Search

Inelastic QED processes, the cross sections of which do not drop with increasing energy, play an important role at high-energy colliders. Such reactions have the form of two-jet processes with the exchange of a virtual photon in the t-channel. We consider them in the region of small scattering angles $m/E \lesssim \theta \ll 1$, which yields the dominant contribution to their total cross sections. A new effective method is presented and applied to QED processes with emission of real photons to calculate the helicity amplitudes of these processes. Its basic idea is similar to the well-known equivalent-lepton method. Compact analytical expressions for those amplitudes up to $e^8$ are derived omitting only terms of the order of $m^2/E^2, \theta^2$, $\theta m/E$ and higher order. The helicity amplitudes are presented in a compact form in which large compensating terms are already cancelled. Some common properties for all jet-like processes are found and we discuss their origin.

A Monte Carlo based on an exact computation of peripheral diagrams is presented. Total cross sections and angular distributions with experimental cuts are shown. This is an updated version of a previous talk at PHOTON’99 [1].

We present predictions for exclusive photoproduction of a $\Phi$ meson on proton at large transfer, where we use a quark–diquark structure model for the proton. Extrapolation from our results to lower transfers is comparable in magnitude with available data in that range. This may support the diquark model in its ability to provide, for that process, an appropriate link between diffractive physics at low transfer and the standard semiperturbative approach of hard exclusive processes at very large transfer, in which the proton recovers its three-quark structure.

In the framework of a relativistic extension of the three‐constituent–quark model of the proton, spinors of third rank in Dirac indices are used to express covariantly the spin structure of S‐state baryons of spin 1/2 or 3/2. These spinors are defined directly in terms of 4‐spinors of the Dirac space D, and classified according to the irreducible representations of both the (internal) Lorentz spin group acting on E=D×D×D, and the permutation group S3. All possible combinations of spin, flavor, and momentum distributions of quarks in S‐state baryons are considered from the point of view of S3. We also suggest the possibility of defining Dirac matrices acting on E, which may prove important in view of studying three‐quark confinement forces through an effective relativistic wave equation for the proton.

The total cross-section of the γγ → 4 leptons proces is known to have a large constant value at high energy, but a complete and exact calculation of related differential spectra does not exist yet. However a complete study of the process is important at NLC for luminosity measurements or for background estimations. We here report on our various steps in that direction, including the implementation of a first new calculation of helicity amplitudes in a Monte-Carlo generator.

Considering the proton either as a three-quark state with its wave function fulfilling QCD sum rules or as a quark-diquark state with wave function parametrizations that are shown to fitGMP, we study the possibility of discriminating between these two competing pictures by comparing their respective predictions for\(J/\Psi \to p\bar p\gamma \) and\(\Upsilon (1S) \to p\bar p\gamma \) decays. ForJ/Ψ decay, thep\(\bar p\) invariantmass distributions derived from either model are found to be of about the same magnitude, but neither of them is able by itself to reproduce present-day data. Differences between the two pictures may eventually show up in the case of Υ(1S) decay, but the predicted counting rates are small. We also consider some other possible production mechanisms for\(p\bar p\gamma \) final states.

High-pT lepton pair production under the conditions of HERA is studied in order to compare the respective contributions of several different production mechanisms, among them the γγ and the Drell-Yan effects. Various distributions are considered. It is shown that the γγ process systematically and largely dominates. Therefore this type of experiment may allow one to determine the equivalent-photon spectrum of the proton.

We here discuss the problem of back-factorization in γγ collision processes; i. e., how to extract, from the experimental measurement of a reaction e e → e e X' the information on the physically interesting process γγ → X in the form of only one term or very few terms (cross section, polarization terms, structure functions). Back-factorization procedures are basically associated with the equivalent-photon approximation.

