arXiv subjects

Stefano Scopel

Publications and source records attributed to Stefano Scopel.

At least 19 recordsLinked to original sources

arXiv2026

Probing Sub-GeV Dark Matter with the Migdal Effect at JUNO

We discuss the sensitivity of the JUNO neutrino detector to the Migdal ionization signal triggered by nuclear scattering events produced by sub-GeV weakly interacting massive particles (WIMPs). Exploiting JUNO's large target mass and the annual modulation effect we find that the aggregate rate from many independent and indistinguishable WIMP events in JUNO's liquid scintillator can be isolated from the total dark rate of the photomultipliers, potentially providing for spin-dependent interactions a world-leading sensitivity across the sub-GeV mass range.

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arXiv2026

Dark Photon mediated Inelastic Dark Matter in Cosmology, Astrophysics and Colliders

We explore the phenomenology of Dark Photon iDM (A$^{\prime}$iDM) where the Standard Model (SM) is extended by a dark sector containing an additional $U(1)_D$ gauge symmetry under which all SM particles are neutral, and that couples to the SM hypercharge gauge boson through a kinetic mixing parameter $\epsilon$. The model contains two Majorana states $\chi_1$ and $\chi_2$ with $\delta=M_{\chi_2}-M_{\chi_1}>0$ and $\chi_1$ the dark matter candidate, and a dark photon $A^{\prime}$ with mass $M_{A^{\prime}}$. Our analysis represents an integration of existing ones, where only specific benchmarks of the A$^{\prime}$iDM scenario have been discussed. In particular, we fix the $U(1)_D$ coupling $\alpha_D$ equal to the electromagnetic one $\alpha_{EM}$ and $\epsilon$ to its experimental upper bound, and perform a complete scan of the remaining parameters $(M_{\chi_1},\delta, M_{A^{\prime}})$, discussing the $\chi_1$ relic abundance, its direct and indirect searches, as well as potential signals from astrophysics and accelerators. Our scan shows that $\alpha_D$ = $\alpha_{EM}$ is not disfavored, as some previous analyses, limited to specific benchmarks, may suggest. We also find that when the $\chi_1$ relic density matches observation direct and indirect searches are not kinematically accessible. On the other hand we find that the projected luminosity of FASER, a detector searching for Long Lived Particles (LLP) decay at the LHC, can probe or rule out the parameters space of the model for $M_{\chi_1}\lesssim$ 7 GeV, 100 MeV $\lesssim \delta\lesssim$ 300 MeV and $M_{A^{\prime}}\lesssim$ 25 GeV. This range of parameter could be significantly extended by the FASER 2 upgrade proposed for the High-Luminosity phase at the LHC. The parameter space probed by LLP seaches partially overlaps with that probed by $\chi_1$ capture in neutron stars.

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arXiv2025

WimPyC: an extension module of WimPyDD for the calculation of WIMP capture in celestial bodies

We introduce WimPyC, a Python code for the calculation of the capture rate of Weakly Interacting Massive Particles (WIMPs) by celestial bodies through nuclear scattering in the optically thin regime. WimPyC is an extension of the WimPyDD code, that calculates WIMP-nucleus scattering signals in direct detection (DD) experiments, and allows to combine DD and capture in celestial bodies in virtually any scenario within the framework of Galilean-invariant non-relativistic effective theory (NREFT), including inelastic scattering, an arbitrary WIMP spin and a generic WIMP velocity distribution in the Galactic halo. WimPyDD and WimPyC are suitable for both top-down approaches, where the interaction operators of a high-energy physics model are matched to those of the NREFT, and to bottom-up studies, where the Wilson coefficients of the NREFT are explored in a model-independent way and/or where the velocity distribution is written in terms of a superposition of streams taken as free parameters. As in the case of WimPyDD WimPyC exploits the factorization of the three main components that enter in the calculation of the capture rate: i) the Wilson coefficients that encode the dependence of the signals on the ultraviolet completion of the effective theory; ii) a response function that depends on the nuclear physics; iii) the halo function that depends on the WIMP velocity distribution. In WimPyC these three components are calculated and stored separately for later interpolation and combined together only as the last step of the signal evaluation procedure. This makes the phenomenological study of the capture rate with WimPyC transparent and improves computational speed.

