Large primordial lepton flavor asymmetries with almost vanishing total baryon-minus-lepton number can evade the usual BBN and CMB constraints if neutrino oscillations lead to perfect flavor equilibration. Solving the momentum averaged quantum kinetic equations (QKEs) describing neutrino oscillations and interactions, we perform the first systematic investigation of this scenario, uncovering a rich flavor structure in stark contradiction to the assumption of simple flavor equilibration. We find (i) a particular direction in flavor space, ∆$\textit{n}$ ≃ – 2/3 (– 1)∆$\textit{n}$ for normal (inverted) neutrino mass hierarchy, in which the flavor equilibration is efficient and primordial asymmetries are essentially unconstrained, (ii) a minimal washout factor, $$ {\left.\Delta {n}_e^2\right|}_{\textrm{BBN}} $$ $\textit{≤}$ 0.03 (0.016) ∑ $$ {\left.\Delta {n}_{\alpha}^2\right|}_{\textrm{ini}} $$ yielding a conservative estimate for the allowed primordial asymmetries in a generic flavor direction, and (iii) particularly strong or weak washout if one of the initial flavor asymmetries vanishes due to non-adiabatic muon- or electron-driven MSW transitions. These results open up the possibility of a first-order QCD phase transition facilitated by large lepton asymmetries as well as baryogenesis from large and compensated ∆$\textit{n}$ = ∆$\textit{n}$ asymmetries. Our systematic approach of deriving momentum averaged QKEs includes collision terms beyond the damping approximation, energy transfer between the neutrino and electron-photon plasma, and provides a fast and reliable way to investigate the impact of primordial lepton asymmetries at the time of BBN. We publicly release the Mathematica code COFLASY-M on https://github.com/mariofnavarro/COFLASY which solves the QKEs numerically.

Switzerland

Valerie Domcke

Miguel Escudero

United Kingdom

Mario Navarro

United States

Stefan Sandner

Lepton flavor asymmetries: from the early Universe to BBN

<math><mi>C</mi><mi>P</mi></math> violation in neutral <math><mi>B</mi></math> meson oscillations is an experimental observable that could be directly related to the baryon asymmetry of the Universe through the <math><mi>B</mi></math>-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 <math><mi>B</mi></math> physics program at the LHC and Belle II, we analyze how large <math><mi>C</mi><mi>P</mi></math> violation in the mixing of neutral <math><msub><mi>B</mi><mi>d</mi></msub></math> and <math><msub><mi>B</mi><mi>s</mi></msub></math> 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 <math><msubsup><mi>M</mi><mn>12</mn><mi>q</mi></msubsup></math>, (ii) vectorlike quark extensions that introduce deviations of <math><mn>3</mn><mo>×</mo><mn>3</mn></math> CKM unitarity, and (iii) light new physics modifying the decay mixing <math><msubsup><mi>Γ</mi><mn>12</mn><mi>q</mi></msubsup></math>. We find that enhancements of the semileptonic asymmetries, that measure the amount of <math><mi>C</mi><mi>P</mi></math> violation in mixing, at the level of <math><msup><mn>10</mn><mrow><mo>−</mo><mn>3</mn></mrow></msup></math> for the <math><msub><mi>B</mi><mi>d</mi></msub></math> system and <math><msup><mn>10</mn><mrow><mo>−</mo><mn>4</mn></mrow></msup></math> for the <math><msub><mi>B</mi><mi>s</mi></msub></math> 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 <math><mi>C</mi><mi>P</mi></math> asymmetries in our analysis puts the <math><mi>B</mi></math>-mesogenesis mechanism in tension. Finally, we conclude that upcoming experimental searches for <math><mi>C</mi><mi>P</mi></math> violation in <math><mi>B</mi></math> meson mixing at LHCb and Belle II are unlikely to detect a new physics signal for the most generic models.

Spain

Carlos Miró

CERN

Miguel Nebot

How large could the <math><mi>C</mi><mi>P</mi></math> violation in neutral <math><mi>B</mi></math>-meson mixing be? Implications for baryogenesis and upcoming searches

Germany

Thomas Schwetz

Jorge Terol-Calvo

Addendum to: A seesaw model for large neutrino masses in concordance with cosmology

