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Natascia Vignaroli

Publications and source records attributed to Natascia Vignaroli.

At least 19 recordsLinked to original sources
arXiv2026

Probing the Tau Anomalous Magnetic Moment at Colliders: From Ultra-Peripheral Collisions to the Precision Frontier

The anomalous magnetic moment of the tau lepton, $a_τ$, represents a fundamental test of the Standard Model (SM) and a high-sensitivity probe for New Physics in the third generation of leptons. Due to the tau's extremely short lifetime, traditional spin-precession measurements remain inaccessible, necessitating innovative experimental strategies at high-energy colliders. This review provides a comprehensive overview of the current experimental landscape, highlighting the recent paradigm shift from LEP-era constraints to the unprecedented precision reached at the LHC. We emphasize the importance of Ultra-Peripheral Heavy-Ion Collisions (UPCs), which act as a ``photon-photon collider'' of extreme intensity. By leveraging the $Z^4$ enhancement of the coherent photon flux in Lead-Lead ($PbPb$) interactions, these collisions provide a theoretically robust ``quasi-static'' environment. These results are critically compared with the latest measurements from proton-proton collisions, including the recent CMS observation of the $γγ\to ττ$ process and the ATLAS constraints from the high-mass Drell-Yan tail. We evaluate their complementarity and the challenges related to Effective Field Theory validity at the TeV scale. Finally, we outline the future prospects for $a_τ$ at Belle II and the Future Circular Collider (FCC) stages. While FCC-hh in $PbPb$ mode provides a theoretically clean environment, its sensitivity remains limited to $\mathcal{O}(10^{-2})$. Conversely, the next generation of lepton facilities, specifically Belle II and FCC-ee, aims for the $\mathcal{O}(10^{-5})$ level, required to probe SM electroweak loop corrections. Long-term projections for a high-energy Muon Collider suggest a potential reach of $\mathcal{O}(10^{-6})$.

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arXiv2025

New avenues for the neutrino dipole portal exploration at the energy frontier

We present a comprehensive and gauge-invariant study of the neutrino dipole portal at the energy frontier. Assuming negligible active-sterile mixing, we analyze sterile neutrino production via dimension-6 dipole operators coupling to the electroweak field strengths. The analysis incorporates both single- and double-gauge-boson effective interactions. We investigate novel collider signatures at the HL-LHC, the FCC-hh -- studied here in this context for the first time -- and a 10 TeV muon collider. Particular emphasis is placed on electroweak boson-initiated processes, which dominate in the high-mass regime above $\sim$1 TeV. At the muon collider, these VBF-like topologies enable production even when the dipole couples to non-muonic flavors, offering a unique and sensitive probe for different flavor scenarios. We derive sensitivity projections for various theoretical benchmarks, reaching dipole couplings down to $d_γ\sim 6\times10^{-7}$ GeV$^{-1}$ at FCC-hh and $d_γ\sim 2\times10^{-7}$ GeV$^{-1}$ at the muon collider.

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arXiv2025

Muon collider probes of Majorana neutrino dipole moments and masses

Majorana neutrinos may have transitional dipole moments, which violate lepton number as well as lepton flavour. We estimate the sensitivity of future colliders to the electron-muon neutrino dipole moment, $λ_{eμ}$, by considering same-sign dilepton final states. We find that hadron colliders, even the proposed FCC-hh, are sensitive only to $|λ_{eμ}|\gtrsim 10^{-9}μ_B$ (with $μ_B$ the Bohr magneton), a value two-three orders of magnitude larger than current bounds from astrophysics and low-energy neutrino-scattering experiments. In the case of a future muon collider, we show that the sensitivity varies from $|λ_{eμ}|\sim 5\cdot 10^{-9}μ_B$ for energy $\sqrt{s}\simeq 3$ TeV, to $\sim 10^{-12}μ_B$ for $\sqrt{s}\simeq 50$ TeV, matching the current laboratory bounds for $\sqrt{s}\simeq 30$ TeV. The singular advantage of the muon collider signal would be a direct, clean identification of lepton number and flavour violation. We also show that a muon collider would improve by orders of magnitude the direct bounds on $m_{eμ}$ and $m_{μμ}$, two of the entries of the Majorana neutrino mass matrix. These bounds could be as strong as $\sim 50$ keV, still far above the neutrino mass scale.

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arXiv2025

Improved precision in inverse beta decay cross section

We analyze the cross section for inverse beta decay, focusing on the moderate energies (a few MeV to hundreds of MeV) relevant for reactor and supernova neutrinos. We discuss the updated evaluations of values and uncertainties in the cross section, and the effect of second-class currents. The estimate of theoretical precision is important for current and future experiments, when large data samples are or become available.

