Multiple Interactions in Two-Photon Collisions

We compute cross sections for events where two pairs of partons scatter off each other in the same $\gamma\gamma$ reaction, giving rise to at least 3 high--{\mbox{$p_T^{}$}} jets. Unlike in {\mbox{$p \bar p$}}\ collisions we find the signal to lie well above the background from higher order QCD processes. If the usual ``eikonaliztion" assumption is correct, the signal should be readily observable at LEP2, and might already be detectable in data taken at TRISTAN.Comment: 8 pages, plain LaTeX, 2 figures (not included). A compressed PS file of the entire paper, including figures, can be obtained via anonymous ftp from ftp://phenom.physics.wisc.edu/pub/preprints/1995/madph-95-921.ps.

Longitudinally Polarized Photoproduction of Inclusive Hadrons Beyond the Leading Order

We present a complete next-to-leading order QCD calculation for single-inclusive large-pT hadron production in longitudinally polarized lepton-nucleon collisions, consistently including ``direct'' and ``resolved'' photon contributions. This process could be studied experimentally at a future polarized lepton-proton collider like eRHIC at BNL. We examine the sensitivity of such measurements to the so far completely unknown parton content of circularly polarized photons.Comment: 15 pages, 7 eps figure

Crystal and magnetic structure of the oxypnictide superconductor LaO(1-x)FxFeAs: evidence for magnetoelastic coupling

High-resolution and high-flux neutron as well as X-ray powder-diffraction experiments were performed on the oxypnictide series LaO(1-x)FxFeAs with 0<x<0.15 in order to study the crystal and magnetic structure. The magnetic symmetry of the undoped compound corresponds to those reported for ReOFeAs (with Re a rare earth) and for AFe2As2 (A=Ba, Sr) materials. We find an ordered magnetic moment of 0.63(1)muB at 2 K in LaOFeAs, which is significantly larger than the values previously reported for this compound. A sizable ordered magnetic moment is observed up to a F-doping of 4.5% whereas there is no magnetic order for a sample with a F concentration of x=0.06. In the undoped sample, several interatomic distances and FeAs4 tetrahedra angles exhibit pronounced anomalies connected with the broad structural transition and with the onset of magnetism supporting the idea of strong magneto-elastic coupling in this material.Comment: 8 pages, 7 figures, regular articl

Higgs and Z boson decays into light gluinos

We calculate the decay rate of scalar and pseudoscalar Higgs bosons into a pair of gluinos, within the Minimal Supersymmetric Standard Model. In the theoretically and experimentally allowed light gluino window, \mg \sim 3--5 GeV, gluino pairs can completely dominate the decays of the light scalar Higgs boson and play a prominent role in the decay of the pseudoscalar Higgs boson. This would alter the limits obtained from $Z$ decays on the lightest CP--even and CP--odd Higgs bosons, and could jeopardize the search for these Higgs particles at future hadron colliders. In contrast, the branching ratio for the two--body decay of $Z$ bosons into pairs of light gluinos is less than 0.1\%.Comment: Latex file, 16 pages of text. 8 uufiled postscript figures included. Compressed postscript version with figures available by anonymous ftp at ftp://phenom.physics.wisc.edu/pub/preprints/current/madph-94-853.ps.

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Looking for the Charged Higgs Boson

This review article starts with a brief introduction to the charged Higgs boson (H^\pm) in the Minimal Supersymmetric Standard Model (MSSM). It then discusses the prospects of a relatively light H^\pm boson search via top quark decay at Tevatron/LHC, and finally a heavy H^\pm boson search at LHC. The viable channels for H^\pm search are identified in both the cases, with particular emphasis on the H^\pm --> tau + nu decay channel. The effects of NLO QCD correction in the SM as well as the MSSM are discussed briefly.Comment: 17 pages with 8 eps figures, Invited review, Reference adde

Scrutinizing LSP Dark Matter at the LHC

We show that LHC experiments might well be able to determine all the parameters required for a prediction of the present density of thermal LSP relics from the Big Bang era. If the LSP is an almost pure bino we usually only need to determine its mass and the mass of the SU(2) singlet sleptons. This information can be obtained by reconstructing the cascade $\tilde{q}_L \to \tilde{\chi}_2^0 q \to \tilde{\ell}_R \ell q \to \tilde{\chi}_1^0 \ell^+ \ell^- q$. The only requirement is that $m_{\tilde{\ell}_R} < m_{\tilde{\chi}_2^0}$, which is true for most of the cosmologically interesting parameter space. If the LSP has a significant higgsino component, its predicted thermal relic density is smaller than for an equal--mass bino. We show that in this case squark decays also produce significant numbers of $\tilde{\chi}_4^0$ and $\tilde{\chi}_2^\pm$. Reconstructing the corresponding decay cascades then allows to determine the higgsino component of the LSP

η\eta Production in Peripheral Heavy-Ion Collisions

We estimate the impact parameter dependence of the production cross section for $\eta_c$ and $\eta_b$ mesons in peripheral heavy-ion collisions collisions. Total and elastic $\gamma\gamma$ cross sections are calculated in an equivalent photon approximation.Comment: 9 pages, uuencoded postscrip

High-pT pi0 Production with Respect to the Reaction Plane Using the PHENIX Detector at RHIC

The origin of the azimuthal anisotropy in particle yields at high pT (pT > 5 GeV/c) in RHIC collisions remains an intriguing puzzle. Traditional flow and parton energy loss models have failed to completely explain the large v2 observed at high pT. Measurement of this parameter at high pT will help to gain an understanding of the interplay between flow, recombination and energy loss, and the role they play in the transition from soft to hard physics. Neutral mesons measured in the PHENIX experiment provide an ideal observable for such studies. We present recent measurements of \piz yields with respect to the reaction plane, and discuss the impact current models have on our understanding of these mechanisms.Comment: Contribnution to the proceedings of Hot Quarks 2006, 15-20 May 2006, Villasimius, Sardini

Radiative Electroweak Breaking with Pseudogoldstone Higgs Doublets

We consider a realistic example of supersymmetric grand unification based on $SU(3)_c \times SU(3)_L \times SU(3)_R$ in which the electroweak (EW) higgs doublets are `light' as a consequence of the `pseudogoldstone' mechanism. We discuss radiative EW breaking in this model, exploring in particular the `small' (order unity) and `large' $(\approx m_t/m_b)$ $\tan \beta$ regions by studying the variations of $r (\equiv \sqrt{\mu^2_{1,2}/\mu^2_3})$, where $\mu^2_{1,2,3}$ are the well known MSSM parameters evaluated at the GUT scale. For $r$ sufficiently close to unity the quantity $\tan \beta$ can be of order unity, but the converse is not always true.Comment: 18 pages plain LaTeX (to be run twice) and 11 figures available separately from uuencoded file