QCD Accuracy Research Articles

Matching NNLO predictions to parton showers using N3LL color-singlet transverse momentum resummation in geneva

We extend the geneva Monte Carlo framework using the transverse momentum of a color-singlet system as the resolution variable. This allows us to use next-to-next-to-next-to-leading-logarithm (${\mathrm{N}}^{3}\mathrm{LL}$) resummation via the radish formalism to obtain precise predictions for any color-singlet production process at the fully exclusive level. Thanks to the implementation of two different resolution variables within the geneva framework, we are able to assess the impact of such a choice on differential observables for the first time. As a first application, we present predictions for Drell-Yan lepton-pair production at next-to-next-to-leading order (NNLO) in QCD interfaced to a parton shower simulation that includes additional all-order radiative corrections. We provide fully showered and hadronized events using pythia8, while retaining the NNLO QCD accuracy for observables which are inclusive over the additional radiation. We also show that it is possible to obtain a numerically good agreement between showered geneva predictions and the ${\mathrm{N}}^{3}\mathrm{LL}$ resummation for the transverse momentum spectrum by choosing a more local recoil scheme. We compare our final predictions to LHC data at 13 TeV, finding good agreement across several distributions.

Scalar-pseudoscalar pair production at the Large Hadron Collider at NLO+NLL accuracy in QCD * *Supported in part by the National Natural Science Foundation of China (11775211, 12061141005, 11805001, 11935001) and the CAS Center for Excellence in Particle Physics (CCEPP)

We thoroughly investigate both transverse momentum and threshold resummation effects on scalar-pseudoscalar pair production via quark-antiquark annihilation at the Large Hadron Collider at QCD NLO+NLL accuracy. A factorization method is introduced to properly supplement the soft-gluon (threshold) resummation contribution from parton distribution functions to the resummed results obtained by the Collins-Soper-Sterman resummation approach. We find that the impact of the threshold-resummation improved PDFs is comparable to the resummation effect of the partonic matrix element and can even predominate in high invariant mass regions. Moreover, the loop-induced gluon-gluon fusion channel in the type-I two-Higgs-doublet model is considered in our calculations. The numerical results show that the electroweak production via quark-antiquark annihilation dominates over the gluon-initiated QCD production by orders of magnitude.

QCD analysis of pion fragmentation functions in the xFitter framework

We present the first open-source analysis of fragmentation functions (FFs) of charged pions (entitled IPM-xFitter) computed at next-to-leading order (NLO) and next-to-next-to-leading order (NNLO) accuracy in perturbative QCD using the xFitter framework. This study incorporates a comprehensive and up-to-date set of pion production data from single-inclusive annihilation (SIA) processes, as well as the most recent measurements of inclusive cross-sections of single pion by the BELLE collaboration. The determination of pion FFs along with their theoretical uncertainties is performed in the Zero-Mass Variable-Flavor Number Scheme (ZM-VFNS). We also present comparisons of our FFs set with recent fits from the literature. The resulting NLO and NNLO pion FFs provide valuable insights for applications in present and future high-energy analysis of pion final state processes.

Nuclear parton distribution functions with uncertainties in a general mass variable flavor number scheme

In this article we obtain a new set of nuclear parton distribution functions (nuclear PDFs) at next-to-leading order and next-to-next-to-leading order accuracy in perturbative QCD. The common nuclear deep-inelastic scattering (DIS) data analyzed in our study are complemented by the available charged-current neutrino DIS data with nuclear targets and data from Drell-Yan cross-section measurements for several nuclear targets. In addition, the most recent DIS data from the Jefferson Lab CLAS and Hall C experiments are also added to our data sample. For these specific datasets, we consider the impact of target mass corrections and higher twist effects which are expected to be important in the region of large $x$ and intermediate-to-low $Q^2$. Our analysis is based on a publicly available open-source tool {\tt APFEL}, which has been modified to be applicable for our analysis of nuclear PDFs. Heavy quark contributions to nuclear DIS are considered within the framework of the {\tt FONLL} general-mass variable-flavor-number scheme. The most recent {\tt CT18} PDFs are used as baseline proton PDFs. The uncertainties of nuclear PDFs are determined using the standard Hessian approach. The results of our global QCD analysis are compared with existing nuclear PDF sets and with the fitted cross-sections, for which our set of nuclear PDFs provides a very good description.

