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Particles traveling in aligned crystals at small angles w.r.t. crystallographic axes or planes are principally steered by the continuous Lindhard potential. This interaction conserves the energy E, the longitudinal momentum p_parallel, the transverse energy of the particle E_perp and is elastic concerning the crystal quantum state. At high enough energy the particle motion is quasi-classical. The time-dependent fluctuations of the positions of the atoms or of the electrons of the crystal create a residual potential, on which the particle can scatter. This interaction does not conserve the previous quantities and is inelastic for the crystal. We compare its treatments with the classical binary collision model and with a phenomenological quantum model. The classical dechanneling rate is estimated to be several ten per cent larger than the quantum one. The influence of correlated vibrations of neighboring atoms is discussed., 8 pages, 2 figures ; Presented at "RREPS-19" Sept. 2019, Belgorod, Russia. This is the Accepted Manuscript version of an article accepted for publication in Journal of Instrumentation. Neither SISSA Medialab Srl nor IOP Publishing Ltd is responsible for any errors or omissions in this version of the manuscript or any version derived from it

We revisit the recursive model for the fragmentation of polarized quarks based on the string $+{^{3}P}_{0}$ mechanism of $q\overline{q}$ pair creation. We make a different choice for one input function of the model that simplifies the implementation in a Monte Carlo program. No new parameters are introduced, and the relevant results are the same apart from the suppression of the spin-independent correlations between successive quarks. The advantage of the present version is that it allows us to study analytically the preservation of positivity bounds and it is more suitable for an interface with external event generators like PYTHIA. The theoretical aspects and the simulation results obtained with a stand alone program are discussed in detail and compared with those of the previous version of the model.

International audience; The strong dependence of the PXR yield on the orientation of the crystal surface, claimed in a recent article by S. Blazhevich and A. Noskov, is questioned. It is argued that, if absorption is neglected, the sum of the ordinary and forward PXR yields does not depend of the surface orientation.

In a crystal, thermal vibrations of neighboring atoms are correlated: long wavelength phonons make them displace in the same direction. These correlations can speed up the dechanneling of axially channeled particles, as shown by a simulation with binary collisions and a Markov chain model of atom vibrations. In the case of e + or e - at small angle with a crystal axis, the correlated vibrations should give rise to a semi-coherent bremsstrahlung on several consecutive atoms of a row.

Several estimators of the initial transverse polarization of a quark fragmenting into a jet are proposed and compared using Monte Carlo simulations. These are based on the simplified version of a recursive model [A. Kerbizi et al, Phys. Rev. D97, 074010 (2018)] which includes the quark spin degree of freedom.

We present a model for Monte Carlo simulation of the fragmentation of a polarized quark. The model is based on string dynamics and the ${}^3P_0$ mechanism of quark pair creation at string breaking. The fragmentation is treated as a recursive process, where the splitting function of the subprocess $q \to h + q'$ depends on the spin density matrix of the quark $q$. The ${}^3P_0$ mechanism is parametrized by a complex mass parameter $\mu$, the imaginary part of which is responsible for single spin asymmetries. The model has been implemented in a Monte Carlo program to simulate jets made of pseudoscalar mesons. Results for single hadron and hadron pair transverse-spin asymmetries are found to be in agreement with experimental data from SIDIS and $e^+e^-$ annihilation. The model predictions on the jet-handedness are also discussed., Comment: 18 pages, 18 figures

Phenomenological aspects of radiation by relativistic electrons in external field, in matter or the vicinity of matter are reviewed, among which: infrared divergence, coherence length effects, shadowing, enhancement in aligned crystals, quantum recoil and spin effects, electron side-slipping, photon impact parameter and the presence of tunnelling in the radiation process., 15 pages, 1 figure. Talk given at 6th International Conference - Channeling 2014 Charged & Neutral Particles Channeling Phenomena October 5-10, 2014 Capri (Naples) Italy

International audience; We describe a Monte Carlo code for the fragmentation of polarized quarks into pseudoscalar mesons. The quark jet is generated by iteration of the splitting q → h + q′ where q and q′ indicate quarks and h a hadron. The splitting function describing the energy sharing between q′ and h is calculated on the basis of the Symmetric Lund Model where the quark spin is introduced through spin matrices as foreseen in the 3 P 0 mechanism. A complex mass parameter is introduced for the parametrisation of the Collins effect. The results for the Collins analysing power and the comparison with the Collins asymmetries measured by the COMPASS collaboration are presented. For the first time preliminary results on the simulated azimuthal asymmetry due to the Boer-Mulders function are also given.

