Andre H. Hoang

Heavy quarkonium physics

This report is the result of the collaboration and research effort of the Quarkonium Working Group over the last three years. It provides a comprehensive overview of the state of the art in heavy-quarkonium theory and experiment, covering quarkonium spectroscopy, decay, and production, the determination of QCD parameters from quarkonium observables, quarkonia in media, and the effects on quarkonia of physics beyond the Standard Model. An introduction to common theoretical and experimental tools is included. Future opportunities for research in quarkonium physics are also discussed.

Thrust at N3LL with power corrections and a precision global fit for αs(mZ)

United States. Dept. of Energy (Director, Office of Science, Office of Nuclear Physics grant DE-FG02-94ER40818)

Jets from massive unstable particles: Top-mass determination

We construct jet observables for energetic top quarks that can be used to determine a short-distance top quark mass from reconstruction in e+e- collisions with accuracy better than LambdaQCD. Using a sequence of effective field theories we connect the production energy, mass, and top width scales, Q>>m>>Gamma, for the top jet cross section, and derive a QCD factorization theorem for the top invariant mass spectrum. Our analysis accounts for alphas corrections from the production and mass scales, corrections due to constraints in defining invariant masses, nonperturbative corrections from the cross talk between the jets, and alphas corrections to the Breit-Wigner line shape. This paper mainly focuses on deriving the factorization theorem for hemisphere invariant mass distributions and other event shapes in e+e- collisions applicable at a future linear collider. We show that the invariant mass distribution is not a simple Breit-Wigner function involving the top width. Even at leading order it is shifted and broadened by nonperturbative soft QCD effects. We predict that the invariant mass peak position increases linearly with Q/m due to these nonperturbative effects. They are encoded in terms of a universal soft function that also describes soft effects for massless dijet events. In a future paper we compute alphas corrections to the jet invariant mass spectrum, including a summation of large logarithms between the scales Q, m, and Gamma.

B decay and the Upsilon mass

Theoretical predictions for inclusive semileptonic B decay rates are rewritten in terms of the {Upsilon}(1S) meson mass instead of the b quark mass, using a modified perturbation expansion. This method gives theoretically consistent and phenomenologically useful results. Perturbation theory is well behaved, and the largest theoretical error in the predictions coming from the uncertainty in the quark mass is eliminated. The results are applied to the determination of {vert_bar}V{sub cb}{vert_bar} , {vert_bar}V{sub ub}{vert_bar} , and {lambda}{sub 1} . {copyright} {ital 1999} {ital The American Physical Society}

Top jets in the peak region: Factorization analysis with next-to-leading-log resummation

We consider top quarks produced at large energy in e+e- collisions, and address the question of what top mass can be measured from reconstruction. The production process is characterized by well-separated scales: the center-of-mass energy Q, the top mass m, the top decay width Gammat, and also LambdaQCD; scales which can be disentangled with effective theory methods. In particular we show how the mass measurement depends on the way in which soft radiation is treated, and that this can shift the mass peak by an amount of order QLambdaQCD/m. We sum large logs for Q>>m>>Gammat>LambdaQCD and demonstrate that the renormalization group ties together the jet and soft interactions below the scale m. Necessary conditions for the invariant mass spectrum to be protected from large logs are formulated. Results for the cross section are presented at next-to-leading order with next-to-leading-log (NLL) resummation, for invariant masses in the peak region and the tail region. Using our results we also predict the thrust distribution for massive quark jets at NLL order for large thrust. We demonstrate that soft radiation can be precisely controlled using data on massless jet production, and that in principle, a short-distance mass parameter can be measured using jets with precision better than LambdaQCD.

Top Mass Measurements from Jets and the Tevatron Top-Quark Mass

Theoretical issues are discussed for the measurement of the top-mass using jets, including perturbative and non-perturbative effects that relate experimental observables to the Lagrangian mass, and appropriate choices for mass schemes. Full account for these issues is given for e + e − → t t ¯ using a factorization theorem for event shapes for massive quarks. Implications for the Tevatron top-mass measurement are discussed. A mass-scheme, the “MSR-mass”, is introduced which allows for a precise description of observables sensitive to scales R ≪ m , but at the same time does not introduce perturbative matching uncertainties in conversion to the MS ¯ mass.

Designing gapped soft functions for jet production

Abstract Distributions in jet production often depend on a soft function, S , which describes soft large angle hadronic radiation between the jets. Near kinematic thresholds S encodes nonperturbative information, while far from thresholds S can be computed with an expansion in operators. We design soft functions for jet factorization theorems that serve this dual purpose, reducing to the perturbative result in the tail region and to a consistent model in the nonperturbative region. We use the MS ¯ scheme, and in both regions S displays the appropriate renormalization group scale dependence. We point out that viable soft functions in jet production should have a gap associated with the minimum soft hadronic energy deposit. This gap is connected to the leading O ( Λ QCD ) renormalon ambiguity in massless jet event shapes. By defining the gap in a suitable scheme we demonstrate that the leading renormalon can be eliminated. This improves the convergence of perturbative results, and also the stability by which nonperturbative parameters encode the underlying soft physics.

Three-loop anomalous dimension of the heavy quark pair production current in nonrelativistic QCD

The three-loop nonmixing contributions to the anomalous dimension of the leading order quark pair production current in nonrelativistic QCD are computed. It is demonstrated that the renormalization procedure can only be carried out consistently if the dynamics of both soft and ultrasoft degrees of freedom is present for all scales below the heavy quark mass and if the soft and ultrasoft renormalization scales are always correlated.

Soft-Collinear Factorization and Zero-Bin Subtractions

We study the Sudakov form factor for a spontaneously broken gauge theory using a (new) {delta}-regulator. To be well defined, the effective theory requires zero-bin subtractions for the collinear sectors. The zero-bin subtractions depend on the gauge boson mass M and are not scaleless. They have both finite and 1/{epsilon} contributions and are needed to give the correct anomalous dimension and low-scale matching contributions. We also demonstrate the necessity of zero-bin subtractions for soft-collinear factorization. We find that after zero-bin subtractions the form factor is the sum of the collinear contributions minus a soft mass-mode contribution, in agreement with a previous result of Idilbi and Mehen in QCD. This appears to conflict with the method-of-regions approach, where one gets the sum of contributions from different regions.

Charm mass determination from QCD charmonium sum rules at order

A bstractWe determine the