Determination of unpolarized pion fragmentation functions using semi-inclusive deep-inelastic-scattering data

We present MAPFF1.0, a determination of unpolarised charged-pion fragmentation functions (FFs) from a set of single-inclusive $e^+e^-$ annihilation and lepton-nucleon semi-inclusive deep-inelastic-scattering (SIDIS) data. FFs are parametrised in terms of a neural network (NN) and fitted to data exploiting the knowledge of the analytic derivative of the NN itself w.r.t. its free parameters. Uncertainties on the FFs are determined by means of the Monte Carlo sampling method properly accounting for all sources of experimental uncertainties, including that of parton distribution functions. Theoretical predictions for the relevant observables, as well as evolution effects, are computed to next-to-leading order (NLO) accuracy in perturbative QCD. We exploit the flavour sensitivity of the SIDIS measurements delivered by the HERMES and COMPASS experiments to determine a minimally-biased set of seven independent FF combinations. Moreover, we discuss the quality of the fit to the SIDIS data with low virtuality $Q^2$ showing that, as expected, low-$Q^2$ SIDIS measurements are generally harder to describe within a NLO-accurate perturbative framework.

Differential heavy quark distributions and correlations in longitudinally polarized deep-inelastic scattering

We present a first calculation of the heavy flavor contribution to the longitudinally polarized deep-inelastic scattering structure function $g_1^{Q}$, differential in the transverse momentum or the rapidity of the observed heavy quark $Q$ or antiquark $\overline Q$. All results are obtained at next-to-leading order accuracy in QCD within the framework of a newly developed parton-level Monte Carlo generator that also allows one to study observables associated with the produced heavy quark pair such as its invariant mass distribution or its correlation in azimuthal angle. First phenomenological studies are carried out for various heavy quark distributions in a kinematic regime relevant for a future Electron-Ion Collider with a particular emphasis on the expected size of the corresponding double-spin asymmetries and their sensitivity to the still poorly constrained helicity gluon distribution. Theoretical uncertainties associated with the choice of the factorization scale are discussed for selected observables.

NLO fragmentation functions for a quark into a spin-singlet quarkonium: same flavor case

In the paper, we calculate the fragmentation functions for c → ηc and b → ηb up to next-to-leading-order (NLO) QCD accuracy. The ultraviolet divergences in the real corrections are removed through operator renormalization under the modified min- imal subtraction scheme. We then obtain the fragmentation functions {D}_{cto {eta}_c} (z, μF) and {D}_{bto {eta}_b} (z, μF) up to NLO QCD accuracy, which are presented as figures and fitting functions. The numerical results show that the NLO corrections are significant. The sensitives of the fragmentation functions to the renormalization scale and the factorization scale are analyzed explicitly.

Signature of the maximally symmetric 2HDM via W±/Z -quadruplet productions at the LHC

We consider the Maximally Symmetric Two-Higgs Doublet Model (MS-2HDM) in which the so-called Standard Model (SM) alignment can be achieved naturally by the virtue of an SO(5) symmetry imposed on the 2HDM. We investigate the signature of the MS-2HDM via $p p \to HX \to VV^* X$ and $p p \to HHX \to VV^*V'V'^* X$ processes at the LHC for different values of $\tan \beta$. We perform our calculations with NLO QCD accuracy, using the \textsf{Herwig~7} multi-purpose event generator at $\sqrt{s}=13$~TeV center-of-mass energy. We show that the production of single SM-like Higgs bosons via $W^{\pm}/Z$-pairs is completely aligned with the SM. Interestingly, the presence of the heavy Higgs states significantly enhances the cross-section for the $W^{\pm}/Z$-quadruplet production channels in the low-$p_{\perp}$ regions. These vital analyses may aid the future discovery of this minimal and very predictive extension of the SM and can be generalised to other realisations of the 2HDM.

