New heavy resonances with sizeable couplings to top quarks can be probed through searches for beyond-the-Standard-Model effects in four-top production at the LHC. In this work, we present the first next-to-leading-order QCD predictions for the full on-shell and off-shell production of four-top events via new electroweak singlet states, along with dedicated analysis strategies based on the reconstruction and tagging of all final-state top quarks. We develop a detector-level simulation incorporating recent advances in top-tagging and boosted object reconstruction. Moreover, we demonstrate that searches at LHC Run 3 and high-luminosity phase in the zero-lepton, one-lepton and same-sign di-lepton channels can improve the sensitivity to the new physics cross sections by up to two orders of magnitude. In particular, colour-octet resonances with masses up to 2–2.5 TeV and colour-singlet states with masses up to 1–1.5 TeV are within reach for coupling values in the 0.1–1 range.

France

Luc Darmé

Benjamin Fuks

Belgium

China

Hao-Lin Li

Matteo Maltoni

Julien Touchèque

Searching for top-philic heavy resonances in boosted four-top final states

<p>We introduce a novel search strategy for heavy top-philic resonances that induce new contributions to four-top production at the LHC. We capitalize on recent advances in top-tagging performance to demonstrate that the final state, that is expected to be boosted based on current limits, can be fully reconstructed and exploited. Notably, our approach promises bounds on new physics cross sections that are a few to 60 times stronger than those obtained with existing searches, showcasing its unprecedented effectiveness in probing top-philic new physics.</p>

Olivier Mattelaer

Novel approach to probing top-philic resonances with boosted four-top tagging

<p>The hadronic vacuum polarization contribution to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mo stretchy="false">(</mo><mi>g</mi><mo>−</mo><mn>2</mn><msub><mrow><mo stretchy="false">)</mo></mrow><mrow><mi>μ</mi></mrow></msub></mrow></math> can be determined via dispersive methods from <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup><mo stretchy="false">→</mo><mtext>hadrons</mtext></mrow></math> data. We propose a novel approach to measure the hadronic cross section <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>σ</mi><mi>had</mi></msub><mo stretchy="false">(</mo><mi>s</mi><mo stretchy="false">)</mo></math> as an alternative to the initial-state radiation and energy scan techniques, which relies on positron annihilation off atomic electrons of a high <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi></math> target (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mmultiscripts><mrow><mi mathvariant="normal">U</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>238</mn></mrow></mmultiscripts></mrow></math>, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi><mo>=</mo><mn>92</mn></math>). We show that by leveraging the relativistic electron velocities of the inner atomic shells, a high-intensity 12 GeV positron beam, such as the one foreseen at JLab, can allow measuring <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>σ</mi><mi>had</mi></msub><mo stretchy="false">(</mo><mi>s</mi><mo stretchy="false">)</mo></math> with high statistical accuracy from the two-pion threshold up to above <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msqrt><mi>s</mi></msqrt><mo>∼</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>.</p>

Italy

Fernando Arias-Aragón

Giovanni Grilli di Cortona

Estonia

Enrico Nardi

Atoms as Electron Accelerators for Measuring the Cross Section of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup><mo stretchy="false">→</mo><mtext>Hadrons</mtext></mrow></math>

Resonant positron annihilation on atomic electrons provides a powerful method to search for light new particles coupled to <math><mrow><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></math>. Reliable estimates of production rates require a detailed characterization of electron momentum distributions. We describe a general method that harnesses the target material Compton profile to properly include electron velocity effects in resonant annihilation cross sections. We additionally find that high-<math><mi>Z</mi></math> atoms can efficiently act as particle physics accelerators, providing a density of relativistic electrons that allows one to extend by several times the experimental mass reach.

Production of Dark Sector Particles via Resonant Positron Annihilation on Atomic Electrons

We introduce the idea of flavour transfer from a non-abelian horizontal SU(2) f  flavour gauge group embedded in the Standard Model flavour structure. The new flavour vector bosons, in the mass range from the tens of GeV to multi-TeV, do not induce large flavour-changing neutral currents and meson oscillations, which usually provide the dominant constraints on this type of structure. Instead, the main constraints arise from “flavour-transfer” operators that we will study in detail. Several explicit models are presented and their prospects are thoroughly explored, including their phenomenology in the lepton and quark sectors at colliders and lower energy experiments. We perform a complete numerical fit in one such scenario, showing that LHC-based lepton-flavour violating searches are competitive with intensity frontier observables.

