arXiv subjects

Silvia Pascoli

Publications and source records attributed to Silvia Pascoli.

At least 37 records · Page 2Linked to original sources

arXiv2023

A Predictive and Testable Unified Theory of Fermion Masses, Mixing and Leptogenesis

We consider a minimal non-supersymmetric $SO(10)$ Grand Unified Theory (GUT) model that can reproduce the observed fermionic masses and mixing parameters of the Standard Model. We calculate the scales of spontaneous symmetry breaking from the GUT to the Standard Model gauge group using two-loop renormalisation group equations. This procedure determines the proton decay rate and the scale of $U(1)_{B-L}$ breaking, which generates cosmic strings and the right-handed neutrino mass scales. Consequently, the regions of parameter space where thermal leptogenesis is viable are identified and correlated with the fermion masses and mixing, the neutrinoless double beta decay rate, the proton decay rate, and the gravitational wave signal resulting from the network of cosmic strings. We demonstrate that this framework, which can explain the Standard Model fermion masses and mixing and the observed baryon asymmetry, will be highly constrained by the next generation of gravitational wave detectors and neutrino oscillation experiments which will also constrain the proton lifetime.

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arXiv2022

DarkNews: a Python-based event generator for heavy neutral lepton production in neutrino-nucleus scattering

We introduce DarkNews, a lightweight Python-based Monte-Carlo generator for beyond-the-Standard-Model neutrino-nucleus scattering. The generator handles the production and decay of heavy neutral leptons via additional vector or scalar mediators, as well as through transition magnetic moments. DarkNews samples pre-computed neutrino-nucleus upscattering cross sections and heavy neutrino decay rates to produce dilepton and single-photon events in accelerator neutrino experiments. We present two case studies with differential distributions for models that can explain the MiniBooNE excess. The aim of this code is to aid the neutrino theory and experimental communities in performing searches and sensitivity studies for new particles produced in neutrino upscattering.

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arXiv2022

The Present and Future Status of Heavy Neutral Leptons

The existence of non-zero neutrino masses points to the likely existence of multiple SM neutral fermions. When such states are heavy enough that they cannot be produced in oscillations, they are referred to as Heavy Neutral Leptons (HNLs). In this white paper we discuss the present experimental status of HNLs including colliders, beta decay, accelerators, as well as astrophysical and cosmological impacts. We discuss the importance of continuing to search for HNLs, and its potential impact on our understanding on key fundamental questions, and additionally we outline the future prospects for next-generation future experiments or upcoming accelerator run scenarios.

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arXiv2022

Higher-order initial conditions with massive neutrinos

The discovery that neutrinos have mass has important consequences for cosmology. The main effect of massive neutrinos is to suppress the growth of cosmic structure on small scales. Such growth can be accurately modelled using cosmological $N$-body simulations, but doing so requires accurate initial conditions (ICs). There is a trade-off, especially with first-order ICs, between truncation errors for late starts and discreteness and relativistic errors for early starts. Errors can be minimized by starting simulations at late times using higher-order ICs. In this paper, we show that neutrino effects can be absorbed into scale-independent coefficients in higher-order Lagrangian perturbation theory (LPT). This clears the way for the use of higher-order ICs for massive neutrino simulations. We demonstrate that going to higher order substantially improves the accuracy of simulations. To match the sensitivity of surveys like DESI and Euclid, errors in the matter power spectrum should be well below 1%. However, we find that first-order Zel'dovich ICs lead to much larger errors, even when starting as early as $z=127$, exceeding 1% at $z=0$ for $k>0.5\text{ Mpc}^{-1}$ for the power spectrum and $k>0.1\text{ Mpc}^{-1}$ for the equilateral bispectrum in our simulations. Ratios of power spectra with different neutrino masses are more robust than absolute statistics, but still depend on the choice of ICs. For all statistics considered, we obtain 1% agreement between 2LPT and 3LPT at $z=0$.

