Polarization and spin correlations in diboson systems serve as powerful tools for precision tests and searches for new physics. Recently, interpreting these observables through the lens of quantum information, for instance by examining whether the diboson systems exhibit entanglement, has introduced a compelling new dimension to these studies. We analyze the angular coefficients in the processes $\textit{pp}$ → $\textit{e}$ $\textit{e}$ $\textit{μ}$ $\textit{μ}$ and $\textit{h}$ → $\textit{e}$ $\textit{e}$ $\textit{μ}$ $\textit{μ}$ , incorporating higher-order QCD and electroweak corrections. Guided by the fundamental properties of the spin density matrix, we assess the stability of the two-qutrit interpretation under radiative effects. For the $\textit{pp}$ → $\textit{e}$ $\textit{e}$ $\textit{μ}$ $\textit{μ}$ process, NLO QCD corrections preserve the two-qutrit structure but weaken entanglement indicators, an effect that can be partially mitigated by jet binning. In contrast, electroweak corrections introduce non-factorizable contributions that modify the quantum properties of the system. While these effects can be largely depleted by selecting events with a double-resonant $\textit{ZZ}$ structure, such a kinematic handle is not available for Higgs decays. In the $\textit{h}$ → $\textit{e}$ $\textit{e}$ $\textit{μ}$ $\textit{μ}$ channel, singly-resonant NLO electroweak corrections substantially distort the angular coefficients, challenging the description of these events as a two-qutrit system.

United States

Dorival Gonçalves

Ajay Kaladharan

United Kingdom

Frank Krauss

Alberto Navarro

Quantum entanglement is quantum: $\textit{ZZ}$ production at the LHC

The Higgs boson decay $\textit{h}$ → $\textit{WW}$ → $$ {\ell}^{+}{\nu}_{\ell }{\ell}^{\prime -}{\overline{\nu}}_{\ell^{\prime }} $$ provides a unique window into the structure of the Higgs couplings to electroweak gauge bosons and has recently gained attention for its potential to unveil quantum properties such as quantum entanglement between the intermediate gauge bosons. In this work, we present a systematic study of next-to-leading order electroweak corrections to the angular coefficients characterizing this decay. While these coefficients are highly constrained at leading order, radiative corrections induce shifts of up to 5% to the existing terms and generate novel structures that vanish at leading order, breaking previous relations among coefficients. While higher-order effects influence the results, the two-qutrit quantum structure in the $\textit{h}$ → $\textit{WW}$ channel exhibits greater stability under such corrections than in the previously studied $\textit{h}$ → $\textit{ZZ}$ decay.

Higher-order corrections to quantum observables in $\textit{h}$ → $\textit{WW}$

Top quark polarization provides an important tool for studying its production mechanisms, spin correlations, top quark properties, and new physics searches. Unlike lighter quarks, the top quark’s polarization remains intact until its decay, enabling precise spin measurements. While the down-type fermions from $\textit{W}$ boson decay are known to be effective spin analyzers, charged leptons have typically been the main target for most analyses. In this paper, we investigate the relevance of global jet dynamics – considering kinematics, jet charges, and particle multiplicity – for hadronic top quark polarimetry. The formalism used allows for analytical derivations obtained throughout the manuscript, offering deeper insights into the corresponding phenomenology.

Zhongtian Dong

Kyoungchul Kong

Andrew Larkoski

Analytical insights on hadronic top quark polarimetry

Precision studies for top quark physics are a cornerstone of the Large Hadron Collider program. Polarization, probed through decay kinematics, provides a unique tool to scrutinize the top quark across its various production modes and to explore potential new physics effects. However, the top quark most often decays hadronically, for which unambiguous identification of its decay products sensitive to top quark polarization is not possible. In this Letter, we introduce a jet flavor tagging method to significantly improve spin analyzing power in hadronic decays, going beyond exclusive kinematic information employed in previous studies. We provide parametric estimates of the improvement from flavor tagging with any set of measured observables and demonstrate this in practice on simulated data using a Graph Neural Network (GNN). We find that the spin analyzing power in hadronic decays can improve by approximately 20% (40%) compared to the kinematic approach, assuming an efficiency of 0.5 (0.2) for the network.

Andrew J. Larkoski

Hadronic top quark polarimetry with ParticleNet

<p>Supercooled first-order phase transition (FOPT) can lead to the formation of primordial black holes (PBHs). This scenario imposes stringent requirements on the profile of the effective potential. In this work, we use the singlet extended Standard Model (xSM) as a benchmark model to investigate this possibility at the electroweak scale. The PBHs formed during a supercooled FOPT have a narrow mass distribution around the mass of Earth. This distribution is closely tied to the temperature at which the PBHs form, corresponding to the FOPT at the electroweak scale. This scenario can be probed with microlensing experiments, space-based gravitational wave detectors, and collider experiments. Remarkably, the future space-based gravitational wave detector LISA will hold the potential to either confirm this scenario that leads to PBH formation across the observable Universe in the xSM or completely rule it out. Interestingly, our findings suggest that PBHs within the xSM framework may align with observations of the six ultrashort timescale events reported by the OGLE microlensing experiment.</p>

