arXiv subjects

Fabian Esser

Publications and source records attributed to Fabian Esser.

11 recordsLinked to original sources

arXiv2026

Probing $\nu$SMEFT at Belle II with displaced dilepton vertices

The existence of right-handed (RH) neutrinos is strongly motivated by the observation of small neutrino masses and as a means to shed light on the origin of parity violation in the Standard Model (SM). Experimental searches for the corresponding mass eigenstates, referred to as heavy neutral leptons (HNLs), have been carried out in a variety of experiments and within different HNL models. In the current work we use the SM effective field theory with added RH neutrinos, known as $\nu$SMEFT, to study GeV-scale HNL production in $e^+e^-$ collisions at the Belle~II experiment. We focus on leptonic decays of long-lived HNLs that produce a displaced vertex in the Belle~II detector. Accounting for detection efficiency and backgrounds, we estimate the sensitivity of Belle~II to the new-physics scale of four-fermion, Higgs-current, and dipole operators in $\nu$SMEFT. We find that for most operators, the search we propose probes large regions of parameter space that are not excluded by other searches.

hep-ph↗

arXiv2026

Cartography of LNV dim-9 SMEFT: Implications for Radiative Neutrino Masses and $0\nu\beta\beta$

We perform a systematic study of lepton-number-violating (LNV) dimension-9 operators in the Standard Model Effective Field Theory (SMEFT) that can mediate neutrinoless double beta decay ($0\nu\beta\beta$) at tree level, and map them to their possible tree-level ultraviolet completions. Using a diagram-based classification, we enumerate all such completions and isolate minimal two-particle models that avoid generating the dimension-5 Weinberg operator or dimension-7 LNV operators at tree level. We then chart how these minimal models populate the operator landscape and organise them by the loop order at which they radiatively induce lower-dimensional LNV operators, highlighting scenarios in which the tree-level dimension-9 contribution can compete with or dominate loop-suppressed neutrino-mass (dimension-5) effects. Representative one-loop and two-loop classes are matched onto the SMEFT, and their implications for neutrino masses, charged-lepton flavour violation, and the relative size of dimension-9 versus dimension-5 contributions to $0\nu\beta\beta$ are analysed, delineating regions of parameter space where upcoming experiments can be sensitive to genuinely short-range LNV dynamics.

hep-ph↗

arXiv2025

A global analysis of ALP-mediated multiboson production at the LHC

Axion-like particles (ALPs) provide a well-motivated framework for physics beyond the Standard Model, coupling to gauge bosons through dimension-five operators protected by an approximate shift symmetry. At the LHC, such interactions lead to distinctive signatures in multiboson production, where the ALP appears as an off-shell mediator rather than a narrow resonance. In this work, we present the first global analysis of ALP-mediated multiboson processes, combining measurements of diphoton, ZZ, $W^+ W^-$, dijet, and vector-boson-fusion final states. On the theory side, motivated from a UV perspective, we assume that the ALP couples only to the gauge sector of the SM, and classify the ALP-multiboson vertices that directly govern collider observables. Our results show that the dijet channel dominates the sensitivity to ALP couplings and determines the limits on $c_{\tilde{G}}$, while diboson and VBF processes provide complementary constraints on the electroweak couplings. We further assess the validity of the EFT expansion given the multi-TeV scales probed in the data. This global study provides the most comprehensive picture to date of ALP-gauge interactions from multiboson production at the LHC, and highlights the opportunities for significant improvements with future high-luminosity datasets.

hep-ph↗

arXiv2025

Probing the coupling of axions to tops and gluons with LHC measurements

We study axion-like particles (ALPs) whose dominant interactions are with gluons and third-generation quarks, and whose couplings to light Standard Model (SM) particles arise at one loop. These loop-induced effects lead to ALP decays and production channels that can be probed at the LHC, even when tree-level couplings are absent. Using an effective field theory (EFT) description that includes momentum-dependent corrections from radiative effects, we reinterpret a wide range of LHC measurements via the CONTUR framework to derive model-independent constraints on the ALP parameter space. We show that LHC data place meaningful bounds in the plane of effective couplings $c^0_t/f_a$ and $c^0_{\tilde G}/f_a$, and that these limits are sensitive to the UV origin of the ALP-top and ALP-gluon couplings. We discuss representative scenarios where either $c^0_t$ or $c^0_{\tilde G}$ vanishes at the matching scale, and highlight the role of EFT running and mixing in generating observable signals. We also assess the domain of validity of the EFT approach by comparing the typical momentum transfer $\sqrt{\hat s}$ in sensitive regions to the underlying scale $f_a$. Our results demonstrate the power of loop-aware EFT reinterpretation of SM measurements in probing otherwise elusive ALP scenarios. The framework presented here can be readily extended to include couplings to other fermions and to accommodate ALP decay or long-lived signatures.

