arXiv subjects

Simone Marzani

Publications and source records attributed to Simone Marzani.

At least 73 records · Page 4Linked to original sources

arXiv2013

Jet substructure with analytical methods

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.

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arXiv2013

Towards an understanding of jet substructure

We present first analytic, resummed calculations of the rates at which widespread 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.

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arXiv2012

On jet mass distributions in Z+jet and dijet processes at the LHC

The mass distribution of jets produced in hard processes at the LHC plays an important role in several jet substructure related studies involving both Standard Model and BSM physics, especially in the context of boosted heavy particle searches. We compute analytically the jet-mass distribution for both Z+jet and dijet processes, for QCD jets defined in the anti-k_t algorithm with an arbitrary radius R, to next-to-leading logarithmic accuracy and match our resummed calculation to full leading-order results. We note the important role played by initial state radiation (ISR) and non-global logarithms explicitly computed here for the first time for hadron collider observables, as well as the jet radius dependence of these effects. We also compare our results to standard Monte Carlo event generators and discuss directions for further studies and phenomenology.

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arXiv2012

Phenomenology of electro-weak bosons at hadron colliders with novel variables

We review the phenomenology of electro-weak bosons produced in hadron-hadron collisions. In particular, we discuss the transverse momentum distribution of lepton pairs with invariant mass close to the Z peak. We describe the theoretical calculation for the magnitude of the transverse momentum Q_T and its comparison to Tevatron and LHC data. We also discuss the related variable ϕ*, describing its experimental advantages as well as its relation to the standard Q_T variable. Finally, we compare resummed predictions for ϕ* to data.

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arXiv2012

Predictions for Drell-Yan ϕ* and Q_T observables at the LHC

We make theoretical predictions for the recently introduced variable ϕ* corresponding to the azimuthal angle between leptons produced in the Drell-Yan process at the LHC. As a consequence of this work we are also able to generate results for the more commonly studied transverse momentum Q_T of the lepton pair. Comparisons of these purely perturbative estimates for the Q_T case yield good agreement with ATLAS and CMS data, as we demonstrate. We anticipate that this work will help stimulate measurements of ϕ* at the LHC.

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arXiv2012

Probing colour flow with jet vetoes

We discuss jet vetoes as a means of probing colour flow in hard-scattering processes in hadronic collisions. As an example, we describe a calculation of the dijet cross-section with a jet veto, which resums the leading logarithms of the veto scale and is matched to a fixed-order computation. We compare this prediction to the measurement performed by the ATLAS collaboration. Finally, we outline future developments in this research area.

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arXiv2012

Probing the low transverse momentum domain of Z production with novel variables

The measurement of the low transverse momentum region of vector boson production in Drell-Yan processes has long been invaluable to testing our knowledge of QCD dynamics both beyond fixed-order in perturbation theory as well as in the non-perturbative region. Recently the DØ collaboration have introduced novel variables which lead to improved measurements compared to the case of the standard QT variable. To complement this improvement on the experimental side, we develop here a complete phenomenological study dedicated in particular to the new ϕ* variable. We compare our study, which contains the state-of-the-art next-to-next-to-leading resummation of large logarithms and a smooth matching to the full next-to-leading order result, to the experimental data and find excellent agreement over essentially the entire range of ϕ*, even without direct inclusion of non-perturbative effects. We comment on our findings and on the potential for future studies to constrain non-perturbative behaviour.

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arXiv2011

The dijet cross section with a jet veto

We study dijet production in proton-proton collisions with a veto on the emission of a third jet in the rapidity region in between the two leading ones. We resum the leading logarithms in the ratio of the transverse momentum of the leading jets and the veto scale and we match this result to leading-order QCD matrix elements. We find that, in order to obtain sensible results, we have to modify the resummation and take into account energy-momentum conservation effects. We compare our theoretical predictions for the gap fraction to experimental data measured by the ATLAS collaboration and find good agreement, although our results are affected by large theoretical uncertainties. We then discuss differences and similarities of our calculation to other theoretical approaches.

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arXiv2011

Accurate QCD predictions for new variables to study dilepton transverse momentum

We report on the computation of the angle-between-leptons distribution in Drell-Yan processes. More precisely we study the recently introduced variable phi*, which provides us with a more accurate probe of the low Q_T domain of Z boson production at hadron colliders. Our theoretical prediction is obtained by matching a next-to-next-to leading logarithmic (NNLL) resummation to a fixed order calculation at next-to-leading order (NLO). We find that the result significantly differs from a pure fixed order calculation in a wide range of the observable we are interested in, clearly indicating the need for resummation. We also perform a first comparison to the measurement by the D0 collaboration, finding good agreement between theory and experiment.

