Francesco D'Eramo
The semi-annihilation reaction takes the schematic form psi_i psi_j -> psi_k phi, where psi_i are stable dark matter particles and phi is an unstable state. Such reactions are allowed when dark matter is stabilized by a larger symmetry than just Z_2. They lead to non-trivial dark matter dynamics in the early universe, and the thermal production of the relic particles can be completely controlled by semi-annihilations. This process might also take place today in the Milky Way, enriching the (semi-)annihilation final state spectrum observed in indirect detection experiments.
Francesco D'Eramo
The Peccei-Quinn solution to the strong CP problem provides a motivated framework rich in cosmological consequence. Thermal axion production is unavoidable if there is a thermal bath at early times. Scattering and decay processes of bath particles can dump relativistic axions in the primordial plasma, and they can leave observable signatures in cosmological observables probing both the early and the late universe if produced with a significant abundance. We present recent and significant improvements for the calculation of the axion production rate for different scenarios and apply these results to predict the abundance of produced axions. Finally, we provide updated cosmological bounds on the QCD axion mass.
Francesco D'Eramo, Eleonora Di Valentino, William Giarè, Fazlollah Hajkarim, Alessandro Melchiorri, Olga Mena, Fabrizio Renzi, Seokhoon Yun
May 16, 2022·astro-ph.CO·PDF We revisit the joint constraints in the mixed hot dark matter scenario in which both thermally produced QCD axions and relic neutrinos are present. Upon recomputing the cosmological axion abundance via recent advances in the literature, we improve the state-of-the-art analyses and provide updated bounds on axion and neutrino masses. By avoiding approximate methods, such as the instantaneous decoupling approximation, and limitations due to the limited validity of the perturbative approach in QCD that forced to artificially divide the constraints from the axion-pion and the axion-gluon production channels, we find robust and self-consistent limits. We investigate the two most popular axion frameworks: KSVZ and DFSZ. From Big Bang Nucleosynthesis (BBN) light element abundances data we find for the KSVZ axion $ΔN_{\rm eff}<0.31$ and an axion mass bound $m_a < 0.53 $ eV (i.e., a bound on the axion decay constant $f_a > 1.07 \times 10^7$ GeV) both at $95\%$ CL. These BBN bounds are improved to $ΔN_{\rm eff}<0.14$ and $m_a< 0.16$ eV ($f_a > 3.56 \times 10^7$ GeV) if a prior on the baryon energy density from Cosmic Microwave Background (CMB) data is assumed. When instead considering cosmological observations from the CMB temperature, polarization and lensing from the Planck satellite combined with large scale structure data we find $ΔN_{\rm eff}<0.23$, $m_a< 0.28$ eV ($f_a > 2.02 \times 10^7$ GeV) and $\sum m_ν< 0.16$ eV at $95\%$ CL. This corresponds approximately to a factor of $5$ improvement in the axion mass bound with respect to the existing limits. Very similar results are obtained for the DFSZ axion. We also forecast upcoming observations from future CMB and galaxy surveys, showing that they could reach percent level errors for $m_a\sim 1$ eV.
Francesco D'Eramo, Krishna Rajagopal, Yi Yin
We present a brief report on a thought experiment in which an incident energetic parton traverses a brick of quark-gluon plasma (QGP), see arXiv:1808.03250 for the full report. We calculate the probability of detecting a parton showing up at a large angle with respect to its initial direction due to scattering with the constituents of QGP, using leading order perturbative QCD. We include all relevant channels, including the Rutherford-like channel as considered in early works, and those that are not Rutherford-like but become important at a large angle. The resulting probability distributions contain information about the short distance structure of QGP. Our results provide key theoretical input toward finding the scatterers within the QGP liquid, which in turn is the necessary first step toward using precise, high-statistics, suitably differential measurements of jet modification in heavy ion collisions to study the evolution of the properties of QGP with changing resolution scale.
Francesco D'Eramo, Nicolas Fernandez, Stefano Profumo
If the dark matter is produced in the early universe prior to Big Bang nucleosynthesis, a modified cosmological history can drastically affect the abundance of relic dark matter particles. Here, we assume that an additional species to radiation dominates at early times, causing the expansion rate at a given temperature to be larger than in the standard radiation-dominated case. We demonstrate that, if this is the case, dark matter production via freeze-in (a scenario when dark matter interacts very weakly, and is dumped in the early universe out of equilibrium by decay or scattering processes involving particles in the thermal bath) is dramatically suppressed. We illustrate and quantitatively and analytically study this phenomenon for three different paradigmatic classes of freeze-in scenarios. For the frozen-in dark matter abundance to be as large as observations, couplings between the dark matter and visible-sector particles must be enhanced by several orders of magnitude. This sheds some optimistic prospects for the otherwise dire experimental and observational outlook of detecting dark matter produced by freeze-in.