If dark matter is composed of axions, then axion stars form in the cores of dark matter halos. These stars are unstable above a critical mass, decaying to radio photons that heat the intergalactic medium, offering a new channel for axion indirect detection. We recently provided the first accurate calculation of the axion decay rate due to axion star mergers. In this work we show how existing data concerning the CMB optical depth leads to strong constraints on the axion photon coupling in the mass range <math><msup><mn>10</mn><mrow><mo>−</mo><mn>14</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi><mo>≲</mo><mspace></mspace><msub><mi>m</mi><mi>a</mi></msub><mo>≲</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>8</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></math>. Axion star decays lead to efficient reionization of the intergalactic medium during the dark ages. By comparing this nonstandard reionization with Planck legacy measurements of the Thomson optical width, we show that couplings in the range <math><msup><mn>10</mn><mrow><mo>−</mo><mn>14</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>GeV</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup><mo>≲</mo><msub><mi>g</mi><mrow><mi>a</mi><mi>γ</mi><mi>γ</mi></mrow></msub><mo>≲</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>10</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>GeV</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> are excluded for our benchmark model of axion star abundance. Future measurements of the 21 cm emission of neutral hydrogen at high redshift could improve this limit by an order of magnitude or more, providing complementary indirect constraints on axion dark matter in parameter space also targeted by direct detection haloscopes.

Charis Kaur Pooni

Malcolm Fairbairn

Diego Blas

Xiaolong Du

David J. E. Marsh

Axion star explosions: A new source for axion indirect detection

The effective number of relativistic neutrino species is a fundamental probe of the early Universe, and its measurement represents a key constraint on many scenarios beyond the Standard Model of Particle Physics. In light of this, an accurate prediction of <math><msub><mi>N</mi><mi>eff</mi></msub></math> in the Standard Model is of pivotal importance. In this work, we consider the last ingredient needed to accurately calculate <math><msubsup><mi>N</mi><mi>eff</mi><mrow><mi>SM</mi></mrow></msubsup></math>: standard zero and finite-temperature QED corrections to <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup><mo>↔</mo><mi>ν</mi><mover><mi>ν</mi><mo>¯</mo></mover></math> interaction rates during neutrino decoupling at temperatures around <math><mi>T</mi><mo>∼</mo><mi>MeV</mi></math>. We find that this effect leads to a reduction of <math><mo>−</mo><mn>0.0007</mn></math> in <math><msubsup><mi>N</mi><mi>eff</mi><mrow><mi>SM</mi></mrow></msubsup></math>. This next-to-leading-order QED correction to the interaction rates, together with finite-temperature QED corrections to the electromagnetic density of the plasma, and the effect of neutrino oscillations, implies that <math><msubsup><mi>N</mi><mi>eff</mi><mrow><mi>SM</mi></mrow></msubsup><mo>=</mo><mn>3.043</mn></math> with a theoretical uncertainty that is much smaller than any projected observational sensitivity.

Italy

Mattia Cielo

Gianpiero Mangano

Ofelia Pisanti

<math><msub><mi>N</mi><mrow><mi>eff</mi></mrow></msub></math> in the Standard Model at NLO is 3.043

James Alvey

Erratum to: The axion quality problem: global symmetry breaking and wormholes

The cosmic microwave background (CMB) has proven to be an invaluable tool for studying the properties and interactions of neutrinos, providing insight not only into the sum of neutrino masses but also the free streaming nature of neutrinos prior to recombination. The CMB is a particularly powerful probe of new eV-scale bosons interacting with neutrinos, as these particles can thermalize with neutrinos via the inverse decay process,  $$\nu \bar{\nu } \rightarrow X$$  <math> <mrow> <mi>ν</mi> <mover> <mrow> <mi>ν</mi> </mrow> <mrow> <mo>¯</mo> </mrow> </mover> <mo>→</mo> <mi>X</mi> </mrow> </math>  , and suppress neutrino free streaming near recombination – even for couplings as small as  $$\lambda _{\nu } \sim {\mathcal {O}}(10^{-13})$$  <math> <mrow> <msub> <mi>λ</mi> <mi>ν</mi> </msub> <mo>∼</mo> <mi>O</mi> <mrow> <mo>(</mo> <msup> <mn>10</mn> <mrow> <mo>-</mo> <mn>13</mn> </mrow> </msup> <mo>)</mo> </mrow> </mrow> </math>  . Here, we revisit CMB constraints on such bosons, improving upon a number of approximations previously adopted in the literature and generalizing the constraints to a broader class of models. This includes scenarios in which the boson is either spin-0 or spin-1, the number of interacting neutrinos is either  $$N_{\textrm{int}} = 1,2 $$  <math> <mrow> <msub> <mi>N</mi> <mtext>int</mtext> </msub> <mo>=</mo> <mn>1</mn> <mo>,</mo> <mn>2</mn> </mrow> </math>   or 3, and the case in which a primordial abundance of the species is present. We apply these bounds to well-motivated models, such as the singlet majoron model or a light  $$U(1)_{L_{\mu }-L_{\tau }}$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <msub> <mi>L</mi> <mi>μ</mi> </msub> <mo>-</mo> <msub> <mi>L</mi> <mi>τ</mi> </msub> </mrow> </msub> </mrow> </math>   gauge boson, and find that they represent the leading constraints for masses  $$m_X\sim 1\, {\textrm{eV}}$$  <math> <mrow> <msub> <mi>m</mi> <mi>X</mi> </msub> <mo>∼</mo> <mn>1</mn> <mspace width="0.166667em"></mspace> <mtext>eV</mtext> </mrow> </math>  . Finally, we revisit the extent to which neutrino-philic bosons can ameliorate the Hubble tension, and find that recent improvements in the understanding of how such bosons damp neutrino free streaming reduces the previously found success of this proposal.

