Vassilis C. Spanos
Current Organizations / Institutes
The organizations/institutes from the scholar's latest article
Previous Organizations / Institutes
The organizations / institutes from the scholar's other articles in the past ten years
Research Subjects
Research Interests
Vassilis C. Spanos has a total of 59 co-authors. Here are the top co-authors by number of shared publications:
- Keith A. Olive
William I. Fine Theoretical Physics Institute, School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
20 shared publications John Ellis
Theoretical Particle Physics and Cosmology Group, Department of Physics, King’s College London, London, UK
12 shared publications- John Ellis
Department of Physics, King’s College London, London WC2R 2LS, United Kingdom
10 shared publications A.B. Lahanas
National and Kapodistrian University of Athens
8 shared publications- Helmut Eberl
Institut für Hochenergiephysik
7 shared publications Yudi Santoso
Department of Physics and Astronomy, University of Victoria, Victoria, BC, V8P 1A1, Canada
7 shared publications
Latest Publications
- 6 June 2025
- Published by Springer Nature
- p. 179-189
Ultraviolet Freeze-in Baryogenesis Scenarios
- 1 January 2025
- Published by IOP Publishing in Journal of Cosmology and Astroparticle Physics
- Vol. 2025 (01)
Gravitino thermal production, dark matter, and reheating of the Universe
We present a full one-loop calculation of the gravitino thermal production rate, beyond the so-called hard thermal loop approximation, using the corresponding thermal spectral functions in numerical form on both sides of the light cone. This framework requires a full numerical evaluation. We interpret our results within the framework of a general supergravity-based model, remaining agnostic about the specifics of supersymmetry breaking. In this context, assuming that gravitinos constitute the entirety of the dark matter in the Universe imposes strict constraints on the reheating temperature. For example, with a gluino mass at the current LHC limit, a maximum reheating temperature of T reh ≃ 109 GeV is compatible with a gravitino mass of m 3/2 ≃ 1 TeV. Additionally, with a reheating temperature an order of magnitude lower at T reh ≃ 108 GeV, the common gaugino mass M 1/2 can range from 2 to 4 TeV within the same gravitino mass range. For much higher values of M 1/2, which are favored by current accelerator and cosmological data in the context of supersymmetric models, such as M 1/2 = 10 TeV, and for m 3/2 ≃ 1 TeV the reheating temperature compatible with the gravitino dark matter scenario is 107 GeV. If other dark matter particles are considered, the reheating temperature could be much lower.
- 1 January 2025
- Published by IOP Publishing in Journal of Cosmology and Astroparticle Physics
- Vol. 2025 (01)
Gravitino thermal production, dark matter, and reheating of the Universe
We present a full one-loop calculation of the gravitino thermal production rate, beyond the so-called hard thermal loop approximation, using the corresponding thermal spectral functions in numerical form on both sides of the light cone. This framework requires a full numerical evaluation. We interpret our results within the framework of a general supergravity-based model, remaining agnostic about the specifics of supersymmetry breaking. In this context, assuming that gravitinos constitute the entirety of the dark matter in the Universe imposes strict constraints on the reheating temperature. For example, with a gluino mass at the current LHC limit, a maximum reheating temperature of T reh ≃ 109 GeV is compatible with a gravitino mass of m 3/2 ≃ 1 TeV. Additionally, with a reheating temperature an order of magnitude lower at T reh ≃ 108 GeV, the common gaugino mass M 1/2 can range from 2 to 4 TeV within the same gravitino mass range. For much higher values of M 1/2, which are favored by current accelerator and cosmological data in the context of supersymmetric models, such as M 1/2 = 10 TeV, and for m 3/2 ≃ 1 TeV the reheating temperature compatible with the gravitino dark matter scenario is 107 GeV. If other dark matter particles are considered, the reheating temperature could be much lower.
