arXiv subjects

Giovanni Ridolfi

Publications and source records attributed to Giovanni Ridolfi.

At least 37 records · Page 2Linked to original sources

arXiv2014

Higgs production in gluon fusion beyond NNLO

We construct an approximate expression for the cross section for Higgs production in gluon fusion at next-to-next-to-next-to-leading order (N$^3$LO) in $α_s$ with finite top mass. We argue that an accurate approximation can be constructed by exploiting the analiticity 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 resummations. 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 N$^3$LO result amounts to a correction of 16% 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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arXiv2014

Updated Higgs cross section at approximate N$^3$LO

We update our estimate of the cross section for Higgs production in gluon fusion at next-to-next-to-next-to-leading order (N$^3$LO) in $α_s$ in view of the recent full computation of the result in the soft limit for infinite top mass, which determines a previously unknown constant. We briefly discuss the phenomenological implications. Results are available through the updated version of the ggHiggs code.

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arXiv2014

Polarized Parton Distributions at an Electron-Ion Collider

We study the potential impact of inclusive deep-inelastic scattering data from a future electron-ion collider (EIC) on longitudinally polarized parton distribution (PDFs). We perform a PDF determination using the NNPDF methodology, based on sets of deep-inelastic EIC pseudodata, for different realistic choices of the electron and proton beam energies. We compare the results to our current polarized PDF set, NNPDFpol1.0, based on a fit to fixed-target inclusive DIS data. We show that the uncertainties on the first moments of the polarized quark singlet and gluon distributions are substantially reduced in comparison to NNPDFpol1.0, but also that more measurements may be needed to ultimately pin down the size of the gluon contribution to the nucleon spin.

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arXiv2013

Signal-background interference effects for $gg \to H \to W^+ W^-$ beyond leading order

We study the effect of QCD corrections to the $gg\to H\to W^+ W^-$ signal-background interference at the LHC for a heavy Higgs boson. We construct a soft-collinear approximation to the NLO and NNLO corrections for the background process, which is exactly known only at LO. We estimate its accuracy by constructing and comparing the same approximation to the exact result for the signal process, which is known up to NNLO, and we conclude that we can describe the signal-background interference to better than ten percent accuracy. We show that our result implies that, in practice, a fairly good approximation to higher-order QCD corrections to the interference may also be obtained by rescaling the known LO result by a K-factor computed using the signal process.

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arXiv2013

Unbiased determination of polarized parton distributions and their uncertainties

We present a determination of a set of polarized parton distributions (PDFs) of the nucleon, at next-to-leading order, from a global set of longitudinally polarized deep-inelastic scattering data: NNPDFpol1.0. The determination is based on the NNPDF methodology: a Monte Carlo approach, with neural networks used as unbiased interpolants, previously applied to the determination of unpolarized parton distributions, and designed to provide a faithful and statistically sound representation of PDF uncertainties. We present our dataset, its statistical features, and its Monte Carlo representation. We summarize the technique used to solve the polarized evolution equations and its benchmarking, and the method used to compute physical observables. We review the NNPDF methodology for parametrization and fitting of neural networks, the algorithm used to determine the optimal fit, and its adaptation to the polarized case. We finally present our set of polarized parton distributions. We discuss its statistical properties, test for its stability upon various modifications of the fitting procedure, and compare it to other recent polarized parton sets, and in particular obtain predictions for polarized first moments of PDFs based on it. We find that the uncertainties on the gluon, and to a lesser extent the strange PDF, were substantially underestimated in previous determinations.

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arXiv2013

b-Initiated processes at the LHC: a reappraisal

Several key processes at the LHC in the standard model and beyond that involve $b$ quarks, such as single-top, Higgs, and weak vector boson associated production, can be described in QCD either in a 4-flavor or 5-flavor scheme. In the former, $b$ quarks appear only in the final state and are typically considered massive. In 5-flavor schemes, calculations include $b$ quarks in the initial state, are simpler and allow the resummation of possibly large initial state logarithms of the type $\log \frac{{\cal Q}^2}{m_b^2}$ into the $b$ parton distribution function (PDF), ${\cal Q}$ being the typical scale of the hard process. In this work we critically reconsider the rationale for using 5-flavor improved schemes at the LHC. Our motivation stems from the observation that the effects of initial state logs are rarely very large in hadron collisions: 4-flavor computations are pertubatively well behaved and a substantial agreement between predictions in the two schemes is found. We identify two distinct reasons that explain this behaviour, i.e., the resummation of the initial state logarithms into the $b$-PDF is relevant only at large Bjorken $x$ and the possibly large ratios ${\cal Q}^2/m_b^2$'s are always accompanied by universal phase space suppression factors. Our study paves the way to using both schemes for the same process so to exploit their complementary advantages for different observables, such as employing a 5-flavor scheme to accurately predict the total cross section at NNLO and the corresponding 4-flavor computation at NLO for fully exclusive studies.