A comparison is made between various equivalent-photon formulas. It is explained, and shown numerically, why the Dalitz-Yennie (or Brodsky Kinoshita-Terazawa) version of the equivalent-photon approximation becomes unappropriate for photon-photon collision processes when the scattered electrons are tagged at finite angles.

The loop level lepton flavor violating signals $\gamma \gamma \to \ell \ell' (\ell=e,\mu,\tau, \ell \neq \ell^\prime)$ are studied in a scenario of low-energy, R-parity conserving, supersymmetric seesaw mechanism within the context of a high energy photon collider. Lepton flavor violation is due to off diagonal elements in the left s-lepton mass matrix induced by renormalization group equations. The average slepton masses ${\widetilde{m}}$ and the off diagonal matrix elements $\Delta m$ are treated as model independent free phenomenological parameters in order to discover regions in the parameter space where the signal cross section may be observable. At the energies of the $\gamma \gamma$ option of the future high-energy linear collider the signal has a potentially large standard model background, and therefore particular attention is paid to the study of kinematical cuts in order to reduce the latter at an acceptable level. We find, for the ($e\tau$) channel, non-negligible fractions of the parameter space ($\delta_{LL}=\Delta m^2/\widetilde{m}^2 \gtrsim 10^{-1}$) where the statistical significance ($SS$) is $SS \gtrsim 3$.

The loop level lepton flavor violating signals {gamma}{gamma}{yields}ll{sup '} (l=e, {mu}, {tau}, l{ne}l{sup '}) are studied in a scenario of low-energy, R-parity conserving, supersymmetric seesaw mechanism within the context of a high-energy photon collider. Lepton flavor violation is due to off-diagonal elements in the left slepton mass matrix induced by renormalization group equations. The average slepton masses m-tilde and the off-diagonal matrix elements {delta}m are treated as model-independent free phenomenological parameters in order to discover regions in the parameter space where the signal cross section may be observable. At the energies of the {gamma}{gamma} option of the future high-energy linear collider the signal has a potentially large standard model background, and therefore particular attention is paid to the study of kinematical cuts in order to reduce the latter to an acceptable level. We find, for the (e{tau}) channel, non-negligible fractions of the parameter space ({delta}{sub LL}={delta}m{sup 2}/m {approx}{sup 2} > or approx. 10{sup -1}) where the statistical significance (SS) is SS > or approx. 3.

High energy photon colliders (γγ,γe) are based on e-e-linear colliders where high energy photons are produced using Compton scattering of laser light on high energy electrons just before the interaction point. This paper is a part of the Technical Design Report of the linear collider TESLA.1Physics program, possible parameters and some technical aspects of the photon collider at TESLA are discussed.

As continuation of our previous paper we further develop our new method for calculating helicity amplitudes of jet-like QED processes described by tree diagrams, applying it to lepton pair production. This method consists in replacing spinor structures for real and weakly virtual intermediate leptons by simple transition vertices. New vertices are introduced for the pair production case, and previous bremsstrahlung vertices are generalized to include virtual photons inside the considered jet. We present a diagrammatic approach that allows to write down in an efficient way the leading helicity amplitudes, at tree level. The obtained compact amplitudes are particularly suitable for numerical calculations in jet-like kinematics. Several examples with up to four particles in a jet are discussed in detail., Comment: 11 pages, 11 figures, Latex, Springer-Latex macros included

The lepton number violating signal of like-sign-dileptons (LSD), p p \to \ell^\pm\ell^\pm + 2 jets, is investigated within a model of mixing in the neutrino sector assuming the existence of heavy Majorana neutrino states with a left--handed coupling to the light leptons. The LSD signal receives contributions both from the resonant production of a heavy Majorana neutrino (N) and from the exchange of a virtual N in the WW fusion mechanisms. These two possibilities are discussed in detail and compared. Helicity amplitudes are given pointing out differences with calculations previously reported by other authors. The signal cross--sections are computed at the energy of the LHC collider (sqrt{S}=14 TeV) at CERN and within the existing experimental limits on the mixing couplings, including those coming from neutrinoless double beta decay. Detailed angular distributions of signal reactions which are complementary to previous studies on the argument are presented., Comment: 27 pages, 7 figures, Revtex3