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arXiv2024

Majorana neutrino dipole moments and masses at a muon collider

A future multi-TeV muon collider would provide an important probe for Majorana neutrinos. A muon collider with a collision energy of $\sim$30 TeV would be sensitive to $ν_e-ν_μ$ transition dipole moments of the order of $\sim 10^{-12}μ_B$ and would be thus competitive with the latest astrophysical observation and laboratory experiments. Contrary to the latter, the muon collider would have the unique advantage of a direct and clean identification of lepton number and flavour violation. This would establish the Majorana nature of the neutrino and would provide crucial complementary information on the neutrino properties in the event of a (near) future observation at low-energy experiments. Additionally, a muon collider would improve by orders of magnitude the direct bounds on the Majorana neutrino mass matrix entries $m_{eμ}$ and $m_{μμ}$.

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arXiv2024

The cross section of inverse beta decay

We discuss the accuracy of the evaluation of the cross section for inverse beta decay at low energies and its relevance in the current experimental framework.

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arXiv2024

New Physics Pathways from B Processes

We re-consider recent measures of $R_{K}$ and $R_{K^*}$, now compatible with the Standard Model expectations, as well as the results for the process $\text{BR}(B_s \rightarrow μ^+ μ^-)$ alongside earlier determinations of $R_{D^{(\ast)}}$ and $\text{BR}(B_c \rightarrow τν)$. We provide analytic constraints on the associated Wilson coefficients in both the $b \to s$ and the $b \to c$ sectors. These allow us to estimate the scale of potential New Physics for generic extensions of the Standard Model. We then use the results to constrain the leptoquark landscape.

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arXiv2024

331 Models and Bilepton Searches at LHC

Despite being remarkable predictive, the Standard Model leaves unanswered several important issues, which motivate an ongoing search for its extensions. One fashionable possibility are the so-called 331 models, where the electroweak gauge group is extended to $SU_L(3)\times U(1)$. We focus on a minimal extension which includes vector-like quarks (VLQs) and new gauge bosons, performing a consistent analysis of the production at LHC of a pair of doubly-charged bileptons. We include for the first time all the relevant processes where VLQs contribute, and in particular the associate production VLQ-bilepton. Finally, we extract the bound on the bilepton mass, $m_Y>$ 1300 GeV, from a reinterpretation of a recent ATLAS search for doubly-charged Higgs bosons in multi-lepton final states.

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arXiv2024

A study of the measurement of the $τ$ lepton anomalous magnetic moment in high energy lead-lead collisions at LHC

The $τ$ lepton anomalous magnetic moment: $a_τ= \frac{g_τ-2}{2}$ was measured, so far, with a precision of only several percents despite its highly sensitivity to physics beyond the Standard Model such as compositeness or Supersymmetry. A new study is presented to improve the sensitivity of the $a_τ$ measurement with photon-photon interactions from ultra-peripheral lead-lead collisions at LHC. The theoretical approach used in this work is based on an effective Lagrangian and on a photon flux implemented in the MadGraph5 Monte Carlo simulation. Using a multivariate analysis to discriminate the signal from the background processes, a sensitivity to the anomalous magnetic moment $\rm{a_τ}$ = 0 $_{+0.011} ^{-0.019}$ is obtained at 95\% CL with a dataset corresponding to an integrated luminosity of 2 nb$^{-1}$ of lead-lead collisions and assuming a conservative 10\% systematic uncertainty. The present results are compared with previous calculations and available measurements.

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arXiv2023

A discussion of the cross section $\barν_e+p\to e^+ + n$

We discuss the interaction cross section $\barν_e-p$ due to charged currents, of major importance for neutrino detection. We present the history of its understanding and highlight the aspects necessary for its precise evaluation. We examine the three most recent determinations and, on the basis of the most recent one, tabulate its updated values and assess its uncertainty.

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arXiv2023

Charged resonances and Minimal Dark Matter bound states at a multi-TeV muon collider

A multi-TeV muon collider proves to be very efficient not only for the search for new heavy neutral particles, but also for the discovery of charged bosons of the $W^\prime$ type. We find that, by analyzing the associated production with a Standard Model W, charged resonances can be probed directly up to multi-TeV mass values close to the collision energy, and for very small couplings with the SM fermions, of the order of $10^{-3}-10^{-4}$ times the SM weak coupling. Additionally, charged bound states of WIMP Minimal Dark Matter, specifically a Majorana fermionic 5-plet, can be discovered with low statistics by running above the kinematic threshold, at a center-of-mass energy just slightly above the mass of the MDM bound state. This opens up a very interesting possibility for the discovery of WIMPs, complementary to the search for the resonant production of the neutral MDM bound state component, which relies on an on-peak search. For 5-plet MDM, indeed, the proposed search strategy is more efficient than the WIMP searches based on mono-X, missing-mass and disappearing tracks signatures.