- Open Access
- 28 October 2024
- Published by Springer Nature in The European Physical Journal C
- Vol. 84 (10) , 1-22
Non-universal SUSY models, $g_\mu -2$, $m_H$ and dark matterWe study the anomalous magnetic moment of the muon, $$g_\mu - 2 \equiv 2 a_\mu $$ , in the context of supersymmetric models beyond the CMSSM, where the unification of either the gaugino masses $$M_{1,2,3}$$ or sfermion and Higgs masses is relaxed, taking into account the measured mass of the Higgs boson, $$m_H$$ , the cosmological dark matter density and the direct detection rate. We find that the model with non-unified gaugino masses can make a contribution $$\Delta a_\mu \sim 20 \times 10^{-10}$$ to the anomalous magnetic moment of the muon, for example if $$M_{1,2} \sim 600\; \textrm{GeV}$$ and $$M_3\sim 8 \; \textrm{TeV}$$ . The model with non-universal sfermion and Higgs masses can provide even larger $$\Delta a_\mu \sim 24 \times 10^{-10}$$ if the sfermion masses for the first and the second generations are $$ \sim 400 \; \textrm{GeV}$$ and that of the third is $$ \sim 8 \; \textrm{TeV}$$ . We discuss the prospects for collider searches for supersymmetric particles in specific benchmark scenarios illustrating these possibilities, focusing in particular on the prospects for detecting the lighter smuon and the lightest neutralino.- 28 October 2024
- Published by Springer Nature in The European Physical Journal C
- Vol. 84 (10) , 1-22
Open AccessNon-universal SUSY models, $g_\mu -2$, $m_H$ and dark matterWe study the anomalous magnetic moment of the muon, $$g_\mu - 2 \equiv 2 a_\mu $$ , in the context of supersymmetric models beyond the CMSSM, where the unification of either the gaugino masses $$M_{1,2,3}$$ or sfermion and Higgs masses is relaxed, taking into account the measured mass of the Higgs boson, $$m_H$$ , the cosmological dark matter density and the direct detection rate. We find that the model with non-unified gaugino masses can make a contribution $$\Delta a_\mu \sim 20 \times 10^{-10}$$ to the anomalous magnetic moment of the muon, for example if $$M_{1,2} \sim 600\; \textrm{GeV}$$ and $$M_3\sim 8 \; \textrm{TeV}$$ . The model with non-universal sfermion and Higgs masses can provide even larger $$\Delta a_\mu \sim 24 \times 10^{-10}$$ if the sfermion masses for the first and the second generations are $$ \sim 400 \; \textrm{GeV}$$ and that of the third is $$ \sim 8 \; \textrm{TeV}$$ . We discuss the prospects for collider searches for supersymmetric particles in specific benchmark scenarios illustrating these possibilities, focusing in particular on the prospects for detecting the lighter smuon and the lightest neutralino. - Open Access
- 29 September 2023
- Published by American Physical Society (APS) in Physical Review D
- Vol. 108 (5) , 055038
Freeze-in baryogenesis and early matter domination
The freeze-in mechanism has been shown to allow the simultaneous generation of cosmic dark matter and a viable matter-antimatter asymmetry in the Universe. When the underlying interactions are described by higher-dimensional, nonrenormalizable operators, the relevant freeze-in processes take place close to the highest considered cosmic temperatures. In this paper we study how the presence of a fluid that temporarily dominates the energy content of the early Universe affects the predictions of this “ultraviolet freeze-in baryogenesis” scenario. We find that this additional cosmic component has a significant impact on the predictions of concrete microscopic models, allowing for reheating temperatures which are much lower than those required in the simplest cosmological scenario. Moreover, we show that inflationary observables can constrain the parameter space of such models, once the latter are examined in conjunction with concrete models of inflation.
- 29 September 2023
- Published by American Physical Society (APS) in Physical Review D
- Vol. 108 (5) , 055038
Open AccessFreeze-in baryogenesis and early matter domination
The freeze-in mechanism has been shown to allow the simultaneous generation of cosmic dark matter and a viable matter-antimatter asymmetry in the Universe. When the underlying interactions are described by higher-dimensional, nonrenormalizable operators, the relevant freeze-in processes take place close to the highest considered cosmic temperatures. In this paper we study how the presence of a fluid that temporarily dominates the energy content of the early Universe affects the predictions of this “ultraviolet freeze-in baryogenesis” scenario. We find that this additional cosmic component has a significant impact on the predictions of concrete microscopic models, allowing for reheating temperatures which are much lower than those required in the simplest cosmological scenario. Moreover, we show that inflationary observables can constrain the parameter space of such models, once the latter are examined in conjunction with concrete models of inflation.