Precise predictions for photon pair production matched to parton showers in GENEVA

We present a new calculation for the production of isolated photon pairs at the LHC with {mathrm{NNLL}}_{{mathcal{T}}_0}^{prime } +NNLO accuracy. This is the first implementation within the Geneva Monte Carlo framework of a process with a nontrivial Born-level definition which suffers from QED singularities. Throughout the computation we use a smooth-cone isolation algorithm to remove such divergences. The higher-order resummation of the 0-jettiness resolution variable {mathcal{T}}_0 is based on a factorisation formula derived within Soft-Collinear Effective Theory which predicts all of the singular, virtual and real NNLO corrections. Starting from this precise parton-level prediction and by employing the Geneva method, we provide fully showered and hadronised events using Pythia8, while retaining the NNLO QCD accuracy for observables which are inclusive over the additional radiation. We compare our final predictions to LHC data at 7 TeV and find good agreement.

Simultaneous extraction of fragmentation functions of light charged hadrons with mass corrections

Achieving the highest possible precision for theoretical predictions at the present and future high-energy lepton and hadron colliders requires a precise determination of fragmentation functions (FFs) of light and heavy charged hadrons from a global QCD analysis with great accuracy. We describe a simultaneous determination of unpolarized FFs of charged pions, charged kaons and protons/antiprotons from single-inclusive hadron production in electron-positron annihilation (SIA) data at next-to-leading order and next-to-next-to-leading order accuracy in perturbative QCD. A new set of FFs, called SGKS20, is presented. We include data for identified light charged hadrons (${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$, ${K}^{\ifmmode\pm\else\textpm\fi{}}$ and $p/\overline{p}$) as well as for unidentified light charged hadrons, ${h}^{\ifmmode\pm\else\textpm\fi{}}$ and show that these data have a significant impact on both size and uncertainties of the fragmentation functions. We examine the inclusion of higher-order perturbative QCD corrections and finite-mass effects. We compare the new SGKS20 FFs with other recent FFs available in the literature and find in general reasonable agreement, but also important differences for some parton species. We show that theoretical predictions obtained from our new FFs are in very good agreement with the analyzed SIA data, especially at small values of $z$. The SGKS20 FF sets presented in this work are available via the LHAPDF interface.

Predictions for Z -Boson Production in Association with a b -Jet at O(αs3)

Precise predictions are provided for the production of a $\mathrm{Z}$-boson and a $\mathrm{b}$-jet in hadron-hadron collisions within the framework of perturbative QCD, at $\mathcal{O}(\alpha_s^3)$. To obtain these predictions we perform the first calculation of a hadronic scattering process involving the direct production of a flavoured-jet at next-to-next-to-leading order accuracy in massless QCD, and extend techniques to also account for the impact of finite heavy-quark mass effects. The predictions are compared to CMS data obtained in $\mathrm{pp}$ collisions at a centre-of-mass energy of $8~\mathrm{TeV}$, which are the most precise data from Run I of the LHC for this process, where a good description of the data is achieved. To allow this comparison we have performed an unfolding of the data, which overcomes the long-standing issue that the experimental and theoretical definitions of jet flavour are incompatible.

Determination of generalized parton distributions through a simultaneous analysis of axial form factor and wide-angle Compton scattering data

In this work, we present a new set of unpolarized ($ H $) and polarized ($\widetilde{H}$) generalized parton distributions (GPDs) that have been determined using a simultaneous $ \chi^2 $ analysis of the nucleon axial form factor (AFF) and wide-angle Compton scattering (WACS) experimental data at the next-to-leading order (NLO) accuracy in QCD. We explore various Ansatzes presented in the literature for GPDs, which use forward parton distributions as input, and choose the ones most suited to our analysis. The experimental data included in our analysis cover a wide range of the squared transverse momentum, which is $ 0.025 < -t < 6.46 $ GeV$ ^2 $. We show that the WACS data affect significantly the large $-t$ behavior of $\widetilde{H}$. The polarized GPDs obtained from the simultaneous analysis of AFF and WACS data differ considerably from the corresponding ones obtained by analyzing AFF and WACS separately, and have less uncertainties. We show that the theoretical predictions obtained using our GPDs are in good agreement with the analyzed AFF and WACS data for the entire range of $ -t $ studied. Finally, we obtain the impact parameter dependent parton distributions, both in an unpolarized and in a transversely polarized proton, and present them as tomography plots.