China

Yongcheng Wu

Primordial black holes from first-order phase transition in the singlet-extended SM

A critical element of the LHC physics program is the search for an additional source of <math><mi>C</mi><mi>P</mi></math> violation. This is largely unexplored in the context of nonlinear Higgs physics, which is naturally described in Higgs effective field theory (HEFT). Relevant new higher-dimensional operators modify the production rate and branching ratios of the Higgs boson, decorrelating different Higgs multiplicities. In this work, we consider single Higgs and Higgs pair production via weak boson fusion from the perspective of gauge-Higgs <math><mi>C</mi><mi>P</mi></math> violation through the lens of Higgs nonlinearity. This generalizes existing rate-based searches and analyses by the ATLAS and CMS experiments. Particular focus is given to the phenomenological differences in the expected beyond Standard Model (BSM) sensitivity pattern when comparing HEFT constraints with Standard Model effective field theory (SMEFT) limits.

Akanksha Bhardwaj

Christoph Englert

Nonlinear gauge-Higgs <math><mi>C</mi><mi>P</mi></math> violation

The Large Hadron Collider provides a unique opportunity to study quantum entanglement and violation of Bell inequalities at the highest energy available today. In this paper, we will investigate these quantum correlations with top quark pair production, which represents a system of two-qubits. The semileptonic top pair channel has a factor of 6 increase in statistics and easier reconstruction with respect to the dileptonic channel. Although measuring the spin polarization of the hadronic top quark is known to be challenging, our study indicates that it is feasible to reconstruct the spin density matrix of the two-qubit system using an optimal hadronic polarimeter. This is achieved with the aid of jet substructure techniques and NN-inspired reconstruction methods, which improve the mapping between subjets and quarks. We find that entanglement can already be observed at more than the <math><mn>5</mn><mi>σ</mi></math> level with existing data, and violation of Bell inequalities may be probed above the <math><mrow><mn>4</mn><mi>σ</mi></mrow></math> level at the HL-LHC with <math><mrow><mn>3</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>ab</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math> of data.

Entanglement and Bell inequalities with boosted <math><mi>t</mi><mover><mi>t</mi><mo>¯</mo></mover></math>

Observation of lepton number (L) violation by two units at colliders would provide evidence for the Majorana nature of neutrinos. We study signals of L-violation in the context of two popular models of neutrino masses, the type-II seesaw model and the Zee model, wherein small neutrino masses arise at the tree-level and one-loop level, respectively. We focus on L-violation signals at the LHC arising through the process pp → ℓ ± ℓ ′± + jets within these frameworks. We obtain sensitivity to L-violation in the type-II seesaw model for triplet scalar masses up to 700 GeV and in the Zee model for charged scalar masses up to 4.8 TeV at the high-luminosity LHC with an integrated luminosity of 3 ab−1.

K. Babu

Rahool Barman

Ahmed Ismail

Probing lepton number violation and Majorana nature of neutrinos at the LHC

Searches for additional sources of <math><mi>C</mi><mi>P</mi></math> violation at the Large Hadron Collider are a central part of the Higgs physics program beyond the Standard Model. Studies employing so-called signed observables that track <math><mi>C</mi><mi>P</mi></math> violation through purpose-built asymmetries bolster efforts based on Higgs boson rate analyses under clear assumptions. A possibility which is, so far, unexplored at the LHC, is a significant nonlinear realization of <math><mi>C</mi><mi>P</mi></math> violation, which is naturally described in nonlinear Higgs effective field theory (HEFT). We perform an analysis of the HL-LHC potential to constrain such interactions considering a large range of single and double Higgs production processes, including differential information where this is statistically and theoretically possible. A particular emphasis of our work is distinguishing expected correlations in the Standard Model effective field theory from those attainable in HEFT.

Nonlinear <math><mi>C</mi><mi>P</mi></math> violation in the top-Higgs sector

The angular moments of the Z boson can be used as analyzers for the underlying production dynamics for the  $$t{{\bar{t}}} Z$$  <math> <mrow> <mi>t</mi> <mover> <mrow> <mi>t</mi> </mrow> <mrow> <mo>¯</mo> </mrow> </mover> <mi>Z</mi> </mrow> </math>   and tZj processes. In this manuscript, we derive these angular moments at leading and next-to-leading order in QCD at the LHC. We show that these observables work as efficient probes to beyond the Standard Model effects, considering the Standard Model Effective Field theory framework. Remarkably, we observe that these probes unveil blind directions to CP-odd operators, providing sizable new physics sensitivity at the 14 TeV LHC with 3 ab $$^{-1}$$  <math> <msup> <mrow></mrow> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </math>   of data.

Roshan Abraham

Boosting new physics searches in  $$t{\bar{t}}Z$$  <math> <mrow> <mi>t</mi> <mover> <mrow> <mi>t</mi> </mrow> <mrow> <mo>¯</mo> </mrow> </mover> <mi>Z</mi> </mrow> </math>   and tZj production with angular moments