This paper deals with the production of orthodimuonium (OM) $({\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ atom in triplet state) in collisions of high-energy electrons with nuclei or atoms. This reaction was previously studied by Holvik and Olsen [Phys. Rev. D 35, 2124 (1987)] on the basis of a bremsstrahlung mechanism where OM is produced by only one virtual photon. In the present paper, we consider a competing three-photon mechanism where the production of OM results from the collision of a photon generated by the electron with two photons emitted by the nucleus. We derive the corresponding energy spectrum and production rate of OM and show that the three-photon mechanism is the dominant one for a heavy atom target.

The production of Like-Sign-Di-leptons (LSD), in the high energy lepton number violating (ΔL=+2) reaction, p p --> 2jets + l+l+ (l=e,\mu,\tau) of interest for the experiments to be performed at the forthcoming Large Hadron Collider (LHC), is investigated in detail, taking up a composite model scenario in which the exchanged virtual composite neutrino is assumed to be a Majorana particle that couples to the light leptons via the SU(2)×U(1) gauge bosons through a magnetic type coupling (σμν). An helicity projection method is used to evaluate exactly the tree-level amplitudes of the contributing parton subprocesses (2→4), which allows to take into account all exchange diagrams and occurring interferences. Numerical estimates of the corresponding signal cross-section that implement kinematical cuts needed to suppress the Standard Model background, are presented which show that in some regions of the parameter space the total number of LSD events is well above the background. Assuming non-observation of the LSD signal it is found that LHC would exclude a composite Majorana neutrino up to 850 GeV (if one requires 10 events for discovery). The sensitivity of LHC experiments to the parameter space is then compared to that of the next generation of neutrinoless double beta decay (ββ0ν) experiment, GENIUS, and it is shown that they will provide constraints of the same order of magnitude and will play a complementary role.

The production of Like-Sign-Di-leptons (LSD), in the high energy lepton number violating ($\Delta L = +2$) reaction, p p --> 2jets + l+l+ (l=e,\mu,\tau) of interest for the experiments to be performed at the forthcoming Large Hadron Collider (LHC), is investigated in detail, taking up a composite model scenario in which the exchanged virtual composite neutrino is assumed to be a Majorana particle that couples to the light leptons via the $SU(2)\times U(1)$ gauge bosons through a magnetic type coupling ($\sigma_{\mu\nu}$). An helicity projection method is used to evaluate exactly the tree-level amplitudes of the contributing parton subprocesses ($2\to 4$), which allows to take into account all exchange diagrams and occurring interferences. Numerical estimates of the corresponding signal cross-section that implement kinematical cuts needed to suppress the Standard Model background, are presented which show that in some regions of the parameter space the total number of LSD events is well above the background. Assuming non-observation of the LSD signal it is found that LHC would exclude a composite Majorana neutrino up to 850 GeV (if one requires 10 events for discovery). The sensitivity of LHC experiments to the parameter space is then compared to that of the next generation of neutrinoless double beta decay ($\beta\beta_{0\nu}$) experiment, GENIUS, and it is shown that they will provide constraints of the same order of magnitude and will play a complementary role., Comment: Revtex style, 31 pages, 8 postscript figures. Few typos corrected

Active Galactic Nuclei are considered as sources of neutrinos, with neutrino energies extending up to 10^{18} eV. It is expected that these highly energetic cosmic neutrinos will be detected by the neutrino telescopes, presently under construction. The detection process is very sensitive to the total muon neutrino cross-section. We examine how the total cross section changes at high energies, by the single production of excited fermions (excited muon and muon-neutrino). For parameters (masses, couplings) of the excited fermions allowed by the experimental constraints, we find that for energies of the incoming muon-neutrino above 100 TeV the cross-section for single production of (excited muon and muon-neutrino) supersedes the standard total cross-section., 12 pages and 2 figures; typset using revtex; postscript files for the figures provided

