TOP-12-022: First Determination of the Strong Coupling Constant from the ttbar Cross Section


Approximate next-to-next-to-leading-order QCD predictions for the cross section of top-quark pair production are used together with different parton distribution functions to extract the strong coupling constant, , from the cross section measured by the CMS experiment in proton-proton collisions at a center-of-mass energy of 7 TeV. This is the first determination of from top-quark production. It probes the strong coupling with remarkable precision and in an energy regime that has been accessible to only a small number of measurements so far. The extracted value of is , in good agreement with the current world average.


Figure Caption
xsec_vs_alpha.png Predicted ttbar cross section as a function of the strong coupling constant, shown for two different calculations at approximate NNLO and four different PDF sets and compared to the cross section measured by CMS. While the uncertainty on the measured cross section is illustrated with a filled band, the uncertainties on the predictions are indicated only at the default value of the corresponding PDF set. For the sake of readability, the comparison between Top++ and HATHOR is only shown for one of the four PDF sets. [Get pdf version]
convolution.png Probability distribution based on the cross-section prediction by Top++ 1.3 and using NNPDF2.1. The distribution, shown here for a top-quark mass of 173.2 GeV and = 0.1190, is obtained by convolving a Gaussian distribution accounting for the PDF uncertainty with a rectangular function covering the scale uncertainties. [Get pdf version]
likelihood.png Final likelihood based on the cross-section prediction by Top++ 1.3 and using NNPDF2.1. This likelihood, shown here for a top-quark mass of 173.2 GeV, is obtained by folding with a Gaussian distribution that represents the measured cross section. [Get pdf version]
summary.png Comparison of the results obtained for from the measured ttbar cross section using the approximate NNLO calculations implemented in Top++ 1.3 and HATHOR 1.3 together with four different PDF sets. The inner error bars include the uncertainties on the measured cross section as well as the PDF and scale uncertainties on the predicted cross section. The outer error bars additionally account for the uncertainty on the top-quark mass. The latest world average for , obtained by the Particle Data Group in June 2012, is also shown, using a dashed line together with a hatched uncertainty band. The filled bands with the dotted lines indicate the default value of the corresponding PDF set. [Get pdf version]


Table Caption
pdfsets.png Default values and provided scan ranges of the four NNLO PDF sets used in this analysis.
results.png Results obtained for by comparing the measured to the predicted ttbar cross section, listed for two different approximate NNLO calculations and four different PDF sets. In addition to the uncertainty due to the top-quark mass, the total uncertainties given here account for the uncertainty on the measured cross section as well as for the uncertainties related to choice and variation of the renormalization and factorization scales and for the uncertainties of the respective PDF set used in the calculation of the expected cross section.

-- SebastianNaumann - 31-Aug-2012

Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf convolution.pdf r2 r1 manage 6.1 K 2012-08-31 - 15:14 SebastianNaumann  
PNGpng convolution.png r1 manage 24.1 K 2012-08-31 - 15:12 SebastianNaumann  
PDFpdf likelihood.pdf r1 manage 5.7 K 2012-08-31 - 19:40 SebastianNaumann  
PNGpng likelihood.png r1 manage 21.0 K 2012-08-31 - 19:40 SebastianNaumann  
PNGpng pdfsets.png r2 r1 manage 55.0 K 2012-08-31 - 20:32 SebastianNaumann  
PNGpng results.png r1 manage 111.6 K 2012-08-31 - 20:35 SebastianNaumann  
PDFpdf summary.pdf r1 manage 5.7 K 2012-08-31 - 19:40 SebastianNaumann  
PNGpng summary.png r1 manage 17.0 K 2012-08-31 - 19:40 SebastianNaumann  
PDFpdf xsec_vs_alpha.pdf r1 manage 7.6 K 2012-08-31 - 19:40 SebastianNaumann  
PNGpng xsec_vs_alpha.png r1 manage 29.1 K 2012-08-31 - 19:40 SebastianNaumann  
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Topic revision: r3 - 2012-09-12 - RobertoTenchini
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