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arXiv2025

Gravitational Wave Signals in a Promising Realization of SO(10) Unification

We investigate gravitational wave signals in a non-supersymmetric grand unified model where the group $SO(10)$ is broken in two steps to the Standard Model gauge group. We calculate the analytical form of the one-loop effective potential responsible for the first step of symmetry breaking and show that it can lead to a first-order phase transition with gravitational wave production. We also determine the gravitational wave background produced by the primordial plasma of relativistic particles. The present experimental sensitivity is still far from the expected signals, but could be in reach of novel detector concepts.

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arXiv2024

Sensitivity of WIMP bounds on the velocity distribution in the limit of a massless mediator

We discuss the sensitivity of the bounds on the spin-independent (SI) and spin-dependent (SD) WIMP-proton and WIMP-neutron interaction couplings $\alpha_{SI, SD}^{p,n}$ on the WIMP velocity distribution for a massless mediator in the propagator by combining direct detection and the neutrino signal from WIMP annihilation in the Sun (fixing the annihilation channel to $b\bar{b}$). We update the bounds in the Standard Halo Model (SHM) and using the halo-independent single-stream method. In the case of a massless mediator the SHM capture rate in the Sun diverges and is regularized by removing the contribution of WIMPs locked into orbits that extend beyond the Sun-Jupiter distance. We discuss the dependence of the SHM bounds on the Jupiter cut showing that it can be sizeable for $\alpha_{\rm SD}^p$ and a WIMP mass $m_\chi$ exceeding 1 TeV. Our updated SHM bounds show an improvement between about two and three orders of magnitude compared to the previous ones in the literature. Our halo-independent analysis shows that, with the exception of $\alpha_{\rm SD}^p$ at large $m_\chi$, the relaxation of the bounds compared to the SHM is of the same order of that for contact interactions, i.e. relatively moderate in the low and high WIMP mass regimes and as large as a few $\times\sim 10^2$ for $m_\chi\simeq$ 20 GeV. On the other hand, the exact determination of the relaxation of the bound becomes not reliable for $\alpha_{\rm SD}^p$ and $m_\chi\gtrsim$ 1 TeV due to the sensitivity of the SHM capture rate in the Sun to the details of the Maxwellian velocity distribution at low incoming WIMP speeds. The halo-independent bounds do not depend on the Jupiter cut needed to regularize the calculation of the capture rate.

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arXiv2024

Low-mass constraints on WIMP effective models of inelastic scattering using the Migdal effect

We use the Migdal effect to extend to low masses the bounds on each of the effective couplings of the non-relativistic effective field theory of a WIMP of mass $m_\chi$ and spin 1/2 that interacts inelastically with nuclei by either upscattering to a heavier state with mass splitting $\delta>0$ or by downscattering to a lighter state with $\delta<0$. In order to do so we perform a systematic analysis of the Migdal bounds in the $m_\chi-\delta$ parameter space comparing them to those from nuclear recoil searches. The Migdal effect allows to significantly extend to low WIMP masses the nuclear recoil bounds for $\delta<0$. In this case the bounds are driven by XENON1T, except when $\delta$ is vanishing or very small, when, depending on the WIMP-nucleus interaction, in the lower end of the $m_\chi$ range either DS50 or SuperCDMS are more constraining. On the other hand, when $\delta>0$ and the WIMP particle upscatters to a heavier state nuclear recoil bounds are stronger than those from the Migdal effect.

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arXiv2024

Gauss-Bonnet Cosmology: large-temperature behaviour and bounds from Gravitational Waves

