We showcase the application of neural importance sampling for the evaluation of NNLO QCD scattering cross sections. We consider Normalizing Flows in the form of discrete Coupling Layers and time continuous flows for the integration of the various cross-section contributions when using the sector-improved residue subtraction scheme. We thereby consider the stratification of the integrands into their positive and negative contributions, and separately optimize the phase-space sampler. We exemplify the novel methods for the case of gluonic top-quark pair production at the LHC at NNLO QCD accuracy. We find significant gains with respect to the current default methods used in STRIPPER in terms of reduced cross-section variances and increased unweighting efficiencies. In turn, the computational costs for evaluations of the integrand needed to achieve a certain statistical uncertainty for the cross section can be reduced by a factor 8.

Germany

Timo Janßen

Poland

Rene Poncelet

Steffen Schumann

Sampling NNLO QCD phase space with normalizing flows

Sherpa is a general-purpose Monte Carlo event generator for the simulation of particle collisions in high-energy collider experiments. We summarise new developments, essential features, and ongoing improvements within the Sherpa 3 release series. Physics improvements include higher-order electroweak corrections, simulations of photoproduction and hard diffraction at NLO QCD, heavy-flavour matching in NLO multijet merging, spin-polarised cross section calculations, and a new model of colour reconnections. In addition, the modelling of hadronisation, the underlying event and QED effects in both production and decay has been improved, and the overall event generation efficiency has been enhanced.

Enrico Bothmann

United Kingdom

Lois Flower

Christian Gütschow

United States

Stefan Höche

Mareen Hoppe

Joshua Isaacson

Max Knobbe

Frank Krauss

Switzerland

Peter Meinzinger

Italy

Davide Napoletano

Alan Price

CERN

Daniel Reichelt

Marek Schönherr

Frank Siegert

Event generation with Sherpa 3

In this work, we present the first full NLO predictions for the process pp → μ + ν  μ e+ ν ejj at the LHC in a typical tri-boson phase space. The NLO corrections reach 50% at the level of the fiducial cross section and have a very different hierarchy with respect to vector-boson-scattering phase spaces. By comparing the cross section of the full off-shell process with the sum of contributing on-shell electroweak-boson production subchannels, we find that the process is dominated by WWW and WH production, while vector-boson-scattering topologies still play a non-negligible role. In addition, NLO QCD predictions matched to parton shower which are supplemented by approximate electroweak corrections are provided. For the fiducial cross section, the electroweak corrections turn out to be small but the QCD corrections reach 47%. For the inclusive cross section, matching to parton shower affects the predictions by 7%. However, for differential distributions corrections due to the parton shower can be much more sizeable, depending on the region of phase space.

Ansgar Denner

Mathieu Pellen

Tri-boson and WH production in the W + W + jj channel: predictions at full NLO accuracy and beyond

We study jet angularities for dijet production at the Relativistic Heavy Ion Collider (RHIC) in proton-proton (pp) and nucleus-nucleus (AA) collisions at 200 GeV nucleon-nucleon center-of-mass collision energy. In particular, we provide NLL resummed predictions for angularity observables of groomed and ungroomed jets produced in pp collisions matched to next-to-leading order QCD calculations resulting in NLO + NLL ′  accuracy. Our parton-level predictions are corrected for non-perturbative effects, such as hadronization and underlying event, using parton-to-hadron level transfer matrices obtained with the Sherpa event generator. Furthermore, we use the Q-Pythia and Jewel generators to estimate the impact of the interaction between quarks and gluons produced by the parton shower with the dense medium formed in heavy-ion collisions on the considered jet angularities.

Yang-Ting Chien

Oleh Fedkevych

Jet angularities in dijet production in proton-proton and heavy-ion collisions at RHIC

We present resummed predictions at next-to-leading logarithmic accuracy matched to the exact next-to-leading order results for a set of classical event-shape observables in hadronic Higgs decays, i.e., for the channels H → gg and H →   <math> <mi>b</mi> <mover> <mi>b</mi> <mo>¯</mo> </mover> </math>  $$ b\overline{b} $$ . We furthermore consider soft-drop grooming of the hadronic final states and derive corresponding NLO+NLL′ predictions for the groomed thrust observable. Differences in the QCD radiation pattern of gluon- and quark-initiated final states are imprinted in the event-shape distributions, offering separation power for the two decay channels. In particular, we show that ungroomed event shapes in H → gg decays develop a considerably harder spectrum than in H →   <math> <mi>b</mi> <mover> <mi>b</mi> <mo>¯</mo> </mover> </math>  $$ b\overline{b} $$  decays. We highlight that soft-drop grooming can substantially alter this behaviour, unless rather inclusive grooming parameters are chosen.

