Dimitrios Karamitros

Bio: Dimitrios Karamitros is an academic researcher from University of Manchester. The author has contributed to research in topics: Dark matter & Higgs boson. The author has an hindex of 3, co-authored 12 publications receiving 52 citations.

Neutrino dark matter and the Higgs portal: improved freeze-in analysis

Abstract: Sterile neutrinos are one of the leading dark matter candidates. Their masses may originate from a vacuum expectation value of a scalar field. If the sterile neutrino couplings are very small and their direct coupling to the inflaton is forbidden by the lepton number symmetry, the leading dark matter production mechanism is the freeze-in scenario. We study this possibility in the neutrino mass range up to 1 GeV, taking into account relativistic production rates based on the Bose-Einstein statistics, thermal masses and phase transition effects. The specifics of the production mechanism and the dominant mode depend on the relation between the scalar and sterile neutrino masses as well as on whether or not the scalar is thermalized. We find that the observed dark matter abundance can be produced in all of the cases considered. We also revisit the freeze-in production of a Higgs portal scalar, pointing out the importance of a fusion mode, as well as the thermalization constraints.

Forbidden frozen-in dark matter

Abstract: We examine and point out the importance of a regime of dark matter pro- duction through the freeze-in mechanism that results from a large thermal correction to a decaying mediator particle mass from hot plasma in the early Universe. We show that mediator decays to dark matter that are kinematically forbidden at the usually considered ranges of low temperatures can be generically present at higher temperatures and actually dominate the overall dark matter production, thus leading to very distinct solutions from the standard case. We illustrate these features by considering a dark Higgs portal model where dark matter is produced via decays of a scalar field with a large thermal mass. We identify the resulting ranges of parameters that are consistent with the correct dark matter relic abundance and further apply current and expected future collider, cosmological, and astrophysical limits.

New opportunities for axion dark matter searches in nonstandard cosmological models

Abstract: We study axion dark matter production from a misalignment mechanism in scenarios featuring a general nonstandard cosmology. Before the onset of Big Bang nucleosynthesis, the energy density of the universe is dominated by a particle field $\phi$ described by a general equation of state $\omega$. The ensuing enhancement of the Hubble expansion rate decreases the temperature at which axions start to oscillate, opening this way the possibility for axions heavier than in the standard window. This is the case for kination, or in general for scenarios with $\omega > 1/3$. However, if $\omega < 1/3$, as in the case of an early matter domination, the decay of $\phi$ injects additional entropy relative to the case of the standard model, diluting this way the preexisting axion abundance, and rendering lighter axions viable. For a misalignment angle $0.5 < \theta_i < \pi/\sqrt{3}$, the usual axion window becomes expanded to $4 \times 10^{-9}$ eV $\lesssim m_a \lesssim 2 \times 10^{-5}$ eV for the case of an early matter domination, or to $2 \times 10^{-6}$ eV $\lesssim m_a \lesssim 10^{-2}$ eV for the case of kination. Interestingly, the coupling axion-photon in such a wider range can be probed with next generation experiments such as ABRACADABRA, KLASH, ADMX, MADMAX, and ORGAN. Axion dark matter searches may therefore provide a unique tool to probe the history of the universe before Big Bang nucleosynthesis.

Forbidden frozen-in dark matter.

Abstract: We examine and point out the importance of a regime of dark matter production through the freeze-in mechanism that results from a large thermal correction to a decaying mediator particle mass from hot plasma in the early Universe. We show that mediator decays to dark matter that are kinematically forbidden at the usually considered ranges of low temperatures can be generically present at higher temperatures and actually dominate the overall dark matter production, thus leading to very distinct solutions from the standard case. We illustrate these features by considering a dark Higgs portal model where dark matter is produced via decays of a scalar field with a large thermal mass. We identify the resulting ranges of parameters that are consistent with the correct dark matter relic abundance and further apply current and expected future collider, cosmological, and astrophysical limits.

Frozen-in fermionic singlet dark matter in non-standard cosmology with a decaying fluid

Abstract: We perform a detailed study of dark matter production via freeze-in under the assumption that some fluid dominates the early Universe before depositing its energy to the plasma causing entropy injection. As a dark matter candidate we consider a fermionic singlet that is produced through its interactions with a scalar particle in the thermal plasma. The fluid alters the expansion rate of the Universe, as well as the scaling of the temperature, which significantly affects the evolution of both the number density and the mean momentum of the dark matter particle. We identify and discuss in detail the effects of the evolution of these quantities by considering several examples representing dark matter production at different stages of expansion and entropy injection. We find that, since the dark matter density is reduced when the entropy injection to the plasma continues after freeze-in, in order to reproduce its observational value an enhanced rate of dark matter production is required relative to standard cosmology. Furthermore, the impact of the assumed non-standard cosmological history on the dark matter mean momentum can result in either a relaxed or a tightened bound on the dark matter mass from large structure formation data.