hep-ph↗

arXiv2024

Faking ZZZ vertices at the LHC

Searches for anomalous neutral triple gauge boson couplings (NTGCs) provide important tests for the gauge structure of the standard model. At the LHC, NTGCs are searched for via the process $pp \to ZZ \to 4l$, where the two $Z$-bosons are on-shell. In this paper, we discuss how the same process can occur through tree-level diagrams just adding a vector-like quark (VLQ) to the standard model. Since NTGCs are generated in standard model effective theory (SMEFT) only at 1-loop order, vector like quarks could be an important alternative interpretation to, and background for, NTGC searches. Here, we construct a simple example model, discuss low-energy constraints and estimate current and future sensitivities on the model parameters from $pp \to ZZ \to 4l$ searches.

hep-ph↗

arXiv2024

Di-Higgs production via Axion-Like Particles

Due to the pseudo-scalar nature of the axion-like particle (ALP), the CP- conserving production of two Higgs bosons via the ALP necessarily involves an additional Z or $\gamma$ boson. We examine the existing constraints from di-Higgs searches at Run 2 of the LHC and find that, despite the presence of extra objects in the final state, these searches are sensitive to a combination of ALP couplings to gluons and three-bosons in the TeV scale range. Additionally, we propose a specialized search strategy incorporating an energetic leptonic Z boson. This refined ALP-induced production process would allow for the identification of the h h $\rightarrow$ 4 b-jet final state and could potentially probe the TeV scale using data from Run 2 of the LHC. This production process can also occur through a coupling between the top quark and the ALP. We translate the current constraints on di-Higgs production into new limits on the ALP-top coupling.

hep-ph↗

arXiv2024

Fermionic UV models for neutral triple gauge boson vertices

Searches for anomalous neutral triple gauge boson couplings (NTGCs) provide important tests for the gauge structure of the standard model. In SMEFT ("standard model effective field theory") NTGCs appear only at the level of dimension-8 operators. While the phenomenology of these operators has been discussed extensively in the literature, renormalizable UV models that can generate these operators are scarce. In this work, we study a variety of extensions of the SM with heavy fermions and calculate their matching to d = 8 NTGC operators. We point out that the complete matching of UV models requires four different CP-conserving d = 8 operators and that the single CPC d = 8 operator, most commonly used by the experimental collaborations, does not describe all possible NTGC form factors. Despite stringent experimental constraints on NTGCs, limits on the scale of UV models are relatively weak, because their contributions are doubly suppressed (being d = 8 and 1-loop). We suggest a series of benchmark UV scenarios suitable for interpreting searches for NTGCs in the upcoming LHC runs, obtain their current limits and provide estimates for the expected sensitivity of the high-luminosity LHC.

hep-ph↗

arXiv2023

On the coupling of axion-like particles to the top quark

In this paper we explore the coupling of a light axion-like particle (ALP) to top quarks. We use high-energy LHC probes, and examine both the direct probe to this coupling in associated production of a top-pair with an ALP, and the indirect probe through loop-induced gluon fusion to an ALP leading to top pairs. Using the latest LHC Run II data, we provide the best limit on this coupling. We also compare these limits with those obtained from loop-induced couplings in diboson final states, finding that the $t\bar t$+MET channel is the best current handle on this coupling.

hep-ph↗

arXiv2023

SMEFT goes dark: Dark Matter models for four-fermion operators

We study ultra-violet completions for d = 6 four-fermion operators in the standard model effective field theory (SMEFT), focusing on models that contain cold dark matter candidates. Via a diagrammatic method, we generate systematically lists of possible UV completions, with the aim of providing sets of models, which are complete under certain, well specified assumptions. Within these lists of models we rediscover many known DM models, as diverse as R-parity conserving supersymmetry or the scotogenic neutrino mass model. Our lists, however, also contain many new constructions, which have not been studied in the literature so far. We also briefly discuss how our DM models could be constrained by reinterpretations of LHC searches and the prospects for HL-LHC and future lepton colliders.

hep-ph↗

arXiv2022

Mapping the SMEFT to discoverable models

The matching of specific new physics scenarios onto the SMEFT framework is a well-understood procedure. The inverse problem, the matching of the SMEFT to UV scenarios, is more difficult and requires the development of new methods to perform a systematic exploration of models. In this paper we use a diagrammatic technique to construct in an automated way a complete set of possible UV models (given certain, well specified assumptions) that can produce specific groups of SMEFT operators, and illustrate its use by generating models with no tree-level contributions to four-fermion (4F) operators. Those scenarios, which only contribute to 4F at one-loop order, can contain relatively light particles that could be discovered at the LHC in direct searches. For this class of models, we find an interesting interplay between indirect SMEFT and direct searches. We discuss some examples on how this interplay would look like when combining low-energy observables with the SMEFT Higgs-fermion analyses and searches for resonance at the LHC.

hep-ph↗

arXiv2022

Generalising Holographic Superconductors

In this paper we propose a generalised holographic framework to describe superconductors. We first unify the description of s-, p- and d-wave superconductors in a way that can be easily promoted to higher spin. Using a semi-analytical procedure to compute the superconductor properties, we are able to further generalise the geometric description of the hologram beyond the AdS-Schwarzschild Black Hole paradigm, and propose a set of higher-dimensional metrics which exhibit the same universal behaviour. We then study the properties of the condensate and the scaling of the critical temperature with the parameters of the higher-dimensional theory, reproducing existing results in the literature and extending them to include a possible description of the newly observed f-wave superconducting systems.

hep-th↗