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G. Heinrich

Publications and source records attributed to G. Heinrich.

At least 73 records · Page 4Linked to original sources
arXiv2010

A proposal for a standard interface between Monte Carlo tools and one-loop programs

Many highly developed Monte Carlo tools for the evaluation of cross sections based on tree matrix elements exist and are used by experimental collaborations in high energy physics. As the evaluation of one-loop matrix elements has recently been undergoing enormous progress, the combination of one-loop matrix elements with existing Monte Carlo tools is on the horizon. This would lead to phenomenological predictions at the next-to-leading order level. This note summarises the discussion of the next-to-leading order multi-leg (NLM) working group on this issue which has been taking place during the workshop on Physics at TeV colliders at Les Houches, France, in June 2009. The result is a proposal for a standard interface between Monte Carlo tools and one-loop matrix element programs.

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arXiv2010

Precise determination of the strong coupling constant at NNLO in QCD from the three-jet rate in electron--positron annihilation at LEP

We present the first determination of the strong coupling constant from the three-jet rate in e+e- annihilation at LEP, based on a next-to-next-to-leading order (NNLO) perturbative QCD prediction. More precisely, we extract alpha_s by fitting perturbative QCD predictions at O(alpha_s^3) to data from the ALEPH experiment at LEP. Over a large range of the jet-resolution parameter ycut this observable is characterised by small non-perturbative corrections and an excellent stability under renormalisation scale variation. For the three-jet rate at ln(ycut) = -3.9 we find alpha_s(MZ) = 0.1175 +/- 0.0020(exp) \+/- 0.0015(theo), which is among the most precise determinations from jet observables at LEP.

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arXiv2009

Nine-Propagator Master Integrals for Massless Three-Loop Form Factors

We complete the calculation of master integrals for massless three-loop form factors by computing the previously-unknown three diagrams with nine propagators in dimensional regularisation. Each of the integrals yields a six-fold Mellin-Barnes representation which we use to compute the coefficients of the Laurent expansion in epsilon. Using Riemann zeta functions of up to weight six, we give fully analytic results for one integral; for a second, analytic results for all but the finite term; for the third, analytic results for all but the last two coefficients in the Laurent expansion. The remaining coefficients are given numerically to sufficiently high accuracy for phenomenological applications.

hep-ph↗
arXiv2009

Determination of the strong coupling constant using matched NNLO+NLLA predictions for hadronic event shapes in e+e- annihilations

We present a determination of the strong coupling constant from a fit of QCD predictions for six event-shape variables, calculated at next-to-next-to-leading order (NNLO) and matched to resummation in the next-to-leading-logarithmic approximation (NLLA). These event shapes have been measured in e+e- annihilations at LEP, where the data we use have been collected by the ALEPH detector at centre-of-mass energies between 91 and 206 GeV. Compared to purely fixed order NNLO fits, we observe that the central fit values are hardly affected, but the systematic uncertainty is larger because the NLLA part re-introduces relatively large uncertainties from scale variations. By combining the results for six event-shape variables and eight centre-of-mass energies, we find alpha_s(MZ)= 0.1224 +/- 0.0009 (stat) +/- 0.0009 (exp) +/- 0.0012 (had) +/- 0.0035 (theo), which improves previously published measurements at NLO+NLLA. We also carry out a detailed investigation of hadronisation corrections, using a large set of Monte Carlo generator predictions.

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arXiv2009

NNLO moments of event shapes in e+e- annihilation

We compute the next-to-next-to-leading order (NNLO) QCD corrections to the first five moments of six event shape variables related to three-particle final states in electron-positron annihilation; the thrust, the heavy jet mass, the C-parameter, the wide and total jet broadenings and the three-to-two-jet transition parameter in the Durham algorithm Y3. The NNLO corrections to the first moment are moderate for all event shapes, while the renormalisation scale dependence of the theoretical prediction is substantially reduced compared to the previously existing NLO results. From a comparison with data from JADE and OPAL, we observe that the energy dependence of the moments of the wide jet broadening and Y3 can be largely explained without any non-perturbative power corrections, while the other observables exhibit a clear need for power-like contributions at low centre-of-mass energy.

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arXiv2009

Photon - Jet Correlations and Constraints on Fragmentation Functions

We study the production of a large-pT photon in association with a jet in proton-proton collisions. We examine the sensitivity of the jet rapidity distribution to the gluon distribution function in the proton. We then assess the sensitivity of various photon + jet correlation observables to the photon fragmentation functions. We argue that RHIC data on photon-jet correlations can be used to constrain the photon fragmentation functions in a region which was barely accessible in LEP experiments.

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arXiv2009

Direct photon production at HERA, the Tevatron and the LHC

We review several most recent prompt-photon measurements at HERA and the Tevatron and discuss their implication for future measurements at the LHC. A comparison to Monte Carlo models, as well as to NLO QCD predictions based on the standard DGLAP and the kT-factorization approaches is discussed. Effects from renormalization and factorization scale uncertainties, as well as uncertainties on the gluon density distribution inside a proton are discussed.