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arXiv2022

Feebly-Interacting Particles:FIPs 2020 Workshop Report

With the establishment and maturation of the experimental programs searching for new physics with sizeable couplings at the LHC, there is an increasing interest in the broader particle and astrophysics community for exploring the physics of light and feebly-interacting particles as a paradigm complementary to a New Physics sector at the TeV scale and beyond. FIPs 2020 has been the first workshop fully dedicated to the physics of feebly-interacting particles and was held virtually from 31 August to 4 September 2020. The workshop has gathered together experts from collider, beam dump, fixed target experiments, as well as from astrophysics, axions/ALPs searches, current/future neutrino experiments, and dark matter direct detection communities to discuss progress in experimental searches and underlying theory models for FIPs physics, and to enhance the cross-fertilisation across different fields. FIPs 2020 has been complemented by the topical workshop "Physics Beyond Colliders meets theory", held at CERN from 7 June to 9 June 2020. This document presents the summary of the talks presented at the workshops and the outcome of the subsequent discussions held immediately after. It aims to provide a clear picture of this blooming field and proposes a few recommendations for the next round of experimental results.

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arXiv2021

Confronting SO(10) GUTs with proton decay and gravitational waves

Grand Unified Theories (GUT) predict proton decay as well as the formation of cosmic strings which can generate gravitational waves. We determine which non-supersymmetric $SO(10)$ breaking chains provide gauge unification in addition to a gravitational signal from cosmic string decay. We calculate the GUT and intermediate scales for these $SO(10)$ breaking chains by solving the renormalisation group equations at the two-loop level. This analysis predicts the GUT scale, hence the proton lifetime, in addition to the scale of cosmic string generation and thus the associated gravitational wave signal. We determine which $SO(10)$ breaking chains survive in the event of the null results of the next generation of gravitational waves and proton decay searches and determine the correlations between proton decay and gravitational waves scales if these observables are measured.

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arXiv2021

A Dark Seesaw Solution to Low Energy Anomalies: MiniBooNE, the muon $(g-2)$, and BaBar

A recent update from MiniBooNE has strengthened the observed $4.8σ$ excess of $e$-like events. Motivated by this and other notable deviations from standard model predictions, such as the muon $(g-2)$, we propose a solution to low energy anomalies through a dark neutrino sector. The model is renormalizable and can also explain light neutrino masses with an anomaly-free and dark $U(1)^\prime$ gauge symmetry broken at the GeV scale. Large kinetic mixing leads to s-channel production of heavy neutral leptons at $e^+e^-$ colliders, where we point out and explain a $\gtrsim 2σ$ excess observed in the BaBar monophoton data. Our model is also compatible with anomalous $e$-like events seen at old accelerator experiments, as well as with an excess of double vertex signatures observed at CCFR.

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arXiv2021

Discovery potential of FASER$ν$ with contained vertex and through-going events

FASER$ν$ is a newly proposed detector whose main mission is to detect the neutrino flux from the collision of the proton beams at the ATLAS Interaction Point (IP) during the run III of the LHC in 2022-2024. We show that this detector can also test certain beyond standard model scenarios, especially the ones in which the neutrino interaction with matter fields can produce new unstable particles decaying back into charged leptons. Models of this kind are motivated by the MiniBooNE anomaly. We show that, if the new physics involves multi-muon production by neutrinos scattering off matter fields, including the neutrino flux interactions in the rock before the detector in the analysis ({\it i.e.,} accounting for the through-going muon pairs) can significantly increase the effective detector mass and its sensitivity to new physics. We propose a concrete model that can give rise to such a multi-muon signal.

