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

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.Comment: 36 pages + appendix, 24 figures. Version accepted for publication in JHE

Phenomenology of New Physics Models at Colliders and in Gravitational Waves

The existence of physics beyond the Standard Model of particle physics is very well motivated. This dissertation studies the phenomenology of models that accommodate such new physics. It mainly covers two aspects: collider phenomenology and gravitational waves. We first present a search for Higgs-portal dark matter at the LHC and its prospective high-luminosity and high-energy upgrades, entertaining the vector-boson fusion channel. We derive the limits on the portal coupling as a function of the dark matter mass, in particular also for masses close to the transition between the on- and off-shell Higgs regime. Subsequently, a study of the h → Zγ decay in top-pair associated production is considered. We evaluate the observational prospects at future proton colliders and derive the corresponding indirect constraints that can be put on the new physics’ contribution to the decay rate. Our exploration of collider probes of physics beyond the Standard Model is then concluded with a comprehensive analysis of the phenomenology of a model in which lepton number is gauged. The model automatically provides a candidate for particle dark matter. We investigate the parameter space in which the measured relic abundance is reproduced, impose constraints from direct and indirect dark matter searches, and assess the limits from collider experiments. We then move on to study the gravitational wave phenomenology of new physics, focusing on stochastic gravitational wave backgrounds generated in cosmological first-order phase transitions. After an introduction to the topic, we return to the gauged-lepton-number-model and investigate the lepton number breaking phase transition. We identify the parameter regions in which the transition is of first order and which are consistent with the collider and dark matter constraints. We then calculate the respective gravitational wave spectrum and evaluate its detectability at LISA and other future gravitational wave observatories. Finally, we consider phase transitions occurring in decoupled dark sectors, particularly focusing on sub-MeV hidden sectors. We investigate the interplay between cosmological constraints on the number of relativistic degrees of freedom and the detectability of the gravitational wave background generated by a phase transition in such a sector.viii, 177 Seite

Prospects of nuclear-coupled-dark-matter detection via correlation spectroscopy of I2+_2^+ and Ca+^+

The nature of dark matter (DM) and its interaction with the Standard Model (SM) is one of the biggest open questions in physics nowadays. The vast majority of theoretically-motivated Ultralight-DM (ULDM) models predict that ULDM couples dominantly to the SM strong/nuclear sector. This coupling leads to oscillations of nuclear parameters that are detectable by comparing clocks with different sensitivities to these nature's constants. Vibrational transitions of molecular clocks are more sensitive to a change in the nuclear parameters than the electronic transitions of atomic clocks. Here, we propose the iodine molecular ion, I$_2^+$, as a sensitive detector for such a class of ULDM models. The iodine's dense spectrum allows us to match its transition frequency to that of an optical atomic clock (Ca$^+$) and perform correlation spectroscopy between the two clock species. With this technique, we project a few-orders-of-magnitude improvement over the most sensitive clock comparisons performed to date. We also briefly consider the robustness of the corresponding "Earth-bound" under modifications of the $Z_N$-QCD axion model.Comment: 10 pages, 4 figures; v2 matches version published in PR

Probing new physics through entanglement in diboson production

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.Comment: 42 pages, 18 figures; v2: added ancillary file

Quantum SMEFT tomography: top quark pair production at the LHC

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 SMEFT 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 non-trivial pattern of effects, which depend on the initial state partons, $q\bar q$ or $gg$, on whether only linear or up to quadratic SMEFT 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.Comment: 8 pages, 5 figures + appendix; v2: minor changes, published versio

Discovering the h→Zγh\to Z \gamma Decay in ttˉt \bar t Associated Production

We explore the prospects to discover the $h \to Z \gamma$ decay in $t\bar t$-associated production, featuring a signal-to-background ratio of ${\cal O}(1)$. Performing a detailed analysis of the semi-leptonic $t \bar t$-decay channel, we demonstrate that the production mode could lead to a $\sim5\,\sigma$ discovery at the high-luminosity LHC, while the effective $h Z \gamma$ coupling could be extracted with a $\sim15\,\%$ accuracy. Extending the analysis to potential future $pp$ 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 CP properties of the Higgs boson.Comment: 7 pages, 4 figures, updated version matches the one published is Phys. Rev.

Dark, Cold, and Noisy: Constraining Secluded Hidden Sectors with Gravitational Waves

We explore gravitational wave signals arising from first-order phase transitions occurring in a secluded hidden sector, allowing for the possibility that the hidden sector may have a different temperature than the Standard Model sector. We present the sensitivity to such scenarios for both current and future gravitational wave detectors in a model-independent fashion. Since secluded hidden sectors are of particular interest for dark matter models at the MeV scale or below, we pay special attention to the reach of pulsar timing arrays. Cosmological constraints on light degrees of freedom restrict the number of sub-MeV particles in a hidden sector, as well as the hidden sector temperature. Nevertheless, we find that observable first-order phase transitions can occur. To illustrate our results, we consider two minimal benchmark models: a model with two gauge singlet scalars and a model with a spontaneously broken $U(1)$ gauge symmetry in the hidden sector.Comment: 37 pages, 12 figures. Noise and PLI sensitivity curves are included in the source director

Cosmology with the Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological applications of gravitational-wave observations extends well beyond these two objectives. This publication presents a summary of the state of the art in LISA cosmology, theory and methods, and identifies new opportunities to use gravitational-wave observations by LISA to probe the universe