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arXiv2013

The scale of soft resummation in SCET vs perturbative QCD

We summarize and extend previous results on the comparison of threshold resummation, performed, using soft-collinear effective theory (SCET), in the Becher-Neubert approach, to the standard perturbative QCD formalism based on factorization and resummation of Mellin moments of partonic cross sections. We show that the logarithmic accuracy of this SCET result can be extended by half a logarithmic order, thereby bringing it in full agreement with the standard QCD result if a suitable choice is made for the soft scale mu_s which characterizes the SCET result. We provide a master formula relating the two approaches for other scale choices. We then show that with the Becher-Neubert scale choice the Landau pole, which in the perturbative QCD approach is usually removed through power- or exponentially suppressed terms, in the SCET approach is removed by logarithmically subleading terms which break factorization. Such terms may become leading for generic choices of parton distributions, and are always leading when resummation is used far enough from the hadronic threshold.

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arXiv2012

The threshold region for Higgs production in gluon fusion

We provide a quantitative determination of the effective partonic kinematics for Higgs production in gluon fusion in terms of the collider energy at the LHC. We use the result to assess, as a function of the Higgs mass, whether the large top mass approximation is adequate and whether Sudakov resummation is advantageous. We argue that our results hold to all perturbative orders. Based on it, we conclude that the full inclusion of finite top mass corrections is likely to be important for accurate phenomenology for a light Higgs with m_H ~ 125 GeV at the LHC with sqrt{s} = 14 TeV.

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arXiv2012

Unbiased Polarised Parton Distribution Functions and their Uncertainties

We present preliminary results on the determination of spin-dependent, or polarised, Parton Distribution Functions (PDFs) from all relevant inclusive polarised DIS data. The analysis is performed within the NNPDF approach, which provides a faithful and statistically sound representation of PDFs and their uncertainties. We describe how the NNPDF methodology has been extended to the polarised case, and compare our results with other recent polarised parton sets. We show that polarised PDF uncertainties can be sizeably underestimated in standard determinations, most notably for the gluon.

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arXiv2012

Theoretical predictions for charm and bottom production at the LHC

We present predictions for a variety of single-inclusive observables that stem from the production of charm and bottom quark pairs at the 7 TeV LHC. They are obtained within the FONLL semi-analytical framework, and with two "Monte Carlo + NLO" approaches, MC@NLO and POWHEG. Results are given for final states and acceptance cuts that are as close as possible to those used by experimental collaborations and, where feasible, are compared to LHC data.

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arXiv2012

Soft gluon resummation of Drell-Yan rapidity distributions: theory and phenomenology

We examine critically the theoretical underpinnings and phenomenological implications of soft gluon (threshold) resummation of rapidity distributions at a hadron collider, taking Drell-Yan production at the Tevatron and the LHC as a reference test case. First, we show that in perturbative QCD soft gluon resummation is necessary whenever the partonic (rather the hadronic) center-of-mass energy is close enough to threshold, and we provide tools to assess when resummation is relevant for a given process. Then, we compare different prescriptions for handling the divergent nature of the series of resummed perturbative corrections, specifically the minimal and Borel prescriptions. We assess the intrinsic ambiguities of resummed results, both due to the asymptotic nature of their perturbative expansion, and to the treatment of subleading terms. Turning to phenomenology, we introduce a fast and accurate method for the implementation of resummation with the minimal and Borel prescriptions using an expansion on a basis of Chebyshev polynomials. We then present results for W and Z production as well as both high- and low-mass dilepton pairs at the LHC, and show that soft gluon resummation effects are generally comparable in size to NNLO corrections, but sometimes affected by substantial ambiguities.

