arXiv subjects

Christopher Schwan

Publications and source records attributed to Christopher Schwan.

At least 19 recordsLinked to original sources

arXiv2026

PineAPPLv1: fast and flexible theory predictions for present and future colliders

We present PineAPPLv1, a library designed to provide accurate and flexible interpolation tables of partonic cross sections that can be convolved with parton distribution functions (PDFs) and fragmentation functions (FFs) for the fast evaluation of high-energy physical observables. The core feature of the new release is the support of multiple convolutions involving initial- and final-state hadronic particles, with any polarisation, that are associated with PDFs and FFs. The library simultaneously supports polarised and unpolarised distributions that obey space-like or time-like evolution, and is developed for an arbitrary number of them, even if physical processes typically only require a few. Control of scale choices when more than one scale characterises a scattering process is also possible. We describe the technical details of the new representation of interpolation coefficients stored in the grid, and we demonstrate the capabilities of the library in a few phenomenological cases of interest. Specifically, we compute predictions for single-inclusive pion production in unpolarised and polarised proton-proton collisions and in semi-inclusive deep-inelastic scattering. We show how, in each case, PDF, FF, and scale uncertainties compare to each other and highlight the potential of PineAPPL as an essential ingredient for precision physics at current and future colliders.

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arXiv2025

Fast interpolation grids for the Drell-Yan process

Modern analyses of experimental data from hadron colliders rely on theory predictions at high orders in perturbation theory and a variety of input settings. Interpolation grids facilitate an almost instant re-evaluation of theory predictions for different input parton distributions functions (PDFs) or scale settings and are thus indispensable in the study of the parton content of the proton. While interpolation grids at next-to-next-to-leading order (NNLO) exist for some key processes relevant for PDF determinations, a notable exception is the Drell-Yan process that constitutes the production of electroweak gauge bosons at hadron colliders and provides important constraints on the quark content of the proton. To address this gap, we report on a new interface between the parton-level Monte Carlo generator NNLOJET and the interpolation grid library PINEAPPL and demonstrate its use for the Drell-Yan process. Accompanying this note, we release Drell-Yan grids covering a wide range of measurements that commonly enter global determinations of PDFs. We use the grids to study accidental cancellation between partonic channels at NNLO and inspect the validity of a K-factor approximation that was widely employed previously.

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arXiv2024

Double-pole approximation for leading-order semi-leptonic vector-boson scattering at the LHC

Measuring vector-boson scattering beyond the fully-leptonic final state is becoming possible at the LHC, which demands to have a solid control on the theory predictions for all final states of this class of processes. In this work we present a full off-shell leading-order calculation for the process $\mathrm{p}\mathrm{p} \to \ell \nu_\ell + 4\mathrm{j}$ in two fiducial regions which are particularly relevant for its experimental measurement. In addition to the fully electroweak order, i.e. $\mathcal{O}(\alpha^6)$, we complement our results with $\mathcal{O}(\alpha_\mathrm{s}\alpha^5)$ and $\mathcal{O}(\alpha_\mathrm{s}^2\alpha^4)$ for inclusive predictions. At $\mathcal{O}(\alpha^6)$ we present for the first time a systematic treatment of the process in double-pole approximation and we perform a detailed study of its range of validity by considering inclusive and differential predictions compared to the full off-shell calculation.

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arXiv2024

Full and approximated NLO predictions for like-sign W-boson scattering at the LHC

We report on a recent calculation of next-to-leading-order (NLO) QCD and electroweak corrections to like-sign W-boson scattering at the Large Hadron Collider, including all partonic channels and W-boson decays in the process $pp \to e^+ ν_e μ^+ ν_μjj + X$. The calculation is implemented in the Monte Carlo integrator Bonsay and comprises the full tower of NLO contributions of the orders $α_s^3α^4$, $α_s^2α^5$, $α_sα^6$, and $α^7$. Our numerical results confirm and extend previous results, in particular the occurrence of large purely electroweak corrections of the order of $\sim-12\%$ for integrated cross sections, which get even larger in distributions. We construct a "VBS approximation" for the NLO prediction based on partonic channels and gauge-invariant (sub)matrix elements potentially containing the vector-boson scattering (VBS) subprocess and on resonance expansions of the Wdecays. The VBS approximation reproduces the full NLO predictions within $\sim1.5\%$ in the most important regions of phase space. Moreover, we discuss results from different versions of "effective vector-boson approximations" at leading order, based on the collinear emission of W bosons of incoming (anti)quarks. However, owing to the only mild collinear enhancement and the design of VBS analysis cuts, the quality of this approximation turns out to be only qualitative at the LHC.

