Search

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.

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.

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.

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

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

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.

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.