My current research activity
focuses on the intersections among
In this broad context, my main current interests lie in exploring the phenomenology of Dark Matter models, particularly via indirect detection methods (gamma rays, charged cosmic rays, neutrinos) and with a special attention to our recent proposal of Minimal Dark Matter, and in Neutrino Cosmology.
In the following, I briefly review my past research activity in some detail, and give an outlook of the open projects and directions. [The numbering follows that of my CV on the homepage.]
I began my work in Particle Theory Beyond the SM by computing some relevant observables in the framework of the theory proposed by R. Barbieri, L. Hall and Y. Nomura (Phys.Rev.D63:105007,2001), an extension of the Standard Model to five dimensions endowed with a supersymmetric structure. The theory is successful in providing a description of the mechanism of electroweak symmetry breaking thanks to the extra dimension, while ensuring calculability for several quantities, a property that is not so common among extra dimensional models.
In collaboration with G. Cacciapaglia and G. Cristadoro, in  I found that the production rate of the Higgs boson via gluon fusion (which is the main channel at a hadron collider) is significantly suppressed, due to cancellations among the additional (Kaluza-Klein) states of the theory.
In  we showed that the theory is compatible with the precision measurements of muon anomalous magnetic moment, by explicitly computing all the relevant additional contributions to such a quantity and finding them small.
In , in collaboration with A. Romanino, Y. Lin and G. Cacciapaglia, I shifted to a more general class of models, characterized by large flat extra dimension accessible to a sterile neutrino. We analysed the effects in the context of supernova physics, where resonant oscillations between the Standard Model electron neutrino and the additional sterile states provide an unconventional escape channel. We showed (via numerical and analytical work) how previous bounds can be largely overcome, thanks to a feedback mechanism that self-limits the energy loss, and we discussed positive effects towards supernova explosion.
In  we completed the previous analysis including the effects of muon and tau neutrinos escape, showing how a feedback prevents an unacceptable energy loss also in this case. For all the different scenarios, we discussed the signatures in the neutrino signal on Earth.
In  we performed a thorough analysis of oscillation signals generated by one extra sterile neutrino, extending previous analyses done in simple limiting cases and including the effects of established oscillations among active neutrinos. Many New Physics candidates act effectively as sterile neutrinos, so that we include them all. We consider as probes the solar, atmospheric, reactor and beam neutrinos, Big-Bang Nucleosynthesis (He4, D), the Cosmic Microwave Background, Large Scale Structure, supernovae and neutrinos from other astrophysical sources. We found no evidence for a sterile neutrino in present data, we identified the still allowed regions, and studied which future experiments can best probe them: sub-MeV solar experiments, more precise studies of CMB or BBN, future supernova explosions... I particularly was involved in the SN and cosmological studies.
In , we addressed the implications on solar neutrino oscillations of the recent proposal that the mass of the neutrinos and the field responsible for dark energy may be connected, leading to the effect of mass varying neutrinos depending on environment. We stressed the model independent consequences, finding in particular that a connection between the effective Delta m^2 in the Sun and the absolute neutrino mass scale is established in these scenarios. This leads to the possibility of explicitly testing the model and to other interesting consequences both for the neutrinos and for the mechanism of dark energy.
In  we presented results on neutrino fluxes from the annihilation of Dark Matter particles accumulated in the center of the Earth and the Sun. They will be hopefully detected in the Neutrino Telescopes (Antares, IceCube, a large Cerenkov detector...). The neutrino fluxes carry precious information on the main properties of DM (its abundance, its mass and its annihilation branching ratios), opening unique windows on its nature and on the theory that encompasses it.
We computed precisely the expected neutrino yield and, especially, the neutrino spectra, which are more free from astrophysical uncertainties. We develop the appropriate formalism to follow the neutrino production, the evolution of the fluxes in the matter of the Earth and the Sun (determined by flavor oscillations, absorptions/scatterings and tau regeneration) and in the vacuum and finally the detection signatures.
In  we explored a new approach to the Dark Matter problem: while Beyond-the-SM theories often provide DM candidates with an obscure phenomenology and an ad-hoc method for stabilization (such as R-parity in SuSy), we looked for a viable candidate just adding to the SM a multiplet in some representation of SU_L(2) x U_Y(1).
We find that a quintuplet with zero hypercharge provides a new minimal candidate for Dark Matter that is fully successful: weakly interacting, electrically neutral and (most importantly) automatically stable on cosmological time scales. We computed its distinctive phenomenology at colliders (the LHC) and in experiments of direct and indirect DM detection, finding that the particle can be detected in the next generation of experiments.
