Electron-photon Collisions Research Articles

Bremsstrahlung Calculations of Single Gauge-Boson Production on Linear Colliders in Context of "New Physics" Searching

Differential and total cross sections of single gauge boson production in high energy electron-photon collisions obtained within the Standard Model in leading order and next-to-leading order are presented. Soft photon bremsstrahlung as well as hard photon bremsstrahlung parts were considered using the dimensional regularization procedure. Special features of receiving the hard bremsstrahlung convergent contribution are discussed. The corresponding anomalous gauge boson couplings were studied in the effective Lagrangian approach. Best conditions for registration of effects beyond the Standard Model are determined.

Modern methods of calculations of bremsstrahlung in the interaction of elementary particles

. The problem of real bremsstrahlung calculation is considered using the modern methods of regularization of divergencies. In particular, we calculate soft photon bremsstrahlung in the most general form using the method of dimensional regularization of infrared divergences. The general calculation algorithm of hard photon bremsstrahlung is described. It is shown that the contribution of hard bremsstrahlung can be separated into the finite and divergent parts. The divergent part can be factorized with the contribution of the initial process in the Born approximation. It is shown that a good choice of kinematic variables makes an analytic covariant calculation of the divergent part of the hard bremsstrahlung possible. In a particular case, an algorithm for determining the kinematic constraints on the invariants is described. A numerical analysis of the radiative corrections for gauge bosons production processes in the case of electron-photon collisions is performed. It is discovered that the contribution of the finite part of bremsstrahlung at high collision energies reaches 20 per cent and must be taken into account in calculations of radiative corrections. The results obtained can be used in various calculations, including covariant ones, performed in the context of confirmation of the Standard Model theoretical predictions or searching for manifestations of alternative gauge models.

W-boson production in the high energy electron-photon collisions

In this paper the analysis of W-boson production process in high-energy electron-photon collisions as a tool to search for deviations from the Standard Model is considered. In particular, a set of extended gauge models, including anomalous multi-boson interactions, are discussed as a promising way for «new physics» study. A numerical analysis of the total cross sections of the processes was carried out. The lowest order radiative corrections in the soft-photon approximation within the Standard Model are taken into account. Calculations beyond the Standard Model was performed, the kinematic features of the cross sections were identified. The restrictions on the anomalous triple gauge boson coupling constants were analyzed and the kinematic areas to the search for their manifestations were obtained during the experiments at the International Linear Collider. The paper shows that the search for «new physics» effects based on electron-photon collisions around the W-boson production peak is the maximal promising. It was also shown that future experiments at high luminosity linear colliders will significantly clarify the constraints on anomalous gauge coupling constants.

Analytical description of photon beam phase spaces in inverse Compton scattering sources

We revisit the description of inverse Compton scattering sources and the photon beams generated therein, emphasizing the behavior of their phase space density distributions and how they depend upon those of the two colliding beams of electrons and photons. Main objective is to provide practical formulas for bandwidth, spectral density, brilliance, which are valid in general for any value of the recoil factor, i.e. both in the Thomson regime of negligible electron recoil, and in the deep Compton recoil dominated region, which is of interest for gamma-gamma colliders and Compton Sources for the production of multi-GeV photon beams. We adopt a description based on the center of mass reference system of the electron-photon collision, in order to underline the role of the electron recoil and how it controls the relativistic Doppler/boost effect in various regimes. Comparisons with Monte Carlo simulations of inverse Compton scattering in various scenarios are presented, showing very good agreement with the analytical formulas: in particular we find that the bandwidth dependence on the electron beam emittance, of paramount importance in Thomson regime, as it limits the amount of focusing imparted to the electron beam, becomes much less sensitive in deep Compton regime, allowing a stronger focusing of the electron beam to enhance luminosity without loss of mono-chromaticity. A similar effect occurs concerning the bandwidth dependence on the frequency spread of the incident photons: in deep recoil regime the bandwidth comes out to be much less dependent on the frequency spread.

