Spin correlations in e^+e^-\to 4 jets

Existence of discrepancies between experimental data and Monte Carlo predictions for angular distributions in four-jet production via annihilation has been known for some time at LEP1. As such QCD processes constitute significant backgrounds to -jet production at LEP2, we consider the possibility that an erroneous modelling of the helicity structure of the final-state partons could affect the accuracy of experimental measurements of the boson parameters. Received: 21 September 1998 / Revised version: 20 January 1999 / Published online: 22 March 1999     

Acoustic phonon anomalies and superstructure of two NbZr alloys

The anomalous dispersion of the [001] transverse phonon branch of Nb_0.9Zr_0.1 and Nb_0.8Zr_0.2 has been measured by neutron inelastic scattering. The anomaly is found to become less pronounced as the Zr content of the alloy is increased. Although the anomaly is strongly temperature-dependent it is not obviously affected by the superconducting transition. Diffraction measurements with the Nb_0.9Zr_0.1 sample show the diffuse scattering which Wakabayashi has recently reported to occur at wave-vectors around [$\text{3}\text{4}$,$\text{3}\text{4}$,$\text{3}\text{4}$] in a 12% alloy. The Nb_0.8Zr_0.2 crystal however displays unbroadened superlattice peaks at wavevectors [$\text{3}\text{4}$,$\text{3}\text{4}$,$\text{3}\text{4}$ in some Brillouin zones.     

Design of Trifluoroalkenyl Iodonium Salts for a Hypervalency‐Aided Alkenylation–Cyclization Strategy: Metal‐Free Construction of Aziridine Rings

The synthesis of fluorinated compounds and their use as pharmaceutical ingredients or synthetic building blocks have been in the focus of chemical and medicinal research. However, the efficient synthesis of trifluoromethylated nitrogen heterocycles is sometimes challenging. Herein, we disclose a simple aziridination process that relies on the use of amines and novel alkenyl iodonium reagents for the synthesis of strained, trifluoromethylated heterocycles. With the utilization of a newly designed and bench‐stable but highly reactive hypervalent alkenyl iodonium species, these three‐membered‐ring heterocyclic compounds can be efficiently constructed from simple amines under mild conditions in the absence of transition‐metal catalysts. The special reactivity of the new trifluoropropenyl synthon towards nucleophilic centers could be exploited in more general cyclization and alkenylation reactions in the future.     

Eikonal regime of gravity-induced scattering at higher energy proton colliders

We compute transplanckian parton scattering in flat extra-dimensional theories at the LHC and at the recently discussed high-energy upgrade (HE LHC). We report new leading-order calculations of the QCD background. We apply appropriate cuts to satisfy the necessary conditions for the eikonal approximation to be valid while at the same time maximising the signal to background ratio at LHC energies. We study the computability of the eikonal signal and consider the effect of a possible 33 TeV high-energy upgrade to the LHC, which serves to extend the calculable region and to enhance the signal to background ratio.     

Diffractive $\gamma \gamma$ production at hadron colliders

We compute the cross section for exclusive double-diffractive production at the Tevatron, , and the LHC. We evaluate both the gg and -channel exchange contributions to the process. The observation of exclusive production at the Tevatron will provide a check on the model predictions and offer an opportunity to confirm the expectations for exclusive double-diffractive Higgs production at the LHC.Received: 10 September 2004, Revised: 8 October 2004, Published online: 1 December 2004     

On radiative QCD backgrounds to exclusive H→bb̄ production at the LHC and a photon collider

Central exclusive Higgs boson production, pp→p⊕H⊕p, at the LHC and s-channel resonant Higgs production in the photon-collider option of the ILC can provide a very important contribution to the comprehensive study of the Higgs sector. Especially attractive is the bb̄ Higgs decay mode, which for certain MSSM scenarios may become the discovery channel in exclusive Higgs production at the LHC and the photon collider (PC). Strongly suppressed and controllable backgrounds is an obvious requirement for the success of these exclusive measurements. One of the main sources of background comes from additional gluon radiation which leads to a three-jet bb̄g final state. We perform an explicit calculation of the subprocesses gg→qq̄g and γγ→qq̄g, where the incoming particles are required to be in a J_z=0 state and the two gluons form a colour singlet, and investigate the salient properties of these potentially important background processes.     

Central exclusive χ c meson production at the Tevatron revisited

Motivated by the recent experimental observation of exclusive χ _c events at the Tevatron, we revisit earlier studies of central exclusive scalar χ _c0 meson production, before generalising the existing formalism to include χ _c1 and χ _c2 mesons. Although χ _c0 production was previously assumed to be dominant, we find that the χ _c1 and χ _c2 rates for the experimentally considered χ _c →J/ψ γ→μ ⁺ μ ^? γ decay process are in fact comparable to the χ _c0 rate. We have developed a new Monte Carlo event generator, SuperCHIC, which models the central exclusive production of the three χ _c states via this decay chain, and have explored possible ways of distinguishing them, given that their mass differences are not resolvable within the current experimental set-up. Although we find that the severity of current experimental cuts appears to preclude this, the acceptance does not change crucially between the three states and so our conclusions regarding the overall rates remain unchanged. This therefore raises the interesting possibility that exclusive χ _c1 and χ _c2 production has already been observed at the Tevatron.     

Magnetic excitations in neodymium,a sinusoidally modulated antiferromagnet

We report the first unambiguous observations of the magnetic excitations in neodymium metal, which exhibits complex antiferromagnetic structures below T_N = 20 K. The wavevector and temperature dependence of the excitations have been determined from neutron inelastic scattering measurements made at the Institut Laue-Langevin. Two district, low-energy, magnetic modes are observed for the [100] basal plane direction (TM) whilst there is only a single mode in the c-direction (TA) These modes are only observed clearly at low temperatures (T?8 K): they broaden and soften as the temperature is raised through T_N. Measurements were also made of the longitudinal and transverse acoustic phonon spectra in neodymium. The predictions of current theories of the excitations in sinusoidally modulated magnetic structures are discussed and compared with the results of our observations.