Coherent Pair Production Research Articles

Accelerator Backgrounds in a Muon Collider

Abstract Muon colliders are considered to be an important future energy frontier accelerator. It is possible to build a large muon collider as a circular machine, even at multi-TeV energies, due to the greatly reduced synchrotron radiation expected from muons. In addition to the same physics processes present in an electron collider, a muon collider will have the potential to produce s-channel resonances such as the various Higgs states at an enhanced rate. For a muon collider with 750 GeV/c μ + μ− with 2 × 10 12 muons per bunch we would expect 8.6 × 10 5 muon decays per meter for the two beams. The energetic electrons from muon decays will produce detector backgrounds that can affect the physics. These backgrounds include electrons from muon decays, synchrotron radiation from the decay electrons, hadrons produced by photo-nuclear interactions, coherent and incoherent beam-beam pair production and Bethe-Heitler muon production. In this paper we will discuss these processes and calculate particle fluxes into the detector volume from these background processes.

COHERENT PAIR PRODUCTION IN CRYSTALS IN PRESENCE OF ACOUSTIC WAVES

Coherent electron-positron pair production by high-energy photons is investigated in a periodically deformed single crystal with a complex base. The formula for the corresponding differential cross-section is derived for an arbitrary deformation field. The conditions are specified under which the influence of the deformation is considerable. The case is considered in detail when the photon enters into the crystal at small angles with respect to a crystallographic axis. The results of the numerical calculations are presented for SiO 2 single crystal in the case of the deformation field generated by the acoustic wave of the S type. It is shown that, in dependence of the parameters, the presence of deformation can either enhance or reduce the pair creation cross-section.

Angular distribution of positrons in coherent pair production in deformed crystals

We investigate the angular distribution of positrons in the coherent process electron–positronpair creation process by high-energy photons in a periodically deformed singlecrystal with a complex base. The formula for the corresponding differential crosssection is derived for an arbitrary deformation field. The case is considered indetail when the photon enters into the crystal at small angles with respect to acrystallographic axis. The results of the numerical calculations are presented forSiO2 and diamond single crystals and Moliere parameterization of the screened atomic potentialsin the case of the deformation field generated by an acoustic wave of S-type.

Coherent bremsstrahlung, coherent pair production, birefringence, and polarimetry in the 20–170 GeV energy range using aligned crystals

The processes of coherent bremsstrahlung (CB) and coherent pair production (CPP) based on aligned crystal targets have been studied in the energy range 20–170 GeV. The experimental arrangement allowed for measurements of single photon properties of these phenomena including their polarization dependences. This is significant as the theoretical description of CB and CPP is an area of active debate and development. With the approach used in this paper, both the measured cross sections and polarization observables are predicted very well. This indicates a proper understanding of CB and CPP up to energies of 170 GeV. Birefringence in CPP on aligned crystals is applied to determine the polarization parameters in our measurements. New technologies for high-energy photon beam optics including phase plates and polarimeters for linear and circular polarization are demonstrated in this experiment. Coherent bremsstrahlung for the strings-on-strings (SOS) orientation yields a larger enhancement for hard photons than CB for the channeling orientations of the crystal. Our measurements and our calculations indicate low photon polarizations for the high-energy SOS photons.20 MoreReceived 2 March 2008DOI:https://doi.org/10.1103/PhysRevSTAB.11.041001This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Coherent pair production by energetic gamma rays incident on quasicrystals

We develop a Born-approximation theory of coherent pair production (CPP) of electrons by energetic gamma rays incident on an icosahedral quasicrystal, described by a schematic model (K model) that includes phonon and phason disorder. Our main result is a formula for the cross-section dσcpp/dε+ for CPP, differential with respect to the positron energy ε+ and of order α2 in the fine-structure constant α≈1/137, but which is otherwise exact. We discuss results of numerical calculations of dσcpp/dε+ versus y=ε+/k for gamma rays of energies k=20MeV, 200MeV, and 3GeV, incident on icosahedral Al–Mn–Si, described as a special case of the K model (vertex model). This consists in placing an Mn atom at each vertex of the relevant Ammann tiles. Our calculations include CPP of types A and B. Both types exhibit vertical intensity drops at irregularly distributed y-values, many of these drops being so large that they should be observable experimentally. They are analogous to the large intensity drops exhibited by coherent bremsstrahlung in quasicrystals. We predict that CPP drops also occur for realistic models of i-Al–Mn–Si at the same y-values as for the vertex model, but whose magnitudes may differ from those predicted by this model.

