Investigating Flavourful New Physics at the TeV Scale

We investigate the possibility of new physics at the TeV scale coupled mainly to the third generation. This arises naturally when trying to address the Standard Model flavour puzzle together with the Higgs hierarchy problem. We show, in a pure Effective Field Theory approach, that this scenario is perfectly compatible with current experimental data, while also being able to accomodate some deviations from the Standard Model observed in semileptonic B decays. From a model-building perspective, we focus on so-called 4321 models, based on a SU(4) × SU(3) × SU(2) × U(1) gauge symmetry, which feature quark-lepton unification at the TeV scale. These models have been generically shown to be the common last step in a sequence of symmetry breakings in deeper UV theories explaining the Standard Model flavour structure. We study important phenomenological implications of 4321 models in the context of leptonic τ decays and electroweak precision tests. Lastly, we analyse the issue of the Higgs mass stability with respect to quantum corrections in generic multi-scale models of flavour

Lepton Flavour Universality in τ\tau decays

The evidence for Lepton Flavour Universality (LFU) violation in semileptonic $B$-decays has been rising over the past few years. Relying on generic effective field theory (EFT) results, it has been shown that models addressing the $B$-anomalies necessarily lead, at one-loop, to deviations from LFU in $\tau$ decays at the few per-mil level. Once a (renormalizable) UV model is specified, the leading-log EFT result receives finite corrections from the matching at the UV scale. We discuss such corrections in a motivated class of models for the B-anomalies, based on an extended $SU(4) \times SU(3) \times SU(2) \times U(1)$ gauge sector. In this scenario, we obtain precise predictions for the effective $W$-boson and $Z$-boson couplings to leptons in terms of the masses and couplings of the new heavy fields. We confirm a few per-mil deviation from universality, within reach of future high-precision experiments.Comment: Contribution to the 56th Rencontres de Moriond and to La Thuile 2022 - Les Rencontres de Physique de la Vall\'ee d'Aost

LFU violations in leptonic τ decays and B-physics anomalies

We present a complete analysis of Lepton Flavor Universality (LFU) violations in leptonic τ decays in motivated models addressing the B-physics anomalies, based on the gauge group. We show that the inclusion of vector-like fermions, required by B-physics data, leads to sizable modifications of the leading-log results derived within an Effective Field Theory approach. In the motivated parameter-space region relevant to the B-physics anomalies, the models predict a few per-mil decrease of the effective W-boson coupling to τ, within the reach of future experiments

Stability of the Higgs Sector in a Flavor-Inspired Multi-Scale Model

We analyze the stability of the Higgs sector of a three-site model with flavor-non-universal gauge interactions, whose spectrum of non-Standard-Model states spans three orders of magnitude. This model is inspired by deconstructing a five-dimensional theory where the generation index is in one-to-one relation to the position in the fifth dimension. It provides a good description of masses and mixing of the SM fermions in terms of scale hierarchies. We demonstrate that, within this construction, the mass term of the SM-like Higgs does not receive large corrections proportional to the highest mass scales. The model suffers only of the unavoidable "little hierarchy problem" between the electroweak scale and the lightest NP states, which are expected to be at the TeV scale.Comment: 17 pages, 1 figure, 2 table

New Physics in the Third Generation: A Comprehensive SMEFT Analysis and Future Prospects

We present a comprehensive analysis of electroweak, flavor, and collider bounds on the complete set of dimension-six SMEFT operators in the $U(2)^5$-symmetric limit. This operator basis provides a consistent framework to describe a wide class of new physics models and, in particular, the motivated class of models where the new degrees of freedom couple mostly to the third generation. By analyzing observables from all three sectors, and consistently including renormalization group evolution, we provide bounds on the effective scale of all 124 $U(2)^5$-invariant operators. The relation between flavor-conserving and flavor-violating observables is analyzed taking into account the leading $U(2)^5$ breaking in the Yukawa sector, which is responsible for heavy-light quark mixing. We show that under simple, motivated, and non-tuned hypotheses for the parametric size of the Wilson coefficients at the high scale, all present bounds are consistent with an effective scale as low as 1.5 TeV. We also show that a future circular $e^+ e^-$ collider program such as FCC-ee would push most of these bounds by an order of magnitude. This would rule out or provide clear evidence for a wide class of compelling new physics models that are fully compatible with present data.Comment: 44 pages, 9 figures, 16 tables; added references, version submitted for publicatio

What is the scale of new physics behind the muon g−2g-2?

We study the constraints imposed by perturbative unitarity on the new physics interpretation of the muon $g-2$ anomaly. Within a Standard Model Effective Field Theory (SMEFT) approach, we find that scattering amplitudes sourced by effective operators saturate perturbative unitarity at about 1 PeV. This corresponds to the highest energy scale that needs to be probed in order to resolve the new physics origin of the muon $g-2$ anomaly. On the other hand, simplified models (e.g.~scalar-fermion Yukawa theories) in which renormalizable couplings are pushed to the boundary of perturbativity still imply new on-shell states below 200 TeV. We finally suggest that the highest new physics scale responsible for the anomalous effect can be reached in non-renormalizable models at the PeV scale.Comment: 16 pages, 4 figure

Understanding the first measurement of B(B→Kννˉ)\mathcal{B}(B\to K \nu \bar{\nu})