In  we investigate the cosmology of ordinary neutrinos and of possible extra light particles. We make use of the most recent data from Cosmic Microwave Background, Supernovae type Ia, Large Scale Structure, Lyman-alpha forest, Baryon Acoustic Oscillation peaks etc. We obtain stringent constraints on the neutrino mass, the effective neutrino density and the properties of proposed new interacting light particles that diminish the neutrino free-streaming. It should be noted that we performed all the analysis making use of numerical codes and tools written and developed by ourselves instead of the commonly used CMBfast-derived tools.
With  we investigated how unconventional cosmologies can relax the stringent bounds on sterile neutrinos. We open the way to a possible primordial leptonic asymmetry, that has the effect of suppressing the production of sterile neutrinos in the Early Universe, therefore modifying the constraints from BBN and from LSS. We identify the portions of the parameter space that can be reopened by introducing a given asymmetry. In the case of the LSND sterile neutrino, we find that a primordial asymmetry of the order of 10e-4 is needed in order to lift the conflicts with cosmology.
With  we revisited the computation of the cosmological relic abundance in the Minimal Dark Matter proposal introduced in , including non-perturbative 'Sommerfeld' corrections. These were found to have a very relevant effect in enhancing the DM annihilations. We also study the peculiar behavior of the DM particles while crossing the Earth at Ultra High Energies, in order to assess the possible detectability in future cosmic ray and neutrino telescopes (e.g. Icecube, Auger, Antares).
In  we precisely calculated the indirect detection signatures of the Minimal Dark Matter model of . We computed the fluxes of positrons, antiprotons and gamma rays from the annihilations of DM particles in the galactic halo and their propagation in the galaxy (designing our own computational tools). We found distinctive and univocal predictions (the model has no free parameters). The enhancement in the annihilation cross section discussed in  put the foreseen fluxes within the reach of those that were upcoming experiments, PAMELA in particular.
When the PAMELA satellite announced preliminary data on the positron flux, showing confirmation for an excess over the expected background that previous experiments had already exposed, we compared in [0808.3867] the fluxes from Minimal DM annihilations predicted in  with such data. We found a remarkably good agreement, and we were able to determine the set of astrophysical parameters that gives the best fit. Later, we summarized in  the status of the model.
In a subsequent paper , we performed a model independent analysis of the PAMELA preliminary data on positrons and anti-protons, together with less known but relevant data from cosmic ray balloon experiments (ATIC and PPB-BETS). We looked for which DM models can explain the signals that appear in the data while remaining compatible with the searches in all other channels. We find that the PAMELA results alone, if due to DM annihilations, individuate a quite unusual DM particle: either very heavy (above 10 TeV) or lighter but annihilating mainly into leptonic channels such as DM DM --> e+ e-. Adding the balloon datasets, only the second possibility is favored.
In  we pursued the model independent analysis of multi-messenger indirect signatures extending to gamma rays and synchrotron radiation from the galactic center and dwarf satellite galaxies. We found that these observations impose stringent constraints: a tension is present with the explanation of the PAMELA and ATIC data in terms of DM annihilations, unless the DM halo profile is significantly more smooth than expected from numerical simulations.
In  we considered another interesting possible signal of DM indirect detection: fluxes of anti-deuterium synthetized in galactic annihilations. We focussed on the `very heavy Dark Matter' scenarios individuated by the recent data, and we found promising perspectives especially for primary annihilation channels into quarks.
Another relevant test of the Dark Matter invoked to explain the positron excess in PAMELA is the flux of gamma rays produced by inverse Compton scattering of such energetic positrons on low energy ambient photons in the galactic halo. This signal has the advantage of being less sensitive to astrophysical details than the gamma rays from the Galactic Center discussed above. We computed this flux for several cases and for a range of DM models in , finding again stringent constraints.
In  we looked once again at the implications of Dark Matter annihilations, this time on the cosmological evolution of the universe. Indeed, the annihilations of Dark Matter during the epoch of galaxy formation inject charged particles and energy, producing reionization and heating of the primordial gas. Comparing with the observed optical depth (from CMB) and the measured temperature of the intergalactic gas we found relevant constraints on DM properties, in the particular for the PAMELA-motivated models. We also found general constraints for more `ordinary' Dark Matter.
When the Fermi satellite started releasing data on diffuse gamma ray, we updated the constraints formerly obtained in  to take the new measurements into account (), at the same time improving and enlarging the analysis (e.g. to the case of decaying DM).