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arXiv2022

Standard model anomalies: Lepton flavour non-universality, g-2 and W-mass

We critically analyze the body of results that hints to the existence of New Physics from possible violations of lepton universality observed by the LHCb experiment in the $μ/e$ ratios $R_{K}$ and $R_{K^*}$ to the $g-2$ lepton anomalies. The analysis begins with a theoretical, in depth, study of the $μ/e$ ratios $R_{K}$ and $R_{K^*}$ as well as the process $B_s \rightarrow μ^+ μ^-$. Here we consider the impact of complex Wilson coefficients and derive constraints on their imaginary and real parts. We then move to a comprehensive comparison with experimental results. We show that, by fitting a single Wilson coefficient, the deviations from the Standard Model are at the $4.7σ$ level when including only the hadronic insensitive observables while it increases to $6.1σ$ when including also the hadronic sensitive ones. When switching on all relevant Wilson coefficients and combining both hadronic sensitive and insensitive data into the fit, the deviation from the Standard Model peaks at $7.2σ$ and decreases at the $4.9σ$ level if we assume that the central values of $R_K$ and $R_{K^{\ast}}$ are taken to be unity. We further estimate other unaccounted for SM contributions and show that their inclusion still requires New Physics to fit the data. We then introduce the $g-2$ lepton anomalies as well as the most recent $W$-mass results. Different theoretical models are considered that can explain the discrepancies from the Standard Model. In the final part of our work we estimate the impact of the forthcoming data from LHCb (coming from LHC Run3) and Belle II, when it will have accumulated about $5~ab^{-1}$.

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arXiv2022

Dark Matter abundance via thermal decays and leptoquark mediators

We explore a new mechanism for reproducing the Dark Matter (DM) abundance: scatterings of one DM particle on light Standard Model particles. Strong bounds on its decays can be satisfied if DM undergoes freeze-in and has a mass around or below the pion mass. This happens, for example, in theories with a right-handed neutrino interacting with charged fermions through a leptoquark exchange. These leptoquarks can be linked to the ones motivated by the B-physics anomalies if assumptions about the flavour structure are made. DM signals are unusual, with interesting possibilities for direct and indirect detection. Achieving thermal freeze-out instead requires models with more than one DM flavour, and couplings parametrically smaller than what needed by the usual pair annihilations.

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arXiv2022

An accurate evaluation of electron (anti-)neutrino scattering on nucleons

We discuss as accurately as possible the cross section of quasi-elastic scattering of electron (anti-)neutrinos on nucleons, also known as inverse beta decay in the case of antineutrinos. We focus on the moderate energy range from a few MeV up to hundreds of MeV, which includes neutrinos from reactors and supernovae. We assess the uncertainty on the cross section, which is relevant to experimental advances and increasingly large statistical samples. We estimate the effects of second-class currents, showing that they are small and negligible for current applications.

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arXiv2022

Off-shell probes of the Higgs Yukawa couplings: light quarks and charm

We review the present status and the future prospects for the measurements of the Higgs Yukawa couplings to light quarks and charm. A special focus is given to novel proposed off-shell probes, which offer promising and complementary opportunities to test light quark Yukawas in triboson final states. Additionally, a new off-shell strategy to test the charm Yukawa coupling in the final state with two bosons plus a charm and a jet is considered. First estimates for the HL-LHC and the FCC-hh sensitivities on the channel are presented, showing encouraging results.

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arXiv2021

Minimal Dark Matter bound states at future colliders

The hypothesis that Dark Matter is one electroweak multiplet leads to predictive candidates with multi-TeV masses that can form electroweak bound states. Bound states with the same quantum numbers as electroweak vectors are found to be especially interesting, as they can be produced resonantly with large cross sections at lepton colliders. Such bound states exist e.g. if DM is an automatically stable fermionic weak 5-plet with mass $M \approx$ 14 TeV such that the DM abundance is reproduced thermally. In this model, a muon collider could resolve three such bound states. Production rates are so large that details of DM spectroscopy can be probed with larger statistics: we compute the characteristic pattern of single and multiple $γ$ lines.

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arXiv2020

Light quark Yukawas in triboson final states

Triple heavy vector boson production, $p p \to VVV$ $(V = W, Z)$, has recently been observed for the first time. We propose that precision measurements of this process provide an excellent probe of the first generation light quark Yukawa couplings. Modified quark interactions with the off-shell Higgs in this process lead to a rapid growth of the partonic cross sections with energy, which manifests in an enhanced $p_T$ distribution of the final state leptons and quarks. We quantify this effect and estimate the present and future 2$σ$ sensitivity to the up, down, and strange Yukawas. In particular, we find that HL-LHC can reach $\mathcal{O}(400)$ sensitivity to the down Yukawa relative to the Standard Model value, improving the current sensitivity in this process by a factor of $10$, and which can be further improved to $\mathcal{O}(30)$ at FCC-hh. This is competitive with and complementary to constraints from global fits and other on-shell probes of the first generation Yukawas. The triboson sensitivity at HL-LHC corresponds to probing dimension-6 SMEFT operators suppressed by an $\mathcal{O}(1)$ TeV scale, similarly to other LHC Higgs probes.

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