We investigate the origin of the unusually large electroweak (EW) radiative effects observed in the extraction of the spin-density matrix and related observables at colliders, focusing on leptonic Z-boson decays. We compute the Z-boson decay spin-density matrix at next-to-leading order (NLO) and find that, while its analytic structure remains essentially unchanged with respect to leading order, the EW corrections induce a sizeable $\textit{−}$35% shift in the spin-analysing power parameter $\textit{η}$ . This effect alone accounts for the striking size of the corrections. For boosted Z bosons, we further show that the treatment of photon radiation in lepton-dressing algorithms significantly affects the extraction of spin-density-matrix coefficients at NLO and must be carefully controlled. To address these challenges, we propose a quantum tomography procedure that is applicable to any final state with one or more on-shell Z bosons that is robust under higher-order corrections. We illustrate its validity and limitations in pp → ZZ → 4$\textit{ℓ}$ and in heavy ($\textit{M}$ > 2$\textit{M}$ ) Higgs boson decay H → ZZ → 4$\textit{ℓ}$.

Italy

Morgan Gratta

Federica Fabbri

Switzerland

Michele Grossi

Belgium

Fabio Maltoni

Davide Pagani

Giovanni Pelliccioli

Alessandro Vicini

Z-boson quantum tomography at next-to-leading order

We present a method for improving next-to-leading order electroweak (EW) predictions for lepton-scattering processes by consistently including double neutral-current corrections arising from vector-boson-fusion topologies, which are formally of higher order. By combining, in a process-independent manner, exact fixed-order results, collinear resummation of QED radiation, and a subtraction procedure, we obtain results which are gauge invariant and valid in the entire phase space, retain any dependence on the masses of electroweak bosons, and can be systematically improved, while avoiding the need for complete next-to-next-to-leading order calculations. This paper is devoted to the development and validation of the formalism; phenomenological applications are presented in a companion study, where we also discuss and motivate why our approach is superior to the one based on EW parton distribution functions for targeting percent-level precision at multi-TeV lepton colliders.

Stefano Frixione

Marco Zaro

Double neutral-current corrections to NLO electroweak leptonic cross sections

Future lepton colliders, such as those based on linear $\textit{e}$ $\textit{e}$ or circular $\textit{μ}$ $\textit{μ}$ accelerators, are expected to attain centre-of-mass energies in the multi-TeV range. In this regime the impact of QED and of weak radiation, in both the initial and the final state, can become a leading effect. By employing a general framework presented in a companion paper — suitable for any flavour of colliding leptons — we improve next-to-leading order electroweak predictions by including higher-order contributions, which encompass, but are not limited to, vector-boson-fusion processes. We apply this approach to the study of $$ t\overline{t} $$ and $\textit{W}$ $\textit{W}$ production at a muon collider operating at centre-of-mass energies up to 10 TeV. We show that such an approach, where both QED and weak contributions are included at fixed order, in addition to the all-order resummation of initial-state QED effects, can provide predictions for arbitrary observables in all of the phase space which are precise at the percent level.

Precision phenomenology at multi-TeV muon colliders

We study the quantum properties of the Higgs-boson decays into four fermions via two vector bosons ($\textit{H}$ → $\textit{VV}$ → 4$\textit{f}$). In particular, we focus on the case of two different-flavour lepton pairs ($\textit{H}$ → $\textit{ZZ}$ → $\textit{μ}$ $\textit{μ}$ $\textit{e}$ $\textit{e}$ ). We compute the quantum-information observables for the corresponding two-qutrit system ($\textit{ZZ}$) at next-to-leading order electroweak (NLO EW) accuracy in the SM. We find that NLO EW corrections lead to giant (order 1) effects in some specific cases, significantly altering the extraction of observables quantifying the quantum correlations. We identify observables that are robust and can be used to extract reliable information. Finally, we discuss possible new physics (NP) effects, parametrised via an effective-field-theory approach. We show how quantum observables can increase the sensitivity to NP also for the process considered in this study.

