It has been claimed that the coherent scattering of relic neutrinos with the Earth will result in a neutrino-antineutrino asymmetry of   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (10−4) on the Earth surface, which is five orders of magnitude larger than the naive model expectation. In this work we show that this overdensity was overestimated for the perfectly round Earth by solving the exact solution with partial waves. After summing over all the angular modes, the maximal asymmetry is only around 10−8 above the ground. To achieve the proposed asymmetry of   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (10−4), a special geography may be needed as the experimental site. The code for a fast evaluation of Bessel functions with large orders through asymptotic expansions has been attached along with this work as supplementary material.

Germany

China

Guo-yuan Huang

Neutrino-antineutrino asymmetry of CνB on the surface of the round Earth

We infer the ultrahigh energy neutrino source by using the Glashow resonance candidate event recently identified by the IceCube Observatory. For the calculation of the cross section for the Glashow resonance, we incorporate both the atomic Doppler broadening effect and initial state radiation   <math> <msub> <mover> <mi>ν</mi> <mo>¯</mo> </mover> <mi>e</mi> </msub> <msup> <mi>e</mi> <mo>−</mo> </msup> <mo>→</mo> <msup> <mi>W</mi> <mo>−</mo> </msup> <mi>γ</mi> </math>  $$ {\overline{\nu}}_e{e}^{-}\to {W}^{-}\gamma $$ , which correct the original cross section considerably. Using available experimental information, we have set a generic constraint on the   <math> <msub> <mover> <mi>ν</mi> <mo>¯</mo> </mover> <mi>e</mi> </msub> </math>  $$ {\overline{\nu}}_e $$  fraction of astrophysical neutrinos, which excludes the μ-damped pγ source around 2σ confidence level under the assumption that neutrino production is dominated by the ∆-resonance. While a weak preference has been found for the pp source, next-generation measurements will be able to distinguish between ideal pp and pγ sources with a high significance assuming an optimistic single power-law neutrino spectrum. The inclusion of multi-pion production at very high energies for the neutrino source can weaken the discrimination power. In this case additional multimessenger information is needed to distinguish between pp and pγ sources.

Manfred Lindner

Nele Volmer

Inferring astrophysical neutrino sources from the Glashow resonance

We explore beta decays in a dark background field, which could be formed by dark matter, dark energy or a fifth force potential. In such scenarios, the neutrino's dispersion relation will be modified by its collective interaction with the dark field, which can have interesting consequences in experiments using tritium beta decays to determine the absolute neutrino mass. Among the most general interaction forms, the (pseudo)scalar and (axial-)vector ones are found to have interesting effects on the spectrum of beta decays. In particular, the vector and axial-vector potentials can induce distinct signatures by shifting the overall electron energy scale, possibly beyond the usually defined endpoint. The scalar and pseudoscalar potentials are able to mimic a neutrino mass beyond the cosmological bounds. We have placed stringent constraints on the dark potentials based on the available experimental data of KATRIN. The sensitivity of future KATRIN runs is also discussed.

Werner Rodejohann

Tritium beta decay with modified neutrino dispersion relations: KATRIN in the dark sea

The question of how heavy a sterile neutrino can be probed in experiments leads us to investigate the Primakoff production of heavy sterile neutrinos from ultrahigh-energy neutrinos via the magnetic dipole portal. Despite the suppression from the small magnetic moment, the transition is significantly enhanced by tiny t-channel momentum transfers, similar to the resonant production of pions and axions in an external electromagnetic field. Based on the current IceCube measurement of astrophysical neutrinos up to PeV energies, strong constraints can already be derived on the transition magnetic moments of sterile neutrinos up to TeV masses. Moreover, we investigate the sensitivity of future tau neutrino telescopes, which are designed for EeV cosmogenic neutrino detection. We find that sterile neutrino masses as large as <math><mn>30</mn><mspace width="0.25em"></mspace><mtext>TeV</mtext></math> can be probed at tau neutrino telescopes such as GRAND, POEMMA, and Trinity.

Sudip Jana

Probing heavy sterile neutrinos at neutrino telescopes via the dipole portal

In light of the exciting campaign of cosmogenic neutrino detection, we investigate the double and multiple tau bangs detectable at future tau neutrino telescopes. Such events are expected from the Standard Model (SM) higher-order processes, which can be easily identified with broad techniques anticipated at future tau neutrino telescopes. We find that SM perturbative processes can already contribute observable double-bang events to telescopes with a sensitivity of collecting  $${\mathcal {O}}(100)$$  <math> <mrow> <mi>O</mi> <mo>(</mo> <mn>100</mn> <mo>)</mo> </mrow> </math>   cosmogenic neutrino events. The detectable but suppressed rate in fact makes the double and multiple bangs an excellent probe of SM unknowns and possible new physics beyond. As a case study, the nonperturbative sphaleron process, which can copiously produce multiple tau bangs, is explored.

