[en] The top quark pairs produced at a polarized muon collider are in a (nearly) pure spin configuration. This result holds for all center-of-mass energies, and is insensitive to the next-to-leading order QCD radiative corrections. The decay products of a polarized top quark show strong angular correlations. The authors describe an interesting interference effect between the left-handed and longitudinally polarized W bosons in top quark decay. This effect is easily observable in the angular distribution of the charged lepton with respect to the beam axis

[en] We compare theoretical expectations for the observation of spin correlations in top quark pair production and decay at the Fermilab Tevatron and the CERN Large Hadron Collider (Lhc). In particular, we note that the differing top quark pair production mechanisms in the two environments test different aspects of the Standard Model and require different strategies to observe the correlations. At the Tevatron, production is dominated by q q → t t and the strategy is to construct a double-decay angle distribution where one decay angle is measured in the t rest frame and the other in the f rest frame. The dominant process at the Lhc is g g → t t with a rich spin structure that allows for a second option in observing spin correlations. Here the strategy is to select events where the if pair is produced at relatively low velocity in the zero momentum frame (ZMF). For these events, there are strong azimuthal correlations between t and f decay products present. This measurement enjoys the advantage that it can be carried out in the laboratory frame.

[en] We examine multiphoton production in the electroweak sector of the standard model in the high-energy limit using the equivalence theorem in combination with spinor helicity techniques. We utilize currents consisting of a charged scalar, spinor, or vector line that radiates n photons. Only one end of the charged line is off shell in these currents, which are known for the cases of like-helicity and one unlike-helicity photons. We obtain a wide variety of helicity amplitudes for processes involving two pairs of charged particles by considering combinations of four currents. We examine the situation with respect to currents which have both ends of the charged line off shell, and present solutions for the case of like-helicity photons. These new currents may be combined with two of the original currents to produce additional amplitudes involving Higgs bosons, longitudinal Z, or neutrino pairs

[en] We apply the solution to the recursion relation for the double-off-shell quark current to the problem of computing one loop amplitudes with an arbitrary number of gluons. We are able to compute amplitudes for photon-gluon scattering, electron-positron annihilation to gluons, and gluon-gluon scattering via a quark loop in the case of like-helicity gluons. In addition, we present the result for the one-loop gluon-gluon scattering amplitude when one of the gluons has opposite helicity from the others

[en] We use the solutions to the recursion relations for double-off-shell fermion currents to compute helicity amplitudes for n-photon scattering and electron-positron annihilation to photons in the massless limit of QED. The form of these solutions is simple enough to allow all of the integrations to be performed explicitly. For n-photon scattering, we find that unless n=4 the amplitudes for the helicity configurations (+++...+) and (-++...+) vanish to one-loop order

[en] We describe a method to help the search for a light top squark (M_t+M_LSPt) at the Fermilab Tevatron. Traditional search methods rely upon a series of stringent background-reducing cuts which, unfortunately, leave very few signal events given the present data set. To avoid this difficulty, we instead suggest using a milder set of cuts, combined with a open-quotes superweight,close-quote close-quote whose purpose is to discriminate between signal and background events. The superweight consists of a sum of terms, each of which are either zero or one. The terms are assigned event-by-event depending upon the values of various observables. We suggest a method for choosing the observables as well as the criteria used to assign the values such that the superweight is open-quotes large close-quote close-quote for the supersymmetric signal and open-quotes small close-quote close-quote for the standard model background. For illustration, we mainly consider the detection of tops squarks coming from top quark decay, making our analysis especially relevant to the W+2 jets top quark sample. copyright 1997 The American Physical Society

[en] We examine multiphoton production in the electroweak sector of the standard model in the high-energy limit using the equivalence theorem in combination with spinor helicity techniques. We obtain recursion relations for currents consisting of a charged scalar, spinor, or vector line that radiates n photons. Closed form solutions to these recursion relations for arbitrary n are presented for the cases of like-helicity and one unlike-helicity photon production. We apply the currents singly and in pairs to obtain amplitudes for processes involving the production of n photons with up to two unlike helicities from a pair of charged particles. The replacement of one or more photons by transversely polarized Z bosons is also discussed

[en] We investigate the conditions under which particle multiplicities in high energy collisions are Boltzmann distributed, as is the case for hadron production in e⁺e^-, pp, ΡΡ¯ and heavy ion collisions. We show that the apparent temperature governing this distribution does not necessarily imply equilibrium (thermal or chemical) in the usual sense. We discuss an explicit example using tree level amplitudes for N photon production in which a Boltzmann-like distribution is obtained without any equilibration. We argue that the failure of statistical techniques based on free particle ensembles may provide a signal for collective phenomena (such as large shifts in masses and widths of resonances) related to the QCD phase transition. (author)

[en] We apply a generalized spin-basis analysis to associated Higgs boson production and gauge boson pair production at CERN LEP. This framework allows us to identify a choice of spin axes for the processes e⁺e^-→ZH, ZZ which leads to strikingly different correlations among the decay products, even well above threshold. This spin basis optimizes the difference in the angular correlations for these two processes. In contrast, the same distributions display little contrast when the helicity basis is used. We also apply this technique to the case of W boson pair production. copyright 1998 The American Physical Society

[en] We show how to observe sizable angular correlations between the decay products of the top quark and those of the top antiquark in top quark pair production and decay at hadron colliders. These correlations result from the large asymmetry in the rate for producing like-spin versus unlike-spin top quark pairs provided the appropriate spin axes are used. The effects of new physics at production or decay on these correlations are briefly discussed. copyright 1996 The American Physical Society