Polarization effects for pair creation by photon in oriented crystals at high energy

Pair creation by a photon in an oriented crystal is considered in the frame of the quasiclassical operator method, which includes processes with polarized particles. Under some quite generic assumptions the general expression is derived for the probability of pair creation of longitudinally polarized electron (positron) by circularly polarized photon in oriented crystal. In the particular cases ϑ 0 ≪ V 0/ m and ϑ 0 ≫ V 0/ m ( ϑ 0 is the angle of incidence, angle between the momentum of the initial photon and axis (plane) of crystal, V 0 is the scale of a potential of axis or a plane relative to which the angle ϑ 0 is defined) one has the constant field approximation and the coherent pair production theory correspondingly. Side by side with coherent process the probability of incoherent pair creation is calculated, which differs essentially from amorphous one. At high energy the pair creation in oriented crystal is strongly enhanced comparing with the amorphous medium. In the corresponding appendixes the integral polarization of positron is found in an external field and for the coherent and incoherent mechanisms.

Results on the coherent interaction of high energy electrons and photons in oriented single crystals

The CERN-NA-59 experiment examined a wide range of electromagnetic processes for multi-GeV electrons and photons interacting with oriented single crystals. The various types of crystals and their orientations were used for producing photon beams and for converting and measuring their polarisation.The radiation emitted by 178GeV unpolarised electrons incident on a 1.5cm thick Si crystal oriented in the Coherent Bremsstrahlung (CB) and the String-of-Strings (SOS) modes was used to obtain multi-GeV linearly polarised photon beams.A new crystal polarimetry technique was established for measuring the linear polarisation of the photon beam. The polarimeter is based on the dependence of the coherent pair production (CPP) cross section in oriented single crystals on the direction of the photon polarisation with respect to the crystal plane. Both a 1mm thick single crystal of Germanium and a 4mm thick multi-tile set of synthetic Diamond crystals were used as analyzers of the linear polarisation.A birefringence phenomenon, the conversion of the linear polarisation of the photon beam into circular polarisation, was observed. This was achieved by letting the linearly polarised photon beam pass through a 10cm thick Silicon single crystal that acted as a “quarter wave plate” (QWP) as suggested by Cabibbo et al.

Formula omitted]-beam propagation in an anisotropic medium

Propagation of a γ -beam in an anisotropic medium is considered. The simplest example of this medium of a general type is a combination of the two linearly polarized monochromatic laser waves with different frequencies (dichromatic wave). The optical properties of this combination are described with the use of the permittivity tensor. The refractive indices and polarization characteristics of normal electromagnetic waves propagating in an anisotropic medium are found. The relations describing variations of γ -beam intensity and Stokes parameters as functions of the propagation length are obtained. The influence of laser wave intensity on the propagation process is calculated. The γ -beam intensity losses in the dichromatic wave depend on the initial circular polarization of γ -quanta. This effect also applies to single crystals which are oriented in some regions of coherent pair production. In principle, the single crystal sensitivity to circular polarization can be used for determination of polarization of high energy (in tens GeV and more) γ -quanta and electrons.

Search for diffractive charm production in 800 GeV/c proton-silicon interactions

A search for charm production in the coherent diffractive dissociation reaction pSi→XSi was carried out for the modes D 0→ K − π +, D 0→ K − π + π + π −, and D +→ K − π + π +. No charm signals were observed, and the 90% confidence level upper limit for coherent charm pair production was determined to be 26 μb per silicon nucleus. The results are interpreted as an upper limit of 0.2% on the amount of intrinsic charm in the proton.

Monte-Carlo study of electromagnetic showers of high energy electrons and photons in aligned crystals

Abstract A Monte–Carlo program to investigate electromagnetic shower formation in single crystals for particle energies up to 300 GeV is presented. It is based on the well known EGS4-code for amorphous materials and accounts for coherent radiation and pair production which arise in a crystal. Channeling radiation and crystal field assisted pair production are calculated within the uniform field approximation. The increase of energy loss of channeled electrons due to radiation cooling is also included. Good agreement with the available experimental data can be reported.

CVC and PCAC in neutrino-lepton interactions

We propose a formalism for the approximate calculation of inelastic neutrino interactions of the form ν + A → ν + B, when the states A and B consist of leptons and/or photons only. Using the conservation properties of the leptonic vector and axial vector currents, the cross section is related to that of photon- and axion-induced reactions ( γ, a 0) + A → B. The procedure is applied to coherent photon emission from atoms ( ν + Atom → ν + Atom + γ), radiative neutrino-electron scattering ( ν + e − → ν + e − + γ), and coherent pair production in the Coulomb field of a nucleus (ν + “γ” → ν + ℓ − + ℓ +) .

