Bethe-Heitler Process Research Articles

Single-spin asymmetries in the Bethe-Heitler process e − + p → e − + γ + p induced by loop corrections

The single-spin beam and target asymmetries in the hard electroproduction process e − + p → e − + γ + p induced by the loop radiative corrections to the vertex part of lepton interaction are considered. The physical reason for the appearance of such asymmetries is the nonzero imaginary part of the amplitude (on the level of radiative corrections) caused by diagrams with photon radiation from the outgoing electron. We calculate the single-spin beam and target asymmetries at a longitudinally polarized electron beam or at arbitrary polarizations of the target proton for the CLAS and HERMES experimental conditions.

Deeply Virtual Compton Scattering measured by HERMES

HERMES has measured azimuthal beam-spin and beam-charge asymmetries in hard electroproduction of photons on nucleons and nuclei. Attributed to the interference of the Bethe-Heitler process and the Deeply Virtual Compton scattering process (DVCS), these asymmetries give access to the latter at the amplitude level. This process provides the theoretically cleanest way to access generalized parton distributions and thus new information on the structure of nucleons and nuclei. The data were accumulated by the HERMES experiment at DESY scattering the HERA 27.6 GeV electron or positron beam off hydrogen, deuterium and neon gas targets.

Time-like compton scattering and the Bethe-Heitler process

Arguments are presented for p(γ, e + e −)p measurements to obtain new information about the off-shell time-like nucleon form factors, especially in the o meson region governing the oN couplings g oNN V,T . Theoretical predictions based upon a Quantum Hadrodynamic model and vector meson dominance are highlighted for both the proton form factor and the time-like Compton scattering cross section. The Bethe-Heitler process is also calculated but is only important at low momentum transfer ¦t¦ permitting a novel high |t| φ enhancement in the Compton cross section related to nucleon strangeness to emerge.

Beam–charge asymmetry associated with DVCS at HERMES

We report the first observation of an azimuthal asymmetry in the hard electroproduction of real photons with respect to the charge of the incoming lepton beam. The asymmetry is attributed to the interference between the Bethe-Heitler process and the deeply-virtual Compton scattering process, which gives access to the latter at the amplitude level. This process appears to be the theoretically cleanest way to access generalized parton distributions. The data have been accumulated by the HERMES experiment at DESY, scattering the HERA 27.6 GeV electron/positron beam off an unpolarized hydrogen gas target.

Numerical study of pair creation by ultraintense lasers

Now that intensity of lasers has reached 1020 W/cm2, electron–positron pairs can be created by the irradiation of such ultraintense lasers on a thin gold foil. The energy of electrons produced by ultraintense lasers reaches more than several tens of MeV. Such high energy electrons become a source for creating electron–positron pairs via interaction with nuclei. There are a few processes that create electron–positron pairs in this situation. Two processes, call the trident process and the Bethe–Heitler process, are considered in this study. A numerical simulation code based on a relativistic Fokker–Planck equation is developed for studying the hot electron transport. The equation is solved by assuming one-dimensional real space and two-dimensional momentum space with axial symmetry. It is found that the total positron yield increases logarithmically with the increase of the laser intensity, and the resultant energy distribution of the created positron is found to have a peak near the energy of 1–2 MeV.

Observation of Exclusive Deeply Virtual Compton Scattering in Polarized Electron Beam Asymmetry Measurements

We report the first results of the beam spin asymmetry measured in the reaction e + p -> e + p + gamma at a beam energy of 4.25 GeV. A large asymmetry with a sin(phi) modulation is observed, as predicted for the interference term of Deeply Virtual Compton Scattering and the Bethe-Heitler process. The amplitude of this modulation is alpha = 0.202 +/- 0.028. In leading-order and leading-twist pQCD, the alpha is directly proportional to the imaginary part of the DVCS amplitude.