In  we took a close look at a class of DM models often advertised as able to explain the cosmic ray excesses, namely models with multistate DM coupled to light hidden sector bosons. With a detailed calculation, we showed instead that such models suffer from several tensions with the gamma ray constraints and in reproducing the cosmological abundance of DM. They are therefore only very marginally viable.
Ref.  represents the coronation of a long effort directed to produce `ingredients' and `recipes' for DM indirect detection, using state of the art calculations and in a consistent framework and provide this to the community for easy use. We computed the energy spectra of e+-, anti-p, anti-d, gamma rays, nu and anti-nu e, mu and tau from DM annihilations or decay in the Galaxy; the propagation functions for charged particles in the halo; the energy spectra of charged particles at the location of the Earth; the gamma ray fluxes from Inverse Compton scattering in the galactic halo and finally the spectra of extragalactic gamma rays. The concrete goal is that now the community has at disposal a suite of results which can make it easier to assess which DM models can explain possible signals that might appear in the data (e.g. the current PAMELA excess in positron fluxes, or future possible results from the Fermi telescope in gamma rays...) while remaining compatible with the searches in other channels, or producing predictions for other channels, in a full multi-messenger approach.
In Ref.  and  I participated in two analyses of the impact of ElectroWeak corrections (i.e. the emission of weak bosons) on the annihilation of DM particles. The process is important, because it can lift the suppression which naturally depresses the cross section of Majorana DM particles annihilating into light fermions (known as helicity suppression), leading to very different resulting spectra.
In Ref.  I worked on the phenomenology of the so-called Asymmetric Dark Matter (aDM) scenario, which assumes the existence of a primordial asymmetry in the dark sector. We studied in particular the effect of oscillations between dark matter and its antiparticle on the re-equilibration of the initial asymmetry before freeze-out, which enable efficient annihilations to recouple. We calculated the evolution of the DM relic abundance and showed how oscillations re-open the parameter space of aDM models, in particular in the direction of allowing large (WIMP-scale) DM masses.
In Ref.  we derived new bounds on decaying Dark Matter from the gamma ray measurements of (i) the isotropic residual (extragalactic) background by the FERMI satellite and (ii) the Fornax galaxy cluster by the HESS telescope.
We found that those from (i) are among the most stringent constraints currently available, for a large range of DM masses and a variety of decay modes, excluding half-lives up to ~10^26 to few 10^27 seconds. In particular, they rule out the interpretation in terms of decaying DM of the e+- spectral features in PAMELA, FERMI and HESS, unless very conservative choices are adopted.
In Ref.  and  we derive constraints on the DM annihilation cross section from the antiproton measurements performed by PAMELA, finding that thay can be competitive with the gamma ray ones. In  we then apply them to specific SuSy models in order to constrain also an `internal' property of the models: the mass splitting between charginos and neutralinos. In  we assess also the sensitivity of the upcoming AMS experiment and its ability to reconstruct DM properties from a possible signal.
With  I moved to the phenomenology of DM direct detection: we discuss a powerful framework (based on non-relativistic operators) and provide a self-contained set of numerical tools to derive the bounds from some current experiments on virtually any arbitrary model of Dark Matter.
In Ref. , we address the often-neglected role of bremsstrahlung processes on the interstellar gas in computing indirect signatures of Dark Matter (DM) annihilation in the Galaxy, particularly for light DM candidates in the phenomenologically interesting O(10) GeV mass range. We find that the effects of bremsstrahlung are important, or even dominant, in determining the gamma-ray spectrum from DM.
Ref.  represents a significant upgrade of Ref. , in particular with the improved knowledge provided by Ref. : we computed the spectra of neutrinos from DM annihilations in the Sun including electroweak correction, state of the art energy losses in solar matter (computed with GEANT) and secondary neutrinos.
In Ref.  we explored the possible yield of anti-Helium nuclei from galactic DM annihilations (in analogy with the work in Ref.  on antideuterium: while in principle the prospect are good since the astrophysical background is very suppressed at the interesting energies, we find that only for very optimistic configurations it might be possible to achieve detection with current generation detectors.