International audience; The hybrid positron source made of a crystal-radiator, as a source of channeling radiation, and an amorphous converter is briefly described. Optimization of the heating effects and thermal shocks led to a granular converter made of small tungsten spheres. Simulation results with application to two linear collider projects CLIC and ILC are presented. A test operated with this kind of source at KEK is described and the results gathered last year, reported. Plans for the future are briefly described.

International audience; The applicability of X-ray radiation for the observation of accelerated particle beam profiles is studied. Three types of quasi-monochromatic X-ray radiation excited by the particles in crystals are considered: characteristic X-ray radiation, parametric X-ray radiation, diffracted transition X-ray radiation. Radiation is collected at the right angle to the particle beam direction. It is show that the most intensive differential yield of X-ray radiation from Si crystal can be provided by characteristic radiation at incident electron energies up to tens MeV, by parametric radiation at incident electron energies from tens to hundreds MeV, by diffracted transition X-ray radiation at GeV and multi-GeV electron energies. Therefore these kinds of radiation are proposed for application to beam profile observation in the corresponding energy ranges of incident electrons. Some elements of X-ray optics for observation of the beam profile are discussed. The application of the DTR as a source of powerful tunable monochromatic linearly polarized X-ray beam excited by a multi-GeV electron beam on the crystal surface is proposed.

International audience; In classical electrodynamics the amplitude of the radiation by a charged particle in non-uniform motion is usually derived from the Liénard-Wiechert retarded potential. We show that it can be derived in a simpler way by supposing an initial radiation field which stimulates the emission and using energy conservation. We also compare the forces which brake (or accelerate) the particle in spontaneous and stimulated emissions (or absorption).

The tunneling ionization of an excited hydrogen atom by a static electric field $\mathbf{E}$ is investigated for the case where the initial electron has an orbital angular momentum $\mathbf{L}$ nonparallel to $\mathbf{E}$. The outgoing electron has a nonzero mean transverse velocity $\ensuremath{\langle}{\mathbf{v}}_{\mathrm{T}}\ensuremath{\rangle}$ in the direction of $\mathbf{E}\ifmmode\times\else\texttimes\fi{}\ensuremath{\langle}\mathbf{L}\ensuremath{\rangle}$. During this process the linear Stark effect makes $\ensuremath{\langle}\mathbf{L}\ensuremath{\rangle}$ and $\ensuremath{\langle}{\mathbf{v}}_{\mathrm{T}}\ensuremath{\rangle}$ oscillate or rotate about $\mathbf{E}$. Measures of the asymmetry are calculated at leading order in $\mathbf{E}$ for an initial state $2P$ state. The generalization to coherent elliptic Rydberg states is outlined. A subset of these states whose classical Kepler ellipses rotate rigidly about $\mathbf{E}$ is particularly interesting for the observation of the asymmetry. The preparation of states with $\mathbf{L}$ nonparallel to $\mathbf{E}$ and the conditions to get a sizable ${\mathbf{v}}_{\mathrm{T}}$ asymmetry are discussed.

In the symmetric string fragmentation recipe adopted by PYTHIA for jet simulations, the transverse momenta of successive quarks are uncorrelated. This is a simplification but has no theoretical reason. Transverse momentum correlations are naturally expected, for instance, in a covariant multiperipheral model of quark hadronization. We propose a simple recipe of string fragmentation which lead to such correlations. The definition of the jet axis and its relation with the primordial transverse momentum of the quark is also discussed., Comment: 7 pages, 3 figures, 1 table

International audience; Light leaking from a bent optical fiber shares many properties with synchrotron radiation : in ray optics, both lights are emitted tangentially to a light cylinder; in wave optics, the emission mechanism involves a tunnel effect. The angular distributions of these two radiations are studied in parallel and found to be similar. The same is done for the impact parameter distributions. The latter show interference fringes of the Airy function type. The far field escaped from the fiber is calculated with the Volume Current Method. An optical system observing the impact parameter profile is proposed.