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arXiv2024

The Path to N$^3$LO Parton Distributions

We extend the existing leading (LO), next-to-leading (NLO), and next-to-next-to-leading order (NNLO) NNPDF4.0 sets of parton distribution functions (PDFs) to approximate next-to-next-to-next-to-leading order (aN$^3$LO). We construct an approximation to the N$^3$LO splitting functions that includes all available partial information from both fixed-order computations and from small and large $x$ resummation, and estimate the uncertainty on this approximation by varying the set of basis functions used to construct the approximation. We include known N$^3$LO corrections to deep-inelastic scattering structure functions and extend the FONLL general-mass scheme to $\mathcal{O}\left( \alpha_s^3\right)$ accuracy. We determine a set of aN$^3$LO PDFs by accounting both for the uncertainty on splitting functions due to the incomplete knowledge of N$^3$LO terms, and to the uncertainty related to missing higher corrections (MHOU), estimated by scale variation, through a theory covariance matrix formalism. We assess the perturbative stability of the resulting PDFs, we study the impact of MHOUs on them, and we compare our results to the aN$^3$LO PDFs from the MSHT group. We examine the phenomenological impact of aN$^3$LO corrections on parton luminosities at the LHC, and give a first assessment of the impact of aN$^3$LO PDFs on the Higgs and Drell-Yan total production cross-sections. We find that the aN$^3$LO NNPDF4.0 PDFs are consistent within uncertainties with their NNLO counterparts, that they improve the description of the global dataset and the perturbative convergence of Higgs and Drell-Yan cross-sections, and that MHOUs on PDFs decrease substantially with the increase of perturbative order.

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arXiv2024

Determination of the theory uncertainties from missing higher orders on NNLO parton distributions with percent accuracy

We include uncertainties due to missing higher order corrections to QCD computations (MHOU) used in the determination of parton distributions (PDFs) in the recent NNPDF4.0 set of PDFs. We use our previously published methodology, based on the treatment of MHOUs and their full correlations through a theory covariance matrix determined by scale variation, now fully incorporated in the new NNPDF theory pipeline. We assess the impact of the inclusion of MHOUs on the NNPDF4.0 central values and uncertainties, and specifically show that they lead to improved consistency of the PDF determination with an ensuing moderate reduction of PDF uncertainties at NNLO.

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arXiv2024

Photons in the proton: implications for the LHC

We construct a set of parton distribution functions (PDFs), based on the recent NNPDF4.0 PDF set, that also include a photon PDF. The photon PDF is constructed using the LuxQED formalism, while QED evolution accounting for O(alpha), O(alpha alphas) and O(alpha^2) corrections is implemented and benchmarked by means of the EKO code. We investigate the impact of QED effects on NNPDF4.0, and compare our results both to our previous NNPDF3.1QED PDF set and to other recent PDF sets that include the photon. We assess the impact of photon-initiated processes and electroweak corrections on a variety of representative LHC processes, and find that they can reach the 5% level in vector boson pair production at large invariant mass.

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arXiv2024

Pineline: Industrialization of High-Energy Theory Predictions

We present a collection of tools automating the efficient computation of large sets of theory predictions for high-energy physics. Calculating predictions for different processes often require dedicated programs. These programs, however, accept inputs and produce outputs that are usually very different from each other. The industrialization of theory predictions is achieved by a framework which harmonizes inputs (runcard, parameter settings), standardizes outputs (in the form of grids), produces reusable intermediate objects, and carefully tracks all meta data required to reproduce the computation. Parameter searches and fitting of non-perturbative objects are exemplary use cases that require a full or partial re-computation of theory predictions and will thus benefit of such a toolset. As an example application we present a study of the impact of replacing NNLO QCD K-factors in a PDF fit with the exact NNLO predictions.

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arXiv2023

Like-Sign W-Boson Scattering at the LHC -- Approximations and Full Next-to-Leading-Order Predictions

