arXiv subjects

S. Kretzer

Publications and source records attributed to S. Kretzer.

At least 19 recordsLinked to original sources

arXiv2007

HERA and the LHC - A workshop on the implications of HERA for LHC physics: Proceedings - Part B

The HERA electron--proton collider has collected 100 pb$^{-1}$ of data since its start-up in 1992, and recently moved into a high-luminosity operation mode, with upgraded detectors, aiming to increase the total integrated luminosity per experiment to more than 500 pb$^{-1}$. HERA has been a machine of excellence for the study of QCD and the structure of the proton. The Large Hadron Collider (LHC), which will collide protons with a centre-of-mass energy of 14 TeV, will be completed at CERN in 2007. The main mission of the LHC is to discover and study the mechanisms of electroweak symmetry breaking, possibly via the discovery of the Higgs particle, and search for new physics in the TeV energy scale, such as supersymmetry or extra dimensions. Besides these goals, the LHC will also make a substantial number of precision measurements and will offer a new regime to study the strong force via perturbative QCD processes and diffraction. For the full LHC physics programme a good understanding of QCD phenomena and the structure function of the proton is essential. Therefore, in March 2004, a one-year-long workshop started to study the implications of HERA on LHC physics. This included proposing new measurements to be made at HERA, extracting the maximum information from the available data, and developing/improving the theoretical and experimental tools. This report summarizes the results achieved during this workshop.

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arXiv2007

Heavy Quarks: Summary Report

The present status of the heavy-quark production theory is critically reviewed in the first contribution. The second contribution summarises the present heavy flavour data from HERA and gives an outlook of what can be expected from HERA-II. The potential of the LHC experiments for charm and beauty physics is reviewed in the 3rd contribution. Then the relevance of saturation and small-x effects to heavy quark production at HERA and at the LHC are discussed. The non-perturbative aspects of heavy-quark fragmentation and their relevance to HERA and LHC are discussed in the next contribution. Finally, a comparison of different theoretical predictions for HERA and LHC based on different approaches is presented.

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arXiv2006

HERA and the LHC - A workshop on the implications of HERA for LHC physics: Proceedings - Part A

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arXiv2004

Neutrino Dimuon Production and the Strangeness Asymmetry of the Nucleon

We have performed the first global QCD analysis to include the CCFR and NuTeV dimuon data, which provide direct constraints on the strange and anti-strange parton distributions, $s(x)$ and $\bar{s}(x)$. To explore the strangeness sector, we adopt a general parametrization of the non-perturbative $s(x), \bar{s}(x)$ functions satisfying basic QCD requirements. We find that the strangeness asymmetry, as represented by the momentum integral $[S^{-}]\equiv \int_0^1 x [s(x)-\bar{s}(x)] dx$, is sensitive to the dimuon data provided the theoretical QCD constraints are enforced. We use the Lagrange Multiplier method to probe the quality of the global fit as a function of $[S^-]$ and find $-0.001 < [S^-] < 0.004$. Representative parton distribution sets spanning this range are given. Comparisons with previous work are made.

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arXiv2004

$σ_{DIS}(νN)$, NLO Perturbative QCD and O(1 GeV) Mass Corrections

The deep-inelastic neutrino-nucleon cross section is one of the components of few GeV neutrino interactions. We present here our results for neutrino-isoscalar nucleon charged current scattering including perturbative next-to-leading order QCD corrections, target mass corrections, charm mass and lepton mass corrections.

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arXiv2004

Longitudinal Gluon Polarization in RHIC Double-Spin Asymmetries

The longitudinally polarized gluon density is probed sensitively in hard collisions of polarized protons under the condition that the dominant dynamics are perturbative and of leading twist origin. First data have recently been presented by Phenix on the double-spin asymmetry A_{LL}^{pi} for pi^0 production at moderate transverse momentum p_\perp \simeq 1 - 4 GeV and central rapidity. By means of a systematic investigation of the relevant degrees of freedom we show that the perturbative QCD framework at leading power in p_\perp produces an asymmetry that is basically positive definite in this kinematic range, i.e. A_{LL}^{pi}\gtrsim O(-10^{-3}) .

