Search for first and second generation leptoquarks at D0

A search for first and second generation pair produced scalar leptoquarks has been done with the DO detector at Fermilab`s p{bar p} machine with {radical}s = 1.8 TeV. Leptoquarks are assumed to be strictly generational; for example, a first generation leptoquark couples only to the electron, its neutrino, and the u and d quarks. 95% C.L. mass limits of 133 GeV/c{sup 2} and 120 GeV/c{sup 2} for respective 100% and 50% decay branching ratios to electron plus quark for first generation scalar leptoquarks have been published. The preliminary results of a search for second generation scalar leptoquarks in the absence of a signal are mass limits of 111 GeV/c{sup 2} and 89 GeV/c{sup 2} for 100% and 50% decay branching ratios to muon plus quark. A feature of these mass limits is that they are independent of the unknown coupling of the leptoquark to leptons and quarks. The detection for e{sup +}e{sup {minus}} and e-p machines depends on the strength of this coupling

Quantum phases of dipolar rotors on two-dimensional lattices

The quantum phase transitions of dipoles confined to the vertices of two dimensional (2D) lattices of square and triangular geometry is studied using path integral ground state quantum Monte Carlo (PIGS). We analyze the phase diagram as a function of the strength of both the dipolar interaction and a transverse electric field. The study reveals the existence of a class of orientational phases of quantum dipolar rotors whose properties are determined by the ratios between the strength anisotropic dipole-dipole interaction, the strength of the applied transverse field, and the rotational constant. For the triangular lattice, the generic orientationally disordered phase found at zero and weak values of both dipolar interaction strength and applied field, is found to show a transition to a phase characterized by net polarization in the lattice plane as the strength of the dipole-dipole interaction is increased, independent of the strength of the applied transverse field, in addition to the expected transition to a transverse polarized phase as the electric field strength increases. The square lattice is also found to exhibit a transition from a disordered phase to an ordered phase as the dipole-dipole interaction strength is increased, as well as the expected transition to a transverse polarized phase as the electric field strength increases. In contrast to the situation with a triangular lattice, on square lattices the ordered phase at high dipole-dipole interaction strength possesses a striped ordering. The properties of these quantum dipolar rotor phases are dominated by the anisotropy of the interaction and provide useful models for developing quantum phases beyond the well-known paradigms of spin Hamiltonian models, realizing in particular a novel physical realization of a quantum rotor-like Hamiltonian that possesses an anisotropic long range interaction.Comment: Updated credit line and changed line spacin

Measurement of the top quark mass in the dilepton channel

This is the publisher's version, also available electronically from http://journals.aps.org/prd/abstract/10.1103/PhysRevD.60.052001

Measurement of the angular distribution of electrons from W⃗ eν decays observed in pp-bar collisions at s√=1.8 TeV

This is the publisher's version, also available electronically from http://journals.aps.org/prd/abstract/10.1103/PhysRevD.63.072001.We present the first measurement of the electron angular distribution parameter α(2) in W⃗ eν events produced in proton-antiproton collisions as a function of the W boson transverse momentum. Our analysis is based on data collected using the DØ detector during the 1994–1995 Fermilab Tevatron run. We compare our results with next-to-leading order perturbative QCD, which predicts an angular distribution of (1±α(1c)osθ*+α(2)cos(2)θ*), where θ* is the polar angle of the electron in the Collins-Soper frame. In the presence of QCD corrections, the parameters α(1) and α(2) become functions of pWT, the W boson transverse momentum. This measurement provides a test of next-to-leading order QCD corrections which are a non-negligible contribution to the W boson mass measurement

Hard single diffractive jet production at D0

Preliminary results from the D{null} experiment on jet production with forward rapidity gaps in {ital p{anti p}} collisions are presented. A class of dijet events with a forward rapidity gap is observed at center-of-mass energies {radical}s = 1800 GeV and 630 GeV. The number of events with rapidity gaps at both center-of-mass energies is significantly greater than the expectation from multiplicity fluctuations and is consistent with a hard single diffractive process. A small class of events with two forward gaps and central dijets is also observed at 1800 GeV. This topology is consistent with hard double pomeron exchange

Studies of WW and WZ production and limits on anomalous WWγ and WWZ couplings

This is the publisher's version, also available electronically from http://journals.aps.org/prd/abstract/10.1103/PhysRevD.60.072002.Evidence of anomalous WW and WZ production was sought in pp-bar collisions at a center-of-mass energy of s√=1.8TeV. The final states WW(WZ)→μν jet jet+X, WZ⃗ μνee+X and WZ⃗ eνee+X were studied using a data sample corresponding to an integrated luminosity of approximately 90pb-1. No evidence of anomalous diboson production was found. Limits were set on anomalous WWγ and WWZ couplings and were combined with our previous results. The combined 95% confidence level anomalous coupling limits for Λ=2TeV are -0.25<~Δκ<~0.39 (λ=0) and -0.18<~λ<~0.19 (Δκ=0), assuming the WWγ couplings are equal to the WWZ couplings