South Africa

Aldo Deandrea

CERN

Farvah Mahmoudi

Gauge SU(2) f  flavour transfers

Recent lattice determinations of the hadronic-vacuum polarization contribution to the muon anomalous magnetic moment <math><msubsup><mi>a</mi><mi>μ</mi><mrow><mi>HVP</mi></mrow></msubsup></math> have confirmed the discrepancy with the data-driven dispersive method. In the meanwhile the CMD-3 Collaboration has reported a result for the <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup><mo>→</mo><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup></math> cross section considerably larger than previous experimental results (and close to the lattice determinations) exacerbating the discordance between different <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> datasets. We explore to what extent these disagreements can be accounted for by some new physics effect altering selectively the individual experimental determinations of <math><mrow><mi>σ</mi><mo>(</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup><mo>→</mo><mi>hadrons</mi><mo>)</mo></mrow></math>. We find that specific effects of GeV-scale new particles are able to shift upwards the KLOE and BABAR results in the low and intermediate energy windows, while leaving the CMD-3 energy scan unaffected. Although these new physics effects cannot fully explain all the discrepancies among the different <math><mrow><mi>σ</mi><mo>(</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup><mo>→</mo><mi>hadrons</mi><mo>)</mo></mrow></math> datasets, they succeed in mitigating the overall tension between data-driven and lattice estimates of <math><msubsup><mi>a</mi><mi>μ</mi><mrow><mi>HVP</mi></mrow></msubsup></math>. Remarkably, the additional loop corrections involving the new particles concur to solve the residual discrepancy with the experimental value of <math><mo>(</mo><mi>g</mi><mo>−</mo><mn>2</mn><msub><mo>)</mo><mi>μ</mi></msub></math>.

Indirect new physics effects on <math><mrow><msub><mrow><mi>σ</mi></mrow><mrow><mi>had</mi></mrow></msub></mrow></math> confront the <math><mrow><mo>(</mo><mi>g</mi><mo>−</mo><mn>2</mn><msub><mrow><mo>)</mo></mrow><mrow><mi>μ</mi></mrow></msub></mrow></math> window discrepancies and the CMD-3 result

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.

Céline Degrande

Germany

Claude Duhr

Mark Goodsell

Gudrun Heinrich

Switzerland

Valentin Hirschi

United States

Stefan Höche

Marius Höfer

Joshua Isaacson

Thorsten Ohl

Davide Pagani

Jürgen Reuter

United Kingdom

Peter Richardson

Steffen Schumann

Hua-Sheng Shao

Frank Siegert

Marco Zaro

UFO 2.0: the ‘Universal Feynman Output’ format

The workshop “Shedding light on X17” brings together scientists looking for the existence of a possible new light particle, often referred to as X17. This hypothetical particle can explain the resonant structure observed at  $$\sim $$  <math> <mo>∼</mo> </math>   17 MeV in the invariant mass of electron-positron pairs, produced after excitation of nuclei such as  $$^8\hbox {Be}$$  <math> <mrow> <msup> <mrow></mrow> <mn>8</mn> </msup> <mtext>Be</mtext> </mrow> </math>   and  $$^4\hbox {He}$$  <math> <mrow> <msup> <mrow></mrow> <mn>4</mn> </msup> <mtext>He</mtext> </mrow> </math>   by means of proton beams at the Atomki Laboratory in Debrecen. The purpose of the workshop is to discuss implications of this anomaly, in particular theoretical interpretations as well as present and future experiments aiming at confirming the result and/or at providing experimental evidence for its interpretation.

Daniele Alves

Daniele Barducci

Gianluca Cavoto

Spain

Luigi Delle Rose

Luca Doria

Jonathan Feng

André Frankenthal

Ashot Gasparian

Evgueni Goudzovski

Carlo Gustavino

Egypt

Shaaban Khalil

Bulgaria

Venelin Kozhuharov

Hungary

Attila Krasznahorkay

Tommaso Marchi

Manuel Meucci

Gerald Miller

Stefano Moretti

Marco Nardecchia

Czechia

Hugo Natal da Luz

Giovanni Organtini

Angela Papa

Ann-Kathrin Perrevoort

Vlasios Petousis

Gabriele Piperno

Mauro Raggi

Francesco Renga

Patrick Schwendimann

Rudolf Sýkora

Claudio Toni

Paolo Valente

Cecilia Voena

Cheuk-Yin Wong

Xilin Zhang

Shedding light on X17: community report

A local flavour symmetry acting on the quarks of the Standard Model can automatically give rise to an accidental global U(1) which remains preserved from sources of explicit breaking up to a large operator dimension, while it gets spontaneously broken together with the flavour symmetry. Such non-fundamental symmetries are often endowed with a mixed QCD anomaly, so that the strong CP problem is automatically solved via the axion mechanism. We illustrate the general features required to realise this scenario, and we discuss a simple construction based on the flavour group SU(3) × SU(2) × U(1) F  to illustrate how mass hierarchies can arise while ensuring at the same time a high quality Peccei-Quinn symmetry.

Clemente Smarra

The axion flavour connection

We discuss the experimental reach of the Frascati PADME experiment in searching for new light bosons via their resonant production in positron annihilation on fixed target atomic electrons. A scan in the mass range around 17 MeV will thoroughly probe the particle physics interpretation of the anomaly observed by the ATOMKI nuclear physics experiment. In particular, for the case of a spin-1 boson, the viable parameter space can be fully covered in a few months of data taking.

Marco Mancini

Resonant search for the X17 boson at PADME