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arXiv2009

NNLO corrections to event shapes in $e^+e^-$ annihilation

We compute the next-to-next-to-leading order (NNLO) QCD corrections to the six most important event shape variables related to three-particle final states in electron-positron annihilation. The corrections are sizeable for all variables, however their magnitude is substantially different for different observables. We observe that the NNLO corrections yield a considerably better agreement between theory and experimental data both in shape and normalisation of the event shape distributions. The renormalisation scale dependence of the theoretical prediction is substantially reduced compared to the previously existing NLO results. Our results will allow a precise determination of the strong coupling constant from event shape data collected at LEP.

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arXiv2008

Infrared structure of e+e- --> 3 jets at NNLO

We describe the calculation of the next-to-next-to-leading order (NNLO) QCD corrections to three-jet production and related event shape observables in electron-positron annihilation. Infrared singularities due to double real radiation at tree level and single real radiation at one loop are subtracted from the full QCD matrix elements using antenna functions, which are then integrated analytically and added to the two loop contribution. Using this antenna subtraction method, we obtain numerically finite contributions from five-parton and four-parton processes, and observe an explicit analytic cancellation of infrared poles in the four-parton and three-parton contributions. All contributions are implemented in a flexible parton-level event generator programme, allowing the numerical computation of any infrared-safe observable related to three-jet final states to NNLO accuracy.

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arXiv2008

Golem95: a numerical program to calculate one-loop tensor integrals with up to six external legs

We present a program for the numerical evaluation of form factors entering the calculation of one-loop amplitudes with up to six external legs. The program is written in Fortran95 and performs the reduction to a certain set of basis integrals numerically, using a formalism where inverse Gram determinants can be avoided. It can be used to calculate one-loop amplitudes with massless internal particles in a fast and numerically stable way.

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arXiv2008

Precise predictions for LHC using a GOLEM

In this talk we present recent next-to-leading order results relevant for LHC phenomenology obtained with the GOLEM method. After reviewing the status of this Feynman diagrammatic approach for multi-leg one-loop calculations we discuss three applications: the loop-induced process gg -> Z^*Z^* and the virtual corrections to the five and six point processes qq -> ZZg and u ubar -> s sbar c cbar. We demonstrate that our method leads to representations of such amplitudes which allow for efficient phase space integration. In this context we propose a reweighting technique of the leading order unweighted events by local K-factors.

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arXiv2008

e^+ e^- -> 3 jets and event shapes at NNLO

We report on the calculation of NNLO corrections to the 3-jet cross section and related event shape distributions in electron-positron annihilation. The corrections are sizable for all variables, however the magnitude of the corrections is substantially different for different observables. We observe that inclusion of the NNLO corrections yields a considerably better agreement between theory and experimental data both in shape and normalization of the event shape distributions in the region where the perturbative result is expected to hold. A new extraction of $α_{s}$ using the event shape variables up to NNLO yields a considerably better consistency between the observables indicating a stabilization of the perturbative corrections at this order.

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arXiv2008

Jet rates in electron-positron annihilation at O(α_s^3) in QCD

We compute production rates for two, three, four and five jets in electron-positron annihilation at the third order in the QCD coupling constant. At this order, three-jet production is described to next-to-next-to-leading order (NNLO) in perturbation theory while the two-jet rate is obtained at next-to-next-to-next-to-leading order (N$^3$LO). Our results yield an improved perturbative description of the dependence of jet multiplicity on the jet resolution parameter, $\ycut$, particularly at small values of $\ycut$.

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arXiv2008

Master Integrals for Fermionic Contributions to Massless Three-Loop Form Factors

In this letter we continue the calculation of master integrals for massless three-loop form factors by giving analytical results for those integrals which are relevant for the fermionic contributions proportional to N_F^2, N_F*N, and N_F/N. Working in dimensional regularisation, we express one of the integrals in a closed form which is exact to all orders in epsilon, containing Gamma-functions and hypergeometric functions of unit argument. In all other cases we derive multiple Mellin-Barnes representations from which the coefficients of the Laurent expansion in epsilon are extracted in an analytical form. To obtain the finite part of the three-loop quark and gluon form factors, all coefficients through transcendentality six in the Riemann zeta-function have to be included.

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arXiv2008

Loop induced interference effects in Higgs Boson plus two jet production at the LHC

We calculate the order O(alpha^2 alpha_s^3) interference effect between the gluon fusion and weak boson fusion processes allowed at the one-loop level in Higgs boson plus 2 jet production at the LHC. The corresponding one-loop amplitudes, which have not been considered in the literature so far, are evaluated analytically using dimensional regularisation and the necessary master integrals with massive propagators are reported. It is discussed in detail how various mechanisms conspire to make this contribution numerically negligible for experimental studies at the LHC.

hep-ph↗
arXiv2008

First determination of the strong coupling constant using NNLO predictions for hadronic event shapes in e^+e^- annihilations

We present the first determination of the strong coupling constant from a fit of next-to-next-to-leading order QCD predictions to event-shape variables, measured in $e^+e^-$ annihilations at LEP. The data have been collected by the ALEPH detector at centre-of-mass energies between 91 and 206 GeV. Compared to results of next-to-leading order fits we observe that the central fit values are lower by about 10%, with considerably reduced scatter among the results obtained with different event-shape variables. The dominant systematic uncertainty from renormalization scale variations is reduced by a factor of two. By combining the results for several event-shape variables and centre-of-mass energies, we find α_s(M_Z^2) = 0.1240+-0.0008(stat)+-0.0010(exp)+-0.0011(had)+-0.0029(theo).

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