Various methods of obtaining longitudinally polarized positrons for future linear colliders are reviewed. Special attention is paid to the schemes using circularly polarized high-energy photons for positron production. Most effectively such photons are obtained from electrons passing through a helical undulator or colliding with a circularly polarized laser wave. Spectrum and polarization of radiation emitted during helical motion of electrons are considered in detail. A new simple presentation of known formulas is used to account for the influence of the wave intensity, of the electron-beam angular divergence, of the collimation of radiation, and of the lateral and temporal profiles of the laser bunch on the radiation properties., 16 pages, 6 figures

Axially oriented crystals, penetrated by high-energy electrons, are powerful photon sources and, hence, intense positron sources. Such kinds of positron sources have been studied experimentally at CERN, with the tertiary electron beam of the SPS having an energy of 6 and 10 GeV. Four and eight millimeters thick tungsten crystals and a compound target made of a 4 mm crystal followed by a 4 mm amorphous disk were used with an orientation along the 〈1 1 1〉 axis. The positrons were detected by a drift chamber, partially immersed in a magnetic field. The reconstructed trajectories allowed the determination of their energy and angular spectra. Significant enhancements were observed for the crystal source when compared to the amorphous one. The gain was about 3 for the 4 mm target and about 2 for the 8 mm and the compound targets. These preliminary results are described after short presentations of the experimental set-up and of the method of track reconstruction.

International audience; This paper gives a short review of the past and recent activities of the Atomic Collisions in Solids Lyon-group, in collaboration with other groups, in the field of high energy channelling. The ion-channelling programme was performed at GANIL-Caen and at GSI-Darmstadt. The electron-channelling programme started at ALS-Saclay for relativistic incident energies and was then extended to SPS-CERN for ultra-relativistic energies. The last part of this paper presents the electron-channelling experiments performed originally at ALS-Saclay, then at BTF-Frascati and more recently at LS-Saga, in order to observe the electron “internal clock” predicted in 1924 by L. de Broglie.

Optical Transition Radiation (OTR) provides an attractive method for diagnostics on electron/positron beams of small dimensions at high energies (GeV). However, some limits on the geometrical resolution at very high energies have been often discussed in the literature and a minimum value given by ‘cj’ has been invoked. In order to bring an experimental contribution to the problem, systematic measurements of electron beam profiles, in the energy range of 1—2 GeV and at optical wavelength between 400 and 700 nm, have been carried out at the Orsay 2 GeV Linear Accelerator. OTR emitted from an aluminium foil at a 30i incidence angle was collected by a two-lens telescope and recorded by an intensified CCD camera. The OTR beam profiles were compared to the profiles obtained by a SEM Grid having a resolution better than 0.5 mm. After a theoretical introduction presenting the di⁄erent resolution limits, which can be invoked with di⁄raction phenomenon, and a presentation of our calibration procedure, the experimental results are presented and compared to these limits. They show that the resolution in OTR measurements is definitely better than the already invoked cj-limit. For a small size electron beam an r.m.s. beam width about 170 lm has been determined. This value can be compared to cj"2.5 mm (E"2 GeV and j"650 nm). Our theoretical analysis also provides a more precise evaluation of the resolution power of OTR, which depends on the sensitivity of the detector; this could be of interest for much higher energies. ( 1998 Elsevier Science B.V. All rights reserved.

International audience; The field of a charged particle passing through or near an optical fiber polarizes the fiber atoms transiently. This produces a particle-induced guided light (PIGL). PIGL can also be assisted by a metallic object stuck to the fiber, via plasmon excitation. Type-I PIGL is produced in a translation-invariant part of the fiber, type-II PIGL is produced at a fiber end, an indentation or a metallic object. Properties of type-I PIGL in a single-mode fiber (spectrum, polarization) are reviewed. Order of magnitudes are given for type-II PIGL from a fiber end or a metallic ball. Interference effects are outlined, in particular the guided Smith-Purcell radiation. Applications of PIGL to beam diagnostics are discussed.

An introduction to channeling and channeling (Kumakhov) radiation is given. Relativistic and quantum-mechanical effects are discussed in function of the electron or positron energy. Phenomena of Quantum Electrodynamics in strong macroscopic field, which can be tested in channeling conditions, are described. Recipees for semi-classical Monte-Carlo simulations are presented and one of them compared with experiments at 10 and 150 GeV.

