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A. Schiller

Publications and source records attributed to A. Schiller.

At least 181 records · Page 10Linked to original sources
arXiv1999

Helicity amplitudes for the small--angle process e^-e^{\pm}\to e^-γγ+ e^{\pm} with both photons along one direction and its cross channel

We study the small--angle double bremsstrahlung in $e^-e^\pm$ scattering for a jet kinematics where both photons move along the electron direction. This region gives the main contribution to the cross section. We present analytic expressions for all 64 amplitudes with arbitrary helicity states of the initial and final leptons and the produced photons convenient for analytic and numerical studies. The accuracy of the obtained amplitudes is given omitting only terms of the order of m^2/E_j^2, θ_j^2 and θ_j m/E_j. The helicity amplitudes for the cross channel γe^\pm \to γe^+e^- + e^\pm are given. Several limits for the helicity amplitudes of hard or soft final particles are considered.

hep-ph↗
arXiv1999

Vortex profiles and vortex interactions at the electroweak crossover

Local correlations of Z-vortex operators with gauge and Higgs fields (lattice quantum vortex profiles) as well as vortex two-point functions are studied in the crossover region near a Higgs mass of 100 GeV within the 3D SU(2) Higgs model. The vortex profiles resemble certain features of the classical vortex solutions in the continuum. The vortex-vortex interactions are analogous to the interactions of Abrikosov vortices in a type-I superconductor.

hep-lat↗
arXiv1999

Search for fourth generation quarks and leptons at the Fermilab Tevatron and CERN Large Hadron Collider

If next generations of heavy quarks and leptons exist within the standard model (SM), they can manifest themselves in Higgs boson production at the Tevatron and the LHC, before being actually observed. This generation leads to an increase of the Higgs boson production cross section via gluon fusion at hadron colliders by a factor 6-9. So, the study of this process at the Tevatron and LHC can finally fix the number of generations in the SM. Using the $WW^*$ Higgs boson decay channel, the studies at the upgraded Tevatron will answer the question about the next generation for mass values 135 GeV $\lsim M_H\lsim 190$ GeV. Studying the $τ\barτ$ channel we show its large potential for the study of the Higgs boson at the LHC even in the standard case of three generations. At the Tevatron, studies in this channel could explore the mass range 110-140 GeV.

hep-ph↗
arXiv1999

Observation of Thermodynamical Properties in the $^{162}$Dy, $^{166}$Er and $^{172}$Yb Nuclei

The density of accessible levels in the ($^3$He,$αγ$) reaction has been extracted for the $^{162}$Dy, $^{166}$Er and $^{172}$Yb nuclei. The nuclear temperature is measured as a function of excitation energy in the region of 0 -- 6 MeV. The temperature curves reveal structures indicating new degrees of freedom. The heat capacity of the nuclear system is discussed within the framework of a canonical ensemble.

nucl-ex↗
arXiv1999

The visible effect of a very heavy magnetic monopole at colliders

If a heavy Dirac monopole exists, the light-to-light scattering below the monopole production threshold is enhanced due to strong coupling of monopoles to photons. At the next Linear Collider with electron beam energy 250 GeV this photon pair production could be observable at monopole masses less than 2.5-6.4 TeV in the $e^+e^-$ mode or 3.7-10 TeV in the $γγ$ mode, depending on the monopole spin. At the upgraded Tevatron such an effect is expected to be visible at monopole masses below 1-2.5 TeV. The strong dependence on the initial photon polarizations allows to find the monopole spin in experiments at $e^+e^-$ and $γγ$ colliders. We consider the $Zγ$ production and the $3γ$ production at $e^+e^-$ and $pp$ or $p\bar{p}$ colliders via the same monopole loop. The possibility to discover these processes is significantly lower than that of the $γγ$ case.

hep-ph↗
arXiv1999

Dynamics of Topological Defects in Electroweak Theory

Embedded defects proposed long ago (Z-vortices and Nambu monopoles) have been successfully searched for in 3D equilibrium lattice studies within the standard model near the electroweak phase transition and the crossover (which follows it for realistic Higgs mass). Gauge independent lattice-vortex operators are proposed. Vortex condensation (percolation) is found to characterize the high-temperature phase. Small vortex clusters are thermally activated with non-negligible density on the low-temperature side only at higher Higgs mass, where preliminary evidence supports their semiclassical nature.

hep-ph↗
arXiv1998

Large Coulomb corrections to the $e^+e^-$ pair production at relativistic heavy ion colliders

We consider the Coulomb correction (CC) to the $e^+e^-$ pair production related to multiphoton exchange of the produced $e^{\pm}$ with nuclei. The contribution of CC to the energy distribution of $e^+$ and $e^-$ as well as to the total pair production cross section are calculated with an accuracy of the order of 1%. The found correction to the total Born cross section is negative and equal to -25% at the RHIC for Au--Au and -14% at the LHC for Pb--Pb collisions.

