2) decay are generalized to all orders of perturbation theory using the structure-function method. An additional source of radiative corrections to the ratio of the positron and muon channels of pion decay, due to emission of virtual and real photons and pairs, is considered. It depends on details of the detection of the final particles and is large enough to be taken into account in theoretical estimates with a level of accuracy of 0.1%.

Generalized dicke model:δm=±2 transitions

The theory of two-atom quantum transitions with frequency doubling of the emitted or absorbed photons in coherent optical processes is constructed. Third-order quantum electrodynamics (QED) effects, including virtual photon exchange and emission (absorption) of a double frequency real photon, are considered. In these effects, the two parts of the interaction accounting for intermediate states with positive or negative frequencies in the atomic spectrum are separated. Correspondingly, the electron and positron polarizations of the medium leading to the appearance of two mechanisms of frequency doubling with different physical properties are distinguished. In this paper, a generalized Dicke model for a system of N two-level atoms is used to describe coherent processes in the electric dipole approximation, allowing quantum transitions with ΔM=±1, ±2, where M is the half-difference of the resonance level populations in the atoms. In the framework of this model, a formula for the intensity of quantum transitions with ΔM=±2 is obtained.

Soft photons in the Lund model

We investigate the emission of real and virtual photons with small transverse momenta close to zero cms rapidity in proton-proton collisions. We consider a model, built upon the space-time structure of the Lund string fragmentation model combined with the Fritiof model for hadronic interactions. The model contains a gauge and Lorentz invariant, albeit semiclassical, treatment of the currents from the final state hadrons as well as their valence constituents. We further investigate the effects of resonance decay, increase of the transverse momentum width of the string fluxtube, gluon emission leading to “bent” strings and finally also the occurrence of polarization charges (virtual q q -production) along the string world-sheet. We show that the spectrum for real and virtual (e +e − pairs) photon emission always will be within a factor of 2 to 3 from the long wavelength limit (the Low limit) for transverse momenta up to a few hundred MeV/ c. We conclude that if the present experimental indications for a much larger source of emission should be confirmed, then a major revision will be necessary in our present understanding of the space-time structure and/or the energy densities available in hadron fragmentation.

THE EQUIVALENT PHOTON APPROXIMATION

We establish a set of conditions which must be satisfied in order to make the equivalent photon spectrum (and also the virtual photon's polarization) Lorentz-invariant. We insist in particular on the condition that the virtual photon's emission angle must be small.