High-energy Electron-positron Research Articles

Anti-Chudakov effect in high-energy electron–positron pair ionization loss in thin target

Abstract The problem of electron–positron pair ionization loss on different distances from its creation point in substance is considered. It is shown that together with the Chudakov effect of pair loss reduction the opposite effect of exceeding by the pair loss of the doubled value of single electron (positron) loss may take place. It is demonstrated that for pair ionization loss in the same substance where the pair is created such effect is rather small but can enhance by the order of magnitude if consider the pair loss in thin plate situated in the direction of pair motion on some distance from this substance. It is pointed out that such effect should be most significant at relatively low pair energies of several hundreds MeV.

On ionization energy losses of high-energy electron–positron pair in thin targets

The present Letter deals with consideration of high-energy electron–positron pair ionization losses in thin dielectric plate which it traverses after being emitted from substance where it is created. It is shown that in this case the Chudakov effect of reduction of pair energy losses takes place on much larger distance from its creation point than in the case of the pair motion in homogeneous infinite medium. It is demonstrated that due to transition radiation which appears during the pair emission from the substance the ratio of the pair energy losses in two plates situated on different distances from it is noticeably different from the case when the radiation is neglected.

1932, a watershed year in nuclear physics

The consequences, for good and ill, of that annus mirabilis of discovery and invention are still very much with us.

Quasiclassical approach to high-energy QED processes in strong laser and atomic fields

An approach, based on the use of the quasiclassical Greenʼs function, is developed for investigating high-energy quantum electrodynamical processes in combined strong laser and atomic fields. Employing an operator technique, we derive the Greenʼs function of the Dirac equation in an arbitrary plane wave and a localized potential. Then, we calculate the total cross section of high-energy electron–positron photoproduction in an atomic field of arbitrary charge number (Bethe–Heitler process) in the presence of a strong laser field. It is shown that the laser field substantially modifies the cross section at already available incoming photon energies and laser parameters. This makes it feasible in principle to observe with present technology the analogous effect in a laser field of the Landau–Pomeranchuk–Migdal effect for the Bethe–Heitler process.

The top quark and Higgs boson masses and the stability of the electroweak vacuum

The ATLAS and CMS experiments observed a particle at the LHC with a mass ≈126 GeV, which is compatible with the Higgs boson of the Standard Model. A crucial question is, if for such a Higgs mass value, one could extrapolate the model up to high scales while keeping the minimum of the scalar potential that breaks the electroweak symmetry stable. Vacuum stability requires indeed the Higgs boson mass to be MH≳129±1 GeV, but the precise value depends critically on the input top quark pole mass which is usually taken to be the one measured at the Tevatron, mtexp=173.2±0.9 GeV. However, for an unambiguous and theoretically well-defined determination of the top quark mass one should rather use the total cross section for top quark pair production at hadron colliders. Confronting the latest predictions of the inclusive pp¯→tt¯+X cross section up to next-to-next-to-leading order in QCD to the experimental measurement at the Tevatron, we determine the running mass in the MS¯-scheme to be mtMS¯(mt)=163.3±2.7 GeV which gives a top quark pole mass of mtpole=173.3±2.8 GeV. This leads to the vacuum stability constraint MH⩾129.4±5.6 GeV to which a ≈126 GeV Higgs boson complies as the uncertainty is large. A very precise assessment of the stability of the electroweak vacuum can only be made at a future high-energy electron–positron collider, where the top quark pole mass could be determined with a few hundred MeV accuracy.

Charge asymmetry in the differential cross section of high-energye+e−photoproduction in the field of a heavy atom

First quasiclassical correction to the differential cross section of high-energy electron-positron photoproduction in the electric field of a heavy atom is obtained with the exact account of the field. This correction is responsible for the charge asymmetry ${\cal A}$ in this process. When the transverse momentum of at least one of the produced particles is much larger than the electron mass $m$, the charge asymmetry can be as large as tens percent. We also estimate the contribution $\tilde{\cal A}$ to the charge asymmetry coming from the Compton-type diagram. For heavy nuclei, this contribution is negligible. For light nuclei, $\tilde{\cal A}$ is noticeable only when the angle between the momenta of electron and positron is of order of $m/\omega$ ($\omega$ is the photon energy) while the transverse momenta of both particles are much larger than $m$.

The International Linear Collider

AbstractA high energy electron positron collider is proposed to complement the large hadron collider, LHC, which has started taking data at CERN, Geneva, Switzerland. Based on superconducting acceleration technology the International Linear Collider ILC is offering a mature proposal. It will deliver high luminosity electron positron collisions at 500 GeV upgradable to 1 TeV. Experimentation at such a collider presents many unique challenges, if the huge physics potential should be harvested efficiently. In this paper after a brief review of the state of the accelerator proposal the state of the preparation of the experimental program is discussed.

