Vector boson fusion at multi-TeV muon colliders

High-energy lepton colliders with a centre-of-mass energy in the multi-TeV range are currently considered among the most challenging and far-reaching future accelerator projects. Studies performed so far have mostly focused on the reach for new phenomena in lepton-antilepton annihilation channels. In this work we observe that starting from collider energies of a few TeV, electroweak (EW) vector boson fusion/scattering (VBF) at lepton colliders becomes the dominant production mode for all Standard Model processes relevant to studying the EW sector. In many cases we find that this also holds for new physics. We quantify the size and the growth of VBF cross sections with collider energy for a number of SM and new physics processes. By considering luminosity scenarios achievable at a muon collider, we conclude that such a machine would effectively be a "high-luminosity weak boson collider," and subsequently offer a wide range of opportunities to precisely measure EW and Higgs coupling as well as to discover new particles.Comment: 58 pages, 17 figures, 9 tables. A contribution to Snowmass 202

Quantum SMEFT tomography: top quark pair

In recent years, an interest in the study of quantum information has grown within the high-energy particle physics community. The possibility to establish the presence of entanglement at particle colliders, such as the Large Hadron Collider (LHC) at CERN, is a novel and thrilling research direction, offering the opportunity to push quantum mechanics to its limits by using the heaviest fundamental particle: the top quark. With this in mind, we propose to use measurements of quantum entanglement to probe the behaviour of fundamental interactions and to search for New Physics at high energy within the Standard Model Effective Field Theory paradigm. Inspired by recent proposals to measure entanglement of top quark pairs produced at the LHC, we examine how the existence of new interactions between fundamental particles modify the Standard Model expectations. By performing analytical calculations, we unveil a non-trivial pattern of effects depending on the kinematical phase space configurations.Comment: Talk at the 15th International Workshop on Top Quark Physics, Durham, UK, 4-9 September 202

Simplified t-channel models for dark matter searches

Una enorme quantità di evidenze sperimentali sulla esistenza di una forma di materia non luminosa nell'Universo, si sono accumulate nel corso di circa un secolo. Chiarire la sua natura è diventata una delle sfide più eccitanti ed urgenti negli sforzi per capire il nostro Universo. In questo lavoro presento uno studio su un approccio per scoprire la Materia Oscura interpretata come particella elementare e sulla possibilità di produrla e rilevarla negli acceleratori. Nella parte introduttiva presento una breve storia delle evidenze astrofisiche e astronomiche che hanno portato alla ipotesi della esistenza di Materia Oscura. Assumendo che la Materia Oscura sia costituita da una particella elementare ulteriore a quelle predette dal Modello Standard, delineo poi i tre principali metodi di rilevazione utilizzati attualmente per identificarla. Nella seconda parte discuto come si possono costruire teorie nelle quali sia possibile interpretare le ricerche attuali ed i risultati corrispondenti. Eseguo un confronto tra approcci diversi, partendo da modelli completi fino a quelli che utilizzano teorie di campo effettive. In particolare, discuto i loro lati positivi e negativi, motivando l'utilizzo di uno schema intermedio, il cosiddetto approccio con modelli semplificati, caratterizzati da un numero limitato di nuovi stati e parametri e che supera le limitazioni intrinseche delle teorie effettive nel contesto delle ricerche negli acceleratori. Nell'ultima parte fornisco una esaustiva classificazione dei modelli semplificati nel canale t, che non sono ancora stati analizzati sistematicamente nella letteratura. Per ciascuno di essi presento un possibile completamento UV e i segnali più promettenti ad LHC. Per questa ragione tutti i modelli considerati sono stati implementati in strumenti Monte Carlo, validati nel confronto con risultati analitici, studiati in dettaglio e resi pronti per un rilascio pubblico per la comunità fenomenologica e sperimentale di LHC

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

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

A comprehensive exploration of t-channel simplified models of dark matter

We analyse six classes of t-channel dark matter simplified models in which the Standard Model field content is extended by a coloured mediator and a dark matter state. The two new states are enforced to be odd under a new parity, while all Standard Model fields are taken even so that dark matter stability is guaranteed. We study several possibilities for the spin of the new particles and the self-conjugate property of the dark matter, and we focus on model configurations in which the dark matter couples to the right-handed up quark for simplicity. We investigate how the parameter spaces of the six models can be constrained by current and future cosmological, astrophysical and collider searches, and we highlight the strong complementary between those probes. Our results demonstrate that scenarios featuring a complex (non self-conjugate) dark matter field are excluded by cosmology and astrophysics alone, the only possibility to avoid these bounds being to invoke very weak couplings and mechanisms such as conversion-driven freeze-out. For models with self-conjugate dark matter, mediator and dark matter masses are pushed deep into the TeV regime, with the lower limits on the mediator mass reaching 3 to 4 TeV and those on the dark matter mass 1 to 2 TeV. Those bounds are much stronger than those obtained in previous studies of t-channel dark matter models, due in particular to overlooked contributions to LHC signal modelling that we incorporate in this work for the first time.Comment: 19 pages, 6 figures, happy holidays

Studying dark matter with MadDM 3.1: a short user guide

MadDM is an automated numerical tool for the computation of dark-matter observables for generic new physics models. We announce version 3.1 and summarize its features. Notably, the code goes beyond the mere cross-section computation for direct and indirect detection. For instance, it allows the user to compute the fully differential nuclear recoil rates as well as the energy spectra of photons, neutrinos and charged cosmic rays for arbitrary $2\to n$ annihilation processes. This short user guide equips researchers with all the relevant information required to readily perform comprehensive phenomenological studies of particle dark-matter models.Comment: 17 pages + references; contribution to "Tools for High Energy Physics and Cosmology" (TOOLS2020), 2-6 Nov. 2020, IP2I Lyon, Franc