hep-ph↗
arXiv1998

Production of para-- and orthopositronium at relativistic heavy ion colliders

We consider the ortho-- and parapositronium production in the process $AA \to AA+$ Ps where A is a nucleus with the charge number Z. The inclusive cross section and the energy distribution of the relativistic Ps are calculated which are of primary interest from the experimental point of view. The accuracy of the corresponding cross sections is given by omitting terms $\sim (Zα)^2/L^2$ for the para--Ps and $\sim (Zα)^2/L$ for the ortho--Ps production where $L=\ln{γ^2} \approx 9$ and 16 for the RHIC and the LHC. Within this accuracy the multiphoton (Coulomb) corrections are taken into account. We show that the RHIC and the LHC will be Ps factories with a productions rate of about $10^5 ÷10^8$ relativistic Ps per day. The fraction of the ortho--Ps is expected to be of the same order as that of the para--Ps for Au--Au and Pb--Pb collisions.

hep-ph↗
arXiv1998

Z-Vortex Percolation in the Electroweak Crossover Region

We study the statistical properties of Z-vortices and Nambu monopoles in the 3D SU(2) Higgs model for a Higgs mass M_H \approx 100 GeV near and above the crossover temperature, where these defects are thermally excited. Although there is no phase transition at that strong selfcoupling, we observe that the Z-vortices exhibit the percolation transition that has been found recently to accompany the first order thermal transition that exists at smaller Higgs mass. Above the crossover temperature percolating networks of Z-vortex lines are ubiquitous, whereas vortices form a dilute gas of closed vortex loops and (Nambu) monopolium states on the low-temperature side of the crossover. The percolation temperature turns out to be roughly independent of the lattice spacing. We find that the Higgs modulus is smaller (the gauge action is larger) inside the vortices, compared to the bulk average. This correlation becomes very strong on the low-temperature side. The percolation transition is a prerequisite of some string mediated baryon number generation scenarios.

hep-lat↗
arXiv1998

Renormalization of four-fermion operators for higher twist calculations

The evaluation of the higher twist contributions to Deep Inelastic Scattering amplitudes involves a non trivial choice of operator bases for the higher orders of the OPE expansion of the two hadronic currents. In this talk we discuss the perturbative renormalization of the four-fermion operators that appear in the above bases.

hep-lat↗
arXiv1998

Nucleon form factors and O(a) Improvement

Nucleon form factors have been extensively studied both experimentally and theoretically for many years. We report here on new results of a high statistics quenched lattice QCD calculation of vector and axial-vector nucleon form factors at low momentum transfer within the Symanzik improvement programme. The simulations are performed at three kappa and three beta values allowing first an extrapolation to the chiral limit and then an extrapolation in the lattice spacing to the continuum limit. The computations are all fully non-perturbative. A comparison with experimental results is made.

hep-lat↗
arXiv1998

Wave Functions and Spectrum in Hot Electroweak Matter for Large Higgs Masses

We present results for the wave functions and the screening mass spectrum for quantum numbers $0^{++}$, $1^{--}$ and $2^{++}$ in the three-dimensional SU(2)-Higgs model near to the phase transition line below the endpoint and in the crossover region. Varying the 3D gauge couplings we study the behaviour along a line of constant physics towards the continuum limit in both phases. In the crossover region the changing spectrum of screening states versus temperature is examined showing the aftermath of the phase transition at lower Higgs mass. Different to smearing concepts we used large sets of operators with various extensions allowing to identify wave functions in position space.

hep-lat↗
arXiv1998

Production of relativistic positronium in collisions of photons and electrons with nuclei and atoms

We consider the production of ultrarelativistic positronium (Ps) in $γA \to Ps + A$ and $e A \to Ps + e A$ processes where $A$ is an atom or a nucleus with charge $Ze$. For the photoproduction of para- and ortho-Ps and the electroproduction of para-Ps we obtain the most complete description compared with previous works. It includes high order $Z α$ corrections and polarization effects. The accuracy of the obtained cross sections is determined by omitted terms of the order of the inverse Ps Lorentz factor squared. The studied high order multi-photon electroproduction of ortho-Ps dominates for the collision of electrons with heavy atoms over the bremsstrahlung production from the electron via a virtual photon proposed by Holvik and Olsen. Our results complete and correct the studies of those authors.

hep-ph↗
arXiv1998

Comment on the absence of Coulomb effects on $e^+e^-$ pair production in ultrarelativistic heavy-ion collisions

In recent works [Segev,Wells; Baltz, McLerran; Eichmann, Reinhardt, Schramm, Greiner] it was claimed that the Coulomb correction to the $e^+e^-$ production cross section in relativistic heavy-ion collisions is absent. We point out that this statement is in obvious contradiction to some well known results. We obtain large Coulomb corrections in the pair production cross section for the RHIC and LHC heavy-ion colliders.

hep-ph↗
arXiv1998

Embedded Topological Defects in Electroweak Theory: From Percolating Networks to Sphalerons

New defects (Z-vortices and Nambu monopoles) are found to become thermodynamically relevant for the broken phase near to the (weakly first order) electroweak phase transition, and below the crossover for higher Higgs mass. The symmetric phase is characterized by vortex condensation (percolation). The percolation transition persists in the crossover region. The quasiclassical nature of the vortices is supported by correlations with Higgs field and gauge field energy. Sphalerons are shown to be related to monopolium bound states.

hep-lat↗
arXiv1998

Composite operators in lattice QCD: nonperturbative renormalization

We investigate the nonperturbative renormalization of composite operators in lattice QCD restricting ourselves to operators that are bilinear in the quark fields. These include operators which are relevant to the calculation of moments of hadronic structure functions. The computations are based on Monte Carlo simulations using quenched Wilson fermions.

hep-lat↗