10.1007/s11448-008-2002-8

Born differential cross sections and the lowest-order radiative correction to them are obtained in the kinematics of large-angle final photon emissions in high-energy electron-positron annihilation processes. Taking into account the possible emission of real soft and hard photons in collinear kinematics, we show the validity of the Drell-Yan form of the differential cross section with the nonsinglet structure functions of the initial leptons. The leading and next-to-leading contribution to the cross sections is obtained. The relevant numerical estimations are presented.

Pulsars as a source of the WMAP haze

The WMAP haze is an excess in the 22 to 93 GHz frequency bands of WMAP extending about 10 degrees from the galactic center. We show thatsynchrotron emission from electron-positron pairs injected into theinterstellar medium by the galactic population of pulsars withenergies in the 1 to 100 GeV range can explain the frequency spectrumof the WMAP haze and the drop in the average haze power withlatitude. The same spectrum of high energy electron-positron pairsfrom pulsars, which gives rise to the haze, may also generate theobserved excesses in AMS, HEAT and PAMELA. We discuss the spatialmorphology of the pulsar synchrotron signal and its deviation fromspherical symmetry, which may provide an avenue to determine the pulsarcontribution to the haze.

On possible interpretations of the high energy electron–positron spectrum measured by the Fermi Large Area Telescope

The Fermi-LAT experiment recently reported high precision measurements of the spectrum of cosmic-ray electrons-plus-positrons (CRE) between 20 GeV and 1 TeV. The spectrum shows no prominent spectral features, and is significantly harder than that inferred from several previous experiments. Here we discuss several interpretations of the Fermi results based either on a single large scale Galactic CRE component or by invoking additional electron–positron primary sources, e.g. nearby pulsars or particle dark matter annihilation. We show that while the reported Fermi-LAT data alone can be interpreted in terms of a single component scenario, when combined with other complementary experimental results, specifically the CRE spectrum measured by H.E.S.S. and especially the positron fraction reported by PAMELA between 1 and 100 GeV, that class of models fails to provide a consistent interpretation. Rather, we find that several combinations of parameters, involving both the pulsar and dark matter scenarios, allow a consistent description of those results. We also briefly discuss the possibility of discriminating between the pulsar and dark matter interpretations by looking for a possible anisotropy in the CRE flux.

Neutron dosimetry at a high-energy electron–positron collider†

Electron–positron colliders with energy of hundreds of MeV per beam have been employed for studies in the domain of nuclear and sub-nuclear physics. The typical structure of such a collider includes an LINAC, able to produce both types of particles, an accumulator ring and a main ring, whose diameter ranges from several tens to hundred meters and allows circulating particle currents of several amperes per beam. As a consequence of the interaction of the primary particles with targets, shutters, structures and barriers, a complex radiation environment is produced. This paper addresses the neutron dosimetry issues associated with the operation of such accelerators, referring in particular to the DAΦ NE complex, operative since 1997 at INFN–Frascati National Laboratory (Italy). Special attention is given to the active and passive techniques used for the spectrometric and dosimetric characterization of the workplace neutron fields, for radiation protection dosimetry purposes. †Paper presented as a contribution to the 10th International Symposium on Solid State Dosimetry, Mexico City (24–28 September 2007).

2γ and 3γ annihilation as calibration processes for high energy e + e − colliders

Born differential cross sections and the lowest-order radiative correction to them are obtained in the kinematics of large-angle final photon emissions in high-energy electron-positron annihilation processes. Taking into account the possible emission of real soft and hard photons in collinear kinematics, we show the validity of the Drell-Yan form of the differential cross section with the nonsinglet structure functions of the initial leptons. The leading and next-to-leading contribution to the cross sections is obtained. The relevant numerical estimations are presented.

Effects of the Cosmic Infrared Background on Delayed High‐Energy Emission from Gamma‐Ray Bursts

Regenerated high-energy emissions from gamma-ray bursts (GRBs) are studied in detail. If the intrinsic primary spectrum extends to the TeV range, these very high-energy photons are absorbed by the cosmic infrared background (CIB). Created high-energy electron-positron pairs up-scatter mainly cosmic microwave background (CMB) photons, and secondary photons are generated in the GeV-TeV range. These secondary delayed photons may be observed in the near future, which are useful for a consistency check of the intrinsic primary spectrum. In this paper, we focus on effects of the CIB on delayed secondary emissions. In particular, we show that not only up-scattered CMB photons but also up-scattered CIB ones are important, especially for low redshift bursts. They also give us additional information on the CIB, whose photon density is not definitely determined so far.