Aude Gehrmann-De Ridder

Christian Preuss

NLO+NLL′ accurate predictions for three-jet event shapes in hadronic Higgs decays

We present a novel strategy for the simultaneous measurement of Higgs-boson branching ratios into gluons and light quarks at a future lepton collider operating in the Higgs-factory mode. Our method is based on template fits to global event-shape observables, and in particular fractional energy correlations, thereby exploiting differences in the QCD radiation patterns of quarks and gluons. In a constrained fit of the deviations of the light-flavour hadronic Higgs-boson branching ratios from their Standard Model expectations, based on an integrated luminosity of  $$5\,\text {ab}^{-1}$$  <math> <mrow> <mn>5</mn> <mspace width="0.166667em"></mspace> <msup> <mtext>ab</mtext> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </mrow> </math>  , we obtain  $$68\%$$  <math> <mrow> <mn>68</mn> <mo>%</mo> </mrow> </math>   confidence level limits of  $$\mu _{gg}=1 \pm 0.05$$  <math> <mrow> <msub> <mi>μ</mi> <mrow> <mi>gg</mi> </mrow> </msub> <mo>=</mo> <mn>1</mn> <mo>±</mo> <mn>0.05</mn> </mrow> </math>   and  $$\mu _{q\bar{q}}&lt;21$$  <math> <mrow> <msub> <mi>μ</mi> <mrow> <mi>q</mi> <mover> <mrow> <mi>q</mi> </mrow> <mrow> <mo>¯</mo> </mrow> </mover> </mrow> </msub> <mo>&lt;</mo> <mn>21</mn> </mrow> </math>  .

Measuring hadronic Higgs boson branching ratios at future lepton colliders

The possibility to reanalyse data taken by the HERA experiments offers the chance to study modern QCD jet and event-shape observables in deep-inelastic scattering. To address this, we compute resummed and matched predictions for the 1-jettiness distribution in neutral current DIS with and without grooming the hadronic final state using the soft-drop technique. Our theoretical predictions also account for non-perturbative corrections from hadronisation through parton-to-hadron level transfer matrices extracted from dedicated Monte Carlo simulations with Sherpa. To estimate parameter uncertainties in particular for the beam-fragmentation modelling we derive a family of replica tunes to data from the HERA experiments. While NNLO QCD normalisation corrections to the NLO+NLL’ prediction are numerically small, hadronisation corrections turn out to be quite sizeable. However, soft-drop grooming significantly reduces the impact of non-perturbative contributions. We supplement our study with hadron-level predictions from Sherpa based on the matching of NLO QCD matrix elements with the parton shower. Good agreement between the predictions from the two calculational methods is observed.

(N)NLO+NLL’ accurate predictions for plain and groomed 1-jettiness in neutral current DIS

We present an update of the Universal FeynRules Output model format, commonly known as the UFO format, that is used by several automated matrix-element generators and high-energy physics software. We detail different features that have been proposed as extensions of the initial format during the last ten years, and collect them in the current second version of the model format that we coin the Universal Feynman Output format. Following the initial philosophy of the UFO, they consist of flexible and modular additions to address particle decays, custom propagators, form factors, the renormalisation group running of parameters and masses, and higher-order quantum corrections.

France

Luc Darmé

Belgium

Céline Degrande

Claude Duhr

Benjamin Fuks

Mark Goodsell

Gudrun Heinrich

Valentin Hirschi

Marius Höfer

Olivier Mattelaer

Thorsten Ohl

Davide Pagani

Jürgen Reuter

Peter Richardson

Hua-Sheng Shao

Marco Zaro

UFO 2.0: the ‘Universal Feynman Output’ format

The photon is the paradigm for a massless particle, and current experimental tests set severe upper bounds on its mass. Probing such a small mass, or equivalently large Compton wavelength, is challenging at laboratory scales, but planetary or astrophysical phenomena may potentially reach much better sensitivities. In this work, we consider the effect of a finite photon mass on Schumann resonances in the Earth-ionosphere cavity, since the transverse magnetic modes circulating Earth have eigenfrequencies of order <math><mrow><mi>O</mi><mo>(</mo><mn>10</mn><mtext> </mtext><mtext> </mtext><mi>Hz</mi><mo>)</mo></mrow></math> that could be sensitive to <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><mi>γ</mi></mrow></msub><mo>≈</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>14</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>eV</mi><mo>/</mo><mi>c</mi></mrow><mrow><mn>2</mn></mrow></msup></mrow></math>. In particular, we update the limit from Kroll [Phys. Rev. Lett. 27, 340 (1971)], <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><mi>γ</mi></mrow></msub><mo>≤</mo><mn>2.4</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>13</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>eV</mi><mo>/</mo><mi>c</mi></mrow><mrow><mn>2</mn></mrow></msup></mrow></math>, by considering realistic conductivity profiles for the atmosphere. We find the conservative upper bound <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><mi>γ</mi></mrow></msub><mo>≤</mo><mn>2.5</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>14</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>eV</mi><mo>/</mo><mi>c</mi></mrow><mrow><mn>2</mn></mrow></msup></mrow></math>, a factor 9.6 more strict than Kroll’s earlier projection.

Brazil

P. C. Malta

J. A. Helayël-Neto

Constraining the photon mass via Schumann resonances

We present the first application of a Nested Sampling algorithm to explore the high-dimensional phase space of particle collision events. We describe the adaptation of the algorithm, designed to perform Bayesian inference computations, to the integration of partonic scattering cross sections and the generation of individual events distributed according to the corresponding squared matrix element. As a first concrete example we consider gluon scattering processes into 3-, 4- and 5-gluon final states and compare the performance with established sampling techniques. Starting from a flat prior distribution Nested Sampling outperforms the Vegas algorithm and achieves results comparable to a dedicated multi-channel importance sampler. We outline possible approaches to combine Nested Sampling with non-flat prior distributions to further reduce the variance of integral estimates and to increase unweighting efficiencies.

David Yallup

Will Handley

Exploring phase space with nested sampling