Simone Marzani

Towards an understanding of jet substructure

A bstractWe present first analytic, resummed calculations of the rates at which wide-spread jet substructure tools tag QCD jets. As well as considering trimming, pruning and the mass-drop tagger, we introduce modified tools with improved analytical and phenomenological behaviours. Most taggers have double logarithmic resummed structures. The modified mass-drop tagger is special in that it involves only single logarithms, and is free from a complex class of terms known as non-global logarithms. The modification of pruning brings an improved ability to discriminate between the different colour structures that characterise signal and background. As we outline in an extensive phenomenological discussion, these results provide valuable insight into the performance of existing tools and help lay robust foundations for future substructure studies.

Higgs production via gluon-gluon fusion with finite top mass beyond next-to-leading order

Abstract We present a computation of the cross section for inclusive Higgs production in gluon–gluon fusion for finite values of the top mass in perturbative QCD to all orders in the limit of high partonic center-of-mass energy. We show that at NLO the high energy contribution accounts for most of the difference between the result found with finite top mass and that obtained in the limit m t → ∞ . We use our result to improve the known NNLO order result obtained at m t → ∞ . We estimate the effect of the high energy NNLO m t dependence on the K factor to be of the order of a few per cent.

Higgs production in gluon fusion at next-to-next-to-leading order QCD for finite top mass

The inclusive Higgs production cross section from gluon fusion is calculated through NNLO QCD, including its top quark mass dependence. This is achieved through a matching of the 1/Mt expansion of the partonic cross sections to the exact large-

Boosted objects and jet substructure at the LHC: Report of BOOST2012, held at IFIC Valencia, 23rd-27th of July 2012

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Boosted objects and jet substructure at the LHC. Report of BOOST2012, held at IFIC Valencia, 23rd–27th of July 2012 - eScholarship

limits which are derived from kT-factorization. The accuracy of this procedure is estimated to be better than 1% for the hadronic cross section. The final result is shown to be within 1% of the commonly used effective theory approach, thus confirming earlier findings.

Higgs production in gluon fusion beyond NNLO

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 first-principle QCD calculations to yield a precise description of the substructure of jets and study the accuracy of state-of-the-art Monte Carlo tools. Limitations of the experiments’ ability to resolve substructure are evaluated, with a focus on the impact of additional (pile-up) proton proton collisions on jet substructure performance in future LHC operating scenarios. A final section summarizes the lessons learnt from jet substructure analyses in searches for new physics in the production of boosted top quarks.

Jet substructure with analytical methods

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 first-principle QCD calculations to yield a precise description of the substructure of jets and study the accuracy of state-of-the-art Monte Carlo tools. Limitations of the experiments’ ability to resolve substructure are evaluated, with a focus on the impact of additional (pile-up) proton proton collisions on jet substructure performance in future LHC operating scenarios. A final section summarizes the lessons learnt from jet substructure analyses in searches for new physics in the production of boosted top quarks.

Resummed Higgs cross section at N

We construct an approximate expression for the cross section for Higgs production in gluon fusion at next-to-next-to-next-to-leading order (N3LO) in αs with finite top mass. We argue that an accurate approximation can be constructed by exploiting the analyticity of the Mellin space cross section, and the information on its singularity structure coming from large N (soft gluon, Sudakov) and small N (high energy, BFKL) all order resummation. We support our argument with an explicit comparison of the approximate and the exact expressions up to the highest (NNLO) order at which the latter are available. We find that the approximate N3LO result amounts to a correction of 17% to the NNLO QCD cross section for production of a 125 GeV Higgs at the LHC (8 TeV), larger than previously estimated, and it significantly reduces the scale dependence of the NNLO result.

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We consider the mass distribution of QCD jets after the application of jet-substructure methods, specifically the mass-drop tagger, pruning, trimming and their variants. In contrast to most current studies employing Monte Carlo methods, we carry out analytical calculations at the next-to-leading order level, which are sufficient to extract the dominant logarithmic behaviour for each technique, and compare our findings to exact fixed-order results. Our results should ultimately lead to a better understanding of these jet-substructure methods which in turn will influence the development of future substructure tools for LHC phenomenology.

LL

A bstractWe present accurate predictions for the inclusive production of a Higgs boson in proton-proton collisions, via gluon-gluon fusion. Our calculation includes nextto-next-to-leading order (NNLO) corrections in perturbative QCD, as well as the resummation of threshold-enhanced contributions to next-to-next-to-next-to-leading logarithmic (N3LL) accuracy, with the inclusion of the recently-determined three-loop constant coefficient (sometimes referred to as N3LL′ accuracy).Our result correctly accounts for finite top, bottom and charm masses at leading order (LO) and next-to-leading order (NLO), and includes top mass dependence at NNLO. At the resummed level the dependence on top, bottom and charm mass is accounted for at NLL, while only the top mass at NNLL. The all-order calculation is improved by a suitable choice of the soft terms, dictated by analyticity conditions and by the inclusion of subleading corrections of collinear origin, which improve the accuracy of the resummation away from the threshold region.We present results for different collider energies and we study perturbative uncertainties by varying renormalization and factorization scales. We find that, at current LHC energies, the resummation corrects the NNLO result by as much as 20% at μR = μF = mH, while the correction is much smaller, 5.5%, at μR = μF = mH/ 2. While the central value of NNLO+N3LL result depends very mildly on the scale choice, we argue that a more realiable estimate of the theoretical uncertainty is found if the perturbative scales are canonically varied about mH.