Francesco D'Eramo, Bradley J. Kavanagh, Paolo Panci
We study vector portal dark matter models where the mediator couples only to leptons. In spite of the lack of tree-level couplings to colored states, radiative effects generate interactions with quark fields that could give rise to a signal in current and future experiments. We identify such experimental signatures: scattering of nuclei in dark matter direct detection; resonant production of lepton-antilepton pairs at the Large Hadron Collider; and hadronic final states in dark matter indirect searches. Furthermore, radiative effects also generate an irreducible mass mixing between the vector mediator and the $Z$ boson, severely bounded by ElectroWeak Precision Tests. We use current experimental results to put bounds on this class of models, accounting for both radiatively induced and tree-level processes. Remarkably, the former often overwhelm the latter.
Yanou Cui, Francesco D'Eramo
We study UV complete theories where the Standard Model (SM) gauge group is extended with a new abelian $U(1)$, and the field content is augmented by an arbitrary number of scalar and fermion SM singlets, potentially including dark matter (DM) candidates. Considerations such as classical and quantum gauge invariance of the full theory and S-matrix unitarity, not applicable within a simplified model approach, are shown to have significant phenomenological consequences. The lack of gauge anomalies leads to compact relations among the $U(1)$ fermion charges, and puts a lower bound on the number of dark fermions. Contrary to naive expectations, the DM annihilation to Zh is found to be p-wave suppressed, as hinted by perturbative unitarity of S-matrix, with dramatic implications for DM thermal relic density and indirect searches. Within this framework, the interplay between dark matter, new vector boson and Higgs physics is rather natural and generic.
Francesco D'Eramo, Alessandro Lenoci, Tommaso Sassi
Quantifying the imprints of freeze-in dark matter (DM) on cosmological structures requires knowledge of its phase-space distribution. We investigate how variations in the cosmological history before nucleosynthesis, the "weather" of that epoch, give rise to distinct "seasons" in the DM momentum distribution that govern its warmness. Studying decay-driven production across diverse cosmological histories, we map how these conditions shape DM phase-space properties. Our study quantifies how the early universe composition plays a key role in determining the mass bound on freeze-in DM.
Francesco D'Eramo
The early universe grants access to energy scales far beyond those achievable in terrestrial experiments and allows unstable Standard Model particles to play an active dynamical role. In this contribution, we focus on recent studies aimed at quantifying the potential of the early universe to probe the properties and interactions of axions. The discussion is organized around four classes of axion scenarios, ordered from long to short lifetimes: (i) stable or long-lived axions contributing to dark radiation; (ii) stable or long-lived axions produced out-of-equilibrium and constituting dark matter; (iii) metastable axions whose decays inject energy into the primordial plasma and leave observable signatures in the global 21 cm signal; and (iv) very short-lived axions that act only as portals to additional degrees of freedom. Together, these scenarios highlight the interplay between axion phenomenology and early universe cosmology and demonstrate the potential of cosmological data to probe axions over a broad range of masses and lifetimes.
Francesco D'Eramo, Seokhoon Yun
Flavor violating axion couplings can be in action before recombination, and they can fill the early universe with an additional radiation component. Working within a model-independent framework, we consider an effective field theory for the axion field and quantify axion production. Current cosmological data exclude already a fraction of the available parameter space, and the bounds will improve significantly with future CMB-S4 surveys. Remarkably, we find that future cosmological bounds will be comparable or even stronger than the ones obtained in our terrestrial laboratories.
Fernando Arias-Aragon, Francesco D'Eramo, Ricardo Z. Ferreira, Luca Merlo, Alessio Notari
Light axions can potentially leave a cosmic background, just like neutrinos. We complete the study of thermal axion production across the electroweak scale by providing a smooth and continuous treatment through the two phases. Focusing on both flavor conserving and violating couplings to third generation quarks, we compute the amount of axions produced via scatterings and decays of thermal bath particles. We perform a model independent analysis in terms of axion effective couplings, and we also make predictions for specific microscopic QCD axion scenarios. This observable effect, parameterized as it is conventional by an effective number of additional neutrinos, is above the $1σ$ sensitivity of future CMB-S4 surveys. Moreover, if one assumes no large hierarchies among dimensionless axion couplings to standard model particles, future axion helioscopes will provide a complementary probe for the parameter region we study.
