Pair production of heavy vector bosons is a key process at colliders: it allows to test our understanding of the Standard Model and to explore the existence of new physics through precision measurements of production rates and differential distributions. New physics effects can be subtle and often require observables specifically designed for their detection. In this study, we focus on quantum information observables that characterise the spin states of the final diboson system. We analyse concurrence bounds, purity, and Bell inequalities for a bipartite qutrit system representing two massive gauge bosons. Our findings show that quantum spin observables can serve as complementary probes for heavy new physics as parametrised by higher dimensional operators in the Standard Model effective field theory. In particular, we find that these observables offer increased sensitivity to operators whose contributions do not interfere with the Standard Model amplitudes at the level of differential cross sections.

Belgium

Rafael Aoude

Israel

Eric Madge

Italy

Fabio Maltoni

United Kingdom

Luca Mantani

Probing new physics through entanglement in diboson production

In recent years, several pulsar timing array collaborations have reported first hints for a stochastic gravitational wave background at nano-Hertz frequencies. Here we elaborate on the possibility that this signal comes from new physics that leads to the generation of a primordial stochastic gravitational wave background. We propose a set of simple but concrete models that can serve as benchmarks for gravitational waves sourced by cosmological phase transitions, domain wall networks, cosmic strings, axion dynamics, or large scalar fluctuations. These models are then confronted with pulsar timing data and with cosmological constraints. With only a limited number of free parameters per model, we are able to identify viable regions of parameter space and also make predictions for future astrophysical and laboratory tests that can help with model identification and discrimination.

Germany

Enrico Morgante

Cristina Puchades-Ibáñez

Nicklas Ramberg

Wolfram Ratzinger

Sebastian Schenk

Pedro Schwaller

Primordial gravitational waves in the nano-Hertz regime and PTA data — towards solving the GW inverse problem

Quantum information observables, such as entanglement measures, provide a powerful way to characterize the properties of quantum states. We propose to use them to probe the structure of fundamental interactions and to search for new physics at high energy. Inspired by recent proposals to measure entanglement of top quark pairs produced at the LHC, we examine how higher-dimensional operators in the framework of the Standard Model effective field theory modify the Standard Model expectations. We explore two regions of interest in the phase space where the Standard Model produces maximally entangled states: at threshold and in the high-energy limit. We unveil a nontrivial pattern of effects, which depend on the initial state partons, <math><mrow><mi>q</mi><mover><mrow><mi>q</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow></math> or <math><mi>g</mi><mi>g</mi></math>, on whether only linear or up to quadratic Standard Model effective field theory contributions are included, and on the phase space region. In general, we find that higher-dimensional effects lower the entanglement predicted in the Standard Model.

Quantum SMEFT tomography: Top quark pair production at the LHC

We investigate the phenomenological implications of the recent W mass measurement by the CDF collaboration, which exhibits tension with the standard model (SM) electroweak fit. Performing the fit to the electroweak observables within the SM effective field theory, we find that the new physics that contributes either to the determination of the electroweak vacuum expectation value, or to the oblique parameters, can improve the agreement with data. The best description is obtained from a fit where flavor universality is not required in the new physics operators, with 2 to 3 σ indications for several nonzero Wilson coefficients. We point out that top partners with order TeV masses could lead to the observed shift in the W mass.

Reuven Balkin

United States

Tony Menzo

Gilad Perez

Yotam Soreq

Jure Zupan

On the implications of positive W mass shift

To solve the hierarchy problem, the relaxion must remain trapped in the correct minimum, even if the electroweak symmetry is restored after reheating. In this scenario, the relaxion starts rolling again until the backreaction potential, with its set of local minima, reappears. Depending on the time of barrier reappearance, Hubble friction alone may be insufficient to retrap the relaxion in a large portion of the parameter space. Thus, an additional source of friction is required, which might be provided by coupling to a dark photon. The dark photon experiences a tachyonic instability as the relaxion rolls, which slows down the relaxion by backreacting to its motion, and efficiently creates anisotropies in the dark photon energy-momentum tensor, sourcing gravitational waves. We calculate the spectrum of the resulting gravitational wave background from this new mechanism and evaluate its observability by current and future experiments. We further investigate the possibility that the coherently oscillating relaxion constitutes dark matter and present the corresponding constraints from gravitational waves.

Abhishek Banerjee

Gravitational wave echo of relaxion trapping

We explore the prospects to discover the <math><mi>h</mi><mo>→</mo><mi>Z</mi><mi>γ</mi></math> decay in <math><mi>t</mi><mover><mi>t</mi><mo>¯</mo></mover></math>-associated production, featuring a signal-to-background ratio of <math><mi>O</mi><mo>(</mo><mn>1</mn><mo>)</mo></math>. Performing a detailed analysis of the semileptonic <math><mi>t</mi><mover><mi>t</mi><mo>¯</mo></mover></math>-decay channel, we demonstrate that the production mode could lead to a <math><mo>∼</mo><mn>5</mn><mi>σ</mi></math> discovery at the high-luminosity LHC, while the effective <math><mi>h</mi><mi>Z</mi><mi>γ</mi></math> coupling could be extracted with a <math><mo>∼</mo><mn>15</mn><mo>%</mo></math> accuracy. Extending the analysis to potential future <math><mi>p</mi><mi>p</mi></math> colliders with 27 TeV and 100 TeV center-of-mass energies, we also show that the latter would allow precision measurements at the few percent level, rendering possible precise extractions of the spin and <math><mi>C</mi><mi>P</mi></math> properties of the Higgs boson.

Florian Goertz

Valentin Titus Tenorth

Discovering the <math><mi>h</mi><mo>→</mo><mi>Z</mi><mi>γ</mi></math> decay in <math><mi>t</mi><mover><mi>t</mi><mo>¯</mo></mover></math> associated production

We constrain the Higgs-portal model employing the vector-boson fusion channel at the LHC. In particular, we include the phenomenologically interesting parameter region near the Higgs resonance, where the Higgs-boson mass is close to the threshold for dark-matter production and a running-width prescription has to be employed for the Higgs- boson propagator. Limits for the Higgs-portal coupling as a function of the dark-matter mass are derived from the CMS search for invisible Higgs-boson decays in vector-boson fusion at 13 TeV. Furthermore, we perform projections for the 14 TeV HL-LHC and the 27 TeV HE-LHC taking into account a realistic estimate of the systematic uncertainties. The respective upper limits on the invisible branching ratio of the Higgs boson reach a level of 2% and constrain perturbative Higgs-portal couplings up to dark-matter masses of about 110 GeV.

Jan Heisig

Michael Krämer

Alexander Mück

Probing Higgs-portal dark matter with vector-boson fusion

New gauge symmetries often appear in theories beyond the Standard Model. Here we study a model where lepton number is promoted to a gauge symmetry. Anomaly cancellation requires the introduction of additional leptons, the lightest of which is a natural leptophilic dark matter candidate. We perform a comprehensive study of both collider and dark matter phenomenology. Furthermore we find that the model exhibits a first order lepton number breaking phase transition in large regions of parameter space. The corresponding gravitational wave signal is computed, and its detectability at LISA and other future GW detectors assessed. Finally we comment on the complementarity of dark matter, collider and gravitational wave observables, and on the potential reach of future colliders.

Leptophilic dark matter from gauged lepton number: phenomenology and gravitational wave signatures