Coherent pair production in linear colliders

The production of an electron-positron pair by a photon impinging on a high-density bunch of electrons is calculated in the deep quantum regime, typically found in linear colliders in the multi-TeV range. We obtain separately a leading contribution proportional to the bunch length and logarithmic edge effects. Pair production is found to be considerable.

Coherent pair production in axial alignment

The differential electron-positron pair-production rate in aligned crystals is predicted to show a measurable resonant structure as a function of the incident photon energy. The resonances are the results of crystal periodicity in the alignment direction. Calculations are presented for diamond and Si, and the expected modification of these results by channeling is discussed.

New Analysis of the Two Competing Processes: Channeling-and Coherent Pair Production and a Discussion of Experimental Data

Pair creation by high energy photons is discussed as the inverse process of bremsstrahlung by electrons. The characteristics of channeling pair production are analysed and compared with those of the coherent analogue. Newly reported experimental findings are examined. Separation of the two pair production mechanisms by energy and temperature variations is discussed.

Search for coherent muon pair production by neutrinos and antineutrinos

A search for coherent μ + μ − pair production has been made using the CHARM neutrino detector exposed to wide-band horn-focussed neutrino and antineutrino beams at the 400 GeV CERN SPS. Out of 3.3 x 10 6 neutrino and antineutrino induced CC events with energy greater than 10 GeV, we find two events which can be attributed to coherent production off the target nuclei (CaCO 3). This allows a limit to be set on the diagonal four-lepton coupling constant G d < 1.5 G f (90% CL).

Screening Corrections in Electron Pair Production by Intermediate- and Low-Energy Photons

The screening correction to the total cross-section for coherent pair production is calculated for photon energies ranging almost from threshold to 45 MeV. The correction is given as the sum of the (negative) Born screening term and a contribution which is obtained by simply shifting the point-Coulomb positron energy spectrum in energy by the amount Vel(0), Vel(r) being the potential caused by the atomic electrons. This contribution is positive and relatively large in the threshold region but decreases fast with increasing photon energy. Results are given for Z ⩽ 92, in terms of tables and simple analytic fits to the numerically calculated results.

Pair production by photons. Screening corrections for intermediate and high energies

The screening correction to the total cross-section for coherent electron pair production by photons is evaluated for intermediate and high energies, by employing the Born-approximation recoil momentum distribution and accurate relativistic form factors. Results are given for all 3≤Z≤92. For each element the results are presented in terms of a seven-parameter formula which is based on a high-energy expansion of the screened cross-section.

A polarized photon beam produced by coherent pair production in oriented graphite

Attenuation by coherent pair production in higly oriented, compression annealed, pyrolytic graphite has been used to polarize a 16-GeV bremsstrahlung beam. The polarizer consists of 61 cm of graphite crystals, whose reciprocal lattice vectors are oriented at 10.5 mrad to the normal to the beam direction, and can be rotated by 90° about the beam line to rotate the plane of polarization. A functionally identical assembly of length 30.5 cm was used as an analyzer to measure the polarization of the beam with the SLAC pair spectrometer. The beam produced intensities greater than 4 × 10 8 equivalent quanta per beam pulse and had a measured polarization of 0.255±0.020.

Production of a High Energy, High Power Polarized Photon Beam at SLAC

Attenuation by coherent pair production in highly oriented compression annealed pyrolytic graphite has been used to polarize a 16 GeV bremsstrahlung beam. Graphite pieces of total volume 0.95 × 0.95 × 5.1 cm3 were assembled in water-cooled precision holders. Eighteen of these units were mounted in two independent coaxial assemblies. Each assembly provided precision angular orientation with respect to the beam direction, rotation of the graphite by 90° about the beam axis to rotate the plane of polarization, and removal from or insertion into the beam. One assembly, with 61 cm of graphite, served as a polarizer; the other, with 30.5 cm of graphite, was used as an analyzer. The complete device was placed between collimators in a sweeping magnet. Polarization of the attenuated beam, measured with the SLAC pair spectrometer, was 0.255 ± 0.020. This number, and the measured spectrum, are in qualitative accord with the calculations. The beam has yielded intensities greater than 4 × 108 equivalent quanta per pulse, and has been used in an experiment to measure asymmetries in pseudoscalar meson photoproduction.

Polarization of High-Energy Photons Using Highly Oriented Graphite

We have demonstrated the feasibility of a new method of polarizing high-energy photons, by preferential attenuation of photons polarized in one plane as a result of coherent pair production on passage through a single crystal of specific orientation. Highly oriented graphite is a particularly favorable material. The method, feasible at presently available energies, is predicted to be especially useful at higher energies.