Littérature scientifique sur le sujet « Generatori NLO+PS »

Abstract The search for new interactions and particles in high-energy collider physics relies on precise background predictions. This has led to many advances in combining precise fixed-order cross-section calculations with detailed event generator simulations. In recent years, fixed-order qcd calculations of inclusive cross sections at n3lo precision have emerged, followed by an impressive progress at producing differential results. Once differential results become publicly available, it would be prudent to embed these into event generators to allow the community to leverage these advances. This note offers some concrete thoughts on me+ps matching at third order in qcd. As a method for testing these thoughts, a toy calculation of e+e− → u$$ \overline{u} $$ u ¯ at $$ \mathcal{O} $$ O ($$ {\alpha}_s^3 $$ α s 3 ) is constructed, and combined with an event generator through unitary matching. The toy implementation may serve also as blueprint for high-precision qcd predictions at future lepton colliders. As a byproduct of the n3lo matching formula, a new nnlo+ps formula for processes with “additional” jets is obtained.

Abstract Motivated by the long-standing hints of lepton-flavour non-universality in the b → cℓν and b → sℓ+ℓ− channels, we study Drell-Yan ditau production at the Large Hadron Collider (LHC). In the context of models with third-generation gauge vector leptoquarks (LQs), we calculate the complete $$ \mathcal{O} $$ O (αs) corrections to the pp → τ+τ− process, achieving next-to-leading order (NLO) plus parton shower (NLO+PS) accuracy using the POWHEG method. We provide a dedicated Monte Carlo code that evaluates the NLO QCD corrections on-the-fly in the event generation and use it to study the numerical impact of NLO+PS corrections on the kinematic distributions that enter the existing experimental searches for non-resonant ditau final states. Based on our phenomenological analysis we derive NLO accurate constraints on the masses and couplings of third-generation gauge vector LQs using the latest LHC ditau search results corresponding to an integrated luminosity of around 140 fb−1 of proton-proton collisions at $$ \sqrt{s} $$ s = 13 TeV. The presented NLO+PS generator allows for an improved signal modelling, making it an essential tool for future ATLAS and CMS searches for vector LQs in τ+τ− final states at LHC Run III and beyond.

Abstract This paper is a follow-up of Ref. Ferrario Ravasio et al. (Eur Phys J C 78:458, 2018. arXiv:1801.03944), where we studied the impact of next-to-leading order calculations merged with parton shower generators (NLO+PS) of increasing accuracy in the extraction of the top mass at hadron colliders. Here we examined results obtained with the older (fortran-based) shower generators and . Our findings are in line with what we found in Ref. Ferrario Ravasio et al. (2018) with the new, c++-based, generators and .

The collinear factorization theorem, combined with finite-order calculations in perturbative QCD, provides a powerful framework to obtain predictions for many collider observables. However, for observables which involve multiple energy scales, transverse degrees of freedom cannot be neglected, and finite-order perturbative calculations have to be combined with resummed calculations to all orders in the QCD running coupling in order to obtain reliable theoretical predictions, capable of describing experimental measurements. This is traditionally done either by analytic resummation methods or by parton shower (PS) Monte Carlo (MC) methods. In this talk we present the Parton Branching (PB) MC method to obtain QCD collider predictions based on Transverse Momentum Dependent (TMD) factorization. The PB provides evolution equations for TMD Parton Distribution Functions (PDFs) which, upon fitting TMD PDFs to experimental data, can be used in TMD MC event generators. We present the basic concepts of the method and illustrate its applications to collider measurements focusing on Drell-Yan (DY) lepton-pair production in different kinematic ranges, from fixed-target to LHC energies. We discuss the latest developments of the method concentrating especially on the matching of next-to-leading-order (NLO) TMD evolution with MC-at-NLO calculations of NLO matrix elements.

Abstract The radial distribution of $$D^0$$D0 mesons in jets probes the diffusion of charm quark relative to the jet axis and provides a new perspective to study the interaction mechanisms between heavy quarks and the medium in the nucleus-nucleus collisions. The in-medium parton propagations are described by a Monte Carlo transport model which uses the next-to-leading order (NLO) plus parton shower (PS) event generator SHERPA as input and includes elastic (collisional) and inelastic (radiative) interaction for heavy quarks as well as light partons. At low $$D^0$$D0 meson $$p_T$$pT, the radial distribution significantly shifts to larger radius indicating a strong diffusion effect which is consistent with the recent experimental data. We demonstrate that the angular deviation of charm quarks declines with $$p_T$$pT and is very sensitive to the collisional more than radiative interaction at $$p_T<5$$pT<5 GeV. As predictions, we present the $$D^0$$D0 meson radial distribution in jets in p + p and $$0{-}10\%$$0-10% Au + Au collisions at $$\sqrt{s_{NN}}=200$$sNN=200 GeV at the RHIC, and also estimate the nuclear modification factor of charm jet in central Au + Au collisions at 200 GeV at the RHIC and central Pb + Pb collisions at 5.02 TeV at the LHC.