- Open Access
Freeze-in baryogenesis and early matter domination
The freeze-in mechanism has been shown to allow the simultaneous generation of cosmic dark matter and a viable matter-antimatter asymmetry in the universe. When the underlying interactions are described by higher-dimensional, non-renormalizable operators, the relevant freeze-in processes take place close to the highest considered cosmic temperatures. In this paper we study how the presence of a fluid that temporarily dominates the energy content of the early universe affects the predictions of this ``Ultraviolet Freeze-In Baryogenesis'' scenario. We find that this additional cosmic component has a significant impact on the predictions of concrete microscopic models, allowing for reheating temperatures which are much lower than those required in the simplest cosmological scenario. Moreover, we show that inflationary observables can constrain the parameter space of such models, once the latter are examined in conjunction with concrete models of inflation.
Open AccessFreeze-in baryogenesis and early matter domination
The freeze-in mechanism has been shown to allow the simultaneous generation of cosmic dark matter and a viable matter-antimatter asymmetry in the universe. When the underlying interactions are described by higher-dimensional, non-renormalizable operators, the relevant freeze-in processes take place close to the highest considered cosmic temperatures. In this paper we study how the presence of a fluid that temporarily dominates the energy content of the early universe affects the predictions of this ``Ultraviolet Freeze-In Baryogenesis'' scenario. We find that this additional cosmic component has a significant impact on the predictions of concrete microscopic models, allowing for reheating temperatures which are much lower than those required in the simplest cosmological scenario. Moreover, we show that inflationary observables can constrain the parameter space of such models, once the latter are examined in conjunction with concrete models of inflation.
- Open Access
- 23 March 2023
- Published by Springer Nature in The European Physical Journal C
- Vol. 83 (3) , 1-31
The CMSSM survives Planck, the LHC, LUX-ZEPLIN, Fermi-LAT, H.E.S.S. and IceCube
We revisit the viability of the CMSSM, searching for regions of parameter space that yield a neutralino dark matter density compatible with Planck measurements, as well as LHC constraints including sparticle searches and the mass of the Higgs boson, recent direct limits on spin-independent and -dependent dark matter scattering from the LUX-ZEPLIN (LZ) experiment, the indirect constraints from Fermi-LAT and H.E.S.S. on dark matter annihilations to photons in dwarf spheroidal galaxies and the Galactic Centre, and the IceCube limits on muons from annihilations to neutrinos in the Sun. For representative values of $$\tan \beta $$ and $$A_0$$ we map in detail the Planck-compatible strips in CMSSM parameter planes, which exhibit multiple distinctive features for large $$\tan \beta $$ , $$A_0 = 0$$ and $$\mu > 0$$ , and identify portions of the strips that survive all the phenomenological constraints. We find that the most powerful constraint is that from $$m_h$$ , followed by the LZ limit on spin-independent scattering, whereas sparticle searches at the LHC and indirect dark matter searches are less restrictive. Most of the surviving CMSSM parameter space features a Higgsino-like dark matter particle with a mass $$\sim 1000{-}1100$$ GeV, which could best be probed with future direct searches for dark matter scattering.- 23 March 2023
- Published by Springer Nature in The European Physical Journal C
- Vol. 83 (3) , 1-31
Open AccessThe CMSSM survives Planck, the LHC, LUX-ZEPLIN, Fermi-LAT, H.E.S.S. and IceCube
