arXiv subjects

Laura Covi

Publications and source records attributed to Laura Covi.

At least 19 recordsLinked to original sources

arXiv2025

Multicomponent Dark Matter with Collider Implications

The present work aims to study an extension of the Standard Model (SM) that addresses the prominent SM shortcomings, i.e., can explain the neutrino mass, the dark matter (DM) content, and the matter-antimatter asymmetry of the Universe. The model introduces the possibility of a multicomponent DM scenario leading to distinctive signals at colliders. The SM is extended by a ``dark" $SU(2)_{D}$ gauge symmetry and new fermions and scalar doublets, charged only under $SU(2)_{D}$, that provide candidates for a multicomponent DM. Previously, we have considered in this model the asymmetric DM scenario, while in the present work, we explore the symmetric DM case. We focus on the parameter region where the dark fermion DM annihilates dominantly into the additional dark gauge bosons and the ``inert" doublet DM annihilates to SM states via the SM Higgs resonance. This particular choice of doublet mass ensures that the heavier BSM Higgs always has one decay mode open to DM leading to the possibility of detecting such particles at the LHC, in the missing energy plus dijet ($\cancel{E}_{T}+2j$) final states. We also discuss the prospects for detecting DM through direct and indirect detection experiments and via Long-Lived-Particle searches. Finally, we show that, as typical of other WIMP models, for low DM mass, signals can be expected in future collider experiments, but for the higher mass range above $500$ GeV we have to rely solely on direct detection experiments. Both types of experiments will be essential to fully cover the allowed parameter space.

hep-ph↗

arXiv2022

Axion and FIMP Dark Matter in a $U(1)$ extension of the Standard Model

In the Standard Model a Dark Matter candidate is missing, but it is relatively simple to enlarge the model including one or more suitable particles. We consider in this paper one such extension, inspired by simplicity and by the goal to solve more than just the Dark Matter issue. Indeed we consider a local $U(1) $ extension of the SM providing an axion particle to solve the strong CP problem and including RH neutrinos with appropriate mass terms. One of the latter is decoupled from the SM leptons and can constitute stable sterile neutrino DM. In this setting, the PQ symmetry arises only as an accidental symmetry but its breaking by higher order operators is sufficiently suppressed to avoid introducing a large $ θ$ contribution. The axion decay constant and the RH neutrino masses are related to the same v.e.v.s and the PQ scale and both DM densities are determined by the parameters of the axion and scalar sector. The model predicts in general a mixed Dark Matter scenario with both axion and sterile neutrino DM and is characterised by a reduced density and observational signals from each single component.

hep-ph↗

arXiv2021

Direct Detection of Dark Matter -- APPEC Committee Report

This Report provides an extensive review of the experimental programme of direct detection searches of particle dark matter. It focuses mostly on European efforts, both current and planned, but does it within a broader context of a worldwide activity in the field. It aims at identifying the virtues, opportunities and challenges associated with the different experimental approaches and search techniques. It presents scientific and technological synergies, both existing and emerging, with some other areas of particle physics, notably collider and neutrino programmes, and beyond. It addresses the issue of infrastructure in light of the growing needs and challenges of the different experimental searches. Finally, the Report makes a number of recommendations from the perspective of a long-term future of the field. They are introduced, along with some justification, in the opening Overview and Recommendations section and are next summarised at the end of the Report. Overall, we recommend that the direct search for dark matter particle interactions with a detector target should be given top priority in astroparticle physics, and in all particle physics, and beyond, as a positive measurement will provide the most unambiguous confirmation of the particle nature of dark matter in the Universe.

hep-ex↗

arXiv2020

Models of decaying FIMP Dark Matter: potential links with the Neutrino Sector

The absolute stability of a dark matter (DM) particle is not a binding requirement. Here we suggest a few scenarios where the DM particle is liable to decay via extremely feeble interactions. This can happen via inexplicably small Yukawa couplings in the simplest conjectures. After setting down such a model, we go beyond it, thus treading onto scenarios where the spontaneous breakdown of some gauged $U(1)$ symmetry may lead to intermediate scales, and suitably suppressed effective operators which allow the DM particle to decay slowly. The constraints from particle physics as well as cosmology are taken into account in each case. The last and more involved scenario, studied in detail, suggest a link between the model parameters that govern neutrino physics on one side, and the dynamics of a quasi-stable DM particle on the other.