Fully exclusive heavy quark-antiquark pair production from a colourless initial state at NNLO in QCD

We present a local subtraction scheme for computing next-to-next-to-leading order QCD corrections to the production of a massive quark-antiquark pair from a colourless initial state. The subtraction terms are built following the CoLoRFulNNLO method and refined in such a way that their integration gives rise to compact, fully analytic expressions. All ingredients necessary for a numerical implementation of our subtraction scheme are provided in detail. As an example, we calculate the fully differential decay rate of the Standard Model Higgs boson to massive bottom quarks at next-to-next-to-leading order accuracy in perturbative QCD.

Precise predictions for double-Higgs production via vector-boson fusion

Theoretical predictions with next-to-next-to-leading order (NNLO) QCD accuracy combined with the next-to-leading order (NLO) electroweak (EW) corrections are presented for differential observables of the double-Higgs production process via vector-boson fusion. While the QCD corrections were previously known, the EW ones are computed here for the first time. The numerical results are obtained for a realistic experimental set-up at the LHC and are presented in the form of fiducial cross sections and differential distributions. Within this setup we find that the VBF approximation employed in the NNLO QCD correction is accurate at the sub-percent level. We find that the NLO EW corrections within the fiducial volume are -,6.1%, making them of almost the same order as the NLO QCD corrections. In some kinematic regions they can grow as large as -,30% making them the dominant radiative corrections. When the EW corrections are combined with the NNLO QCD corrections we find a total correction of -,14.8%. The results presented here thus comprise the state-of-the-art theoretical predicition for the double-Higgs production via vector-boson fusion, which will be of value to the high-luminosity programme at the LHC.

Defiducialization: providing experimental measurements for accurate fixed-order predictions

An experimental procedure is proposed to perform measurements of differential cross sections for vector boson production which can be compared to fixed-order QCD predictions with improved accuracy. The procedure relies on applying theoretical acceptance corrections computed as a function of the transverse momentum of the W/Z boson, p_T, to the experimental measurement, rather than comparing data directly against fiducial fixed-order predictions. It is demonstrated that, contrary to standard fiducial computations, these acceptance factors vary little at low p_T, so they can be reliably computed using fixed-order perturbation theory. An example analysis is performed using the ATLAS measurement of the Z-boson production cross section at center-of-mass energy of 8 TeV. The resulting full phase space measurement of the cross section differential in the boson rapidity is compared to theoretical predictions computed with next-to-next-to leading-order accuracy in QCD. Further extensions of the approach which include different types of measurements and improved theoretical predictions are discussed.

The simplest of them all: toverline{t}{W}^{pm } at NLO accuracy in QCD

Recent measurements of the pp → toverline{t}{W}^{pm } process in multi-lepton final states, as performed by the ATLAS collaboration in the context of the Higgs boson studies in the toverline{t}H channel, have shown discrepancies between theoretical predictions and experimental data. Such discrepancies have been observed both in the overall normalisation as well as in the modelling of the toverline{t}{W}^{pm } process. With the goal of understanding and resolving the modelling issues within the SM toverline{t}{W}^{pm } process we report on the state-of-the-art NLO QCD computation for this process. Specifically, we calculate higher-order corrections to the {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }{e}^{+}{nu}_eboverline{b} and {e}^{-}{overline{nu}}_e{mu}^{+}{nu}_{mu }{e}^{-}{overline{nu}}_eboverline{b} final state at the LHC with sqrt{s} = 13 TeV. In the computation off-shell top quarks are described by Breit-Wigner propagators, furthermore, double-, single- as well as non-resonant top-quark contributions along with all interference effects are consistently incorporated at the matrix element level. Results at NLO QCD accuracy are presented in the form of fiducial integrated and differential cross sections for two selected renormalisation and factorisation scale choices and three different PDF sets. The impact of the top quark off-shell effects on the toverline{t}{W}^{pm } cross section is also examined by an explicit comparison to the narrow-width approximation.