We provide a transparent discussion of the high temperature asymptotic behaviour of Cosmology in a dilaton-Einstein-Gauss-Bonnet (dEGB) scenario of modified gravity with vanishing scalar potential. In particular, we show that it has a clear interpretation in terms of only three attractors (stable critical points) of a set of autonomous differential equations: $w=-\frac{1}{3}$, $w=1$ and $1<w<\frac{7}{3}$, where $w\equiv p/\rho$ is the equation of state, defined as the ratio of the total pressure and the total energy density. All the possible different high-temperature evolution histories of the model are exhausted by only eight paths in the flow of the set of the autonomous differential equations. Our discussion clearly explains why five out of them are characterized by a swift transition of the system toward the attractor, while the remaining three show a more convoluted evolution, where the system follows a meta-stable equation of state at intermediate temperatures before eventually jumping to the real attractor at higher temperatures. Compared to standard Cosmology, the regions of the dEGB parameter space with $w=-\frac{1}{3}$ show a strong enhancement of the expected Gravitational Wave stochastic background produced by the primordial plasma of relativistic particles of the Standard Model. This is due to the very peculiar fact that dEGB allows to have an epoch when the energy density $\rho_{\rm rad}$ of the relativistic plasma dominates the energy of the Universe while at the same time the rate of dilution with $T$ of the total energy density is slower than what usually expected during radiation dominance. This allows to use the bound from BBN to put in dEGB a constraint $T_{\rm RH}\lesssim 10^8 - 10^9$ GeV on the reheating temperature of the Universe $T_{\rm RH}$. Such BBN bound is complementary to late-time constraints from compact binary mergers.

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arXiv2024

WIMP constraints from black hole low-mass X-ray binaries

The abnormally fast orbital decay observed in the black hole (BH) Low-Mass X-ray binaries (BH-LMXB) A0620-00 and XTE J1118+480 can be explained by the dynamical friction between Dark Matter (DM) and the companion star orbiting around the low-mass BH (of a few $M_\odot$) of the system. In this case the value of the index $γ_{\rm sp}$ of the DM spike surrounding the BH can be pinned down with an accuracy of a few percent, way better than that for much bigger systems such as the super massive BHs (SMBHs) in the Galactic Center or in M87. We have used data from XTE J1118+480 to put bounds on the WIMP annihilation cross section times velocity $\langle σv\rangle$, assuming that DM annihilation is driven by the $b\bar{b}$ annihilation channel and that it proceeds in $s$-wave. The bounds are driven by the radio synchrotron signal produced by $e^\pm$ final states propagating in the magnetic field near the BH. For DM masses $m_χ$ up to the TeV scale XTE J1118+480 allows to constrain $\langle σv\rangle$ well below $\langleσv\rangle_{\rm thermal}$, corresponding to the observed DM relic density in the Universe for a thermal WIMP. On the other hand, for $m_χ\gtrsim$ 15 GeV, the bounds from the SMBHs in the GC or in M87 do not reach $\langleσv\rangle_{\rm thermal}$ when the very large uncertainties on the corresponding spike indices are taken into account, in spite of potentially producing much larger DM densities compared to XTE J1118+480. Our bounds for XTE J1118+480 have a mild sensitivity on spatial diffusion, but diffusion enhances the sensitivity of the results upon the intensity of the magnetic field. Taken at face value the bound from XTE J1118+480 on $\langle σv\rangle$ is the most constraining compared to all others for $m_χ\lesssim$ 1 TeV, unless the intensity of the magnetic field is significantly smaller than its equipartition estimation.

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arXiv2023

Halo-independent bounds on Inelastic Dark Matter

We discuss halo-independent constraints on the Inelastic Dark Matter (IDM) scenario, in which a Weakly Interaction Massive Particle (WIMP) state $χ$ with mass $m_χ$ interacts with nuclear targets by upscattering to a heavier state $χ^{\prime}$ with mass $m_χ+δ$. In order to do so we adopt the single-stream method, that exploits the complementarity of Direct Detection (DD) and Capture in the Sun to extend the experimental sensitivity to the full range of incoming WIMP speeds. We show that a non-vanishing mass splitting $δ$ modifies such range, and that for particular combinations of $m_χ$ and $δ$ the complementarity between the two detection techniques required by the method is lost. Specifically, assuming for the escape velocity in our Galaxy $u_{esc}$ the reference value $u_{esc}^{ref}$ = 560 km/s a halo-independent bound is possible when $δ\lesssim$ 510 keV for a Spin-Independent interaction and when $δ\lesssim$ 245 keV for a Spin-Dependent interaction (with the Spin-Independent value slightly reduced to $δ\lesssim$ 490 keV when $u_{esc}>u_{esc}^{ref}$). In the low-mass regime the bound from capture in the Sun is always more constraining than that for DD and is sufficient alone to provide a halo-independent constraint, while for large WIMP masses the halo-independent bound is given by a combination of capture in the Sun and DD. We also find that, for $u_{esc}$ = $u_{esc}^{ref}$, unless the mass of the target used in DD is larger than about four times that of the target driving capture in the Sun, DD does not play any role in the determination of the maximal value of $δ$ for which a halo-independent bound is possible. We also discuss the issue of thermalization of IDM within the Sun and show that its impact on our results is mild.