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arXiv2011

High energy resummation for rapidity distributions

We discuss the generalisation of high-energy resummation to rapidity distributions to leading logarithmic accuracy. We test our procedure applying it to Higgs production in gluon-gluon fusion both with finite top mass and in the infinite mass limit. We check that they reproduce the known results at fixed order and we estimate finite top mass corrections to the NLO distribution.

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arXiv2011

QCD predictions for new variables to study dilepton transverse momenta at hadron colliders

The D0 collaboration has recently introduced new variables, a_T and phi* to more accurately probe the low Q_T domain of Z boson production at hadron colliders than had been previously possible through a direct study of the Q_T distribution. The comparison of such accurate data to precise theoretical predictions from QCD perturbation theory will yield important information on the ability of resummed QCD predictions as well as parton shower models to describe the low Q_T domain and should enable more stringent constraints on non-perturbative effects. In the present paper we provide analytical predictions for the above mentioned variables, that contain resummation of large logarithms, including next-to-next-to leading logarithmic (NNLL) terms, supplemented by exact next-to-leading order calculations from MCFM.

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arXiv2011

Finite fermion mass effects in pseudoscalar Higgs production via gluon-gluon fusion

We compute the leading logarithmic behaviour of the cross-section for the production of a pseudoscalar Higgs boson in gluon-gluon fusion to all-orders in perturbation theory, in the limit of large partonic centre of mass energy. We also calculate the Higgs rapidity distribution to the same accuracy. We include the contributions of top and bottom quarks, together with their interference. Our results are given in terms of single and double integrals, evaluated explicitly up to next-to next-to leading order (NNLO). We use our results to improve the known NNLO inclusive cross-section computed in the effective theory where the fermions in the loop are integrated out. The size of finite fermion mass effects on the inclusive cross-section is found to be small, reaching a few percent only for large values of the pseudoscalar mass.

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arXiv2011

Small x resummation of rapidity distributions: the case of Higgs production

We provide a method for the all order computation of small x contributions at the leading logarithmic level to cross-sections which are differential in rapidity. The method is based on a generalization to rapidity distributions of the high energy (or k_T) factorization theorem hitherto proven for inclusive cross-sections. We apply the method to Higgs production in gluon-gluon fusion, both with finite top mass and in the infinite mass limit: in both cases, we determine all-order resummed expressions, as well as explicit expressions for the leading small x terms up to NNLO. We use our result to construct an explicit approximate analytic expression of the finite-mass NLO rapidity distribution and an estimate of finite-mass corrections at NNLO.

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arXiv2010

Non-global logarithms and jet algorithms in high-pT jet shapes

We consider jet-shape observables of the type proposed recently, where the shapes of one or more high-pT jets, produced in a multi-jet event with definite jet multiplicity, may be measured leaving other jets in the event unmeasured. We point out the structure of the full next-to-leading logarithmic resummation specifically including resummation of non-global logarithms in the leading-Nc limit and emphasising their properties. We also point out differences between jet algorithms in the context of soft gluon resummation for such observables.

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arXiv2010

High-energy resummation at the LHC: the case of Drell-Yan processes

We first review the general framework which enables one to resum high-energy logarithms in hadronic processes, both in the evolution of parton densities and in the coefficient functions. We then present an all-order calculation in perturbative QCD of the inclusive Drell Yan and vector boson production in hadron-hadron collisions, in the limit where centre of mass energy is much bigger than the invariant mass of the final state. Our calculation resums leading non-trivial logarithms in the ratio of these two scales. We also study the phenomenological relevance of the high-energy corrections for Drell-Yan processes at the LHC. We find that the resummation corrects the NNLO result by a few percent, for values of the invariant mass of the lepton pair below 100 GeV. Corrections to W, Z production are expected to be of the same order.

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arXiv2010

Higgs production in gluon fusion at NNLO for finite top quark mass

The evaluation of the top quark mass suppressed terms to the Higgs production cross section in gluon fusion at next-to-next-to-leading order is reported on. In the region below threshold, the Feynman diagrams are evaluated using asymptotic expansions. The result is then matched to the high-energy limit derived from kT factorization. The result shows that the heavy-top limit used so far approximates the full result to better than 1%.

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arXiv2009

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/mtop expansion of the partonic cross sections to the exact large s-hat limits which are derived from k_T-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.

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arXiv2009

Jet vetoing at the LHC

We study the effect of a veto on additional jets in the rapidity region between a pair of high transverse momentum jets at the LHC. We aim to sum the most important logarithms in the ratio of the jet transverse momentum to the veto scale and to that end we attempt to assess the significance of the super-leading logarithms that appear at high orders in the perturbative expansion. We also compare our results to those of HERWIG++, in an attempt to ascertain the accuracy of the angular ordered parton shower. We find that there are large corrections that arise for large enough jet transverse momenta as a consequence of Coulomb gluon exchanges.

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