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Register a new userC-Parameter and Jet Broadening at PETRA Energies
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O. Biebel
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e^+e^- annihilation data recorded by the JADE detector at PETRA were used to measure the C-parameter for the first time at sqrt{s}= 35 and 44 GeV. The distributions were compared to a resummed QCD calculation. In addition, we applied extended resummed calculations to the total and wide jet broadening variables, B_T and B_W. We combined the results on alpha_s with those of our previous study of differential 2-jet rate, thrust, and heavy jet mass, obtaining alpha_s(35 GeV) = 0.1448 +0.0117 -0.0070 and alpha_s(44 GeV) = 0.1392 +0.0105 -0.0074. Moreover power corrections to the mean values of the observables mentioned above were investigated considering the Milan factor and the improved prediction for the jet broadening observables. Our study, which considered e^+e^- data of five event shape observables between sqrt{s}= 14 and 183 GeV, yielded alpha_s(M_{Z^0})=0.1177 +0.0035 -0.0034.
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e^+e^- annihilation data recorded by the JADE detector at PETRA were used to measure the C-parameter for the first time at sqrt{s}= 35 and 44 GeV. The distributions were compared to a resummed QCD calculation which recently became available for this observable. % In addition, we applied extended resummed calculations to the heavy and wide jet broadening variables, B_T and B_W, which now include a proper treatment of the quark recoil against multi-gluon emission with single-logarithmic accuracy. % We further investigated power corrections to the mean values of the observables mentioned above. In this study, we considered all available e^+e^- data between sqrt{s}= 35 and 172 GeV.
This report of the BOOST2012 workshop presents the results of four working groups that studied key aspects of jet substructure. We discuss the potential of the description of jet substructure in first-principle QCD calculations and study the accuracy of state-of-the-art Monte Carlo tools. Experimental limitations of the ability to resolve substructure are evaluated, with a focus on the impact of additional proton proton collisions on jet substructure performance in future LHC operating scenarios. A final section summarizes the lessons learnt during the deployment of substructure analyses in searches for new physics in the production of boosted top quarks.
Recent results on jet physics and tests of QCD from hadronic final states in $e^+e^-$ annihilation at PETRA and at LEP are reviewed, with special emphasis on hadronic event shapes, charged particle production rates, properties of quark and gluon jets and determinations of $alpha_s$. The data in the entire energy range from PETRA to LEP-2 are in broad agreement with the QCD predictions. The world summary of measurements of $alpha_s$ is updated and a detailed discussion of various methods to determine the overall error of $alpha_s (M_Z)$ is presented. The new world average is $alpha_s (M_Z) = 0.119 pm 0.004$. The size of the error depends on the treatment of correlated uncertainties.
The Linear Boltzmann Transport (LBT) model coupled to hydrodynamical background is extended to include transport of both light partons and heavy quarks through the quark-gluon plasma (QGP) in high-energy heavy-ion collisions. The LBT model includes both elastic and inelastic medium-interaction of both primary jet shower partons and thermal recoil partons within perturbative QCD (pQCD). It is shown to simultaneously describe the experimental data on heavy and light flavor hadron suppression in high-energy heavy-ion collisions for different centralities at RHIC and LHC energies. More detailed investigations within the LBT model illustrate the importance of both initial parton spectra and the shapes of fragmentation functions on the difference between the nuclear modifications of light and heavy flavor hadrons. The dependence of the jet quenching parameter $hat{q}$ on medium temperature and jet flavor is quantitatively extracted.
Central lead-lead collisions at the LHC energies may pose a particular challenge for jet identification as multiple jets are produced per each collision event. We simulate the jet evolution in central Pb-Pb events at $sqrt{s_{rm NN}} = 2.76$ GeV collision energy with EPOS3 initial state, which typically contains multiple hard scatterings in each event. Therefore the partons from different jets have a significant chance to overlap in momentum space. We find that 30% of the jets with $p_perp > 50$ GeV, identified by the standard anti-$k_perp$ jet finding algorithm with jet cone size R=0.3, contain `intruder particles from overlapping generator-level jets. This fraction increases with increasing beam energy and increasing R. The reconstructed momentum of the jet differs from that of the modelled jet by the loss due to jet partons which are outside of the jet cone and by the gain due to intruder partons. The sum of both may be positive or negative. These intruder partons particularly affect the radial jet momentum distribution because they contribute mostly at large angles $Delta r$ with respect to the jet centre. The study stresses the importance of the jet overlap effect emerging in central lead-lead collisions at the LHC energies, while being negligible in peripheral PbPb or $p$Pb/$pp$ collisions.
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