Miguel Escudero

The vast majority of extensions of the Standard Model affecting the number of effective relativistic neutrino species ( N eff ) do so additively, namely, they enhance this quantity with some light state contributing to dark radiation. In this work, we consider precisely the opposite case: new physics scenarios that can lead to N eff < 3 that are consistent with all known cosmological, astrophysical, and laboratory data. We are motivated by three main reasons: (1) a recent measurement from Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT) in combination with Planck that leads to N eff = 2.81 ± 0.12 , (2) by a new and powerful measurement of the primordial helium abundance, which anchors N eff to be very close to the Standard Model value one second after the big bang, (3) by the deployment of the Simons Observatory which will provide precise tests of the radiation content in the Universe and which may detect with a high significance cosmologies with N eff < 3 . We survey the main theoretical possibilities and find that only a few simple scenarios can consistently give N eff = 2.81 ± 0.12 . One class consists of thermal electrophilic relics with masses m ∼ 8 – 13 MeV . Another consists of out-of-equilibrium particles decaying to e + e − or γ γ , with a rather particular lifetime 0.05 s ≲ τ ≲ 3 min , mass 250 MeV ≲ m ≲ 600 MeV , and abundance ρ / ρ γ ∼ 0.1 at decay. Thermal electrophilic particles are especially interesting because they can account for the dark matter in the Universe and can be tested in experiments such as SENSEI, DAMIC-M, and Oscura, and their portals to the visible sector at experiments such as NA64 and LDMX. We conclude that if the Simons Observatory confirms that N eff ≃ 2.8 , it will point to very specific extensions of the Standard Model.

Cosmological determinations of the number of relativistic neutrino species, N _eff , are becoming increasingly accurate, and further improvements are expected both from CMB and BBN data. Given this context, we update the evaluation of N _eff and the current entropy density via the momentum-averaged approach. This allows for a numerically fast description of neutrino decoupling, easily portable to an array of new physics scenarios. We revisit all aspects of this approach, including collision terms with full electron mass dependence, finite temperature QED corrections to the equation of state, neutrino oscillations, and the modelling of neutrino ensembles with effective chemical potentials. For integrated observables, our results differ by less than 0.04% from the solution of the momentum-dependent evolution equation. We outline how to extend the approach to BSM settings, and will highlight its power in Part II. To facilitate the practical implementation, we release a Mathematica and python code within nudec_BSM_v2 , easily linkable to BBN codes.

At temperatures below the QCD phase transition, any substantial lepton number in the Universe can only be present within the neutrino sector. In this work, we systematically explore the impact of a non-vanishing lepton number on Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB). Relying on our recently developed framework based on momentum averaged quantum kinetic equations for the neutrino density matrix, we solve the full BBN reaction network to obtain the abundances of primordial elements. We find that the maximal primordial total lepton number L allowed by BBN and the CMB is -0.12 (-0.10) ≤ L ≤ 0.13 (0.12) for NH (IH), while specific flavor directions can be even more constrained. This bound is complementary to the limits obtained from avoiding baryon overproduction through sphaleron processes at the electroweak phase transition since, although numerically weaker, it applies at lower temperatures and is obtained completely independently. We publicly release the C++ code COFLASY-C on GitHub ( https://github.com/mariofnavarro/COFLASY/tree/COFLASY-C ) which solves for the evolution of the neutrino quantum kinetic equations numerically.

Cosmological neutrino mass bounds are becoming increasingly stringent. The latest limit within Λ CDM from Planck 2018 + ACT lensing + DESI is ∑ m ν < 0.072 eV at 95% CL, very close to the minimum possible sum of neutrino masses ( ∑ m ν > 0.06 eV ), hinting at vanishing or even “negative” cosmological neutrino masses. In this context, it is urgent to carefully evaluate the origin of these cosmological constraints. In this paper, we investigate the robustness of these results in three ways: (i) we check the role of potential anomalies in Planck CMB and DESI BAO data; (ii) we compare the results for frequentist and Bayesian techniques, as very close to physical boundaries subtleties in the derivation and interpretation of constraints can arise; (iii) we investigate how deviations from Λ CDM , potentially alleviating these anomalies, can alter the constraints. From a profile likelihood analysis, we derive constraints in agreement at the ∼ 10 % level with Bayesian posteriors. We find that the weak preference for negative neutrino masses is mostly present for Planck 18 data, affected by the well-known “lensing anomaly.” It disappears when the new Planck 2020 HiLLiPoP is used, leading to significantly weaker constraints. Additionally, the pull toward negative masses in DESI data stems from the z = 0.7 bin, which contains a BAO measurement in ∼ 3 σ tension with Planck expectations. Without this bin, and in combination with HiLLiPoP, the bound relaxes to ∑ m ν < 0.11 eV at 95% CL. The recent preference for dynamical dark energy alleviates this tension and further weakens the bound. As we are at the dawn of a neutrino mass discovery from cosmology, it will be very exciting to see if this trend is confirmed by future data. Published by the American Physical Society 2024

C P violation in neutral B meson oscillations is an experimental observable that could be directly related to the baryon asymmetry of the Universe through the B -mesogenesis mechanism. As this phenomenon is highly suppressed in the Standard Model, it could also be a sensitive probe for many new physics scenarios that modify neutral meson mixing. Motivated by these facts, and the timely B physics program at the LHC and Belle II, we analyze how large C P violation in the mixing of neutral B d and B s meson systems could be. We answer this question, in light of current experimental data, within three different scenarios, namely: (i) generic heavy new physics only affecting the mass mixing M 12 q , (ii) vectorlike quark extensions that introduce deviations of 3 × 3 CKM unitarity, and (iii) light new physics modifying the decay mixing Γ 12 q . We find that enhancements of the semileptonic asymmetries, that measure the amount of C P violation in mixing, at the level of 10 − 3 for the B d system and 10 − 4 for the B s system can be achieved within scenarios (i) and (ii), while they are much more suppressed in realistic UV completions triggering scenario (iii). With respect to cosmology, the difficulty of finding large C P asymmetries in our analysis puts the B -mesogenesis mechanism in tension. Finally, we conclude that upcoming experimental searches for C P violation in B meson mixing at LHCb and Belle II are unlikely to detect a new physics signal for the most generic models. Published by the American Physical Society 2024