A non-conventional positron source using 1.5–20 GeV electron beam impinging on a tungsten crystal oriented along its 〈111〉 axis is described. Computer simulation taking into account photon creation in the crystal, positron generation in a subsequent amorphous target and also positron capture and transport, allowed extensive investigation of this novel type of source. The possible performances are discussed hereafter. Positron yield has been calculated for different operating conditions. Comparisons with classical sources are presented.

A string model of quark hadronization, taking the quark spin degree of freedom into account, is proposed. The method for using the model in a Monte-Carlo code for jet generation is given.

The spin correlations in a reaction involving initial and final polarized particles are encoded in a cross section matrix which is taken as an example of multipartite density matrix. Quantum positivity stipulates that the cross section is positive for any spin states - separable or entangled - of the particles. A weaker condition, classical positivity only requires that the cross section is positive for separable spin state. Classical and Quantum positivity are represented by diagrams. The domains of classical positivity and of separability are dual under a reciprocal polar transformation. This property can be usefull in quantum information theory.

An alternative way to the conventional positron source using intense electron beams, of some GeV, impinging on thick amorphous targets is presented. This source is using two successive targets: the first one is a tungsten crystal, which axis is aligned with the beam direction, and the second is an amorphous target put at some distance from the crystal. The enhanced radiation due to electron channelling produces a large amount of photons which, consecutively, create high quantity of e +- e - pairs in the amorphous target. Between the two targets a sweeping magnet takes off all or part of the charged particles coming out from the crystal. An optimization procedure is carried out based on two important parameters: a) the distance between the two targets; b) the minimum energy above which the charged particles coming out from the crystal are allowed to hit the amorphous target. This optimization, which first step is presented here, aims to obtain the highest accepted yield (> 1 e +/ e -) after the capture system and the minimum Peak Energy Density Distribution (PEDD) (PEDD < 35 J/g) in order to avoid thermal gradients, which are destructive for the target.

In radiation processes such as a transition radiation, diffraction radiation, etc. based on relativistic electrons passing through or near an opaque screen, the electron self-field is partly shadowed after the screen over a distance of the order of the formation length $\gamma^2\lambda$. This effect has been investigated on coherent diffraction radiation (DR) by electron bunches. Absorbing and conductive half-plane screens were placed at various distances L before a standard DR source (inclined half-plane mirror). The radiation intensity was reduced when the screen was at small L and on the same side as the mirror. No reduction was observed when the screen was on the opposite side. It is worth noting that absorbing and conductive half-plane screens produce the same shadowing effect. The shadowing effect is responsible for a bound on the intensity of Smith-Purcell radiation., Comment: VIII International Symposium Radiation from Relativistic Electrons in Periodic Structures (RREPS-09), Zvenigorod, Russia, Sept. 7-11, 2009

Travelling wave in a helical wave guide is considered for electron acceleration. A first determination of the travelling wave modes using a partial wave expansion (PWE) and a point matching method is presented. It gives a rapid solution for moderate deformation of the guide relative to a straight cylinder. Strong deformations will give higher accelerating gradient but the PWE diverges. Two methods overcoming this difficulties are suggested., VIII International Symposium Radiation from Relativistic Electrons in Periodic Structure (RREPS-09) Zvenigorod, Russia, September 7-11, 2009

Positivity reduces substantially the allowed domain for spin observables. We briefly recall some methods used to determine these domains and give some typical examples for exclusive and inclusive spin-dependent reactions., Comment: 7 pages, 3 figures, Talk presented at CIPANP 2009, San Diego California, USA, May 26-31, 2009 To be published in AIP Conference Proceedings

Two applications of semi-inclusive electroproduction of fast baryons are presented: (i) The steepness of the baryon spectrum at medium rapidity can give information about the general mechanisms of baryon production in soft and hard processes. (ii) With a polarized target and a detector which analyses the polarization of final Λ's, the aptitude of quarks to carry transverse polarization can be tested for the first time.