Since 2010 another anomaly has attracted a lot of attention in the community: a GeV diffuse excess from the Galactic Center region, which can be explained in terms of DM with a mass of tens of GeV annihilating in quarks or leptons with thermal annihilation cross section. In Ref.  we investigated the antiproton constraints on such an explanation. We treated with particular care the uncertainties related to the propagation of antiprotons in the Galaxy and in the proximity of the solar system (the so-called solar modulation effects). We found that, while the DM interpretation is ruled out for stringent assumptions, in the full case (thin propagation halos and/or a conservative treatment of solar modulation) no firm conclusion can be reached.
Ref.  approached a rather different topic: we considered an electroweak triplet as an extension of the Standard Model (a good DM candidate, if the B-L symmetry is enforced) and perform an analysis of the reach for such a particle at the high-luminosity LHC and at a futuristic 100 TeV pp collider. We do so for the monojet, monophoton, vector boson fusion and disappearing tracks channels. For the large mass region, high energy and high luminosity conditions will be necessary.
In ref.  and ref.  I went back again to antiprotons. First we revisited the computation of the astrophysical background and of the DM antiproton fluxes fully including some effects which are often considered subleading but actually prove to be quite relevant: diffusive reacceleration, energy losses (including tertiary component) and solar modulation. Then, as soon as the data from AMS-02 came out, in  we reevaluated again the secondary astrophysical antiproton to proton ratio and its uncertainties, finding that there is no unambiguous evidence for a significant excess with respect to expectations, and we provided a first assessment of the updated Dark Matter constraints.
Ref.  constitutes an upgrade and complement of , the collection of tools and recipes for DM indirect detection. We here focussed on secondary radiation: bremsstrahlung and Inverse Compton gamma rays and synchrotron radiation.
Ref.  we reconsider the model of Minimal Dark Matter, almost ten years after we proposed it in , and compute its updated gamma ray signatures. We find that the model is constrained by the line searches from the Galactic Center: it is ruled out if the Milky Way possesses a cuspy profile such as NFW but it is still allowed if it has a cored one. We also explore a wider mass range, which applies to the case in which the relic abundance requirement is relaxed.
The work in ref.  derives bounds on annihilating or decaying Dark Matter from yet another aspect: the synchrotron radiation constrained by the radio surveys of the Galaxy. We employed the tools developed in  and we compared with standard and new surveys (such as e.g. those by the CMB satellite Planck). The derived bounds turn out to be not very stringent, but complementary to those obtained with other indirect detection methods, especially for dark matter annihilating into leptonic channels.
In ref.  we addressed a different kind of DM model, in which the DM particle (taken to be a Dirac fermion here) is charged under a dark force and couples to a dark photon. We studied thoroughly the cosmology and the phenomenology of this model, spanning large ranges for the DM mass and the dark photon mass. We focussed in particular on the issue of the formation of DM bound states, and its impact on the relic density computations, cosmology, direct and indirect detection signals. In ref.  we extended the same analysis to the case of asymmetric Dark Matter. Finally in ref.  we considered the case in which the DM density is diluted by the late decay of the dark photons ('Homeopatic DM'): this allows to consider very heavy DM (above 100 TeV) and provides for an interesting phenomenology in cosmic rays, which can be reliably computed.
Ref.  dealt with a completely different kind of Dark Matter: Primordial Black Holes which could have been created by exotic processes in the Early Universe. We considered the production of sub-GeV electrons and positrons from their Hawking evaporation and compared the predicted flux to the data of the Voyager-1 spacecraft, the only probe capable of measuring them since it has crossed the edge of the heliosphere that acts as a shield to low-energy cosmic rays. We slightly improved and complemented the constraints for PBHs of mass <10^16 grams.
Ref.  and ref. [1812.07831] are somewhat similar in spirit, but different in contents. I teamed up with two large experimental collaborations to test models again inspired by Minimal Dark Matter but then enlarged to more general cases. In , as associate scientists of the HESS Collaboration, we derive constraints from gamma-ray lines and 'pure WIMP' spectra from Dark Matter annihilations in dwarf galaxies. In [1812.07831] we make the analysis already sketched in  more concrete: with an Analysis Consultant Experts (ACE) status within the ATLAS Collaboration, we investigated the reach of the high-luminosity LHC to an electroweak triplet, in the Vector Boson Fusion and mono-photon channels.
~~During my laurea thesis work I also dealt with the physics of Strong Interactions and Quantum Chromodynamics, under the supervision of P. Nason and G. Marchesini. We derived a formula for a particular regime in Drell-Yan processes (the production of a lepton--antilepton pair in proton--antiproton collisions). Namely, the intersection of threshold production and small transverse momentum regimes. I had the opportunity of studying in a certain detail the resummation of soft gluon emissions.