Diffractive production of real photons

The process eP → ePγ has been investigated by both the H1 and ZEUS collaborations at HERA. Contributions beyond the normal QED Bethe-Heitler process have been seen within the measurable kinematic range. These contributions are consistent with a calculation of Deeply Virtual Compton Scattering. A cross section, differential in Q 2 or W, has been measured by the H1 collaboration and compared with this calculation.

Bethe-Heitler process in polarized photon-nucleon interactions

We calculate the Bethe-Heitler cross section for the production of lepton pairs in the field of a longitudinally polarized nucleon, taking into account the lepton masses and the target mass. This process is a dominant background to the detection of open charm from semi-leptonic decay modes, which is a potential probe of the polarized gluon distribution in the nucleon.

Deeply virtual Compton scattering

We study in QCD the physics of deeply-virtual Compton scattering (DVCS)---the virtual Compton process in the large s and small t kinematic region. We show that DVCS can probe a new type of off-forward parton distributions. We derive an Altarelli-Parisi type of evolution equations for these distributions. We also derive their sum rules in terms of nucleon form-factors of the twist-two quark and gluon operators. In particular, we find that the second sum rule is related to fractions of the nucleon spin carried separately by quarks and gluons. We estimate the cross section for DVCS and compare it with the accompanying Bethe-Heitler process at CEBAF and HERMES kinematics.

Lepton-pair production in virtual Compton scattering off the proton

We show that lepton-pair production in virtual Compton scattering offers, through interference with the well-known Bethe-Heitler process, a sensitive probe to separate longitudinal and transverse response at positive photon invariant masses. This interference can be measured directly in terms of an asymmetry. The role of off-shell effects in the NNγ vertices is investigated as well. An additional NNγγ contact term in the amplitude, included to ensure gauge invariance of the model, cancels a substantial part of the off-shell effects, but introduces ambiguities which affect observables. The response of nucleon resonances is also investigated.

LIMITS ON THE MASS OF LIGHT (PSEUDO)SCALAR PARTICLES FROM BETHE-HEITLER e+e− and μ+μ− PAIR PRODUCTION IN A PROTON-IRON BEAM DUMP EXPERIMENT

A search has been performed for weakly interacting neutral light scalar and pseudoscalar particles in a proton-iron beam dump experiment via the Bethe-Heitler process. No positive signal was observed. Limits are derived on the masses of these particles in the framework of the Standard Model and its minimal supersymmetric extension (MSSM) comparing the experimental results with expectations for Bethe-Heitler e+e− and μ+μ− pair production. The Higgs particle of the SU2L×U1 Standard Theory is excluded for masses in the range mH<0.9 MeV at 95% CL. Limits on light Higgs particles of the MSSM and Peccei-Quinn like axions are derived.

Neutrino tridents, conserved vector current, and partially conserved axial-vector current.

The trident reaction ..nu..+Z..--> nu..+l/sup +/+l/sup -/+Z is analyzed from the viewpoint of the conservation properties of the weak lepton current. The conserved part of the current yields a cross section that is related to the Bethe-Heitler process ..gamma..+Z..-->..l/sup +/+l/sup -/+Z. The nonconserved part, with the divergence partial/sub ..mu../A/sub ..mu../ = 2m/sub l/il-bar..gamma../sub 5/l, gives an additional contribution which is expressible in terms of an axionlike reaction a/sup 0/+Z..-->..l/sup +/+l/sup -/+Z. We calculate and compare the two pieces using an equivalent-photon approximation.

A numerical formula for the total cross section of the lepton-pair production via the Bethe-Heitler process of a light pseudoscalar particle

We present in detail the calculations of the total cross section for coherent lepton-pair production of a light pseudoscalar particle off a nucleus via the Bethe-Heitler process. For future reference to experiments which are aimed to search for the pseudoscalar particle, we construct a numerical formula for the total cross section as a function of the incident energy of the pseudoscalar particle and the mass number of the target nucleus.