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arXiv2004

Neutrino-Production of Charm and the Strangeness Asymmetry of the Nucleon

Interest in the strange nucleon sea has been renewed when it was realized that the strangeness asymmetry $s^-=s-\bar{s}$ plays a prominent role in the interpretation of the NuTeV weak mixing angle anomaly. I review the NLO QCD calculation of the neutrino-production of opposite-sign dimuons as the experimental signature of the strange quark parton density. Results from a recent CTEQ fit are presented and discussed with respect to their stability under NLO corrections and their impact on the NuTeV measurement.

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arXiv2003

The Parton Structure of the Nucleon and Precision Determination of the Weinberg Angle in Neutrino Scattering

A recently completed next-to-leading-order program to calculate neutrino cross sections, including power-suppressed mass correction terms, has been applied to evaluate the Paschos-Wolfenstein relation, in order to quantitatively assess the validity and significance of the NuTeV anomaly. In particular, we study the shift of $\sin^2 θ_{\mathrm{W}}$ obtained in calculations with a new generation of PDF sets that allow $s(x)\neq \bar{s}(x)$, enabled by recent neutrino dimuon data from CCFR and NuTeV, as compared to the previous $s = \bar{s}$ parton distribution functions like CTEQ6M. The extracted value of $\sin^2 θ_{\mathrm{W}}$ is closely correlated with the strangeness asymmetry momentum integral $\int_{0}^{1}x[s(x)-\bar{s}(x)] dx$. We also consider isospin violating effects that have recently been explored by the MRST group. The results of our study suggest that the new dimuon data, the Weinberg angle measurement, and other data sets used in global QCD parton structure analysis can all be consistent within the Standard Model.

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arXiv2003

Target Mass Corrections to Electro-Weak Structure Functions and Perturbative Neutrino Cross Sections

We provide a complete and consistent framework to include subasymptotic perturbative as well as mass corrections to the leading twist (tau=2) evaluation of charged and neutral current weak structure functions and the perturbative neutrino cross sections. We revisit previous calculations in a modern language and fill in the gaps that we find missing for a complete and ready-to-use "NLO xi-scaling" formulary. In particular, as a new result we formulate the mixing of the partonic and hadronic structure function tensor basis in the operator approach to deep inelastic scattering. As an underlying framework we follow the operator product expansion a la Georgi & Politzer that allows the inclusion of target mass corrections at arbitrary order in QCD and we provide explicit analytical and numerical results at NLO. We compare this approach with a simpler collinear parton model approach to xi-scaling. Along with target mass corrections we include heavy quark mass effects as a calculable leading twist power suppressed correction. The complete corrections have been implemented into a Monte Carlo integration program to evaluate structure functions and/or integrated cross sections. As applications, we compare the operator approach with the collinear approximation numerically and we investigate the NLO and mass corrections to observables that are related to the extraction of the weak mixing angle from a Paschos-Wolfenstein-like relation in neutrino-iron scattering. We expect that the interpretation of neutrino scattering events in terms of oscillation physics and electroweak precision physics will benefit from our results.

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arXiv2003

CTEQ6 Parton Distributions with Heavy Quark Mass Effects

Previously published CTEQ6 parton distributions adopt the conventional zero-mass parton scheme; these sets are most appropriate for use with massless hard-scattering matrix elements commonly found in most physics applications. For precision observables which are sensitive to charm and bottom quark mass effects, we provide in this paper an additional CTEQ6HQ parton distribution set determined in the more general variable flavor number scheme which incorporates heavy flavor mass effects. The results are obtained by combining these parton distributions with consistently matched DIS structure functions computed in the same scheme. We describe the analysis procedure, examine the predominant features of the new distributions, and compare with previous distributions.

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arXiv2003

Deep Inelastic Neutrino Interactions

Understanding neutrino interactions is an important task in searches for neutrino oscillations; e.g. the nu_{mu} -> nu_{tau} oscillation hypothesis will be tested through nu_{tau} production of tau in long-baseline experiments as well as underground neutrino telescopes. An anomaly in the deep inelastic interaction of neutrinos has recently been observed by the NuTeV collaboration -- resulting in a measured weak mixing angle sin^2 Theta_{W} that differs by ~ 3 sigma from the standard model expectation. In this contribution to the proceedings of NUINT02, we summarize results on the NLO neutrino structure functions and cross sections in which charm quark mass and target mass effects in the collinear approximation are included.

