Gonzalo Alonso-Álvarez

Bio: Gonzalo Alonso-Álvarez is an academic researcher from McGill University. The author has contributed to research in topics: Dark matter & Sterile neutrino. The author has an hindex of 3, co-authored 6 publications receiving 17 citations. Previous affiliations of Gonzalo Alonso-Álvarez include Heidelberg University.

Unleashing the full power of LHCb to probe Stealth New Physics

Abstract: In this paper, we describe the potential of the LHCb experiment to detect Stealth physics. This refers to dynamics beyond the Standard Model that would elude searches that focus on energetic objects or precision measurements of known processes. Stealth signatures include long-lived particles and light resonances that are produced very rarely or together with overwhelming backgrounds. We will discuss why LHCb is equipped to discover this kind of physics at the Large Hadron Collider and provide examples of well-motivated theoretical models that can be probed with great detail at the experiment.

Sterile neutrino dark matter catalyzed by a very light dark photon

Abstract: Sterile neutrinos ($ u_s$) that mix with active neutrinos ($ u_a$) are interesting dark matter candidates with a rich cosmological and astrophysical phenomenology. In their simplest incarnation, their production is severely constrained by a combination of structure formation observations and X-ray searches. We show that if active neutrinos couple to an oscillating condensate of a very light $L_{\mu}-L_{\tau}$ gauge field, resonant $ u_a$-$ u_s$ oscillations can occur in the early universe, consistent with $ u_s$ constituting all of the dark matter, while respecting X-ray constraints on $ u_s\to u_a\gamma$ decays. Interesting deviations from standard solar and atmospheric neutrino oscillations can persist to the present.

Collider signals of baryogenesis and dark matter from B mesons: A roadmap to discovery

Abstract: Low-scale baryogenesis could be discovered at $B$-factories and the LHC. In the $B$-Mesogenesis paradigm, the CP violating oscillations and subsequent decays of $B$ mesons in the early Universe simultaneously explain the origin of the baryonic and the dark matter of the Universe. This mechanism for baryo- and dark matter-genesis from $B$ mesons gives rise to distinctive signals at collider experiments, which we scrutinize in this paper. We study CP violating observables in the $B^0_q-\bar{B}_q^0$ system, discuss current and expected sensitivities for the exotic decays of $B$ mesons into a visible baryon and missing energy, and explore the implications of direct searches for a TeV-scale colored scalar at the LHC and in meson-mixing observables. Remarkably, we conclude that a combination of measurements at BaBar, Belle, Belle II, LHCb, ATLAS and CMS can fully test $B$-Mesogenesis.

Leading Logs in QCD Axion Effective Field Theory

Abstract: The axion is much lighter than all other degrees of freedom introduced by the Peccei-Quinn mechanism to solve the strong CP problem. It is therefore natural to use an effective field theory (EFT) to describe its interactions. Loop processes calculated in the EFT may however explicitly depend on the ultraviolet cutoff. In general, the UV cutoff is not uniquely defined, but the dimensionful couplings suggest to identify it with the Peccei-Quinn symmetry-breaking scale. An example are K+ → π+ + a decays that will soon be tested to improved precision in NA62 and KOTO and whose amplitude is dominated by the term logarithmically dependent on the cutoff. In this paper, we critically examine the adequacy of using such a naive EFT approach to study loop processes by comparing EFT calculations with ones performed in complete QCD axion models. In DFSZ models, for example, the cutoff is found to be set by additional Higgs degrees of freedom and to therefore be much closer to the electroweak scale than to the Peccei-Quinn scale. In fact, there are non-trivial requirements on axion models where the cutoff scale of loop processes is close to the Peccei-Quinn scale, such that the naive EFT result is reproduced. This suggests that the existence of a suitable UV embedding may impose restrictions on axion EFTs. We provide an explicit construction of a model with suitable fermion couplings and find promising prospects for NA62 and IAXO.