Color fluctuations in high energy hadron- and photon-nucleus collisions

We explain that coherence of high energy QED and QCD processes implies existence of new kind of phenomena which are beyond a framework based on Regge poles (cuts). New phenomena emerge as the consequence of compositeness of the bound states and the Lorentz slowing down of interaction. We focus on the color fluctuations phenomena predicted earlier for [Formula: see text] collisions within QCD and recent evidence for this phenomenon from [Formula: see text] LHC run, significant modification of nuclear shadowing phenomenon in the diffractive photoproduction of vector mesons observed recently in the ultra peripheral collisions at LHC. We outlined briefly general properties of color fluctuations phenomena and perspectives of future studies of this phenomenon in electron (photon) collisions with nuclei.

Biocompatible silver nanoparticles embedded in a PEG–PLA polymeric matrix for stimulated laser light drug release

The laser-induced release of a well-known hepatoprotective drug (silibinin, SLB) from a temperature-sensitive polymeric composite loaded with silver nanoparticles (Ag NPs) was investigated. The surface chemistry tuning and the specific design of Ag NPs are fundamental in view of the engineering of specific stimuli-responsive systems, able to control drug release in response to external stimuli. The release profiles of SLB from the newly synthesized PEG–PLA@Ag composite show strong dependences on laser wavelength and Ag NPs’ Surface Plasmon Resonance (SPR). The resonant laser light excites the SPR of the NPs and the absorbed energy is converted into heat due to electron–photon collisions. The heat generated from the nanometer-sized metal particles embedded within the polymer is efficient and strongly localized. The nanovector, irradiated by a relatively low-intensity laser but tuned specifically to the metal NPs’ SPR, releases the encapsulated drug with a higher efficiency than that not irradiated or irradiated with a laser wavelength far from the metal SPR. A combination of analytical techniques including UV–Vis, NMR, and FT-IR spectroscopy and scanning/transmission electron microscopy has been used to study the structural and morphological properties of the composite. The controllable specificity of this approach and the possibility of the SPR-mediated localized photothermal effect to be usefully applied in aqueous environments are the relevant advances of the proposed system for photothermal therapies that make use of visible optical radiation or for the drug delivery in proximity of the tumor cells.

Higgs production in two-photon process and transition form factor

The Higgs production in the two-photon fusion process is investigated where one of the photons is off-shell while the other one is on-shell. This process is realized in either electron–positron collision or electron–photon collision where the scattered electron or positron is detected (single tagging) and described by the transition form factor. We calculate the contributions to the transition form factor of the Higgs boson coming from top-quark loops and W-boson loops. We then study the Q2 dependence of each contribution to the total transition form factor and also of the differential cross section for the Higgs production.

Sum rule for photon target

The amplitude of zero angle scattering of an electron on photon in the 3rd QED order of fine structure constant with ${\ensuremath{\gamma}}^{*}\ensuremath{\gamma}$ intermediate state converting into quark-antiquark is considered. Utilizing analytic properties of the elastic photon-photon scattering amplitude, an explicit expression for the differential cross section of quark-antiquark pair production at the electron-photon collision in peripheral kinematics is apparently derived. The limiting case of small transferred momenta with an application of the Weizs\acker-Williams like relation gives the sum rule for the photon target, bringing into the relation the sum of ratios of the four powers of the quark charges to squared quark masses with an integral over the total $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}2\text{ }\text{ }\mathrm{jets}$ cross section.

Electron and phonon distribution relaxation in metal films under a femtosecond laser pulse

The simultaneous time evolution of electron and phonon distributions in a gold film submitted to a femtosecond laser pulse has been theoretically investigated. A system of two coupled time-dependent Boltzmann equations describing electron and phonon dynamics has been solved by considering electron–electron (e–e) and electron–phonon (e–p) collisions. Phonon–phonon (p–p) collisions have been neglected. The collision probability of each process is derived by quantum-mechanical first order perturbation theory. The injected laser energy (source term) is directly absorbed by the electron gas through electron–photon collisions and a perturbation term is added to the Boltzmann equation for electrons. However, phonons do not directly absorb energy from the laser but they are affected by the irradiation due to their collisional interaction with electrons. The results emphasize the transient non-equilibrium behavior of the electron distribution function and details of electron and phonon relaxation dynamics.

