Physics Quick Hits

Lots of interesting papers today. Little time to write, so only minimal commentary for now.

The reactor antineutrino anomaly still isn't real.

The Reactor Antineutrino Anomaly refers to the deficit observed between the average event rate measured in reactor antineutrino experiments with respect to the theoretical prediction. This anomaly was first identified in 2011 (2.5σ) as a consequence of the Huber-Muller reactor antineutrino flux calculation. It was thought to be resolved in 2021 as a result of new reactor antineutrino flux calculations, with a reduction to about 1σ. In this work, we examine the latest reactor antineutrino flux calculation published in 2023 by a French research group. This work represents the first summation model to include a comprehensive uncertainty budget. The result indicates a revival of the Reactor Antineutrino Anomaly at the level of 2.2σ. We also consider the usual simplest explanation of the Reactor Antineutrino Anomaly by active-sterile neutrino oscillations. We present the constraints on the oscillation parameters and we derive a tension of 3.8σ with the results of gallium source experiments (Gallium Anomaly) taking into account also the solar neutrino and KATRIN bounds, that of the combined short-baseline reactor spectral ratio measurements, and that of the Daya Bay search for a sub-eV sterile neutrino. Since the tension may be due to underestimated systematic uncertainties and the main tension is between the gallium data and the other data, we finally present the results of a global analysis with enlarged gallium uncertainties, which reduce the global tension to 1.3σ.

C. Giunti, Y.F. Li, R.P. Zhang, "Revival of the Reactor Antineutrino Anomaly" arXiv:2605.10353 (May 11, 2026).

Intriguing.

The charged-lepton Koide relation remains a striking empirical regularity in Standard-Model flavor data. We prove that for any positive mass set with Koide ratio Q0, the one-particle extension Q(m1,…,mN,x) has a unique global minimum Qmin=Q0/(1+Q0) at m∗=[(∑imi)/(∑imi‾‾‾√)]2. This exact kinematic result defines a unique extension benchmark. For the measured charged leptons it gives mℓ∗=1.25534(16)GeV and Qexp4,min=0.3999978(43); in the ideal Koide limit QKℓ=2/3, the corresponding minimum is exactly 2/5. In the effective-participant language Neff≡1/Q, the optimal one-particle extension increases Neff by one, while the equal-k multiplet extension increases it by k. The one-particle Neff profile is exactly Lorentzian in a dimensionless share-mismatch coordinate u, which we interpret kinematically rather than dynamically. Using charged-lepton pole masses with the PDG~2024 own-scale MS⎯⎯⎯⎯⎯⎯⎯⎯⎯ charm mass gives Q(e,μ,τ,c)=0.4000025(64), i.e. 11.7ppm above the measured-input benchmark and 6.2ppm above 2/5. This intentionally mixed-definition comparison is treated only as a phenomenological coincidence. To calibrate it within a stated benchmark class, we perform an exhaustive common-scale scan over non-neutrino Standard Model 2-body and 3-body seeds with one added mass. The charged-lepton-plus-charm continuation ranks 33/12,720 in the raw trial set, 24/2,640 after collapsing repeated scale realizations, and 6/756 within the fermion-only collapsed subset. We present the charm case as an empirically calibrated example of the theorem, not as a dynamical flavor model.

K. Hübner, "A minimization theorem for the Koide ratio and its Standard Model calibration" arXiv:2605.09651 (May 10, 2026).

So what?

Koide's charged-lepton relation suggests that (me‾‾‾√,mμ‾‾‾√,mτ‾‾‾√) is the natural family vector. We construct an effective compact-cycle model in which this vector is sampled from one real amplitude Z(ϕ) on an internal circle, while the masses are quadratic overlaps, ma∝|Z(2πa/3)|2. The amplitude is built from the two lowest antiperiodic modes on the circle; their symmetric square is periodic and gives the minimal three-harmonic family space e^iϕ,1,e^−iϕ. A reality condition together with the requirement that the amplitude comes from the square of one two-component spinor fixes the relative weights required by Koide's 45º geometry. The remaining orientation angle is fixed by matching one C3 family shift to transport on the full circle: integrating out the higher Fourier harmonics gives the Berry dressing that enters the determinant term and selects θℓ=−2/9. Using me and mμ as inputs, the model predicts mτ=1776.97MeV.