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arXiv2023

WIMPs in Dilatonic Einstein Gauss-Bonnet Cosmology

We use the Weakly Interacting Massive Particle (WIMP) thermal decoupling scenario to probe Cosmologies in dilatonic Einstein Gauss-Bonnet (dEGB) gravity, where the Gauss-Bonnet term is non-minimally coupled to a scalar field with vanishing potential. We put constraints on the model parameters when the ensuing modified cosmological scenario drives the WIMP annihilation cross section beyond the present bounds from DM indirect detection searches. In our analysis we assumed WIMPs that annihilate to Standard Model particles through an s-wave process. For the class of solutions that comply with WIMP indirect detection bounds, we find that dEGB typically plays a mitigating role on the scalar field dynamics at high temperature, slowing down the speed of its evolution and reducing the enhancement of the Hubble constant compared to its standard value. For such solutions, we observe that the corresponding boundary conditions at high temperature correspond asymptotically to a vanishing deceleration parameter q, so that the effect of dEGB is to add an accelerating term that exactly cancels the deceleration predicted by General Relativity. The bounds from WIMP indirect detection are nicely complementary to late-time constraints from compact binary mergers. This suggest that it could be interesting to use other Early Cosmology processes to probe the dEGB scenario.

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arXiv2023

Bracketing the direct detection exclusion plot for a WIMP of spin one half in non-relativistic effective theory

Assuming a standard Maxwellian velocity distribution for the WIMPs in the halo of our Galaxy we use the null results of an exhaustive set of 9 direct detection experiments to calculate the maximal variation of the exclusion plot for each Wilson coefficient of the most general Galilean-invariant effective Hamiltonian for a WIMP of spin one half due to interferences. We consider 56 Wilson coefficients $c_i^{p,n}$ and $α_i^{n,p}$ for WIMP-proton and WIMP-neutron contact interactions ${\cal O}_i^{p,n}$ and the corresponding long range interaction ${\cal O}_i^{p,n}/q^2$, parameterized by a massless propagator $1/q^2$. For each coupling we provide a different exclusion plot when the following set of operators is allowed to interfere: proton-neutron, i.e. $c_i^{p}$-$c_i^{n}$ or $α_i^{p}$-$α_i^{n}$; contact-contact or long range-long range, i.e. $c_i^{p,n}$-$c_j^{p,n}$ or $α_i^{p,n}$-$α_j^{p,n}$; contact-long range, i.e. $c_i^{p,n}$-$α_j^{p,n}$. For each of the 56 Wilson coefficients $c_i^{p,n}$ and $α_j^{p,n}$ and for the largest number of interfering operators the exclusion plot variation can reach 3 orders of magnitude and reduces to a factor as small as a few for the Wilson coefficients of the effective interactions where the WIMP couples to the nuclear spin, thanks to the combination of experiments using proton-odd and neutron-odd targets. Some of the conservative bounds require an extremely high level of cancellation, putting into question the reliability of the result. We analyze this issue in a systematic way, showing that it affects some of the couplings driven by the operators ${\cal O}_{1}$, ${\cal O}_{3}$, ${\cal O}_{11}$, ${\cal O}_{12}$ and ${\cal O}_{15}$, especially when interferences among contact and long range interactions are considered.