Stochastics of multiple electron–photon head-on collisions

The problem of stochastic in multiple electron–photon head-on collisions has been considered in the paper. A kinetic theory for the distributions over the electron energy and collisions number and the equations for these distribution moments are used for this study instead of frequently used approximate estimations. An iterative method has been suggested for solving the equations for the distribution moments. It has been shown that the variance of the energy distribution as a function of the mean number of collisions has a maximum at some value of n ̄ if the incident energy spread of electrons is less than some limit value. That is, the energy spread damping due to the head-on electron–photon collisions takes place for electrons with large enough variance of energy distribution. Analytical formulas and Monte Carlo simulation show that multiple scattering of electrons influences on the spectra both electrons and photons even for the photon target of small thickness. In particular, multiple scattering of electrons leads to broadening of spectra of photons with small emission angle. This fact has to be taken into account in the nuclear and high energy physics experiments where such photon beams can be used and beam monochromaticity is important.

Superlight gravitinos in electron–photon collisions

Motivated by recent studies of supersymmetry in higher-dimensional spaces, we discuss the experimental signatures of a superlight gravitino ( mass ⩽10 −3 eV ). We concentrate on the process e − γ→ e ̃ R G ̃ as a probe of supersymmetry, where a single heavy superpartner and a superlight gravitino are produced. The fact that there is only one heavy superpartner in the final state in this process would require a lower center-of-mass energy for on-shell production compared to conventional pair production. For instance, for a 500 GeV machine, we find that a positive signal will be found if the supersymmetry breaking scale is less than about 2 TeV. If no positive signal is found, this process puts a bound on the supersymmetry breaking scale.

Laser influence on magnetoresistance

We present the model of interaction between the electrons inside a ferromagnetic-normal metal-ferromagnetic trilayer sample and the high intensity short pulse laser light. The incident electromagnetic field of laser on sample surface perturbs the Boltzmann distribution function determined by the electron–electron and electron–photon collision terms. The numerical results confirm that the laser pulse can influence significantly on the electron distribution function and as a consequence, the giant magnetoresistance.

Can large-scale magnetic fields survive during the pre-recombination era of the universe?

The influence of a dense photon field on the generation of electric currents and magnetic fields in the early phases of the universe is considered. It is shown that due to the strong interaction of charged particles and photons, any directed relative drift velocity of positive and negative charges is immediately thermalized and currents are shut off. Large-scale magnetic fields caused by phase transitions evolve only under the influence of the expanding plasma, which is strongly coupled to the photons—an effect well known as Silk-damping. Immediately after the epoch of baryon–antibaryon annihilation, this damping mechanism results in a radiation dominated electric conductivity that is mainly determined by the proton–photon collisions. After the next phase transition (electron–positron annihilation), the electron–photon collisions provide the effective damping. Whereas the electric conductivity due to Coulomb collisions scales as the temperature T3/2, the photon-driven conductivity scales as T−3. The corresponding magnetic diffusion time scale in very early phases of the universe is too fast to allow for any significant macroscopic magnetic field.

HIGH ENERGY PHOTON-PHOTON AND ELECTRON-PHOTON COLLISIONS

The advent of a next linear e±e- collider and back-scatterd laser beams will allow the study of a vast array of high energy processes of the Standard Model through the fusion of real and virtual photons and other gauge bosons. As examples, I discuss virtual photon scattering γ*γ*→X in the region dominated by BFKL hard Pomeron exchange and report the predicted cross sections at present and future e±e- colliders. I also discuss exclusive γγ reactions in QCD as a measure of hadron distribution amplitudes and a new method for measuring the anomalous magnetic and quadrupole moments of the W and Z gauge bosons to high precision in polarized electron-photon collisions.

