Zeroing in on more photons and gluons

We discuss radiation zeros that are found in gauge tree amplitudes for processes involving multi-photon emission. Previous results are clarified by examples and by further elaboration. The conditions under which such amplitude zeros occur are identical in form to those for the single-photon zeros, and all radiated photons must travel parallel to each other. Any other neutral particle likewise must be massless (e.g. gluon) and travel in that common direction. The relevance to questions like gluon jet identification and computational checks is considered. We use examples to show how certain multi-photon amplitudes evade the zeros, and to demonstrate the connection to a more general result, the decoupling of an external electromagnetic plane wave in the ``null zone". Brief comments are made about zeros associated with other gauge-boson emission.Comment: 26 page

Extracting W Boson Couplings from the e+e−e^{+}e^{-} Production of Four Leptons

We consider the processes $e^{+}e^{-}\rightarrow \ell^{+} \ell^{\prime -}\nu \bar{\nu}^{\prime}$, including all possible charged lepton combinations, with regard to measuring parameters characterizing the $W$ boson. We calculate at what level these processes can be used to measure anamolous triple-boson vertice coupling parameters for the cases of $e^{+}e^{-}$ colliders at 500 $GeV$ and 1 $TeV$ center of mass energies.Comment: 13 pages,OCIP/C-93-

Nuclear shadowing at low Q^2

We re-examine the role of vector meson dominance in nuclear shadowing at low Q^2. We find that models which incorporate both vector meson and partonic mechanisms are consistent with both the magnitude and the Q^2 slope of the shadowing data.Comment: 7 pages, 2 figures; to appear in Phys. Rev.

Single-photon events in e^+ e^- collisions

We provide a detailed investigation of single-photon production processes in $e^+e^-$ collisions with missing momenta carried by neutrinos or neutralinos. The transition amplitudes for both processes can be organized into a generic simplified, factorized form; each neutral V$\pm$A vector current of missing energy carriers is factorized out and all the characteristics of the reaction is solely included in the electron vector current. Firstly, we apply the generic form to give a unified description of a single-photon production with a Dirac-type or Majorana-type neutrino-pair and to confirm their identical characteristics as suggested by the so-called Practical Dirac-Majorana Confusion Theorem. Secondly, we show that the generic amplitude form is maintained with the anomalous P- and C-invariant WW$\gamma$ couplings in the neutrino-associated process and it enables us to easily understand large contributions of the anomalous WW$\gamma$ couplings at higher energies and, in particular, at the points away from the Z-resonance peak. Finally, the neutralino-associated process, which receives modifications in both the left-handed and right-handed electron currents due to the exchanges of the left-handed and right-handed selectrons, can be differentiated from the neutrino-associated ones through the left-right asymmetries and/or the circular polarization of the outgoing photon.Comment: 20 pages, REVTeX, epsfig.sty, 7 figures (7 eps files

Shadowing in Inelastic Scattering of Muons on Carbon, Calcium and Lead at Low XBj

Nuclear shadowing is observed in the per-nucleon cross-sections of positive muons on carbon, calcium and lead as compared to deuterium. The data were taken by Fermilab experiment E665 using inelastically scattered muons of mean incident momentum 470 GeV/c. Cross-section ratios are presented in the kinematic region 0.0001 < XBj <0.56 and 0.1 < Q**2 < 80 GeVc. The data are consistent with no significant nu or Q**2 dependence at fixed XBj. As XBj decreases, the size of the shadowing effect, as well as its A dependence, are found to approach the corresponding measurements in photoproduction.Comment: 22 pages, incl. 6 figures, to be published in Z. Phys.

QCD Corrections and Non-standard Three Vector Boson Couplings in W+W−W^+W^- Production at Hadron Colliders

The process p\,p\hskip-7pt\hbox{^{^{(\!-\!)}}} \rightarrow W^{+} W^{-} + X \rightarrow \ell^+_1 \nu_1 \ell^-_2 \bar \nu_2 + X is calculated to ${\cal O}(\alpha_s)$ for general $C$ and $P$ conserving $WWV$ couplings ($V=\gamma,\, Z$). The prospects for probing the $WWV$ couplings in this reaction are explored. The impact of ${\cal O}(\alpha_s)$ QCD corrections and various background processes on the observability of non-standard $WWV$ couplings in $W^+ W^-$ production at the Tevatron and the Large Hadron Collider (LHC) is discussed in detail. Sensitivity limits for anomalous $WWV$ couplings are derived at next-to-leading order for the Tevatron and LHC center of mass energies, and are compared to the bounds which can be achieved in other processes. Unless a jet veto or a cut on the total transverse momentum of the hadrons in the event is imposed, the ${\cal O}(\alpha_s)$ QCD corrections and the background from top quark production decrease the sensitivity of p\,p\hskip-7pt\hbox{^{^{(\!-\!)}}} \rightarrow W^{+} W^{-} + X \rightarrow \ell^+_1 \nu_1 \ell^-_2 \bar \nu_2 + X to anomalous $WWV$ couplings by a factor two to five.Comment: REVTEX 3, 62 pages, 21 Figures (not included available upon request), the postscript file of the complete paper is available at ftp://ucdhep.ucdavis.edu/han/ww/ww_paper.p

Q(2) dependence of nuclear shadowing

We reexamine the predictions of a two-phase model of shadowing in nuclear deep-inelastic scattering in light of new NMC data on the x, A, and Q² dependence of the ratios of structure functions. The model, which combines vector meson dominance at low Q² with diffractive Pomeron exchange for large Q², agrees with the observed small, but nonzero, slopes in logQ², which indicate the importance of higher twist shadowing effects in the transition region, 0.1≲Q²≲1 GeV². We note also that the latest E665 data on the deuteron to proton ratio suggests the presence of a small amount of shadowing in the deuteron.W. Melnitchouk, A. W. Thoma