We revisit the viability of the CMSSM, searching for regions of parameter space that yield a neutralino dark matter density compatible with Planck measurements, as well as LHC constraints including sparticle searches and the mass of the Higgs boson, recent direct limits on spin-independent and -dependent dark matter scattering from the LUX-ZEPLIN (LZ) experiment, the indirect constraints from Fermi-LAT and H.E.S.S. on dark matter annihilations to photons in dwarf spheroidal galaxies and the Galactic Centre, and the IceCube limits on muons from annihilations to neutrinos in the Sun. For representative values of $$\tan \beta $$ and $$A_0$$ we map in detail the Planck-compatible strips in CMSSM parameter planes, which exhibit multiple distinctive features for large $$\tan \beta $$ , $$A_0 = 0$$ and $$\mu > 0$$ , and identify portions of the strips that survive all the phenomenological constraints. We find that the most powerful constraint is that from $$m_h$$ , followed by the LZ limit on spin-independent scattering, whereas sparticle searches at the LHC and indirect dark matter searches are less restrictive. Most of the surviving CMSSM parameter space features a Higgsino-like dark matter particle with a mass $$\sim 1000{-}1100$$ GeV, which could best be probed with future direct searches for dark matter scattering. - Open Access
- 4 January 2023
- Published by Springer Nature in The European Physical Journal C
- Vol. 83 (1) , 1-17
Gravitational waves from no-scale supergravity
In this paper we study four concrete models, based on no-scale supergravity with SU(2,1)/SU(2) $$\times $$ U(1) symmetry. We modify either the Kähler potential or the superpotential, which are related to the no-scale theory with this symmetry. In this scenario, the induced Gravitational Waves, are calculated to be detectable by the future space-based observations such as LISA, BBO and DECIGO. The models under study are interrelated, as they all yield the Starobinsky effective-like scalar potential in the unmodified case. We evaluate numerically the scalar power spectrum and the stochastic background of the Gravitational Waves, satisfying the observational Planck cosmological constraints for inflation.- 4 January 2023
- Published by Springer Nature in The European Physical Journal C
- Vol. 83 (1) , 1-17
Open AccessGravitational waves from no-scale supergravity
In this paper we study four concrete models, based on no-scale supergravity with SU(2,1)/SU(2) $$\times $$ U(1) symmetry. We modify either the Kähler potential or the superpotential, which are related to the no-scale theory with this symmetry. In this scenario, the induced Gravitational Waves, are calculated to be detectable by the future space-based observations such as LISA, BBO and DECIGO. The models under study are interrelated, as they all yield the Starobinsky effective-like scalar potential in the unmodified case. We evaluate numerically the scalar power spectrum and the stochastic background of the Gravitational Waves, satisfying the observational Planck cosmological constraints for inflation. - Open Access
- 26 September 2022
- Published by American Physical Society (APS) in Physical Review D
- Vol. 106 (6) , 063532
Primordial black holes and gravitational waves in multiaxion-Chern-Simons inflation
We study aspects of inflation and the possibility of enhanced production of primordial black holes (PBHs) and gravitational waves (GWs) in a string-inspired model of two axion fields coupled to Chern-Simons gravity, which results in a running-vacuum-model inflation. Fluctuations of the scale-invariant spectrum, consistent with the cosmological data, are provided in this model by world-sheet (nonperturbative) instanton terms of the axion field arising from string compactification. As a result of such modulations, there is an enhanced production of PBHs and GWs in such cosmologies, which may lead to observable in principle patterns in the profile of GWs during the radiation era. Moreover, we demonstrate that the PBHs may provide a significant amount of dark matter in this Universe. For comparison, we also discuss a two-stage inflation cosmological model of conventional string-inspired axion monodromy, involving again two axion fields. The resulting modifications imprinted on the GWs spectra between these two classes of models are distinct, and can, in principle, be distinguished by future interferometers. We consider models with more or less instantaneous reheating. We also make some remarks on the effects of a prolonged reheating period in leading to further enhancement of the power spectrum and thus fractions of PBHs that play the role of dark matter.