Leading Logs in QCD Axion Effective Field Theory

Abstract: The axion is much lighter than all other degrees of freedom introduced by the Peccei-Quinn mechanism to solve the strong CP problem. It is therefore natural to use an effective field theory (EFT) to describe its interactions. Loop processes calculated in the EFT may, however, explicitly depend on the ultraviolet cutoff. In general the UV cutoff is not uniquely defined, but the dimensionful couplings suggest to identify it with the Peccei-Quinn symmetry-breaking scale. An example are $K \rightarrow \pi + a$ decays that will soon be tested to improved precision in NA62 and KOTO and whose amplitude is dominated by the term logarithmically dependent on the cutoff. In this paper, we critically examine the adequacy of using such a naive EFT approach to study loop processes by comparing EFT calculations with ones performed in complete QCD axion models. In DFSZ models, for example, the cutoff is found to be set by additional Higgs degrees of freedom and to therefore be much closer to the electroweak scale than to the Peccei-Quinn scale. In fact, there are non-trivial requirements on axion models where the cutoff scale of loop processes is close to the Peccei-Quinn scale, such that the naive EFT result is reproduced. This suggests that the existence of a suitable UV embedding may impose restrictions on axion EFTs. We provide an explicit construction of a model with suitable fermion couplings and find promising prospects for NA62 and IAXO.

New physics searches at kaon and hyperon factories

Abstract: Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

Flavor probes of axion-like particles.

Abstract: Axions and axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model (SM). We investigate the phenomenology of an ALP with flavor-changing couplings, and present a comprehensive analysis of quark and lepton flavor-changing observables within a general ALP effective field theory. Observables studied include rare meson decays, flavor oscillations of neutral mesons, rare lepton decays, and dipole moments. We derive bounds on the general ALP couplings as a function of its mass, consistently taking into account the ALP lifetime and branching ratios. We further calculate quark flavor-changing effects that are unavoidably induced by running and matching between the new physics scale and the scale of the measurements. This allows us to derive bounds on benchmark ALP models in which only a single (flavorless or flavor-universal) ALP coupling to SM particles is present at the new physics scale, and in this context we highlight the complementarity and competitiveness of flavor bounds with constraints derived from collider, beam dump and astrophysical measurements. We find that searches for ALPs produced in meson decays provide some of the strongest constraints in the MeV-GeV mass range, even for the most flavorless of ALP models. Likewise, we discuss the interplay of flavor-conserving and flavor-violating couplings of the ALP to leptons, finding that constraints from lepton flavor-violating observables generally depend strongly on both. Additionally, we analyze whether an ALP can provide an explanation for various experimental anomalies including those observed in rare B-meson decays, measurements at the ATOMKI and KTeV experiments, and in the anomalous magnetic moments of the muon and the electron.

Flavor probes of axion-like particles

Abstract: Axions and axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model (SM). We investigate the phenomenology of an ALP with flavor-changing couplings, and present a comprehensive analysis of quark and lepton flavor-changing observables within a general ALP effective field theory. Observables studied include rare meson decays, flavor oscillations of neutral mesons, rare lepton decays, and dipole moments. We derive bounds on the general ALP couplings as a function of its mass, consistently taking into account the ALP lifetime and branching ratios. We further calculate quark flavor-changing effects that are unavoidably induced by running and matching between the new physics scale and the scale of the measurements. This allows us to derive bounds on benchmark ALP models in which only a single (flavorless or flavor-universal) ALP coupling to SM particles is present at the new physics scale, and in this context we highlight the complementarity and competitiveness of flavor bounds with constraints derived from collider, beam dump and astrophysical measurements. We find that searches for ALPs produced in meson decays provide some of the strongest constraints in the MeV-GeV mass range, even for the most flavorless of ALP models. Likewise, we discuss the interplay of flavor-conserving and flavor-violating couplings of the ALP to leptons, finding that constraints from lepton flavor-violating observables generally depend strongly on both. Additionally, we analyze whether an ALP can provide an explanation for various experimental anomalies including those observed in rare B-meson decays, measurements at the ATOMKI and KTeV experiments, and in the anomalous magnetic moments of the muon and the electron.

Two Sides of the Same Coin: Sterile Neutrinos and Dark Radiation, Status and Perspectives

Abstract: The presence of light sterile neutrinos is one of the unanswered questions of particle physics. The cosmological counterpart is represented by dark radiation, i.e., any form of radiation present in the early Universe besides photons and standard (active) neutrinos. This short review provides a comprehensive overview of the two problems and of their connection. We review the status of neutrino oscillation anomalies, commenting on the most recent oscillation data and their mutual tensions, and we discuss the constraints from other terrestrial probes. We show the shortcomings of translating light sterile neutrinos in cosmology as additional thermalised relativistic species, produced by neutrino oscillations, and we detail alternative solutions, specifically focusing on neutrino nonstandard interactions, and on their link to the Hubble constant problem. The impact of a new force leading to dark radiation–dark matter interactions is also discussed in the realm of new physics in the dark sector.