arXiv subjects

Giulia Zanderighi

Publications and source records attributed to Giulia Zanderighi.

At least 55 records · Page 3Linked to original sources

arXiv2021

Anomalous couplings in $Zγ$ events at NNLO+PS and improving $ν\barνγ$ backgrounds in dark-matter searches

The measurement of the triple gauge couplings (TGCs) is a central part of diboson studies at the LHC. In this letter we consider the $Zγ$ process and include anomalous TGCs (aTGCs) in the event generation at next-to-next-to-leading order QCD accuracy (NNLO+PS) within the MiNNLO$_{\rm PS}$ framework. While our implementation is fully general and applies to both $Z\to \ell^+\ell^-$ and $Z\to ν\barν$ decays, we focus here on the $ν\barνγ$ final state. After validation of our simulation of $ν\barνγ$ events, for which NNLO+PS accuracy is achieved for the first time, the effects of aTGCs on various distributions are studied. Moreover, we show the relevance of NNLO+PS accuracy for the $ν\barνγ$ background to photon plus missing energy signatures in dark-matter searches, and we compare MiNNLO$_{\rm PS}$ predictions for $ν\barνγ$ production to recent 13 TeV data.

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arXiv2021

$ZZ$ production at nNNLO+PS with MiNNLO$_{\text{PS}}$

We consider $ZZ$ production in hadronic collisions and present state-of-the-art predictions in QCD perturbation theory matched to parton showers. Next-to-next-to-leading order corrections to the quark-initiated channel are combined with parton showers using the MiNNLO$_{\text{PS}}$ method, while next-to-leading order corrections to the loop-induced gluon fusion channel are matched using the POWHEG method. Their combination, dubbed nNNLO+PS, constitutes the best theoretical description of $ZZ$ events to date. Spin correlations, interferences and off-shell effects are included by calculating the full process $pp \to \ell^+\ell^-\ell^{(\prime)+}\ell^{(\prime)-}$. We show the crucial impact of higher-order corrections for both quark- and gluon-initiated processes as well as the relevance of the parton shower in certain kinematical regimes. Our predictions are in very good agreement with recent LHC data.

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arXiv2021

$W^+W^-$ production at NNLO+PS with MiNNLO$_{\rm PS}$

We consider $W^+W^-$ production in hadronic collisions and present the computation of next-to-next-to-leading order accurate predictions consistently matched to parton showers (NNLO+PS) using the MiNNLO$_{\rm PS}$ method. Spin correlations, interferences and off-shell effects are included by calculating the full process $pp \to e^+ν_e μ^-\barν_μ$. This is the first NNLO+PS calculation for $W^+W^-$ production that does not require an a-posteriori multi-differential reweighting. The evaluation time of the two-loop contribution has been reduced by more than one order of magnitude through a four-dimensional cubic spline interpolation. We find good agreement with the inclusive and fiducial cross sections measured by ATLAS and CMS. Both NNLO corrections and matching to parton showers are important for an accurate simulation of the $W^+W^-$ signal, and their matching provides the best description of fully exclusive $W^+W^-$ events to date.

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arXiv2020

Lepton-quark collisions at the Large Hadron Collider

Processes commonly studied at the Large Hadron Collider (LHC) are induced by quarks and gluons inside the protons of the LHC beams. In this letter we demonstrate that, since protons also contain leptons, it is possible to target lepton-induced processes at the LHC as well. In particular, by picking a lepton from one beam and a quark from the other beam, we present for the first time a comprehensive analysis of resonant single leptoquark (LQ) production at a hadron collider. In the case of minimal scalar LQs, we derive novel bounds that arise from the LHC Run II considering all possible flavour combinations of an electron or a muon and an up ($u$), a down ($d$), a strange or a charm quark. For the flavour combinations with a $u$ or a $d$ quark, the obtained limits represent the most stringent constraints to date on LQs of this type. The prospects of our method at future LHC runs are also explored. Given the discovery reach of the proposed LQ signature, we argue that dedicated resonance searches in final states featuring a single light lepton and a single light-flavour jet should be added to the exotics search canon of both the ATLAS and the CMS collaboration.

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arXiv2020

Advancing MiNNLO$_{\rm PS}$ to diboson processes: $Zγ$ production at NNLO+PS

We consider $Z γ$ production in hadronic collisions and present the first computation of next-to-next-to-leading order accurate predictions consistently matched to parton showers (NNLO+PS). Spin correlations, interferences and off-shell effects are included by calculating the full process $pp\to\ell^+\ell^-γ$. We extend the recently developed MiNNLO$_{\rm PS}$ method to genuine $2\to 2$ hard scattering processes at the LHC, which paves the way for NNLO+PS simulations of all diboson processes. This is the first $2\to2$ NNLO+PS calculation that does not require an a-posteriori multi-differential reweighting. We find that both NNLO corrections and matching to parton showers are crucial for an accurate simulation of the $Z γ$ process. Our predictions are in very good agreement with recent ATLAS data.

