Photon Cross Sections at Ecm = 2 TeV

Photon production rates have been studied by the D0 and CDF experiments in Run II of the Fermilab Tevatron Collider. Measurements of the inclusive isolated photon cross section and the di-photon cross section are presented, based on integrated luminosities of 0.3 fb-1 and 0.2 fb-1, respectively. The results are compared to perturbative QCD calculations in various approximations.Comment: 4 pages 2 figures, presented at XLIrst Rencontres de Moriond on ``QCD and High Energy Hadronic Interactions'', La Thuile, Italy, March 18-25, 2006, on behalf of the CDF and D0 collaboration

Dijet azimuthal decorrelations for Δϕdijet<2π/3\Delta \phi_{\rm dijet} < 2\pi/3 in perturbative QCD

We point out an inconsistency in perturbative QCD predictions previously used for dijet azimuthal decorrelations for azimuthal angles of $\Delta\phi_{\rm dijet} < 2\pi/3$ between the two jets. We show how the inconsistency arises and how the calculations can be modified to provide more accurate results that exhibit a smaller scale dependence and give a better description of the data than the inconsistent results. We also explain how the quality of the predictions strongly depends on a perceivedly minor detail in the definition of the dijet phase space and give recommendations for future measurements.Comment: 12 pages, 4 figures, 3 tables, submitted to JHE

Direct determination of the gluon density in the proton from jet cross sections in deep-inelastic scattering

A QCD analysis of jet production in deep-inelastic scattering is presented. The studies are based on recent measurements of jet cross sections in the H1 experiment at HERA which are directly sensitive to the gluon density in the proton. The inclusion of H1 data on the inclusive deep-inelastic scattering cross section enables us to perform a consistent, simultaneous determination of the quark and gluon densities in the proton. The gluon density has been determined at fairly large values of $0.01 < x < 0.1$ at a scale of the order of the transverse jet energies \mu^2_f = 200\GeV^2. The result is consistent with indirect determinations from HERA structure function data and extends their range of sensitivity to larger $x$-values

A new quantity for studies of dijet azimuthal decorrelations

We introduce a new measurable quantity, $R_{\Delta \phi}$, for studies of the rapidity and transverse momentum dependence of dijet azimuthal decorrelations in hadron-hadron collisions. In pQCD, $R_{\Delta \phi}$ is computed as a ratio of three-jet and dijet cross sections in which the parton distribution functions cancel to a large extent. At the leading order, $R_{\Delta \phi}$ is proportional to $\alpha_s$, and the transverse momentum dependence of can therefore be exploited to determine $\alpha_s$. We compute the NLO pQCD theory predictions and non-perturbative corrections for $R_{\Delta \phi}$ at the LHC and the Tevatron and investigate the corresponding uncertainties. From this, we estimate the theory uncertainties for $\alpha_s$ determinations based on $R_{\Delta \phi}$ at both colliders. The potential of $R_{\Delta \phi}$ measurements for tuning Monte Carlo event generators is also demonstrated.Comment: 20 pages, 11 figures, 1 table, submitted to JHE

Using Jet Substructure at the LHC to Search for the Light Higgs Bosons of the CP-Violating MSSM

The CP-violating version of the Minimal Supersymmetric Standard Model (MSSM) is an example of a model where experimental data do not preclude the presence of light Higgs bosons in the range around 10 -- 110 GeV. Such light Higgs bosons, decaying almost wholly to b-bbar pairs, may be copiously produced at the LHC, but would remain inaccessible to conventional Higgs searches because of intractable QCD backgrounds. We demonstrate that a significant number of these light Higgs bosons would be boosted strongly enough for the pair of daughter $b$-jet pairs to appear as a single `fat' jet with substructure. Tagging such jets could extend the discovery potential at the LHC into the hitherto-inaccessible region for light Higgs bosons.Comment: LaTeX, 33 pages, 5 eps figures and 5 tables embedded. minor changes, to appear in Physical Review

Progress in the Understanding of Dijet Production in Deep-Inelastic Scattering

Recent results on dijet production in deep-inelastic scattering from the H1 experiment at the ep-collider HERA are presented. Internal jet structure has been studied in terms of jet shapes and subjet multiplicities in the Breit frame. Both observables are seen to be well described by QCD models. We observe a broadening of the jets towards the proton direction and at lower transverse jet energies. Dijet rates and dijet cross sections have been measured over a wide range of four momentum transfers (5 < Q^2 < 5000 GeV^2) and transverse jet energies (25 < E^2_t,Breit < 1200 GeV^2) with different jet algorithms. Perturbative QCD calculations in next-to-leading order in the strong coupling constant give a good description of the data.Comment: 8 pages, 3 figures, to appear in the proceedings of the 6th International Workshop on Deep Inelastic Scattering and QCD (DIS98), Brussels 199

Determination of the Strong Coupling Constant and Multijet Cross Section Ratio Measurements

Concepts and results of determinations of the strong coupling constant in hadron collisions are discussed. A recent alpha_s result from the inclusive jet cross section in pp-bar collisions at sqrt(s)=1.96 TeV is presented which is based on perturbative QCD calculations beyond next-to-leading order. Emphasis is put on the consistency of the conceptual approach. Conceptual limitations in the approach of extracting alpha_s from cross section data are discussed and how these can be avoided by using observables that are defined as ratios of cross sections. For one such observable, the multijet cross section ratio R3/2, preliminary results are presented.Comment: 4 pages, 2 figures, proceedings of DIS201