La tesi si occupa delle correzioni di potenza calcolate su un parametro-soglia di momento trasverso che interessano calcoli perturbativi di ordine superiore nella costante di accoppiamento forte. In particolare, si impone un parametro-soglia di momento trasverso, qt_cut, su uno stato finale bianco e si calcolano le correzioni di potenza sul parametro-soglia, fino alla quarta potenza, per la sezione d'urto inclusiva al primo ordine sottodominante in QCD. Inoltre, si considera lo stesso processo al secondo ordine sottodominante in QCD, limitandosi al contributo reale-virtuale, e si calcolano le correzioni di potenza per la sezione d'urto inclusiva sino alla seconda potenza sul parametro-soglia. Lo studio della dipendenza della sezione d'urto dal parametro-soglia permette di approfondire il comportamento della prima nei limiti dello spazio delle fasi, dando indizi sulla struttura a tutti gli ordini nella costante di accoppiamento forte e sull'identificazione di caratteristiche universali. La conoscenza delle correzioni di potenza è poi un ingrediente fondamentale per ridurre la dipendenza delle sezioni d'urto di QCD dal parametro-soglia quando viene applicato un metodo slicing come la sottrazione-qt. Si presentano, quindi, risultati analitici per la produzione di un bosone vettore e per la produzione di un bosone di Higgs al primo ordine sottodominante in QCD, e per la parte reale-virtuale del canale qg per la produzione di un bosone vettore al secondo ordine. Nel dettaglio, si illustra una procedura generale per il calcolo delle correzioni di potenza sul parametro-soglia. Per mostrare l'impatto numerico di tali correzioni, si presentano dei risultati numerici e si discute di come la dipendenza residuale da qt_cut interessi la sezione d'urto totale della produzione di un bosone Z o di un bosone di Higgs presso LHC. La seconda parte della tesi, complementare alla prima, è dedicata allo sviluppo di un'interfaccia tra MadGraph5_aMC@NLO e POWHEG BOX, al fine di coniugare la flessibilità di MadGraph quando genera gli elementi di matrice per processi del Modello Standard o di sue estensioni, con tutte le caratteristiche di POWHEG BOX. Tra di esse, la possibilità di generare eventi con peso positivo è fondamentale e rende POWHEG uno dei metodi più usati quando sono necessari campioni molto numerosi. Al fine di testare l'interfaccia, si studia la produzione di un bosone di spin 0 assieme a due getti, con accoppiamenti che violano CP. Si discutono delle distribuzioni che caratterizzano le proprietà del bosone e alcuni risultati ottenuti con la funzione di ripesamento di POWHEG BOX. Infine, si presentano alcune distribuzioni ottenute con il metodo MiNLO.
The thesis deals with the power corrections in a transverse-momentum cutoff that affect perturbative calculations at higher orders in the strong coupling constant. In particular, we impose a transverse-momentum cutoff, qt_cut, on a colourless final state and we compute the power corrections for the inclusive QCD next-to-leading-order cross section in the cutoff, up to the fourth power. We also consider the same production process at next-to-next-to-leading order in QCD, restricting ourselves to the real-virtual contribution, and we compute the power corrections up to the second power in the cutoff for the inclusive cross section. The study of the dependence of the cross section on qt_cut allows for an understanding of its behaviour at the boundaries of the phase space, giving hints on the structure at all orders in the strong coupling constant and on the identification of universal patterns. The knowledge of such power corrections is also a required ingredient in order to reduce the dependence on the transverse-momentum cutoff of the QCD cross sections at higher orders, when the qt-subtraction method, i.e. a slicing scheme, is applied. We present analytic results for both Drell-Yan vector-boson and Higgs-boson production in gluon fusion at next-to-leading order in QCD, and for the real-virtual part of the qg-initiated channel of vector-boson production at NNLO in QCD. In particular, we illustrate a process-independent procedure for the calculation of the all-order power corrections in the cutoff. In order to show the impact of the power-correction terms, we present selected numerical results and discuss how the residual dependence on qt_cut affects the total cross section for Drell-Yan Z production and Higgs boson production via gluon fusion at the Large Hadron Collider. A second and complementary part of the thesis is devoted to the development of an interface between MadGraph5_aMC@NLO and the POWHEG BOX framework, in order to match the flexibility of MadGraph for the generation of matrix elements for Standard-Model processes and for several of its extensions, to all features of the POWHEG BOX framework. Among those, it is essential the possibility, via the POWHEG method, to generate events with positive weights, which makes it the method of choice when large samples of events are needed. As a proof of concept, we provide a phenomenological study for the production of a spin-0 Higgs-like boson, in association with up to two jets, with CP-violating couplings. We discuss a few distributions able to characterise the spin-0 boson CP properties, and discuss a few results obtained using the POWHEG BOX reweighting feature. We also present a few distributions obtained with the MiNLO method.