Parton-shower effects in Higgs production via vector-boson fusion

We present a systematic investigation of parton-shower and matching uncertainties of perturbative origin for Higgs-boson production via vector-boson fusion. To this end we employ different generators at next-to-leading order QCD accuracy matched with shower Monte Carlo programs, PYTHIA8, and HERWIG7, and a next-to-next-to-leading order QCD calculation. We thoroughly analyse the intrinsic sources of uncertainty within each generator, and then compare predictions among the different tools using the respective recommended setups. Within typical vector-boson fusion cuts, the resulting uncertainties on observables that are accurate to next-to-leading order are at the 10% level for rates and even smaller for shapes. For observables sensitive to extra radiation effects uncertainties of about 20% are found. We furthermore show how a specific recoil scheme is needed when PYTHIA8 is employed, in order not to encounter unphysical enhancements for these observables. We conclude that for vector-boson fusion processes an assessment of the uncertainties associated with simulation at next-to-leading order matched to parton showers based only on the variation of renormalisation, factorisation and shower scales systematically underestimates their true size.

Scalar leptoquark pair production at hadron colliders

We revisit scalar leptoquark pair-production at hadron colliders and significantly improve the level of precision of the cross section calculations. Apart from QCD contributions, we include lepton t-channel exchange diagrams that turn out to be relevant in the light of the recent B-anomalies. We evaluate all contributions at next-to-leading-order accuracy in QCD and resum, in the threshold regime, soft-gluon radiation at next-to-next-to-leading logarithmic accuracy. Our predictions consist hence in the most precise leptoquark cross section calculations available to date, and are necessary for the best exploitation of leptoquark searches at the LHC.

QCD analysis of non-singlet structure functions at NNLO accuracy, based on the Laplace transform

In this work, using the Laplace transformation technique we present our results for non-singlet quark distributions as well as nucleon structure function $F_2(x,Q^2)$ in unpolarized case at next-to-next-to-leading order (NNLO) QCD accuracy. We shall particularly compare our results for the sets of valence-quark parton distribution functions with the contemporary collaborations like CT14, CT18, MMHT14, MKAM16 and NNPDF. To construct the nucleon structure function we employ the expansion of Jacobi polynomials which is a suitable transform to convert the results of non-singlet structure function from the Laplace $s$-space to Bjorken $x$-space. We shall also consider the contributions of target mass correction as well as the higher twist effects at large-$x$ region for the proton and deuteron structure functions. Our results for the unpolarized quark distribution functions and nucleon structure functions are in good agreement with recent theoretical models and available experimental data.

The studies on Z rightarrow Upsilon (1S)+g+g at the next-to-leading-order QCD accuracy

In this paper, we carry out the next-to-leading-order (NLO) studies on Z rightarrow Upsilon (1S)+g+g via the color-singlet (CS) b{bar{b}} state. We find the newly calculated NLO QCD corrections to this process can significantly influence its leading-order (LO) results and greatly improve the dependence on the renormalization scale. By including the considerable feeddown contributions, the branching ratio {mathcal {B}}_{Z rightarrow Upsilon (1S)+g+g} is predicted to be (0.56–0.95) times 10^{-6}, which can reach up to 19–31% of the LO predictions given by the CS dominant process Z rightarrow Upsilon (1S)+b+{bar{b}}. Moreover, Z rightarrow Upsilon (1S)+g+g also seriously affects the CS predictions on the Upsilon (1S) energy distributions, especially when z is relatively small. In summary, for the inclusive Upsilon (1S) productions in Z decay, besides Z rightarrow Upsilon (1S)+b+{bar{b}}, the gluon radiation process Z rightarrow Upsilon (1S)+g+g can provide indispensable contributions as well.