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arXiv2012

Threshold resummation in SCET vs. perturbative QCD: an analytic comparison

We compare threshold resummation in QCD, as performed using soft-collinear effective theory (SCET), to the standard perturbative QCD formalism based on factorization and resummation of Mellin moments of partonic cross-sections. We consider various forms of the SCET result, which correspond to different choices of the soft scale mu_s that characterizes this approach. We derive a master formula that relates the SCET resummation to the QCD result for any choice of mu_s. We then use it first, to show that if SCET resummation is performed in N-Mellin moment space by suitable choice of mu_s it is equivalent to the standard perturbative approach. Next, we show that if SCET resummation is performed by choosing for mu_s a partonic momentum variable, the perturbative result for partonic resummed cross-sections is again reproduced, but like its standard perturbative counterpart it is beset by divergent behaviour at the endpoint. Finally, using the master formula we show that when mu_s is chosen as a hadronic momentum variable the SCET and standard approach are related through a multiplicative (convolutive) factor, which contains the dependence on the Landau pole and associated divergence. This factor depends on the luminosity in a non-universal way; it lowers by one power of log the accuracy of the resummed result, but it is otherwise subleading if one assumes the luminosity not to contain logarithmically enhanced terms. Therefore, the SCET approach can be turned into a prescription to remove the Landau pole from the perturbative result, but the price to pay for this is the reduction by one logarithmic power of the accuracy at each order and the need to make assumptions on the parton luminosity.

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arXiv2008

Borel resummation of transverse momentum distributions

We present a new prescription for the resummation of contributions due to soft gluon emission to the trasverse momentum distribution of processes such as Drell-Yan production in hadronic collisions. We show that familiar difficulties in obtaining resummed results as a function of transverse momentum starting from impact-parameter space resummation are related to the divergence of the perturbative expansion of the momentum-space result. We construct a resummed expression by Borel resummation of this divergent series, removing the divergence in the Borel inversion through the inclusion of a suitable higher twist term. The ensuing resummation prescription is free of numerical instabilities, is stable upon the inclusion of subleading terms, and the original divergent perturbative series is asymptotic to it. We compare our results to those obtained using alternative prescriptions, and discuss the ambiguities related to the resummation procedure.

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arXiv2008

Updated predictions for the total production cross sections of top and of heavier quark pairs at the Tevatron and at the LHC

We present updated predictions for the total production cross section of top-quark pairs at the Tevatron and at the LHC, and, at the LHC, of heavy-quark pairs with mass in the range 0.5-2 TeV. For t\bar{t} production at the LHC we also present results at \sqrt{S}= 10 TeV, in view of the expected accelerator conditions during the forthcoming 2008 run. Our results are accurate at the level of next-to-leading order in alpha_s, and of next-to-leading threshold logarithms (NLO+NLL). We adopt the most recent parametrizations of parton distribution functions, and compute the corresponding uncertainties. We study the dependence of the results on the top mass, and we assess the impact of missing higher-order corrections by independent variations of factorisation and renormalisation scales.

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arXiv2007

A new prescription for soft gluon resummation

We present a new prescription for the resummation of the divergent series of perturbative corrections, due to soft gluon emission, to hard processes near threshold in perturbative QCD (threshold resummation). This prescription is based on Borel resummation, and contrary to the commonly used minimal prescription, it does not introduce a dependence of resummed physical observables on the kinematically unaccessible x->0 region of parton distributions. We compare results for resummed deep-inelastic scattering obtained using the Borel prescription and the minimal prescription and exploit the comparison to discuss the ambiguities related to the resummation procedure.

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arXiv2007

A Positive-Weight Next-to-Leading-Order Monte Carlo for Heavy Flavour Hadroproduction

We present a next-to-leading order calculation of heavy flavour production in hadronic collisions that can be interfaced to shower Monte Carlo programs. The calculation is performed in the context of the POWHEG method. It is suitable for the computation of charm, bottom and top hadroproduction. In the case of top production, spin correlations in the decay products are taken into account.

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arXiv2007

The POWHEG-hvq manual version 1.0

This note documents the use of the POWHEG-hvq package, a generator for heavy flavour hadroproduction at next-to-leading order in QCD, that can be easily interfaced to shower Monte Carlo programs, in such a way that NLO and shower accuracy are both maintained.

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arXiv2007

Higgs Boson Properties in the Standard Model and its Supersymmetric Extensions

We review the realization of the Brout-Englert-Higgs mechanism in the electroweak theory and describe the experimental and theoretical constraints on the mass of the single Higgs boson expected in the minimal Standard Model. We also discuss the couplings of this Higgs boson and its possible decay modes as functions of its unknown mass. We then review the structure of the Higgs sector in the minimal supersymmetric extension of the Standard Model (MSSM), noting the importance of loop corrections to the masses of its five physical Higgs bosons. Finally, we discuss some non-minimal models.

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