Francesco D'Eramo, Nejc Košnik, Federico Pobbe, Aleks Smolkovič, Olcyr Sumensari
We investigate scenarios with $\mathcal{O}(1\mathrm{\,TeV})$ scalar leptoquarks that act as portals between the Standard Model and Dark Matter. We assume that Dark Matter is a scalar singlet $S$ which couples to a scalar leptoquark $Δ$ and the Higgs boson via the terms in the scalar potential. In addition, the leptoquark is endowed with Yukawa couplings to quarks and leptons that may address the anomalies in $B$ meson decays. We consider the $SS$ annihilation cross sections to estimate the Dark Matter relic abundance and explore the interplay between astrophysical, collider and flavour physics bounds on such models. In the heavy Dark Matter window, $m_S > m_Δ$, the leptoquark portal becomes the dominant mechanism to explain the Dark Matter abundance. We find that the leptoquark Yukawa couplings, relevant for quark and lepton flavour physics, are decoupled from the dark matter phenomenology. By focussing on a scenario with a single leptoquark state, we find that relic density can only be explained when both $Δ$ and $S$ masses are lighter than $\mathcal{O}(10\mathrm{\,TeV})$.
Francesco D'Eramo, Krishna Rajagopal, Yi Yin
By finding rare (but not exponentially rare) large-angle deflections of partons within a jet produced in a heavy ion collision, or of such a jet itself, experimentalists can find the weakly coupled short-distance quark and gluon particles (scatterers) within the strongly coupled liquid quark-gluon plasma (QGP) produced in heavy ion collisions. This is the closest one can come to probing QGP via a scattering experiment and ultimately learning how a strongly coupled liquid emerges from an asymptotically free gauge theory. The short-distance, particulate, structure of liquid QGP can be revealed in events in which a jet parton resolves, and scatters off, a parton from the droplet of QGP. The probability for picking up significant transverse momentum via a single scattering was calculated previously, but only in the limit of infinite parton energy which means zero angle scattering. Here, we provide a leading order perturbative QCD calculation of the Molière scattering probability for incident partons with finite energy, scattering at a large angle. We set up a thought experiment in which an incident parton with a finite energy scatters off a parton constituent within a "brick" of QGP, which we treat as if it were weakly coupled, as appropriate for scattering with large momentum transfer, and compute the probability for a parton to show up at a nonzero angle with some energy. We include all relevant channels, including those in which the parton that shows up at a large angle was kicked out of the medium as well as the Rutherford-like channel in which what is seen is the scattered incident parton. The results that we obtain will serve as inputs to future jet Monte Carlo calculations and can provide qualitative guidance for how to use future precise, high statistics, suitably differential measurements of jet modification in heavy ion collisions to find the scatterers within the QGP liquid.
Francesco D'Eramo, Ricardo Z. Ferreira, Alessio Notari, José Luis Bernal
Scattering and decay processes of thermal bath particles involving heavy leptons can dump hot axions in the primordial plasma around the QCD phase transition. We compute their relic density, parameterized by an effective number $ΔN_{\rm eff}$ of additional neutrinos. For couplings allowed by current bounds, production via scattering yields $ΔN_{\rm eff} \lesssim 0.6$ and $ΔN_{\rm eff} \lesssim 0.2$ for the cases of muon and tau, respectively. Flavor violating tau decays to a lighter lepton plus an axion give $ΔN_{\rm eff} \lesssim 0.3$. Such values of $ΔN_{\rm eff}$ can alleviate the tension between the direct local measurement of the Hubble constant $H_0$ and the inferred value from observations of the Cosmic Microwave Background, assuming $Λ$CDM. We analyze present cosmological data from the Planck collaboration and baryon acoustic oscillations with priors given in terms of the axion-lepton couplings. For axions coupled to muons, the tension can be alleviated below the 3$σ$ level. Future experiments will measure $ΔN_{\rm eff}$ with higher precision, providing an axion discovery channel and probing the role of hot axions in the $H_0$ tension.
Francesco D'Eramo, Stefano Profumo
We propose a novel framework where light (sub-GeV) dark matter (DM) is detectable with future MeV gamma-ray telescopes without conflicting with Cosmic Microwave Background (CMB) data. The stable DM particle $χ$ has a very low thermal relic abundance due to its large pair-annihilation cross section. The DM number density is stored in a heavier, meta-stable partner $ψ$ with suppressed pair-annihilation rates, that does not perturb the CMB, and whose late-time decays $ψ\rightarrow χ$ fill the universe with $χ$ DM particles. We provide explicit, model-independent realizations for this framework, and discuss constraints on late-time decays, and thus on parameters of this setup, from CMB, Big Bang Nucleosynthesis, and Large Scale Structure.
Francesco D'Eramo
The Strongly Coupled Standard Model predicts a rich spectrum of excited states at the Fermi scale. We study the first radial excitations of the vector bosons. The inclusion of these new states affects the low energy phenomenology of the model. We put constraints on the effective couplings by performing a global fit with the electroweak observables, and we find that the excitations have to be rather decoupled from the low-energy states.