Sparticle production in electron-photon collisions

We explore the potential of electron-photon colliders to measure fundamental supersymmetry parameters via the processes $e\gamma \to \tilde e \tilde\chi^0$ (selectron-neutralino) and $e\gamma\to \tilde\nu \tilde\chi^-$ (sneutrino-chargino). Given the $\tilde\chi^0$ and $\tilde\chi^-$ masses from $e^+e^-$ and hadron collider studies, cross section ratios $\sigma(\gamma_-)/\sigma(\gamma_+)$ for opposite photon helicities determine the $\tilde\nu_L$, $\tilde e_L$ and $\tilde e_R$ masses, independent of the sparticle branching fractions. The difference $m_{\tilde\nu_L}^2-m_{\tilde e_L}^2$ measures $M_W^2\cos2\beta$ in a model-independent way. The $\tilde e_L$ and $\tilde e_R$ masses test the universality of soft supersymmetry breaking scalar masses. The cross section normalizations provide information about the gaugino mixing parameters.

Metal surface photoelectric effect: dependence on the dynamic electron distribution function

We present here an aspect of the generalized Sommerfeld model that includes all previous results obtained within this model and we extend it by including the effect of the deviation of the electron distribution function from equilibrium when taking into account the electron-electron and the electron-photon collision terms. The results allow an interesting insight into the photoelectric effect which is clearly seen as a stepwise process. The calculations are in good agreement with experimental results for the case of gold and show the strong dependence of the magnitude of the photosensitivity on the experimentally estimated value for the electron relaxation time among other parameters.

Chargino-sneutrino production in electron-photon collisions

We study the production of charginos and sneutrinos in electron-photon collisions ${(e}^{\ensuremath{-}}\ensuremath{\gamma}\ensuremath{\rightarrow}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{j}^{\ensuremath{-}}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\nu}}}_{e})$ within the minimal supersymmetric standard model. The high energy photons can be generated by Compton backscattering of intense laser pulses off one of the beams of the Next Linear Collider. This process could offer a significant opportunity to identify sneutrinos, which exclusively decay invisibly into a neutrino and the LSP, since the cross sections are two orders of magnitude higher than for the radiative production of invisible sneutrinos ${(e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\nu}}\overline{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\nu}}}\ensuremath{\gamma})$. For three scenarios of gaugino-Higgsino mixing the cross sections and polarization asymmetries are computed and the resulting signatures are compared with the SM background.

Laser Assisted Electron - Alkali Atom Collisions

Lasar assisted inelastic scattering of electrons by alkali atoms is studied theoretically. The non-perturbative quasi-energy method, which is generalised for many atomic states, is used to describe the laser–atom interaction, and the electron–atom interaction is treated within the first Born approximation. We have calculated the total cross section for the excitation of sodium atoms due to simultaneous electron–photon collisions. We show the effect of laser and collision parameters, e.g. laser intensity, polarisation and incident electron energy, on the excitation process.

Identifying the Higgs boson in electron-photon collisions.

We analyze the production and detection of the Higgs boson in the next generation of linear $e^+e^-$ colliders operating in the $e\gamma$ mode. In particular, we study the production mechanism $e + \gamma \rightarrow e \gamma \gamma \rightarrow e + H$, where one photon is generated via the laser backscattering mechanism, while the other is radiated via the usual bremsstrahlung process. We show that this is the most important mechanism for Higgs boson production in a $500$ GeV $e\gamma$ collider for $M_H\raisebox{-.4ex}{\rlap{$\sim$}} \raisebox{.4ex}{$>$}140$ GeV. We also study the signals and backgrounds for detection of the Higgs in the different decay channels, $b \bar b$, $W^+W^-$, and $ZZ$, and suggest kinematical cuts to improve the signature of an intermediate mass Higgs boson.

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.