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T. Teubner

Publications and source records attributed to T. Teubner.

At least 55 records · Page 3Linked to original sources
arXiv2001

The Threshold t-tbar Cross Section at NNLL Order

The total cross section for top quark pair production close to threshold in e+e- annihilation is investigated. Details are given about the calculation at next-to-next-to-leading logarithmic order. The summation of logarithms leads to a convergent expansion for the normalization of the cross section, and small residual dependence on the subtraction parameter nu. A detailed analysis of the residual nu dependence is carried out. A conservative estimate for the remaining uncertainty in the normalization of the total cross section from QCD effects is $\lesssim \pm 3%$. This makes precise extractions of the strong coupling and top width feasible, and further studies of electroweak effects mandatory.

hep-ph↗
arXiv2001

A Renormalization Group Improved Calculation of Top Quark Production near Threshold

The top quark cross section close to threshold in $e^+e^-$ annihilation is computed including the summation of logarithms of the velocity at next-to-next-to-leading-logarithmic order in QCD. The remaining theoretical uncertainty in the normalization of the total cross section is at the few percent level, an order of magnitude smaller than in previous next-to-next-to-leading order calculations. This uncertainty is smaller than the effects of a light standard model Higgs boson.

hep-ph↗
arXiv2000

Skewed Parton Distributions and F_2^D at beta -> 1

We show that the diffractive structure function is perturbatively calculable in the domain where the diffractive mass is small but still outside the resonance region. In this domain, which can be characterized by Lambda^2/Q^2 << 1-beta << (Lambda^2/Q^2)^1/2, the structure function represents a new observable, which is highly sensitive to the small-x skewed gluon distribution. Our leading order calculation and the estimate of next-to-leading order corrections are consistent with available data and demonstrate the potential of more precise data to put further constraints on skewing effects.

hep-ph↗
arXiv2000

Top-Antitop Pair Production close to Threshold: Synopsis of recent NNLO Results

Using non-relativistic effective theories, new next-to-next-to-leading order (NNLO) QCD corrections to the total $t\bar t$ production cross section at the Linear Collider have been calculated recently. In this article the NNLO calculations of several groups are compared and the remaining uncertainties are discussed. The theoretical prospects for an accurate determination of top quark mass parameters are discussed in detail. An outlook on possible future improvements is given.

hep-ph↗
arXiv1999

Q^2 dependence of diffractive vector meson electroproduction

We give a general formula for the cross section for diffractive vector meson electroproduction, gamma^* p -> Vp. We first calculate diffractive qqbar production, and then use parton-hadron duality by projecting out the J^P = 1^- state in the appropriate mass interval. We compare the Q^2 dependence of the cross section for the diffractive production of rho and J/psi mesons with recent HERA data. We include the characteristic Q^2 dependence associated with the use of the skewed gluon distribution. We give predictions for sigma_L/sigma_T for both rho and J/psi production.

hep-ph↗
arXiv1999

Theory of Elastic Vector Meson Production

The elastic production of vector mesons at HERA is discussed from the theoretical point of view. We briefly review different models, their successes and shortcomings. Main emphasis is put on recent issues in perturbative QCD calculations. Models including the vector meson wave function are compared with an approach based on parton-hadron duality. We discuss several refinements of these models in some detail, including the important role of off-diagonal parton distributions.

hep-ph↗
arXiv1999

Top Quark Pair Production close to Threshold: Top Mass, Width and Momentum Distribution

The complete NNLO QCD corrections to the total cross section $σ(e^+e^- \to Z*,γ*\to t\bar t)$ in the kinematic region close to the top-antitop threshold are calculated by solving the corresponding Schroedinger equations exactly in momentum space in a consistent momentum cutoff regularization scheme. The corrections coming from the same NNLO QCD effects to the top quark three-momentum distribution $dσ/d |\vec k_t|$ are determined. We discuss the origin of the large NNLO corrections to the peak position and the normalization of the total cross section observed in previous works and propose a new top mass definition, the 1S mass M_1S, which stabilizes the peak in the total cross section. If the influence of beamstrahlung and initial state radiation on the mass determination is small, a theoretical uncertainty on the 1S top mass measurement of 200 MeV from the total cross section at the linear collider seems possible. We discuss how well the 1S mass can be related to the $\bar{MS}$ mass. We propose a consistent way to implement the top quark width at NNLO by including electroweak effects into the NRQCD matching coefficients, which then can become complex.

hep-ph↗
arXiv1999

Axial Contributions at the Top Threshold

We calculate the contributions of the axial current to top quark pair production in e+ e- annihilation at threshold. The QCD dynamics is taken into account by solving the Lippmann-Schwinger equation for the P wave production using the QCD potential up to two loops. We demonstrate that the dependence of the total and differential cross section on the polarization of the e+ and e- beams allows for an independent extraction of the axial current induced cross section.

hep-ph↗
arXiv1999

Upsilon photoproduction at HERA compared to estimates of perturbative QCD

We estimate the cross section for gamma p -> Upsilon p by two independent methods. First, by studying the corrections to the naive leading-order QCD formula and, second, by using parton-hadron duality. The estimates are in good agreement with each other and with the recent measurements of the cross section at HERA.