Double and multiple bangs at tau neutrino telescopes

Whether the neutrino mass ordering is normal or inverted remains an experimentally open issue in neutrino physics. The knowledge of neutrino mass ordering has great importance for neutrinoless double-beta ( $$ 0\nu \beta \beta $$  <math> <mrow> <mn>0</mn> <mi>ν</mi> <mi>β</mi> <mi>β</mi> </mrow> </math>  ) decay experiments, which can establish the nature of massive neutrinos, i.e., whether they are Dirac or Majorana fermions. Recently, the KamLAND-Zen 800 measurement has reached for the first time the parameter space of the inverted ordering with a vanishing lightest neutrino mass. By assuming the inverted ordering, we attempt to derive the physical information of the neutrino nature and Majorana CP phases from a negative or positive observation of  $$ 0\nu \beta \beta $$  <math> <mrow> <mn>0</mn> <mi>ν</mi> <mi>β</mi> <mi>β</mi> </mrow> </math>   decays in the near future. Moreover, the possibility of extracting the nuclear matrix element in the case of a positive observation is also examined.To avoid the ambiguity from unknown priors of neutrino masses, we adopt the maximum likelihood method instead of the Bayesian approach usually considered in previous works.

Italy

Newton Nath

Inference of neutrino nature and Majorana CP phases from  $$\mathbf{0}{\nu \beta \beta }$$  <math> <mrow> <mn>0</mn> <mrow> <mi>ν</mi> <mi>β</mi> <mi>β</mi> </mrow> </mrow> </math>   decays with inverted mass ordering

We investigate a consistent scenario of time-varying neutrino masses, and discuss its impact on cosmology, beta decay, and neutrino oscillation experiments. Such time-varying masses are assumed to be generated by the coupling between a sterile neutrino and an ultralight scalar field, which in turn affects the light neutrinos by mixing. We demonstrate how various cosmological bounds, such as those coming from big bang nucleosynthesis, the cosmic microwave background, as well as large scale structures, can be evaded in this model. This scenario can be further constrained using multiple terrestrial experiments. In particular, for beta-decay experiments like KATRIN, nontrivial distortions to the electron spectrum can be induced, even when time-variation is fast and it gets averaged. Furthermore, the presence of time-varying masses of sterile neutrinos will alter the interpretation of light sterile neutrino parameter space in the context of the reactor and gallium anomalies. In addition, we also study the impact of such time-varying neutrino masses on results from the BEST collaboration, which have recently strengthened the gallium anomaly. If confirmed, we find that the time-varying neutrino mass hypothesis could give a better fit to the recent BEST data.

Spain

Pablo Martínez-Miravé

Manibrata Sen

Cosmology-friendly time-varying neutrino masses via the sterile neutrino portal

The LHCb measurements of the <math><mi>μ</mi><mo>/</mo><mi>e</mi></math> ratio in <math><mi>B</mi><mo>→</mo><mi>K</mi><mo>ℓ</mo><mo>ℓ</mo></math> decays <math><mo>(</mo><msub><mi>R</mi><mrow><mi>K</mi></mrow></msub><mo>)</mo></math> indicate a deficit with respect to the Standard Model prediction, supporting earlier hints of lepton universality violation observed in the <math><msub><mi>R</mi><msup><mi>K</mi><mrow><mo>(</mo><mo>*</mo><mo>)</mo></mrow></msup></msub></math> ratio. Possible explanations of these <math><mi>B</mi></math>-physics anomalies include heavy <math><msup><mi>Z</mi><mo>′</mo></msup></math> bosons or scalar and vector leptoquarks mediating <math><mi>b</mi><mo>→</mo><mi>s</mi><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup></math>. We note that a muon collider can directly measure this process via <math><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup><mo>→</mo><mi>b</mi><mover><mi>s</mi><mo>¯</mo></mover></math> and can shed light on the lepton nonuniversality scenario. Investigating currently discussed center-of-mass energies <math><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>3</mn></math>, 6 and 10 TeV, we show that the parameter space of <math><msup><mi>Z</mi><mo>′</mo></msup></math> and leptoquark solutions to the <math><msub><mi>R</mi><msup><mi>K</mi><mrow><mo>(</mo><mo>*</mo><mo>)</mo></mrow></msup></msub></math> anomalies can be mostly covered. Effective operators explaining the anomalies can be probed with the muon collider setup <math><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>6</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math> and integrated luminosity <math><mrow><mi>L</mi><mo>=</mo><mn>4</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>ab</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math>.