Netherlands

Samuel Witte

Precision CMB constraints on eV-scale bosons coupled to neutrinos

Using a new sample of extremely metal poor systems, the EMPRESS survey has recently reported a primordial helium abundance that is <math><mn>3</mn><mi>σ</mi></math> smaller than the prediction from the standard big bang nucleosynthesis (BBN) scenario. This measurement could be interpreted as a hint for a primordial lepton asymmetry in the electron neutrino flavor. Motivated by the EMPRESS results, we present a comprehensive analysis of the lepton asymmetry using measurements of the abundances of primordial elements, along with cosmic microwave background (CMB) data from Planck. Assuming that there is no dark radiation in our Universe, we find an electron neutrino chemical potential <math><mrow><msub><mrow><mi>ξ</mi></mrow><mrow><msub><mrow><mi>ν</mi></mrow><mrow><mi>e</mi></mrow></msub></mrow></msub><mo>=</mo><mn>0.043</mn><mo>±</mo><mn>0.015</mn></mrow></math>, which deviates from zero by <math><mn>2.9</mn><mi>σ</mi></math>. If no assumption is made on the abundance of dark radiation in the Universe, the chemical potential is <math><mrow><msub><mrow><mi>ξ</mi></mrow><mrow><msub><mrow><mi>ν</mi></mrow><mrow><mi>e</mi></mrow></msub></mrow></msub><mo>=</mo><mn>0.046</mn><mo>±</mo><mn>0.021</mn></mrow></math>, which deviates from zero by <math><mn>2.2</mn><mi>σ</mi></math>. We also find that this result is rather insensitive to the choice of nuclear reaction rates. If the true helium abundance corresponds to the EMPRESS central value, future CMB observations from the Simons Observatory and CMB-S4 will increase the significance for a nonzero lepton asymmetry to <math><mn>4</mn><mi>σ</mi></math> and <math><mn>5</mn><mi>σ</mi></math> respectively, assuming no dark radiation, or to <math><mn>3</mn><mi>σ</mi></math> when no assumption is made on the abundance of dark radiation.

Alejandro Ibarra

Victor Maura

Primordial lepton asymmetries in the precision cosmology era: Current status and future sensitivities from BBN and the CMB

Cosmological constraints on the sum of the neutrino masses can be relaxed if the number density of active neutrinos is reduced compared to the standard scenario, while at the same time keeping the effective number of neutrino species N eff ≈ 3 by introducing a new component of dark radiation. We discuss a UV complete model to realise this idea, which simultaneously provides neutrino masses via the seesaw mechanism. It is based on a U(1) symmetry in the dark sector, which can be either gauged or global. In addition to heavy seesaw neutrinos, we need to introduce 𝒪(10) generations of massless sterile neutrinos providing the dark radiation. Then we can accommodate active neutrino masses with ∑m  ν  ~ 1 eV, in the sensitivity range of the KATRIN experiment. We discuss the phenomenology of the model and identify the allowed parameter space. We argue that the gauged version of the model is preferred, and in this case the typical energy scale of the model is in the 10 MeV to few GeV range.

A seesaw model for large neutrino masses in concordance with cosmology

Dark sector particles at the GeV scale carrying baryon number provide an attractive framework for understanding the origin of dark matter and the matter-antimatter asymmetry of the universe. We demonstrate that dark decays of hadronic states containing strange quarks—hyperons—offer excellent prospects for discovering such dark baryons. Building up on novel calculations of the matrix elements relevant for hyperon dark decays, and in view of various collider, flavor, and astrophysical constraints, we determine the expected rates at hyperon factories like BESIII and LHCb. We also highlight the interesting theoretical connections of hyperon dark decays to the neutron lifetime anomaly and mesogenesis.