The Forward Physics Facility at the High-Luminosity LHC

Abstract: High energy collisions at the High-Luminosity Large Hadron Collider (LHC) produce a large number of particles along the beam collision axis, outside of the acceptance of existing LHC experiments. The proposed Forward Physics Facility (FPF), to be located several hundred meters from the ATLAS interaction point and shielded by concrete and rock, will host a suite of experiments to probe standard model (SM) processes and search for physics beyond the standard model (BSM). In this report, we review the status of the civil engineering plans and the experiments to explore the diverse physics signals that can be uniquely probed in the forward region. FPF experiments will be sensitive to a broad range of BSM physics through searches for new particle scattering or decay signatures and deviations from SM expectations in high statistics analyses with TeV neutrinos in this low-background environment. High statistics neutrino detection will also provide valuable data for fundamental topics in perturbative and non-perturbative QCD and in weak interactions. Experiments at the FPF will enable synergies between forward particle production at the LHC and astroparticle physics to be exploited. We report here on these physics topics, on infrastructure, detector, and simulation studies, and on future directions to realize the FPF’s physics potential.

Minimal spontaneously broken hidden sector and its impact on Higgs boson physics at the Large Hadron Collider

Abstract: We have studied a hidden sector of the SM with spontaneous symmetry breaking that opens many different scenarios for Higgs physics. We have shown that this hidden sector can affect the SM Higgs detection. In some specific regimes it is still possible to detect the Higgs; in other scenarios the hidden sector would completely eclipse it. We have performed a study based on the paper by R. Schabinger and J. D. Wells (Physical Review, D72 (2005), p. 093007). We consider a hidden gauge U(1) symmetry, meaning that this sector does not mix with the usual gauge groups of the Standard Model, except, maybe, with the Higgs sector. The Lagrangian under consideration for this case is LHiggs = |DμH| + |DμΦ| +mH |H| +mΦ|Φ| − λ|H| − ρ|Φ| + η|H||Φ| . (1) We are interested in the spontaneous symmetry breaking scenario. Therefore, we write these fields as H = 1 √ 2 ( h+ v + iG0 G± ) , Φ = 1 √ 2 (φ+ ξ + iG′) (2) where v(' 246 GeV) and ξ are vacuum expectation values; H and Φ are the physical fields. The G fields are Goldstone bosons absorbed by the vector bosons. By just replacing Eq. (2) into Eq. (1) we arrive at the following Lagrangian. − 4 h − ρ 4 Φ − λvh − ρψΦ + [ − 2 λv + mH 2 + ηψ2 4 ] h + [ − 2 ρψ + mΦ 2 + ηv2 4 ] Φ + [ mHv − λv + η 2 vψ ] h+ [ mΦψ − ρψ + η 2 ψv ] Φ + constants . (3) By looking at the {h, φ} sector, one can write the mass matrix that has to be diagonalized in order to obtain the mass eigenstates. This matrix is given by M = ( 2λv2 ηvξ ηvξ 2ρξ2 ) (4) and is diagonalized by the mixing angle tanω = ηvξ (ρξ2 − λv2) + √ (ρξ2 − λv2)2 + η2v2ξ2 (5) with h = cosω s1 + sinω s2 (6) *Work performed as a student project under the supervision of A. Zerwekh.

Dark Matter production from relativistic bubble walls

Abstract: In this paper we present a novel mechanism for producing the observed Dark Matter (DM) relic abundance during the First Order Phase Transition (FOPT) in the early universe. We show that the bubble expansion with ultra-relativistic velocities can lead to the abundance of DM particles with masses much larger than the scale of the transition. We study this non-thermal production mechanism in the context of a generic phase transition and the electroweak phase transition. The application of the mechanism to the Higgs portal DM as well as the signal in the Stochastic Gravitational Background are discussed.

The Dark Light

Abstract: Book synopsis: Rebekah has lived on the island her whole life, and it's only now that she's starting to wonder what she might experience outside her strict religious community. Alex has been sent to the island to escape her dark past, and through her eyes it's a dark and sinister place. Thrown together by chance, Rebekah and Alex strike up an unlikely friendship and it's together that they attempt to break free of their worlds and make a world of their own. But when a kiss between the girls is witnessed by an islander there is no escape they can make - the Rapture is coming for them all.

Extending the reach of FASER, MATHUSLA, and SHiP towards smaller lifetimes using secondary particle production

Abstract: Many existing or proposed intensity-frontier search experiments look for decay signatures of light long-lived particles (LLPs), highly displaced from the interaction point, in a distant detector that is well-shielded from SM background. This approach is, however, limited to new particles with decay lengths similar to or larger than the baseline of those experiments. In this study, we discuss how this basic constraint can be overcome in BSM models that go beyond the simplest scenarios. If more than one light new particle is present in the model, an additional secondary production of LLPs may take place right in front of the detector, opening this way a new lifetime regime to be probed. We illustrate the prospects of such searches in the future experiments FASER, MATHUSLA and SHiP, for representative models, emphasizing possible connections to dark matter or an anomalous magnetic moment of muon. We also analyze additional advantages from employing dedicated neutrino detectors placed in front of the main decay volume.