We present a new calculation of next-to-leading-order corrections of the strong and electroweak interactions to like-sign W-boson scattering at the Large Hadron Collider, implemented in the Monte Carlo integrator Bonsay. The calculation includes leptonic decays of the $\mathrm{W}$ bosons. It comprises the whole tower of next-to-leading-order contributions to the cross section, which scale like $α_\mathrm{s}^3α^4$, $α_\mathrm{s}^2α^5$, $α_\mathrm{s}α^6$, and $α^7$ in the strong and electroweak couplings $α_\mathrm{s}$ and $α$. We present a detailed survey of numerical results confirming the occurrence of large pure electroweak corrections of the order of $\sim-12\%$ for integrated cross sections and even larger corrections in high-energy tails of distributions. The electroweak corrections account for the major part of the complete next-to-leading-order correction, which amounts to $15{-}20\%$ in size, depending on the details of the event selection chosen for analysing vector-boson-scattering. Moreover, we compare the full next-to-leading-order corrections to approximate results based on the neglect of contributions that are not enhanced by the vector-boson scattering kinematics (VBS approximation) and on resonance expansions for the $\mathrm{W}$-boson decays (double-pole approximation); the quality of this approximation is good within $\sim 1.5\%$ for integrated cross sections and the dominating parts of the differential distributions. Finally, for the leading-order predictions, we construct different versions of effective vector-boson approximations, which are based on cross-section contributions that are enhanced by collinear emission of $\mathrm{W}$ bosons off the initial-state (anti)quarks; in line with previous findings in the literature, it turns out that the approximative quality is rather limited for applications at the LHC.

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arXiv2023

Precise predictions for same-sign W-boson scattering at the LHC

Vector-boson scattering processes are of great importance for the current run-II and future runs of the Large Hadron Collider. The presence of triple and quartic gauge couplings in the process gives access to the gauge sector of the Standard Model (SM) and possible new-physics contributions there. To test any new-physics hypothesis, sound knowledge of the SM contributions is necessary, with a precision which at least matches the experimental uncertainties of existing and forthcoming measurements. In this article we present a detailed study of the vector-boson scattering process with two positively-charged leptons and missing transverse momentum in the final state. In particular, we first carry out a systematic comparison of the various approximations that are usually performed for this kind of process against the complete calculation, at LO and NLO QCD accuracy. Such a study is performed both in the usual fiducial region used by experimental collaborations and in a more inclusive phase space, where the differences among the various approximations lead to more sizeable effects. Afterwards, we turn to predictions matched to parton showers, at LO and NLO: we show that on the one hand, the inclusion of NLO QCD corrections leads to more stable predictions, but on the other hand the details of the matching and of the parton-shower programs cause differences which are considerably larger than those observed at fixed order, even in the experimental fiducial region. We conclude with recommendations for experimental studies of vector-boson scattering processes.

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arXiv2023

Theory prediction in PDF fitting

Continuously comparing theory predictions to experimental data is a common task in analysis of particle physics such as fitting parton distribution functions (PDFs). However, typically, both the computation of scattering amplitudes and the evolution of candidate PDFs from the fitting scale to the process scale are non-trivial, computing intesive tasks. We develop a new stack of software tools that aim to facilitate the theory predictions by computing FastKernel (FK) tables that reduce the theory computation to a linear algebra operation. Specifically, I present PineAPPL, our workhorse for grid operations, EKO, a new DGLAP solver, and yadism, a new DIS library. Alongside, I review several projects that become available with the new tools.

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arXiv2022

Parton Distributions and New Physics Searches: the Drell-Yan Forward-Backward Asymmetry as a Case Study

We discuss the sensitivity of theoretical predictions of observables used in searches for new physics to parton distributions (PDFs) at large momentum fraction $x$. Specifically, we consider the neutral-current Drell-Yan production of gauge bosons with invariant masses in the TeV range, for which the forward-backward asymmetry of charged leptons from the decay of the gauge boson in its rest frame is a traditional probe of new physics. We show that the qualitative behaviour of the asymmetry depends strongly on the assumptions made in determining the underlying PDFs. We discuss and compare the large-$x$ behaviour of various different PDF sets, and find that they differ significantly. Consequently, the shape of the asymmetry observed at lower dilepton invariant masses, where all PDF sets are in reasonable agreement because of the presence of experimental constraints, is not necessarily reproduced at large masses where the PDFs are mostly unconstrained by data. It follows that the shape of the asymmetry at high masses may depend on assumptions made in the PDF parametrization, and thus deviations from the traditionally expected behaviour cannot be taken as a reliable indication of new physics. We demonstrate that forward-backward asymmetry measurements could help in constraining PDFs at large $x$ and discuss the accuracy that would be required to disentangle the effects of new physics from uncertainties in the PDFs in this region.

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arXiv2022

Theory pipeline for PDF fitting

Fitting PDFs requires the integration of a broad range of datasets, both from data and theory side, into a unique framework. While for data the integration mainly consists in the standardization of the data format, for the theory predictions there are multiple ingredients involved. Different providers are developed by separate groups for different processes, with a variety of inputs (runcards) and outputs (interpolation grids). Moreover, since processes are measured at different scales, DGLAP evolution has to be provided for the PDF candidate, or precomputed into the grids. We are working towards the automation of all these steps in a unique framework, that will be useful for any PDF fitting groups, and possibly also for phenomenological studies.