hep-ph↗

arXiv2019

Common origin of baryon asymmetry, dark matter and neutrino mass

In this work, we explain three beyond standard model (BSM) phenomena, namely neutrino masses, the baryon asymmetry of the Universe and Dark Matter, within a single model and in each explanation the right handed (RH) neutrinos play the prime role. Indeed by just introducing two RH neutrinos we can generate the neutrino masses by the Type-I seesaw mechanism. The baryon asymmetry of the Universe can arise from thermal leptogenesis from the decay of lightest RH neutrino before the decoupling of the electroweak sphaleron transitions, which redistribute the $ B-L $ number into a baryon number. At the same time, the decay of the RH neutrino can produce the Dark Matter (DM) as an asymmetric Dark Matter component. The source of CP violation in the two sectors is exactly the same, related to the complex couplings of the neutrinos. By determining the comoving number density for different values of the CP violation in the DM sector, we obtain a particular value of the DM mass after satisfying the relic density bound. We also give prediction for the DM direct detection (DD) in the near future by different ongoing DD experiments.

hep-ph↗

arXiv2018

Dark Matter Sommerfeld-enhanced annihilation and Bound-state decay at finite temperature

Traditional computations of the dark matter (DM) relic abundance, for models where attractive self-interactions are mediated by light force-carriers and bound states exist, rely on the solution of a coupled system of classical on-shell Boltzmann equations. This idealized description misses important thermal effects caused by the tight coupling among force-carriers and other charged relativistic species. We develop for the first time a comprehensive ab-initio derivation for the description of DM long-range interactions in the presence of a hot and dense plasma background directly from non-equilibrium quantum field theory. Most importantly, the scattering and bound states get strongly mixed in the thermal plasma environment, representing a characteristic difference from a pure vacuum theory computation. The main result of this work is a novel differential equation for the DM number density, written down in a form which is manifestly independent under the choice of what one would interpret as a bound or a scattering state at finite temperature. The collision term, unifying the description of annihilation and bound state decay, turns out to have in general a non-quadratic dependence on the DM number density. This generalizes the form of the conventional Lee-Weinberg equation which is typically adopted to describe the freeze-out process. We prove that our general number density equation is consistent with previous literature results under certain limits.

hep-ph↗

arXiv2018

Time-dependent Features in the Primordial Spectrum

In a Quantum Field Theory with a time-dependent background, time-translational symmetry is broken. We therefore expect time-dependent loop corrections to cosmological observables after renormalization for an interacting field, with the consequent physical implications. In this paper we compute and discuss such radiative corrections to the primordial spectrum within simple models, both for massless and massive virtual fields, and we disentangle the time-dependence caused by the background and by the initial state after renormalization. For the investigated models the departure from near-scale-invariance is very small and there is full compatibility with the current Planck data constraints. Future CMB measurements may improve the current constraints on feature-full primordial spectra and possibly observe these effects in the most optimistic scenario of hybrid inflation, revealing the interacting nature of the inflaton field.

gr-qc↗

arXiv2018

Explaining the 3.5 keV X-ray Line in a ${L_μ-L_τ}$ Extension of the Inert Doublet Model