Priyanka Lamba

Quantum properties of $\textit{H → VV}$

We study the effects of the one-loop renormalisation group running/mixing of the Wilson coefficients in the standard model effective field theory on the predictions for , $$ t\overline{t}H $$ and production at the LHC. We focus on a subset of operators closed under QCD corrections and explore the differences between employing a fixed or dynamical scale on the SMEFT predictions for key observables such as the Higgs transverse momentum and the Higgs pair invariant mass. We then study the impact of consistently taking into account renormalisation group effects on the constraints that can be obtained on the Wilson coefficients through current and future measurements at the LHC.

United Kingdom

Giuseppe Ventura

Eleni Vryonidou

Impact of SMEFT renormalisation group running on Higgs production at the LHC

We compute the effects due to the virtual exchange (or the soft emission) of a scalar particle with generic couplings to the top quark in   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$  pair production at the LHC. We apply the results to two cases of interest, extending and completing previous studies. First, we consider the indirect search for light (m  S  &lt; 2m  t ) top-philic scalars with CP-even and/or CP-odd interactions. Second, we investigate how to set constraints on anomalous Yukawa couplings of the Higgs boson to the top quark. Our results show that the current precision of experimental data together with the accuracy of the SM predictions make such indirect determinations a powerful probe for new physics.

Simone Tentori

Top-quark pair production as a probe of light top-philic scalars and anomalous Higgs interactions

We study the quantum properties of top quark pairs in lepton colliders with unpolarised beams, including spin correlations, entanglement, and violation of Bell inequalities. We present analytical results in the SM and in the SMEFT and discuss several practical aspects, like the choice of quantisation axes and   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$  threshold effects. We also note a correspondence between parity symmetry and entanglement. We find that quantum observables exhibit a rich phenomenology in the SM, and can also provide additional leverage in detecting new physics residing at higher scales.

Claudio Severi

Quantum tops at circular lepton colliders

We study the capabilities of a muon collider, at 3 and 10 TeV center-of-mass energy, of probing the interactions of the Higgs boson with the muon. We consider all the possible processes involving the direct production of EW bosons (W, Z and H) with up to five particles in the final state. We study these processes both in the HEFT and SMEFT frameworks, assuming that the dominant BSM effects originate from the muon Yukawa sector. Our study shows that a Muon Collider has sensitivity beyond the high-luminosity LHC, especially as it does not rely on the Higgs-decay branching fraction to muons. A 10 TeV muon collider provides a unique sensitivity on muon and (multi-) Higgs interactions, significantly better than the 3 TeV option. Particularly, we find searches based purely on multi-Higgs production to be particularly effective in probing these couplings.

Eugenia Celada

United States

Tao Han

Germany

Wolfgang Kilian

Nils Kreher

Yang Ma

Jürgen Reuter

Tobias Striegl

Keping Xie

Probing Higgs-muon interactions at a multi-TeV muon collider

We study the LHC phenomenology of an Axion Like Particle (ALP) that couples only derivatively with the top quark at tree level. We inspect the radiatively induced couplings with the SM fermions and (gauge) bosons and the associated production and decay mechanisms of the ALP. We focus on the most challenging mass window that remains open for a top-philic ALP, i.e., the range between tens and hundreds of GeV. Not only ALP production processes but also virtual ALP contributions to final states with top quarks are considered in detail. We show how searches through resonant production, such as ALP production in association with a   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$  pair, are complementary to precision measurements of   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$  and   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t}t\overline{t} $$  final states, the latter being competitive or even more powerful for a top-philic ALP in this mass window. Finally, we explore the scenario where the top-philic ALP acts as a mediator to a dark-matter sector, resulting in missing energy signatures at the LHC. We find that the LHC constraints from   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$ ,   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t}t\overline{t} $$  and ALP + jet production, together with those from   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$  + ALP production, can already exclude a large fraction of the parameter space leading to the correct relic abundance.

Simone Blasi

Alberto Mariotti

Ken Mimasu

Top-philic ALP phenomenology at the LHC: the elusive mass-window

The recent observation of entanglement between top and anti-top quarks at the LHC opens the way to interpreting collider data with quantum information tools. In this work we investigate the relevance of quantum observables in searches of new physics. To this aim, we study spin correlations of  $$t\overline{t }$$  pairs originating from various intermediate resonances, and compare the discovery reach of quantum observables compared to classical ones. We find that they provide complementary information and, in several notable cases, also the additional leverage necessary to detect new effects.

Quantum detection of new physics in top-quark pair production at the LHC