On a $Z^{\prime}$ signature at next high energy electron-positron colliders

The associated production of a $Z^{\prime}$ and a final hard photon in high energy electron-positron colliders is studied. It is shown that the hard-photon spectrum contains useful information on the $Z^{\prime}$ properties. This suggests that, if a new neutral gauge boson exists for $M_{Z^{\prime}} < \sqrt{s}$ , it will not be necessary to make a new energy run at the $Z^{\prime}$ mass in order to get most of its properties.

Hard X-Ray and Gamma-Ray Emission Induced by Ultra-High-Energy Protons in Cluster Accretion Shocks

All sufficiently massive clusters of galaxies are expected to be surrounded by strong accretion shocks, where protons can be accelerated to $\sim 10^{18}$-$10^{19}$ eV under plausible conditions. Such protons interact with the cosmic microwave background and efficiently produce very high energy electron-positron pairs, which then radiate synchrotron and inverse Compton emission, peaking respectively at hard X-ray and TeV gamma-ray energies. Characterized by hard spectra (photon indices $\sim 1.5$) and spatial distribution tracing the accretion shock, these can dominate over other nonthermal components depending on the shock magnetic field. HESS and other Cerenkov telescopes may detect the TeV emission from nearby clusters, notwithstanding its extended nature. The hard X-rays may be observable by future imaging facilities such as NeXT, and possibly also by ASTRO-E2/HXD. Such detections will not only provide a clear signature of ultra-high energy proton acceleration, but also an important probe of the accretion shock itself as well as magnetic fields in the outermost regions of clusters.

Ion backflow in the Micromegas TPC for the future linear collider

We present ion backflow measurements in a Micromegas (MICRO-MEsh GASeous detector) TPC device developed for the next high-energy electron–positron linear collider under study and a simple explanation for this backflow. A Micromegas micromesh has the intrinsic property to naturally stop a large fraction of the secondary positive ions created in the avalanche. It is shown that under some workable conditions on the pitch of the mesh and on the gas mixture, the ion feedback is equal to the field ratio (ratio of the drift electric field to the amplification field). Measurements with an intense X-ray source are in good agreement with calculations and simulations. The conclusion is that in the electric field conditions foreseen for the Micromegas TPC (drift and amplification fields, respectively, equal to 150–200 V/cm and 50–80 kV/cm) the expected ion backflow will be of the order of 2–3×10-3. In addition, measurements have been done in a 2 T magnetic field: as expected the ion backflow is not altered by the magnetic field.

Ion backflow in the Micromegas TPC for the future linear collider

We present ion backflow measurements in a Micromegas (MICRO-MEsh GASeous detector) TPC device developed for the next high-energy electron–positron linear collider under study and a simple explanation for this backflow. A Micromegas micromesh has the intrinsic property to naturally stop a large fraction of the secondary positive ions created in the avalanche. It is shown that under some workable conditions on the pitch of the mesh and on the gas mixture, the ion feedback is equal to the field ratio (ratio of the drift electric field to the amplification field). Measurements with an intense X-ray source are in good agreement with calculations and simulations. The conclusion is that in the electric field conditions foreseen for the Micromegas TPC (drift and amplification fields, respectively, equal to 150–200 V/cm and 50–80 kV/cm) the expected ion backflow will be of the order of 2–3 × 10 - 3 . In addition, measurements have been done in a 2 T magnetic field: as expected the ion backflow is not altered by the magnetic field.

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.

New heavy charged leptons at future high energy electron-positron colliders

New heavy charged lepton production and decay signatures at future electron-positron colliders are investigated at \(\sqrt {s}=500\) GeV. The consequences of model dependence for vector singlets and vector doublets are studied. Distributions are calculated including hadronization effects and experimental cuts that suppress the standard model background. The final state leptonic energy distributions are shown to give a very clear signature for heavy charged leptons.

Precision Tests of the Electroweak Theory

Electroweak data from the high energy electron–positron and proton–antiproton colliders are reviewed. On the whole the data is consistent with and supports the predictions of the electroweak theory. However, a crucial prediction of the theory remains to be verified: the existence of the Higgs boson and its light mass, less than 193 GeV, obtained from a fit to all the data within the electroweak framework. The lower limit on its mass from direct searches being 114 GeV, the mass of the Higgs is fixed within a narrow range which is expected to be explored at the Fermilab Tevatron experiments or later at the Large Hadron Collider at CERN.