We study in detail the contribution of heavy composite Majorana neutrinos to neutrino-less double beta decay. Our analysis confirms the result of a previous estimate by two of the authors. Excited neutrinos couple to the electroweak gauge bosons through a magnetic type effective Lagrangian. The relevant nuclear matrix element is related to matrix elements available in the literature and current bounds on the half-life of neutrino-less double beta decay are converted into bounds on the compositeness scale and/or the heavy neutrino mass. Our bounds are of the same order of magnitude as those available from accelerator experiments., Comment: 26 pages, 3 figures. RevTex file and 3 postscript figures. Numerical error corrected. This implies a slight change on the numerical results and in the figures. The conclusions are not affected. Uses an updated experimental input relative to the version to be published by the Physical Review D

Using the general helicity formula for $\gamma^* \gamma^*$ collisions, we are showing that it should be possible to determine a number of independent ``structure functions'', i.e. linear combinations of elements of the two-photon helicity tensor, through azimuthal correlations in two-body or quasi two-body reactions induced by the photon-photon interaction, provided certain experimental conditions are satisfied. Numerical results of our computations are presented for some particular processes and dynamic models., Comment: 23 pages; Revtex file; Postscript files of the 6 figures appended in a uuencoded file

It is shown that photon-gluon scattering via the quark-box diagram, which may be considered a critical test of higher-order quantum chromodynamics, should become a predominant and measurable effect in an experiment designed to study the reaction ep → e + (high- p T ) γ + (high- p T ) jet + X under certain kinematic conditions at a superhigh-energy electron-proton storage ring such as HERA.

We show that it is possible, in principle, to study the scattering between an "almost real" muon or pion and an electron in an e– e+ colliding beam experiment where one large-angle electron and one large-angle muon or pion would be measured in the final state. In the muon case, the counting rate should be high enough to allow this experiment to be performed with electron-positron storage rings of the next generation.

The contribution of higher-twist mechanisms to π+ π−,K+K− and ρ° ρ° production in high-energy\(\bar p\)p collisions, with the two mesons emitted back to back at 90° in the c.m. frame and at moderate or largepT, has been calculated and compared with the minimal-twist contribution, i.e. double jet fragmentation. The signal/background ratio is shown to depend critically on Δp, a parameter defining the upper limit for non-detection of accompanying particles; and, to a lesser extent, on the meson wave function. It is shown that, while there is little hope for checking higher-twist contributions in pion pair production, such contributions may show up clearly in kaon and rho pair production. In the last case, it may be possible to use polarization effects, in addition to cross-section measurements, in order to show the occurrence of higher-twist mechanisms.

For lepton pair production in nucleon-nucleon collisions, the mechanism of ..gamma gamma.. materialization is compared with that of quark-antiquark annihilation (Drell-Yan process). The predictions compared show that, whereas the ..gamma gamma.. background of the Drell-Yan process should not exceed a few percent in present accelerator experiments, it might become an almost 100% effect with future proton-proton colliding beams of superhigh energies (s approx. 10/sup 6/ GeV/sup 2/) in the measurable range of lepton pair masses. It is also shown that the rate of the ..gamma gamma.. effect should be almost independent of the particular model chosen for the inelastic structure function, and that the double equivalent-photon approximation works very well in computing that effect.