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arXiv2002

Fragmentation functions and their role in determining the polarized parton densities

We extract the pion fragmentation functions and their uncertainties from a judicious choice of e+e- and semi-inclusive DIS data. These are used to study the error propagation in the extraction of polarized parton densities from semi-inclusive DIS asymmetries. We conclude that the uncertainties on polarized PDs have been underestimated in the past.

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arXiv2002

Tau neutrino deep inelastic charged current interactions

The nu_mu -> nu_tau oscillation hypothesis will be tested through nu_tau production of tau in underground neutrino telescopes as well as long-baseline experiments. We provide the full QCD framework for the evaluation of tau neutrino deep inelastic charged current (CC) cross sections, including next-leading-order (NLO) corrections, charm production, tau threshold, and target mass effects in the collinear approximation. We investigate the violation of the Albright-Jarlskog relations for the structure functions F_4,5 which occur only in heavy lepton (tau) scattering. Integrated CC cross sections are evaluated naively over the full phase space and with the inclusion of DIS kinematic cuts. Uncertainties in our evaluation based on scale dependence, PDF errors and the interplay between kinematic and dynamical power corrections are discussed and/or quantified.

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arXiv2001

Differential Distributions for NLO Neutrino-Production of Charm

Charged current DIS charm production measures the strange-quark PDF. A complete analysis requires both a fully differential theoretical calculation, and a Monte Carlo detector simulation. We present analytic and numeric results at NLO suitable for experimental analysis.

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arXiv2001

Differential Distributions for NLO Analyses of Charged Current Neutrino-Production of Charm

Experimental analyses of charged current deep inelastic charm production -- as observed through dimuon events in neutrino-iron scattering -- measure the strangeness component of the nucleon sea. A complete analysis requires a Monte Carlo simulation to account for experimental detector acceptance effects; therefore, a fully differential theoretical calculation is necessary to provide complete kinematic information. We investigate the theoretical issues involved in calculating these differential distributions at Next-Leading-Order (NLO). Numerical results are presented for typical fixed target kinematics. We present a corresponding FORTRAN code suitable for experimental NLO analysis.

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arXiv2001

Fragmentation functions from semi-inclusive DIS pion production and implications for the polarized parton densities

By combining recent HERMES data on semi-inclusive DIS $π^\pm$-production with the singlet fragmentation function $D_Σ^{π^+}$, which is well determined from $e^+e^-$ data, we are able to extract, for the first time, the flavoured fragmentation functions $D_u^{π^+}$, $D_d^{π^+}$ and $D_s^{π^+}$ without making any assumptions about favoured and unfavoured transitions. Whereas $D_u^{π^+}$ and $D_d^{π^+}$ are very well determined, the accuracy of $D_s^{π^+}$ is limited by the uncertainty in evolving $D_Σ^{π^+}$ from the $Z^0$ pole down to the SIDIS scale of a few $(GeV)^2$. We discuss how the precision on $D_s^{π^+}$ could be improved. Knowledge of the $D_{q=u,d,s}^{π^+}$ will permit the extraction of the polarized parton densities from future polarized SIDIS asymmetry measurements. We study the precision that can be expected in such an extraction.

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arXiv2001

An Alternative Approach to Semi-inclusive DIS and A Model Independent Determination of D_u^{π^+}, D_d^{π^+} and D_s^{π^+}

We discuss how to extract the polarized parton densities and fragmentation functions in a model independent way from SIDIS, and present results for D_{u,d,s}^{π^+}.

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arXiv2001

Predictions for Neutrino Structure Functions

The first measurements of Delta- xF3 are higher than current theoretical predictions. We investigate the sensitivity of these theoretical predictions upon a variety of factors including: renormalization scheme and scale, quark mass effects, higher twist, isospin violation, and PDF uncertainties.

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