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arXiv2023

Low-mass extension of direct detection bounds on WIMP-quark and WIMP-gluon effective interactions using the Migdal effect

Updating a previous analysis where we used elastic nuclear recoils we study the Migdal effect to extend to low WIMP masses the direct detection bounds to operators up to dimension 7 of the relativistic effective field theory describing WIMP interactions with quarks and gluons. To this aim we include in our analysis the data of the XENON1T, SuperCDMS, COSINE-100, and DarkSide-50 experiments and assume a standard Maxwellian for the WIMP velocity distribution. We find that the bounds can reach down to a WIMP mass $\simeq$20 MeV, although in the case of higher-dimension operators the energy scale of the ensuing constraints may be inconsistent with the validity of the effective theory.

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arXiv2023

Halo-independent bounds on the non-relativistic effective theory of WIMP-nucleon scattering from direct detection and neutrino observations

We combine experimental constraints from direct detection searches and from neutrino telescopes looking for WIMP annihilations in the Sun to derive halo-independent bounds on each of the 28 WIMP-proton and WIMP-neutron couplings of the effective non-relativistic Hamiltonian that drives the scattering process off nuclei of a WIMP of spin 1/2. The method assumes that the velocity distribution is normalized to one and homogeneous at the the solar system scale, as well as equilibrium between WIMP capture and annihilation in the Sun, and requires to fix the WIMP annihilation channels (we assume $b\bar{b}$). We consider a single non-vanishing coupling at a time, and find that for most of the couplings the degree of relaxation of the halo-independent bounds compared to those obtained by assuming the Standard Halo Model is with few exceptions relatively moderate in the low and high WIMP mass regimes, where it can be as small as a factor of $\simeq 2$, while in the intermediate mass range between 10 GeV and 200 GeV it can be as large as $\sim 10^3$. An exception to this general pattern, with more moderate values of the bound relaxation, is observed in the case of spin-dependent WIMP-proton couplings with no or a comparatively small momentum suppression, for which WIMP capture is strongly enhanced because it is driven by scattering events off $^1H$, which is the most abundant target in the Sun. Within this class of operators the relaxation is particularly small for interactions that are driven by only the velocity-dependent term, for which the solar capture signal is enhanced compared to the direct detection one, thanks to the highest speed of scattering WIMPs within the Sun due to the larger gravitational acceleration.

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arXiv2022

Improved White Dwarves Constraints on Inelastic Dark Matter and Left-Right Symmetric Models

WIMPs can be captured in compact stars such as white dwarves (WDs) leading to an increase in the star luminosity through their annihilation process. We show that when the WIMP interacts with the nuclear targets within the WD through inelastic scattering and its mass exceeds a few tens GeV the data on low-temperature large-mass WDs in the Messier 4 globular cluster can probe values of the mass splitting $δ\lesssim$ 40 MeV. Such value largely exceeds those ensuing from direct detection and from solar neutrino searches. We apply such improved constraint to the specific DM scenario of a self-conjugate bi-doublet in the Left-Right Symmetric Model (LRSM), where the standard $SU(2)_L$ group with coupling $g_L$ is extended by an additional $SU(2)_R$ with coupling $g_R$. We show that bounds from WDs significantly reduce the cosmologically viable parameter space of such scenario, in particular requiring $g_R>g_L$. For instance, for $g_R/g_L$ = 1.8 we find the two viable mass ranges 1.2 TeV $\lesssim m_χ\lesssim$ 3 TeV and 5 TeV $\lesssim m_χ\lesssim$ 10 TeV, when the charged $SU(2)_R$ gauge boson mass $M_{W_2}$ is lighter than $\simeq$ 12 TeV. We also discuss the ultraviolet completion of the LRSM model, when the latter is embedded in a Grand Unified Theory. We show that such low-energy parameter space and compatibility to proton-decay bounds require a non-trivial extension of the particle content of the minimal model. We provide a specific example where $M_{W_2}\lesssim$ 10 TeV is achieved by extending the LRSM at high energy with color triplets that are singlets under all other groups, and $g_R/g_L>$1 is obtained by introducing $SU(2)_L$ triplets with no $SU(2)_R$ counterparts, i.e. by breaking the symmetry between the multiplets of $SU(2)_L$ and $SU(2)_R$.