hep-ph↗
arXiv1998

Analytic Calculation of Two Loop Corrections to Heavy Quark Pair Production Vertices Induced by Light Quarks

Non-singlet ${\cal{O}}(α_s^2)$ corrections from light quarks are calculated analytically for the massive quark-antiquark pair production rates through the vector, axial-vector, scalar and pseudoscalar currents for all energies above the threshold. The results are presented in terms of the moments of the absorptive part of the vacuum polarization function of the light quarks which allows for an immediate determination of ${\cal{O}}(α_s^2)$ corrections coming from other types of light particles. The kinematic regime close to the threshold is examined.

hep-ph↗
arXiv1998

The Perturbative QCD Potential and the ttbar Threshold

We include the full second-order corrections to the static QCD potential in the analysis of the ttbar threshold cross section. There is an unexpectedly large difference between the QCD potential improved by the renormalization-group equation in momentum space and the potential improved by the renormalization-group equation in coordinate space. This difference remains even at a fairly short distance 1/r \simeq 100 GeV and its origin can be understood within perturbative QCD. We scrutinize the theoretical uncertainties of the QCD potential in relation to the ttbar threshold cross section. In particular there exists a theoretical uncertainty which limits our present theoretical accuracy of the ttbar threshold cross section at the peak to be not better than 6% within perturbative QCD.

hep-ph↗
arXiv1998

QCD Potential and ttbar Threshold Cross Section --- Status Report ---

We include the full second-order corrections to the static QCD potential in the analysis of the ttbar threshold cross section. Then we examine the difference between the results obtained in the momentum-space approach and in the coordinate-space approach, which was found recently. Contrary to our expectation, the reduction of the difference by the inclusion of the second-order corrections is very small. There still remains a non-negligible deviation, which originates from the difference in the construction of the potentials in the two spaces. We scrutinize this problem. In particular, we estimate our present theoretical uncertainty of the ttbar threshold cross section at the peak to be at least 6% within perturbative QCD.

hep-ph↗
arXiv1998

Top Quark Pair Production at Threshold: Complete Next-to-next-to-leading Order Relativistic Corrections

The complete next-to-next-to-leading order (i.e. $O(v^2)$, $O(v α_s)$ and $O(α_s^2)$) relativistic corrections to the total photon mediated $t\bar t$ production cross section at threshold are presented in the framework of nonrelativistic quantum chromodynamics. The results are obtained using semi-analytic methods and ``direct matching''. The size of the next-to-next-to-leading order relativistic corrections is found to be comparable to the size of the next-to-leading order ones.

hep-ph↗
arXiv1997

Massive Quark Production in Electron Positron Annihilation to Order $α_s^2$

Recent analytical and numerical results for the three-loop polarization function allow to present a phenomenological analysis of the cross section for massive quark production in electron positron annihilation to order $α_s^2$. Numerical predictions based on fixed order perturbation theory are presented for charm and bottom production above 5 and 11.5 GeV, respectively. The contribution from these energy regions to $α(M_Z^2)$, the running QED coupling constant at scale M_Z, are given. The dominant terms close to threshold, i.e. in an expansion for small quark velocity $β$, are presented.

hep-ph↗
arXiv1997

Extracting alpha_s from Electron-Positron Annihilation around 10 GeV

The total cross section for electron-positron annihilation into hadrons is calculated in the region just below the B meson threshold. QCD corrections up to third order, quark mass effects, initial state radiation, the running of alpha_QED and the tails of the Upsilon resonances are included in the prediction. For given alpha_s the prediction is accurate to 0.5 percent. An experimental measurement with the corresponding precision would allow to determine alpha_s with high accuracy at intermediate energies.

hep-ph↗
arXiv1997

Diffractive electroproduction of rho meson excitations

We use perturbative QCD and parton-hadron duality to estimate the cross sections for the diffractive electroproduction of rhoprime(1-) and rho(3-) resonances which occur in the 1.3-1.8 GeV mass interval. We present the cross sections and the ratios sigma_L/sigma_T as a function of Q^2. We compare the predictions with those for the diffractive electroproduction of rho mesons. We show how such diffractive electroproduction measurements at HERA can probe features of the perturbative QCD 'Pomeron'.

hep-ph↗
arXiv1996

The QCD description of diffractive rho meson electroproduction

We critically review the QCD predictions for the cross sections sigma_L and sigma_T for diffractive rho meson electroproduction in longitudinally and transversely polarised states in the HERA energy region. We show that both perturbative and non-perturbative approaches which involve convolution with the rho meson wave function predict values of sigma_T which fall-off too quickly with increasing Q^2, in comparison with the data. We present a perturbative QCD model based on the open production of light qqbar pairs and parton-hadron duality, which describes all features of the data for rho electroproduction at high Q^2 and, in particular, predicts a satisfactory Q^2 behaviour of sigma_L/sigma_T. We find that precise measurements of the latter can give valuable information on the Q^2 behaviour of the gluon distribution at small x.

hep-ph↗