Brazil

Farinaldo S. Queiroz

Probing the <math><msub><mi>R</mi><msup><mi>K</mi><mrow><mo>(</mo><mo>*</mo><mo>)</mo></mrow></msup></msub></math> anomaly at a muon collider

The Hubble parameter inferred from cosmic microwave background observations is consistently lower than that from local measurements, which could hint towards new physics. Solutions to the Hubble tension typically require a sizable amount of extra radiation <math><mi>Δ</mi><msub><mi>N</mi><mi>eff</mi></msub></math> during recombination. However, the amount of <math><mi>Δ</mi><msub><mi>N</mi><mi>eff</mi></msub></math> in the early Universe is unavoidably constrained by big bang nucleosynthesis (BBN), which causes problems for such solutions. We present a possibility to evade this problem by introducing neutrino self-interactions via a simple Majoron-like coupling. The scalar is slightly heavier than 1 MeV and allowed to be fully thermalized throughout the BBN era. The rise of neutrino temperature due to the entropy transfer via <math><mrow><mi>ϕ</mi><mo>→</mo><mi>ν</mi><mover><mrow><mi>ν</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow></math> reactions compensates for the effect of a large <math><mi>Δ</mi><msub><mi>N</mi><mi>eff</mi></msub></math> on BBN. Values of <math><mi>Δ</mi><msub><mi>N</mi><mi>eff</mi></msub></math> as large as 0.7 are in this case compatible with BBN. We perform a fit to the parameter space of the model.

Solving the Hubble tension without spoiling big bang nucleosynthesis

We investigate the sensitivity of the projected TeV muon collider to the gauged <math><msub><mi>L</mi><mi>μ</mi></msub><mo>−</mo><msub><mi>L</mi><mi>τ</mi></msub></math> model. Two processes are considered: <math><msup><mi>Z</mi><mo>′</mo></msup></math>-mediated two-body scatterings <math><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup><mo>→</mo><msup><mo>ℓ</mo><mo>+</mo></msup><msup><mo>ℓ</mo><mo>−</mo></msup></math> with <math><mo>ℓ</mo><mo>=</mo><mi>μ</mi></math> or <math><mi>τ</mi></math> and scattering with initial state photon emission, <math><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup><mo>→</mo><mi>γ</mi><msup><mi>Z</mi><mo>′</mo></msup><mo>,</mo><msup><mi>Z</mi><mo>′</mo></msup><mo>→</mo><mo>ℓ</mo><mover><mo>ℓ</mo><mo>¯</mo></mover></math>, where <math><mo>ℓ</mo></math> can be <math><mi>μ</mi></math>, <math><mi>τ</mi></math>, or <math><msub><mi>ν</mi><mrow><mi>μ</mi><mo>/</mo><mi>τ</mi></mrow></msub></math>. We quantitatively study the sensitivities of these two processes by taking into account possible signals and relevant backgrounds in a muon collider experiment with a center-of-mass energy <math><mrow><msqrt><mrow><mi>s</mi></mrow></msqrt><mo>=</mo><mn>3</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math> and a luminosity <math><mrow><mi>L</mi><mo>=</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>ab</mi></mrow><mrow><mi>−</mi><mn>1</mn></mrow></msup></mrow></math>. For two-body scattering, one can exclude <math><msup><mi>Z</mi><mo>′</mo></msup></math> masses <math><mrow><msub><mrow><mi>M</mi></mrow><mrow><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></mrow></msub><mo>≲</mo><mn>100</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math> with <math><mi>O</mi><mo>(</mo><mn>1</mn><mo>)</mo></math> gauge couplings. When <math><mrow><msub><mrow><mi>M</mi></mrow><mrow><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></mrow></msub><mo>≲</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi><mo>&lt;</mo><msqrt><mrow><mi>s</mi></mrow></msqrt></mrow></math>, one can exclude <math><msup><mi>g</mi><mo>′</mo></msup><mo>≳</mo><mn>2</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>2</mn></mrow></msup></math>. The process with photon emission is more powerful than the two-body scattering if <math><msub><mi>M</mi><msup><mi>Z</mi><mo>′</mo></msup></msub><mo>&lt;</mo><msqrt><mi>s</mi></msqrt></math>. For instance, a sensitivity of <math><msup><mi>g</mi><mo>′</mo></msup><mo>≃</mo><mn>4</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>3</mn></mrow></msup></math> can be achieved at <math><mrow><msub><mrow><mi>M</mi></mrow><mrow><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></mrow></msub><mo>=</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math>. The parameter spaces favored by the <math><mrow><mo>(</mo><mi>g</mi><mo>−</mo><mn>2</mn><msub><mrow><mo>)</mo></mrow><mrow><mi>μ</mi></mrow></msub></mrow></math> and <math><mi>B</mi></math> anomalies with <math><mrow><msub><mrow><mi>M</mi></mrow><mrow><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></mrow></msub><mo>&gt;</mo><mn>100</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math> are entirely covered by a muon collider.