Sum rules are derived which relate moments ∫ ωn(dN/dω d2[ncirc]n of the differential photon spectrum emitted by a classically moving point charge to the integral of the square of the apparent position, velocity or acceleration, as seen by the observer. Smoothing of the trajectory by means of a convolution with a function f(τ), where τ is the observer's time, results in a filtering of the spectrum by being the fourier transform of f(τ); this allows us to write filtered sum rules. Examples of applications of unfiltered and filtered sum rules to computer simulation of channeling radiation are presented.

The electromagnetic wave field propagating in a helical wave guide is decomposed in an angular momentum basis. Eigenmodes are calculated using a truncation in $l$ and a discretisation of the boundary condition. Modes slightly slower than light are considered for relativistic particle acceleration., Comment: talk given at Channeling 2008 conference; Charged and Neutral Particles Channeling Phenomena - Channeling 2008, Erice : Italie (2008)

In coherent radiation sources (diffraction radiation, Smith-Purcell effect, etc.) based on relativistic electrons passing by a material radiator, the electron self-field is partly shadowed after each part of the radiator over a distance of the order of the formation length g2l. This effect has been investigated on coherent diffraction radiation (DR) by electron bunches. An absorbing half-plane screen was placed at various distances L before a standard DR source (inclined half-plane mirror). The DR intensity was reduced when the screen was at small L and on the same side as the mirror. No reduction was observed when the screen was on the opposite side. The shadowing effect can significantly reduce the total energy radiated in a long radiator., Comment: Was Presented at the symposium Channeling 2008 (Charged and Neutral Particles Channeling Phenomena), October 25 - November 1, 2008 Erice (Trapani - Sicily), Italy, and was accepted for publication in proceedings of symposium

A two-step positron facility using a multi-GeV electron beam is under study for electron–positron colliders as ILC or the SuperB factory. In the first step, the electron beam impinges on an oriented tungsten crystal. Channelling radiation provides a large number of γ-rays and consequently creates a large number of pairs in an amorphous target placed after the crystal. Such a method has been successfully tested at CERN and KEK and is now operating at KEK. As a second step and as an upgrade of the system, the electron beam, instead of being sent to the crystal, collides with a circularly polarized laser beam, creating circularly polarized Compton backscattered γ-rays, which impinge on a thin amorphous target where they generate longitudinally polarized positrons. Simulation results are provided for the two steps and the choice of the parameters is discussed for ILC and for the SuperB factory. Particularly, the choice of an ERL instead of a classical linac is recommended for the ILC.

A charged particle passing through or near a narrow optical fiber induces, by polarisation, coherent light guided by the fiber. In the limit of zero crossing angle, the radiation tends towards a Cherenkov radiation with a discrete spectrum, studied by different authors. If the particle crosses a bent fiber at regularly spaced points, interference gives quasi-monochromatic lines. If the particle passes near an end of the fiber, light is produced by the capture of virtual photons through the end face. An alternative way consists in sticking a metallic ball to the fiber: the passing particle induces plasmons which are then evacuated as light in the fiber. Interferences can occur between lights from several ends or balls. Applications of these various light signals to beam diagnostics are discussed. The shadow effect, which reduces the photon yield when the particle runs parallel to a row of balls, is pointed out and an upper bound $-dE/dz\le C(Ze/b)^2$ for the particle energy loss is conjectured ($Ze$ is the particle charge, $b$ the impact parameter and $C$ a numerical constant). This bound should also apply to other kinds of light sources, in particular to Smith-Purcell radiation., Comment: Presented at the VII Symposium Radiation by Relativistic Electrons in Periodic Structures (RREPS'O7, Prague, Sept. 24-27, 2007, submitted to Nucl. Inst. Methods B

Théorie; Thanks to the Dirac equation, the hydrogen-like atom at high $Z$ offers a precise model of relativistic bound state, allowing to test properties of unpolarized and polarized structure functions analogous to the hadronic ones, in particular: Sivers effect, sum rules for the vector, axial, tensor charges and for the magnetic moment, positivity constraints, sea contributions and fracture functions