Ultra high energy cosmic ray neutrinos

Neutrinos, produced by the collisions of ultra high energy cosmic rays and the 3 K background radiation, require careful treatment of the evolving cosmic ray spectrum. The resulting neutrino differential energy spectrum is flat up to energies of order 10 19eV, thus most events are expected at this energy. The total ν e-flux should be significantly larger than the proton flux at ∼5 × 10 19eV when the recoil proton is correctly treated in photomeson production and when the Bethe-Heitler process (e +e − pair production) is incorporated. Based upon the observed primary proton spectrum we obtain a lower limit on the neutrino flux of ∼5.2/km 2yr sr implying roughly 0.4 detected upward moving events per year in the present Fly's Eye range of sensitivity.

The hadronless muon-pair events from charm as the Bethe-Heitler process of a weakly interacting pseudoscalar particle

The hadronless muon-pair events from CHARM may be interpreted as the Bethe-Heitler process of a weakly interacting pseudoscalar particle. We consider three cases: (i) the pseudoscalar particle is the standard axion, (ii) it is the supersymmetric U boson, and (iii) it is a hypothetical particle, which couples to the fermion number B + L or B − L.

Lepton-pair production of a light pseudoscalar particle via the Bethe-Heitler process

Bethe-Heitler processes of light pseudoscalar particles off nuclei are at present very important experimentally. For these processes we present our results which seem to differ from previous theoretical calculations found in the literature.

Photoproduction of pairs of neutral fermions with a magnetic moment in the field of a nucleus of arbitrary spin

We have investigated the electromagnetic (Bethe-Heitler) process of the formation of a pair of neutral baryons of spin 1/2 by a photon in the field of a nucleus possessing electric quadrupole and magnetic dipole and octupole moments. The cross section for photoproduction of neutron-antineutron pairs has been found as a function of the electric (Ze, Q) and magnetic (μI, Ω) multipole moments of the target nucleus. The angular distribution and energy spectrum of the neutron of the pair produced have been studied for9Be,12C, and40Ca nuclei.

Photoproduction of μ pairs and large- pT particles in QCD

We calculate cross sections for γ N → μ μ X, γ N → ( jet)X and γ N → π + X using the distribution functions for quarks in the photon predicted by QCD. The Drell-Yan contribution to the μ-pair cross section is masked by the Bethe-Heitler process in the region where it is interesting. However, the characteristic cross section for photoproduction of large- p T particles, which is predicted by QCD, is measurable.

Phenomenology of the new light Higgs boson search

We study the feasibility of two experiments to test the existence of the new light Higgs boson (Higglet) h: (1) the observation servation of Higglets produced in high energy proton reactions using the Bethe-Heitler process h + Fe → ℓ +ℓ − + Fe for detection; (2) the production of Higglets using low energy intensive electron beam with detection using the two photon decay mode.

Characteristics of charged-lepton spectra from a neutrino-produced sequential heavy lepton

The production of a charged sequential heavy lepton, ${L}^{+}$, in high-energy neutrino scattering off nucleons via the Bethe-Heitler process and its subsequent leptonic decay into an ${e}^{+}{\ensuremath{\nu}}_{e}{\overline{\ensuremath{\nu}}}_{L}$ triplet is calculated. The production cross section for a 2-GeV/${\mathit{c}}^{2}$ heavy lepton with the usual strength of Fermi coupling, ${G}_{F}$, is 9.4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}44}$ ${\mathrm{cm}}^{2}$ at 50 GeV incoming neutrino energy. Details of the ${e}^{+}$ spectra and also that of a negative muon from the incoming neutrino are presented. Among others, one characteristic feature is that ${e}^{+}$ is more energetic than the ${\ensuremath{\mu}}^{\ensuremath{-}}$ at 50 GeV incoming energy, due to threshold effects. Comparison of the single-particle spectra and also various ${\ensuremath{\mu}}^{\ensuremath{-}}{e}^{+}$ correlations with several background processes are discussed.