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arXiv2020

Leptons in the Proton

As is the case for all light coloured Standard Model particles, also photons and charged leptons appear as constituents in ultrarelativistic hadron beams, and admit a parton density function (PDF). It has been shown recently that the photon PDF can be given in terms of the structure functions and form factors for electron-proton scattering. The same holds for lepton PDFs. In the present work we set up a calculation of the lepton PDFs at next-to-leading order, using the same data input needed in the photon case. A precise knowledge of the lepton densities allows us to study lepton-initiated processes even at a hadron collider, with all possible combinations of same-charge, opposite-charge, same-flavour, different-flavour leptons and leptons-quarks, most of which cannot be realized in any other foreseeable experiment. The lepton densities in the proton are extremely small, so that their contribution to Standard Model processes is generally shadowed by processes initiated by coloured partons. We will show, however, that there are cases where these processes can be relevant, giving rise to rare Standard Model signatures and to new production channels, that can enlarge the discovery potential of New Physics at the LHC and future high energy colliders with hadrons in the initial state.

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arXiv2020

NNLOPS description of the H$\to\! b\bar{b}$ decay with MINLO

We present an event generator that describes the Higgs boson decay into $b$-quarks at next-to-next-to-leading-order (NNLO) in QCD and allows for a consistent matching to parton shower. For this purpose, we work within the POWHEG framework and employ the MINLO method to produce an NNLO-accurate H$\to\! b\bar{b}$ event sample which can further be interfaced with a generic Higgs boson production mode. The resulting events, that combine Higgs production and decay, can be matched to parton shower by means of a vetoed shower. As an example, we present a short phenomenological study for associated Higgs production (HZ) and vector boson fusion (VBF) processes. We release the source code that was used to obtain these results as an $\text{h_bbg}$ user process in POWHEG-BOX-V2. We also provide tools that enable interfacing the decay events with events for an arbitrary Higgs production mode and subsequent matching of the combined Les Houches (LH) events with a parton shower.

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arXiv2019

High precision determination of $α_s$ from a global fit of jet rates

We present state-of-the-art extractions of the strong coupling based on N$^3$LO+NNLL accurate predictions for the two-jet rate in the Durham clustering algorithm at $e^+e^-$ collisions, as well as a simultaneous fit of the two- and three-jet rates taking into account correlations between the two observables. The fits are performed on a large range of data sets collected at LEP and PETRA colliders, with energies spanning from $35$ GeV to $207$ GeV. Owing to the high accuracy of the predictions used, the perturbative uncertainty is considerably smaller than that due to hadronization. Our best determination at the $Z$ mass is $α_s(M_Z) = 0.11881 \pm 0.00063(exp.) \pm 0.00101(hadr.) \pm 0.00045(ren.) \pm 0.00034(res.)$, which is in agreement with the latest world average and has a comparable total uncertainty.

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arXiv2018

MINLO t-channel single-top plus jet

We present a next-to-leading order accurate simulation of t-channel single-top plus jet production matched to parton showers via the POWHEG method. The calculation underlying the simulation is enhanced with a process-specific implementation of the multi-scale improved NLO (MINLO) method, such that it gives physical predictions all through phase space, including regions where the jet additional to the t-channel single-top process is unresolved. We further describe a tuning procedure for the MINLO Sudakov form factor, fitting the coefficient of the first subleading term in its exponent using an artificial neural-network. The latter tuning, implemented as a straightforward event-by-event reweighting, renders the MINLO simulation NLO accurate for t-channel single-top observables, in addition to those of the analogous single-top plus jet process.

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arXiv2018

NNLOPS accurate predictions for $W^+W^-$ production

We present novel predictions for the production of $W^+W^-$ pairs in hadron collisions that are next-to-next-to-leading order accurate and consistently matched to a parton shower (NNLOPS). All diagrams that lead to the process $pp\to e^- \bar ν_e\;μ^+ν_μ+X$ are taken into account, thereby including spin correlations and off-shell effects. For the first time full NNLOPS accuracy is achieved for a $2\to 4$ process. We find good agreement, at the 1$σ$ level, with the $W^+W^-$ rates measured by ATLAS and CMS. The importance of NNLOPS predictions is evident from differential distributions sensitive to soft-gluon effects and from the large impact ($10$% and more) of including next-to-next-to-leading order corrections on top of MiNLO. We define a charge asymmetry for the $W$ bosons and the leptons in $W^+W^-$ production at the LHC, which is sensitive to the $W$ polarizations and hence can be used as a probe of new physics.

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arXiv2018

The 2016 European School of High-Energy Physics

The European School of High-Energy Physics is intended to give young physicists an introduction to the theoretical aspects of recent advances in elementary particle physics. These proceedings contain lecture notes on the theory of the Weak interaction and Higgs physics, flavour physics and CP violation, neutrinos, theories beyond the Standard Model, physics at the LHC Run-2 and beyond, practical statistics for high-energy physicists, and cosmology and dark matter.

