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Register a new userAnalysis note: jet reconstruction, energy spectra, and substructure analyses with archived ALEPH data
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The first measurements of anti-$k_{T}$ jet energy spectrum and substructure in hadronic $Z$ decays are presented. The archived $e^+e^-$ annihilation data at a center-of-mass energy of 91 GeV were collected with the ALEPH detector at LEP in 1994. The jet substructure was analyzed as a function of jet energy. The results are compared with the perturbative QCD calculations and predictions from the PYTHIA v6.1, SHERPA, and HERWIG v7.1.5 event generators. In this note, jet reconstruction procedure, jet energy calibration, and the performance with archived ALEPH data and Monte Carlo simulations are also documented.
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We discuss the latest results from jet fragmentation and jet substructure measurements performed with the ALICE experiment in proton-proton and heavy-ion collisions in a wide range of jet transverse momentum. The jet production cross sections and cross section ratios for different jet resolution parameters will be shown in a wide range of $p_{textrm{T}}$. Results will be compared to next-to-leading order pQCD calculations.
We use the SHERPA Monte Carlo generator to simulate the process $e^+e^-rightarrowmbox{hadrons}$ using matrix elements with up to six partons in the final state. Two samples of SHERPA events are generated. In the LO sample, all final states are generated with leading order matrix elements; in the NLO sample, matrix elements for final states with up to four partons are generated at next-to-leading order, while matrix elements for final states with five or six partons are generated at leading order. The resulting samples are then passed through the ALEPH detector simulation. We compare the Monte Carlo samples to each other, to samples generated using the KK2f generator interfaced with PYTHIA, and to the archived ALEPH data at both LEP1 and LEP2 energies. We focus on four-jet observables with particular attention given to dijet masses. The LO and NLO SHERPA samples show significant improvement over the KK2f generation for observables directly related to clustering events into four jets, while maintaining similar performance to KK2f for event-shape variables. We additionally reweight the dijet masses using LEP1 data and find that this greatly improves the agreement between the three Monte Carlo samples at LEP2 energies for these observables.
We investigate an excess observed in hadronic events in the archived LEP2 ALEPH data. This excess was observed at preselection level during data-MC comparisons of four-jet events when no search was being performed. The events are clustered into four jets and paired such that the mass difference between the two dijet systems is minimized. The excess occurs in the region $M_1+M_2sim 110mbox{ GeV}$; about half of the excess is concentrated in the region $M_1sim 80mbox{ GeV}$, $M_2sim 25mbox{ GeV}$, with a local significance between $4.7sigma$ and $5.5sigma$, depending on assumptions about hadronization uncertainties. The other half of the events are in a broad excess near $M_1sim M_2sim 55mbox{ GeV}$; these display a local significance of $4.1-4.5sigma$. We investigate the effects of changing the SM QCD Monte Carlo sample, the jet-clustering algorithm, and the jet rescaling method. We find that the excess is remarkably robust under these changes, and we find no source of systematic uncertainty that can explain the excess. No analogue of the excess is seen at LEP1.
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.
The striking suppression and modification patterns that are observed in jet observables measured in heavy-ion collisions with respect to the proton-proton baseline have the potential to constrain the spatio-temporal branching process of energetic partons in a dense QCD medium. The mechanism of jet energy loss is intricately associated with medium resolution of jet substructure fluctuations. This naturally affects the behavior of the suppression of jets at high-pT, inducing an explicit dependence on jet scales. In this contribution, we review recent work on using the insight from multi-parton quenching to calculate leading-logarithmic corrections to the single-inclusive jet spectrum, and discuss its impact on a wide range of observables, including jet substructure.
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