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arXiv2022

NLO QCD and EW corrections to off-shell $\mathrm{t}\mathrm{Z}\mathrm{j}$ production at the LHC

The production of a single top quark in association with a $\mathrm{Z}$ boson ($\mathrm{t}\mathrm{Z}\mathrm{j}$ production) at the LHC is a relevant probe of the electroweak sector of the Standard Model as well as a window to possible new-physics effects. The growing experimental interest in performing differential measurements for this process demands an improved theoretical modelling in realistic fiducial regions. In this article we present an NLO-accurate $\mathrm{t}\mathrm{Z}\mathrm{j}$ calculation that includes complete off-shell effects and spin correlations, combining QCD and electroweak radiative corrections to the LO signal. Integrated and differential cross-sections are shown for a fiducial setup characterized by three charged leptons, two jets, and missing energy.

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arXiv2022

NLO QCD and EW corrections to vector-boson scattering into $\rm W^+W^-$ at the LHC

We present the full next-to-leading-order electroweak and QCD corrections to vector-boson scattering into a pair of off-shell opposite-sign W bosons decaying into leptons of different flavour at the LHC. We include full leading-order predictions for the irreducible background. Explicitly, we investigate the process ${\rm pp \to e^+ ν_eμ^-\barν_μjj} + X$ at leading orders $O(α^6)$, $O(\rmα_sα^5)$, $O(\rmα_s^2α^4)$, supplemented by the loop-induced $O(\rmα_s^4α^4)$ contribution, and at next-to-leading orders $O(α^7)$ and $O(\rmα_sα^6)$ in two setups providing fiducial cross sections as well as differential distributions. We take full account of photon-induced next-to-leading-order contributions, which prove to be non negligible. With $-11.4\%$ and $-6.7\%$ in the two setups, the electroweak corrections are smaller than for other vector-boson-scattering processes. This can be traced back to the presence of the Higgs-boson resonance in the fiducial phase space, whose effects we analyse within an additional unphysical, but manifestly gauge-invariant setup. The QCD corrections amount to $-5.1\%$ and $-21.6\%$ in the two setups. The large size of the latter correction, compared to other vector-boson scattering processes, is explained by a very restrictive definition of its fiducial phase space.

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arXiv2022

The Path to Proton Structure at One-Percent Accuracy

We present a new set of parton distribution functions (PDFs) based on a fully global dataset and machine learning techniques: NNPDF4.0. We expand the NNPDF3.1 determination with 44 new datasets, mostly from the LHC. We derive a novel methodology through hyperparameter optimisation, leading to an efficient fitting algorithm built upon stochastic gradient descent. We use NNLO QCD calculations and account for NLO electroweak corrections and nuclear uncertainties. Theoretical improvements in the PDF description include a systematic implementation of positivity constraints and integrability of sum rules. We validate our methodology by means of closure tests and "future tests" (i.e. tests of backward and forward data compatibility), and assess its stability, specifically upon changes of PDF parametrization basis. We study the internal compatibility of our dataset, and investigate the dependence of results both upon the choice of input dataset and of fitting methodology. We perform a first study of the phenomenological implications of NNPDF4.0 on representative LHC processes. The software framework used to produce NNPDF4.0 is made available as an open-source package together with documentation and examples.

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arXiv2022

Vector Boson Scattering Processes: Status and Prospects

Insight into the electroweak (EW) and Higgs sectors can be achieved through measurements of vector boson scattering (VBS) processes. The scattering of EW bosons are rare processes that are precisely predicted in the Standard Model (SM) and are closely related to the Higgs mechanism. Modifications to VBS processes are also predicted in models of physics beyond the SM (BSM), for example through changes to the Higgs boson couplings to gauge bosons and the resonant production of new particles. In this review, experimental results and theoretical developments of VBS at the Large Hadron Collider, its high luminosity upgrade, and future colliders are presented.

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arXiv2021

An open-source machine learning framework for global analyses of parton distributions

We present the software framework underlying the NNPDF4.0 global determination of parton distribution functions (PDFs). The code is released under an open source licence and is accompanied by extensive documentation and examples. The code base is composed by a PDF fitting package, tools to handle experimental data and to efficiently compare it to theoretical predictions, and a versatile analysis framework. In addition to ensuring the reproducibility of the NNPDF4.0 (and subsequent) determination, the public release of the NNPDF fitting framework enables a number of phenomenological applications and the production of PDF fits under user-defined data and theory assumptions.

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