- 26 September 2022
- Published by American Physical Society (APS) in Physical Review D
- Vol. 106 (6) , 063532
Open AccessPrimordial black holes and gravitational waves in multiaxion-Chern-Simons inflation
We study aspects of inflation and the possibility of enhanced production of primordial black holes (PBHs) and gravitational waves (GWs) in a string-inspired model of two axion fields coupled to Chern-Simons gravity, which results in a running-vacuum-model inflation. Fluctuations of the scale-invariant spectrum, consistent with the cosmological data, are provided in this model by world-sheet (nonperturbative) instanton terms of the axion field arising from string compactification. As a result of such modulations, there is an enhanced production of PBHs and GWs in such cosmologies, which may lead to observable in principle patterns in the profile of GWs during the radiation era. Moreover, we demonstrate that the PBHs may provide a significant amount of dark matter in this Universe. For comparison, we also discuss a two-stage inflation cosmological model of conventional string-inspired axion monodromy, involving again two axion fields. The resulting modifications imprinted on the GWs spectra between these two classes of models are distinct, and can, in principle, be distinguished by future interferometers. We consider models with more or less instantaneous reheating. We also make some remarks on the effects of a prolonged reheating period in leading to further enhancement of the power spectrum and thus fractions of PBHs that play the role of dark matter.
- Open Access
Ultraviolet freeze-in baryogenesis
We study a mechanism through which the cosmic dark matter density can be explained simultaneously with the observed baryon asymmetry of the Universe. At the core of our proposal lie the out-of-equilibrium scattering processes of bath particles which are responsible for the production of feebly-interacting dark matter. The same processes violate $CP$, which further leads to an asymmetry between matter and antimatter being generated in the visible sector. We focus on the possibility that these interactions are described through non-renormalizable operators, which leads to both dark matter and the baryon asymmetry being produced at high temperatures. The mechanism is exemplified by studying two concrete scenarios, one involving scalar and one involving fermion dark matter. We find that in both cases it is, indeed, possible to achieve a common explanation for the dark matter content and the matter-antimatter asymmetry of the Universe, provided that dark matter is in the keV mass range.Open AccessUltraviolet freeze-in baryogenesis
We study a mechanism through which the cosmic dark matter density can be explained simultaneously with the observed baryon asymmetry of the Universe. At the core of our proposal lie the out-of-equilibrium scattering processes of bath particles which are responsible for the production of feebly-interacting dark matter. The same processes violate $CP$, which further leads to an asymmetry between matter and antimatter being generated in the visible sector. We focus on the possibility that these interactions are described through non-renormalizable operators, which leads to both dark matter and the baryon asymmetry being produced at high temperatures. The mechanism is exemplified by studying two concrete scenarios, one involving scalar and one involving fermion dark matter. We find that in both cases it is, indeed, possible to achieve a common explanation for the dark matter content and the matter-antimatter asymmetry of the Universe, provided that dark matter is in the keV mass range. - Open Access
- 21 July 2022
- Published by American Physical Society (APS) in Physical Review D
- Vol. 106 (2) , 023515
Ultraviolet freeze-in baryogenesis
We study a mechanism through which the cosmic dark matter density can be explained simultaneously with the observed baryon asymmetry of the Universe. At the core of our proposal lie the out-of-equilibrium scattering processes of bath particles which are responsible for the production of feebly-interacting dark matter. The same processes violate , which further leads to an asymmetry between matter and antimatter being generated in the visible sector. We focus on the possibility that these interactions are described through nonrenormalizable operators, which leads to both dark matter and the baryon asymmetry being produced at high temperatures. The mechanism is exemplified by studying two concrete scenarios, one involving scalar and one involving fermion dark matter. We find that in both cases it is, indeed, possible to achieve a common explanation for the dark matter content and the matter-antimatter asymmetry of the Universe, provided that dark matter is in the keV mass range.- 21 July 2022
- Published by American Physical Society (APS) in Physical Review D
- Vol. 106 (2) , 023515
Open AccessUltraviolet freeze-in baryogenesis
We study a mechanism through which the cosmic dark matter density can be explained simultaneously with the observed baryon asymmetry of the Universe. At the core of our proposal lie the out-of-equilibrium scattering processes of bath particles which are responsible for the production of feebly-interacting dark matter. The same processes violate , which further leads to an asymmetry between matter and antimatter being generated in the visible sector. We focus on the possibility that these interactions are described through nonrenormalizable operators, which leads to both dark matter and the baryon asymmetry being produced at high temperatures. The mechanism is exemplified by studying two concrete scenarios, one involving scalar and one involving fermion dark matter. We find that in both cases it is, indeed, possible to achieve a common explanation for the dark matter content and the matter-antimatter asymmetry of the Universe, provided that dark matter is in the keV mass range.