It is shown that, in first approximation, radiative corrections in photon-photon collision experiments without electron tagging can be estimated by using a double equivalent-photon approximation of the same type as (but, of course, analytically much more complicated than) that used for computation of $\ensuremath{\gamma}\ensuremath{\gamma}$ cross sections without radiative corrections. It is obvious that, in such an approximation, radiative corrections\char22{}for a given beam energy ${E}_{0}$ and a given invariant mass $M$ produced\char22{}become independent of the specific process $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}X$ considered. The invariant-mass spectrum, corrected for radiation, will be written in the general form $\frac{d{\ensuremath{\sigma}}^{\mathrm{corr}}}{\mathrm{dM}}=(1+\ensuremath{\delta})\frac{d{\ensuremath{\sigma}}^{0}}{\mathrm{dM}}$ where $\frac{d{\ensuremath{\sigma}}^{0}}{\mathrm{dM}}$ is the uncorrected spectrum. Values obtained for $\ensuremath{\delta}$ at typical beam energies ${E}_{0}=1.5, 3, 15, \mathrm{and} 70$ GeV, and for $\frac{M}{{E}_{0}}$ ranging between 0.1 and 0.6, are systematically of the order of less than \ifmmode\pm\else\textpm\fi{} 1%.

In the framework of the helicity method, we propose a simple way of analyzing in first approximation the brute data of a ..gamma gamma.. collision experiment in the deep-inelastic configuration in terms of structure functions pertaining to the virtual ..gamma gamma.. interaction. The conditions of such an approximation are defined, and its validity is numerically checked for a wide variety of values of the kinematic parameters. One concludes that, in order to make the back-factorization procedure precise enough, tagging angles of the forward-scattered electrons should not exceed a few mrad.

It is shown that the Dalitz-Yennie version of the equivalent-photon approximation, originally established for electroproduction in the target rest frame, becomes inappropriate, to some extent, for application to ..gamma gamma.. collision experiments in electron storage rings, when the outgoing electrons are tagged at finite angles. For this type of experiment, a new Dalitz-Yennie-type formula is given, the use of which is, however, restricted in principle to single-tagging measurements. In the configurations thus considered, quantitative differences between the original and the new Dalitz-Yennie-type formula may become significant; a numerical comparison shows indeed that the new formula definitely provides a better approximation, with respect to an ''effective-photon spectrum'' derived from the exact calculation for various processes, than the former one.

In the framework of the helicity formalism, we show that single-tagged events in ..gamma gamma.. experiments performed with electron storage rings involving finite-angle tagging systems may be analyzed, with a very good accuracy, as electroproduction on a free photon target, i. e., through a four- or six-term formula.

Reactions hh→γγX are computed, assuming both photons to be omitted, with equal and opposite momenta, at 90° in the overall c.m. frame. Contributions of qq and gg interactions are considered. Such measurements might provide a check of gluon distribution functions.

For electron-positron colliding-beam processes of the type e−e+ → e−e+, where one of the e± particles and the photon are detected at large angle, a number of theoretical predictions are given and compared with some data of a recent experiment performed at Orsay.

Radiative corrections of order α are computed for an experiment measuring the photon's hadronic or leptonic structure function F 2 λ ( x , Q 2 ). For the hard-photon contribution the peaking approximation is used. The total correction, for a typical experiment performed with present high-energy e + e − colliding-beam machines (PETRA, PEP), appears to vary with x and Q 2 , ranging between minus and plus a few percent. It tends to increase in absolute value with the beam energy.

Direct-photon pair production in high-energy hadron collisions is considered. After discussing general aspects of such reactions and giving a brief historical survey of the subject, we present some calculations on the contributions from $q\overline{q}$ and $\mathrm{gg}$ collisions (the latter via the quark box) to the $\ensuremath{\gamma}\ensuremath{\gamma}$ continuum, and on possible resonant contributions. Finally, an estimation of the indirect-photon background (mainly due to ${\ensuremath{\pi}}^{0}$ and $\ensuremath{\eta}$ decay, and to quark bremsstrahlung) is given for colliding-beam conditions at high energy; assuming both photons to be measured at 90\ifmmode^\circ\else\textdegree\fi{} with equal and opposite momenta, and (within experimental limits) unaccompanied by any hadrons or additional photons, it is shown that this background can be sharply reduced.