Francesco D'Eramo, Giuseppe Lucente, Newton Nath, Seokhoon Yun
Solar nuclear reactions can occasionally produce sub-MeV elusive beyond the Standard Model particles that escape the solar interior without further interactions. This study focuses on massive spin-one particles. We construct the general theoretical framework and identify two crucial mixing sources involving the photon, which facilitate communication between the hidden and visible sectors: kinetic mixing with the photon, and plasma-induced mixing due to thermal electron loops. For both cases, we focus on the second stage of the solar proton-proton chain and evaluate the fluxes of monochromatic 5.49~MeV hidden vectors produced by the $p(d, ^3{\rm He})γ^\prime$ nuclear reaction. We then investigate their terrestrial detection via Compton-like scatterings. The incoming fluxes are polarized, and we evaluate the cross sections for Compton-like scatterings for transverse and longitudinal vectors. Finally, we apply this framework to a concrete case by investigating the sensitivity of the forthcoming Jiangmen Underground Neutrino Observatory (JUNO) experiment and identifying parameter space where current terrestrial bounds will be improved.
Raymond T. Co, Francesco D'Eramo, Lawrence J. Hall, Keisuke Harigaya
In all supersymmetric theories, gravitinos, with mass suppressed by the Planck scale, are an obvious candidate for dark matter; but if gravitinos ever reached thermal equilibrium, such dark matter is apparently either too abundant or too hot, and is excluded. However, in theories with an axion, a saxion condensate is generated during an early era of cosmological history and its late decay dilutes dark matter. We show that such dilution allows previously thermalized gravitinos to account for the observed dark matter over very wide ranges of gravitino mass, keV < $m_{3/2}$ < TeV, axion decay constant, $10^9$ GeV < $f_a$ < $10^{16}$ GeV, and saxion mass, 10 MeV < $m_s$ < 100 TeV. Constraints on this parameter space are studied from BBN, supersymmetry breaking, gravitino and axino production from freeze-in and saxion decay, and from axion production from both misalignment and parametric resonance mechanisms. Large allowed regions of $(m_{3/2}, f_a, m_s)$ remain, but differ for DFSZ and KSVZ theories. Superpartner production at colliders may lead to events with displaced vertices and kinks, and may contain saxions decaying to $(WW,ZZ,hh), gg, γγ$ or a pair of Standard Model fermions. Freeze-in may lead to a sub-dominant warm component of gravitino dark matter, and saxion decay to axions may lead to dark radiation.
Francesco D'Eramo, Nicolas Fernandez, Stefano Profumo
We consider a modification to the standard cosmological history consisting of introducing a new species $φ$ whose energy density red-shifts with the scale factor $a$ like $ρ_φ\propto a^{-(4+n)}$. For $n>0$, such a red-shift is faster than radiation, hence the new species dominates the energy budget of the universe at early times while it is completely negligible at late times. If equality with the radiation energy density is achieved at low enough temperatures, dark matter can be produced as a thermal relic during the new cosmological phase. Dark matter freeze-out then occurs at higher temperatures compared to the standard case, implying that reproducing the observed abundance requires significantly larger annihilation rates. Here, we point out a completely new phenomenon, which we refer to as $\textit{relentless}$ dark matter: for large enough $n$, unlike the standard case where annihilation ends shortly after the departure from thermal equilibrium, dark matter particles keep annihilating long after leaving chemical equilibrium, with a significant depletion of the final relic abundance. Relentless annihilation occurs for $n \geq 2$ and $n \geq 4$ for s-wave and p-wave annihilation, respectively, and it thus occurs in well motivated scenarios such as a quintessence with a kination phase. We discuss a few microscopic realizations for the new cosmological component and highlight the phenomenological consequences of our calculations for dark matter searches.
Francesco D'Eramo, Jesse Thaler, Zoe Thomas
Anomaly mediation is a ubiquitous source of supersymmetry (SUSY) breaking which appears in almost every theory of supergravity. In this paper, we show that anomaly mediation really consists of two physically distinct phenomena, which we dub "gravitino mediation" and "Kahler mediation". Gravitino mediation arises from minimally uplifting SUSY anti-de Sitter (AdS) space to Minkowski space, generating soft masses proportional to the gravitino mass. Kahler mediation arises when visible sector fields have linear couplings to SUSY breaking in the Kahler potential, generating soft masses proportional to beta function coefficients. In the literature, these two phenomena are lumped together under the name "anomaly mediation", but here we demonstrate that they can be physically disentangled by measuring associated couplings to the goldstino. In particular, we use the example of gaugino soft masses to show that gravitino mediation generates soft masses without corresponding goldstino couplings. This result naively violates the goldstino equivalence theorem but is in fact necessary for supercurrent conservation in AdS space. Since gravitino mediation persists even when the visible sector is sequestered from SUSY breaking, we can use the absence of goldstino couplings as an unambiguous definition of sequestering.