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arXiv2018

NNLOPS accurate associated HZ production with NLO decay ${\rm{H}} \to b\bar{b}$

We present a next-to-next-to-leading order (NNLO) accurate description of associated HZ production, followed by the Higgs boson decay into a pair of $b$-quarks treated at next-to-leading order (NLO), consistently matched to a parton shower (PS). The matching is achieved by performing reweighting of the $\texttt{HZJ-MiNLO}$ events, using multi-dimensional distributions that are fully-differential in the HZ Born kinematics, to the NNLO results obtained by using the $\texttt{MCFM-8.0}$ fixed-order calculation. Additionally we include the $gg\to\rm{HZ}$ contribution to the discussed process that appears at the $\mathcal{O}(α_s^2)$. We present phenomenological results obtained for 13 TeV hadronic collisions.

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arXiv2018

Fully differential VBF Higgs production at NNLO

We calculate the fully differential next-to-next-to-leading-order (NNLO) corrections to vector-boson fusion (VBF) Higgs production at proton colliders, in the limit in which there is no cross-talk between the hadronic systems associated with the two protons. We achieve this using a new "projection-to-Born" method that combines an inclusive NNLO calculation in the structure-function approach and a suitably factorised next-to-leading-order (NLO) VBF Higgs plus 3-jet calculation, using appropriate Higgs plus 2-parton counter events. An earlier calculation of the fully inclusive cross section had found small NNLO corrections, at the 1% level. In contrast, the cross section after typical experimental VBF cuts receives NNLO contributions of about 4%, while differential distributions show corrections of up to 6-7% for some standard observables. The corrections are often outside the NLO scale-uncertainty band.

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arXiv2017

Constraints on the trilinear Higgs coupling from vector boson fusion and associated Higgs production at the LHC

We examine the constraints on the trilinear Higgs coupling $λ$ that originate from associated ($Vh$) and vector boson fusion (VBF) Higgs production in $pp$ collisions in the context of the Standard Model effective field theory. The 1-loop contributions to $pp \to V h$ and $pp \to jj h$ that stem from insertions of the dimension-6 operator $O_6 = - λ\left (H^\dagger H \right )^3$ are calculated and combined with the ${\cal O} (λ)$ corrections to the partial decay widths of the Higgs boson. Employing next-to-next-to-leading order QCD predictions, we analyse the sensitivity of current and forthcoming measurements of the signal strengths in $Vh$ and VBF Higgs production to changes in $λ$. We show that future LHC runs may be able to probe modifications of $λ$ with a sensitivity similar to the one that is expected to arise from determinations of double-Higgs production. The sensitivity of differential $Vh$ and VBF Higgs distributions to a modified $h^3$ coupling is also studied.

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arXiv2017

The Photon Content of the Proton

The photon PDF of the proton is needed for precision comparisons of LHC cross sections with theoretical predictions. In a recent paper, we showed how the photon PDF could be determined in terms of the electromagnetic proton structure functions $F_2$ and $F_L$ measured in electron-proton scattering experiments, and gave an explicit formula for the PDF including all terms up to next-to-leading order. In this paper we give details of the derivation. We obtain the photon PDF using the factorisation theorem and applying it to suitable BSM hard scattering processes. We also obtain the same PDF in a process-independent manner using the usual definition of PDFs in terms of light-cone Fourier transforms of products of operators. We show how our method gives an exact representation for the photon PDF in terms of $F_2$ and $F_L$, valid to all orders in QED and QCD, and including all non-perturbative corrections. This representation is then used to give an explicit formula for the photon PDF to one order higher than our previous result. We also generalise our results to obtain formulæ for the polarised photon PDF, as well as the photon TMDPDF. Using our formula, we derive the $P_{γi}$ subset of DGLAP splitting functions to order $αα_s$ and $α^2$, which agree with known results. We give a detailed explanation of the approach that we follow to determine a photon PDF and its uncertainty within the above framework.

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arXiv2017

On the excess in the inclusive $W^+W^-\rightarrow l^+l^-ν\barν$ cross section

In this note we analyse the excess in the $W^+W^-$ inclusive cross section recently measured by the LHC experiments. We point out that in fact for the ATLAS fiducial cross sections there is no excess in the measurements compared to the NLO QCD predictions. We also argue that higher order effects to the fiducial cross section are small, and tend to cancel each other, hence the inclusion of NNLO and NNLL corrections will not modify this agreement significantly. We find that at 8 TeV a substantial part of the disagreement with the NLO prediction for the total cross section observed by ATLAS is due to the extrapolation carried out with POWHEG.

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arXiv2017

A general method for the resummation of event-shape distributions in e^+e^- annihilation

We present a novel method for resummation of event shapes to next-to-next-to-leading-logarithmic (NNLL) accuracy. We discuss the technique and describe its implementation in a numerical program in the case of e^+e^- collisions where the resummed prediction is matched to NNLO. We reproduce all the existing predictions and present new results for oblateness and thrust major.

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arXiv2017

Quark masses in Higgs production with a jet veto

We study the impact of finite mass effects due to top and bottom loops in the jet-veto distribution for Higgs production. We discuss the appearance of non-factorizing logarithms in the region pt > m_b. We study their numerical impact and argue that these terms can be treated as a finite remainder. We therefore detail our prescription for resumming the jet-vetoed cross section and for assessing its uncertainty in the presence of finite mass effects. Resummation for the jet-veto, including mass effects, has been implemented in the public code JetVHeto.

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