We explain the existence of neutrino masses and their flavor structure, dark matter relic abundance and the observed 3.5 keV X-ray line within the framework of a gauged $U(1)_{L_μ - L_τ}$ extension of the "scotogenic" model. In the $U(1)_{L_μ - L_τ}$ symmetric limit, two of the the RH neutrinos are degenerate in mass, while the third is heavier. The $U(1)_{L_μ - L_τ}$ symmetry is broken spontaneously. Firstly, this breaks the $μ-τ$ symmetry in the light neutrino sector. Secondly, this results in mild splitting of the two degenerate RH neutrinos, with their mass difference given in terms of the $U(1)_{L_μ - L_τ}$ breaking parameter. Finally, we get a massive $Z_{μτ}$ gauge boson. Due to the added $Z_2$ symmetry under which the RH neutrinos and the inert doublet are odd, the canonical Type-I seesaw is forbidden and the tiny neutrino masses are generated radiatively at one loop. The same $Z_2$ symmetry also ensures that the lightest RH neutrino is stable and the other two can only decay into the lightest one. This makes the two nearly-degenerate lighter neutrinos a two-component dark matter, which in our model are produced by the freeze-in mechanism via the decay of the $Z_{μτ}$ gauge boson in the early universe. We show that the next-to-lightest RH neutrino has a very long lifetime and decays into the lightest one at the present epoch explaining the observed 3.5 keV line.

hep-ph↗

arXiv2016

Matter Power Spectrum in Hidden Neutrino Interacting Dark Matter Models: A Closer Look at the Collision Term

Dark Matter (DM) models providing possible alternative solutions to the small- scale crisis of standard cosmology are nowadays of growing interest. We consider DM interacting with light hidden fermions via well motivated fundamental operators showing the resultant matter power spectrum is suppressed on subgalactic scales within a plausible parameter region. Our basic description of the evolution of cosmological perturbations relies on a fully consistent first principles derivation of a perturbed Fokker-Planck type equation, generalizing existing literature. The cosmological perturbation of the Fokker-Planck equation is presented for the first time in two different gauges, where the results transform into each other according to the rules of gauge transformation. Furthermore, our focus lies on a derivation of a broadly applicable and easily computable collision term showing important phenomenological differences to other existing approximations. As one of the main results and concerning the small-scale crisis, we show the equal importance of vector and scalar boson mediated interactions between DM and light fermions.

hep-ph↗

arXiv2016

A possible relation between leptogenesis and PMNS phases

We propose a new scenario for baryogenesis through leptogenesis, where the CP phase relevant for leptogenesis is connected directly to the PMNS phase(s) in the light neutrino mixing matrix. The scenario is realized in case only one CP phase appears in the full theory, originating from the complex vacuum expextation value of a standard model singlet field. In order to realize this scheme, the electroweak symmetry is required to be broken during the leptogenesis era and a new loop diagram with an intermediate $W$ boson exchange including the low energy neutrino mixing matrix should play the dominant contribution to the CP violation for leptogenesis. In this letter, we discuss the new basic mechanism, which we call type-II leptogenesis, and give an estimate for maximally reachable baryon asymmetry depending on the PMNS phases.

hep-ph↗

arXiv2015

LHC constraints on Gravitino Dark Matter

Gravitino Dark Matter represents a compelling scenario in Supersymmetry, which brings together a variety of data from cosmology and collider physics. We discuss the constraints obtained from the LHC on supersymmetric models with gravitino dark matter and neutralino NLSP, which is the case most difficult to disentangle at colliders from a neutralino LSP forming DM. The phenomenological SUSY model with 19+1 free parameters is adopted. Results are obtained from broad scans of the phase space of these uncorrelated parameters. The relation between gravitino mass, gluino mass and reheating temperature as well as the derived constraints on these parameters are discussed in detail. This relation offers a unique opportunity to place stringent bounds on the cosmological model, within the gravitino dark matter scenario, from the results of the LHC searches in Run-2 and the planned High-Luminosity upgrade.