An alternative way to the conventional positron sources - those which use high intensity electron beams impinging on thick amorphous targets - is presented here. It makes use of a compound target with an aligned crystal as a radiator , where the incident electron beam radiates a large number of photons (as channeling radiation or coherent bremsstrahlung) and an amorphous converter where these photons materialize in e+ e- pairs. This new kind of target has been studied experimentally at CERN and the main results providing an accurate description of this positron source are given here. The ability of such a source to work in a linear collider scheme depending on its radiation resistance, the dedicated test carried out at SLAC on a W crystal are also described and the results are reported.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Théorie, CAS,; New kind of positron sources for future linear colliders, where the converter is a tungsten crystal oriented on the axis, has been studied at CERN in the WA103 experiment. In such sources the photons which create the $e^+ e^-$ pairs result from channeling radiation and coherent bremsstrahlung. In this experiment electron beams of 6 and 10 GeV were sent on different kinds of targets: a 4 mm thick crystal, a 8 mm thick crystal and a compound taraget (4 mm crystal + 4 mm amorphous disk). An amorphous tungsten target 20 mm thick was also used for comparison with the 8 mm crystal. Tracks of outgoing charged particles were detected and analyzed by a drift chamber in a magnetic field. The energy and angle spectra of the positrons were obtained for energies up to 150 MeV and angles up to 30 degrees. The measured positron distribution in momentum space (longitudinal versus transverse) is also presented, giving a full momentum space description of the source. Results on outgoing photons are also presented. A significant enhancement of both photon and positron production is clearly observed. At 10 GeV incident energy, the positron enhancement factor is 4 for the 4 mm crystal, about 2 for the 8 mm crystal. Besides, the simulation code for the crystal processes is validated by a quite good agreement between the simulated and experimental spectra, both for positrons and photons.

The results of theoretical investigation of angular distributions of radiation from a relativistic electron passing through a thin crystal at a small angle to the crystal axis are presented. The electron trajectories in crystal were simulated using the binary collision model which takes into account both coherent and incoherent effects at scattering. The angular distribution of radiation was calculated as a sum of radiation from each electron. It is shown that there are non-trivial angular distributions of the emitted photons, which is connected to the superposition of the coherent scattering of electrons by atomic rows (“doughnut scattering” effect) and the suppression of the radiation due to the multiple scattering effect (similar to the Landau–Pomeranchuk–Migdal effect in an amorphous matter). The orientation dependence of angular distribution of radiation is also analyzed.

The characteristics of the electron-photon showers initiated by 2 to 10 GeV electrons aligned along the -axis of tungsten crystals are compared with those for the amorphous tungsten . In this energy range, as known, the positron yield at the optimal target thicknesses is larger in a crystal case only by several percent. However, the amount of the energy deposition in a crystal turns out to be considerably (by 20 - 50 %) lower than in an amorphous target providing the same positron yield, while the peak energy-deposition density is approximately of the same magnitude in the both cases., 9 pages, 5 figures

In the present article the available formulas for diffraction radiation (DR) by an electron near a perfectly conducting half plane are generalized to any direction of the electron velocity, using Lorentz transformation. This allows to take into account electron beam divergence in an exact way. A new method for determining both horizontal and vertical beam divergence using one slit DR radiator, but two optical wavelengths, is proposed.

Simulations of a nonconventional positron source, using a 1.5-20-GeV electron beam impinging on an assemblage of a tungsten crystal oriented along its axis and an amorphous target of the same material, are presented. An overall simulation taking into account photon creation in the optimum length of the crystal, positron generation in the target, and positron capture and transport allows optimization of the source. Positron yield has been calculated for different operating conditions. This crystalline positron source features a higher level of photon radiation compared to classical bremsstrahlung; a positron production enhancement of the same order as the radiation increase; and the possibility of using thin amorphous targets behind the crystal, which allows weak positron transverse momentum and lowers the amount of deposited energy in the target. >

The setup based on a moderate energy linear accelerator (linac) is proposed to provide users with a tunable, quasi-monochromatic, linearly polarized X-ray beam. The effect of parametric X-ray radiation (PXR) from relativistic electrons in a crystal is used in an X-ray source. The application of the setup for calibration of X-ray space telescopes and other equipment is considered. The setup allows calibration of angular, spectral, polarization parameters of telescopes with smooth tuning of X-ray energy from several keV to hundreds of keV. Estimates of X-ray flux intensities and spectral line widths on the telescope aperture at a distance of several hundred meters are presented. Besides, the PXR-based facility for simultaneous generation of several X-ray beams is proposed.