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arXiv2021

Gravitational wave signatures from discrete flavor symmetries

Non-Abelian discrete symmetries have been widely used to explain the patterns of lepton masses and flavor mixing. In these models, a given symmetry is assumed at a high scale and then is spontaneously broken by scalars (the flavons), which acquire vacuum expectation values. Typically, the resulting leading order predictions for the oscillation parameters require corrections in order to comply with neutrino oscillation data. We introduce such corrections through an explicit small breaking of the symmetry. This has the advantage of solving the cosmological problems of these models without resorting to inflation. The explicit breaking induces an energy difference or "bias" between different vacua and drives the evolution of the domain walls, unavoidably produced after the symmetry breaking, towards their annihilation. Importantly, the wall annihilation leads to gravitational waves which may be observed in current and/or future experiments. We show that a distinctive pattern of gravitational waves with multiple overlapped peaks is generated when walls annihilate, which is within the reach of future detectors. We also show that cosmic walls from discrete flavor symmetries can be cosmologically safe for any spontaneous breaking scale between 1 and $10^{18}$ GeV, if the bias is chosen adequately, without the need to inflate the walls away. We use as an example a particular $A_4$ model in which an explicit breaking is included in right-handed neutrino mass terms.

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arXiv2021

Gravitational waves and proton decay: complementary windows into GUTs

Proton decay is a smoking gun signature of Grand Unified Theories (GUTs). Searches by Super-Kamiokande have resulted in stringent limits on the GUT symmetry breaking scale. The large-scale multipurpose neutrino experiments DUNE, Hyper-Kamiokande and JUNO will either discover proton decay or further push the symmetry breaking scale above $10^{16}$ GeV. Another possible observational consequence of GUTs is the formation of a cosmic string network produced during the breaking of the GUT to the Standard Model gauge group. The evolution of such a string network in the expanding Universe produces a stochastic background of gravitational waves which will be tested by a number of gravitational wave detectors over a wide frequency range. We demonstrate the non-trivial complementarity between the observation of proton decay and gravitational waves produced from cosmic strings in determining $SO(10)$ GUT breaking chains. We show that such observations could exclude $SO(10)$ breaking via flipped $SU(5)\times U(1)$ or standard $SU(5)$, while breaking via a Pati-Salam intermediate symmetry, or standard $SU(5)\times U(1)$, may be favoured if a large separation of energy scales associated with proton decay and cosmic strings is indicated. We note that recent results by the NANOGrav experiment have been interpreted as evidence for cosmic strings at a scale $\sim 10^{14}$ GeV. This would strongly point towards the existence of GUTs, with $SO(10)$ being the prime candidate. We show that the combination with already available constraints from proton decay allows to identify preferred symmetry breaking routes to the Standard Model.

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arXiv2020

An optimal nonlinear method for simulating relic neutrinos

Cosmology places the strongest current limits on the sum of neutrino masses. Future observations will further improve the sensitivity and this will require accurate cosmological simulations to quantify possible systematic uncertainties and to make predictions for nonlinear scales, where much information resides. However, shot noise arising from neutrino thermal motions limits the accuracy of simulations. In this paper, we introduce a new method for simulating large-scale structure formation with neutrinos that accurately resolves the neutrinos down to small scales and significantly reduces the shot noise. The method works by tracking perturbations to the neutrino phase-space distribution with particles and reduces shot noise in the power spectrum by a factor of $\mathcal{O}\left(10^2\right)$ at $z=0$ for minimal neutrino masses and significantly more at higher redshifts, without neglecting the back-reaction caused by neutrino clustering. We prove that the method is part of a family of optimal methods that minimize shot noise subject to a maximum deviation from the nonlinear solution. Compared to other methods we find permille level agreement in the matter power spectrum and percent level agreement in the large-scale neutrino bias, but large differences in the neutrino component on small scales. A basic version of the method can easily be implemented in existing N-body codes and makes it possible to run neutrino simulations with significantly reduced particle load. Further gains are possible by constructing background models based on perturbation theory. A major advantage of this technique is that it works well for all masses, enabling a consistent exploration of the full neutrino parameter space.