hep-ph↗

arXiv2015

Gravitino Dark Matter and low-scale Baryogenesis

A very simple way to obtain comparable baryon and DM densities in the early Universe is through their contemporary production from the out-of-equilibrium decay of a mother particle, if both populations are suppressed by comparably small numbers, i.e. the CP violation in the decay and the branching fraction respectively. We present a detailed study of this kind of scenario in the context of a R-parity violating realization of the MSSM in which the baryon asymmetry and the gravitino Dark Matter are produced by the decay of a Bino. The implementation of this simple picture in a realistic particle framework results, however, quite involving, due to the non trivial determination of the abundance of the decaying Bino, as well as due to the impact of wash-out processes and of additional sources both for the baryon asymmetry and the DM relic density. In order to achieve a quantitative determination of the baryon and Dark Matter abundances, we have implemented and solved a system of coupled Boltzmann equations for the particle species involved in their generation, including all the relevant processes. In the most simple, but still general, limit, in which the processes determining the abundance and the decay rate of the Bino are mediated by degenerate right-handed squarks, the correct values of the DM and baryon relic densities are achieved for a Bino mass between 50 and 100 TeV, Gluino NLSP mass in the range 15-60 TeV and a gravitino mass between 100 GeV and few TeV. These high masses are unfortunately beyond the kinematical reach of LHC. On the contrary, an antiproton signal from the decays of the gravitino LSP might be within the sensitivity of AMS-02 and gamma-ray telescopes.

hep-ph↗

arXiv2015

Model independent limits on an ultra-light gravitino from Supernovae

We revisit astrophysical constraints on models with an ultra-light gravitino, in particular extending the analysis to more general models and the case of R-parity breaking. These constraints allow to restrict the value of the gravitino mass depending on the masses of scalar moduli. We perform both a frequentist and bayesian analysis and we find comparable results, even if the bayesian analysis is in part affected by volume effects. Only a small window of gravitino masses can be excluded by the SN constraints in a model-independent way, while limits obtained with a definite assumption on the scalar masses result more stringent.

hep-ph↗

arXiv2014

3.55 keV line in Minimal Decaying Dark Matter scenarios

We investigate the possibility of reproducing the recently reported $3.55\,\mbox{keV}$ line in some simple decaying dark matter scenarios. In all cases a keV scale decaying DM is coupled with a scalar field charged under SM gauge interactions and thus capable of pair production at the LHC. We will investigate how the demand of a DM lifetime compatible with the observed signal, combined with the requirement of the correct DM relic density through the freeze-in mechanism, impacts the prospects of observation at the LHC of the decays of the scalar field.

hep-ph↗

arXiv2014

LHC prospects for minimal decaying Dark Matter

We study the possible signals at LHC of the minimal models of decaying dark matter. Those models are characterized by the fact that DM interacts with SM particles through renormalizable coupling with an additional heavier charged state. Such interaction allows to produce a substantial abundance of DM in the early Universe via the decay of the charged heavy state, either in- or out-of-equilibrium. Moreover additional couplings of the charged particle open up decay channels for the DM, which can nevertheless be sufficiently long-lived to be a good DM candidate and within reach of future Indirect Detection observations. We compare the cosmologically favored parameter regions to the LHC discovery reach and discuss the possibility of simultaneous detection of DM decay in Indirect Detection.

hep-ph↗

arXiv2013

Out-of-equilibrium Baryogenesis and SuperWIMP Dark Matter

The experimental fact that the energy density in Dark Matter and in Baryons is of the same order is one of the most puzzling in cosmology. In this letter we suggest a new mechanism able to explain this coincidence in the context of out-of-equilibrium baryogenesis with DM production "à la" SuperWIMP starting from the same initial particle. We then discuss two simple implementations of this scenario within supersymmetric models with gravitino DM.

hep-ph↗

arXiv2013

SUSY dark matter(s)

We review here the status of different dark matter candidates in the context of supersymmetric models, in particular the neutralino as a realization of the WIMP-mechanism and the gravitino. We give a summary of the recent bounds in direct and indirect detection and also of the LHC searches relevant for the dark matter question. We discuss also the implications of the Higgs discovery for the supersymmetric dark matter models and give the prospects for the future years.

hep-ph↗

arXiv2013

Minimal Decaying Dark Matter and the LHC

We consider a minimal Dark Matter model with just two additional states, a Dark Matter Majorana fermion and a colored or electroweakly charged scalar, without introducing any symmetry to stabilize the DM state. We identify the parameter region where an indirect DM signal would be within the reach of future observations and the DM relic density generated fits the observations. We find in this way two possible regions in the parameter space, corresponding to a FIMP/SuperWIMP or a WIMP DM. We point out the different collider signals of this scenario and how it will be possible to measure the different couplings in case of a combined detection.

hep-ph↗