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arXiv2022

Complementarity of experiments in probing the non-relativistic effective theory of dark matter-nucleon interactions

The non-relativistic effective theory of dark matter-nucleon interactions depends on 28 coupling strengths for dark matter spin up to 1/2. Due to the vast parameter space of the effective theory, most experiments searching for dark matter interpret the results assuming that only one of the coupling strengths is non-zero. On the other hand, dark matter models generically lead in the non-relativistic limit to several interactions which interfere with one another, therefore the published limits cannot be straightforwardly applied to model predictions. We present a method to determine a rigorous upper limit on the dark matter-nucleon interaction strength including all possible interferences among operators. We illustrate the method to derive model independent upper limits on the interaction strengths from the null search results from XENON1T, PICO-60 and IceCube. For some interactions, the limits on the coupling strengths are relaxed by more than one order of magnitude. We also present a method that allows to combine the results from different experiments, thus exploiting the synergy between different targets in exploring the parameter space of dark matter-nucleon interactions.

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arXiv2021

WimPyDD: an object-oriented Python code for WIMP-nucleus scattering direct detection in virtually any scenario

We introduce WimPyDD, a modular, object-oriented and customisable Python code that accurately predicts the expected WIMP-nucleus scattering rates in WIMP direct-detection experiments including the response of the detector. WimPyDD utilises the framework of Galilean-invariant non-relativistic effective theory, allowing to handle an arbitrary number of effective operators, and can perform the calculation of the excepted rate in virtually any scenario, including inelastic scattering, WIMPs with an arbitrary spin, and a generic velocity distribution in the Galactic halo. The power and flexibility of WimPyDD is discussed in some explicit examples.

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arXiv2021

WimPyDD: an object-oriented Python code for the calculation of WIMP direct detection signals

We introduce WimPyDD, a modular, object-oriented and customizable Python code that calculates accurate predictions for the expected rates in Weakly Interacting Massive Particle (WIMP) direct-detection experiments within the framework of Galilean-invariant non-relativistic effective theory in virtually any scenario, including inelastic scattering, an arbitrary WIMP spin and a generic WIMP velocity distribution in the Galactic halo. WimPyDD exploits the factorization of the three main components that enter in the calculation of direct detection signals: i) the Wilson coefficients that encode the dependence of the signals on the ultraviolet completion of the effective theory; ii) a response function that depends on the nuclear physics and on the main features of the experimental detector (acceptance, energy resolution, response to nuclear recoils); iii) a halo function that depends on the WIMP velocity distribution and that encodes the astrophysical inputs. In WimPyDD these three components are calculated and stored separately for later interpolation and combined together only as the last step of the signal evaluation procedure. This makes the phenomenological study of the direct detection scattering rate with WimPyDD transparent and fast also when the parameter space of the WIMP model has a large dimensionality.

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arXiv2021

The phenomenology of nuclear scattering for a WIMP of arbitrary spin

We provide a first systematic and quantitative discussion of the phenomenology of the non-relativistic effective Hamiltonian describing the nuclear scattering process for a Weakly Interacting Massive Particle (WIMP) of arbitrary spin $j_χ$. To this aim we obtain constraints from a representative sample of present direct detection experiments assuming the WIMP-nucleus scattering process to be driven by each one of the 44 effective couplings that arise for $j_χ\le$ 2. We find that a high value of the multipolarity $s\le 2 j_χ$ of the coupling, related to the power of the momentum transfer $q$ appearing in the scattering amplitude, leads to a suppression of the expected rates and pushes the expected differential spectra to large recoil energies $E_R$. For $s\le$ 4 the effective scales probed by direct detection experiments can be suppressed by up to 5 orders of magnitude compared to the case of a standard spin-independent interaction. For operators with large $s$ the expected differential spectra can be pushed to recoil energies in the MeV range, with the largest part of the signal concentrated at $E_R\gtrsim$ 100 keV and a peculiar structure of peaks and minima arising when both the nuclear target and the WIMP are heavy. As a consequence the present bounds on the effective operators can be significantly improved by extending the recoil energy intervals to higher recoil energies. Our analysis assumes effective interaction operators that are irreducible under the rotation group. Such operators drive the interactions of high-multipole dark matter candidates, i.e. states that possess only the highest multipole allowed by their spin. As a consequence our analysis represents also the first phenomenological study of the direct detection of quadrupolar, octupolar, and hexadecapolar dark matter.

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