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arXiv2020

Unravelling the richness of dark sector by FASER$ν$

FASER$ν$ is a newly proposed experiment which will take data in run III of the LHC during 2021-2023. It will be located in front of the FASER detector, 480~m away from the ATLAS interaction point in the forward direction. Its main goal is to detect neutrinos of all flavors produced at the interaction point with superb precision in reconstructing charged tracks. This capability makes FASER$ν$ an ideal setup for uncovering the pattern and properties of a light dark sector. We demonstrate this capability for a well-motivated class of models with a dark matter candidate of mass around a few GeV. Dark matter annihilates to a pair of intermediate neutral particles that subsequently decay into the standard model charged fermions. We show how FASER$ν$ can shed light on the structure of the dark sector by unravelling the decay chain within such models.

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arXiv2020

Dark Neutrinos and a Three Portal Connection to the Standard Model

We introduce a dark neutrino sector which respects a hidden $U(1)^\prime$ gauge symmetry, subsequently broken by the vacuum expectation value of a dark scalar. The model is a self-consistent realisation of an extended hidden sector that communicates with the SM only via the three renormalizable portals, namely neutrino, vector and scalar mixing. The interplay between portal couplings leads to several novel signatures in heavy neutrino, dark photon, and dark scalar searches, typically characterised by multi-leptons plus missing energy and displaced vertices. A striking signature arises in kaon factories such as NA62, where $K^+\to\ell_α^+ν\ell_β^+\ell_β^-$ decays could reveal a heavy neutrino and a light dark photon resonance above backgrounds. Given the open parameter space, we also comment on recent ideas to explain outstanding experimental anomalies, and how they would fit in our proposed model. A minimal extension of the model, possibly motivated by anomaly cancellation, can accommodate a dark matter candidate strongly connected to the neutrino sector.

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arXiv2020

Constraining structure formation using EDGES

The experiment to detect the global epoch of reionization signature (EDGES) collaboration reported the detection of a line at 78MHz in the sky-averaged spectrum due to neutral hydrogen (HI) 21-cm hyperfine absorption of cosmic microwave background photons at $z\sim 17$. This requires that the spin temperature of HI be coupled to the kinetic temperature of the gas at this $z$ through the scattering of Ly-$α$ photons emitted by massive stars. To explain the experimental result, star formation needs to be sufficiently efficient at $z\sim 17$ and this can be used to constrain models in which small-scale structure formation is suppressed (DMF models), either due to dark matter free-streaming or non-standard inflationary dynamics. We combine simulations of structure formation with a simple recipe for star formation to investigate whether these models emit enough Ly-$α$ photons to reproduce the experimental signal for reasonable values of the star formation efficiency, $f_\star$. We find that a thermal warm dark matter (WDM) model with mass $m_\mathrm{WDM}\sim 4.3\,\mathrm{keV}$ is consistent with the timing of the signal for $f_\star\lesssim 2\%$. The exponential growth of structure around $z\sim 17$ in such a model naturally generates a sharp onset of the absorption. A warmer model with $m_\mathrm{WDM}\sim 3\,\mathrm{keV}$ requires a higher star formation efficiency, $f_\star\sim 6\%$, which is a factor of few above predictions of current star formation models and observations of satellites in the Milky Way. However, uncertainties in the process of star formation at these $z$ do not allow to derive strong constrains on such models using 21-cm absorption line. The onset of the 21-cm absorption is generally faster in DMF compared to cold dark matter (CDM), unless some process significantly suppresses star formation in halos with masses below $\sim 10^8\,h^{-1}\,\mathrm{M}_\odot$.

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arXiv2020

Heavy Neutral Leptons from low-scale seesaws at the DUNE Near Detector

Heavy nearly-sterile neutrinos are a common ingredient in extensions of the Standard Model which aim to explain neutrino masses, like for instance in Type I seesaw models, or one of its variants. If the scale of the new Heavy Neutral Leptons (HNLs) is sufficiently low, observable signatures can arise in a range of current and upcoming experiments, from the LHC to neutrino experiments. In this article, we discuss the phenomenology of sterile neutrinos in the MeV to GeV mass range, focusing on their decays. We embed our discussion in a realistic mass model and consider the resulting implications. We focus in particular on the impact on the signal of the strong polarisation effects in the beam for Majorana and (pseudo-)Dirac states, providing formulae to incorporate these in both production and decay. We study how the Near Detector of the upcoming Deep Underground Neutrino Experiment (DUNE) can constrain HNL states by searching for their decay products inside the detector. We conduct a Monte Carlo background analysis for the most promising signatures, incorporating the detector's particle identification capabilities, and estimate the experimental sensitivity of DUNE to these particles. We also present an estimate of the $ν_τ$-derived HNL flux at DUNE, currently missing in the literature, which allows us to discuss searches for HNLs at higher masses.