Recently, Belle II reported on the first measurement of $\mathcal{B}(B^\pm\to K^\pm \nu\bar{\nu})$ which appears to be almost $3\sigma$ larger than predicted in the Standard Model. We point out the important correlation with $\mathcal{B}(B\to K^{\ast} \nu\bar{\nu})$ so that the measurement of that decay mode could help restraining the possible options for building the model of New Physics. We then try to interpret this new experimental result in terms of physics beyond the Standard Model by using SMEFT and find that a scenario with coupling only to $\tau$ can accommodate the current experimental constraints but fails in getting a desired $R_{D^{(\ast )}}^\mathrm{exp}/R_{D^{(\ast )}}^\mathrm{SM}$, unless one turns the other SMEFT operators that are not related to $b\to s\ell\ell$ or/and $b\to s\nu\nu$.Comment: 8 pages, 6 figure

HighPT: A Tool for high-pTp_T Drell-Yan Tails Beyond the Standard Model

HighPT is a Mathematica package for the analysis of high-energy data of semileptonic transitions at hadron colliders. It allows to compute high-$p_T$ tail observables for semileptonic processes, i.e. Drell-Yan cross sections, for dilepton and monolepton final states at the LHC. These observables can be calculated at tree level within the Standard Model Effective Field Theory, including the relevant operators up to dimension eight to ensure a consistent description of the cross section including terms of $\mathcal{O}(\Lambda^{-4})$ in the cutoff scale $\Lambda$. For New Physics models with new mediators that can be resolved at LHC energies, HighPT can also account for the full propagation effects of these new bosonic states at tree level. Using the available data from the high-$p_T$ tails in the relevant LHC run-II searches by the ATLAS and CMS collaborations, HighPT can also construct the corresponding likelihoods for all possible flavors of the leptonic final states. As an illustration, we derive and compare constraints on Wilson coefficients at different orders in the Effective Field Theory expansion, and we investigate lepton flavor violation for the $S_3$ leptoquark model. The HighPT code is publicly available at https://github.com/HighPT/HighPT.Comment: 34 page

Drell-Yan tails beyond the Standard Model

We investigate the high-p$_{T}$ tails of the pp → ℓν and pp → ℓℓ Drell-Yan processes as probes of New Physics in semileptonic interactions with an arbitrary flavor structure. For this purpose, we provide a general decomposition of the 2 → 2 scattering amplitudes in terms of form-factors that we match to specific scenarios, such as the Standard Model Effective Field Theory (SMEFT), including all relevant operators up to dimension-8, as well as ultraviolet scenarios giving rise to tree-level exchange of new bosonic mediators with masses at the TeV scale. By using the latest LHC run-II data in the monolepton (eν, μν, τν) and dilepton (ee, μμ, ττ, eμ, eτ, μτ) production channels, we derive constraints on the SMEFT Wilson coefficients for semileptonic four-fermion and dipole operators with the most general flavor structure, as well as on all possible leptoquark models. For the SMEFT, we discuss the range of validity of the EFT description, the relevance of $$\mathcal{O}$$(1/Λ$^{2}$) and $$\mathcal{O}$$(1/Λ$^{4}$) truncations, the impact of d = 8 operators and the effects of different quark-flavor alignments. Finally, as a highlight, we extract for several New Physics scenarios the combined limits from high-p$_{T}$ processes, electroweak pole measurements and low-energy flavor data for the b → cτν transition, showing the complementarity between these different observables. Our results are compiled in HighPT (https://highpt.github.io), a package in Mathematica which provides a simple way for users to extract the Drell-Yan tails likelihoods for semileptonic effective operators and for leptoquark models

Drell-Yan Tails Beyond the Standard Model

We investigate the high-$p_T$ tails of the $pp\to \ell \nu$ and $pp \to \ell \ell$ Drell-Yan processes as probes of New Physics in semileptonic interactions with an arbitrary flavor structure. For this purpose, we provide a general decomposition of the $2\to2$ scattering amplitudes in terms of form-factors that we match to specific scenarios, such as the Standard Model Effective Field Theory (SMEFT), including all relevant operators up to dimension-$8$, as well as ultraviolet scenarios giving rise to tree-level exchange of new bosonic mediators with masses at the TeV scale. By using the latest LHC run-II data in the monolepton ($e\nu$, $\mu\nu$, $\tau\nu$) and dilepton ($ee$, $\mu\mu$, $\tau\tau$, $e\mu$, $e\tau$, $\mu\tau$) production channels, we derive constraints on the SMEFT Wilson coefficients for semileptonic four-fermion and dipole operators with the most general flavor structure, as well as on all possible leptoquark models. For the SMEFT, we discuss the range of validity of the EFT description, the relevance of $\mathcal{O}(1/\Lambda^2)$ and $\mathcal{O}(1/\Lambda^4)$ truncations, the impact of $d=8$ operators and the effects of different quark-flavor alignments. Finally, as a highlight, we extract for several New Physics scenarios the combined limits from high-$p_T$ processes, electroweak pole measurements and low-energy flavor data for the $b\to c\tau\nu$ transition, showing the complementarity between these different observables. Our results are compiled in {\tt HighPT}, a package in {\tt Mathematica} which provides a simple way for users to extract the Drell-Yan tails likelihoods for semileptonic effective operators and for leptoquark models.Comment: 61 pages, 19 figure