Kirill Shulga, "Charged-Lepton Koide Geometry from a Green-Dressed Compact Family Cycle" arXiv:2605.10245 (May 11, 2026).

Similar to another recent paper.

We show how, by exploiting the process of Coherent Elastic neutrino (v) Nucleus Scattering (CEvNS), neutrinos produced by nuclear reactor experiments appear to corroborate the evidence of the so-called X17 particle, which has been invoked to explain the ATOMKI anomaly. We base our analysis primarily on CONUS+ and Dresden-II data, which, when combined with CEvNS data from COHERENT and neutrino oscillation data from IceCube, single out a unique region of couplings to neutrinos and nuclei.

Johan Rathsman, Joakim Cederkäll, Yasar Hicyilmaz, Else Lytken, Stefano Moretti, "The X17 Existence Hinted at by Nuclear Reactor Neutrinos" arXiv:2605.10689 (May 11, 2026) (Short version of 2603.15246 using a different model for the X17).

Neutrinos do not have negative mass, so something isn't quite right in the model to estimate its masses from cosmology.

Recent baryon acoustic oscillation (BAO) distance measurements, when combined with Cosmic Microwave Background (CMB) observations in the ΛCDM framework, lead to a preference for negative neutrino masses. We investigate whether this neutrino mass anomaly can be alleviated by a class of astrophysically motivated reionization histories. Using a frequentist analysis, we find that some reionization histories can move the best-fit value of ∑mν to a positive value and bring ∑mν ≃ 0.06 eV into the 95% confidence interval. To separate the effect of the total optical depth from that of the details of the reionization history, we compare a high-τ history with a two-step tanh-like reionization history of the same τ. The resulting Δχ2(∑mν) profiles are nearly identical. This indicates that the effect is mainly driven by the total optical depth, while the details of the reionization history play only a minor role.

Yi Cheng Dai, Wei Liao, "Reionization History and Neutrino Mass" arXiv:2605.10116 (May 11, 2026).

As expected.

Modified Newtonian Dynamics (MOND) is a paradigm that can do away with dark matter at galaxy scales, but displays a residual missing mass discrepancy in galaxy clusters. Prompted by the updated JWST-based gravitational lens model of the Bullet Cluster, I confirm here that this cluster exhibits the same residual missing mass discrepancy as other clusters of similar mass in the MOND context. Moreover, this missing mass should be mostly collisionless, since it is centred on the galaxies of the Bullet Cluster.

Benoit Famaey, "On the residual missing mass of the Bullet Cluster" arXiv:2605.10022 (May 11, 2026).

Color me skeptical. It will take a closer look to poke holes in it, however. I suspect that while it may point out problems in toy-model MOND and some other models, this data could actually point the way towards a better modified gravity theory rather than towards dark matter particles which have myriad problems of their own that are ignored in this study.

Modified gravity theories such as Modified Newtonian Dynamics (MOND) and Scalar-Tensor-Vector Gravity (STVG) have been proposed as alternatives to dark matter, but decisive tests have been hindered by degeneracies between baryonic structure and gravitational laws. Here we break this degeneracy using independent, high-precision constraints: the Milky Way radial rotation curve, vertical phase-space spirals from Gaia, and a broken-exponential stellar disk. A joint reconstruction of the radial and vertical gravitational fields reveals a structural inconsistency in modified gravity -- no model can simultaneously reproduce both observations. Our results strongly disfavor MOND at >13σ and STVG at >4σ. In contrast, dark matter halo models naturally explain the observations, providing a self-consistent test of gravity on galactic scales.

Zheng-long Wang, Yue-Lin Sming Tsai, Lan Zhang, Yin Wu, Haining Li, Xiang-Xiang Xue, Hongsheng Zhao, Yi-Zhong Fan, "Milky Way Dynamics Favor Dark Matter over Modified Gravity Models" arXiv:2605.10857 (May 11, 2026).