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arXiv2019

Heavy Neutrinos with Dynamic Jet Vetoes: Multilepton Searches at $\sqrt{s} = 14,~27,$ and $100$ TeV

Heavy neutrinos $(N)$ remain one of most promising explanations for the origin of neutrinos' tiny masses and large mixing angles. In light of broad advances in understanding and modeling of hadron collisions at large momentum transfer, we revisit the long-standard search strategy for heavy $N$ decaying to multiple charged leptons $(\ell)$, $pp \to N\ell X \to 3\ell νX$. For electroweak and TeV-scale $N$, we propose a qualitatively new collider analysis premised on a dynamic jet veto and discriminating, on an event-by-event basis, according to the relative amount of hadronic and leptonic activity. We report that the sensitivity to $N$ at the Large Hadron Collider (LHC) can be improved by roughly an order of magnitude over the collider's lifetime. At $\sqrt{s}=14$ TeV with $\mathcal{L}=3~{\rm ab}^{-1}$, active-sterile mixing as small as $\vert V_{\ell N}\vert^2 = 10^{-2} ~(10^{-3})~[5\times10^{-4}]$ can be probed at $95\%$ CL for Dirac neutrinos masses $m_N \lesssim 1200~(300)~[200]$ GeV, well beyond present constraints for such heavy states. The improvement holds also for Majorana $N$, and is largely independent of whether charged lepton flavor is conserved or violated. The analysis, built almost entirely from inclusive, transverse observables, is designed to be robust across increasing collider energies, and hence serves as a basis for searches at future colliders: With $\mathcal{L}=15~{\rm ab}^{-1}$ at $\sqrt{s}=27$ TeV, one can probe mixing below $\vert V_{\ell N}\vert^2 = 10^{-2} ~(10^{-3})~[2\times10^{-4}]$ for $m_N \lesssim 3500~(700)~[200]$ GeV. At a hypothetical 100 TeV $pp$ collider with $\mathcal{L}=30~{\rm ab}^{-1}$, one can probe below $\vert V_{\ell N}\vert^2 = 9\times10^{-5}~(10^{-3})~[10^{-2}]$ for $m_N \lesssim 200$ GeV ($4$ TeV) [$15$ TeV]. We anticipate these results can be further improved with detector-specific tuning and application of machines learning techniques.

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arXiv2019

Neutrino Masses from a Dark Neutrino Sector below the Electroweak Scale

We consider a minimal extension of the Standard Model which advocates a dark neutrino sector charged under a hidden $U(1)^\prime$. We show that neutrino masses can arise radiatively in this model. The observed values are compatible with a light dark sector below the electroweak scale and would imply new heavy fermions which may be testable in the next generation of beam dump searches at DUNE, NA62 and SHIP.

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arXiv2019

Leptogenesis via Varying Weinberg Operator: a Semi-Classical Approach

In this Letter, we introduce leptogenesis via a varying Weinberg operator from a semi-classical perspective. This mechanism is motivated by the breaking of an underlying symmetry which triggers a phase transition that causes the coupling of the Weinberg operator to become dynamical. Consequently, a lepton anti-lepton asymmetry arises from the interference of the Weinberg operator at two different spacetime points. Using this semi-classical approach, we treat the Higgs as a background field and show a reflection asymmetry between the leptons and anti-leptons is generated in the vicinity of the bubble wall. We solve the equations of motion of the lepton and anti-lepton quasiparticles to obtain the final lepton asymmetry.

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